International Journal of Automotive Technology, Vol. 15, No. 6, pp. 945−951 (2014) DOI 10.1007/s12239−014−0099−2
Copyright © 2014 KSAE/ 080−10 pISSN 1229−9138/ eISSN 1976−3832
FRACTURE PROPERTIES OF ALUMINUM FOAM CRASH BOX M. S. HAN1), B. S. MIN2) and J. U. CHO3)* 1)
Department of Mechanical and Automotive Engineering, Keimyung University, Daegu 704-701, Korea 2) SMT R&D Team CTO BU, Mirae Corporation, 12 3Gongdan 2-ro, Seobuk-gu, Cheonan-si, Chungnam 331-220, Korea 3) Division of Mechanical and Automotive Engineering, Kongju National University, Chungnam 331-717, Korea (Received 17 October 2012; Revised 7 January 2013; Accepted 21 October 2013) ABSTRACT−This study investigates the compression property experiment to examine impact absorption when aluminum foam is applied to crash box in order to absorb impact energy in car crash with low speed. The result of compression property experiment shows that case 6, which involves the buckling that collapses into 5-layer structure, is the best model with regard to impact absorption. This study analyzes impact characteristics according to the structure of crash box which influences such factors as damage and safety of vehicles. As the simulation result can be agreed with experimental graph, all experimental data at this study are verified. These experimental results can be applied into real field effectively. It also proposes the effective design to improve impact performance by analyzing the property of crash box through its compressive test. KEY WORDS : Compression property, Aluminum foam, Crash box, Layer structure, Safety
1. INTRODUCTION As more durable automobiles as high class have been demanded, the amount of car and the extra devices must be changed accordingly. Since the international issue of environmental pollution is currently highlighted, it is of great urgency to improve fuel efficiency in preparation for stricter regulations on the average fuel efficiency and automobile emissions of each manufacturer. The measures to improve fuel efficiency include factors connected to the efficiencies of engine and driving system, reduction of driving resistance, light vehicular weight, etc. The transmission efficiency of engines and driving systems has almost reached its limitation in terms of technology with lightweight supplies. And so it is difficult to expect drastic improvement for fuel efficiency although the lighter vehicular weight may significantly improve fuel efficiency by modifying the automotive structure and replacing existing materials by reasonable parts (Park et al., 2010; Elmarakbi and Sennah, 2006). RCAR, the international organization that consists of insurance research centers around the world, suggests the effective design to improve safety performance against collisions at low speed and vehicular damage repair. This design has been contributed to the optimization ability of automotive manufacturer (Peroni et al., 2009). As shown in Figure 1, the concept of crash is introduced into the design to reduce car body damage upon impact under low speed
Figure 1. Impact absorption system of bumper, side member (cross member) and crash box.
Figure 2. Aluminum foam (closed cell) × 30. and to enhance performance, basic impact-related efficiency and impact absorption devices such as bumper, side member (crossmember), crash box, etc (Rusinek et al.,
*Corresponding author. e-mail:
[email protected] 945
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2008). As crash box has the moderate space at bumper joint of front and rear bumper, the damage of car body can be reduced. And the capability to absorb impact force is improved through bending deformation of the part applicable at low speed impact. The structure of crash box which is manufactured thinly tends to show diverse impact behavior. There is the good prospect that aluminum alloy, magnesium alloy, plastic resin and composites as well as steel are applied to crash box. Interest in aluminum foams has grown with their potential to deliver excellent performance in innovative parts where low weight, high specific stiffness, and strength combined with good energy absorption are strict design parameters. These features make metallic foams the potential material for absorbing impact energy during the vehicular crash against either another vehicle or a pedestrian. Potential applications of these foams also exist in shipbuilding, the aerospace industry, and civil engineering (Mitrevski et al., 2006; Mondal et al., 2009; Belingardi et al., 2002; Cho et al., 2012). A lot of foam components are commonly made by injecting gas or foaming agents into molten metal inside a proper closed die. Secondary operations, which can affect the behavior of metallic foams during formation, must be taken into account to have good components. In addition, manufacturing costs must be significantly reduced. Though quite a number of applications are being explored for closedcell foams, it is insufficient to study it systematically. Aluminum foam in particular has more impact energy absorption than solid steel or aluminum. Aluminum foam (closed cell) as in Figure 2 is applied to the crash box which absorbs impact upon car crash at low speed(Lee et al., 2006; Toksoy and Guden, 2010; Ramamurty and Kumaran, 2004; Zhou and Soboyejo, 2004). Figure 3 shows the aluminum foam specimen penetrated perfectly by impact. Figure 4 also shows cross-sectional views of un-penetrated and penetrated aluminum foam specimens by impact. The striker with impact velocity of 0.938 m/s does not penetrate the specimen as shown in Figure 4 (a). On the other hand, the striker with impact velocity of 1.72 m/s penetrates the specimen as shown in Figure 4 (b). This aluminum foam crash box has the relation to energy
Figure 3. Aluminum foam specimen penetrated perfectly by impact.
