Confinement effects on the measurement of the dynamic increase factor of brittle materials

This paper summarises the previous publications on the consideration of strain-rate enhancement of the uniaxial compressive strength of brittle materials. Based on examples of concrete and mortar, it is shown that a significant contribution is made from radial-inertia-induced confinement to the apparent dynamic uniaxial compressive strength from split Hopkinson pressure bar (SHPB) measurements. The dynamic increase factor (DIF) based on these measurements has been published widely and adopted in many design considerations and some numerical codes. Since the confinement effect on the axial compressive strength has been taken into consideration in the quasi-static material model of brittle materials, the neglection of such effects in the interpretation of SHPB testing results and the implementation of measured DIF into dynamic constitutive equations will lead to double consideration of the lateral confinement effects. This is non-conservative for the structural design against impact and blast loads. This paper will give clear recommendation on how to correct SHPB testing results and estimate strain-rate effects on the compressive strength of brittle materials.