Anatoliy Zavdoveev
email: avzavdoveev@gmail.com

Peter Zok
email: peter.zok@rub.de

Valeriy Pozniakov

Massimo Rogante

Thierry Baudin
email: thierry.baudin@universite-paris-saclay.fr

Mark Heaton
email: mark.heaton@antsltd.com>

Alex Gaivoronskiy

Sergiy Zhdanov

Philippe Acquier
email: philippeacquier@yahoo.fr

Tatiana Solomijchuk

Valeriy Kostin

Mykola Skoryk
email: mykolaskor@gmail.com

Ilya Klochkov

Sviatoslav Motrunich

Sviatoslav

Methodology Philippe Aquier Conceptualization

Valeriy Poznyakov

Alex Gajvoronskiy Supervision

Effect of pulse current GMAW on the yield stress of the S460M TMCP steel welded joints

Keywords: gas metal arc welding, pulsed arc welding, heat affected zone microstructure, mechanical properties

Thermomechanically cold processed (TMCP) steels with a high level of strength are actively used in various constructions. The high strength of the TMCP steels is acquired due to the formation of the fine-grained structure. Such steels have relatively low carbon percentage. These greatly simplify the solution of the problem of improving the quality and reliability of metal structures. At the same time this raises new questions in terms of the technology for welding such steels. In the first instance, it is conditioned by the complex behavior of the TMCP structure under the welding thermal cycle effect. The most important property of the welded joint is yield stress (YS) which characterizes the workability of the whole joint. The estimation of the YS while developing the welding technology is a valuable task for design. This current research has made a complex investigation of the modern welding technique's effect on the behavior of the yield stress formation in TMCP steel welded joints. For progressive technology development, pulse arc welding was used.

Introduction

In recent years, the amount of welded steel constructions with higher strength abilities increased continuously. Quality requirements in many industries such as shipbuilding, civil engineering, hydropower, etc. dictate new rules for the development of welding technologies for metal structures while maintaining a high set of performance properties [START_REF] Lee | Evaluation of high strength TMCP steel weld for use in cold regions[END_REF][START_REF] Medina | Effects of v and Nb on static recrystallisation of austenite and precipitate size in microalloyed steels[END_REF][START_REF] Fossaert | The effect of niobium on the hardenability of microalloyed austenite[END_REF][START_REF] Zavdoveev | Effect of heat treatment on the mechanical properties and microstructure of HSLA steels processed by various technologies[END_REF][START_REF] Zavdoveev | Influence of thermal cycle of welding on structure and mechanical properties of haz metal in high-strength steel produced by controlled rolling[END_REF]. The addition of alloys in micro doses has the potential to be a key factor for higher strength [START_REF] Moon | Microstructure and High-Temperature Strength in the Weld Coarse-Grained Heat-Affected Zone of Fire-Resistant Steels and the Effects of Mo and Nb Additions[END_REF][START_REF] Rajan | Heat treatment: principles and techniques[END_REF]. The increase in carbon concentration offers a significant increase in strength at the same time has limitations associated with a decrease in ductility [START_REF] Sim | On the Strengthening Effects Affecting Tensile and Low Cycle Fatigue Properties of Low-Alloyed Seismic/Fire-Resistant Structural Steels[END_REF]. The further additional doping with elements that offer solid solution hardenings such as Mn and Si increases the strength of the metal, however, leads to a deterioration in weldability [START_REF] Park | Fusion Weldabilities of Advanced High Manganese Steels: A Review[END_REF] (CE=%C+%Mn/6+(%Cr+%Mo+%V)/5+(%Cu+%Ni)/15) [START_REF] Medina | Effects of v and Nb on static recrystallisation of austenite and precipitate size in microalloyed steels[END_REF]. Microalloying with carbide and nitride-forming elements (Nb, V, Ti) is limited by the increasing cost of rolled steel.

The welding process has subjected the metal to heating that intersects critical points Ac1 Ac3, forming a heat-affected zone (HAZ) in the final welded joint [START_REF] Janardhan | Fatigue Properties of Resistance Spot Welded Dissimilar Interstitial-Free and High Strength Micro-Alloyed Steel Sheets[END_REF]. Thus, the behavior of the metal under such thermal effect (welding thermal cycle) is crucial in the welded joints' mechanical properties formation, firstly due to structural transformation occurred. In this case, the mechanism of further structure evolution [START_REF] Zavdoveev | Effect of heat treatment on the mechanical properties and microstructure of HSLA steels processed by various technologies[END_REF][START_REF] Liu | Optimized Properties of a Quenching and Partitioning Steel by Quenching at Fine Martensite Start Temperature[END_REF][START_REF] Nasiri | Thermal Mechanisms of Grain Refinement in Steels: A Review[END_REF] is determined by a number of factors. During being in the temperature interval above critical points [START_REF] Shome | Effect of simulated thermal cycles on the microstructure of the heat-affected zone in HSLA-80 and HSLA-100 steel plates[END_REF] Ac1, Ac3, the dispersion carbides (formed by doping elements) and carbonitrides of microalloying elements [START_REF] Sakthivel | Creep Properties of Intercritical Heat Treated Boron Added Modified 9Cr-1Mo Steel[END_REF][START_REF] Li | Outstanding Synergy of Superior Strength and Ductility in Heterogeneous Structural 1045 Carbon Steel[END_REF] reduce the tendency of the austenite to grow the grain size [START_REF] Kim | Misorientation Characteristics at the Growth Front of Abnormally-Growing Goss Grains in Fe-3%Si Steel[END_REF], either by blocking the migration of austenite grain boundaries (Zener pinning effect) [START_REF] Moon | Pinning efficiency of austenite grain boundary by a cubic shaped TiN particle in hot rolled HSLA steel[END_REF]. In contrast to the situation for high-strength steel grades, C-Mn the concentration of the Mn is not enough for delaying grain growth of the austenite during overheating to critical points, and a high level of strength, in this case, can be reachable only for increased concentration of C to provide needed strength level.

Here in the case of microalloying, for obtaining the needed strength level, the carbon content can be reduced which thus improves their weldability (C provides the most significant input in CE), but for further increase in strength, the special techniques and approaches are required. Further increases in the yield strength of structural steels can be achieved using a special rolling method, including thermomechanical cold processing (TMCP). This requires systematic control of both temperature and degree of deformation during metal formation [START_REF] Nasiri | Thermal Mechanisms of Grain Refinement in Steels: A Review[END_REF][START_REF] Chadha | Effect of Double Hit Hot Deformation on the Evolution of Dynamically Transformed Ferrite[END_REF][START_REF] Kumar | Comparative Assessment on the Hot Deformation Behaviour of 9Cr-1Mo Steel with 1Cr-1Mo Steel[END_REF][START_REF] Jiang | Effect of Transformation Temperature on the Ferrite-Bainite Microstructures, Mechanical Properties and the Deformation Behavior in a Hot-Rolled Dual Phase Steel[END_REF].