Figure 4. Cross-sectional views of un-penetrated (a) and penetrated (b) aluminum foam specimens by impact.
Figure 5. Energy absorption capacity vs. weight due to elapsed time about Al foam. absorption or reaction force. The absorbed total energy vs. weight and the strength (load) vs. weight due to elapsed time are shown by Figure 5. Total energy and load according to weight show the energy absorption capacity about aluminum foam in case of impacting velocity of 1.4 m/s. Aluminum foam has not been applied at many vehicles yet because of complex design and configuration. But crash box has already been the indispensable element for manufacturing automobile at vehicle industry. In this study, 6 kinds of crash box models with aluminum foam are designed and manufactured. When these crash boxes are
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Figure 6. Configurations and dimensions of crash boxes (cases 1, 2, 3, 4, 5 and 6; Unit: mm). applied with the force at low speed impact, it is investigated which the model of case 6 has the most durability on impact among 6 kinds of crash box models. This study proposes the plan to improve impact performance by analyzing the property of crash box through compressive test.
2. EXPERIMENTAL RESULTS AND DISCUSSIONS 2.1. Experimental Specimens Aluminium foams are put into two different forms of assemblies exhibiting the energy absorption analyses as much as heights of 30 mm and 50 mm filled with foam by comparing the compression properties in cases of six different cases are compared. The dimensions of six cases 1, 2, 3, 4, 5 and 6 are shown in Figure 6. It is effective that the foam is bonded at the middle of crash box. As shown by Figure 6, aluminum foam is inserted to be positioned at the middle of crash box so that middle part could be bent at least more than one face among side faces. In case of the design that aluminum foam is inserted in the middle of crash box, crash box is deformed sequentially by absorbing impact energy. As it is possible to absorb high impact energy, the load transmitted to car body becomes lower. The direction of load stroke is z axis. As thin crash box and aluminium foam become only contact, this foam is filled as diagonal lines as shown by Figure 6. Two vertical sides at thin crash box have projecting parts with the height of 15 mm. Projecting part has the influence on fracture property of crash box. Case 1, 3 and 5 have one same crash box. Cases 2, 4 and 6 have the other same crash box. The direction on projecting part at cases 1, 3 and 5 is different from case 2, 4 and 6. The structures of cases 1 and 2 are Ushaped aluminum sheets of the same size crossed to each other. The internal parts of cases 1 and 2 contain no aluminum foam. For cases 3 and 4, aluminum foam as much as the height of 30 mm is inserted into cases 1 and 2, respectively. For cases 5 and 6, aluminum foam as much as the height of 50 mm is inserted into cases 1 and 2,
Figure 7. Aluminum sheet joint part of crash box.
Figure 8. Crash box inserted with aluminum foam.
respectively. Figure 7 shows the joint part of the aluminum sheet for the crash box and Figure 8 shows the crash box into which aluminum foam is inserted. To examine the compression property, A5052 as the material type of aluminum sheet commonly used as molding material due to its excellent weldability, strength and fatigue resistance is adopted for the crash box. The properties of these materials are investigated in the experiment conducted in this study and are shown in Table 1. 2.2. Experimental Setup To examine compression properties of crash box, the
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Table 1. Material properties of model. Item
A5052
AL foam
Young's modulus (GPa)
71.7
0.46
Poisson's ratio
0.33
0.11
2.68 × 10
3 × 10-7
Tensile yield strength (MPa)
325
1.65
Tensile ultimate strength (MPa)
330
2.3
3
Mass density (Kg/mm )
-6
Figure 10. Specimen at compression procedure and experimental result graph of load-displacement compression (case 1).
