Stress and strain fields are generally assumed homogeneous in high strain rate experiments. Therefore, only mean stress and strain are measured. However, this assumption is not valid for soft materials and/or very high strain rates. For heterogeneous dynamic tests, inverse techniques are usually proposed to identify the stress and strain fields. In this paper, a non-parametric method to identify the stress field is presented. This method is based on a force measurement on the specimen boundary and a displacement (strain) field measurement carried out by a digital image correlation technique. It is applied to measure the strain rate sensitivity of a synthetic rubber (Criblex 80). This new technique estimates the stress field with good accuracy.

International audienceStress and strain homogeneity, in tested samples, is a crucial assumption during a dynamic test. Whenever this assumption is true, the conventional recovery of the mean strain and stress is valid. However, when the stress and strain fields in the sample are not homogeneous, more sophisticated treatment must be considered. Inverse problem techniques are then proposed. Nevertheless, they may yield a non-physical result. In this paper, a non-parametric solution to the problem of non-homogeneity in dynamic tests is presented. The stress field is deduced from the displacement field measured via a Digital Speckle Photography (DSP) technique and a force boundary condition

Keywords:High strain rate Soft material Stress heterogeneity Elastomer Hopkinson bar Because of their low mechanical wave speed, high strain rate testing of rubber is highly difficult. Indeed, stress and strain homogeneity is hard to achieve. In this paper, a semi-analytic inverse solution is proposed. This solution is based on a uni-axial stress state assumption in the specimen. Moreover, a new-Hookean law is assumed for rubber. The new method is successfully applied to a high strain rate test on a synthetic rubber.