TMCP requires that the rolling process must be carried out in such a way that the individual stages of deformation of the steel take place at specified temperatures. There are two main effects:

-the development of fine-grained structures that improves mechanical properties and increases impact strength [START_REF] Mayo | On the characterization procedure to quantify the contribution of microstructure on mechanical properties in intercritically deformed low carbon HSLA steels[END_REF];

-the limitation or delay of recrystallization, which is achieved by the introduced microalloying elements [START_REF] Medina | Effects of v and Nb on static recrystallisation of austenite and precipitate size in microalloyed steels[END_REF][START_REF] Larzabal | Effect of Microstructure on Post-Rolling Induction Treatment in a Low C Ti-Mo Microalloyed Steel[END_REF] (Nb, Ti).

TMCP steels with a high level of strength and relatively low carbon equivalent greatly simplify the solution of the problem of improving the quality and reliability of metal structures [START_REF] Lee | Evaluation of high strength TMCP steel weld for use in cold regions[END_REF][START_REF] Meester | The weldability of modern structural TMCP steels[END_REF][START_REF] Lee | Correlation of microstructure and fracture properties in weld heat-affected zones of thermomechanically controlled processed steels[END_REF][START_REF] Zavdoveev | Welding Thermal Cycle Impact on the Microstructure and Mechanical Properties of Thermo-Mechanical Control Process Steels[END_REF][START_REF] Porter | Performance of TMCP steel with respect to mechanical properties after cold forming and post-forming heat treatment[END_REF]. However, this raises new questions in terms of welding technologies for such steels. Namely, the carbon equivalent alone is no longer enough for studies of the heat of TMCP steels. This is primarily due to the peculiarities of the structural state of such steels, which is not taken into account by the carbon equivalent. Technical and economic aspects arising from the ability to make products from these steels and their use in energy efficient industries, as well as their suitability for the construction of various structures, including those operating in extreme climatic conditions are of great importance for materials science. Issues related to this group of steels in order to improve the technologies used for the manufacture of metal structures from these steels by welding methods needs to be addressed. One of the promising ways to solve the problems of weldability of TMCP steels is the use of pulse-arc welding, which allows controlling the modes and thermal cycles of welding in a wide range [START_REF] Needham | Material transfer characteristics with pulsed current[END_REF][START_REF] Palani | Selection of parameters of pulsed current gas metal arc welding[END_REF][START_REF] Tong | Quality and productivity improvement in aluminium alloy thin sheet welding using alternating current pulsed metal inert gas welding system[END_REF][START_REF] Zavdoveev | Development of the PC-GMAW welding technology for TMCP steel in accordance with welding thermal cycle, welding technique, structure, and properties of welded joints[END_REF][START_REF] Rajasekaran | Weld Bead Characteristics in Pulsed GMA Welding of AI-Mg Alloys[END_REF][START_REF] Zavdoveev | Features of structure formation and properties of joints of s460m steel made by pulsed-arc welding[END_REF].

Thus, weldability is determined as the formation of an equal strength joint to that of the substrate. The measurement of mechanical properties is essential, as well as structural analysis for understanding the nature of formation properties [START_REF] Nazarov | Comparison of weldability of high-strength pipe steels microalloyed with niobium, niobium and vanadium[END_REF][START_REF] Górka | Weldability of Thermomechanically Treated Steels Having a High Yield Point[END_REF][START_REF] Nam | Effect of post weld heat treatment on weldability of high entropy alloy welds[END_REF][START_REF] Górka | Assessment of Steel Subjected to the Thermomechanical Control Process with Respect to Weldability[END_REF]. Since there are several parts in welded joints to be estimated, namely, weld zone (WEZ), heataffected zone (HAZ), and base metal (BM), it puts the task of firstly mechanical properties determination for each part. The estimation of the yield stress (YS), which is the most important characteristic, meets difficulties exactly in the HAZ because of the small length of this zone. The tensile test samples can be easily cut from weld and BM, but the issue of the HAZ scale factor makes this task much more problematic. From this point of view, the indentation technique opens the way to solving this problem. In the current contribution, precise measurement is taken of the yield stress for welded joints especially in the HAZ zone, as well as a comparison with tensile tests and Vickers hardness tests made for BM and welds. The microstructural investigation shows a good correlation with mechanical properties.

Experimental

The thermomechanical cold processing strengthened steel S460M [START_REF] Zavdoveev | Weldability of high-strength microalloyed steel S460M[END_REF],

manufactured in accordance with EN 10025-4: 2007 is used. In all the received conditions it possesses the following mechanical properties: yield stress (YS) R' p0.2 = 460 MPa, ultimate tensile stress (UTS) Rm = 600 MPa,  = 25% (uniform elongation for failure),  = 58% (area reduction during tensile test). In case of using the TMCP steel, it is necessary to mention, that depending on the level of cold hardening effect throughout the crosssection of the material the yield strength can vary [START_REF] Lee | Evaluation of high strength TMCP steel weld for use in cold regions[END_REF][START_REF] Porter | Performance of TMCP steel with respect to mechanical properties after cold forming and post-forming heat treatment[END_REF].

For the welding procedure, the G3Si1 welding wire of 1.2mm in diameter was chosen. The actual chemical compositions of the investigated steel and welding consumables measured using an optical emission spectrometer are given in Table 1. The welding power source used was an inverter type ewm Phoenix Pulse 501 (company "MULTIMATRIX"), which provides a different frequency of pulses in pulse-arc welding [START_REF] Nasiri | Thermal Mechanisms of Grain Refinement in Steels: A Review[END_REF]. To determine the welding and technological characteristics of the current source, a digital oscilloscope UTD2000CEX-II was used. It records the current characteristic of the power supply over a wide range. A 75mA shunt with a resistance of For the basic comparative samples, the welding in shielding gas (Ar+18% CO 2 ) of S460M steel joints with a thickness of 16 mm and a V-shaped opening of the edges was used. Prototypes using PC-GMAW in the forced mode (PC-GMAW F) were performed for butt joints without edge machining. It should be noted that in this case, a high-quality weld formation was provided with complete welding, which is impossible in the case of using the traditional welding process. The welding modes used in the present work are presented in Table 2. From common practice, the welding current is responsible for penetration depth and welding voltage for the width of the weld [START_REF] Górka | Weldability of Thermomechanically Treated Steels Having a High Yield Point[END_REF][START_REF] Zavdoveev | Weldability of high-strength microalloyed steel S460M[END_REF]. At the same time, the welding speed is governing the heat input which is closely connected with the cooling rate of the welded joint. If the welding speed chased high, the cooling rate will be also high and this fact consequently will lead to the formation of a more hardened structure and thus a more high level of the strength of the WEZ and HAZ metal [START_REF] Park | Fusion Weldabilities of Advanced High Manganese Steels: A Review[END_REF][START_REF] Shome | Effect of simulated thermal cycles on the microstructure of the heat-affected zone in HSLA-80 and HSLA-100 steel plates[END_REF]. , where І pulse = 220 А, І pause = 0.8*І pulse t pulse = 0.5s , and t pause = 0.25s are the pulse duration and pause, respectively.

