Explosively-driven fracture in polymethyl methacrylate (PMMA)*

Explosively-driven fracture of polymethyl methacrylate (PMMA) was explored using high-speed shadowgraphy and Photon Doppler velocimetry (PDV). Monolithic 0.3 x 0.3 x 0.3 m cubes of PMMA were machined so that RP-80 electronic bridgewire detonators could be positioned in the middle of the cube. Experiments measured fracture produced with one and two simultaneously-initiated detonators. The cubes were subjected to 2MPa of uni-axial stress in both the vertical direction and horizontal directions and baseline test with no external stress. The particle velocity history on one surface, aligned with the detonators, was recoded using PDV and was compared to the shock response recorded in the high-speed videos. The high-speed video records were then analyzed to extract the fracture front, which was related to a fracture radius and area in the samples. Post-test images of the PMMA cubes aided in the determination of three-dimensional effects not directly imaged by the cameras. The results show that the fractures are primarily driven by the high pressure gases produced by the explosive detonation, and the fracture propagation is minimally affected by wave interactions. The fracture fronts are observed to change direction to align with the externally applied stresses. The application of external stress results in shorter fracture lengths.