Axial Crushing Behavior of EPP Foam-Filled Nested GFRP Tubes

Polymer matrix composites are used in many components in the automotive sector due to their high specific mechanical properties, and their use in energy-absorbing structures provides significant reductions in vehicle weight. In this study, the energy absorption behavior and load-carrying capacity of nested glass fiber reinforced epoxy composite (GFRP) tubes with and without expanded polypropylene (EPP) foam filling were investigated under quasi-static axial loading. Three nesting arrangements of three concentric tubes were tested: equal-length, short-to-long and long-to-short. Foam filling increased the load-carrying capacity and the absorbed energy of all nested configurations. For the equal-length configuration, EPP foam filling increased the mean crushing force and absorbed energy by approximately 34%, while the specific energy absorption (SEA) improved by about 12%. In the short-to-long configuration, foam filling increased the absorbed energy by approximately 30%, whereas in the long-to-short configuration the increase was approximately 19%; however, the SEA increased by only 6% in the short-to-long configuration and remained practically unchanged in the long-to-short configuration, and the maximum force increased by 24–40% with foam filling. Among the investigated arrangements, the equal-length nested tubes exhibited the highest energy absorption, while the short-to-long configuration provided the lowest initial force and a stepwise load progression due to sequential tube activation. The long-to-short configuration demonstrated intermediate behavior with relatively higher force levels and moderate energy absorption. The results indicate that both foam filling and nesting arrangement govern the quasi-static crushing response of nested composite tubes and that the gain in absorbed energy due to foam filling must be weighed against the added mass and the higher transmitted force.