A STUDY OF THE TWO-DIMENSIONAL IMPACT RESPONSE OF CRUSHABLE FOAM

Abstract

An experimental and numerical study of the two-dimensional response of crushable foam to low velocity impact is undertaken. Rigid polyurethane foam blocks are subjected to normal and oblique impact by gravity-driven impactors of different geometries, at velocities ranging from 2 m/s to 4 m/s. The impactors comprise a rectangular block, a wedge-tipped block and a cylinder. Quantities measured during impact are the impactor deceleration, velocity, displacement and energy dissipated. The effects of impact velocity and impactor geometry on the deformation induced and energy absorbed are identified. Experimental results show that resistance to deformation is dictated by the projected contact area and less dependent on the geometry of the impactor nose. The energy dissipated exhibits a linear relationship with the volume of material crushed, regardless of impactor geometry; the latter only has some influence on the slope, which varies from 1.3 to 1.6x 105 J/m3. The essentially linear dependence of energy absorbed on material crushed facilitates a priori estimation of the amount of foam required to dissipate a given impact energy. Some strain rate effect is evident from variations in the initial peak decelerations for impact by the flat-ended impactor, the ensuing deceleration plateau level and energy absorbed. However, strain rate effects are small for the range of impact velocities in the present study. Visual examination of specimens after impact shows that the resulting deformation profile is governed by impactor geometry and the impactor nose profile determines the occurrence of splitting. A common feature for all impactors is that gross deformation is confined to their immediate vicinity and there is a well defined boundary with undeformed material. In conjunction with the experimental study, a two-dimensional numerical model is proposed to simulate the gross deformation induced in the impact process. It employs a lumped mass approach and is formulated in terms of finite deformation. Appropriate equations of motion, stress strain relations, a material failure model and contact algorithms are developed. Results generated by this model exhibit good correlation with experiments, thus substantiating its validity. The proposed model demonstrates advantages over traditional finite element approaches, in that it accommodates severe deformation and extensive structural failure without the problems of excessive mesh distortion and untenable time step reduction which accompany finite element simulations. Limitations of the numerical model are that it does not account for the strain-rate sensitivity and assumes an uncoupled constitutive relationship and simple material failure model. Refinements of the numerical model in these respects are envisaged to yield improvements in the representation of actual physical response and widen the scope of applicability.