Experimental Techniques for the Investigation of the Elasto-Plastic Transition Zone of Foamed Materials

The classical assumption in solid materials, i.e. that the plastic behaviour is incompressible, does no longer hold in the case of cellular materials. The plastic behaviour is pressure‐sensitive due to the cellular structure even when the pure base material is independent of the hydrostatic pressure in the plastic range. Therefore, the yield criterion needs to incorporate the hydrostatic pressure. In many cases, the yield criterion can be simplified to an additive form where an arbitrary scalar function weights the influence of the hydrostatic stress. The yield stress can be obtained from uniaxial tests but the determination of the weighting function for the hydrostatic stress requires the realisation of multi‐axial stress states. This work presents two experimental procedures, i.e. an experiment under plane strain conditions and the axial compression, for the determination of the parameters of the yield criterion in the elasto‐plastic transition zone. Furthermore, both experiments aim to determine a second elastic constant if for example Young's modulus is known from uniaxial compression tests. The proposed procedures are numerically applied to a material obeying the Deshpande‐Fleck yield criterion.[1,2] The classical assumption in solid materials, i.e. that the plastic behaviour is incompressible, does no longer hold in the case of cellular materials. The plastic behaviour is pressure‐sensitive due to the cellular structure. Therefore, the yield criterion needs to incorporate the hydrostatic pressure. The yield stress can be obtained from uniaxial tests but the determination of the weighting function for the hydrostatic stress requires the realisation of multi‐axial stress states. This work presents two experimental procedures for the determination of the parameters of the yield criterion in the elasto‐plastic transition zone.