The common welding parameter i.e wire diameter, tip to work distance (is equal to 10 times wire diameter), interpass temperature, and welding speed are chosen based on practice and previous experiments [START_REF] Zavdoveev | Development of the PC-GMAW welding technology for TMCP steel in accordance with welding thermal cycle, welding technique, structure, and properties of welded joints[END_REF][START_REF] Zavdoveev | Weldability of high-strength microalloyed steel S460M[END_REF].

In order to study the influence of the welding modes on welding thermal cycles (WTC), appropriate experiments were performed. Steel plates, 10 mm thick were used to register the WTC, into which chromel-alumel thermocouples were drilled to a depth of 7.5-8 mm. This value of the drilling depth is based on previous studies [START_REF] Zavdoveev | Welding Thermal Cycle Impact on the Microstructure and Mechanical Properties of Thermo-Mechanical Control Process Steels[END_REF] and is due to the need to register the WTC in the area of the HAZ.

The pointwise mechanical characterization was carried out through minor destructive indentation techniques, 3D measurements of these indents and inverse analyses by finite-element material models. This method is called "Eindruckverfahren" or Imprint-Testing [START_REF] Schmaling | Determination of plastic material properties by analysis of residual imprint geometry of indentation[END_REF] and provides the following values (Figure 2.): The Rockwell cone indentation measurements of the investigated samples were made with the following mesh consisting of 3 lines equidistant from each other on 0.5 mm and 20 points per line (the distance between points is 0.5mm).

 comparative yield strength (Imprint) strength
Standard cylindrical tension samples were made of welded joints for mechanical testing of the weld metal for comparison purposes. Three samples for each measurement with a 6 mm diameter for their working parts were mechanically manufactured (type II following ISO 6892:1998). The tests were carried out at an ambient temperature. The tests were carried out at an ambient temperature.

Metallographic studies were performed using a scanning electron microscope on the Miga C LMU (Tescan). The samples for the metallographic studies were prepared according to standard methods using diamond pastes of different dispersion. The detection of microstructure was performed by chemical etching in 4% alcohol solution with nitric acid. The Vickers microhardness of the individual structural components and the integral hardness of the metal was measured on a hardness tester M-400 company "LECO" at a load of 100 g (HV).

The ability of the metal to resist brittle fracture was determined using approaches to fracture mechanics. The values of the criteria K 1C and δ C defined below were determined by the standard methods and formulas given in [START_REF] Zavdoveev | Features of structure formation and properties of joints of s460m steel made by pulsed-arc welding[END_REF].

To determine the values of the critical stress intensity factor K 1C and the critical crack opening δ С , samples of rectangular cross section 10x20x90 mm 3 with a notch length of 7 mm and a fatigue crack of 3 mm length were used. These samples were tested for three-point bending at room temperature.

Results and discussion

Structure and properties of the base metal

Owing to TMCP with controlled cooling in the temperature range 900-700C in ferrous S460M ferritic-pearlitic steel, a striped structure with a grain size of about 10  is formed with a hardness of 195 HV (Figure 3). The study of the hardness (Figure 4) of welded joints revealed that when PC-GMAW F is used, the WEZ metal hardness is 15-20% lower than the hardness of the weld metal made by standard PC-GMAW and PA-GMAW respectively. When for GMAW mode they are almost equal value. This is due to the peculiarities of the structure formation caused by the welding thermal cycle (WTC) which will be considered below. In the HAZ metal, the hardness values of PC-GMAW F are also lower than in standard PC-GMAW.

This fact is an additional advantage of PC-GMAW F, as a uniform distribution of mechanical properties will be formed in the welded joint. It should be noted that, with PC-GMAW F, the hardness in the HAZ decreases to the level of the base metal more slowly. Loading curves for measuring mesh are presented in figure 5, where it can be observed a clear tendency for homogeneous distribution for PC-GMAW-F mode. This effect is conditioned by structure formation and will be considered below. From this point of view, there is a good correlation with hardness measurements. As well as for loading curves and calculated data of yield stress and ultimate tensile stress are also in good correlation (figure 6). For the determination of the UTS, a strong correlation exists with microhardness. Similar behavior of the yield stress on a lower strength level is observed. The value of the error is conditioned by the number of measurements per point on the graph. Here, averaged values were determined from 3 lines to get one point. In the HAZ due to the V type edge development the calculation can take part in several zones. For a better visualization, the R'p0.2 and Rm maps for welded joints are built in From the given YS and UTS maps (Figure 7 When using PC-GMAW F, given that the hardness of the HAZ metal is not more than 20-25% of the base metal, this ensures the uniformity of the welded joint. The behavior of mechanical properties described above is explained by microstructural evolution in dependence on the welding mode and presented below. with GMAW mode and slightly larger in comparison with PC-GMAW mode (Figure 9 c).

The most significant changes are observed in the case of the PC-GMAW F mode, namely the microstructure of the WEZ metal differs significantly from the other welding modes.

The needle-like ferrite (NF) with large plates is observed, and the allocation of polygonal ferrite became wider and their share increased (Figure 9 d). For the HAZ metal (Figure 10) the microstructure of the upper and lower bainite for GMAW and PA-GMAW mode, a bainite structure with a small proportion (up to 3… 5%) of the martensite for PC-GMAW mode, and finally, mainly needle ferrite with an ordered second phase for PC-GMAW F mode are observed. Such differences are due to the peculiarities of the WTC during pulse-arc welding (Figure 11). Arc welding is characterized by physical and metallurgical processes so that one understands the number of complex objects that occurred in the molten metal [START_REF] Singh | Influences of Latent Heat on Temperature Field, Weld Bead Dimensions and Melting Efficiency During Welding Simulation[END_REF],

and the metal of the HAZ under the effect of the welding thermal cycle (WTC), so the main factor affecting it is the WTC. The WTC is characterized by parameters like maximum temperature, heating and cooling rate, time above the set temperatures and in the temperature range. It is possible to control the WTC by changing the modes of the pulse GMAW, namely the main components of current, welding voltage, frequency and duty cycle, and welding speed. in the case of PC-GMAW and PC-GMAW-F, the rate of increase in the temperature is greater than in the case of GMAW; in the high-temperature range from 1350 o C to 1000 o C, the cooling of the metal is faster, and in the temperature range less than 1000 °Cslower. The peculiarities of the WTC flow during PC-GMAW and PC-GMAW-F, allowed to establish that the cooling rate of the metal in the HAZ areas heated to temperatures of 1000 C and more is higher than in stationary arc welding. In the HAZ metal, where the metal is heated to temperatures below 1000 C, the cooling rate of the metal is lower than at GMAW. This promotes the flow of diffusion processes during structural transformations.

Similar patterns are observed in PA-GMAW (Figure 11 c), namely, the cooling rate of HAZ metal in the temperature range of 600-500 o C is less than in standard GMAW.

The WTC plays a key role in the hardening of the weld metal, as it affects the refinement of structural components. During pulsed arc welding there is an interruption in an arc which causes a thermal "blow" that leads to increasing the number of the centers of crystallization. As a result, the structure is refined and the strength characteristics are increased.

