An investigation of the crash performance of

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An investigation of the crash performance of magnesium, aluminum and advanced high strength steels and different cross-sections for vehicle thin-walled energy absorbers

Keywords Crash analysis, lightweight vehicle design, lightweight alloys, new generation steels, crashworthiness

In this paper, the effect of conventional steel, new generation DP-TRIP steels, AA7108 – AA7003 aluminum alloys, AM60 – AZ31 magnesium alloys and crash-box cross-sections on crash performance of thin-walled energy absorbers are investigated numerically for the lightweight design of vehicle structures. According to finite element analysis results, crash performance parameters such as total energy absorption, specific energy absorption, reaction forces and crush force efficiencies are compared for the above-mentioned materials. The energy absorption capability of steel energy absorbers is better than that of aluminum and magnesium absorbers. On the other hand, the energy absorption capacity per unit mass of energy absorbers made from lightweight materials is higher than that of steel energy absorbers. This advantage of lightweight alloys encourages automobile manufacturers to use them in designing structural vehicle components.

The development of new structural components is a challenging issue due to new regulations on safety and emissions in the automotive industry. The main aim of automotive engineers is to develop advanced safety systems that satisfy international crash test regulations such as ECE, NHTSA standards [1-3]. While adding new safety systems to a vehicle, designers also need to reduce fuel consumption. Replacing traditional materials with lighter alternatives in vehicle design is one of the best solutions for improving fuel economy and reducing greenhouse gas emissions [4-13]. A 10 % weight reduction of a vehicle can result in 4-8 % improvement in fuel effi-

ciency [14]. Expressed differently, a 100 kg weight reduction represents a fuel savings and CO2 reduction of about 3.5-8.5 g CO2 × km-1 for a passenger car [15]. Nowadays, there is a great deal of interest in new generation materials such as advanced high strength steel and low density aluminum-magnesium alloys to reduce vehicle weight. The body (body-in-white) and chassis components constitute nearly one-quarter of the overall weight of a vehicle. Advanced materials can be used in different vehicle components to lighten the chassis and body structure. Advanced high-strength steels (AHSS) such as dualphase (DP) and transformation-induced

Emre Demirci and Ali Rıza Yıldız, Bursa, Turkey

Article Information Correspondence Address Prof. Dr. Ali Rıza Yildiz Department of Automotive Engineering Uludağ University Görükle, Bursa, Turkey E-mail: [email protected]

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plasticity (TRIP) steels provide extremely high strength compared to conventional steel. In addition, AHSS can provide the same level of safety provided by conventional steel with less thickness [3]. The World Steel Association’s automotive group World Auto Steel has been working on a project called Future Steel Vehicle, which uses AHSS for automotive steel applications and reduces body structure weight by 39 % [16]. Aluminum and magnesium alloys, by contrast, are becoming popular in the automotive industry for light vehicle design due to their low density [3, 16-18]. The densities of aluminum and magnesium alloys are 65 % and 77 %

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lower than that of steel, respectively. However, the elastic moduli of these alloys are also lower than that of steel, which means that to achieve equal bending stiffness, aluminum or magnesium components must be thicker than a steel component. Many studies have been carried out on thin-walled tubes and these studies have been published in the literature. It has been observed that the crash performance of thin-walled tubes changes with the cross-sectional geometries of the tubes. Witteman [18] studied square, rectangular, circular, hexagonal and octagonal crosssection steel energy absorbers, all under the same impact conditions. It was observed that the energy absorbers with polygonal and circular cross-sections absorbed more energy than square and rectangular cross-section absorbers. In another study, Guler et al. [20] found that the crush force efficiency of a circular energy absorber is higher than that of other energy absorbers. Jensen et al. [21] investigated the deformation modes of thin-walled tubes and

their effect on energy absorption. They found that the global buckling of a tube causes a reduction in energy absorption. Conical tube design is one of the methods to avoid global buckling. Nagel and Thambiratnam [22, 23] reported that the maximum crush force in conical energy absorbers is lower and the mean crush force higher due to progressive axial collapse, compared with straight energy absorbers. In the literature, there are some studies on the application of aluminum and magnesium alloys in vehicle design. Zhou et al. [24] replaced the steel vehicle hood with an optimized aluminum hood and reduced the weight of the hood by 46.4 %. In another study, Kohar et al. [25] studied high crush efficient aluminum vehicle rails to lighten a vehicle. Logan et al. [26] used magnesium in an automobile body structure resulting in a weight reduction of more than 40 % compared with conventional steel structures, while improving structural performance. In this study, various designs of thinwalled tubes are investigated for light-

