Tensile deformation of anisotropic porous copper with directional pores
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The tensile deformation of anisotropic porous copper with unidirectionally oriented cylindrical pores was investigated by an acoustic emission method. In the loadings parallel and perpendicular to the orientation direction of the pores, many cracks are formed after yielding and they strongly affect the deformation. The formed cracks rapidly grow and connect with each other near the peak stress of the stress–strain curve, thereby leading to final fracture. Crack formation is easier under perpendicular loading than under parallel loading, because high stress concentration and stress triaxiality occurs around the pores. As a result, the strength and elongation for perpendicular loading are much smaller than those for parallel loading. Furthermore, in the case of perpendicular loading, the localized deformation around pores drastically decreases the plastic Poisson’s ratio. These results indicate that a porous copper macroscopically behaves as a semibrittle material under perpendicular loading, while the porous copper exhibits ductility under parallel loading. I. INTRODUCTION
Porous metals exhibit a variety of properties by virtue of their porosity.1,2 These properties include sound and energy absorption, fluid permeability, and biocompatibility, as well as low thermal conductivity and large surface area. Thus, porous metals are expected to be used in a variety of applications such as sound absorbers, energy absorbers, filters, heat insulators, and biomedical implants. Thus, various kinds of porous metals have been developed and their properties have been widely studied.3 The compressive deformation of porous metals has been studied intensively,4–8 because it is closely related to energy absorption, which is one of the most important properties of porous metals. Less attention has been paid to the tensile deformation behavior, despite the fact that this area of study is important in a wide variety of applications. However, previous researches have clarified some important features of the tensile deformation mechanism. In closed-cell metal foams,4,9 for example, cracks are formed around defects or wiggles in the cell walls at the initial stage of the stress–strain curve. As a result, the peak stress is low and fractures result from considerably small strains. Moreover, in open-cell metal foams,10 cracks are formed in the weak regions at the initial stage of the stress–strain curve, resulting in low tensile strength and elongation. Low strength and elongation are closely associated with crack formation in both closed-cell and open-cell metal foams; as a result, the suppression of crack formation is an important means to improve the tensile properties. a)
Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/JMR.2010.0261 J. Mater. Res., Vol. 25, No. 10, Oct 2010
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Recently, porous metals with unidirectionally oriented cylindrical pores,11–13 also known as lotus-type porous metals (or lotus metals)14,15 or gasar metals,11 have ga
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