Dynamic characterization of rigid foam used in finite element sports ball simulations

Abstract

Numerical simulation of sport ball impacts is challenging due to the varied contact conditions involved and the difficulty in characterizing nonlinear materials at high strain rates. The following considers rigid polyurethane foam used in softballs. Past works have shown that load displacement curves from viscoelastic material models do not completely agree with experiment, suggesting incorrect mechanisms of compressive deformation. Additionally, dynamic testing using a pressure bar apparatus was unable to achieve strain rates low enough, and dynamic mechanical analysis was unable to achieve strain magnitudes large enough to represent play conditions.A method was developed to impact polyurethane foam samples over a range of displacement rates and magnitudes representative of play conditions. A characteristic stress–strain loading curve of polyurethane foam was produced and incorporated into a finite element model. Comparison of instrumented and numerical impact properties produced agreeable results. The foam material model was then applied to a simulated softball which impacted surfaces of varying geometry. Results showed that the foam material model better predicted softball deformation mechanisms than previous linear–viscoelastic models both as a function of speed and surface geometry. Persistent discrepancies in rate dependence indicate a lack of complete characterization, however.