A Belhadj Ahmed

C Baudouin
email: cyrille.baudouin@ensam.eu

S Leleu

R Bigot

P Secordel

Methodology to Straighten the end Parts of Long Workpieces

Keywords: long workpiece, cold rolling, straightness, methodology

A workpiece is considered as "long" when one dimension is larger than the two others, for example, railways or guides of manufacturing machines. The customers' requirements' evolutions asks the question of workpieces end parts straightness. This paper deals with the means to measure and correct the straightness, a way to prevent flaws in adapting the process and it is especially focused on improving quality for the end parts of the long workpiece. At first, to get the right measure of the straightness, we take into account the fact that the workpiece is deformable; consequently, we assumed the hypothesis that the measured straightness is composed of the intrinsic straightness and the elastic one. Then an approach was developed in order to measure any straightness. Secondly, after getting the proof that the measures are reliable; the correction of the straightness has been studied. The method of semi-automated straightening has been developed in order to improve the productivity. Some experiments have been realized and the results have been compared to the theoretical ones. The conclusions will help to find ways to modify the process in order to prevent the flaws of straightness of long workpieces.

Introduction

Long workpieces are characterized by one dimension greatly longer than the two others. Square and cylindrical cross-sections bars or rails belong to this category. These workpieces are generally produced by hot rolling process. During cooling, thermal stresses produce residual deformations evaluated through straightness error. Straightening machines with rollers are usually used to correct these geometrical variations [1,2], but it remains straightness error at each end of the workpiece. To avoid cutting the ends [3], a loop "measuring-straightening" could be repeated up to ensure workpiece conformity. Often, end straightening is done using an hydraulic press manually driven by constraining the bar into three points bending in order to introduce a plastic deflection. Due to more and more tight requirements on straightness on the one hand, and a request to increase productivity on the other hand, the optimization of quality and time on straightening process are required. In an industrial context, in-line straightening model is desired to avoid slowing down production rates. Moreover, the success of straightening will not depend on the operator experiment.

In this work, a methodology has been developed to optimize the straightening process. This approach is based on a coupling between inspection and mechanics. First, improvements have been proposed to evaluate straightness from untreated measurements. Then, an analytical model based on elastic-plastic bending of beams was established to compute straightening parameters: distance between the press supports, load, displacement, residual deflection and springback. If the result is not good after one straightening, additional measurements should be used to improve the implemented model. To illustrate this methodology, experiments were done on square and cylindrical cross-sections bars or rails.

Methodology to Straighten the end Parts of Workpieces

In an industrial context, the proposed methodology, based on a loop "measuring -straightening", adds propositions to reduce the number of iterations to improve productivity. This paper gives some details on these specific propositions located in gray boxes on the global flow chart of straightening process (Fig. 1).

Figure. 1. Flow chart for straightening process.

The straightening of the ends of bars should be done with several press strokes if necessary, but without intermediate measurement. Only two measurements are required. The first must be done to initialize computation algorithm that will determine straightening parameters (activities 2.1 and 2.2). After a first step of straightening, a second measurement is necessary to record the conformity report of controlled bar. However, if this second measurement conclude to straightness nonconformity, it will be necessary to do another loop, including, at least, a third measurement. This additional loop is considering as an unsuccess of straightening because it decreases the productivity. To ensure that this situation is not only unfavourable, the additional measurement will be treated differently than the first one in order to bring new knowledges. The third measurement not only initializes the straightening algorithm, but also its result computation will be incorporated into a database of knowledge to improve the straightening efficiency for similar configurations (activity 1.3). The parameters of the material model are the principal adjustment values in the database. This activity can also be called "improving straightening by learning".

As finite element simulation takes too much time, an online analytical model in elastic-plastic bending is used to predict straightening parameters. Details of this model are described in mechanical straightening section. Meanwhile, during the physical straightening (activity 2.3), a theoretical profile after straightening is computed (activity 2.4). If this computed profile remains in non-conformity, other steps of press strokes should be applied until getting a satisfying straightness. In practice, according to the profile waveform, multi-step straightening is sometimes inevitable. The efficiency of straightening process depends on a good representation of the measured profile theoretically straight. Because of production rates requires a non-contact inspection, several technologies of measurement could be used [START_REF] Huart | Method for straightening rolled metal sections[END_REF]. Then, profile representation quality depends on data processing. As the product length makes it deformable and the cross-section influences the evaluation of straightness, the computation of the representative workpiece profile is discussed in next section.

Straightness of Long Workpieces

Measurement of Workpiece. The total length of evaluated workpieces is between 15 and 100 meters, the length of end parts depends both on the total length of workpiece and the distance between straightening machine rollers. Usually it should be between one and four meters. In this paper, the aim is to measure the end parts of long workpiece to evaluate its straightness. This geometric characteristic is defined as the minimum distance between two parallel lines enclosing the measured profile [5], as shown in Fig. 2. Due to high evaluation length, the measurement is relatively complex. In fact, it is difficult to make accurate measuring machines with good repeatability over several meters. The best way to guarantee the conformity of such a workpiece is to control manufacturing parameters. In theory, the online control of straightening machine parameters (wear of rollers, compliant rollers positioning, load on the rollers, etc.) should be good enough to ensure straightness conformity. However, this process cannot straighten the workpiece end parts, so a specific inspection is required. Moreover, customers usually require direct inspections because they do not trust in indirect measurements. That leads to the development of particular measuring machines and associated numerical processing in order to correct imperfections: they come from mechanical guidance deviations or sensors errors (often considered insignificant). To correct mechanical guidance deviations, it is possible to design measuring machines that are able to dissociate metrological structure from that which supports the effort. As a result, obtained measurements are independent from machine components' deformation [START_REF] Lahousse | Application d'une nouvelle conception d'architecture à une machine de mesure de résolution nanométrique[END_REF]. Nevertheless, this possible design increases the number of workpieces in these measuring machines (two structures, sensors to evaluate structures' deformations) so their cost too. Another way is to use a multi-probes system which makes possible to separate straightness profile of workpiece from the errors generated by measuring machine after computing [7]. However, the measure reference is done by the sensors alignment on a small length, and the smallest error could spread to the evaluated length with a large extent. Indeed the correctness and uncertainty of multi-probe scanning system is inversely proportional to the measured length and it is almost impossible to achieve high level of zero-calibration. For instance, to evaluate straightness on three meters with an accuracy of 0.3 mm, laser probe sensors must have 2.72 10 mm calibration. So to make possible separation straightness profile of workpiece from the errors generated by measuring machine, workpiece must be measured in several complementary positions (reversal technique). This technique is easily applicable for the measurement of bars with a cylindrical cross-sections but it remains more complex in the case of rail, because it is not possible to rotate a long rail.