Fracture toughness

To assess the sensitivity of metal parts of welded joints to the stress concentration under conditions of flat deformation under static load, a force criterion is used -the critical stress intensity factor K 1C . As the values of K 1C increase, the sensitivity of the metal to the stress concentration decreases. Deformation fracture criteria are used to determine the crack resistance of fracture materials, which are accompanied by significant plastic deformations in the area near the top of the crack and precedes its spread. This criterion is called the critical opening of the crack tip - C . If the shore opening is greater than  C , then the adhesion stress is zero.

The values of the criteria K 1C and δ C were determined by standard methods. To determine the values of the critical stress intensity factor K 1C the next formula is used [START_REF] Zavdoveev | Features of structure formation and properties of joints of s460m steel made by pulsed-arc welding[END_REF]:

(1), where: P is the critical load at which the sample collapses; L is the distance between the supports; t is the thickness of the sample; b is the width of the sample; Y is the shape factor of the sample.

The critical opening of the crack (crack opening displacement) δ c is determined by the formula:

δ С = (2)
, where: YSyield stress (is determined by indentation), E is the Young's modulus which is equal to 200 GPa for low carbon steel [START_REF] Zavdoveev | Effect of heat treatment on the mechanical properties and microstructure of HSLA steels processed by various technologies[END_REF].

It is used to estimate the resistance of the metal to brittle fracture under conditions of large plastic deformation, when the crack at its apex reaches the critical size of  C and begins to propagate rapidly, using the energy released during its further growth. The indicators of resistance to brittle fracture of HAZ metal of welded joints made of standard and forced pulse-arc welding are at a high level, due to the peculiarities of the formation of the structure (Figure 12). As crack opening displacement (COD,  c ) can give additional information about ductility and the ability of the metal to resist crack propagation, and based on the indentation results it becomes possible to estimate (using eq.2) the COD.

Here it is seen that despite K1C slight difference (within 5 %) the value of the COD has more significant dispersion up to 75 % (Figure 12 b). From the data it is clear that from point of view of plasticity mechanics the PC-GMAW -F provides the best characteristics. The pulse-arc welding allows to effectively regulate the structure in the welded joints of high-strength TMCP steels. Under conditions of approximately the same running energies, the effective welding current of the pulse-arc process is 25% higher than when welding with GMAW. This allows the depth of penetration to be increased. Features of the pulse change of the welding current significantly change the nature of the WTC and as a consequence the structure of the thermal zone. The formation of a mixed structure in HAZ allows high strength and resistance to brittle fracture to be obtained. The use of the PC-GMAW F gives the opportunity to make quality welded joints with uniform distribution of the yield stress, which is explained by the features of structural formation governed by WTC. These above-described results allow the following conclusions to be made:

Conclusions

Due to the change of WTC (increases in cooling rate in the temperature range > 1000 o С), the analysis of experimental data has shown that usage of PC-GMAW leads to a fine-grained structure in the weld in comparison with GMAW, and metal hardening structures in the HAZ. The cooling rate of HAZ metal in the temperature range of 600-500 o C is reduced by almost 1.5 times, which reduced the width of the HAZ by 40%. As a result of tests, it was found that the metal of the weld and the HAZ of the welded joints of S460M steel, made by pulse-arc welding, have a sufficiently high resistance to brittle fracture, and 20% higher strength.

Studies of the influence of standard and forced pulse-arc welding on the structure and mechanical properties of TMCP and treated steel S460M allowed to establish the following advantages of the latter:

-in comparison with standard pulse-arc welding, the forced PC-GMAW allows carrying out welding without the development of edges when receiving high-quality welded joints -30% lower values of microhardness, as well as yield stress, in the HAZ of the welded joint in comparison with the standard GMAW due to the peculiarities of the thermal cycle of welding.

-15% lower values of microhardness of the weld metal compared to PC-GMAW and PA-GMAW and close to the values of the base metal, which is provided by the formation of a favorable microstructure -higher homogenous distribution of YS and UTS over the WEZ and HAZ close to the strength level of BM -high level of resistance to brittle fracture of the welded joints, as well as crack opening displacement calculated based on the yield stress data after indentation.

These features of forced pulse-arc welding ensure high mechanical properties of welded joints of TMCP hardened steel S460M, which is the main task for this class of steel. It is demonstrated that the possibility of conducting PC-GMAW F without the development of edges, in addition, can increase the productivity of the process by 40%.

  figure 1. A schematic representation of the oscillogram as well as the live one. The peaks correspond to pulse current and  pulse , and  pause is the time of the pulse and pause respectively.

Figure 1 .

 1 Figure 1. Pulse current characteristics.

Figure 2 .

 2 Figure 2. The plastic flow schematic curve determined with indentation technique called "Eindruckverfahren" (Imprintec) (a); The testing tool in general view (b)

Figure 3 .

 3 Figure 3. Microstructure of the base metal in as-received condition (P: pearlite, F: ferrite) -(a) high magnification (b) -low magnification

Figure 4

 4 Figure 4 Hardness of S460M steel welded joints: GMAW (a), PC-GMAW (b), PA-GMAW (c), PC-GMAW -F (d)

Figure 5 .

 5 Figure 5. Loading curves for welded joints of S460M steel made by different welding modes: GMAW (a), PC-GMAW (b), PA-GMAW (c), PC-GMAW -F (d). (Number of curves correspond to mesh nodes)

Figure 6 .

 6 Figure 6. The distribution of the YS (green) and UTS (blue) for welded joints of S460M steel made by different welding modes : GMAW (a), PC-GMAW (b), PA-GMAW (c), PC-GMAW -F (d).

Figure 7 and

 7 Figure 7 and Figure 8, respectively. It is clearly seen that PC-GMAW and PA-GMAW provide more high level of stress (R'p0.2, Rm). The most favorable distribution of the stress (R'p0.2, Rm) is provided by PC-GMAW F mode, because of dispersion of the yield stress and ultimate tensile stress are minimal in comparison with other welding techniques.

Figure 7 .

 7 Figure 7. The YS maps for welded joints of S460M steel made by different welding modes : GMAW (a), PC-GMAW (b), PA-GMAW (c), PC-GMAW -F (d).

Figure 8 .

 8 Figure 8. The UTS maps for welded joints of S460M steel made by different welding modes : GMAW (a), PC-GMAW (b), PA-GMAW (c), PC-GMAW -F (d).

3. 2 . 3 .

 23 Microstructure analysisThe explanation of the mechanical properties evolution during different welding modes used in the present work, namely GMAW, PC-GMAW, PA-GMAW, and PC-GMAW-F gave by microstructural analysis. The main consideration is focused on the WEZ and HAZ metal. The following microstructural features can be observed. The ferrite structure of various modifications (polygonal, large-needle) in the WEZ metal is formed during GMAW. The characteristic size of the polyhedral ferrite in length is more than 100 μm with coarse-grained needle-like ferrite (Figure9 a).

Figure 9 .

 9 Figure 9. The microstructure of the WEZ metal for S460M steel joints : GMAW (a), PC-GMAW (b), PA-GMAW (c), PC-GMAW -F (d).

