Change of Young’s Modulus with Increasing Applied Tensile Strain in Open Cell Nickel and Copper Foams

Abstract

The change of Young’s modulus with increasing applied tensile strain in the open cell nickel and copper foams, fabricated by the slurry foaming process at Mitsubishi Materials Corporation, Japan, was investigated experimentally and analytically. Stress-strain curves were measured at room temperature, in which the loading-unloading-reloading process was performed on the sample at various applied strains. For measurement of the strain of the sample itself accurately, the non-contact extensometer was employed. The Young’s modulus values at various applied tensile strains were estimated from the slope of the elastic region in the stress-strain curve in the unloading process. It was revealed that two competitive factors affect on the Young’ modulus value. One is the irreversible morphology-change of cells arising from the plastic deformation of the metal as a results of which, the cells are elongated and struts are straight forwarded in the tensile direction. This factor acts to raise the Young’s modulus with increasing applied strain. Another factor is the failure of struts, which tends to reduce the Young’s modulus with increasing applied strain, especially beyond the strain where the struts are failed successively. Accordingly, due to the competition of the former and latter factors, the Young’s modulus increases, reaching maximum and then decreases with increasing applied strain in both of the foams. The increase in Young’s modulus with increasing applied strain due to the change of geometry of struts was confirmed by the experimental test of micro-samples composed of several struts and nodes and by the simulation based on the finite element analysis using a model micro-sample.