In this paper, the measuring machine considered has only one structure and uses only one laser sensor. It can be assumed that the motion deviations from the guidance system are known by an appropriate calibration carried out periodically. To evaluate straightness directly, the workpiece is motionless and sensors move along the workpiece. The untreated measurement includes intrinsic deviations of workpiece and process measurement deviations. So, numerical computation is required to dissociate these two origins of deviations. For instance, the length of the workpiece involves hyperstatic conditions, which generates stresses on workpiece that is not in accordance with ISO 13565-1 standards [START_REF]Geometrical Product Specifications (GPS). Surface texture: profile method: surface having stratified functional properties. Part 1: filtering and general measurement conditions[END_REF]. In the opposite, if the workpiece is in an isostatic position on the measuring space, an elastic deformation occurs. The generated deflection is not an intrinsic error because it could be considered as a degree of freedom for the workpiece. Moreover, bad alignments of the workpiece in the measuring space could also conclude to wrong interpretations of results. The following two subsections propose methodologies to correct acquisitions errors mentioned above and related to the measurement process.

Influence of Elastic Deformation in the Evaluation of Straightness (activity 1.6).

The measurement process begins with positioning the long workpiece in the measuring space. As a result of its length, the position in this space is uncertain. In other words, the angle α between the workpiece and the theoretical line of measurement is variable (Fig. 3). To achieve a reliable measurement, a laser-scanning sensor with a wide measurement field could be used to cover the workpiece during inspection as line laser sensor or a CCD camera for instance [START_REF] Oliver | Vision system for gauging and automatic straightening of steel bars[END_REF]. Another solution is to move the workpiece with centring touch until it was under the sensor field. The best way for a quality acquisition is to make a combination of these two solutions. However, after moving the workpiece, it must be clamped in order to avoid elastic springback. Therefore, the inspection is done under stress. Assuming that guidance measuring machine has been cancelled (activity 1.4); the measured profile is a combination of both intrinsic deviations of the workpiece and elastic curvature deviation as a result of clamping. The elastic curvature can be modelled by an elastic deflection of beam that fits a cubic polynomial law.

Real workpiece

= - -- -+ . 1.1 = -6 . 1.2
where K is constant. To separate elastic deviation from global measurement, parameter K can be minimized by least squares criterion for example. The main drawback of this methodology is the probability to accept an intrinsic cubic polynomial deviation. The effort to guarantee that workpiece is under the measurement field is linked to parameter K. So, the measurement of this effort enables to dissociate intrinsic deviations from elastic ones.

Influence of Cross-Section in the Evaluation of the Straightness. Consider a cylindrical crosssection bar perfectly straight. A laser sensor moves along this specimen according a line without deviation. If the workpiece is not correctly aligned with the inspection direction, the measured profile indicates a straightness deviation as shown in Fig. 4. In fact, the inspection of a theoretical cylinder in a plane that does not include its axis generates an elliptical error. Inappropriate correction of this deviation can lead to scrap a good workpiece. Line-laser sensors or CCD camera are more adapted by measuring the highest point of each measurement to compute the straightness profile.

Figure. 4. Laser deviation during measurement.

In case of end parts of rail, straightness must be evaluated in the vertical and horizontal plane. Two laser sensors are oriented in these two directions and they moved to scan two orthogonal straightness profiles simultaneously. The combined analysis of the two signals could be used to correct the misalignment between workpiece and inspection direction. Indeed, two sections of a same cylindrical cross-section bar shifted in horizontal direction generate variations on laser measurement not only in the horizontal direction but also on the vertical ones (Fig. 5).

Figure. 5. Impact of horizontal offset on the vertical straightness measurement.

If the cylindrical cross-section is considered constant over the length (assumption), the impact of an horizontal shifts on vertical acquisition and vice versa leads to solve a two equations system with two unknowns: the translations in horizontal and vertical directions. For a cylindrical cross-section, the system is not linear, so numerical iterations were used to compute the representation of the workpiece profile. Partial Conclusion. In this section, two improvements to evaluate straightness with accuracy on long workpieces were described: the elastic correction due to workpiece maintaining in the measuring space and the coupled computation with horizontal and vertical direction. Applied on the evaluation of straightness for cylindrical, rectangular and rail cross-sections, theses proposed improvements have reduced repeatability on straightness deviations by 11%.

Mechanical Straightening

Supporting Points Locations for Straightening Stroke. For each workpiece, two parallel lines enclose the measured profile (activity 1), Fig. 2. There are only three points in coincidence between these two lines and the measured profile. These three points are designated as optimal support locations for bending straightening process: two are designed for supporting points and the third is designed for the controlled die. As a result, press load is not always at the middle of supporting points. However, if one of the supporting points is at the end of the profile, it should be relocated to make straightening process possible. Other critical cases could be considered:

• If the supporting points are not adjustable (commonly the press load is at the middle of supporting points), the load will be applied on the highest point of profile and supporting points are automatically located.

• If the length between the supporting points is larger than the possible setting, the load is applied on the highest point and the supporting points are positioned proportionally. The new analytical profile is computed, even if straightening is not physically performed. If the new computed profile is not straight enough, the process described in this paragraph must be repeated several times. Straightening Stroke Parameters. Several studies have been conducted to model the straightening process by elastic-plastic bending [10,11]. However, in these models the relationship between load and displacement to apply for straightening in plastic range are obtained by approximated formula adjusted by numerical simulations. These models are limited by both the cross-section geometry and the material behaviour. In case of small deformations, an analytical model for the straightening process of rectangular and cylindrical cross-section bars have been developed [START_REF] Jun | Study on calculation method of press straightening stroke based on straightening process model[END_REF]. This model takes into account only centring load between supporting points. In the present work, this model has been extended to the case of an arbitrary location of controlled die (Fig. 6). The total displacement required for straightening process depends on material behaviour and on cross-section of workpiece. The material law is assumed to be elastic, perfectly plastic (no work hardening). Workpiece is stress-free and after loading only uni-axial stress is considered. During straightening, there are three stages (Fig. 7):

1. The load generates a fully elastic deformation. Under the yield load point, the displacement is proportional to the applied force. After springback, the workpiece returns to its initial state.

2. The load generates an elastic-plastic deformation: relationship between displacement and load for a square cross-section workpiece is described by Eq. 2.1 to Eq. 2.6. where E is Young modulus, / 5 yield stress, moment of inertia, * coefficient that depends on the level of plastic deformation and ' 67 bending moment at the initial yielding.

= ! + " . 2.1 ! = # 2 3 %& -# 2 . 2.2 " = # 2 ' ()
3. The load generates fully plastic deformation: a plastic hinge appears in cross-section under load location. In industrial context, this case is not considered compared to straightness deviations to reduce. The total displacement 8 9:9 = 8 ; + 8 < is introduced with the help of a vertical punch. After springback, the workpiece rebounds with 8 ; and the total deflection will be reduced to < . If the initial deflection that has to be corrected is equal to 8 < , then the workpiece will be correctly straightened.

Results and Discussions. The methodology described in this paper was applied to straighten square cross-section bars. The Fig. 8 shows profiles of two different bars:

• In case of profiles with a large curvature, one straightening shot can reduce straightness deviation, Fig. 8 (a).

• In case of profiles with parabolic form (small) curvature, a plastic hinge appears below the load location. This case requires additional steps of straightening, Fig. The residual stresses introduced a small deviation between computed and measured profile in the bars after straightening. This difference is computed to adjust parameters of straightening process.

