Strain rate dependence of anisotropic compression behavior in porous iron with unidirectional pores
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Hiroyuku Yamada, Keitaro Horikawa, and Hidetoshi Kobayashi Graduate School of Engineering Science, Osaka University, Toyonaka, Osaka 560-8531, Japan
Hideo Nakajima The Institute of Scientific and Industrial Research, Osaka University, Ibaraki, Osaka 567-0047, Japan (Received 9 November 2009; accepted 8 March 2010)
The strain rate dependence of anisotropic compression behavior in porous iron with cylindrical pores oriented in one direction was investigated. Through high strain rate (103 s1) compression tests along the orientation direction of pores using the split Hopkinson pressure bar method, it was shown that the stress–strain curve exhibits a unique plateau-stress region where deformation proceeds with almost no stress increase. The appearance of the plateau-stress region is related to the buckling deformation of the iron matrix and provides superior energy absorption. However, for the middle (101 s1) and low strain rates (104 s1), compression along the same direction produces no such plateau region. In fact, in contrast to compression in the parallel direction, compression perpendicular to the orientation direction of pores produces no plateau-stress regions in any of the three strain rates.
I. INTRODUCTION
Porous metals exhibit various properties by the very nature of being porous. These include sound absorption, permeability of fluid, and large surface areas.1,2 Thus, they are expected to be used in a variety of applications in fields such as the automobile and railway industries,3 where they will be subjected to high strain rate deformation. Therefore, their high strain rate compression behaviors and energy absorption characteristics have been the focus of a number of investigations in recent times. Almost all of the studies done so far have been conducted on metal foams with spherical or isotropic pores of high porosity exceeding approximately 80%.4–12 The stress– strain curves of metal foams have been shown to exhibit a plateau-stress region where deformation proceeds at nearly constant nominal stress, in contrast to that of nonporous metals, where stress continuously increases as deformation progresses due to work hardening. In the plateau-stress region, the energy is absorbed by the bending and collapse of cell walls. However, aluminum alloys foams (typical metal foams) do not absorb so much energy since the nonuniformity of the porous structure correlates to low mechanical strength.13 Therefore, various studies have been conducted with the aim of increasing the mechanical strength of metal foams by improving a)
Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/JMR.2010.0147 J. Mater. Res., Vol. 25, No. 6, Jun 2010
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their porous structure14 and the strength of the matrix metals.5 Recently, porous metals with cylindrical pores oriented in one direction,15–17 so-called lotus-type porous metals (lotus metals)18,19 or gasar metals,15 have garnered much interest. The lotus metals exhibit the anisotrop
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