Quasi-static behaviour of crash components with steel skins and polymer foam cores

Abstract

Different types of crash components are extensively used by the automotive industry to absorb energy during car accidents. Such components typically consist of sandwich structures with thin metal or composite plates as skins and a cellular foam as core to dissipate the energy. In this study, the quasi-static behaviour of two types of polymeric foams with different densities, namely extruded polystyrene (XPS) and expanded polypropylene (EPP), utilized as core material in a crash component, has been investigated. First, an extensive material test program involving compression tests on cubic specimens loaded in different material directions was carried out to reveal the mechanical properties of the foams. Second, quasi-static impact tests were conducted on various target configurations consisting of 0.8 mm thick skins of Docol 600DL steel and the various foam materials as core. In these tests, the applied force and the displacement of the striker were registered by the test machine, while digital cameras and 3D-DIC were used to measure the back-plate out-of-plane displacement of the various components.It seems clear from the presented results that if low weight combined with maximum energy absorption are the primary interests, an XPS foam as core seems to be beneficial, while if force reduction and minimum back-plate displacement are most important, an EPP foam as core seems to be a better choice. Thus, the response of the crash component is largely determined by the properties of the core material during quasi-static loading.

Low velocity impact on crash components with steel skins and polymer foam cores

“…Nominal mechanical properties of both XPS and EPP are provided in Table 1. SEM images of the foams revealed that the microstructure of XPS and EPP in this study is similar, but the cell size seems to be somewhat smaller for the latter [30].…”

“…The dynamic material tests were carried out in a split-Hopkinson tensile bar (SHTB). Force-elongation curves and true stress-plastic strain curves until necking in different material directions from quasi-static tensile tests on the steel skins used in this study can be found in [30]. These tests revealed that the skin material can be considered isotropic with a small variation in elongation to failure, while the dynamic tensile tests showed that this steel alloy is only moderately strain-rate sensitive.…”

“…Experimental results in terms of true stress-strain curves from Series 1 and 2 are presented in [30], but some of the main findings are included in the following for completeness. All samples showed typical closed-cell foam behaviour [9], i.e., a linear elastic region, a plateau region with large deformations, and finally a densification region where the cells become more and more compacted.…”

“…As the compression increases, the localised areas extend and propagate outwards [17]. Typical strain fields of both XPS and EPP foams were established using the DIC-code eCorr [35], and can be found in [30] and [37]. These fields revealed a distinct difference in the deformation mode for the two types of foam when crushed, even though the global stress-strain response looks similar.…”

“…Then, quasi-static and dynamic impact tests on sandwich structures were performed. While the quasi-static tests have been reported in a recent publication by Reyes and Børvik [30], this paper focuses on the dynamic response of the same materials and components. The sandwich panels applied in the dynamic impact tests consisted of 50 mm thick polymeric foam cores and skins of 0.8 mm thick Docol 600DL steel plates.…”