Abstract
Effect of hydrostatic pressure was studied on the fracture velocity and morphology of cracked PMMA tensile plate specimens (108mm×30mm×5mm) of different molecular weights: Mv=2.3×106 and 2.0×105. Ultrasonic fractography was employed to precisely measure fracture velocity under a pressure of less than 150MPa; fracture instability occurred only under pressure of less than 10MPa. Fracture velocity, monitored in the middle part of a specimen, decreased with increasing pressure up to 50MPa, and above this remained almost constant. The way in which a specimen was chucked affected the velocity decrease. The molecular weight change had no discernible effect on fracture velocity under pressure. There was a morphological change in the fracture of specimens with the high and the low molecular weights loaded at pressures of over 100MPa and 50MPa, respectively; a cleavage type fracture tended to be replaced by a mixed cleavage-non cleavage type, particularly during the initial stage of fracture. The affected area featured many longitudinal fine streaks, each of which was composed of an array of small shear cusps.