Conclusion

In an industrial context, based on a coupling between mechanics and inspection, the proposed methodology describes an online process for improving straightness of long workpieces. To sum up:

• Improvements have been proposed to evaluate straightness of untreated measurements considering elastic deviations due to clamping and the influence of cross-section geometry on straightness evaluation

• Analytical model based on elastic-plastic bending of beams was established for the computation of straightening parameters: the distance between the press supports, load, displacement, residual deflection and springback.

• Two measurements (one to evaluate residual deflection, the other to check the conformity after pressing) should be enough to determine the straightening of an end bar. Additional measures should be taken into account to improve the implemented model in the proposed methodology.

• The experimental tests confirm that the methodology described in this paper would be of considerable help to the operator or even can be automated to improve productivity.

Figure. 2 .

 2 Figure. 2. Straightness evaluation of 3.2 m long end part for the workpiece of 36 m length.

  Model (a) Before clamping. (b) After clamping.

Figure. 3 .

 3 Figure. 3. Elastic deviation of workpiece before and after clamping.

Figure. 6 .

 6 Figure. 6. Elastic plastic bending of rectangular cross-section.

Figure. 7 .

 7 Figure. 7. Load deflection for rectangular cross-section.

  8 (b). In order to improve straightening of large curved workpiece, a model based on four points bending should be more appropriated.(a) Large curvature.(b) Small curvature.

Figure. 8 .

 8 Figure. 8. Straightening of two square cross-section bars.

Acknowledgements

Authors would like to thank Tata Steel Rail France and the region "Lorraine" for their technical and financial contributions.


Najla Lasoued
email: lassoued_najla@yahoo.fr

Essaid Bilal
email: bilalessaid@gmail.com

Saloua Rejeb

Issam Guenole-Bilal

Nejib Rejeb

Frédéric Gallice

Fernando Machado De Mello

Resumo

Transfer heavy metal from soil treated to colza

Keywords: Mud, heavy metals, colza, bio-accumulator, wastewater. ---------

The impact of heavy metals were tested in sludge from urban and industrial wastewater treatment plants, trying to understand their influence on colza growth and their bioaccumulation. We chose the colza to their specific characteristics. The colza is a plant of the family Brassica Napus, is an excellent bio-accumulator of heavy metals. The mean levels found in the soil are organized in the following order: Fe >> Mn> Zn> Pb> Cu> Ni> Co> Cd. The contents of heavy metals in the sludge are very high and exceed European values allowed for that type of use. The effects of the contribution of sludge are manifested by a significant increase in the heavy metals of the colza, these results in a variation of the ratio between the aerial part and roots of the plant; this ratio tends to increase with the dose of mud brought in soil. The roots of plants show high levels of Zn even on the ground of untreated soil. The contents of Ni, Pb and Zn, compared to Cu and Co, are higher in the roots of colza.

INTRODUCTION

Colza is primarily grown for its oil, accounted for 12% of the oil consumption in the world, behind palm oil (27%) and soybeans (24%). In Europe, its markets are divided between food and non-food outlets, in particular biofuels. Hence the importance of studying the effects of additions of sewage sludge on the development of colza and transfer of heavy metals from soil to colza. The main objective of this study is to evaluate effect of urban and industrial sewage sludge containing heavy metals especially lead and chromium. These sewage sludge are made at different doses (5, 25, 50 and 100 t/ha). We are therefore interested in the growth and absorption of heavy metals by rapeseed Colza and follow the fate of the latter in the ground to prevent pollution events and toxicity.

MATERIALS AND METHODS

The experimental protocol was installed in the field of the Agricultural Experiment Station of Oued Souhil -Nabeul, situated about 60 kilometers from Tunis and belonging to the National Institute for Research in Rural Engineering Water and Forest. The urban mud used in this study is taken from the wastewater treatment plant in Korba with a treatment system at low load activated sludge followed by maturation. The sludge from this station underwent a stabilization in aerobic followed by a drying on beds. The dry sludge is removed from the drying bed.

The mud is from the industrial wastewater treatment plant in Bou Argoub which hosts two industrial zones, companies in the Refrigeration and Brasserie Company of Tunis (SFBT) specialized in the food industry, and Assad specialized in the electrical industry. Sludge from this station underwent a stabilization in aerobic followed by drying on beds. This sludge is loaded with heavy metals especially lead and chromium. The plant material used in this experiment is the rapeseed (Brassica napus) is an annual plant with yellow flowers of the family Brassicaceae. The colza is chosen for their ability to accumulate metals and it is widely cultivated for the production of food oil diesel fuel (biodiesel). That is, with sunflower and olive, one of the three main sources of edible vegetable oil.

The test device is installed on two juxtaposed plots reserved for each crop (wheat or rapeseed). For each type of sludge, four doses (5, 25, 50 and 100 t / ha) were brought into play and compared to control soil without any addition. Field work began in September 2010 with the spreading of sludge realized September 20, 2010. They were manually dug into the ground. Before application, the sludge was analyzed. The soil was sampled twice, first before the application of sludge and the second time after harvest. The samples were taken between the lines by auger at four depths (0-10, 10-20, 20-40 and 40-60 cm).

In the laboratory, soil samples were dried in open air and sieved to 2mm or 0.2mm depending on the type of analysis required. Soil testing is in progress. The main parameters are determined particle size, total calcium, conductivity, carbon, organic matter, total nitrogen and determination of heavy metals. For the particle size we used the method of the International pipette Robinson, it is primarily the destruction of soil organic matter using H 2 O 2 and dispersion of clays by sodium hexametaphosphate. Clays and silts are measured in the suspension of land following the decay time that depends on particle diameter (NF X 31-107). The settling velocities of particles can be calculated by the formula of Stokes.

For plants, we performed the semi rapeseed (50 seeds /m²) December 29, 2010 and semi wheat (350 seeds /m²) January 5, 2011. The rapeseed harvest was performed after the formation of slices May 25; 2011.We weighed the aerial part and the root. The same work was done with wheat June 9, 2011. The samples were subsequently dried and crushed ore to determine the mix of metals in different parts of the plant.

The Mud and soil samples were analyzed by XRF (X-Ray Fluorescence) and ICP-AES (Inductive Coupled Plasma Atomic Emission Spectrometry Activa-Horiba Jobin Yvon Spectrometer) in the Geosciences and environment Department of Ecole Nationale Supérieure des Mines de Saint Etienne. The soil pH was measured using a 1:2 soil to water ratio. Metal accumulation in plant tissues was evaluated from the most abundant species. Plant samples were thoroughly washed in tap water and rinsed three times with distilled water. Samples were then separated into leaves, stems and roots, dried at 40 °C to constant weight and grounded and sieved at 2 mm. Digestion of plant samples was performed using hot nitric concentrated acid, according to [START_REF] Zarcinas | Nitric acid digestion and multielement analysis of plant material by inductively coupled plasma spectrometry[END_REF], Petrescu andBilal (2006, 2007), [START_REF] Secu | Lead, zinc and copper in the bioacumulative horizon of soils from Iaşi and the surrounding areas[END_REF] and Lăcătusu et al. (2009). The plant extracts were analysed by ICP-AES.

