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Linkage disequilibrium

In population genetics, linkage disequilibrium (LD) is a measure of non-random association between segments of DNA (alleles) at different positions on the chromosome (loci) in a given population based on a comparison between the frequency at which two alleles are detected together at the same loci and the frequencies at which each allele is detected at that loci overall, whether it occurs with or without the other allele of interest. Loci are said to be in linkage disequilibrium when the frequency of being detected together (the frequency of association of their different alleles) is higher or lower than expected if the loci were independent and associated randomly.

While the pattern of linkage disequilibrium in a genome is a powerful signal of the population genetic processes that are structuring it, it does not indicate why the pattern emerges by itself. Linkage disequilibrium is influenced by many factors, including selection, the rate of genetic recombination, mutation rate, genetic drift, the system of mating, population structure, and genetic linkage.

In spite of its name, linkage disequilibrium may exist between alleles at different loci without any genetic linkage between them and independently of whether or not allele frequencies are in equilibrium (not changing with time). Furthermore, linkage disequilibrium is sometimes referred to as gametic phase disequilibrium; however, the concept also applies to asexual organisms and therefore does not depend on the presence of gametes.

Formal definition
Suppose that among the gametes that are formed in a sexually reproducing population, allele A occurs with frequency $$ p_A $$ at one locus (i.e. $$ p_A $$ is the proportion of gametes with A at that locus), while at a different locus allele B occurs with frequency $$ p_B $$. Similarly, let $$ p_{AB} $$ be the frequency with which both A and B occur together in the same gamete (i.e. $$ p_{AB} $$ is the frequency of the AB haplotype).

The association between the alleles A and B can be regarded as completely random—which is known in statistics as independence—when the occurrence of one does not affect the occurrence of the other, in which case the probability that both A and B occur together is given by the product $$ p_{A} p_{B} $$ of the probabilities. There is said to be a linkage disequilibrium between the two alleles whenever $$ p_{AB} $$ differs from $$ p_A p_B $$ for any reason.

The level of linkage disequilibrium between A and B can be quantified by the coefficient of linkage disequilibrium $$ D_{AB}$$, which is defined as

 * $$ D_{AB} = p_{AB} - p_A p_B,$$

Linkage disequilibrium corresponds to $$ D_{AB} \neq 0 $$. In the case $$D_{AB}=0$$ we have $$ p_{AB} = p_A p_B $$ and the alleles A and B are said to be in linkage equilibrium. The subscript "AB" on $$ D_{AB} $$ emphasizes that linkage disequilibrium is a property of the pair $$\{A, B\}$$ of alleles and not of their respective loci. Other pairs of alleles at those same two loci may have different coefficients of linkage disequilibrium.

For two biallelic loci, where a and b are the other alleles at these two loci, the restrictions are so strong that only one value of D is sufficient to represent all linkage disequilibrium relationships between these alleles. In this case, $$D_{AB} = -D_{Ab} = -D_{aB} = D_{ab}$$. Their relationships can be characterized as follows.

$$D = P_{AB} -P_{A}P_{B}$$

$$-D = P_{Ab} -P_{A}P_{b}$$

$$-D = P_{aB} -P_{a}P_{B}$$

$$D = P_{ab} -P_{a}P_{b}$$

The sign of D in this case is chosen arbitrarily. The magnitude of D is more important than the sign of D because the magnitude of D is representative of the degree of linkage disequilibrium. However, positive D value means that the gamete is more frequent than expected while negative means that the combination of these two alleles are less frequent than expected.

Linkage disequilibrium in asexual populations can be defined in a similar way in terms of population allele frequencies. Furthermore, it is also possible to define linkage disequilibrium among three or more alleles, however these higher-order associations are not commonly used in practice.

Normalization
The linkage disequilibrium $$D$$ reflects both changes in the intensity of the linkage correlation and changes in gene frequency. This poses an issue when comparing linkage disequilibrium between alleles with differing frequencies. Normalization of linkage disequilibrium allows these alleles to be compared more easily.

D' Method
Lewontin suggested calculating the normalized linkage disequilibrium (also referred to as relative linkage disequilibrium) $$D'$$ by dividing $$D$$ by the theoretical maximum difference between the observed and expected allele frequencies as follows:

 * $$D' = \frac D {D_\max}$$

where

 * $$D_\max= \begin{cases}

\min\{p_A p_B,\,(1-p_A)(1-p_B)\} & \text{when } D < 0\\ \min\{p_A (1-p_B),\,p_B(1-p_A)\} & \text{when } D > 0 \end{cases} $$

The value of $$D'$$ will be within the range $$-1\leq D'\leq 1$$. When $D' = 0$, the loci are independent. When $-1 \leq D' < 0$, the alleles are found less often than expected. When $0 < D' \leq 1$, the alleles are found more often than expected.

Note that $$|D'|$$ may be used in place of $$D'$$ when measuring how close two alleles are to linkage equilibrium.

r² Method
An alternative to $$D'$$ is the correlation coefficient between pairs of loci, usually expressed as its square, $$r^2$$.

 * $$r^2=\frac{D^2}{p_A(1-p_A)p_B (1-p_B)}$$

The value of $r^2$ will be within the range $-1 \leq r^2 \leq 1$. When $r^2 = 0$, there is no correlation between the pair. When $|r^2| = 1$, the correlation is either perfect positive or perfect negative according to the sign of $r^2$.

d Method
Another alternative normalizes $$D$$ by the product of two of the four allele frequencies when the two frequencies represent alleles from the same locus. This allows comparison of asymmetry between a pair of loci. This is often used in case-control studies where $$B$$ is the locus containing a disease allele.

$$d =\frac{D}{p_B (1-p_B)}$$

ρ Method
Similar to the d method, this alternative normalizes $$D$$ by the product of two of the four allele frequencies when the two frequencies represent alleles from different loci.

$$\rho =\frac{D}{(1-p_A) p_B}$$

Limits for the ranges of linkage disequilibrium measures
The measures $$r^2$$ and $$D'$$ have limits to their ranges and do not range over all values of zero to one for all pairs of loci. The maximum of $$r^2$$ depends on the allele frequencies at the two loci being compared and can only range fully from zero to one where either the allele frequencies at both loci are equal, $$P_A=P_B$$ where $$D>0$$, or when the allele frequencies have the relationship $$P_A=1-P_B$$ when $$D<0$$. While $$D'$$ can always take a maximum value of 1, its minimum value for two loci is equal to $$|r|$$ for those loci.

Example: Two-loci and two-alleles
Consider the haplotypes for two loci A and B with two alleles each—a two-loci, two-allele model. Then the following table defines the frequencies of each combination:

Note that these are relative frequencies. One can use the above frequencies to determine the frequency of each of the alleles:

If the two loci and the alleles are independent from each other, then we would expect the frequency of each haplotype to be equal to the product of the frequencies of its corresponding alleles (e.g. $$x_{11} = p_1 q_1$$).

The deviation of the observed frequency of a haplotype from the expected is a quantity called the linkage disequilibrium and is commonly denoted by a capital D:

 * $$D = x_{11} - p_1q_1$$

Thus, if the loci were inherited independently, then $$x_{11} = p_1 q_1$$, so $$D = 0$$, and there is linkage equilibrium. However, if the observed frequency of haplotype $$A_1B_1$$ were higher than what would be expected based on the individual frequencies of $$A_1$$ and $$B_1$$ then $$x_{11} > p_1 q_1$$, so $$D > 0$$, and there is positive linkage disequilibrium. Conversely, if the observed frequency were lower, then $$x_{11} < p_1 q_1$$, $$D < 0$$, and there is negative linkage disequilibrium.

The following table illustrates the relationship between the haplotype frequencies and allele frequencies and D.

Additionally, we can normalize our data based on what we are trying to accomplish. For example, if we aim to create an association map in a case-control study, then we may use the d method due to its asymmetry. If we are trying to find the probability that a given haplotype will descend in a population without being recombined by other haplotypes, then it may be better to use the ρ method. But for most scenarios, $$r^2$$ tends to be the most popular method due to the usefulness of the correlation coefficient in statistics. A couple examples of where $$r^2$$ may be very useful would include measuring the recombination rate in an evolving population, or detecting disease associations.

Role of recombination
In the absence of evolutionary forces other than random mating, Mendelian segregation, random chromosomal assortment, and chromosomal crossover (i.e. in the absence of natural selection, inbreeding, and genetic drift), the linkage disequilibrium measure $$D$$ converges to zero along the time axis at a rate depending on the magnitude of the recombination rate $$c$$ between the two loci.

Using the notation above, $$D= x_{11}-p_1 q_1$$, we can demonstrate this convergence to zero as follows. In the next generation, $$x_{11}'$$, the frequency of the haplotype $$A_1 B_1$$, becomes

 * $$x_{11}' = (1-c)\,x_{11} + c\,p_1 q_1$$

This follows because a fraction $$(1-c)$$ of the haplotypes in the offspring have not recombined, and are thus copies of a random haplotype in their parents. A fraction $$x_{11}$$ of those are $$A_1 B_1$$. A fraction $$c$$ have recombined these two loci. If the parents result from random mating, the probability of the copy at locus $$A$$ having allele $$A_1$$ is $$p_1$$ and the probability of the copy at locus $$B$$ having allele $$B_1$$ is $$q_1$$, and as these copies are initially in the two different gametes that formed the diploid genotype, these are independent events so that the probabilities can be multiplied.

This formula can be rewritten as

 * $$x_{11}' - p_1 q_1 = (1-c)\,(x_{11} - p_1 q_1)$$

so that

 * $$D_1 = (1-c)\;D_0$$

where $$D$$ at the $$n$$-th generation is designated as $$D_n$$. Thus we have

 * $$D_n = (1-c)^n\; D_0.$$

If $$n \to \infty$$, then $$(1-c)^n \to 0$$ so that $$D_n$$ converges to zero.

If at some time we observe linkage disequilibrium, it will disappear in the future due to recombination. However, the smaller the distance between the two loci, the smaller will be the rate of convergence of $$D$$ to zero.

Visualization
Once linkage disequilibrium has been calculated for a dataset, a visualization method is often chosen to display the linkage disequilibrium to make it more easily understandable.

The most common method is to use a heatmap, where colors are used to indicate the loci with positive linkage disequilibrium, and linkage equilibrium. This example displays the full heatmap, but because the heatmap is symmetrical across the diagonal (that is, the linkage disequilibrium between loci A and B is the same as between B and A), a triangular heatmap that shows the pairs only once is also commonly employed. This method has the advantage of being easy to interpret, but it also cannot display information about other variables that may be of interest.

