Temperature effect on non-stationary compressive loading response of polymethacrylimide solid foam

This paper presents the results of experimental investigation and modeling the polymethacrylimide (PMI) solid foam response on slightly non-stationary compressive loads at different loading speed and temperature. The PMI represents a typical material used as a core component of composite sandwich structures. For all conditions, the strain–stress curves exhibit three definite regions, such as linearly elastic, plateau, and densification as it is known from the literature. However, it is shown in the present study that even minor fluctuations of the loading speed may lead to significant qualitative effects within the plateau segment. In particular, spike-wise load drops occur in a regular way during the loading phase when the specimen temperature is below −20 °C. Acoustic emission tests lead to the conclusion that the load drops are associated with generation of brittle fractures in the clusters of broken foam cells. Also, cyclic loading tests were conducted to evaluate the energy loss in cells permanent deformation during the loading cycle. The amount of energy dissipated for each cycle is reduced at higher temperatures. Finally, based on the experimental results, a phenomenological model of the foam load–displacement response at different loading speeds and temperatures is presented in the form of a single analytical expression.