Measurement of Opening Displacement near Rapidly Propagating Crack Tip in PMMA

Pulsed holographic microscopy was applied to take an instantaneous microscopic photograph of a rapidly propagating crack in PMMA. The crack was in the opening mode and its speed was at several hundred m/s. The crack and craze opening displacement, which are both called COD, were measured as a function of distance r from the craze tip within the region of r up to 8mm. The crack opening displacement was nearly proportional to √r, while the craze opening displacement deviated from the curve of √r. The crack tip opening displacement (CTOD) was determined from the measured COD data, and the dynamic stress intensity factor KI(v) was also obtained. Furthermore, the value of J-integral was calculated by assuming small scale yielding. It happens that cracks which have the same KI(v) value and crack speed show different COD profile in the craze layer.