More robust visualization options are also available, like the textile plot. In a textile plot, combinations of alleles at a certain loci can be linked with combinations of alleles at a different loci. Each genotype (combination of alleles) is represented by a circle which has an area proportional to the frequency of that genotype, with a column for each loci. Lines are drawn from each circle to the circles in the other column(s), and the thickness of the connecting line is proportional to the frequency that the two genotypes occur together. Linkage disequilibrium is seen through the number of line crossings in the diagram, where a greater number of line crossings indicates a low linkage disequilibrium and fewer crossings indicate a high linkage disequilibrium. The advantage of this method is that it shows the individual genotype frequencies and includes a visual difference between absolute (where the alleles at the two loci always appear together) and complete (where alleles at the two loci show a strong connection but with the possibility of recombination) linkage disequilibrium by the shape of the graph.

Another visualization option is forests of hierarchical latent class models (FHLCM). All loci are plotted along the top layer of the graph, and below this top layer, boxes representing latent variables are added with links to the top level. Lines connect the loci at the top level to the latent variables below, and the lower the level of the box that the loci are connected to, the greater the linkage disequilibrium and the smaller the distance between the loci. While this method does not have the same advantages of the textile plot, it does allow for the visualization of loci that are far apart without requiring the sequence to be rearranged, as is the case with the textile plot.

This is not an exhaustive list of visualization methods, and multiple methods may be used to display a data set in order to give a better picture of the data based on the information that the researcher aims to highlight.

Resources
A comparison of different measures of LD is provided by Devlin & Risch

The International HapMap Project enables the study of LD in human populations online. The Ensembl project integrates HapMap data with other genetic information from dbSNP.

Analysis software

 * PLINK – whole genome association analysis toolset, which can calculate LD among other things
 * LDHat
 * Haploview
 * LdCompare — open-source software for calculating LD.
 * SNP and Variation Suite – commercial software with interactive LD plot.
 * GOLD – Graphical Overview of Linkage Disequilibrium
 * TASSEL – software to evaluate linkage disequilibrium, traits associations, and evolutionary patterns
 * rAggr – finds proxy markers (SNPs and indels) that are in linkage disequilibrium with a set of queried markers, using the 1000 Genomes Project and HapMap genotype databases.
 * SNeP – Fast computation of LD and Ne for large genotype datasets in PLINK format.
 * LDlink – A suite of web-based applications to easily and efficiently explore linkage disequilibrium in population subgroups. All population genotype data originates from Phase 3 of the 1000 Genomes Project and variant RS numbers are indexed based on dbSNP build 151.
 * Bcftools – utilities for variant calling and manipulating VCFs and BCFs.

Simulation software

 * Haploid — a C library for population genetic simulation (GPL)

title: Linkage disequilibrium


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Norland, Ontario

Norland is a small rural farming community, from 1854-1967 known as Norland, Victoria, Canada West, British Colonial America, then on the 1 July 1867 it became known as Norland, Laxton Township, Victoria, Ontario, Canada until 1 July 2001 when it was amalgamation to became part of the city of Kawartha Lakes, in the Canadian province of Ontario. It is located in a scenic spot on the Gull River at the north end of Shadow Lake, the northernmost Kawartha lake in the Trent River chain. Here, a series of three waterfalls reflect the change in elevation of the land leading north to the Haliburton Highlands.

Norland was named after Norland, England, the birthplace of one of the first settlers, William H. Smith.

Located within the city of Kawartha Lakes, Norland falls under the leadership of Mayor Andy Letham. Originally elected as Mayor in 2014, Letham has been elected to serve his second term in the 2018 municipal election. The city of Kawartha Lakes is currently split into 16 different wards, each with their own councillor. Norland is located in Ward 1, councillor being Rob Macklem. The 2018 Municipal elections bring change to the political structure of Norland, and Kawartha Lakes as a whole. Previously being 16 wards, the new political term, beginning on December 3, 2018, will result in the reduction of Kawartha Lakes to 8 wards, bringing Norland a new councillor, Emmet Yeo.

Features

 * Large mural featuring a nature scene with animals and native people located on the largest building in town situated at the crossroads of Highway 35 and Kawartha Lakes Road 45
 * Elliot Falls, a hydroelectric power generating station.
 * "The falls" a falls and dam near the town centre. Recently a popular tourist spot during the summer for swimming and fishing, the dam has recently been converted into a power facility, with the addition of a powerhouse and a bypass gate to maintain water levels. This is a result of Parks Canada's issuing a license to Timber Run Hydropower Corporation, allowing the company to build at the site.
 * The "Old School", the building which now houses the public library, used to also be the municipal building for the tri-township of Laxton-Digby-Longford. The Townships no longer exist since the amalgamation of Victoria County into the City of Kawartha Lakes.
 * Municipal water system owned and operated by the City of Kawartha Lakes.
 * Municipal wharf located on the Gull River below the falls and upstream of Shadow Lake. Boat access south to Coboconk.
 * Several municipal waterfront parks.
 * The Links at Monck's Landing Golf Club is located two kilometres northwest of Norland.
 * The Norland Recreation Centre, located on the grounds of Ward Memorial Park, is a staple in the small community. The building is the main property for locals to host personal celebrations such as large parties and dances. During the winter months, the centre was formerly transformed into a public indoor natural ice skating rink until COKL environmentalists insulated the exterior walls preventing the rink from freezing, and during the summer months hosts the popular Norland Festival.
 * The Norland Summer Festival, a popular summer festival in the community. One of the biggest events of the year, the festival is a weekend long ordeal in which locals and tourists can participate in multiple tournaments such as LOB ball and horseshoes, enjoy different food trucks and vendors, as well as watch a demolition derby.
 * The Riverside Inn, a popular restaurant located in the heart of the town. Operating under new ownership, the restaurant now boosts modern features attracting tourists to the rural community, such as a Tesla charging station, installed in 2016.
 * The Norland General Store, located at the end of town, the store hosts the town's only gas station. Coming under new ownership in 2018, the Esso location is modernizing, now including pay-at-the-pump gas pumps.

The post office services locals with lockboxes and one rural route.

Norland was once mentioned on NBC's Late Night with Conan O'Brien when the show was in Toronto for a week, Mike Myers mentioned the village by name including the only still operating gas station and a resident of Norland.

license: Creative Commons - Attribution Share-Alike - https://creativecommons.org/licenses/by-sa/4.0/

title: Norland, Ontario


text:
Gilbert Blane

Sir Gilbert Blane of Blanefield, 1st Baronet FRSE FRS MRCP (29 August 1749 – 26 June 1834) was a Scottish physician who instituted health reform in the Royal Navy. He saw action against both the French and Spanish fleets, and later served as a Commissioner on the Sick and Wounded Board of the Admiralty.

Life
Born in Blanefield, by Kirkoswald, in Ayrshire, he was the fourth son of Gilbert Blane of Blanefield (d.1771) and Agnes McFadzen.

He studied medicine at Edinburgh University and Glasgow University (MD 1778) before moving to London. The challenge of establishing a practice in London was eased by his friendship with Dr William Hunter, elder brother of the famous John Hunter who is now widely regarded as the father of modern surgery in Britain. Dr William Hunter introduced Blane to Lord Rodney who appointed Blane as his personal physician aboard HMS Sandwich.

Blane was appointed Physician to the Fleet (1779–1783) and accompanied Rodney, initially to pursue the Spanish squadron besieging Gibraltar and engaging them at the Battle of Cape St. Vincent, and then to the West Indies. Blane did much to improve the health of sailors by improving their diet and enforcing proper sanitary precautions. He demanded monthly reports from other ships' surgeons which enabled him to build up a detailed picture of the high levels of sickness that affected the squadron. Despite James Lind's 1753 publication of A treatise on scurvy, which established the importance of fresh fruit and vegetables in preventing scurvy, the Admiralty had not implemented his recommendations, and scurvy remained a significant cause of sickness in the Fleet. Blane published a pamphlet for the benefit of ships' surgeons in 1780 entitled On the most effective means for preserving the health of seamen, particularly in the Royal Navy. He advocated the use of citrus juice as a preventative and cure for scurvy in the squadron and eventually, as Commissioner of the Sick and Wounded Board, persuaded the Admiralty to go against the theories of the medical establishment and introduce lemon juice as daily addition to the naval diet in 1795. Later lemons were replaced by limes which could be obtained from Britain's Caribbean colonies, and for this reason, "limey" became a common slang word for a British person.

On his return to Britain, he became Physician to St Thomas' Hospital (1783–1795), Physician Extraordinary to the Prince of Wales (1786) and Physician in Ordinary to the King (George IV and William IV). He became a Fellow of the Royal Society in 1784 and delivered their Croonian lecture in 1788 On the Nature of the Muscles, and on the Theory of Muscular Motion. By virtue of these court and hospital appointments, he built up a good practice for himself in London, and the government constantly consulted him on questions of public hygiene. In 1795 Blane was appointed as Commissioner on the Sick and Wounded Board of the Admiralty; the provision of soap, lemons, adequate ventilation and standardised medical stores have all been attributed to his reforms. He was an instrumental advisor to the government in the drafting of the 1799 Quarantine Act. Blane left the Commission in 1802.In 1809 Blane provided expert medical advice with regards to the Walcheren expedition. This involved a British occupation of Walcheren Island with 40,000 men. The island was surrounded by marshland and many troops were struck down with 'Walcheren Fever', most likely malaria. The General Staff heeded Blane's advice that the occupying forces should be evacuated due to the likelihood of sickness. As a reward for his services to the Walcheren expedition and his other naval contributions, he was made baronet of Blanefield in the County of Ayr, on 26 December 1812.

His printed works include Observations on the Diseases of Seamen (1785) and Elements of Medical Logic (1819).

He was an advocate for issuing each Royal Navy sailor with a tourniquet to stem catastrophic bleeding in battle.

Blane lived at Burghfield in Berkshire and at Kirkoswald in Ayrshire.

He was elected a member of the Institute of France in 1826.

Controversy
Blane attended the anatomy course of Prof Alexander Monro Secundus when he studied medicine at Edinburgh University. Monro later accused Blane of plagiarising some of the information in publications, going so far as to write a pamphlet about the incident.

Death
He died at Sackville Street in the Piccadilly area of London on 26 June 1834.

Family
Blane married Elizabeth Gardiner in 1786. She died in 1832.

