STUDY OF CRACK-TIP MOTION IN DYNAMIC FRACTURE BY MICROSCOPIC TIME-RESOLVED HOLOGRAPHY

A novel laser system has been designed and developed for the time-reso lved holographic analysis of dynamic crack propagation in thin (150 mu m) glass plates. The system provides a frame-to-frame resolution of f rom 28.3 to 161.6 ns, with a spatial resolution of better than 25 mu m . Holographic analyses were conducted using both diffuse and direct la ser illumination, demonstrating a broad range of applicability for thi s method. A series of specimens with pre-crack lengths of from 3.5 to 10.5 mm were tested using various pre-load levels and crack-growth-ini tiating explosive sizes. All specimens exhibited similar dynamic fract ure behavior, particularly those outside of the region of shock of the initiating explosive (those of the longer pre-crack set). A best-fit to the theoretical prediction for mode I crack growth revealed a good match to the case for a specimen 'overloaded' by 64 percent. However, no variation with applied load was evident, indicating that an immedia te acceleration to a velocity of approximate to 1200 m/s (near the emp irically determined terminal velocity) was the more plausible fit. Sin ce the terminal velocity was approximately the same for all specimens, this investigation also demonstrated that the terminal velocity is in dependent of crack-tip stresses, even for the most extreme explosive p ulses.