Figure 10 .

 10 Figure 10. The microstructure of the HAZ metal for S460M steel joints : GMAW (a), PC-GMAW (b), PA-GMAW (c), PC-GMAW -F (d).

Figure 11 .

 11 Figure 11. Thermal welding cycles for different welding modes : GMAW (a), PC-GMAW (b), PA-GMAW (c), PC-GMAW -F (d)

Figure 12

 12 Figure 12 Fracture toughness and crack opening displacement

Table 1 .

 1 Chemical composition, wt. %

		C	Si	Mn	Cr	Ni	V	Nb	Al	S	P
	S460M 0.15	0.23	1.3	0.09	0.019	0.01	0.05	0.025 0.013 0.017
	G3Si1	0.08	0.8	1.4	0.09	0.02	0.01	-	-	0.020 0.020

Table 2 .

 2 Welding parameters used in this work

	Welding	Wire	Current (A)	Voltage (V)	Welding	Tip to work	Gas flow	Interpass
	mode	diameter			speed (mm/s)	distance	(l/min)	temperature
		(mm)				(mm)		(C)
	GMAW	1.2	200	16	4	12	15-20	50
	PC-GMAW	1.2	220	26	4	12	15-20	50
	PA-GMAW	1.2	205*	18	4	12	15-20	50
	PC-GMAW F	1.2	320	28	5	12	15-20	-

*Here: I av = I pulse t pulse + I pause t pause t pulse + t pause

  and 8) the distribution of strength values is shown pointwise. Especially the HAZ from PC-GMAW and PA-GMAW becomes evident for its increased values. The pointwise determined YS and UTS confirm and expand the expression of heterogeneous or homogeneous mechanical properties in the different welding joints.3.2.3 Tensile test analysisFor comparison with indentation, standard tensile tests were performed and the slight difference in yield stress (measured by tensile test) is explained by the scale factor of the tested samples. From the data given in table3it is seen that under the conditions of pulse-arc welding that higher values of the strength of the weld metal are achieved in comparison with the base metal. Plasticity remains at a fairly high level. The values of the strength of the weld metal are in good agreement with the hardness values for the area of the weld metal, namely do not exceed the values of the base metal by more than 20%.

Table 3 .

 3 Mechanical properties of the weld metal of S460M steel.

		Welding technique	YS , МPа	UTS , МPа		
		PC-GMAW	570	667	24	68
	Weld metal	PC-GMAW F	590	675	23	55
	(WEZ)	PA-GMAW	590	680	26	70
		GMAW	477	586	28	73
	BМ		452	581	26	60

Acknowledgments

This work was supported by the National Academy of Sciences of Ukraine [grant number 0117U001666]. The authors are grateful to Dr A. Shishkevich for his help in the experiments.

Statements and Declarations

Funding

This work was supported by the National Academy of Sciences of Ukraine [grant number 0117U001666].


Nabil Benamar
email: benamar73@gmail.com

Maria Benamar
email: mariabenamar@gmail.com

Sara Ahnana
email: sara.ahnana@gmail.com

Fatima Zahrae Saiyari
email: saiyari.f.z@gmail.com

Moulay Driss

E L Ouadghiri

Jean Marie Bonnin
email: 2jm.bonnin@telecom-bretagne.eu

ARE VDTN ROUTING PROTOCOLS SUITABLE FOR DATA COLLECTION IN SMART CITIES: A PERFORMANCE ASSESSMENT

Keywords: Vehicular Delay-Tolerant Networks, Routing, Performance Evaluation, ONE

Vehicular Delay-Tolerant Networks (VDTNs) are composed of mobile nodes (vehicles) that communicate wirelessly to transfer data between nodes despite connectivity issues. It enables network connectivity in sparse or partitioned opportunistic networks, characterized by the low node density where the vehicular traffic is sparse and direct end-to-end paths between communicating parties do not always exist. Routing in such environments is challenging due to the absence, for nodes, of information about the state of the partitioned network, and because transfer opportunities between nodes are of limited duration. This paper focuses on the study of the performance of some well-known VDTN routing protocols in different scenarios to assess their suitability of use in the case of collecting sensor data in cities. In this paper we study the case of stationary nodes that represent urban sensor measuring different types of data, and transmitting collected information to stationary destination nodes, which are connected to the Internet. The transmission mechanism is performed through a set of mobile nodes in a VDTN context. We study the impact of different parameters on the routing protocols performances, using a large set of simulations and two scenarios. The results show that there is no perfect routing protocolthat is the best for all scenarios.

INTRODUCTION

One of the main issues of Smart Cities is the collection of data through the use of low cost sensors wirelessly. These sensors can be interconnected through different media, however the concept of "smart" leads essentially in the use of the infrastructure wisely and efficiently. As a consequence, the interconnection of the low cost sensors in Smart Cities should be solved using other networks than 3G, Wi-Fi, etc. Under this optic, Vehicular Networks could be a smart solution.

Vehicular Networks are a new class of wireless networks that are formed between moving vehicles equipped with wireless interfaces and can exchange traffic and road safety information with nearby cars and/or roadside units [1], [2], [3]. We often consider the introduction of car-to-car communication for the service they provide, with regard to safety and traffic management, thus the improvement of driving experience [START_REF] Sichitiu | Inter-vehicle communication systems: a survey[END_REF]. Nevertheless communicating cars and other vehicles (bus, tram, etc…) could also be considered a new class of wireless network and then offers a communication service for various kinds of applications. Due to the important issues that can be realized in such environment, vehicular networks have become popular research topic during the last years [START_REF] Lee | Survey of Routing Protocols in Vehicular Ad Hoc Networks[END_REF], [START_REF] Soares | Performance assessment of a geographic routing protocol for vehicular delay-tolerant networks[END_REF], [START_REF] Casaca | From Delay-Tolerant Networks to Vehicular Delay-Tolerant Networks[END_REF], [START_REF] Khabbaz | Disruption-Tolerant Networking: A Comprehensive Survey on Recent Developments and Persisting Challenges[END_REF].

Vehicular Ad Hoc Networks (VANETs) -a subclass of Mobile Ad Hoc Networks (MANETs)face highly variable density of traffic, which affects drastically the connectivity of such networks. In rush hours, Vehicular networks attain high delivery probabilities, and when the traffic quiets down, endto-end connections via intermediate nodes cannot be guaranteed any more. In such scenarios, researchers have proposed the use of Delay Tolerant Networks (DTN) [START_REF] Fall | A delay-tolerant network architecture for challenged internets[END_REF], based on the storecarry-and forward paradigm to solve the problem of intermittent and opportunistic connectivity, which have lead to Vehicular Delay Tolerant Networks (VDTNs) as shown in figure 1.