Figure 9. Experimental setup at compression test. Table 2. Specifications of SHIMADZU AG-X. Items
Specifications
Crosshead-table Clearance (Stroke)
Max. 1440 mm
Crosshead speed
0.0005~500 mm/min
Allowed test force
250 kN
Test force measurement
Within ±1% of Displayed Test Force
Crosshead position Detection precision
Within ±1% of Indicated Value
Crosshead speed Precision
±0.1%
graph of load-displacement with marks of ①, ②, ③ and ④. As shown in the figures, when the displacement quantity is about 3 mm, the load reaches the maximum of 35034N, the load decreases until the displacement of 20 mm at mark 1as ① layer. It continues to fluctuate thereafter. Marks of ②, ③ and ④ are also shown by 2, 3 and 4 layers. The specimen shows that the buckling of the crash box turns to 4-layer structure. 2.3.2. Case 2 specimen As case 2 specimen, Figure 11 shows specimen configurations at compression procedure and the experimental result graph of load-displacement with marks of ①, ②, ③. The result of case 2 compression test shows the same aspects as case 1. When the amount of displacement is about 3 mm, the load reaches the peak up
versatile test equipment, SHIMADZU AG-X is used for the compression test. Figure 9 shows the experimental setup. The crosshead speed at the compression test is 2 mm/min as quasi-static state and camera is used to photograph the buckling of specimen and experimental process as in Figure 9. Table 2 shows specifications of SHIMADZU AG-X. The crosshead speed limits with 500 mm/mim as specifications at Table 2 remarked as static tester. Low speed can be seen to become within 500 mm/min as the research scope in this study. 2.3. Experimental Result 2.3.1. Case 1 specimen As case 1 specimen, Figure 10 shows specimen configurations at compression procedure and the experimental result
Figure 11. Specimen at compression procedure and experimental result graph of load-displacement compression (case 2).
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Figure 12. Specimen at compression procedure and experimental result graph of load-displacement compression (case 3).
Figure 14. Specimen at compression procedure and experimental result graph of load-displacement compression (case 5).
to 31799N at mark ① as 1 layer. The load decreases up to the point where the displacement becomes 20 mm, and then cycles of increases and decreases are repeated thereafter. Marks of ② and ③ are also shown by 2 and 4 layers. The buckling of crash box as well turns to 4-layer structure just as case 1.
2.3.4. Case 4 specimen As case 4 specimen, Figure 13 shows specimen configurations at compression procedure and the experimental result graph of load-displacement with marks of ①, ②, ③, ④ and ⑤. As for the compression experimental result of case 4, the aspects of load graph are similar to that of case 3 except that there are more load increases and decreases than what is shown in case 3. Marks of ①, ②, ③, ④ and ⑤ are also shown by 1, 2, 3, 4 and 5 layers. The structure of buckling turns to 5-layer structure just as case 3.
2.3.3. Case 3 specimen As case 3 specimen, Figure 12 shows specimen configurations at compression procedure and the experimental result graph of load-displacement with marks of ①, ②, ③, ④ and ⑤. The aspects of graph are similar to those of cases 1 and 2 up to the point where the displacement quantity is 50 mm, from which point the load increases and decreases once again. Marks of ①, ②, ③, ④ and ⑤ are also shown by 1, 2, 3, 4 and 5 layers. The structure of buckling turns to 5-layer structure.
Figure 13. Specimen at compression procedure and experimental result graph of load-displacement compression (case 4).
2.3.5. Case 5 specimen As case 5 specimen, Figure 14 shows specimen configurations at compression procedure and the experimental result graph of load-displacement with marks of ①, ②, ③, ④ and ⑤. Marks of ①, ②, ③, ④ and ⑤ are also shown by 1, 2, 3, 4 and 5 layers. The compression experimental result
Figure 15. Specimen at compression procedure and experimental result graph of load-displacement compression (case 6).