The Gilbert Blane Medal
In 1830 Sir Gilbert Blane established, with the sanction of the Board of the Admiralty, a fund for the encouragement of Naval Medical science, which was vested in the corporation of the Royal Colleges of Surgeons of London. The fund of £300 was to be used to confer gold medals, to be awarded jointly by the Presidents of the Royal Colleges of Physicians and Surgeons, to the Medical Officer of the Royal Navy who was judged to demonstrate "the most distinguished proofs of skill, diligence, humanity and learning in the exercise of their professional duties". The medal was designed by Benedetto Pistrucci who had designed the 1817 St George and dragon reverse of the British gold sovereign which is still used today. It is currently struck in gold-plated Sterling silver; the reverse side is inscribed with the motto 'Mente Manuque '; with mind and hand. Under the original terms, a pair of medals would be awarded biennially. Blane himself judged the first award of a pair of medals in 1832; to Dr John Liddell of HMS Asia for his preparations for the 1827 Battle of Navarino and to Dr William Donnelly of HMS Hussar for his examinations of the nature of syphilis and rheumatic fever. These were the only medals adjudicated by Blane prior to his death in 1834. In 1913 the rules of the prize were amended and the medal was to be awarded to the Medical Officer who achieved the highest mark in his professional examinations for promotion to Staff Surgeon. In 1936 the regulations were again changed to a form that remains extant to this day:"This fund is employed for the purposes of conferring a Gold Medal on the Medical Officer of the Royal Navy who, to a degree which is considered worthy of recognition, has brought about an advance in any branch of Medical Science in its application to Naval Service, or has contributed to an improvement in any matter affecting the health or living conditions of Naval personnel. In adjudicating the award of the Medal consideration will be given to achievement by Medical Officers in research, in original articles and reports; criticisms of a constructive character of existing conditions; and information which is brought to notice or work performed, or suggestions made, by Medical Officers within the scope of the regulations governing the award as stated above."The Gilbert Blane Medal is awarded annually to this day, alternately by the Royal College of Surgeons of England and the Royal College of Physicians, upon the recommendation of the Medical Director General (Navy). The medal was last awarded in 2020 to Surgeon Commander Anton Fries, consultant plastic surgeon at Oxford University Hospitals, for his work on reconstructive transplantation surgery.

title: Gilbert Blane


text:
Creep

Creep, Creeps, CREEP, or Creeping may refer to:

Films

 * Creeps (film), a 1956 short starring the Three Stooges
 * The Creeps (film), a 1997 film directed by Charles Band
 * Creep (2004 film), a 2004 British-German horror film
 * Creep (2014 film), an American found-footage horror film
 * Creep 2, a 2017 American found-footage horror film; sequel to the 2014 film

Gaming

 * "Creep", a carpet of bio-matter Zerg colonies produce in the StarCraft video game franchise
 * The Creeps!, a 2008 tower defense video game

Literature

 * Creeps (novel), a 2013 young adult novel by Darren Hynes
 * The Creeping, a 2015 young adult novel by Alexandra Sirowy

Groups

 * Creep (band), an American electronic music band
 * The Creeps, a 1980s Swedish band that won the Rockbjörnen prize in 1990
 * The Creeps, a 1960s South African band featuring David Kramer

Albums

 * Creeps, 2015 album from Indian Handcrafts

Songs

 * "Creep" (Mobb Deep song)
 * "Creep" (Radiohead song)
 * "Creep" (Stone Temple Pilots song)
 * "Creep" (TLC song)
 * "Creep", a song by Dannii Minogue on the album Neon Nights
 * "C.R.E.E.P.", a song by the Fall
 * "Creepin'", a song by Eric Church
 * "The Creep" (song), by the Lonely Island
 * "The Creep", a 1950s instrumental by Ken Mackintosh
 * "The Creeps" (Camille Jones song), remixed by, among others, Fedde le Grand
 * "The Creeps (Get on the Dancefloor)", song by the Freaks
 * "Creeping" (song), a 2018 single by Lil Skies featuring Rich the Kid
 * "Creepin'" (Metro Boomin, the Weeknd and 21 Savage song), a 2022 song by Metro Boomin, the Weeknd and 21 Savage

Project management

 * Feature creep, the gradual and unmanaged addition of features to software
 * Instruction creep, the gradual and unmanaged addition of unnecessary instructions
 * Mission creep, the gradual and unmanaged addition of additional tasks in a mission
 * Scope creep, the gradual and unmanaged addition of additional tasks in a project

Science

 * Creep (deformation), the tendency of a solid material to slowly move or deform permanently under the influence of stresses
 * Creep, the advancing of a railway wheel more or less than is expected from rolling, without large-scale slip; see rail adhesion
 * Idle creep, the tendency of a car with an automatic transmission to roll without the brakes engaged or the gear set to neutral
 * Aseismic creep, a slow, steady movement along an earthquake fault
 * Downhill creep, the slow progression of soil and rock down a low-grade slope
 * Location creep, an erratic effect in real-time locating systems
 * Superfluid creep, the tendency for a superfluid to "crawl" up the walls of its container; see Superfluidity

Other

 * Creep, a creepy person
 * Committee for the Re-Election of the President (CRP), mockingly abbreviated as CREEP, an fundraising organization for Richard Nixon's 1972 re-election campaign
 * "The Creeps", an episode of the TV series Adventure Time

title: Creep


text:
User talk:Sumanbhatta

Thanks for fixing the link on the ANZ New Zealand page. I don't have the hang of that yet. Acad Ronin 10:35, 26 May 2004 (UTC)

Please claim your upload(s): File:External micrometer.jpg
Hi, This image was seemingly uploaded prior to current image polices, Thank you.

However, as part of ongoing efforts to ensure all media on English Wikipedia is correctly licensed and attributed it would be appreciated if you were able to confirm, that it was your own work, by marking it as own, amending the information added by a third party, and by changing the license to an appropriate "self" variant. You can also add  to the Media by uploader or Presumed self tag(s) if  present to indicate that you've acknowledged the image, and license shown (and updated the information where appropriate).

This will assist those reviewing the many many "free" images on commons that have not yet been transferred to Commons. Sfan00 IMG (talk) 09:18, 10 August 2016 (UTC)

title: User talk:Sumanbhatta


text:
Creep (deformation)

In materials science, creep (sometimes called cold flow) is the tendency of a solid material to undergo slow deformation while subject to persistent mechanical stresses. It can occur as a result of long-term exposure to high levels of stress that are still below the yield strength of the material. Creep is more severe in materials that are subjected to heat for long periods and generally increases as they near their melting point.

The rate of deformation is a function of the material's properties, exposure time, exposure temperature and the applied structural load. Depending on the magnitude of the applied stress and its duration, the deformation may become so large that a component can no longer perform its function – for example creep of a turbine blade could cause the blade to contact the casing, resulting in the failure of the blade. Creep is usually of concern to engineers and metallurgists when evaluating components that operate under high stresses or high temperatures. Creep is a deformation mechanism that may or may not constitute a failure mode. For example, moderate creep in concrete is sometimes welcomed because it relieves tensile stresses that might otherwise lead to cracking.

Unlike brittle fracture, creep deformation does not occur suddenly upon the application of stress. Instead, strain accumulates as a result of long-term stress. Therefore, creep is a "time-dependent" deformation.

Creep or cold flow is of great concern in plastics. Blocking agents are chemicals used to prevent or inhibit cold flow. Otherwise rolled or stacked sheets stick together.

Temperature dependence
The temperature range in which creep deformation occurs depends on the material. Creep deformation generally occurs when a material is stressed at a temperature near its melting point. While tungsten requires a temperature in the thousands of degrees before the onset of creep deformation, lead may creep at room temperature, and ice will creep at temperatures below 0 °C. Plastics and low-melting-temperature metals, including many solders, can begin to creep at room temperature. Glacier flow is an example of creep processes in ice. The effects of creep deformation generally become noticeable at approximately 35% of the melting point (in Kelvin) for metals and at 45% of melting point for ceramics.

Theoretical framework
Creep behavior can be split into three main stages.

In primary, or transient, creep, the strain rate is a function of time. In Class M materials, which include most pure materials, primary strain rate decreases over time. This can be due to increasing dislocation density, or it can be due to evolving grain size. In class A materials, which have large amounts of solid solution hardening, strain rate increases over time due to a thinning of solute drag atoms as dislocations move.

In the secondary, or steady-state, creep, dislocation structure and grain size have reached equilibrium, and therefore strain rate is constant. Equations that yield a strain rate refer to the steady-state strain rate. Stress dependence of this rate depends on the creep mechanism.

In tertiary creep, the strain rate exponentially increases with stress. This can be due to necking phenomena, internal cracks, or voids, which all decrease the cross-sectional area and increase the true stress on the region, further accelerating deformation and leading to fracture.

Mechanisms of deformation
Depending on the temperature and stress, different deformation mechanisms are activated. Though there are generally many deformation mechanisms active at all times, usually one mechanism is dominant, accounting for almost all deformation.

Various mechanisms are:
 * Bulk diffusion (Nabarro–Herring creep)
 * Grain boundary diffusion (Coble creep)
 * Glide-controlled dislocation creep: dislocations move via glide and climb, and the speed of glide is the dominant factor on strain rate
 * Climb-controlled dislocation creep: dislocations move via glide and climb, and the speed of climb is the dominant factor on strain rate
 * Harper–Dorn creep: a low-stress creep mechanism in some pure materials

At low temperatures and low stress, creep is essentially nonexistent and all strain is elastic. At low temperatures and high stress, materials experience plastic deformation rather than creep. At high temperatures and low stress, diffusional creep tends to be dominant, while at high temperatures and high stress, dislocation creep tends to be dominant.

Deformation mechanism maps
Deformation mechanism maps provide a visual tool categorizing the dominant deformation mechanism as a function of homologous temperature, shear modulus-normalized stress, and strain rate. Generally, two of these three properties (most commonly temperature and stress) are the axes of the map, while the third is drawn as contours on the map.

To populate the map, constitutive equations are found for each deformation mechanism. These are used to solve for the boundaries between each deformation mechanism, as well as the strain rate contours. Deformation mechanism maps can be used to compare different strengthening mechanisms, as well as compare different types of materials.

$$ \frac{\mathrm{d}\varepsilon}{\mathrm{d}t} = \frac{C\sigma^m}{d^b} e^\frac{-Q}{kT}$$

where ε is the creep strain, C is a constant dependent on the material and the particular creep mechanism, m and b are exponents dependent on the creep mechanism, Q is the activation energy of the creep mechanism, σ is the applied stress, d is the grain size of the material, k is Boltzmann's constant, and T is the absolute temperature.

Dislocation creep
At high stresses (relative to the shear modulus), creep is controlled by the movement of dislocations. For dislocation creep, Q = Q(self diffusion), 4 ≤ m ≤ 6, and b < 1. Therefore, dislocation creep has a strong dependence on the applied stress and the intrinsic activation energy and a weaker dependence on grain size. As grain size gets smaller, grain boundary area gets larger, so dislocation motion is impeded.

