Journal of Physics: Conference Series OPEN ACCESS Indentation tests of aluminium honeycombs To cite this article: A Ashab et al 213 J. Phys.: Conf. Ser. 451 123 View the article online for updates and enhancements. This content was downloaded from IP address 148.251.232.83 on 17//218 at 18:38
D2FAM 213 Indentation tests of aluminium honeycombs A Ashab 1, Y C Wong 1, G Lu 2 and D Ruan 1, 3 1 Faculty of Engineering and Industrial Sciences, Swinburne University of Technology, Hawthorn, VIC 3122, Australia 2 School of Mechanical and Aerospace Engineering, Nanyang Technological University, Singapore 639798 Email: druan@swin.edu.au Abstract. Aluminium honeycomb is a type of cellular material which has high strength to weigh ratio and is a good energy absorber. They are used as structural components in various engineering applications. Comprehensive study has been conducted on the compressive behavior of aluminium honeycombs. However, the research of aluminium honeycombs subjected to other type of loading, such as indentation, is still limited. In this paper, quasi-static and dynamic indentation tests were conducted to study the deformation and energy absorption of three types of HEXCELL aluminium honeycombs with different cell sizes and cell wall thicknesses. Quasi-static tests were conducted by using a universal MTS machine at velocities of.5 mm/s,.5 mm/s and 5 mm/s, respectively. Dynamic tests were conducted by using a high speed INSTRON machine at a velocity of 5 m/s. Force-displacement curves were plotted in which the total energy absorbed was calculated. The deformation of aluminium honeycombs in indentation tests includes the compression of honeycomb cells under the indenter and tearing of honeycomb cell walls along the indenter edges. The energy dissipated in compression and tearing were calculated and discussed. The effects of cell size, cell wall thickness and loading velocity or strain rate on the plateau stress and energy absorption were analyzed. 1. Introduction Aluminium honeycombs are lightweight and have good energy absorption capabilities. They are widely used as core materials in various fields of engineering such as aerospace and automotive because of their extensive engineering properties. Some of the applications of aluminium honeycombs are landing gear door and flaps of aircraft. For example crushable aluminium honeycombs had been used in the Apollo 11 lading module. The large plastic deformation in hexagonal honeycombs is the main energy absorption region. It has been demonstrated that the plastic deformation and energy absorption characteristics of aluminium honeycombs depends on both the geometrical configuration and mechanical properties of cell wall materials [1, 2]. Cell size, cell wall thickness and strain rate are important parameters that affect plateau stress as well as impact response of aluminium honeycombs. Gibson and Ashby [1] pointed out that the deformation pattern and compressive strength varied with the direction of loads, for example, aluminium honeycombs strength is higher in the out-of plane direction compared with the two in-plane directions. A schematic diagram of aluminum honeycomb structure is shown in Figure 1. The cell wall thickness, t and 2t are known as single wall and double wall respectively. The cell edge length is denoted as l while h is the thickness of honeycomb. 3 To whom any correspondence should be addressed. Content from this work may be used under the terms of the Creative Commons Attribution 3. licence. Any further distribution of this work must maintain attribution to the author(s) and the title of the work, journal citation and DOI. Published under licence by Ltd 1
