Cyclic-dynamic properties of particle foams

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Particle foams are characterized by a unique combination of low density, high mechanical energy absorption under pressure, great part-design freedom and low manufacturing costs. They are thus predestined for a variety of applications, which currently include athletic shoes and safety-relevant parts of vehicle interiors. By selection of the processing parameters during foaming, the properties of the cell structure and thus the behavior of the molded parts can be specifically adjusted to the application. According to the current state of research, however, there are significant uncertainties regarding the structure-property relationships of particle foams. Thus, although calculation methods are available that are suitable for predicting macroscopic material behavior in any multiaxial stress states, they are based on substantial simplifications, such as neglecting local stress and strain maxima at the mesolevel of the foam. Hence, in the case of global pressure loading, local bending, buckling or tensile failure occur within the cell structure. The mechanical behavior is also significantly influenced by the enclosed cell gas and its compression. If the final part is subjected to cyclic load over a longer period of time, empirical studies determined a cyclical creep. Both, the interaction between cell gas and cell structure and the phenomenology of cyclic creep have not been adequately investigated yet.The project focuses on the experimental and numerical investigation of the mechanical behavior of particle foams under quasi-static as well as cyclic compressive loading and unloading. With the help of a to be developed test stand for performing compression tests under ambient pressure and the application of X-ray tomographic analyzes, the interactions between cell gas, cell structure and base polymer as well as the time-dependent mechanical properties of the particle foam are going to be investigated. The main focus is on the viscoelastic properties and the creep behavior of the particle foam as well as the global and local stress-strain curves with repetitive loading and unloading. The analysis of the cell morphology forms the basis for the numerical simulation of the particle foams. Within the project, first single loading and unloading steps are to be simulated taking into account cell structure, cell gas and viscoelasticity. In particular, inelastic effects and instabilities on the local level are to be detected and their effect on global behavior to be identified. Thus, the work is going to create important foundations for an improved material understanding, which is required for a more resource-efficient and reliable design of mechanical long-term-loaded particle foam parts.