High-speed photography [166,666 to 333,330 frames/second] has been used to observe and define the pattern of destructive stresses generated by an explosive charge detonated on the end of a right circular cylinder. The loading is somewhat more severe, but not substantially unlike, the loading a drill rod receives during percussive drilling operations. It was found that the internally convex cylindrical surface transforms and focuses the energy of the blow first in the form of a converging tensile wave, which is followed shortly by a similarly converging shear wave. Damage is successively wrought by the two converging waves that arrive one after the other, the damage being due to the shear wave extending further down the cylinder. The diameter of the cylinder regulates the extent of the damage, limiting severe damage to the first three diameters of length.
When a solid cylinder is struck a sharp and strong blow on one end, the end of the cylinder generally suffers severe damage, appearing as if an internal axial explosion had blown it apart. Major damage is usually limited to a distance along the cylinder equal to about 2 or 3 cylinder diameters. A well defined teardrop-shaped symmetrical plug forms in Plexiglas [Fig. 1] and a similar pattern of fracturing is observed in aluminum, steel, glass and copper cylinders. The pattern is controlled principally by the geometry of the specimen rather than the material and scales almost perfectly with cylinder diameter.
Only a small fraction of the energy of the original blow remains in the cylinder, the rest being carried away by the pieces that fly off. This small remaining fraction moves down the cylinder as an elastic wave with rod velocity, [E/p] 1/2, where E is the Young's modulus and rho is the density of the rod material.
Previous tests with essentially point explosive charges, charges whose area of contact was very small compared with the area of the end of the cylinder, have shown that internally reflected and transformed stress waves can cause fracturing when they are focused and thus caused to interact.
The present study differs from previous ones in several respects. The entire end of the cylinder was covered with explosive so that the impulsive load acted simultaneously over a large area. The simple ray-tracing techniques used before are inadequate to describe the wave interactions since the phenomenon must now be treated as a diffraction problem. Damage is much more severe and the intensity of the elastic wave developed with the larger explosive is high enough to make visible the deformation produced during action of the wave. But most important, the present tests were monitored with high-speed photographic equipment up to framing rates of 333,330/second. This made it possible to observe the actual formation of the fractures, the propagation of the waves generated by the explosion and subsequent reflections and, from these observations, develop a better understanding of the genesis of rod waves.
