This high-speed photography image recorded a very special fracture mechanics phenomenon: two fast cracks (as demonstrated by two shear shock waves) just met at the specimen center. After a steel projectile hit a model sandwich plate (steel/transparent Homalite -100 polymer/steel), stress wave propagation was observed in the form of photo-elasticity fringe movement. Two interfacial cracks from the two ends of the model sandwich plate, entered the field of view with very high speeds (> 1400m/s) and formed two shock waves (since the crack tip speed exceeded the shear wave speed of the polymer). For further technical details and more photos, click here to read the related paper (Xu and Rosakis, IJSS, 2002) For more real movies recorded from a high-speed camera( click here). It will take a few minutes to access my movie site since the size of each movie is quite large. But the movie resolution and layout from my site is much better than the movie from YouTube (below). © Dr. L. R. Xu (Vanderbilt University) and Dr. A. J. Rosakis (California Institute of Technology)