Some alloys exhibit a very large stress exponent (m > 10), and this has typically been explained by introducing a "threshold stress," σth, below which creep can't be measured. The modified power law equation then becomes:

$$\frac{\mathrm{d}\varepsilon}{\mathrm{d}t} = A \left(\sigma-\sigma_{\rm th}\right)^m e^\frac{-Q}{\bar R T}$$

where A, Q and m can all be explained by conventional mechanisms (so 3 ≤ m ≤ 10), and R is the gas constant. The creep increases with increasing applied stress, since the applied stress tends to drive the dislocation past the barrier, and make the dislocation get into a lower energy state after bypassing the obstacle, which means that the dislocation is inclined to pass the obstacle. In other words, part of the work required to overcome the energy barrier of passing an obstacle is provided by the applied stress and the remainder by thermal energy.

Nabarro–Herring creep
Nabarro–Herring (NH) creep is a form of diffusion creep, while dislocation glide creep does not involve atomic diffusion. Nabarro–Herring creep dominates at high temperatures and low stresses. As shown in the figure on the right, the lateral sides of the crystal are subjected to tensile stress and the horizontal sides to compressive stress. The atomic volume is altered by applied stress: it increases in regions under tension and decreases in regions under compression. So the activation energy for vacancy formation is changed by ±σΩ, where Ω is the atomic volume, the positive value is for compressive regions and negative value is for tensile regions. Since the fractional vacancy concentration is proportional to exp(−$Q_{f} ± σΩ⁄RT$), where Qf is the vacancy-formation energy, the vacancy concentration is higher in tensile regions than in compressive regions, leading to a net flow of vacancies from the regions under tension to the regions under compression, and this is equivalent to a net atom diffusion in the opposite direction, which causes the creep deformation: the grain elongates in the tensile stress axis and contracts in the compressive stress axis.

In Nabarro–Herring creep, k is related to the diffusion coefficient of atoms through the lattice, Q = Q(self diffusion), m = 1, and b = 2. Therefore, Nabarro–Herring creep has a weak stress dependence and a moderate grain size dependence, with the creep rate decreasing as the grain size is increased.

Nabarro–Herring creep is strongly temperature dependent. For lattice diffusion of atoms to occur in a material, neighboring lattice sites or interstitial sites in the crystal structure must be free. A given atom must also overcome the energy barrier to move from its current site (it lies in an energetically favorable potential well) to the nearby vacant site (another potential well). The general form of the diffusion equation is $$D = D_0e^{\frac{E}{KT}}$$ where D0 has a dependence on both the attempted jump frequency and the number of nearest neighbor sites and the probability of the sites being vacant. Thus there is a double dependence upon temperature. At higher temperatures the diffusivity increases due to the direct temperature dependence of the equation, the increase in vacancies through Schottky defect formation, and an increase in the average energy of atoms in the material. Nabarro–Herring creep dominates at very high temperatures relative to a material's melting temperature.

Coble creep
Coble creep is the second form of diffusion-controlled creep. In Coble creep the atoms diffuse along grain boundaries to elongate the grains along the stress axis. This causes Coble creep to have a stronger grain size dependence than Nabarro–Herring creep, thus, Coble creep will be more important in materials composed of very fine grains. For Coble creep k is related to the diffusion coefficient of atoms along the grain boundary, Q = Q(grain boundary diffusion), m = 1, and b = 3. Because Q(grain boundary diffusion) is less than Q(self diffusion), Coble creep occurs at lower temperatures than Nabarro–Herring creep. Coble creep is still temperature dependent, as the temperature increases so does the grain boundary diffusion. However, since the number of nearest neighbors is effectively limited along the interface of the grains, and thermal generation of vacancies along the boundaries is less prevalent, the temperature dependence is not as strong as in Nabarro–Herring creep. It also exhibits the same linear dependence on stress as Nabarro–Herring creep. Generally, the diffusional creep rate should be the sum of Nabarro–Herring creep rate and Coble creep rate. Diffusional creep leads to grain-boundary separation, that is, voids or cracks form between the grains. To heal this, grain-boundary sliding occurs. The diffusional creep rate and the grain boundary sliding rate must be balanced if there are no voids or cracks remaining. When grain-boundary sliding can not accommodate the incompatibility, grain-boundary voids are generated, which is related to the initiation of creep fracture.

Solute drag creep
Solute drag creep is one of the mechanisms for power-law creep (PLC), involving both dislocation and diffusional flow. Solute drag creep is observed in certain metallic alloys. In these alloys, the creep rate increases during the first stage of creep (Transient creep) before reaching a steady-state value. This phenomenon can be explained by a model associated with solid-solution strengthening. At low temperatures, the solute atoms are immobile and increase the flow stress required to move dislocations. However, at higher temperatures, the solute atoms are more mobile and may form atmospheres and clouds surrounding the dislocations. This is especially likely if the solute atom has a large misfit in the matrix. The solutes are attracted by the dislocation stress fields and are able to relieve the elastic stress fields of existing dislocations. Thus the solutes become bound to the dislocations. The concentration of solute, C, at a distance, r, from a dislocation is given by the Cottrell atmosphere defined as

$$ C_r = C_0 \exp\left(-\frac{\beta\sin\theta}{rKT}\right) $$

where C0 is the concentration at r = ∞ and β is a constant which defines the extent of segregation of the solute. When surrounded by a solute atmosphere, dislocations that attempt to glide under an applied stress are subjected to a back stress exerted on them by the cloud of solute atoms. If the applied stress is sufficiently high, the dislocation may eventually break away from the atmosphere, allowing the dislocation to continue gliding under the action of the applied stress. The maximum force (per unit length) that the atmosphere of solute atoms can exert on the dislocation is given by Cottrell and Jaswon

$$ \frac{F_{\rm max}}{L} = \frac{C_0 \beta^2}{bkT} $$

When the diffusion of solute atoms is activated at higher temperatures, the solute atoms which are "bound" to the dislocations by the misfit can move along with edge dislocations as a "drag" on their motion if the dislocation motion or the creep rate is not too high. The amount of "drag" exerted by the solute atoms on the dislocation is related to the diffusivity of the solute atoms in the metal at that temperature, with a higher diffusivity leading to lower drag and vice versa. The velocity at which the dislocations glide can be approximated by a power law of the form

$$ v = B {\sigma^*}^m B =B_0 \exp\left(\frac{-Q_{\rm g}}{RT}\right)$$

where m is the effective stress exponent, Q is the apparent activation energy for glide and B0 is a constant. The parameter B in the above equation was derived by Cottrell and Jaswon for interaction between solute atoms and dislocations on the basis of the relative atomic size misfit εa of solutes to be

$$B= \frac{9kT}{MG^2b^4\ln\frac{r2}{r1}} \cdot \frac{D_{\rm sol}}{\varepsilon_{\rm a}^2c_0}$$

where k is Boltzmann's constant, and r1 and r2 are the internal and external cut-off radii of dislocation stress field. c0 and Dsol are the atomic concentration of the solute and solute diffusivity respectively. Dsol also has a temperature dependence that makes a determining contribution to Qg.

If the cloud of solutes does not form or the dislocations are able to break away from their clouds, glide occurs in a jerky manner where fixed obstacles, formed by dislocations in combination with solutes, are overcome after a certain waiting time with support by thermal activation. The exponent m is greater than 1 in this case. The equations show that the hardening effect of solutes is strong if the factor B in the power-law equation is low so that the dislocations move slowly and the diffusivity Dsol is low. Also, solute atoms with both high concentration in the matrix and strong interaction with dislocations are strong gardeners. Since misfit strain of solute atoms is one of the ways they interact with dislocations, it follows that solute atoms with large atomic misfit are strong gardeners. A low diffusivity Dsol is an additional condition for strong hardening.

Solute drag creep sometimes shows a special phenomenon, over a limited strain rate, which is called the Portevin–Le Chatelier effect. When the applied stress becomes sufficiently large, the dislocations will break away from the solute atoms since dislocation velocity increases with the stress. After breakaway, the stress decreases and the dislocation velocity also decreases, which allows the solute atoms to approach and reach the previously departed dislocations again, leading to a stress increase. The process repeats itself when the next local stress maximum is obtained. So repetitive local stress maxima and minima could be detected during solute drag creep.

Dislocation climb-glide creep
Dislocation climb-glide creep is observed in materials at high temperature. The initial creep rate is larger than the steady-state creep rate. Climb-glide creep could be illustrated as follows: when the applied stress is not enough for a moving dislocation to overcome the obstacle on its way via dislocation glide alone, the dislocation could climb to a parallel slip plane by diffusional processes, and the dislocation can glide on the new plane. This process repeats itself each time when the dislocation encounters an obstacle. The creep rate could be written as:

$$\frac{\mathrm{d}\varepsilon}{\mathrm{d}t} = \frac{A_{\rm CG}D_{\rm L}}{\sqrt M}\left(\frac{\sigma\Omega}{kT}\right)^{4.5}$$

where ACG includes details of the dislocation loop geometry, DL is the lattice diffusivity, M is the number of dislocation sources per unit volume, σ is the applied stress, and Ω is the atomic volume. The exponent m for dislocation climb-glide creep is 4.5 if M is independent of stress and this value of m is consistent with results from considerable experimental studies.

Harper–Dorn creep
Harper–Dorn creep is a climb-controlled dislocation mechanism at low stresses that has been observed in aluminum, lead, and tin systems, in addition to nonmetal systems such as ceramics and ice. It was first observed by Harper and Dorn in 1957. It is characterized by two principal phenomena: a power-law relationship between the steady-state strain rate and applied stress at a constant temperature which is weaker than the natural power-law of creep, and an independent relationship between the steady-state strain rate and grain size for a provided temperature and applied stress. The latter observation implies that Harper–Dorn creep is controlled by dislocation movement; namely, since creep can occur by vacancy diffusion (Nabarro–Herring creep, Coble creep), grain boundary sliding, and/or dislocation movement, and since the first two mechanisms are grain-size dependent, Harper–Dorn creep must therefore be dislocation-motion dependent. The same was also confirmed in 1972 by Barrett and co-workers where FeAl3 precipitates lowered the creep rates by 2 orders of magnitude compared to highly pure Al, thus, indicating Harper–Dorn creep to be a dislocation based mechanism.