D2FAM 213 Buckling, folding, stretching, de-bonding and tearing are the major failure mechanisms of aluminium honeycombs. Many researchers [3-6] studied aluminium honeycombs experimentally, numerically and theoretically, to characterize their crushing behaviour and failure mechanism under both quasi-static and dynamic compression. Zhou and Mayer [7] conducted compressive and indentation punch tests on two types of aluminium honeycombs with different cell sizes (6.4 mm and 19.1 mm respectively). In compression, the honeycomb with larger cell size was found to be strain rate dependent, while the 3honeycomb with smaller cell size did not show evident strain rate sensitivity. Quasi-static tearing strength of honeycomb was defined, measured and calculated from the indentation punch tests. Two types of honeycombs deformed differently. However, since no dynamic indentation punch test was conducted, the strain rate sensitivity is still unknown. Figure 1. Schematic diagram of an aluminium honeycomb structure. In the present paper, out-of-plane quasi-static and dynamic indentations, and compressive tests have been conducted using MTS and high speed INSTRON machines to investigate tearing and compressive strengths. Three types of aluminium honeycombs having different cell sizes and wall thicknesses have been used in the experiment. For quasi-static tests, velocities of.5 mm/s,.5 mm/s and 5 mm/s were used and for dynamic tests a velocity of 5 m/s was used. The tearing and compressive strengths were calculated from the experimental results and the dependency of the dissipated energy on the loading speed was also discussed. 2. Experiments 2.1. Specification of materials and specimens HEXCELL aluminium honeycombs with different cell sizes and wall thicknesses were used in the experiments. The specific material used is aluminium alloy 52 with an H39 temper. The properties of three types of honeycombs are listed in Table 1. Each of the hexagonal honeycomb cell contain four single walls and two adjacent double walls of which all the cells are inter-connected to each other. The thickness of the double walls is twice of the single wall thickness. In compressive tests, honeycomb specimens used were 9 mm 9 mm. In indentation tests, honeycomb specimens used were 18 mm 18 mm. All honeycomb specimens were mm in height. The dimension of the indenter used in the indentation tests was a 9 mm 9 mm which covers at least 9 9 cells of all types of honeycombs studied. This cross-section of the indenter used was to ensure almost uniform compression of honeycomb cells. A circular plate with holes was used as fixed 2
D2FAM 213 platen where the honeycomb specimen was placed. In order to avoid specimen dis-connected with the platen rubber bands double sided tape was used to fasten the specimen in the platen. The holes in the circular plate are for the entrapped air to escape during testing. Therefore in this experiment the effect of entrapped air could be ignored. Table 1. HEXCELL 52 aluminium hexagonal honeycomb. Name Material Cell size D (mm) Cell wall thickness t (mm) Nominal Density ρ (kg/m 3 ) Modulus (GPa) Crush Strength (MPa) No. of cells covered by Indenter A 3.1-3/16-52-.1N 4.763.254 49.66.52.9 19 19 B 4.2-3/8-52-.3N 9.525.762 67.28.93 1.52 9 9 C 4.5-1/8-52-.1N 3.175.254 72.9 1.3 1.79 28 28 2.2. Quasi-static tests Figure 2. Out-of-plane quasi-static indentation tests conducted on the kn MTS machine. Out-of-plane quasi-static tests were conducted on an MTS machine (Model LPS.4) which had a load capacity of kn that could reach a velocity of mm/s. In the quasi-static tests, the specimens were placed on the lower fixed circular platen. The fixed platen consisted of holes around the centre to allow the entrapped air to escape during testing. Figure 2 shows the experimental setup of the MTS machine for quasi-static tests. The indenter was connected to the upper load cell which moved downwards to crush the specimens. Fixed position of the specimens on the circular platen was maintained for all the tests. Both indentation and compressive tests were conducted using the MTS machine at velocities of.5 mm/s,.5 mm/s and 5 mm/s respectively. The corresponding nominal strain rates were -3, -2 and -1 s -1 respectively for the specimen with a thickness of mm. Compressive force, F c, was measured in the compressive tests. The total force measured in the 3
