Dynamic Strength and Fragmentation Experiments on Brittle Materials Using Theta-Specimens

Characterization of the strength and fragmentation response of brittle materials poses unique challenges related to specimen gripping and alignment. These challenges are often exacerbated when the characterization is to be conducted at elevated strain rates. Tensile strength of brittle samples are often characterized using the Brazilian disk testing geometry. While this ameliorates issues related to specimen alignment, the stress field in the specimen is not uniform, complicating the analysis of the test results. The theta specimen geometry was designed specifically to provide a uniform state of uniaxial stress in the specimen gage section when the exterior of the sample is subjected to compressive loading. Here we evaluate the use of the theta specimen geometry with a compressive Kolsky bar to measure the dynamic tensile strength and fragmentation response of a brittle polymer, Poly(Methyl methacrylate). Finite element simulations are used to investigate the effect of geometry and loading pulse shape on the ability to establish a state of uniaxial stress in the gage section. Particular attention is given the excitation of lateral vibrations in the gage section, which would perturb the desired uniaxial stress state.