Harper–Dorn creep is typically overwhelmed by other creep mechanisms in most situations, and is therefore not observed in most systems. The phenomenological equation which describes Harper–Dorn creep is

$$\frac{\mathrm{d}\varepsilon}{\mathrm{d}t} = \rho_0 \frac{D_{\rm v} G b^3}{k T} \left(\frac{\sigma_{\rm s}^n} G \right)$$

where ρ0 is dislocation density (constant for Harper–Dorn creep), Dv is the diffusivity through the volume of the material, G is the shear modulus and b is the Burgers vector, σs, and n is the stress exponent which varies between 1 and 3.

Later investigation of the creep region
Twenty-five years after Harper and Dorn published their work, Mohamed and Ginter made an important contribution in 1982 by evaluating the potential for achieving Harper–Dorn creep in samples of Al using different processing procedures. The experiments showed that Harper–Dorn creep is achieved with stress exponent n = 1, and only when the internal dislocation density prior to testing is exceptionally low. By contrast, Harper–Dorn creep was not observed in polycrystalline Al and single crystal Al when the initial dislocation density was high.

However, various conflicting reports demonstrate the uncertainties at very low stress levels. One report by Blum and Maier, claimed that the experimental evidence for Harper–Dorn creep is not fully convincing. They argued that the necessary condition for Harper–Dorn creep is not fulfilled in Al with 99.99% purity and the steady-state stress exponent n of the creep rate is always much larger than 1.

The subsequent work conducted by Ginter et al. confirmed that Harper–Dorn creep was attained in Al with 99.9995% purity but not in Al with 99.99% purity and, in addition, the creep curves obtained in the very high purity material exhibited regular and periodic accelerations. They also found that the creep behavior no longer follows a stress exponent of n = 1 when the tests are extended to very high strains of >0.1 but instead there is evidence for a stress exponent of n > 2.

 * Requirements for the occurrence
 * Harper–Dorn creep is usually regarded as a Newtonian viscous process with n = 1. Some very recent experimental evidence suggests that the stress exponent may be closer to ~2. Harper–Dorn creep should be observed at a low-stress creep regime where the stress exponent is lower than in the conventional power-law regime where n ≈ 3–5.
 * Unlike Nabarro–Herring diffusion depending on grain size, the Harper–Dorn flow process is independent of grain size. In the initial experiments of Harper and Dorn, identical creep rates are recorded either over a wide range of grain sizes in polycrystalline samples or in a combination of polycrystalline samples and single crystals.
 * The measured creep rates should be significantly faster, typically by more than two orders of magnitude, than the creep rates anticipated for Nabarro–Herring diffusion creep. At very high testing temperatures, Coble diffusion creep will be of negligible significance under these conditions.
 * The volumetric activation energy indicates that the rate of Harper–Dorn creep is controlled by vacancy diffusion to and from dislocations, resulting in climb-controlled dislocation motion. Unlike in other creep mechanisms, the dislocation density here is constant and independent of the applied stress.
 * The dislocation density must be low for Harper–Dorn creep to dominate. The density has been proposed to increase as dislocations move via cross-slip from one slip-plane to another, thereby increasing the dislocation length per unit volume. Cross-slip can also result in jogs along the length of the dislocation, which, if large enough, can act as single-ended dislocation sources.

Sintering
At high temperatures, it is energetically favorable for voids to shrink in a material. The application of tensile stress opposes the reduction in energy gained by void shrinkage. Thus, a certain magnitude of applied tensile stress is required to offset these shrinkage effects and cause void growth and creep fracture in materials at high temperature. This stress occurs at the sintering limit of the system.

The stress tending to shrink voids that must be overcome is related to the surface energy and surface area-volume ratio of the voids. For a general void with surface energy γ and principle radii of curvature of r1 and r2, the sintering limit stress is

$$\sigma_{\rm sint} = \frac{\gamma}{r_1}+\frac{\gamma}{r_2}$$

Below this critical stress, voids will tend to shrink rather than grow. Additional void shrinkage effects will also result from the application of a compressive stress. For typical descriptions of creep, it is assumed that the applied tensile stress exceeds the sintering limit.

Creep also explains one of several contributions to densification during metal powder sintering by hot pressing. A main aspect of densification is the shape change of the powder particles. Since this change involves permanent deformation of crystalline solids, it can be considered a plastic deformation process and thus sintering can be described as a high temperature creep process. The applied compressive stress during pressing accelerates void shrinkage rates and allows a relation between the steady-state creep power law and densification rate of the material. This phenomenon is observed to be one of the main densification mechanisms in the final stages of sintering, during which the densification rate (assuming gas-free pores) can be explained by:

$$\dot{\rho}=\frac{3A}{2}\frac{\rho(1-\rho)}{\left(1-(1-\rho)^\frac1n\right)^n}\left(\frac32\frac{P_{\rm e}}{n}\right)^n$$

in which ρ̇ is the densification rate, ρ is the density, Pe is the pressure applied, n describes the exponent of strain rate behavior, and A is a mechanism-dependent constant. A and n are from the following form of the general steady-state creep equation,

$$\dot{\varepsilon}=A\sigma^n$$

where ε̇ is the strain rate, and σ is the tensile stress. For the purposes of this mechanism, the constant A comes from the following expression, where A′ is a dimensionless, experimental constant, μ is the shear modulus, b is the Burgers vector, k is Boltzmann's constant, T is absolute temperature, D0 is the diffusion coefficient, and Q is the diffusion activation energy:

$$A = A'\frac{D_0 \mu b}{kT} \exp\left(-\frac{Q}{kT}\right)$$

Polymers

Creep can occur in polymers and metals which are considered viscoelastic materials. When a polymeric material is subjected to an abrupt force, the response can be modeled using the Kelvin–Voigt model. In this model, the material is represented by a Hookean spring and a Newtonian dashpot in parallel. The creep strain is given by the following convolution integral:

$$\varepsilon(t) = \sigma C_0 + \sigma C \int_0^\infty f(\tau)\left(1-e^{-t/\tau}\right) \,\mathrm{d} \tau$$

where σ is applied stress, C0 is instantaneous creep compliance, C is creep compliance coefficient, τ is retardation time, and f(τ) is the distribution of retardation times.

When subjected to a step constant stress, viscoelastic materials experience a time-dependent increase in strain. This phenomenon is known as viscoelastic creep.

At a time t0, a viscoelastic material is loaded with a constant stress that is maintained for a sufficiently long time period. The material responds to the stress with a strain that increases until the material ultimately fails. When the stress is maintained for a shorter time period, the material undergoes an initial strain until a time t1 at which the stress is relieved, at which time the strain immediately decreases (discontinuity) then continues decreasing gradually to a residual strain.

Viscoelastic creep data can be presented in one of two ways. Total strain can be plotted as a function of time for a given temperature or temperatures. Below a critical value of applied stress, a material may exhibit linear viscoelasticity. Above this critical stress, the creep rate grows disproportionately faster. The second way of graphically presenting viscoelastic creep in a material is by plotting the creep modulus (constant applied stress divided by total strain at a particular time) as a function of time. Below its critical stress, the viscoelastic creep modulus is independent of the stress applied. A family of curves describing strain versus time response to various applied stress may be represented by a single viscoelastic creep modulus versus time curve if the applied stresses are below the material's critical stress value.

Additionally, the molecular weight of the polymer of interest is known to affect its creep behavior. The effect of increasing molecular weight tends to promote secondary bonding between polymer chains and thus make the polymer more creep resistant. Similarly, aromatic polymers are even more creep resistant due to the added stiffness from the rings. Both molecular weight and aromatic rings add to polymers' thermal stability, increasing the creep resistance of a polymer.

Both polymers and metals can creep. Polymers experience significant creep at temperatures above around -200 °C; however, there are three main differences between polymeric and metallic creep. In metals, creep is not linearly viscoelastic, it is not recoverable, and it is only present at high temperatures.

Polymers show creep basically in two different ways. At typical work loads (5% up to 50%) ultra-high-molecular-weight polyethylene (Spectra, Dyneema) will show time-linear creep, whereas polyester or aramids (Twaron, Kevlar) will show a time-logarithmic creep.

Wood
Wood is considered as an orthotropic material, exhibiting different mechanical properties in three mutually perpendicular directions. Experiments show that the tangential direction in solid wood tend display a slightly higher creep compliance than in the radial direction. In the longitudinal direction, the creep compliance is relatively low and usually do not show any time-dependency in comparison to the other directions.

It has also been shown that there is a substantial difference in viscoelastic properties of wood depending on loading modality (creep in compression or tension). Studies have shown that certain Poisson's ratios gradually go from positive to negative values during the duration of the compression creep test, which does not occur in tension.

Concrete
The creep of concrete, which originates from the calcium silicate hydrates (C-S-H) in the hardened Portland cement paste (which is the binder of mineral aggregates), is fundamentally different from the creep of metals as well as polymers. Unlike the creep of metals, it occurs at all stress levels and, within the service stress range, is linearly dependent on the stress if the pore water content is constant. Unlike the creep of polymers and metals, it exhibits multi-months aging, caused by chemical hardening due to hydration which stiffens the microstructure, and multi-year aging, caused by long-term relaxation of self-equilibrated microstresses in the nanoporous microstructure of the C-S-H. If concrete is fully cured, creep effectively ceases.

Metals
Creep in metals primarily manifests as movement in their microstructures. While polymers and metals share some similarities in creep, the behavior of creep in metals displays a different mechanical response and must be modeled differently. For example, with polymers, creep can be modeled using the Kelvin-Voigt model with a Hookean spring dashpot but with metals, the creep can be represented by plastic deformation mechanisms such as dislocation glide, climb and grain boundary sliding. Understanding the mechanisms behind creep in metals is becoming increasingly more important for reliability and material lifetime as the operating temperatures for applications involving metals rise. Unlike polymers, in which creep deformation can occur at very low temperatures, creep for metals typically occur at high temperatures. Key examples would be scenarios in which these metal components like intermetallic or refractory metals are subject to high temperatures and mechanical loads like turbine blades, engine components and other structural elements. Refractory metals, such as tungsten, molybdenum, and niobium, are known for their exceptional mechanical properties at high temperatures, proving to be useful materials in aerospace, defense and electronics industries.

Case studies

Although mostly due to the reduced yield strength at higher temperatures, the collapse of the World Trade Center was due in part to creep from increased temperature.

The creep rate of hot pressure-loaded components in a nuclear reactor at power can be a significant design constraint, since the creep rate is enhanced by the flux of energetic particles.

Creep in epoxy anchor adhesive was blamed for the Big Dig tunnel ceiling collapse in Boston, Massachusetts that occurred in July 2006.

The design of tungsten light bulb filaments attempts to reduce creep deformation. Sagging of the filament coil between its supports increases with time due to the weight of the filament itself. If too much deformation occurs, the adjacent turns of the coil touch one another, causing local overheating, which quickly leads to failure of the filament. The coil geometry and supports are therefore designed to limit the stresses caused by the weight of the filament, and a special tungsten alloy with small amounts of oxygen trapped in the crystallite grain boundaries is used to slow the rate of Coble creep.