Figure 1: Energy absorbers on vehicle body [27]

Figure 2: Straight tube geometries, a) S1: square, b) S2: rectangular, c) S3: circular, d) S4: polygonal

Figure 3: 5° tapered tube geometries, a) T1: square, b) T2: rectangular, c) T3: circular, d) T4: polygonal

weight vehicle design. The crash performances of the thin-walled tubes made of different materials such as conventional SPC 440 steels, advanced high strength DP – TRIP steel, Al 7108 – Al 7003 aluminum and AM60 – AZ31 magnesium low density alloys are investigated numerically. Crashworthiness of different materials have been investigated separately in the literature. In this study, conventional steel, AHSS, low density aluminum and magnesium alloys are examined and compared under the same conditions. In this way, the use of advanced materials as energy absorbers is investigated.

Energy absorbers and crashworthiness parameters In the automotive industry, vehicle safety systems have been designed to protect occupants during an accident and can be considered in two classes: active safety systems and passive safety systems. The active safety systems intervene before an accident occurs while the passive safety systems are designed to protect occupants during a crash. One of the passive safety systems are the energy absorbers in the vehicle chassis. Figure 1 shows the location of the energy absorbers on a vehicle [27]. The main purpose of using energy absorbers is to control the impact energy of a vehicle during impact and to prevent the transfer of high forces from bumper to main body. The crashworthiness of energy absorbers has been proven in various research works in the literature. Most studies on energy absorbers focus on thinwalled tube structures [28-32]. In this study, square, rectangular, circular and polygonal cross-section thin-walled tube geometries are designed as straight and tapered in CAD software. The geometric forms and dimensions of the tubes can be seen in Figures 2 and 3. All the straight tubes have the same perimeter as s = 200 mm. Similarly, the tapered tubes are designed with the same perimeter as s = 200 mm at the front of the tube. A constant tapered angle 5°of has been chosen for the tapered tubes. All the straight and tapered tube models have the same length of the undeformed tube L = 180 mm and thickness t = 1.6 mm. To evaluate the crashworthiness of the energy absorber structures several criteria have been defined in literature. The crashworthiness criteria used in this study are total energy absorption (EA), peak crush force (PCF), mean crush force (MCF), spe-

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cific energy absorption (SEA). More explanations about crashworthiness indicators can be found in [28, 33-35]

Materials

Young’s modulus (MPa)

Density (kg × m-3)

Poisson’s ratio

Yield strength (MPa)

C (s-1)