D2FAM 213 indentation tests, F T, was the sum of the compressive force, F c, and tearing force, F t. Thus the tearing force could be calculated as, F t = F T F c. 2.3. Dynamic Tests Figure 3. Out-of-plane dynamic indentation tests conducted on the high speed INSTRON machine. Out-of-plane dynamic tests were conducted on a high speed INSTRON (VHS88) machine which has capacity to generate a maximum velocity of m/s. Both indentation and compressive tests were conducted at a velocity of 5 m/s. Figure 3 shows the experimental setup of the INSTRON machine for dynamic indentation tests. In the experiment the circular fixed platen was connected to the upper load cell. The specimens were attached to the fixed upper platen by using rubber bands for the indentation tests and specimens were fasten to the indenter by double-sided tape in compressive tests. The indenter was connected to the lower piston of the machine which moved upwards to apply the impact force to the specimens. 3. Results and Discussions The force-displacement curves of three types of aluminium honeycombs subjected to indentation and compressive loads at velocities of.5 mm/s,.5 mm/s 5 mm/s and 5 m/s are shown in Figures 4-7 respectively. It can be seen from Figures 4-7 that the plateau forces in the quasi-static tests (Figures 4-6) for all types of honeycomb specimens are generally constant. While in the dynamic tests (5 m/s, Figure 7) the transient forces fluctuate significantly in the plateau region. The average plateau force was calculated by taking the average force within a displacement range of 3-38 mm. The mean plateau stress was calculated as the ratio of average force to the crosssectional area of the honeycomb specimen. The mean plateau stresses of three different honeycomb materials at different velocities,.5 mm/s,.5 mm/s, 5mm/s and 5m/s are listed in Table 2. It has been observed that the mean plateau stress in both indentation and compressive tests increases with the increase in velocity. However, the tearing strength, F t = F T F c, did not show any trend with the change of the velocity. The area under the force-displacement curve is the total energy absorbed by honeycombs. The energy absorbed by honeycombs in compression, E c, is the area under the force-displacement curves recorded in compressive tests. 4
D2FAM 213 4 A1 B1 C1 2 2 4 (a) 4 A2 B2 C2 2 2 4 (b) Figure 4. Quasi-static force-displacement curves of three types of aluminium honeycombs at a loading velocity of.5 mm/s in: (a) indentation tests; (b) compressive tests. 5
D2FAM 213 4 A3 B3 C3 2 2 4 6 (a) 4 A4 B4 C4 2 2 4 (b) Figure 5. Quasi-static force-displacement curves of three types of aluminium honeycombs at a loading velocity of.5 mm/s in: (a) indentation tests; (b) compressive tests 6
D2FAM 213 4 A5 B5 C5 2 2 4 (a) 4 A6 B6 C6 2 2 4 (b) Figure 6. Quasi-static force-displacement curves of three types of aluminium honeycombs at a loading velocity of 5 mm/s in: (a) indentation tests; (b) compressive tests 7
D2FAM 213 6 4 A7 B7 C7 2-2 4 (a) 6 4 A8 B8 C8 2-2 4 (b) Figure 7. Dynamic force-displacement curves of three types of aluminium honeycombs at a loading velocity of 5 m/s in: (a) indentation tests; (b) compressive tests The total energy absorbed by honeycombs in indentation, E T, is the area under the forcedisplacement curves recorded in indentation tests and listed in Table 3. Since both compression of 8
