Dynamic Fracture Instability: Monitoring Crack Velocity with Analog Electronics

This lecture explores the use of a simple analog electronic circuit to measure crack speed in dynamic fracture experiments, reaching speeds close to 1,000 m/sec. The instructor discusses the background theory of crack propagation, the experimental setup involving a Wheatstone bridge and high-speed op-amp, and the use of profilometry to analyze crack surface features. Discrepancies between theoretical predictions and experimental data on crack tip speeds are highlighted, showing unexpected spikes and accelerations. The critical transition to unstable crack propagation is examined through cross-correlation of velocity with surface profilometry, revealing a uniquely valued critical velocity. The lecture concludes by emphasizing the importance of monitoring crack velocity fluctuations for a deeper understanding of dynamic fracture behavior.