P

SPC440

206000

7850

0.3

318

6844

3.91

Finite element models and crash analysis

DP600

206000

7850

0.3

379

1587

2.33

TRIP780

206000

7850

0.3

503

11410

3.28

AA7108 – T6

71000

2770

0.33

340

-

-

AA7003 – T1

71000

2770

0.33

202

-

-

AM60

45000

1770

0.35

138

3300

1.1

AZ31

45000

1770

0.35

165

24124

3.09

The energy absorber geometries designed are imported into pre-process software and required definitions for finite element model such as mesh type, boundary conditions and loads are defined. The finite element model is analyzed by solver software and the results evaluated by post-process software. In this study, Altair HypeMesh, Ls-Dyna and Ls-Prepost are used as preprocessor, solver and post-processor, respectively. Materials. The crashworthiness of the thin-walled tubes designed using new generation metals are compared with conventional steel tubes for lightweight vehicle design. For this purpose, SPC440 steel, DP600-TRIP780 steels, AA7108-AA7003 aluminum alloys and AM60-AZ31 magnesium alloy are investigated as conventional steel, advanced high strength steels and low density alloys, respectively. The mechanical properties of the materials have been obtained from the literature [16, 36-39] and given in Table 1. DP600 is a dual phase steel consisting of a TRIP steel with a multi-phase microstructure consisting of a primary ferrite matrix and hard phases such as retained austenite, martensite and bainite, a soft ferritic matrix as the primary phase and discrete hard martensitic islands as a second phase. The combination of these two phases yields an excellent strength-ductility balance, so DP steel is widely used in the automotive industry. TRIP steel has a multi-phase microstructure consisting of a primary ferrite matrix and hard phases such as retained austenite, martensite and bainite [3]. TRIP steel has begun to be used in the automotive industry in recent years due to its high energy absorption capacity and excellent durability. The mechanical properties of 7xxx aluminum alloys depend on chemical composition and microstructure [40]. In particular, the microstructure obtained during the thermomechanical process directly affects the strength and durability of the alloys. In this study, AA7108 temper T6 and AA7003 temper T1 aluminum alloys were chosen because of their good mechanical properties and applicability to the automotive industry. In recent years, automotive manufacturers have been interested

60 (2018) 7-8

Table 1: Mechanical properties of materials used

in magnesium alloys because of their high strength-to-weight ratio. However, the sheet formability of magnesium alloys is still limited due to some difficulties. AM60 magnesium alloy has been selected in this study because it has higher mechanical strength than most magnesium alloys. In addition, AZ31 magnesium alloy has been investigated because it manifests good stress-strain behavior. The magnesium alloys and steels used in this study are sensitive to strain rate. The strain rate effects are considered with the Cowper-Symonds model as shown in Equation (1)

σ dy

1⎤ ⎡ ⎢ ⎛ ε⋅ ⎞ P ⎥ = σ y ⎢1+ ⎜⎜ ⎟⎟ ⎥ε⋅ > 0 ⎢ ⎝C⎠ ⎥ ⎣ ⎦

(1)

with ε⋅: strain rate, σ dy: dynamic yield strength, σ y : static yield strength, C and P: Cowper – Symonds’ coefficients. The C and P parameters for the materials used in this study have been selected from the literature [41-44] and are given in Table 1. On the other hand, AA7108 and AA7003 aluminum alloys manifest strainrate-insensitive behavior [36]. Some studies show that aluminum alloys have very high C parameter values. This shows that

Figure 4: True stress – strain curve of steel and aluminum materials

Figure 5: Stress – strain curves of magnesium alloys under tension and compression

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Materials Model

S1

SPC440


S3

T1

8.33

10.95

10.10

5.26

4.49

3.87

3.21

160.72

233.52

229.31

123.32

83.211

77.20

47.58

MCF (kN)

66.64

87.60

80.80

42.08

35.92

30.96

25.68

18.42

24.22

22.34

32.96

28.13

37.94

31.47

EA (kJ)

6.90

9.34

9.44

4.22

3.53

3.69

2.68

PCF (kN)

154.53

232.65

222.58

123.94

83.71

76.33

46.93

T3

T4

kg-1)

MCF (kN)

55.20

74.72

75.52

33.76

28.24

29.52

21.44

SEA (kJ × kg-1)

15.26

20.65

20.88

26.44

22.12

36.18

26.28

EA (kJ)

9.49

13.72

13.71

5.71

4.87

5.14

3.78

PCF (kN)

157.32

248.35

230.63

120.27

80.129

82.55

50.21

MCF (kN)

75.92

109.76

109.68

45.68

38.96

41.12

30.24

20.99

30.35

30.33

35.79

30.53

50.39

37.06

EA (kJ)

10.13

14.51

14.28

6.01

5.27

5.42

4.01

kg-1)

PCF (kN)

158.29

243.90

228.92

127.22

83.52

81.41

50.79

MCF (kN)

81.04

116.08

114.24

48.08

42.16

43.36

32.08

SEA (kJ × kg-1)

22.40

32.09

31.58

37.66

33.02

53.14

39.31

EA (kJ)

8.59

11.67

12.11

5.44

4.69

3.94

3.36

PCF (kN)

159.49

228.67

233.80

132.57

83.22

75.37

49.76

MCF (kN)

68.72

93.36

96.88

43.52

37.52

31.76

26.88

14.39

19.55

20.29

25.83

22.27

29.27

24.96

EA (kJ)

