Insights into the formulation of PU foam and its mechanical behavior under quasi-static and impact loads

Abstract

In today’s world, cellular materials find extensive use in many engineering applications including being deployed as energy?absorbing countermeasures for impact safety protection. Polymeric foam is a popular category of such a material. A particularly common type of polymeric foam is polyurethane (PU) foam which is the subject of study here. Driven by the versatility of PU foam in terms of engineering attributes, the ease of fabrication including moldability, and relative environment?friendliness apart from the issue of recyclability, a great deal of research is still being done on this material for gaining deeper understanding of its behavior and improvement of chemical and manufacturing processes. The present research is a contribution in this direction and has thrown light on the following aspects of PU foam especially from the perspective of information available—or more appropriately the lack of same—in published literature: • Clarification of viscoelastic, high resilient and semi?rigid categorization of PU foam on the basis of load?displacement responses including unloading • Development of a novel mechanical strategy for arriving at the desirable mixing ratio of principal reactants (i.e., a polyol and an isocyanate) of a PU foam by optimizing its mechanical properties and corroborating the effectiveness of the approach with microstructure?based findings and recommended stoichiometric ratios from supplier of foam chemicals • Demonstrating the monotonic dependence of foam mechanical properties on effective density for a given mixing ratio that can be valuable for ascertaining the foam of right density for a mechanical design application • Characterization of rigid PU foam of a given density formed with the optimum values of density is achieved for desirable mixing ratio and the same is used in the subsequent analysis under quasi?static compressive and tensile loading conditions • Development of a novel methodology for quantification of the strain?rate sensitivity of the yield stress of a PU foam by combining UTM?based tests at low strain rates and drop?weight impact tests at medium strain rates relevant for day?to?day impact protection applications such as for higher occupant safety in car collisions • Development of a finite element modeling procedure for advanced contact?impact simulations using the explicit LS?DYNA solver by capturing the effect of strain rate on foam strength in a suitable constitutive model for crushable foam • Utilization of foam as a core material for sandwiched construction with jute?polyester laminates, and as a matrix for a reinforced composite with PVC tubes, giving rise to quantum increase in load?carrying and energy?absorption capacities of plain foam or of the individual reinforcing materials