Figure 1: VDTN Scenario

One of the major concerns in such environment is the persistence/absence of connectivity between communicating parties. Due to nodes density and traffic variations, highly dynamic topology, short contact durations, limited transmission ranges, radio obstacles, and interferences, these networks are prone to intermittent connectivity, and significant loss rates [START_REF] Casaca | From Delay-Tolerant Networks to Vehicular Delay-Tolerant Networks[END_REF]. As a consequence, the use of conventional ad hoc routing protocols designed for connected networks become inadequate. In fact, Routing in vehicular networks presents a particularly challenging problem due to the unique characteristics of these networks [START_REF] Benamar | Routing protocols in vehicular delay-tolerant networks : a comprehensive survey[END_REF].

In this paper, our objective is the study of the performances of some well-known VDTN routing protocols in different scenarios. Through the present study, our concern is the evaluation of these protocols when the number of mobile nodes becomes small which leads to sparse and disconnected networks. This study focuses on the case of stationary nodes that represent urban sensor measuring and transmitting different types of data, to stationary destination nodes, which are connected to the Internet. Sensors are supposed to present no complexity in their operation mode. Thus, the transmission mechanism is performed through a set of mobile nodes in a VDTN context, and we suppose a one-way communication between sensors and gateways. We do not care about the gateway used in a given transmission: the first gateway reached in the best. We study the impact of different parameters on the routing protocols performances, using a large set of simulations and two scenarios. The results show that no protocol among those we have studied performs significantly better than others in all scenarios we used. This confirms the intuition that each protocol exhibits good performances in very specific situation it has been designed to operate in.

The remainder of this paper is organized as follows. Section 2 is an overview of DTNs and VDTNs focusing on routing protocols used in such networks. Section 3 describes the scenarios used in our present study and all related parameters. Section 4 provides the performance assessment part of the paper and is divided into two parties: one for network setting and the second one presents simulations results and a comprehensive discussion and interpretation of graphics. Finally Section 5 concludes the paper and suggests further research works.

DELAY TOLERANT NETWORKS OVERVIEW

DTNs, are a kind of networks which have been taking a huge attention and been the subject of many researches in the last years, because of their challenging applications [START_REF] Cerf | Delay Tolerant Network Architecture[END_REF], [START_REF] Scott | Bundle Protocol Specification[END_REF], [START_REF] Khabbaz | Disruption-Tolerant Networking: A Comprehensive Survey on Recent Developments and Persisting Challenges[END_REF]. They were designed for the first time for the interplanetary networking (IPN) [START_REF] Wood | A bundle of problems[END_REF], to establish communication between nodes on the different solar system planets after the failure of traditional protocols used in the internet communication for many reasons, then spread across several applications in many critical environments such as military networking, sparse sensor networks [START_REF] Khabbaz | Disruption-Tolerant Networking: A Comprehensive Survey on Recent Developments and Persisting Challenges[END_REF], vehicular networks, which is one of the growing field of DTNs.

DTN take part of the wireless ad hoc networks with new added features that make it suitable for these applications [START_REF] Scott | Delay-Tolerant Networking: An Approach to Interplanetary Internet[END_REF]. The transfer of data is based on a topology where it exist intermittent connection, high node mobility, and no end to end path between nodes this exchange can be done only if the nodes are in the same transmission range.

The architecture of DTN

The DTN architecture was the center of interest for researchers and network designers, during the last ten years. The DTN Research Group (DTNRG) has designed a complete architecture to support various protocols in DTNs [START_REF] Cerf | Delay Tolerant Network Architecture[END_REF]. DTN architecture was targeted to support the network connection disruption, and also to offer structure that deal with the heterogeneity of various regions by the addition of an overlay called bundle layer between transport and application layer [START_REF] Scott | Bundle Protocol Specification[END_REF]. In other words, the bundle layer consists of forming an overlay network to transfer the messages/bundles by using the hop-by-hop transfer. The buffer corresponding to nodes supposed to be large to allow the storage of bundles [START_REF] Wood | A bundle of problems[END_REF]. With all those capabilities, DTNs are considered strong enough to connect isolated heterogeneous regions together with a good interoperability between them, regardless of their technologies or region features.

Store-Carry-Forward routing paradigm

To deal with the new challenging situations, the traditional store-and-forward paradigm used in the Internet was not required anymore because of the lack of infrastructure in such networks, but it is store-carry-forward (SCF) routing which used [START_REF] Casaca | From Delay-Tolerant Networks to Vehicular Delay-Tolerant Networks[END_REF], [START_REF] Khabbaz | Disruption-Tolerant Networking: A Comprehensive Survey on Recent Developments and Persisting Challenges[END_REF], [START_REF] Scott | Delay-Tolerant Networking: An Approach to Interplanetary Internet[END_REF], [START_REF] Fall | DTN : An Architectural Retrospective[END_REF].

The idea behind SCF is simple, the message (bundle) will be stored in the buffer of an intermediate node when the next hop is not available, until it finds the opportunity to be forwarding to another host and so on. The process continues till the bundle reaches the destination or its time to live (TTL) expires and the message get dropped. It should be noted that the message could be not only stored but also replicated by multiple nodes before reaching its destination.

Routing protocols in DTNs and VDTNs

The vehicle delay tolerant network make opportunistic communications by utilizing the mobility of vehicles, the node makes delay tolerant based on the paradigm of "store-carry and forward" to deliver packets to the destination, which implies some degree of cooperation among nodes, as nodes route other node messages, or pick them in one place and deliver them in another. In order to overcome the lack of end-to-end paths, the protocols replicate messages, if necessary, in each contact.

Classifications of Routing Protocols In DTNs

Different classifications [START_REF] Mangrulkar | Routing protocol for Delay Tolerant Network: A survey and comparison[END_REF], [START_REF] Wisitpongphan | Routing in Sparse Vehicular Ad Hoc Wireless Networks[END_REF], [START_REF] Benamar | Routing protocols for DTN in vehicular environment[END_REF], [START_REF] Barros | Routing Protocol for Delay Tolerant Network: A Survey and Comparison[END_REF], [START_REF] Kurhinen | Delay tolerant routing in sparse vehicular ad hoc networks[END_REF]have been done by researchers for routing protocols in DTNs, there are many advantages and disadvantages to each approach, the use of the appropriate approach is probably dependent on the scenario at hand. Based on the methodology used to find destinations, and whether replication of messages is used or not, routing in DTN can be classified according to several categories:

Flooding or Forwarding

Flooding strategy: In flooding strategy, messages are replicated to enough nodes so that destination nodes must receive it so it increases the probability of message delivery to the destination but Flooding based approach increases the contention for network resources like bandwidth and storage, and thus can not cope with network congestions and does not scale well [START_REF] Ali | Routing protocols in Delay Tolerant Networks -a survey[END_REF].

Forwarding-based: In this approaches there are much less wasteful of network resources, as only a single copy of a message is stored in the network at any given time. Knowledge about network is used to select the best path to the destination (Fig 2). This category is also known as knowledge-based [START_REF] Lebrun | Knowledge-Based Opportunistic Forwarding in Vehicular Wireless Ad Hoc Networks[END_REF].

Single copy or multiple copies

Single-copy category maintains a single copy of a bundle in the network that is forwarded between network nodes. Multiple-copy category replicates bundles at contact opportunities.

