In situ deformation characterization of density-graded foams in quasi-static and impact loading conditions

Abstract

Digital image correlation is utilized to characterize deformation and strain fields developed within the layers of density-graded multilayered foam structures subjected to uniaxial quasi-static and dynamic compression. Three-layered graded structures fabricated from rigid polyurethane foams with nominal densities of 160, 240, and 320 kg/m3 are subjected to quasi-static and dynamic loading. The quasi-static measurements show that, irrespective of the loading direction, the densification is initiated in the lowest density layer and propagates into other layers later once the first layer is fully densified. The deformation mechanisms are different in the case of dynamic loading conditions than quasi-static loading. In the case of dynamic loading, the deformation mechanism depends on the sample orientation relative to the direction of the applied load. In cases where the higher density layers are impacted, the propagation of the elastic and compaction waves leads to partial deformation of the lowest density layer. Sample deformation continues in all layers upon the reflection of the stress waves from the distal end of the sample. A completely different deformation response is observed in cases where the lowest density layer is oriented towards the impact face. A detailed full-field analysis of strain and stress is performed. The mechanisms associated with the formation and propagation of stress waves from the impact end to the samples' distal end are discussed.