Experimental investigation of initial yield surfaces of solid foams and their evolution under subsequent loading

Metal foams exhibit excellent characteristics suitable for a variety of light weight construction applications, based on outstanding specific mechanical properties and the ability to provide a nearly constant force during energy absorption. All components are subjected to multiaxial stress states during the application. In order to ensure a safe design, yield surfaces constitute the fundamental basis for simulations. The determination of yield surfaces for metal foams is a complex challenge. In the literature yield surfaces have been usually simulated using model assumptions and rarely experimentally validated. The present contribution provides a comprehensive representation of experimentally determined yield surfaces for open-cell nickel/polyurethane (Ni/PU) hybrid metal foams. For the first time in the literature, the effect of different nickel coating thicknesses on the shape of the yield surface is systematically examined. Determining the yield surface parameters as function of the effective foam density allows for a prediction of the minimum layer thickness required for any complex loading condition. Furthermore, the development of the post-yield surfaces under increasing strain is investigated. Post-yield surfaces provide reliable information about the mechanical material behaviour under applied multiaxial stress after a certain pre-damage.