RESULTS

Sand is the most representative size fraction in this soil which is a sandy loam soil texture. The analytical results show that the soil of the plot used is characterized by an alkaline pH, conductivity ranging from 1.06 to 1.52 mmho/cm resulting low salinity, soil saturation is between 30.4 and 31.8 ml/100g. Limestone is totally inherited components of soil, possibly modified by repeated and massive supply of amendments basic. The analysis of the limestone is needed to refine the characterization of soil constituents. The percentages of limestone (FIGURE 1) in the different horizons are less than 5% so it is a non-calcareous soil, with very low organic matter content. Contents of total nitrogen are relatively low. The C/N ratio is widely used to characterize and classify types of organic matter in soil. This ratio C/N is about 7 at the first horizon (0-10cm) indicates that organic matter will be quickly mineralized. Concerning trace elements, iron is the most representative. The main levels found in the soil are organized in the following order: Fe >> Mn> Zn> Pb> Cu> Ni> Co> Cd main concentrations of heavy metals introduced by the sludge. The factor and the degree of contamination allow evaluation of soil contamination. The calculation of the contamination factor (CF) is calculated by dividing the concentration of each metal in the soil concentration in unpolluted soil (Muller, 1969;Hakanson, 1980;Rashed, 2010). The values of CF (Bu) of most metals are less than 1 except for Cu and Zn from 25t/ha treatment (FIGURE 2), soil contamination is low. In contrast, the heavy metals CF (BI) are greater than 1, except for Cd (FIGURE 3). The contamination is very high for Cr and Pb and moderate for Co, Ni and Zn. The sum of all factors of contamination for all elements examined is the degree of contamination (Cdeg) of the environment. According to a classification by Hakanson, 1980 andLoska et al., 2003, the Cdeg (Bu) is less than 8 (FIGURE 4) corresponds to low level contamination and the Cdeg (BI) is moderate of soil treated about 5t/ha, considerable for 25t/ha and very high degree of contamination for 50t/ha and 100t/ha. Figure 5. Heavy metals behavior in control soil and treated soils before and after the harvest of crops.

Treated soils and control soil were analyzed before and after the harvest of crops. We note that in the control soil levels of Pb, Ni and Zn were reduced by half, and the Co and Cu contents from 35 to 30 % and 17% for Cd and 10% for Cr (FIGURE 5). In soils treated with sewage sludge, we observed in soils treated with industrial sludge lower metal contents of 57% for Zn 53% for Pb, 45% for Ni, 38% Co, 28% for Cu and 17% for Cd. The levels of these metals remain constant regardless of the treatment except for contents of 45% Cr and 43% for Cd treated soil 100t/ha. For soils treated with urban sewage sludge (FIGURE 5), we have the same tendency for soils treated with sewage sludge industry, the levels of some metals remain constant is that the sludge spreading rate. Though with some differences due to the nature of the sludge, 57% loss for Zn, 45% for Ni, 29% for Pb, 17% for Cd and 10% for Co, Cu and Cr, with however, a decrease of 38% Co contents for soils treated about 100t/ha.

Figure 6: Distribution of heavy metals in the root of colza in soils with addition (5t/ha, 25t/ha, 50t/ha and 100t/ha) of urban (BU) and industrial (BI) sludge and control soil (T).

The Pb concentrations in roots and seed of colza ranged respectively from 65 to 86 ppm and 53 to 71 ppm (FIGURE 6) from industrial sludge treatment and 40 to 59 ppm and 46 to 67 ppm from urban sludge treatment. The Ni (68 to 93 ppm and 74 to 174 ppm) and Zn (67 to 79 ppm and 71 to 97 ppm) are high concentration in roots and seed colza from industrial sludge. The Ni and Zn concentrations from soil treated with urban sludge are ranged from 65 to 70 ppm in root and 71 to 86 ppm. The bioconcentration factor (BCF) can be used to estimate the potential of a plant for phytoremediation. The ability of a plant to accumulate metals from soil can be estimated using the bioconcentration factor (BCF) is defined as the ratio of metal concentrations in the roots compared to those in the soil (Dowdy et al. 1997;Yoon et al. 2006;Gupta & Sinha 2008). Species that showed BCF values lower than 1 are not suitable for phytoextraction. The ability to move the metals from root to shoot, is measured using the FT (translocation factor), which is defined as the ratio of metal concentrations in shoots compared to those in roots (Yoon et al. 2006. Gupta andSinha, 2007). According to McFarlane et al. (2007), FT is defined as the ratio of metal concentrations in leaves compared to those in the roots. The ability to move the metals from root to shoot, is measured using the TF (translocation factor or transfer factor), which is defined as the ratio of the concentrations of metals in the shoots by compared to those in the roots (Yoon et al., 2006;Gupta and Sinha, 2007). According to McFarlane et al. (2007), TF is defined as the ratio of metal concentrations in leaves compared with those in the roots. TF> 1 indicates the preferential distribution of metals in roots. Colza Zn accumulates preferentially in the roots (TF> 1), while it appears to have the ability to transfer to its aerial parts (TF <1) Co, Ni, Cr, Pb and to a lesser extent Cu. [START_REF] Deng | Accumulation of lead, zinc, copper and cadmium by 12 wetland plant species thriving in metal-contaminated sites in China[END_REF] also found that Zn, Cu and Cd accumulated, were mainly distributed at the root parts compared to aerial parts. Translocation Factors weaker were examined in the case of accumulation of Cr, Ni and Pb (FIGURE 7). This result highlights the ability of colza to transfer the metals of these roots to its aerial parts. Fe and Mn were significantly decreased in the stems, roots, siliques and leaves by the addition of sewage sludge. However, this decrease was significantly marked with the contribution of urban sludge in the siliques and leaves colza.

DISCUSSION AND CONCLUSION

Metal concentrations in plants vary with plant species [START_REF] Secu | Lead, zinc and copper in the bioacumulative horizon of soils from Iaşi and the surrounding areas[END_REF], Petrescu and Bilal, 2007and Lacatusu et al., 2009). Plant uptake of heavy metals from soil occurs either passively with the mass flow of water into the roots, or through active transport, crosses the plasma membrane of root epidermal cells.

The mean metal contents found in the soil are organized in the following order: Fe >> Mn> Zn> Pb> Cu> Ni> Co> Cd, main concentrations of heavy metals were introduced by the sludge. The contamination factor (CF) of most metals in urban sludge soil is less than 1 except for Cu and Zn from 25t/ha treatment, the contamination of soil treated by urban sludge is low comparatively with soil treated by industrial sludge where CF (BI) is greater than 1, except for Cd. In this soil, the degree of contamination is very higher from 25 ha/t and levels observed for Cr and Pb is high and moderate for Co, Ni and Zn. After the harvest of crops, the control soil contents of Pb, Ni and Zn are reduced by half, and the Co and Cu contents from 35 to 30 % and 17% for Cd and 10% for Cr. In soils treated with sewage sludge, we observed in soils treated with industrial sludge lower metal contents of 57% for Zn 53% for Pb, 45% for Ni, 38% Co, 28% for Cu and 17% for Cd. The levels of these metals remain constant regardless of the treatment except for contents of 45% Cr and 43% for Cd treated soil 100t/ha.

