Effect of density on the fatigue behaviour of EPP and ETPU bead foams.

Previous research has shown that the static mechanical properties of bead foams are highly dependent on the base material, the materials the foam is made of. Knowledge on the mechanical behaviour is used to produce resource-efficient components tailored to the application. However, components made from bead foams are often subjected to cyclic dynamic loads during their lifetime. The extent to which this changes the mechanical response over time is still unclear. To close that gap in knowledge, foam blocks were made from commercially available expanded thermoplastic polyurethane (ETPU) and expanded polypropylene (EPP) of the same density. The elastic stress of the two materials was determined in quasi-static mechanical tests. To compare the fatigue behaviour, long-term hysteresis measurements were performed in stepwise increasing strain tests (deformation-controlled) and single-stage (stress-controlled) compression tests. The results of the mechanical tests show excellent fatigue behaviour of ETPU as the material maintains its progressive stress-strain behaviour in the stepwise increasing strain test up to 80 % deformation. Dynamic creep is significantly lower compared to EPP. The one-step test illustrates the different fatigue behaviour at a load of 150 % of the respective elastic stress. EPP shows a compaction of 27 % after 1,000 load cycles and ETPU a compaction of 7.4 % after 500,000 load cycles. The stiffness of EPP increases significantly due to densification, while the stiffness of ETPU remains constant over the entire test duration after settling at the beginning. [ABSTRACT FROM AUTHOR]