6.67

9.69

9.88

4.27

3.60

3.77

2.87

SEA (kJ ×

T2

AA7108 AA7003 AM60 AZ31

EA (kJ)

SEA (kJ ×

S4

TRIP780

PCF (kN) SEA (kJ ×

S2

DP600

kg-1)

PCF (kN)

163.14

234.41

233.34

133.51

83.90

77.52

50.44

MCF (kN)

53.36

77.52

79.04

34.16

28.80

30.16

22.96

SEA (kJ × kg-1)

11.17

16.23

16.55

20.28

17.09

28.01

22.96

EA (kJ)

9.95

14.40

13.83

6.17

4.87

4.85

3.76

PCF (kN)

150.98

212.19

214.10

116.79

78.09

71.46

49.30

MCF (kN)

79.60

115.20

110.64

49.36

38.96

38.80

30.08

SEA (kJ × kg-1)

17.58

25.44

24.43

30.89

24.39

38.00

29.47

EA (kJ)

10.04

14.45

14.09

6.65

5.36

5.47

4.38

PCF (kN)

187.44

249.79

238.58

129.47

88.88

73.15

51.74

MCF (kN)

80.32

115.6

112.72

53.20

42.88

43.76

35.04

17.56

25.27

24.64

32.95

26.56

42.44

33.98

SEA (kJ ×

kg-1)

Table 2: Crash analysis results for all energy absorber models (EA: energy absorption, PCF: peak Crush force, MCF: mean crush force, SEA: specific energy absorption)

Figure 6: Finite element model of energy absorber T4

the strain sensitivity of aluminum can be neglected [45]. The plastic deformation behavior of the steel and aluminum materials can be defined by true stress-true strain curves. The effective true stress-true strain curves of the steel and aluminum materials used in the finite element model are shown in Figure 4. The crash analysis of tubes under dynamic load depends on many nonlinear conditions such as large deformations, strain hardening and strain rate effects. Nonlinear material behavior of steel and aluminum tubes is defined by a Type 24 – piecewise linear isotropic plasticity material model in Ls-Dyna [46]. The material behavior in the elastic – plastic region is defined by the effective stress – strain curve in a Type 24 material model. Also, the strain rate effects can be included in the material model using Cowper-Symonds C-P parameters. Magnesium alloys exhibit a different stress-strain behavior under tension and compression tests because of anisotropic plastic deformation. A type 124 – modified piecewise linear plasticity material model [46] has been selected to define both the tension and compression behavior of magnesium alloys. Stress – strain curves of AM60 and AZ31 magnesium alloys, defined in the material model, are shown in Figure 5. The anisotropic behavior of magnesium alloys is not included in the material model because it is negligible at high strain rates [17]. The strain rate sensitivity of the magnesium alloys is defined using Cowper-Symonds parameters in the Type 124 material model. Finite element models. The CAD data of the energy absorber models are imported into the HyperMesh Ls-Dyna interface to create the finite element models. The finite element mesh of energy absorbers are carried out by the quadrilateral shell elements by means of five integration points throughout the thickness. The shell element size is selected as 3 × 3 mm to ensure the accuracy of the results. The BelytschkoLin-Tsay element formulation, the default formulation of Ls-Dyna, is used for the shell elements. To simulate a crash of the energy absorbers, the straight tubes are fixed at one end in all directions and axially impacted by a movable rigid wall at the other end. Tapered tubes are fixed at the wider end. The rigid wall is modeled as having a mass of 200 kg and an initial velocity of 13.89 m × s-1. The finite element model of T4 with the boundary conditions is shown

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in Figure 6. The contact between the energy absorbers and the rigid wall is defined by an “Automatic Single Surface” card in Ls-Dyna with a static friction coefficient of 0.3 and a dynamic friction coefficient of 0.2 [47].