D2FAM 213 honeycomb cells and tearing along the 4 edges of honeycomb specimen occurred simultaneously in the indentation of honeycombs, the total energy dissipated in tearing, E t, is calculated by the difference of E T and E c. The percentages of compressive energy, E c, and tearing energy, E t, at different strain rates are listed in the Table 3. For honeycombs material 3.1-3/16-52-.1N (A1, A3, A5, A7) the percentage of compression and tearing energies varied in the range of 83% - 87% and 13% - 17% respectively. For the honeycomb material 4.5-1/8-52-.1N (B1, B3, B5, B7) the percentages for compression energy and tearing energies were similar to those for honeycombs material 3.1-3/16-52-.1N. However, for honeycombs material 4.2-3/8-52-.3N the percentages of compression and tearing energies varied in the range of 7% - 74% and 26% - % respectively, i.e. the tearing energy was higher than that of the other two types honeycomb materials. This is mainly due to the thicker cell walls of honeycomb material (4.2-3/8-52-.3N) compared with the other two types of honeycomb materials. However, the tearing energy, E t = E T E c, did not change significantly with loading velocity. Table 2. Summary of experimental results Test no. Material Test type t/l Velocity (m/s) Mean plateau stress σ pl (MPa) Total Dissipated energy (J) A1 3.1-3/16-52-.1N Indentation.924 5-5 1.7 4 A2 3.1-3/16-52-.1N Compression.89 252 A3 3.1-3/16-52-.1N Indentation.924 5-4 1.8 5 A4 3.1-3/16-52-.1N Compression.93 264 A5 3.1-3/16-52-.1N Indentation.924 5-3 1.12 323 A6 3.1-3/16-52-.1N Compression.89 268 A7 3.1-3/16-52-.1N Indentation.924 5 1.2 338 A8 3.1-3/16-52-.1N Compression 1.4 295 B1 4.2-3/8-52-.3N Indentation.139 5-5 1.95 546 B2 4.2-3/8-52-.3N Compression 1.35 382 B3 4.2-3/8-52-.3N Indentation.139 5-4 2.9 586 B4 4.2-3/8-52-.3N Compression 1.44 49 B5 4.2-3/8-52-.3N Indentation.139 5-3 2.1 598 B6 4.2-3/8-52-.3N Compression 1.53 444 B7 4.2-3/8-52-.3N Indentation.139 5 2.27 643 B8 4.2-3/8-52-.3N Compression 1.74 46 C1 4.5-1/8-52-.1N Indentation.139 5-5 2.18 616 C2 4.5-1/8-52-.1N Compression 1.84 522 C3 4.5-1/8-52-.1N Indentation.139 5-4 2.19 619 9
D2FAM 213 C4 4.5-1/8-52-.1N Compression 1.76 526 C5 4.5-1/8-52-.1N Indentation.139 5-3 2.28 661 C6 4.5-1/8-52-.1N Compression 1.98 553 C7 4.5-1/8-52-.1N Indentation.139 5 2.42 683 C8 4.5-1/8-52-.1N Compression 2.3 576 Test no. Table 3. Effect of strain rate on the dissipated energy Strain rate (s -1 ) Compression energy % Tearing energy % A1 3 83 17 A3 2 86 14 A5 1 83 17 A7 87 13 B1 3 7 B3 2 7 B5 1 74 26 B7 71 29 C1 3 84 16 C3 2 84 16 C5 1 83 17 C7 84 16 4. Conclusions The out-of-plane indentation and compression behaviours of three types of aluminium honeycombs were investigated experimentally by using an MTS machine and a high speed INSTRON machine. In both quasi-static and dynamic tests, constant velocity was achieved in all tests. Force-displacement curves were recorded in all tests and presented. Mean plateau stress and energy absorbed in compression and indentation of honeycombs were calculated. Results indicated that both mean plateau stress and total energy absorbed in compression and indentation increased with the density of honeycombs and loading velocity or strain rate. The tearing strength and tearing energies increased with the cell wall thickness of honeycomb, no trend was observed with the change of loading velocity or strain rate in this study. Further detailed study will be carried out in the near future with a focus on the strain rate effect on tearing strength and energy. Acknowledgements The authors are grateful to Swinburne University of Technology for the financial support through a postgraduate scholarship, the Australia Research Council for the financial support through a Discovery grant and Dr Shanqing Xu for his help in the experiments.
D2FAM 213 References [1] Gibson L J and Ashby M F 1997 Cellular solids: structure and properties 2nd ed. (Cambridge: Cambridge University Press) [2] Yamashita M and Gotoh M 25 Int. J. of Impact Eng. 32 618 [3] Zhang J and Ashby M F 1992 Int. J. of Mech. Sci. 34 475 [4] Xu S, Beynon J H, Ruan D and Lu G 212 Compos. Struct. 94 2326 [5] Masters I G and Evans K E 1996 Compos. Struct. 35 43 [6] Wierzbicki T 1983 Int. J. of Impact Eng. 1 157 [7] Zhou Q and Mayer R R 22 J. Eng. Mater.-T ASME 124 412 11