Effects of pore morphology on fatigue strength and fracture surface of lotus-type porous copper
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M. Otsuka Shibaura Institute of Technology, Koto-ku, Tokyo 135-8548, Japan
H. Nakajima The Institute of Scientific and Industrial Research, Osaka University, Ibaraki, Osaka 567-0047, Japan (Received 26 September 2006; accepted 26 January 2007)
We studied the effect of anisotropic pore morphology on the fatigue behavior and fracture surface of lotus-type porous copper, which was fabricated through unidirectional solidification in pressurized hydrogen and argon atmospheres. The fatigue strength at finite life is closely related to the pore morphology. The fatigue strength decreases with increasing porosity, and the strength depends on applied-stress direction. The fatigue life is the longest in the direction parallel to the longitudinal axis of cylindrical pores. The fatigue strength at finite life is proportional to the ultimate tensile strength and can be expressed by a simple power-law formula. Anisotropic pores affect the fracture surface of lotus copper; crack-initiation site depends on applied-stress direction, and the anisotropic shape pores affect the direction of crack propagation and final fracture surface.
I. INTRODUCTION
Porous metals including foamed metals possess unique features such as low density, high surface area, sound absorption, etc.1,2 Therefore, the porous metals are expected to be used in various fields. For example, opencell porous nickel with high surface area is used as the electrode of a battery, and closed-cell porous aluminum with high-energy absorption is used as a shock absorber. However, conventional porous metals possess a shortcoming: low strength. This is because their pore shape is irregular and the distribution of pores is random.3,4 Hence, the conventional porous metals with low strength cannot be used as structural materials, although they show unique features. Lotus-type porous metals (lotus metals) possessing cylindrical pores elongated in one direction can be fabricated by unidirectional solidification of metals in pressurized gas atmospheres.5 Because the cylindrical pores elongate in one direction, there is little stress concentration around the pores when stress is applied in the longitudinal direction of the pores. Furthermore, the porosity of lotus metals is small compared with conventional porous metals, and thus the thickness of a cell wall between pores is wider than that of the conventional porous metals. Therefore, the mechanical propera)
Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/JMR.2007.0164 J. Mater. Res., Vol. 22, No. 5, May 2007
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ties of lotus metals are superior to those of the conventional porous metals.6–9 This superior strength allows lotus metals to be used as structural materials with unique features. For practical use, the knowledge of their fatigue behavior is important because the effect of pore morphology on the fatigue behavior probably differs from that on tensile (or compressive) strength. So far, the fatigue behavior of conventional poro
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