In soils treated with industrial sludge, the metal concentrations decreased by 57% for Zn, 53% for Pb, 45% for Ni, 38% Co, 28% for Cu and 17% for Cd. The level of these metals remains constant regardless of the treatment, except for 100t/ha treated soils. We have a super saturation of the plant from a threshold beyond which the colza cannot assimilate metals. We have a threshold effect that occurs to 100t/ha where some metals such as Co, Cr and Cd react differently. The result of this study highlights the ability of colza to transfer the metals of these roots to its aerial parts. The Zn, Cd and more or less Cu accumulates preferentially in the colza roots, while it appears to have the ability to transfer to its aerial parts Co, Ni, Cr, Pb. [START_REF] Deng | Accumulation of lead, zinc, copper and cadmium by 12 wetland plant species thriving in metal-contaminated sites in China[END_REF] also found that Zn, Cu and Cd accumulated, were mainly distributed at the root parts compared to aerial parts. Weaker translocation factors were observed in the case of accumulation of Cr, Ni and Pb, this result highlights the ability of colza to transfer these metals of these roots to its aerial parts. According to [START_REF] Deng | Lead and zinc accumulation and tolerance in populations of six wetland plants[END_REF], this phenomenon could be explained by the anatomical characteristics of the plant. Unlike Ni and Zn, which play an essential role in plant nutrition and enzymatic processes, Cd is a highly toxic metal that can affect the growth and metabolism of plants.

According to [START_REF] Raskin | Phytoremediation of toxic metals: using plants to clean-up the environment[END_REF] cadmium uptake by plants can be explained by the existence of competition between Cd and Zn mainly because of their close chemistries. In addition, studies conducted by [START_REF] Clemens | Toxic metal accumulation, responses to exposure and mechanisms of tolerance in plants[END_REF] and [START_REF] Verbruggen | Mechanisms to cope with arsenic or cadmium excess in plants[END_REF] indicate that Cd uptake by plants seems to occur through ion channels and transporters Zn molecules. These observations are consistent with the strong correlation obtained in our study between the concentrations of Cd and Zn measured in the colza roots (FIGURE 7). The Pb, Ni and Zn concentrations are very high in roots and seed of colza from soil treated. The seed colza are used for product oil, we don't know what the reaction of the human body of the colza oil from this treated soil with sewage sludge.

The stress generated by heavy metals in industrial sludge spreading, produces in colza cells programmed death. Several authors (Gilchrist, 1998;[START_REF] Larsen | The BCL2 gene, prototype of a gene family that controls programmed ce11 death (apoptosis)[END_REF][START_REF] White | Life, death, and the pursuit of apoptosis[END_REF][START_REF] Wyllie | Ce11 death the significance of apoptosis[END_REF] observed this type of behavior in tobacco and mammals. Heavy metals inhibit the absorption of nutrients Ca, Mg, Mn, Zn, Cu and Fe and cause phenomena that we have observed in TEM cells colza. [START_REF] Srivastava | Biochemical changes in duck weed after cadmium treatment[END_REF]; [START_REF] Breckle | Growth under stress: heavy metals, plant roots: the hidden half[END_REF]; [START_REF] Cataldo | Organic constituents and complexation of nickel (II), iron (III), cadmium (II), and plutonium (IV) in soybean xylem exudates, Plant Physiology[END_REF][START_REF] Cataldo | Organic constituents and complexation of nickel (II), iron (III), cadmium (II), and plutonium (IV) in soybean xylem exudates, Plant Physiology[END_REF]; [START_REF] Ouariti | Responses of bean and tomato Plants to cadmium -growth, mineral nutrition and nitrate reduction[END_REF]; [START_REF] Burzynski | Influence of lead on NO 3 uptake and reduction in cucumber seedlings[END_REF] and [START_REF] Sandalio | Cadmium-induced changes in the growth and oxidative metabolism of pea plants[END_REF] showed the close relationship between metals heavy, the nutrient deficiency and structural aberrations in colza cells.

We must complete these observations in a larger study in cell biology and biochemistry to better understand the phenomenon of colza cell autophagy and its relations with the spreading of industrial sludge rich in heavy metals. These transformations will have a significant impact on the colza oil produced by this type of culture and therefore an impact on the human body.

Figure 1 .

 1 Figure 1. Chemical composition of different horizons of the soil profile four depths (0-10, 10-20, 20-40 and 40-60 cm) and sludge (Urban sludge (UB) and Industrial sludge (BI)). MO: organic matter.

Figure 2 .

 2 Figure 2. Influence of different doses (5, 25, 50 and 100 t/ha) of urban sewage sludge treatment on contamination factor (CF).

Figure 3 .

 3 Figure 3. Influence of different doses (5, 25, 50 and 100 t/ha) of industrial sewage sludge treatment on contamination factor (CF).

Figure 4 .

 4 Figure 4. Influence of different doses (5, 25, 50 and 100 t/ha) of urban (BU) and industrial (BI) sewage sludge treatment on contamination degree of soil (Cdeg).

Figure 7 :

 7 Figure 7: The canola translocation factor (TF) in soil with the contribution different doses of industrial and urban sewage sludge.


J Sebastian Arrieta

Dr Julie Diani

Pierre Gilormini

Dr Msc J Sebastian Arrieta

Sebastian Arrieta
email: juan-sebastian.arrieta-escobar@ensam.eu

Cyclic and monotonic testing of free and constrained recovery properties of a chemically crosslinked acrylate

Keywords: Amorphous network, Shape memory polymer, Recovery, Cyclic tests, Thermal properties

A chemically crosslinked acrylate network was submitted to various thermomechanical strain and stress recoveries while changing the experimental parameters in order to better define the benefits and the limits of using chemically crosslinked polymers for multicycle applications or applications under external constraints. The results showed that the free recoveries and the constrained recoveries remained the same at the first cycle and during the next ones. It was also shown that the low stress usually generated by a crosslinked network when submitted to a constrained recovery can be significantly increased by choosing suitable experimental conditions.

INTRODUCTION

Shape memory polymers (SMPs) have drawn substantial attention recently due to their actual and potential applications. [1][2][START_REF] Lin | Shape Memory Rigidizable Inflatable (RI) Structures for Large Space Systems Applications[END_REF][START_REF] Sokolowski | [END_REF][START_REF] Hu | Shape Memory Polymers and Textiles[END_REF][START_REF] Hu | [END_REF][7] Research interests focus on both experimental characterization 8- addresses, from an experimental perspective, two major problems that may be encountered when designing shape memory applications for chemically crosslinked polymers. First, are the shape memory properties exhibited by these materials at the first thermomechanical cycle reproducible during subsequent cycles? This question is of interest when these smart materials are used for multicycle applications. Second, chemically crosslinked polymers are known to generate a small stress when recovering their temporary shape, which is a main drawback when they are submitted to an external constraint like in the case of a stent within an artery.

Actually, the external constraint might prevent the material shape recovery. Therefore, the issue is to define the best experimental conditions in favor of increasing the stress generated by the material during shape recoveries.