Results and discussions The dynamic axial crash analyses of all models are simulated using the Ls-Dyna explicit solver. Crash simulation is carried out by considering the impact of the rigid wall. The total energy absorption, the peak crush force and the mean crush force values are calculated for a 125 mm deformation of the tubes. Moreover, the specific energy absorptions of all models are calculated for all materials. The results and comparison of the models are shown in Table 2. The results indicate that the total energy absorption and the specific energy absorption of circular and polygonal tubes are better than those of square and rectangular tubes for both straight and tapered models independently of the material used. However, the peak crush force values of circular and polygonal tubes are higher than those of square and rectangular tubes for all models. When the straight and tapered tubes are compared, it can be seen that the specific energy absorption of the tapered tubes are lower than that of the straight tubes. At the same time, the peak crush force values of the tapered tubes are lower than that of the straight tubes for all materials used. In this case, the crush force efficiency of all models can be calculated to find a more efficient energy absorber. The crush force efficiencies of the straight and tapered tubes for all used materials are shown in Figure 7. The figure demonstrates that the tapered tubes are more efficient than the straight tubes. As can be seen in Figure 7, the AZ31 magnesium alloy tapered – polygonal energy absorber (T4) has a maximum crash efficiency value of 0.677. An absorbed energy – displacement graph and a reaction force – displacement graph are shown in Figure 8 for selected energy absorber model T4. As shown in the force – displacement graph, the T4 tubes are subjected to progressive axial collapse, which causes high crush efficiency. In particular, the magnesium alloys show a less wavy curve characteristic behavior with respect to the force – displacement graph, showing that magnesium alloys are more efficient than other materials.

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Examining the effect of materials to the crashworthiness of all energy absorbers is the main purpose of this study. The total energy absorption and the peak crush force results can be misleading when comparing steel, aluminum and magnesium alloys due to the thickness of the tubes. Although all models have the same thickness of t = 1.6 mm, aluminum and magnesium alloys are lighter than steel. In order to compare all materials correctly, the specific energy absorption results are discussed. The specific energy absorption of the straight and tapered tubes for all used materials is shown in Figure 9. As seen in Figure 9, the conventional steel energy absorbers have the lowest specific energy absorption of all models. Advanced high strength steel DP600 and

TRIP780 are better than conventional steel but not to the same extent as low density alloys. The results show that aluminum and magnesium alloys manifest better energy absorption than steel with respect to weight. The AM60 magnesium alloy straight – hexagonal (S4) energy absorber manifests the best specific energy absorption of 49,76 kj × kg-1. The absorbed energy – displacement graph and the reaction force – displacement graph of the selected energy absorber model S4 are shown in Figure 10. The reaction force – displacement graphs also show the deformation characteristics of tubes for each material. Figure 10 shows that all models with a hexagonal tapered energy absorber have the same deformation modes, especially at the initial 50 mm.

Figure 7: Comparison of the crush force efficiency for all models

Figure 8: Absorbed energy – displacement and reaction force – displacement graph of model T4 for all materials

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As the strength of the material increases, the reaction force increases but the curve characteristic remains similar.


Conclusions In this study, the crashworthiness of straight and tapered thin-walled tubes out of different materials was investigated numerically. According to the finite element analysis results, crash performance parameters such as total energy absorption, specific energy absorption, reaction forces and crush force efficiency are compared for steel, aluminum and magnesium. According to the results, the total energy absorption and the specific energy absorption of the polygonal and circular energy

Figure 9: Comparison of the specific energy absorption for all models

absorbers are better than that of square and rectangular energy absorbers. This means that for all materials, the crashworthiness of the polygonal and circular energy absorbers is better. On the other hand, the tapered tubes are more efficient than the straight tubes due to the absorption of higher energy per unit mass. As a result of this comparison of materials, we conclude that advanced high strength steel and low density alloys can be used in vehicle bodies instead of conventional steel. The DP600 and TRIP780 energy absorbers manifest a higher specific energy absorption than that of a conventional steel energy absorber. Therefore, the energy absorbers can be designed thinner when using high strength steel. This

advantage will help automotive engineers to lighten vehicle safety components. However, high peak force values must still be reduced by triggering mechanisms for high crush efficiency. Although the energy absorption capability of the steel energy absorbers is better than that of aluminum and magnesium absorbers, energy absorbers made from low density materials can absorb more energy per unit mass than those made using conventional and advanced high strength steels. In this study the AA7108-T6 energy absorber shows maximum specific energy absorption. This advantage of aluminum and magnesium alloys has been encouraging automotive engineers to use them for designing lightweight structural vehicle components while reducing fuel consumption and emissions. If the difficulties and costs in the production methods of aluminum and magnesium are reduced, these materials will offer a good alternative for steel to design lightweight structural components.

