Precise evaluation of fast fracture velocities in acrylic polymers at the slow-to-fast transition

Transient fast fracture velocities at the onset of the slow-to-fast transition in polymethylmethacrylate (PMMA) have been measured precisely in single-edge-notched specimens of various geometries by using ultrasonic fractography. Little sign of the fracture velocity of the order of 10° m sec^–1 have been detected on the instability onset. The fast fracture starts from a point source almost instantaneously. The initial velocity has been shown to fall in a small range, 90 to 150 m sec^–1, almost independent of the loading speeds from 0.1 to 100 mm min^–1 and specimen temperature from –50 to 40° C, with exceptional cases for specimens loaded slowly (0.1 mm min^–1) at a low temperature (–50° C). As the final minimum velocity of an arresting crack, a value of 42 m sec^–1 has been obtained under room temperature. Crack propagation in low molecular weight PMMA has been shown to be more temperature, as well as loading rate, dependent than in higher molecular weight PMMA.