73 Postscript

Two final, cautionary, comments. Insufficient experimental data are available to be fully confident about the accuracy of the isotropic hardening law (7.18). There is some evidence that the rate of hardening for hydrostatic compression may be greater than that in simple compression. Indeed, experimental measurements of the hydrostatic and uniaxial compression responses of Alporas and Duocel suggest that the hardening rate is faster for hydrostatic compression (see Figure 7.5 for data presented in the form of true stress versus logarithmic plastic strain curves).

Figure 7.5 Compression and hydrostatic stress-strain curves for Alporas and Duocel foams: true stress and logarithmic strain

Some metallic foams harden in compression but soften (and fail prematurely) after yielding in tension. This behavior, not captured by the present constitutive law, is caused by the onset of a new mechanism, that of cell-wall fracture, which progressively weakens the structure as strain increases.

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