Creep can cause gradual cut-through of wire insulation, especially when stress is concentrated by pressing insulated wire against a sharp edge or corner. Special creep-resistant insulations such as Kynar (polyvinylidene fluoride) are used in wire wrap applications to resist cut-through due to the sharp corners of wire wrap terminals. Teflon insulation is resistant to elevated temperatures and has other desirable properties, but is notoriously vulnerable to cold-flow cut-through failures caused by creep.

In steam turbine power plants, pipes carry steam at high temperatures (566 C) and pressures (above 24.1 MPa). In jet engines, temperatures can reach up to 1400 C and initiate creep deformation in even advanced-design coated turbine blades. Hence, it is crucial for correct functionality to understand the creep deformation behavior of materials.

Creep deformation is important not only in systems where high temperatures are endured such as nuclear power plants, jet engines and heat exchangers, but also in the design of many everyday objects. For example, metal paper clips are stronger than plastic ones because plastics creep at room temperatures. Aging glass windows are often erroneously used as an example of this phenomenon: measurable creep would only occur at temperatures above the glass transition temperature around 500 C. While glass does exhibit creep under the right conditions, apparent sagging in old windows may instead be a consequence of obsolete manufacturing processes, such as that used to create crown glass, which resulted in inconsistent thickness.

Fractal geometry, using a deterministic Cantor structure, is used to model the surface topography, where recent advancements in thermoviscoelastic creep contact of rough surfaces are introduced. Various viscoelastic idealizations are used to model the surface materials, including the Maxwell, Kelvin–Voigt, standard linear solid and Jeffrey models.

Nimonic 75 has been certified by the European Union as a standard creep reference material.

The practice of tinning stranded wires to facilitate the process of connecting the wire to a screw terminal, though having been prevalent and considered standard practice for quite a while, has been discouraged by professional electricians, owing to the fact that the solder is likely to creep under the pressure exerted on the tinned wire end by the screw of the terminal, causing the joint to lose tension and hence create a loose contact over time. The accepted practice when connecting stranded wire to a screw terminal is to use a wire ferrule on the end of the wire.

Prevention
Generally, materials have better creep resistance if they have higher melting temperatures, lower diffusivity, and higher shear strength. Close-packed structures are usually more creep resistant as they tend to have lower diffusivity than non-close-packed structures. Common methods to reduce creep include:
 * Solid solution strengthening: adding other elements in solid solution can slow diffusion, as well as slow dislocation motion via the mechanism of solute drag.
 * Particle dispersion strengthening: adding particles, often incoherent oxide or carbide particles, block dislocation motion.
 * Precipitation hardening: precipitating a second phase out of the primary lattice blocks dislocation motion.
 * Grain size: increasing grain size decreases the amount of grain boundaries, which results in slower creep due to the high diffusion rate along grain boundaries. This is opposite low-temperature applications, where increasing grain size decreases strength by blocking dislocation motion. In very high temperature applications such as jet engine turbines, single crystals are often used.

Superalloys

Materials operating in high-performance systems, such as jet engines, often reach extreme temperatures surpassing 1000 °C, necessitating specialized material design. Superalloys based on cobalt, nickel, and iron have been engineered to be highly resistant to creep. The term ‘superalloy’ generally refers to austenitic nickel-, iron-, or cobalt-based alloys that use either γ′ or γ″ precipitation strengthening to maintain strength at high temperature.

The γ′ phase is a cubic L12-structure Ni3(Al,Ti,Ta,Nb) phase that produces cuboidal precipitates. Superalloys often have a high (60–75%) volume fraction of γ′ precipitates. γ′ precipitates are coherent with the parent γ phase, and are resistant to shearing due to the development of an anti-phase boundary when the precipitate is sheared. The γ″ phase is a tetragonal Ni3Nb or Ni3V structure. The γ″ phase, however, is unstable above 650 °C, so γ″ is less commonly used as a strengthening phase in high temperature applications. Carbides are also used in polycrystalline superalloys to inhibit grain boundary sliding.

Many other elements can be added to superalloys to tailor their properties. They can be used for solid solution strengthening, to reduce the formation of undesirable brittle precipitates, and to increase oxidation or corrosion resistance. Nickel-based superalloys have found widespread use in high-temperature, low stress applications. Iron-based superalloys are generally not used at high temperatures as the γ′ phase is not stable in the iron matrix, but are sometimes used at moderately high temperatures, as iron is significantly cheaper than nickel. Cobalt-based γ′ structure was found in 2006, allowing the development of cobalt-based superalloys, which are superior to nickel-based superalloys in corrosion resistance. However, in the base (cobalt–tungsten–aluminum) system, γ′ is only stable below 900 °C, and cobalt-based superalloys tend to be weaker than their Ni counterparts.

1. Stages of creep
Based on the description of creep mechanisms and its three different stages mentioned earlier, creep resistance generally can be accomplished by using materials in which their tertiary stage is not active since, at this stage, the strain rate increases significantly by increasing stress. Therefore, a sound component design should satisfy the primary stage of creep, which has a relatively high initial creep rate that decreases with increasing exposure time, leading to the second stage of creep, in which the creep rate in the material is decelerated and reaches its minimum value via work hardening. The minimum value of creep rate is actually a constant creep rate, which plays a crucial role in designing a component, and its magnitude depends on temperature and stress. The minimum value of creep rate that is commonly applied to alloys is based on two norms: (1) the stress required to produce a creep rate of $0.1e−3 %$ and (2) the stress required to produce a creep rate of $0 %$, which takes roughly about 11.5 years. The former standard has widely been used in the component design of turbine blades, while the latter is frequently used in designing steam turbines. One of the primary goals of creep tests is to determine the minimum value of the creep rate at the secondary stage and also to investigate the time required for an ultimate failure of a component. However, when it comes to ceramic applications, there are no such standards to determine their minimum creep rates, but it's safe to say that ceramics are usually picked up to operate at high-temperature applications under loads mainly because they possess a long lifetime. However, by acquiring information obtained from creep tests, proper ceramic material(s) can be selected for desired application to assure the safe service and evaluate the time period of secure service in high-temperature environments that the structural thermostability is essential. Therefore, by making the proper choice, suitable ceramic components may be selected, capable of operating at various conditions of high temperature and creep deformation.

2. Materials selection
Generally speaking, the structure of materials is different from one another. Metallic materials have different structures compared to polymer or ceramics, and even within the same class of materials, different structures might have existed at different temperatures. The difference in the structure includes the difference in grains (for example, their sizes, shapes, distributions), their crystalline or amorphous nature, and even dislocation and/or vacancy contents that are prone to change following a deformation. So, since creep is a time-dependent process and differs from one material to another, all these parameters must be considered in materials selection for a specific application. For instance, materials with low-dislocation content are among the suitable candidates for creep resistance. In other words, the dislocation glide and climb can be reduced if the proper material is selected (for example, ceramic materials are very popular in this case). In terms of vacancies, it's noteworthy to mention that the vacancy content not only depends on the chosen material but also on the component's service temperature. Vacancy-diffusion-controlled processes that promote creep can be categorized into grain boundary diffusion (Coble creep) and lattice diffusion (Nabarro–Herring creep). Therefore, the properties of dislocations and vacancies, their distributions throughout the structure, and their potential change due to long-time exposure to stress and temperature must be considered seriously in materials selection for components design. Therefore, the role of dislocations, vacancies, wide range of obstacles that can retard the dislocation motion, including grain boundaries in polycrystalline materials, solute atoms, precipitates, impurities, and strain fields originating from other dislocations or their pile-ups which increase the lifetime of materials and make them more resistive with respect to creep are always at the top list of materials selection and component design. For instance, different types of vacancies in ceramic materials have different charges stemming from their dominant chemical boding. So, existing or newly-formed vacancies must be charge-balanced to maintain the overall neutrality of the final structure. Besides paying attention to individual dislocation and vacancy contents, the correlation between them is also worth exploring since dislocation's ability to climb depends heavily on how many vacancies are available in its vicinity. To recap, materials must be selected and developed that possess low dislocation and vacancy contents to have a practical creep resistance component.

3. Various working conditions
● Temperature

Creep is a phenomenon that is related to a material's melting point (Tm). Generally, a high lifetime is expected when we have a higher melting point, and the reason behind selecting materials with high melting points originates from the fact that diffusion processes are related to temperature-dependent vacancy concentrations. The diffusion rate is slower in high-temperature materials. Ceramic materials are well known for having high melting points, and that's the reason why ceramics gravitated considerable attention to creep resistance applications. Although it's widely known that creep starts at a temperature equal to 0.5Tm, the safe temperature to avoid the initiation of creep is 0.3Tm. Creep that starts below or at 0.5Tm is called "low temperature creep" because diffusion is not very progressive at such low temperatures, and the kind of creep that occurs is not diffusion-dominant and is related to other mechanisms. ● Time

As mentioned previously, creep is a time-dependent deformation. Fortunately, creep doesn't occur suddenly in brittle materials as it does under tension and other forms of deformation, and it's an advantage for designers. Over time, creep strain develops in a material that is exposed to stress at the temperature of the application, and it depends on the duration of the exposure. Thus, besides temperature and stress, the creep rate is also a function of time, and it can be generalized as this function ε = F(t, T, σ) that tells the designer all the three parameters, including time, temperature, and stress acting in concert, and all of them must be considered if a successful creep-resistance component is to be attained.

title: Creep (deformation)


text:
LinuxTag

LinuxTag was an annual Free and Open Source Software (FOSS) conference and exposition with an emphasis on Linux but also BSD descendants located in Germany. The name is a compound with the German Tag meaning "Day", as it was initially a single day conference, but soon extended to multiple days, then always including a weekend. LinuxTag was the world's largest FLOSS conference and exhibition for years and aimed to provide a comprehensive overview of the Linux and Free Software market as well to promote contacts between users and developers. With this broad approach LinuxTag was one of the most important events of this kind.

LinuxTag's slogan, "Where .COM meets .ORG", refers to its stated aim of bringing together commercial and non-commercial groups in the IT sector. Each year's event also had its own motto, often reflecting a specific topic which was focused in that year.

Promotion of free software
LinuxTag regards itself as part of the Free Software movement and hence promoted this community by an extraordinary degree through supporting numerous Open Source Software projects. LinuxTag offered these projects to promote their software and their concepts, and thus present themselves to the public in an appropriate manner, with their own booths, forums and lectures, free of charge. The goal is to encourage projects to share concepts and content to the benefit of other groups and companies, and to provide forums for in-depth discussions of new technologies and opportunities.

