The properties of copper foams with negative Poisson's ratio via resonant ultrasound spectroscopy

The Poisson's ratios of re-entrant Cu foams with different initial relative densities were studied by resonant ultrasound spectroscopy in the work by Li, Dong, and Lakes (pp. 1983–1987). The transformation into the re-entrant foam was accomplished by applying a sequence of permanent triaxial compression deformations. The Poisson's ratio first decreases and then increases with increasing compression strain. A minimum in Poisson's ratio of approximately -0.7 for all initial densities was achieved with an appropriate permanent compression strain, Foams with higher values of initial density attained minima in Poisson's ratio at lower permanent volumetric compression. The shear modulus increased with increasing volumetric compression ratio, and showed a small hump near the point corresponding to Poisson's ratio minimum.