| Polymeric foams, a type of porous material made in large volume and being capable of large recoverable defornation, have long been widely used in non-structural applications. For specific applications in vehicle safety and crashworthiness, the stress-strain relationship of these materials must be well understood so as to provide required inputs for finite element models. It is desirable that the mechanical properties be calculated from a single constitutive equation that describes the complete behavior under various loading conditions. This research is focused on the crushability of polymeric structural foams, under various strain rates (loading conditions). Five topics were covered in this dissertation. (1) Experimental investigation was conducted on the structural foams under uniaxial-stress and uniaxial-strain quasistatic conditions. A nonlinear multi-parameter phenomenological constitutive model, that is continuously differentiable and defined in the entire real domain such that the stress-strain responses can be characterized through a single-variable multi-parameter function, is developed and validated for the foam materials under large deformation. (2) A specialized quasistatic compressive test approach, named multi-step loading was then designed to check the uniformity of deformation in a foam specimen subjected to unixial-strain. A graphical analysis tool, named "crushability map" was developed, which was intended to characterize the crushability of foams completely, to facilitate selection of the appropriate candidates according to specific design criteria, to evaluate the residual crushability of foams. (3) The constitutive model was employed then to parametrically characterize the crushability and residual crushability of foams. The parameters in the model were expressed in terms of initial bulk density and/or porosity. It was demonstrated that the functional form of the parameters could be used to retrieve stress-strain response and crushability map, to parametrically represent the crushability and residual crushability. (4) A new data reduction procedure for high strain rate test on foams using polymer split Hopkinson pressure bar (SHPB) based on a novel iterative deconvolution algorithm to correct attenuation and dispersion including viscoelasticity characterization and data reduction was developed and validated. (5) High strain rate behavior of foams in comparison with quasistatic response was conducted. The failure modes and strain rate sensitivity parameter were analyzed and presented. |