Deterministic or stochastic DTN routing

Deterministic routing is characterized by the knowledge of the current topology and future changes can be predicted. Stochastic DTN routing is generally used when node movement is random or unknown and nodes know very little or nothing about the future evolution of the topology.

VDTN routing Protocols

Prophet (Probabilistic Routing Protocol using History of Encounters and Transitivity) uses a probabilistic metric: delivery predictability y, that attempts to estimate, based on node encounter history, which node has the higher probability of successful delivery of a message to the final destination [START_REF] Lindgren | Probabilistic Routing Protocol for Intermittently Connected Networks draftirtf-dtnrg-prophet-09[END_REF]. When two nodes are in communication range, a new message copy is transferred only if the other node has a better probability of delivering it to the destination.

Direct Delivery are single copy DTN routing protocols where only one copy of each message exists in the network [START_REF] Spyropoulos | Single-copy routing in intermittently connected mobile networks[END_REF]. In Direct Delivery, the message is kept in the source and delivered only to the final destination, if the nodes meet. In First Contact, the message is forwarded to the first node encountered and deleted. The message is forwarded until it reaches the intended destination [25]. Epidemic Routing protocol [26]is flooding-based protocol, where nodes continuously replicate and transmit messages to newly discovered contacts that do not already possess a copy of the message. Consequently, epidemic routing protocol minimizes the delivery delay and maximizes the delivery ratio as messages may reach the destination on multiple paths, but spoils storage and bandwidth in comparison with other protocols [START_REF] Benamar | Routing protocols in vehicular delay-tolerant networks : a comprehensive survey[END_REF]. Spray and Wait [27]is an n -copy routing protocol with two phases: (1) spray phase, where a message created by the source node is initially spread by the source to encountered nodes until the n copies are exhausted ; (2) wait phase, where every node containing a copy of the message performs a direct delivery to the destination. There are two variants of the protocol: normal mode, where a node gives one copy of the message to each node it discovers that does not have the message; and binary mode, where half of the n copies are given in each encounter. MaxProp protocol attempts to transfer all messages not held by the other node, when it is in communication range [28]. The protocol uses acknowledgments to clear the remaining copies of a message in the network when the destination node receives it. When nodes discover each other, MaxProp exchanges messages in a specific priority order, taking into account message hop counts and the delivery likelihood to a destination based on previous encounters. New packets are assigned higher priority, and the protocol attempts to avoid reception of duplicate packets. RAPID (Resource Allocation Protocol for Intentional DTN), routing packets are opportunistically replicated until a copy reaches the destination node [29]. The protocol models DTN routing as a utility -driven resource allocation problem. The routing metric is a per-packet utility function. When nodes are in communication range, RAPID replicates the packet that results locally in the highest increase in utility. The corresponding utility Ui of packet I, is defined as the expected contribution of i to the given utility routing metric. A comprehensive surveys related to routing protocols in VDTN can be found in [START_REF] Casaca | From Delay-Tolerant Networks to Vehicular Delay-Tolerant Networks[END_REF], [START_REF] Khabbaz | Disruption-Tolerant Networking: A Comprehensive Survey on Recent Developments and Persisting Challenges[END_REF], [START_REF] Benamar | Routing protocols in vehicular delay-tolerant networks : a comprehensive survey[END_REF]. Table 1, summarizes some specific characteristics of these protocols. unlimited-copy hight attempt to limit replication

PERFORMANCE ASSESSMENT

This section presents a study with the performance comparison of some well known routing protocols in the context of VDTN: MAXPROP, SPRAY and WAIT, EPIDEMIC and PROPHET. Also, some routing protocols have not been included in our simulations like First Contact (FC) and Direct Delivery (DD). In fact, DD protocol cannot be used in our case since the source and destination node will never meet. They are stationary nodes. FC protocol has been discarded from the current study due to its poor performances in regards to delivery probability and Latency as it has been studied in [30].

Furthermore, Six performance metrics are considered. The number of initiated bundle transmissions is defined as the number of started transmissions between nodes. The number of dropped bundles is defined as the number of bundles that have been discarded from the nodes' buffers due to overflow or TTL expiration. The delivery probability is measured as the relation of the number of unique delivered bundles to the number of bundles created. It tells the percentage of successfully received bundles among all sent. The average delivery delay is defined as the average time between bundles generation and bundles delivery. The overhead ratio measures how many transfers are needed for each bundle delivery. Finally, the average hop count is defined as the average number of hops counts between the source and the destination node of bundles.

The aim of the present simulations is to show different key performances of used routing protocols. The simulation study was performed using the Opportunistic Network Environment (ONE) simulator [31], which is one of the major simulation tools used to validate DTN routing protocols [32], [33] . It is a JAVA based simulator, which uses the Helsinki city map and allows node movement modeling, inter-node opportunistic contact using different interface types. The ONE offers a framework for implementing routing protocols in DTN environment, and permits graphical visualization of mobile nodes. Furthermore, we have observed some memory limitations of ONE when the number of nodes exceeds 300. However, the choice of using the ONE simulator still one of the best provided the number of nodes still below this limit, which is generally the case in DTN context.

NETWORK SETTINGS

The network scenario is based on a part of the city of Helsinki (Finland) presented in Fig. 1 the total number of all nodes in all simulations has been kept fix and equal to 100. Ten stationary destination nodes and ten stationary source nodes are placed at the map positions presented in this figure. Fig. 3 shows the localization of all stationary nodes before running the simulation. In Fig. 4, we can see all nodes of the present simulations. Source nodes represent sensors used in urban area to collect different types of information and measures, whereas destinations nodes are gateways connected to Internet. The use of VDTN networks in such a context can be a promising low-cost solution for urban sensing and information/entertainment applications.

Figure 3: The Position Of Sources And Destinations Nodes In The Helsinki City Map Figure 4: The Position Of All Nodes In The Network

All nodes in the network have a 5M in their buffer size. During the simulated 12 hours period of time, the group of mobile nodes (vehicles) moves along the map roads with a speed ranging from 10 km/h to 50 km/h, between random locations. Simulations were done using two mobility models, namely: the shortest path map based movement model, and the map based movement model. Mobility models define the algorithms and rules that generate the vehicle movement paths. Three types of synthetic movement models are included in the ONE simulator: 1) random movement, 2) mapconstrained random movement, and 3) human behavior based movement. Since we did not find significant differences between simulation results in the two different mobility models, we just present results related to the shortest path map based movement model. Thus, even if the Random Way Point mobility model exhibits slightly different results for all simulations in comparison to the two other models, it is not relevant for our use case. The number of mobile nodes in the first scenario is 80 representing a dense network while in the second scenario this number is 20. The idea is to assess different performances of VDTN protocols in cases where usually the vehicle density is low (rural area) and a city where the traffic is dense.

