In this study the finite element (FE) predictions of the physical response of foams during impact by a rigid body such as the Hybrid Ill headform is determined by material law equations generally approximated based on the elastoplasticity theory. However, the structural foam aspect, its discontinuous nature, makes it difficult to apply the laws of continuum mechanics and to construct constitutive equations for foam-like material. One part of the problem relates to the state of stress. In materials such as steel, the state of hydrostatic stress does not affect the stress strain behaviour under uniaxial compression or tension in plastic regime. In other words, when steel is subject to hydrostatic pressures the stress strain characteristic can be predicted from a uniaxial test. However, if the stresses acting on a section of foam are triaxial, the response of a headform may be different than predicted from uniaxial test data. The experimental data presented indicate that the state of hydrostatic stress may affect the response of a Hybrid III headform interacting with foam padding. Based on these results and using elastoplasticity theory, an idealized computational material model of foam is generated and used to quantify the state of stress effects. (A)
Abstract