No abstract

Dynamic tensile-splitting experiments were conducted on brittle polyester disks, manufactured from Homalite ®-100, to photoelastically visualize stress field development in the specimens and, ultimately, specimen fracture. A Split Hopkinson Pressure Bar (SHPB) was used to load the specimen dynamically and record load data as a function of time. Images of the stress field were captured with high-speed photography and interpreted using photoelastic analysis. The experiments determined that the specimens reached an equilibrium state relatively quickly, and remained in equilibrium until fracture. Fracture began with in-plane cracks forming on either side of the specimen. Transverse cracks emanated from the in-plane cracks, then propagated across the specimen mid-plane at velocities up to 60% of the shear wave velocity in the material. Contact loads calculated from photoelasticity agreed favorably with SHPB results. These experiments also allowed for the determination of the dynamic splitting strength of the Homalite ®-100 as a function of strain rate. The dynamic splitting strength of the material increased with the increasing strain rate, reaching a maximum value of about twice the static splitting strength.

Static and dynamic compression experiments were performed on concrete and granite specimens with various levels of induced damage. Damage was induced into the specimens by repeated impacts from a falling weight and quantified as a measure of damage crack surface area per volume using a statistical microscopy technique. The static experiments were performed following ASTM standard procedures. The static compressive strength of both materials decreased with increasing levels of damage. The reduction in strength is due to the induced damage causing the activation and propagation of failure cracks in the specimens. The dynamic experiments were conducted using a 50.5 mm diameter Split Hopkinson Pressure Bar. The undamaged dynamic compressive strengths of both the concrete and granite were approximately twice their static values at average strain rates of 380/s and 400/s, respectively. However, as the damage levels were increased, the dynamic compressive strength remained unchanged. For the dynamic experiments, the strain energy is stored in the specimen faster than the cracks can develop and coalesce to completely fail the specimen. Therefore, the relatively small amounts of induced damage did not affect the dynamic compressive strength.