LinuxTag e.V.
The non-profit association "LinuxTag e.V." was founded in preparation for LinuxTag's move from Kaiserslautern to the University of Stuttgart in 2000. The association plans and organizes the LinuxTag event by volunteer work and guides its ideological development. The association LinuxTag e.V. is registered in Association Register VR 2239 of the Kaiserslautern District Court. The association manages the LinuxTag name and word mark. The purpose of the association, according to its bylaws, is "the promotion of Free Software", and is pursued primarily by organizing the LinuxTag events.

The association is represented by a three-person executive board, supplemented by several representatives with delegated authority. All members of the LinuxTag association are volunteers and receive no remuneration for their service, with the sole exception from 1 April to 31 July 2005 when the First Chairman and CFO were employed and remunerated by the association. Surpluses resulting from the LinuxTag events or sponsorship are reinvested in the association's non-profit activities.

History
LinuxTag was launched in 1995 by a handful of active members of the "Linux project group (PG Linux)" of the Unix-AG, a student's workgroup at the University of Kaiserslautern. They wanted to inform the fellow students and the public about Linux and Open Source Software, which both were in their early stages at this time. However, since then the event has changed venues several times to keep pace with rapidly growing numbers of speakers, exhibitors and visitors.

Kaiserslautern
The first couple of LinuxTag events were held at the University of Kaiserslautern and organized under the auspices of its Unix-AG PG Linux.

LinuxTag 1996
The very first LinuxTag in January 1996 was a theme night on Linux attended by 70 participants, comprising six talks and a "Linux install party".

LinuxTag 1997/1 and 1997/2
In 1997 was the only year in which two LinuxTag events were organized; the talks of these and consecutive LinuxTag events are still online.

LinuxTag 1997/1 was attended by 150 participants, despite being disturbed by freezing rain which brought most private and public transport to a halt. Hence the LinuxTag team decided to move the LinuxTag events into the summer and to organize another LinuxTag for 1997.

LinuxTag 1997/2 had a couple of hundred visitors and brought some firsts: Two tracks with talks running in parallel, occupying two days, tutorial sessions for acquiring a deeper understanding of a topic and a small exhibition at which Open Source Software projects presented their goals and achievements.

LinuxTag 1998
The LinuxTag 1998 conference and exhibition was attended by circa 3,000 visitors. For the first time external exhibitors were present, e.g., the KDE project.

LinuxTag 1999
The LinuxTag 1999 was announced nation-wide for the first time and hence drew some 7,000 visitors. Due to the ".com" boom which also triggered many major players in the IT industry to embrace Open Source Software, the booths from companies filled a whole building of the university, among then Hewlett-Packard, IBM, SuSE etcetera. The booths of non-commercial projects occupied another, smaller building, while the talks used all major lecture rooms. It was also the first time an opportunity to take the LPI Exam 101 was offered. This was the last time LinuxTag was held in Kaiserslautern, because the number of attendees equalled the number of students at this time at the University of Kaiserslautern and there was no space left to grow.

In the aftermath of this event, the LinuxTag association was founded.

Stuttgart
In 2000 and 2001 LinuxTag was held in Stuttgart.

LinuxTag 2000
LinuxTag 2000 took place at the Stuttgart Trade Fair from 29 June to 2 July (i.e., four days) and was attended by approximately 17,000 visitors. The conference included a business track for the first time, devoted to such topics as IT security, legal aspects of Free Software, potential applications of Linux and Open Source Software in commercial applications. IT decision-makers presented case studies of applications of Free Software. Among the speakers were J. D. Frazer aka Illiad, the creator of User Friendly, Rob "Cmdr Taco" Malda, initiator of Slashdot and keynote speaker was Richard M. Stallman.

LinuxTag 2001
LinuxTag 2001 was held at Stuttgart Trade Fair from 5 to 8 July, with 14,870 visitors. The event was held under the patronage of the German Ministry of Economy. Keynote speakers were Eric S. Raymond, Rob Malda of Slashdot and John "Maddog" Hall of Linux International.

Karlsruhe
From 2002 to 2005, the LinuxTag conference and exhibition was held in Karlsruhe.

LinuxTag 2002
LinuxTag 2002 happened from 6 June to 9 July 2002 for the first time at the Messe Karlsruhe with about 13,000 visitors. The motto of the conference was "Open your mind, open your heart, open your source!". Approximately 100 booths comprised the exhibition. Keynote speaker was Matthias Kalle Dalheimer.

LinuxTag 2003
LinuxTag 2003 was titled Open Horizons and was held from 10 to 13 July 2003 at Messe Karlsruhe. Along the admission ticket for € 10, visitors received the LinuxTag edition of the initial release of the Knoppix DVD and a Tux pin. With 19,500 visitors, the number of visitors increased by 40 percent over the previous year.

Circa 150 exhibitors, both businesses and non-profit groups were represented. Apple showed Mac OS X running Open Source Software. Other highlights included the release of OpenGroupware.org, OpenOffice.org, a conversion to run Linux of several dozen Xboxes, some by a hardware modification by two solder points, some by utilizing so-called MechInstallers.

There was also a Programming Competition and on Sunday, 13 July 2003 from 13 to 14 o'clock a world record attempt took place: On a server 100 Linux desktop sessions with Gnome and KDE were simultaneously running, which could be used by everyone on the Internet.

Besides the exhibition also sub-conferences were held at which renowned experts discussed a subject. There was, for example, a Debian conference and on Sunday there was a lecture on TCPA, followed by a discussion. The Business and Government sub-conference with 400 participants grew by about 60% compared to 2002. The free lecture program was opened on Friday by the Parliamentary State Secretary, Federal Ministry of Economics and Labour, Rezzo Schlauch. Keynote speaker was Jon "Maddog" Hall.

With Webcams one could take a virtual visit of the fair. The "Pingu-Cam" recorded pictures of Penguin in the Karlsruhe Zoo, which is located right next to the location of the Messe Karlsruhe at that time.

LinuxTag 2004
LinuxTag 2004 took place from 23 to 26 June for the third time at the Messe Karlsruhe in 2004. Those who applied on the LinuxTag homepage got in for free. For the regular entrance fee of € 10 one received a Tux-pin, a Knoppix DVD and a DVD with FreeBSD, NetBSD and OpenBSD.

LinuxTag 2004 had the slogan "Free source - free world". 16,175 visitors were counted. A record number of about 170 exhibitors, among them many freelance projects, but also numerous large and medium-sized enterprises attended. Hewlett-Packard was the third time Official Cornerstone Partner. Other major companies at the exhibition were the C & L Verlag, Intel, Novell, Oracle, SAP and Sun Microsystems. For the first time Microsoft was represented with a booth.

On the single-day business and administration sub-conference on 24 June business and government case studies plus success stories of the use of open source software were presented. Among other things, also issues with computer viruses and worms waere addressed.

For the free conference, a record of about 350 proposals from 20 countries were handed in, of which 130 could be accommodated in the program. The fundamental threat of software patents for Open Source Software was an important topic. As already in 1999 one could undergo a LPI 101 certification at LinuxTag. As at most LinuxTag events, a coding marathon and a hacking contest were performed.

LinuxTag 2005
LinuxTag 2005 took place from 22 to 25 June at the Messe Karlsruhe. It was the 11th LinuxTag and titled "Linux everywhere". As usual a conference with far more than 100 talks and presentations was accompanied by a large exhibition with booths from more than 160 companies and projects, plus on 22 June 2005 the business and administration sub-conference. Additionally workshops and tutorials were offered.

Keynote speaker Jimmy Wales announced a collaboration between Wikipedia and KDE in his opening speech. Through a web API every program can directly access Wikipedia, which was used by the KDE Media Player Amarok version 1.3 to access the Wikipedia articles of artists.

The organizers counted about 12,000 visitors, a little less than in previous years, which was mainly caused by the newly introduced mandatory entrance fee and the hottest week of the year 2005.

Wiesbaden
In 2006 LinuxTag moved to Wiesbaden.

LinuxTag 2006
LinuxTag 2006 took place from 3 to 6 May 2006 in the Rhein-Main-Hallen in Wiesbaden under the motto „See, what's ahead“. Over 9,000 people from over 30 nations attended LinuxTag 2006. 150 exhibitors were present, among others IBM, Avira and Sun Microsystems, but some other regular exhibitors as Hewlett-Packard or Red Hat were missing. Three teams participated in the annual hacking contest.

The most visited talk of was the Keynote of Ubuntu founder Mark Shuttleworth. The "chief dreamer of Ubuntu" referred to himself and the good cooperation of users with developers. He stressed that Kubuntu and Ubuntu should be treated equally and that there is a good cooperation between the developers of both. The second keynote speaker was Andrew Morton There was also the opportunity to watch some of the talks by video stream, which was used by approximately 1,800 people.

Berlin
Since 2007 the LinuxTag took place in Berlin, from 2007 to 2013 at the Messe Berlin, located under the Funkturm Berlin.

LinuxTag 2007
LinuxTag 2007 took place from 30 May to 2 June 2007 with the motto "Come in: We're open". It was attended by about 9,600 people and comprised more than 200 talks.

The event was held under the auspices of Interior Minister Wolfgang Schäuble. This sparked a lively debate due to the political attitude of Wolfgang Schäuble in his administrative role, which triggered calls to boycott LinuxTag. This was even reported and discussed outside of Germany.

LinuxTag 2008
LinuxTag 2008 took place from 28 to 31 May at the Messe Berlin with 11,612 visitors from 31 countries and more than 200 exhibitors. The second LinuxTag in the German capital was under the patronage of (then) German Foreign Minister and Vice Chancellor Frank-Walter Steinmeier and was labelled as a part of a six-day "IT Week in the Capital Region", which also included the IT business trade fair IT Profits under the patronage of the Federal Minister of transport as a co-event to LinuxTag. LinuxTag 2008 hosted as sub-conferences the second German Asterisk day, a user and developer conference for IP Telephony and the 8th @Kit Congress, on which legal issues of professional IT use are discussed. Core topics of LinuxTag 2008 were "Highlights of digital lifestyle" and the "Mobile + Embedded Area".

LinuxTag 2009
LinuxTag 2009 took place from 24 to 27 June at the Messe Berlin, attracting over 10,000 visitors. As in 2008 it was placed under the patronage of German Foreign Minister and Vice Chancellor Frank-Walter Steinmeier. The new president of the Free Software Foundation Europe Karsten Gerloff attended LinuxTag. A focal point was "business processes using Linux" and "Open Source Software in the colours of the Tricolor" for which 14 Open Source Software suppliers from France displayed their product and service ranges.