Data bundles are originated at specific stationary source nodes and are destined to specific stationary terminal nodes. No random transmissions have been used in all simulations. Furthermore, we have used external event generator to create 1000 messages with sizes varying from 100KB to 1MB. Those messages are created in an interval time between the start of the simulation and 900s. Data bundles TTL changes between 50, 100, 150, 200, 300, 400, and 500 minutes, across the simulations. Increasing the TTL will lead to contention for network resources. All network nodes use a Bluetooth link connection with a transmission data rate of 2 Mbps and an Omni-directional transmission range of 10 meters. The configuration of PRoPHET protocol parameters is set according to the default values proposed in default setting and the number of copies parameter (L) of Spray and Wait is equal to 15% of the total nodes. The different parameters are grouped in the table below. We ran thirty separated simulations using differentseeds for each protocol in each scenario, and the results were averaged. Simulations in ONE, run much faster than in real-time. It should be noted thatsome simulations take much time than others depending on the number of nodes used, and also on the protocol being simulated. Simulation speeds are ranging from 3 to 35 min per simulation, and these values can greatly change depending on physical resources of the computer used. Fig. 5 shows the different paths of the bundles sent from source node S8 to destination node D96, using Maxprop protocol.

RESULTS ANALYSIS

As it should be expected, the number of mobile nodes in the simulation area has a direct effect on all measurable metrics. Figure 6-a shows the performances of the four studied protocols in terms of delivery probability vs. the time to live (TTL). Increasing TTL values leads to hold bundles in buffers for longer time while waiting for the opportunity to forward or deliver them.

Maxprop protocol shows the best delivery probabilityresults when the number of nodes is high, followed by Spray and Wait. However, in terms of Latency, fig. 6-b shows that Maxprop has the worst performances comparing to the other protocols, for both scenarios, and Prophet protocol comes first exhibiting the lowest Latency average overall.

Maxprop can play an important role in our case study, since it shows the best results in terms of delivery probability and addresses scenarios in which either transfer duration or buffer sizes are a limited resource in the network [28]. However, one of the main characteristics of this protocol is the use of acknowledgements sent from the destination to the source and network-wide. This component of the protocol is not needed in our scenarios since we are mainly interested by a low-coast sensors (static nodes) and a one-way communication: sensor to Internet gateway.

In the other hand, Prophet presents a great overhead ratio comparing to the rest of protocols, as we can see in Fig. 6-c, followed by epidemic, as both protocols make multiple copies of a given bundle to deliver it to the final destination. The same message generation period for all sources requires higher message traffic and thus a larger number of transmissions. Consequently, the overhead ratio increases when the number of sources increases. Spray and Wait exhibits the lowest magnitude in term of overhead ratio, because of its direct transmission mechanism. This protocol limits the number of bundle copies created per bundle in order to control flooding. In addition, the hop count is a factor, which contributes to the overall end-to-end delay. It is an important metric that allows us to get a better understanding of how a routing protocol should be designed so that it can deliver acceptable performances. Fig 6-d shows the performance of the studied routing protocols regarding hop count metric. Epidemic protocol exhibits the highest values for the dense scenario. Indeed, as the network becomes dense, the hop count values increase. This is due to the nature of this protocol consisting on duplicating bundles to reach the final destination as fast as possible.

Furthermore, as expected, Epidemic protocol shows the maximum started messages since each packet will be duplicated in each contact opportunity. Fig. 6-e shows that the number of mobile nodes in the simulation area has a clear effect on the number of initiated bundles transmissions due to the increased number of contact opportunities. The number of dropped and started messages is high in the case of Epidemic protocol (Fig. 6-f), which is a direct consequence of duplication mechanism of this protocol and the limited size of the buffer. If we increase the buffer capacity the number of dropped bundles will be significantly reduced. However, the buffer capacity should be small, at least moderated in our case, to remain in the low-cost sensors category.

CONCLUSION

This paper evaluated the performances of different VDTN routing protocols using two scenarios: sparse and dense network. Our main goal is to decide if one of this routing protocols could be suitable for data collection in smart cities. The ONE simulator was used for this study, which is a dedicated tool for DTN and opportunistic networks. The results observed show that there is no protocol adequate for all cases and contexts. Our simulations show that each protocol can exhibit good performances in some metrics but shows a different behavior regarding the other metrics, as studied protocols perform better or worse on different scenarios. Moreover, all the protocols we studied are probably too complex to be implemented in sensor and in all vehicles for the very simple usage we envision. Maxprop shows the best delivery probability results when the number of nodes is high, and the worst latency values for sparse and dense traffic, while Prophet protocol shows best values in term of latency. Maxprop can play an important role in our case study, however, due to its use of acknowledgements sent from the destination to the source and networkwide, this protocol cannot be a good candidate for our purpose since our focus is toward low-cost sensors and a one-way communication: sensor to Internet gateway. Also, Prophet protocol presents a great overhead ratio followed by epidemic, as both protocols use multiple copies of bundles, while Spray and Wait exhibits the lowest values in term of overhead ratio. This suggests the need for further research in the area, to develop a simpler routing protocol that can present good performances in terms of the different metrics used in this study, which can lead to its standardization in intelligent transport systems.

REFERENCE:

[

Figure 2 :

 2 Figure 2 : Illustration Of Flooding Based And Forwarding Based Strategies

Figure 5 :

 5 Figure 5: Example Of Message Paths From Node S8 To D96

Figure 6 :

 6 Figure 6: Bundle Delivery Probability (A), Latency (B), Overhead_Ratio (C), Hopcount_Avg (D), Number Of Dropped Bundles (E) And Number Of Started Bundles (F) As Function Of Bundles TTL In A Scenario With 20 And 100 Nodes Using Map Based Movement For Epidemic, Spray And Wait, Prophet And Maxprop Routing Protocols

Figure 7 :

 7 Figure 7 : Summary of the performances of the four VDTN routing protocols.

Table 1 :

 1 DTN and VDTN Routing Protocols And Their Characteristics.

	Routing Protocols	single/multiple or n	Replication Rate	Functions, Objectives
		Copy
	Epidemic	unlimited-copy	Very High	Rapid propagation of data
	Direct Delivery	single-copy	none	Source moves and delivers the bundle
				directly
	Prophet	unlimited-copy	Medium	Probabilistic
	SprayAndWait	n-copy	Medium	Sets a limit on the number of copies
	(SnW)
	MaxProp	unlimited-copy	High	use of the delivery likelihood as a cost
				assigned to each destination
	Resource
	Allocation Protocol
	for Intentional DTN
	(Rapid)

Table 2 :

 2 Summarization Of The Different Parameter Used In The Scenario

	Simulation time	43200 seconds
	Buffer size	5 M
		Shortest Path Map Based
	Movement Model	Movement; Map Based
		Movement
	TTL	[50;100;150;200;300;400;500]
	Routing protocol	[MaxProp; Epidemic; Prophet; Spray And Wait]
	Interface type	Simple Broadcast Interface
		100 nodes (10 sources, 10
	Number of nodes	destinations, 80 and 20 mobile nodes (respectively for the
		first and second scenario)
	Velocity of mobile nodes	2.7, 13.9
	Size of bundles	100KB, 1MB
	Event used	External Event Queue
	Size of map	4500m, 3400m
	Transmission range	10 meters
	Transmission speed	2 Mbps
	Number of copies (L)	15, the binary mode = true