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M. S. HAN, B. S. MIN and J. U. CHO
of case 5 is different from former cases in that the aluminum foam causes the structure of the buckling to maintain the constant gap and then it collapses into 5-layer structure. 2.3.6. Case 6 specimen As case 6 specimen, Figure 15 shows specimen configurations at compression procedure and the experimental result graph of load-displacement with marks of ①, ②, ③, ④ and ⑤. Marks of ①, ②, ③, ④ and ⑤ are also shown by 1, 2, 3, 4 and 5 layers. Just as case 5, case 6 shows that the aluminum foam causes buckling to maintain the constant gap and then it is led to collapse into 5-layer structure. For compression property in cases 1 and 2, which contained no aluminum foam, the buckling is led to collapse into 4-layer structure, and maximum loads are 35034N and 31799N, respectively. As for cases 3, 4, 5, and 6, which contain aluminum foam, the buckling is led to collapse into 5-layer structure. The maximum loads are 33980N, 34523N, 35743N, and 30710N, respectively. The maximum load in case 6 becomes lowest among six cases. As the result, the aspects of graphs turns out to be similar to each other. With regard to the structure of buckling, cases 1 and 2 are of 4-layer structure while cases 3, 4, 5, and 6 turn to be of 5-layer structure. Case 6, which shows the buckling led to collapse into 5-layer structure with the maximum load of 30710N, is thought to have the most excellent impact absorption effect. 2.4. Simulation Result to Verify Experiment To verify experiments of this study, Figure 16 shows the model of impact box and fixed plate in case 6 on
Figure 16. Simulation model of impact box and fixed plate.
Figure 17. Meshes of impact box and fixed plate.
Figure 18. Boundary conditions of impact box and fixed plate.
Figure 19. Deformation contour in case 6 at the elapsed time of 5 ms with configurations of simulation and experiment.
Figure 20. Load due to displacement at experiment and simulation in case 6.
simulation analysis with ANSYS 13.0 (Swanson, 2010; De Giorgi et al., 2010; Konstantinidis et al., 2005). This model has the same size and dimension as real specimen. The finite element model is divided with hexagonal elements as shown by Figure 17. The numbers of nodes and elements are 42262 and 12422. The material of impact box is structural steel. The contact condition is friction between impact box, crash box and fixed plate. As the boundary condition shown by Figure 18, the impact box is applied
FRACTURE PROPERTIES OF ALUMINUM FOAM CRASH BOX
with the displacement speed of 1mm/s and the lower plate is fixed. Figure 19 shows deformation contour in case 6 at the elapsed time of 5ms with configurations of simulation and experiment. The simulation model configuration becomes similar to experimental specimen. Figure 20 shows the graph of load due to time at experiment and simulation in case 6. As shown by Figure 20, simulation result approaches experimental curve. As these simulation results can be agreed with experimental graph, all experimental data at this study are verified. These experimental results can be applied into real field effectively.
3. CONCLUSION This study applies aluminum foam to the crash box that absorbs the impact at car crash with low speed and investigates the compression property experiment. The research findings are as follows: (1) For compression property in cases 1 and 2, which contains no aluminum foam, the buckling is led to collapse into 4-layer structure, and the maximum loads are 35034N and 31799N, respectively. (2) As for cases 3, 4, 5, and 6, which contained aluminum foam, the buckling is led to collapse into 5-layer structure. The maximum loads are 33980N, 34523N, 35743N and 30710N, respectively. (3) Case 6, which shows the buckling led to collapse into 5-layer structure, is thought to have the most excellent impact absorption effect. (4) As simulation results can be agreed with experimental graph, all experimental data in this study are verified. These experimental results can be applied into real field effectively. (5) This study proposes the effective design to improve impact performance by investigating the property of crash box through compressive test. ACKNOWLEDGEMENT−This work was supported by the research grant of the Kongju National University in 2012. This research was supported by Basic Science Research Program through the National Research Foundation of Korea(NRF) funded by the Ministry of Education, Science and Technology (20110006548).
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