LinuxTag 2010
LinuxTag 2010 took place from 9 to 12 June 2010 at the Messe Berlin. This 16th LinuxTag was attended by about 11,600 people and stood under the patronage of Cornelia Rogall-Grothe, Federal Government Commissioner for Information Technology. Keynote speakers included Microsoft's general manager James Utzschneider who stunned the audience with his open approach to Open Source Software, the CEO of SugarCRM, Larry Augustin, who underlined the economic impact of FLOSS and its leverage in cloud computing, the "director of open source" at Google, Chris DiBona, who underlined the professional level of LinuxTag, the kernel developer Jonathan Corbet who gave an outlook on the next Linux kernel 2.6.35, and Ubuntu founder Mark Shuttleworth laid out the next milestones for the Ubuntu desktop distribution.

LinuxTag 2011
The 17th LinuxTag was held from 11 to 14 May 2011 at the Messe Berlin under the motto "Where .com meets .org". It was attended by 11,578 visitors and was placed under the patronage of Cornelia Rogall-Grothe of the Federal Government Commissioner for Information Technology, again. Keynotes were given by Wim Coekaerts (Oracle), Bradley Kuhn (Software Freedom Conservancy) and Daniel Walsh (Red Hat).

LinuxTag 2012
The 18th LinuxTag happened from 23 to 26 May 2012 at the Messe Berlin with the motto "Open minds create effective solutions", another time under the patronage of Cornelia Rogall-Grothe of the German Federal Government Commissioner for Information Technology. At LinuxTag, the "Open MindManager Morning" premiere took place, on which industry experts and educators across the IT discussed changes in society and Open Source philosophy.

In addition, there was the premiere of the new lecture series "Open Minds Economy", organized by the Open Source Business Alliance (OSBA) and Messe Berlin, which presented successful business models and applications of open source software in the fields of economy and society.

Keynotes were given by Jimmy Schulz, chairman of the project group "Interoperability, Standards and Open Source" of the commission for research on internet and digital society of the German Bundestag, Ulrich Drepper, maintainer of the GNU C standard library Glibc and Lars Knoll, employee at Nokia and chief maintainer of the QT library.

LinuxTag 2013
The 19th LinuxTag was held from 22 to 25 May 2013 on the Berlin Exhibition Grounds with the motto "In the cloud - triumph of the free software goes on". Again it stood under the patronage of Cornelia Rogall-Grothe, the Federal Government Commissioner for Information Technology.

It was also the premiere of the Open IT Summit, which was organized as a parallel conference at LinuxTag of the Open Source Business Alliance (OSBA) and Messe Berlin with the goal to discuss the topic open source in the business environment. Additionally the OpenStack Day took place in cooperation with the OpenStack Foundation as a first major sub-conference on the subject OpenStack in Europe. The Foundation has its headquarters in the U.S. and sees itself as a global reservoir of the same name, scalable cloud management platform.

Keynotes were given by kernel developer Matthew Garrett on the subject of Unified Extensible Firmware Interface (UEFI) and Secure Boot, and Benjamin Mako Hill, a researcher at the Massachusetts Institute of Technology (MIT), who marked so-called anti-features as unacceptable, where manufacturers build usage restrictions into devices.

LinuxTag 2014
Since about 2012, the number of visitors of LinuxTag declined despite the ever increasing use of Open Source Software. This decline in visitor numbers was interpreted as a side effect of the ubiquity of Free Software and Linux, hence not being of extraordinary interest any longer. In addition, for some years, there were many similar regional events, in Germany and other European nations, which have drawn from the concept of LinuxTag.

In order to adapt to the changes, the LinuxTag focused on the core issue of the professional use of Open Source Software, as in 2014. Therefore, LinuxTag started a strategic partnership with the droidcon.

The 20th LinuxTag took place on 8 and 10 May 2014 at the STATION Berlin. In spatial and temporal proximity were the Media Convention Berlin (6 to 7 May), the re:publica (6 to 8 May) and the droidcon (8 to 10 May 2014). All events aimed towards a close relationship in order to achieve a high level of appreciation of the combined effort.

title: LinuxTag


text:
User talk:Paulbmann

Proposed deletion of DFAC lexer generator

The article DFAC lexer generator has been proposed for deletion  because of the following concern:
 * Software program that does not meet GNG.

While all contributions to Wikipedia are appreciated, content or articles may be deleted for any of several reasons.

You may prevent the proposed deletion by removing the  notice, but please explain why in your edit summary or on the article's talk page.

Please consider improving the article to address the issues raised. Removing  will stop the proposed deletion process, but other deletion processes exist. The speedy deletion process can result in deletion without discussion, and articles for deletion allows discussion to reach consensus for deletion. Qwyrxian (talk) 04:38, 9 July 2010 (UTC)

Long time no see
Nice to see you here on Wikipedia (re Talk:LR parser, et al). You may remember me (and Mark Harrison) from VISystems circa 1988. — Loadmaster (talk) 21:33, 24 May 2011 (UTC)

File:LRGenOverview.jpg
Hi, it is my understanding you were trying to delete File:LRGenOverview.jpg. As it is unused and you were the original uploader, I've deleted it, hope that's what you meant.  Snowolf How can I help? 21:13, 4 December 2011 (UTC)

ArbCom elections are now open!
Hi, You appear to be eligible to vote in the current Arbitration Committee election. The Arbitration Committee is the panel of editors responsible for conducting the Wikipedia arbitration process. It has the authority to enact binding solutions for disputes between editors, primarily related to serious behavioural issues that the community has been unable to resolve. This includes the ability to impose site bans, topic bans, editing restrictions, and other measures needed to maintain our editing environment. The arbitration policy describes the Committee's roles and responsibilities in greater detail. If you wish to participate, you are welcome to review the candidates' statements and submit your choices on the voting page. For the Election committee, MediaWiki message delivery (talk) 22:17, 30 November 2015 (UTC)

LR(*) parser
Paul, unfortunately for the article LR(*) parser, Wikipedia can not accept original research. If you are indeed the only person to use this algorithm, it is your original research and is therefore not appropriate for Wikipedia. Because of this, I will be nominating the article for deletion. If you wish to have your idea published, you should pursue this in computer science journals or possibly the news media. Oiyarbepsy (talk) 01:14, 1 April 2018 (UTC)

Proposed deletion of LR(*) parser

The article LR(*) parser has been proposed for deletion because of the following concern: "This article is original research. The author of the article claims credit for developing this, see the talk page."

While all constructive contributions to Wikipedia are appreciated, pages may be deleted for any of several reasons.

You may prevent the proposed deletion by removing the notice, but please explain why in your edit summary or on the article's talk page.

Please consider improving the page to address the issues raised. Removing will stop the proposed deletion process, but other deletion processes exist. In particular, the speedy deletion process can result in deletion without discussion, and articles for deletion allows discussion to reach consensus for deletion. Oiyarbepsy (talk) 01:15, 1 April 2018 (UTC)

title: User talk:Paulbmann


text:
Talk:Hoarding disorder

Merger discussion
I propose that hoarder house be merged into this article. I'm currently working on an epidemiology section for the article which will include the relevant information from hoarder house. Luiysia (talk) 21:22, 10 August 2023 (UTC)

 * Merging with hoarder house is not relatable because article has been  redirected to Hoarding disorder as a separate disorder. Dcotos (talk) 14:55, 18 September 2023 (UTC)

Linking Hoarding Disorder to Disability page
Hoarding disorder has been recognized as a disability and is now covered uner the Americans with Disabilities Act (and likely other countries' similar governance agencies). I think it would be helpful to have a formal link to Disability page. There should also be more formal references to the International Obsessive Compulsive Disorder Foundation (IOCDF), which has excellent resources for both professionals and those looking for help. This article would also benefit from updated works by Dr.Frost et al. MTshepherdess (talk) 04:58, 29 May 2024 (UTC)

title: Talk:Hoarding disorder


text:
Skivvy

Skivvy may refer to:
 * Undergarments, (primarily in the United States)
 * The upper body clothing also known as a Polo neck or turtle neck
 * Maid, servant, or person at the bottom of the social order (primarily in Britain)

title: Skivvy


text:
Janet Akyüz Mattei

Janet Hanula Mattei ( Akyüz; January 2, 1943 – March 22, 2004) was a Turkish-American astronomer who was the director of the American Association of Variable Star Observers (AAVSO) from 1973 to 2004.

Biography
Janet Hanula Akyüz was born in Bodrum, Turkey, the eldest of five siblings born to Turkish Jewish parents, Bella and Baruh Akyüz. She was educated at the American Collegiate Institute, İzmir. She came to the United States for university studies, and attended Brandeis University in Waltham, MA on the Wien Scholarship. Later, she was offered a job by Dorrit Hoffleit at the Maria Mitchell Observatory in Nantucket, Massachusetts.

She worked at Leander McCormick Observatory in Charlottesville, Virginia from 1970 to 1972 and received her M.A. in Astronomy from the University of Virginia in 1972 and her Ph.D. in Astronomy from Ege University in İzmir, Turkey, 1982.

As head of the AAVSO for over 30 years, she collected observations of variable stars by amateur astronomers from around the world. She coordinated many important observing programs between amateur observers and professional astronomers. She was also keenly interested in education and student science projects. Under her direction, the database of the association was made available to educators and also assisted non-professional astronomers access the Hubble Space Telescope.

Awards and honors
Mattei won many awards, including the Centennial Medal of the Société Astronomique de France, 1987; George Van Biesbroeck Prize, American Astronomical Society, 1993; Leslie Peltier Award, Astronomical League, 1993; first Giovanni Battista Lacchini Award for collaboration with amateur astronomers, Unione Astrofili Italiani, 1995; and the Jackson-Gwilt Medal of the Royal Astronomical Society, 1995. Asteroid 11695 Mattei was named in her honor on January 9, 2001 (M.P.C. 41938).

Death
She died of leukemia in Boston in 2004, aged 61.

title: Janet Akyüz Mattei


text:
Sky Valley

Sky Valley may refer to:
 * Sky Valley, California
 * Sky Valley, Georgia
 * Welcome to Sky Valley, Kyuss album
 * Skykomish Valley, also known as Sky Valley

title: Sky Valley


text:
Skippack

Skippack may refer to the following in the U.S. state of Pennsylvania

 * Skippack Township, Pennsylvania
 * Skippack, Pennsylvania, a census-designated place in the above township
 * Skippack Bridge, a stone arch bridge in Montgomery County
 * Skippack Creek, a tributary of Perkiomen Creek
 * Skippack Pike, a historical section of Pennsylvania Route 73

title: Skippack