In terms of multicycle thermomechanical shape recovery tests, physically crosslinked polyurethane reported shape recoveries that could reduce with the number of cycles. 15 A composite silicone-poly(-caprolactone) polymer submitted to three uniaxial tension strain memory cycles, showed strain recoveries with respect to temperatures that superpose for the material deformed at high temperatures. 16 Chemically crosslinked polyurethanes submitted to four thermomechanical cycles reported qualitatively good shape fixity and complete shape recovery. 17 The effect of strain recovery cycles for epoxy networks deformed to their temporary shape at temperatures above and below the rubbery states was tested. Differences in the material cyclic behavior at moderately low temperatures, driving to lower pre-strain at the second cycle when the temporary shape was set at constant stress were noted. 18,19 Finally, cyclic shape memory thermocycles with free or constrained recovery steps on a crosslinked polyurethane submitted to its temporary shape at high temperatures above the glass transition were presented by Volk et al. 20 The comparison of the free recoveries and the constrained recoveries between cycles 1 and 2 appears in the form of a table comparing only two specific stress-strain states along the thermomechanical cycles. In order to complement the literature experimental data, free recoveries and constrained recoveries are compared in this paper for up to four cycles when the temporary deformation had been applied at high or moderately low temperatures. Results are discussed in regards to the material stress-strain behavior evolution along the cycles.

The resulting stress with respect to temperature while the pre-strain is maintained constant was first displayed for shape memory polymers maintained in a bending state of strain. 21 The tests were run on crosslinked acrylate network and the pre-strain was applied below and above the glass transition temperature. When the strain was applied at a temperature below the rubbery state, the specimen exhibited a significant stress peak at the glass transition temperature. For samples deformed in the rubbery state, the stress curve with respect to temperature showed a sigmoidal form and the final stress matched with the material behavior at the final temperature. An epoxy network, uniaxially constrained in tension and in compression at a temperature well above the rubbery state was studied. 9 During constrained recovery tests, measures of stress with respect to temperature exhibited a compression state at moderately low temperature due to the impeded thermal expansion, then a sigmoidal form ending with the stress resulting from the material behavior at the applied strain and temperature. Later, the effects of the cooling rate and of the heating rate on the stress vs. temperature response of samples submitted to compressions at moderately low temperatures was reported, 11 and a little effect of the cooling rate and an increase of the stress peak when increasing the heating rate. However, it may be noted that at the end of the cooling and at the beginning of the heating these experimental results do not show the expected stress and the experimental data are truncated. This is due to the mixed conditions including constrained recovery at high temperatures and free recovery at low temperatures. Finally, it was observed on a crystalline polymer that increasing the applied strain increases the stress peak. 22 Extensive experimental results reveal that the stress peak increases when lowering the temperature at which the strain is applied and with reducing stress relaxation duration at the loading temperature. It was also observed that these variations did not depend on the storage time at low temperature. 23 The latter experimental data were obtained on specimen strained at 100% and very little or no compression was measured at the beginning of the heating, which is inconsistent and remains to be understood. 9 However, an extended storage time at low temperature (up to 14 days), should affect the material shape recovery. 24 Therefore, the largest possible pre-strain should be applied at the lowest temperature and the shape memory polymer should be heated fast in order to generate the maximum stress peak. These three experimental parameters have been tested on our acrylate network and although the above statement is theoretically right, it should be balanced by the poor fixity that may be obtained at low temperatures and by the sample break during heating when highly deformed at low temperatures. The effect of structural relaxation has been tested also.

In the following, we present a series of original experimental results obtained on an acrylate network that was synthesized in lab. The material was submitted to uniaxial thermomechanical cycles. The pre-strain was applied at various temperatures, in the rubbery state and below, and stored at room temperature (well below the glass transition). Then, the pre-strained samples were submitted to free recoveries or constrained recoveries at various heating rates from room temperature to high temperature. The material, experiments and results are presented and discussed in the next sections.

EXPERIMENTAL Material

An acrylate polymer network was synthetized in the laboratory using a composition previously proposed. 25 The material is obtained by photopolymerization of benzyl methacrylate (BMA) added with 550 g/mol molar weight poly(ethylene glycol) dimethacrylate (PEGDMA) that serves as crosslinker. A mix of 90% in mol of BMA, 10% in mol of PEGDMA and 0.5% of 2-dimethoxy-2-phenylacetophenone (DMPA) as photoinitiator is set to polymerize for 50 minutes in an ultra violet chamber UVP CL-1000. Final products are rectangular plates of 1.3 mm thickness. The material viscoelasticy temperature range was characterized in tension by performing a temperature sweep of 2 °C/min while submitting the material to dynamic mechanical analysis (DMA) at 1 Hz. The test was performed on a TMA Q800 from TA instruments. Figure 1

Thermomechanical cycle experimental set up

Dogbone samples 4 mm wide and 25 mm long were cut in the acrylate plates. Specimens were submitted to classic thermomechanical recovery cycles performed on an Instron 5881 tension machine equipped with an environmental chamber capable of applying accurate heat ramps. Local strains were measured by video extensometry, and stresses were measured with a 1 kN load cell. Except when mentioned otherwise, strains were applied at a constant crosshead speed of 10 mm/min. A typical thermocycle consists of 4 steps:

Step 1: Sample heating at the desired temperature and pre-straining,

Step 2: Sample cooling to a temperature below the onset of the glass transition temperature while its strain is maintained constant,

Step 3: Stresses releasing; the strain applied during step 1 keeps stored due to the material lower molecular mobility in the glassy state,

Step 4: The sample undergoes a free recovery when heated, or the stored strain is maintained and constrained recovery is recorded as the sample is heated. The heating procedure consists in applying a constant temperature ramp.

The thermomechanical cycle depends on various experimental parameters. In the following, the main focus will be on the temperature at which the strain is applied, which classically should be chosen above the temperature of the onset of the glass transition (40 °C for our material), on the amount of applied strain, and on the temperature heating rate during step 4.

Material stress relaxation during or after the applied pre-strain will also be monitored as in the case of constrained recovery.

First, we consider cyclic free recoveries and constrained recoveries for samples submitted to temporary stretching at high temperature (65 °C) and moderately low temperature (45 °C).

We define a moderately low temperature as the temperature where the material deformation mechanisms are no longer due to entropy changes only. For this purpose, each sample is submitted to four consecutive thermomechanical cycles during which the material stressstrain responses are recorded during step 1, and the free recoveries or constrained recoveries during step 4 are also monitored. Such experimental results will provide complete data in order to discuss the re-usability of chemically crosslinked polymer networks for shape memory applications.

Second, some specimens are submitted to thermomechanical constrained recoveries varying the temperature of pre-strain application from 35 °C to 65 °C, the applied strain from 20% to 55% during step 1, and the heating rates from 1°C/min to 10 °C/min during step 4. During the heating ramp, a compression stress may appear due to the hindered thermal expansion, and therefore the sample dimensions were chosen in order to prevent buckling. For samples submitted to 20% strain at 40 °C, the stretching time was also varied in order to study the impact of material relaxation on the stress peak recorded during heating. Another way to study this effect was to let the stress relax at 40 °C during one hour just after deforming the sample. The results will assess the best experimental conditions to produce the maximum stress during recovery while preserving the sample integrity. Only one experimental result is shown for each test for reading clarity, but every test, cyclic or not, was repeated at least twice in order to discard experimental errors.