Acknowledgement The authors gratefully acknowledge the financial assistance of the Turkish Academy of Sciences, Turkey (A. R. Y./TUBA- GEBIP/2015), Young Scientist Award program.

References

Figure 10: Absorbed energy – displacement and reaction force – displacement graph of model S4 for all materials

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DOI 10.3139/120.111201 Materials Testing 60 (2018) 7-8, pages 661-668 © Carl Hanser Verlag GmbH & Co. KG ISSN 0025-5300

The authors of this contribution Dr. Ali Rıza Yıldız is a Professor at the Department of Automotive Engineering, Uludağ University, Bursa, Turkey. He worked in the field of multi-component topology optimization of structures as Research Associate at the University of Michigan, Ann Arbor, USA. Furthermore, he worked on a NSF and DOE funded research projects at the Center for Advanced Vehicular Systems (CAVS), Mississippi State University in Starkville, USA. In 2015, he was a winner of TÜBA-GEBİP Young Scientist Outstanding Achievement Award given by the Turkish Academy of Sciences (TÜBA). He also received the METU (Middle East Technical University) Prof. Mustafa N. Parlar Foundation Research Incentive Award in 2015. In 2017, The TUBITAK Incentive Award, given to scientists who under the age of 40 who have proved to have the necessary qualifications to contribute to science in the future at an international level, was given to him. His research interests are the finite element analysis of automobile components, lightweight design, composite materials, vehicle design, vehicle crashworthiness, shape and topology optimization of vehicle components, meta-heuristic optimization

Abstract Eine vergleichende Untersuchung zur Kollisionssicherheit von dünnwandigen Rohren aus Stahl, Aluminium und Magnesium für den Fahrzeugleichtbau. In den letzten Jahren ist die Gewichtsreduktion von Fahrzeugen eine der bedeutendsten Eigenschaften in Hinblick auf die Kraftstoffeffizienz und niedrige Emissionen in der Automobilindustrie. Daher versuchen Automobilingenieure neue Sicherheitssysteme für beides, eine Leichtbaustruktur und zur Erfüllung neuer internationaler Crashversuchsstandards, wie zum Beispiel ECE und NHTSA, zu entwickeln. Dünnwandige Rohre werden als Crashboxen oder Energieabsorber in Fahrzeugrahmen verwandt. Für den vorliegenden Beitrag wurde die Kollisionssicherheit von verschieden geformten dünnwandigen Rohren aus verschiedenen Werkstoffen, eine neue Generation von DP-TRIP-Stählen, den Aluminiumlegierungen AA7108 und AA7003 sowie der Magnesiumlegierung AM60 für den Leichtbau von Fahrzeugenergieabsorbern numerisch untersucht. Entsprechend der Ergebnisse der Finite Elemente Analysen wurden die Crashperformanz-Parameter wie die totale Energieabsorption, die spezifische Energieabsorption, die Reaktionskräfte und die Effizienz hinsichtlich der Eindrückkraft für die verschiedenen Werkstoffe verglichen. Obwohl die Energieabsorbierungskapazität für die Energieabsorber aus Stahl besser als die für die Aluminium- und Magnesiumabsorber war, können die Energieabsorber aus den Leichtbauwerkstoffen mehr Energie pro Masseeinheit des Werkstoffes absorbieren als die Stähle. Dieser Vorteil der Leichtbaulegierungen hat Automobilbauer dazu veranlasst, diese im Design von Fahrzeugstrukturkomponenten zu verwenden.

techniques and sheet metal forming. He has been serving as a technical consultant for R&D Projects of Oyak-Renault Automobile Factory. Emre Demirci, born 1988, received his Bachelor’s and Master’s degree at the Department of Mechanical Engineering, Yıldız Technical University in İstanbul in 2010 and Bursa Technical Uni-

versity, Turkey, in 2014, respectively. He has been working during his master studies in the field of optimum design of automobile crash boxes. His master study was supported by the Ministry of Science, Industry and Technology of Turkey. He is currently a research assistant at Bursa Technical University.

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