An Image-Based Inertial Impact Test for the High Strain Rate Properties of Brittle Materials

Abstract

Testing ceramics at high strain rates presents many experimental difficulties due to the brittle nature of the material being tested. When using a split Hopkinson pressure bar (SHPB) for high strain rate testing, adequate time is required for stress wave effects to damp out. For brittle materials, with small strains to failure, it is difficult to satisfy this constraint. Thus, most available high strain rate data for ceramics focuses on using the SHPB for strength testing in compression. Due to the limitations of the SHPB technique, there is minimal data on the stiffness and tensile strength of ceramics at high strain rates. Recently, a new image-based inertial impact (IBII) test method has shown promise for analysing the high strain rate behaviour of brittle materials. This test method uses a reflected compressive stress wave to generate tensile stress and failure in an impacted specimen. Throughout the propagation of the stress wave, full-field displacement measurements are taken. Strain fields and acceleration fields are derived from the displacement fields. The acceleration fields are then used to reconstruct stress information and identify the material properties. The aim of this study is to apply IBII test methodology to analyse the stiffness and strength of ceramics at high strain rates. Preliminary results have shown that it was possible to use the IBII test method to identify the elastic modulus and strength of tungsten carbide at strain rates on the order of 1000/s. For a tungsten carbide with 13% cobalt binder the elastic modulus was identified as 520 GPa and the tensile strength was 1400 MPa at nominal strain rate of 1000/s. Further tests are planned on several different grades of tungsten carbide and other ceramics including boron carbide and sapphire.