RESULTS AND DISCUSSION

Strain and stress recoveries under cyclic loading conditions

Pre-strain applied at high temperature in the rubbery state

This section studies the strain and stress recoveries of the acrylate network submitted to 4 consecutive identical thermomechanical cycles characterized by an applied strain of 20% at 65 °C (in the rubbery state) during step 1, and a heating ramp of 5°C/min during step 4.

Figure 2 shows the material stress-strain responses recorded during each step 1 of the four consecutive cycles. One notices that the material behavior remains identical and unaffected by the consecutive cycles.

Figure 3 shows the material free recoveries during the heating steps. The material strain fixity, characterizing the strain after step 3 over the applied strain, remains 99% from cycle 1 to cycle 4. Moreover, the material strain recovery is shown to be similar for all cycles, displaying a full strain recovery after 65 °C. These results corroborate the results reported on an epoxy network for tests extending to 20 cycles. 18 Constrained recovery cycles were also applied during step 4 for the same test conditions and are reported in Figure 4. Only three cycles were obtained, since the samples broke systematically during the fourth cycle. It may be noticed that there is no significant difference between the cycles, and that the material exhibits a compression state at the beginning of the heating step, which results from the constrained thermal expansion while heating. At higher temperatures the material undergoes a change of state from glassy to rubbery characterized by a stress evolution toward positive values. At high temperatures, the final stress is in good agreement with the material stress-strain response at the same temperatures. Figure 4 is in accordance with previous results that showed cyclic independency for the constrained recovery of a polyurethane network pre-strained in the rubbery state. 20 Moreover, such results were expected in regards of the material behavior repeatedly recorded at 65 °C over the cycles (Figure 2). Figures 2 to 4 show that when submitted to shape memory cycles with a temporary shape applied in the rubbery state, the acrylate exhibits no sign of memory of the previous cycles. Therefore, as long as the material is submitted to cyclic free recovery, identical recoveries at the first cycle and at the subsequent ones are expected. For constrained recoveries, the response does not evolve from one cycle to the next one, but the number of possible cycles is limited.

Pre-strain applied at moderately low temperature within the viscoelastic transition

The acrylate network is submitted to four consecutive thermomechanical cycles characterized by 20% strain applied at 45°C during step 1 and a constant heating rate of 5 °C/min during step 4. This test differs from the test run in the previous section by the temperature at which the strain is applied. Temperature 45 °C stands within the glass transition range (Figure 1). At such a temperature, the macromolecular chains are significantly less mobile than at the higher temperature of 65 °C, which leads to a significantly different material behavior. The material stress-strain responses in uniaxial tension measured at 45 °C during the steps 1 of the four thermomechanical cycles are presented in Figure 5. The large stress reached at 45 °C may be noted, compared to the maximum stress of 1 MPa measured at 65 °C. The shape of the material stress-strain response reminds of the behavior of an elasto-plastic behavior. However, the material slowly recovers the whole applied strain when the stress is released. Two hours were required to recover 20% strain at 45 °C. Therefore, the material behavior is more viscoelastic than elastic-plastic. At the second cycle, the initial stiffness and the elastic limit are enhanced by the previous loading. After the second cycle, the material behavior stabilized and the stress-strain responses corresponding to cycles 3 and 4 superimpose with cycle 2. Such a cyclic behavior is consistent with the lower strains recorded by Feldkamp and Rousseau 10 after the first cycle when the same stress was applied during consecutive strain memory cycles. Figure 6 shows four consecutive free recoveries (step 4) when heating the sample at 5 °C/min.

The material shows a fixity of approximately 97% along the cycles. Full strain recovery is observed at 45°C for the four cycles. More interestingly, the kinetics of recovery is independent of the cycle number. The material strain recovery is complete. The latter result indicates that the material behavior alteration is likely to happen during the first loading only, and the material state does not evolve after the maximum loading is first reached. Similar results were obtained on an epoxy network. 10 The plot of the constrained recovery of a polymer network pre-strained more than once at moderately low temperature has never been shown in the literature. Considering the change in the material behavior recorded along the cycles (Figure 5), one could intuitively expect a constrained recovery change from cycle 1 to cycle 2. Figure 7 displays the stress recovery (step 4) at constant 20%-strain. Unlike when the pre-strain is applied at high temperatures, the stress recovery presents a stress peak at the DSC glass transition temperature, approximately 46 °C. At the beginning of the heating ramp, a compression stress is induced by the impeded thermal expansion. Such a compression has never been shown in the literature for samples strained at moderately low temperatures, because of experimental set up limits 2,11 or large applied strain. 23 Actually, when increasing the applied strain, the stress takes and keeps positive values at the beginning of heating. Next section will address these results in more details. It is to be noticed that the maximum compression is significantly larger in Figure 7 than in Figure 4.

Despite the stress-strain response change from cycle 1 to cycle 2 displayed in Figure 5, the constrained recovery is similar from one cycle to the next one. This reveals that either the stress-strain alteration due to the first load does not affect the material stress recovery with respect to temperature, or this alteration is not significant enough to appear in the stress recovery curve with the precision of the experimental set up.

Finally, constrained recovery seemed a highly damaging test for both loading temperatures 45 °C and 65 °C, since the samples broke at the fourth cycle consistently, whereas samples submitted to free recovery never broke. An interpretation of the systematic sample failure in cyclic constrained recovery, may be based on the failure envelope of Smith. 26 The latter characterizes the time to failure in constant strain conditions and would correspond to the total time at constant strain over the cycles during our experiments. This interpretation is supported by some tests (not shown), where the material was stretched in the rubbery state and then kept at constant strain at high temperature (65°C): the material showed consistent failure after few minutes.

The experimental data plotted in this section showed that the measured constrained or free recoveries at the first cycle are representative of the recoveries of the subsequent cycles. This result was shown to apply whether the material had been deformed at low or high temperatures. Therefore, chemically crosslinked polymers can be good candidates for multicycle applications and a single cycle is needed to characterize their thermomechanical recovery properties.

Favorable experimental conditions for a high stress peak during constrained recovery

In the previous section, it was shown that after submitting the material to a pre-strain at a moderately low temperature, a stress peak was recorded at the glass transition temperature during constrained recovery conditions. This section aims at determining the favorable conditions for enhancing this stress peak, which may be an asset in the case of free recovery under external constraint.

Temporary shape fixation temperature

The effect of the pre-strain temperature on the constrained recovery has been investigated previously. 23 In order to confirm these results, the acrylate network was submitted to thermomechanical tests with step 1 characterized by 20% strain applied at various temperatures from the glassy state (35 °C) to the rubbery state (65°C). Samples were all submitted to a 5 °C/min heating ramp while maintaining the sample length constant during step 4. The stress values recorded during step 4 are plotted in Figure 8. A stress peak is observed for samples strained at 50°C and below, which increases as the pre-strain temperature decreases. Moreover, it does not appear necessarily at the glass transition temperature, and it is shifted toward lower temperatures as the temperature of strain application decreases. All these results are in good agreement with the experimental data obtained on a thermoset polyurethane, 23 and similar results were also obtained on a Poly(methyl methacrylate) (PMMA). 27 Nevertheless, Sun et al. 27 did not find the overshoot evolution shown in Figure 8.

At 35 °C, which stands below the onset of the glass transition, an interestingly large stress peak is noted but when pre-strained at such a temperature, the material presents a strain fixity of 90%, while it increases to 93% at 40 °C and to 97% at 45 °C. Therefore, a trade off exists between good shape fixity and high resulting stress when looking for the optimal pre-strain temperature.

Temporary applied strain

Another way to increase the resulting stress peak during constrained recovery, that has not been studied previously, is to increase the applied pre-strain. In this section, the specimen is submitted to different strains at 40 °C (within the glass transition) during step 1. After releasing the stress, similar strain fixities of 93% were recorded for samples strained 20, 40 and 55%. Figure 9 presents the stress evolution when heating the sample at 5 °C/min and maintaining its length constant. The stress peak increases significantly with the applied strain.

The compression state observed at the beginning of the heating ramp disappears, as the applied strain increases. This explains why Azra et al. 23 who had submitted their materials to a large strain (100%), did not measure any compression during their experiments. Increasing the applied strain appears to have a favorable impact on the amount of resulting stress when heating the material, but it may end with sample break at high temperatures. Actually, each sample submitted to 55% strain broke before reaching 60 °C. These sample failures are consistent with the maximum strains at break recorded during uniaxial tension tests at constant temperatures. The material is expected to break around 55% strain at 60 °C from Figure 10, where the material uniaxial strain at break with respect to temperature is shown.

Note that Figure 10 is consistent with the strain at break with respect to temperature obtained for acrylate networks submitted to uniaxial tension and also displayed by Safransky and Gall 25 and by Yackaki et al. 28 Therefore, applying a large strain increases the stress generated during constrained recovery, but the maximum strain that is sustainable by the material at higher temperatures has to be taken into account in order to avoid undesired material breaks.

Heating rate during constrained recovery

A steeper heating ramp during constrained recovery tests tends to increase the stress overshoot. 11 Therefore, acrylate samples were submitted to 20% strain at 45 °C during step 1 and, after shape fixation and stress release, the samples were heated at 1 °C/min, 5 °C/min and 10 °C/min while maintaining their lengths constant. The uniaxial stresses recorded while heating the samples are shown in Figure 11. An increase of the stress overshoot and its shift toward higher temperatures when the temperature heating rate is increased has been observed which is consistent with Castro el al. 11 and Qi et al. 29 These results could not be explained by the material thermo-viscoelasticity only, and the coefficient of thermal expansion has been shown in the simulations of Chen and Nguyen 30 to affect the stress overshoot. Since the coefficient of thermal expansion of our material depends strongly on the heating rate, this may explain the observed effect on the overshoot shown in Figure 11. It may also be noted that the samples were then submitted to the same heating rate of 5 °C/min while maintaining their length constant. The resulting stress with respect to temperature is displayed in Figure 12.

Allowing the material to undergo some stress relaxation leads to a significantly lower stress overshoot. It may also be also noted that the temperature of stress inversion toward positive values increases when the stress relaxation increases, which results in more compressive states at the beginning of the heating ramp. These experimental results show that the stress overshoot is significantly dependent of structural relaxation, as asserted by some models of the literature. 31,32

CONCLUSION

A rather complete experimental study on the free recovery and constrained recovery of a shape memory crosslinked polymer network has been presented in order to validate and supplement the experimental data of the literature. The study aimed at better understanding the potentiality and the limits of chemically crosslinked polymers for future shape memory applications.

On one hand, it was shown that crosslinked networks are good candidates for multicycle shape memory applications since the kinetics of their free or constrained recoveries are the same at the first cycle and at the subsequent cycles. Therefore, a single thermocycle is enough to characterize the shape memory kinetics. On the other hand, for shape memory applications with external constraints, it was shown that it is possible to increase the stress response of the material during constrained recovery. The ideal conditions are summarized as follows: (i) decreasing the temporary shape fixation temperature below the glass transition temperature while keeping a reasonable large shape fixation, (ii) increasing the pre-strain as much as possible without being too close to the strain at break, (iii) limiting the strain relaxation during pre-strain application, fixation and recovery, and finally (iv) increasing the heating rate during shape recovery. Among these parameters, lowering the temperature of pre-strain fixation was noticed to have the strongest effect on the constrained stress recovery.

  show the storage Young modulus and the damping factor, resulting from the DMA temperature sweep. It may be noted that the material viscoelasticity extends from 40 to 80°C. The glass transition temperature measured by differential scanning calorimetry (DSC) at a heating ramp of 10 °C/min was located at 46 °C.

Figure 1 :

 1 Figure 1: Dynamic mechanical analysis temperature sweep from 20°C to 100°C at 2 °C/min

Figure 2 :

 2 Figure 2: Material stress-strain response in uniaxial tension when deformed up to 20% at

Figure 3 :

 3 Figure 3: Step 4 free recovery during four consecutive thermomechanical cycles for a sample

Figure 4 :

 4 Figure 4: Step 4 constrained recovery during consecutive thermomechanical cycles for a

Figure 5 :

 5 Figure 5: Material stress-strain response in uniaxial tension when deformed up to 20% at

Figure 6 :

 6 Figure 6: Step 4 free recovery during four consecutive thermomechanical cycles for a sample

Figure 7 :

 7 Figure 7: Step 4 constrained recovery during consecutive thermomechanical cycles for a

Figure 8 :

 8 Figure 8: Stress vs. temperature during constrained recovery for samples submitted to a

Figure 9 :

 9 Figure 9: Stress vs. temperature during constrained recovery at 5°C/min heating rate for

Figure 10 :

 10 Figure 10: Strain at break with respect to temperature resulting from uniaxial tension tests run

Figure 12 :

 12 Figure 12: Stress vs. temperature during constrained recovery at 5 °C/min following various

among others, and modeling development (see reviews

13,14 ). The current contribution

ACKNOWLEDGMENT

This work was supported by the French "Agence Nationale de la Recherche" through project REFORM 10-JCJC-0917.

heating rate seems to have little effect on the compression exhibited at moderately low temperature.

Molecular relaxation during deformation

The stress peak observed on samples strained at moderately low temperatures can be interpreted by the limited structural relaxation in the glassy state. 31 In order to validate this interpretation, four samples were submitted to 20% strain at 40 °C while allowing more or less molecular relaxation before cooling at constant strain. The first sample was strained at 10 mm/min and slowly cooled down to room temperature in approximately 30 minutes. The second sample was strained at 0.3 mm/min, thus allowing more molecular relaxation during the loading step, and then it was cooled down slowly. The third sample was strained at 10 mm/min and maintained at 20% strain during one hour at 40 °C before cooling down slowly.

Finally, the last sample was deformed at a slow crosshead speed of 0.3 mm/min and held for one hour at 40°C before letting the sample cooling down slowly to room temperature. All