Role of Shear and Longitudinal Waves in Stone Comminution by Lithotripter Shock Waves

Abstract

Mechanisms of stone fragmentation by lithotripter shock waves were studied. Numerically, an isotropic‐medium, elastic‐wave model was employed to isolate and assess the importance of individual mechanisms in stone comminution. Experimentally, cylindrical U‐30 cement stones were treated in an HM‐3‐style research lithotripter. Baffles were used to block specific waves responsible for spallation, squeezing, or shear. Surface cracks were added to stones to simulate the effect of cavitation, and then tested in water and glycerol (a cavitation suppressive medium). The calculated location of maximum stress compared well with the experimental observations of where cracks naturally formed. Shear waves from the shock wave in the fluid traveling along the stone surface (a kind of dynamic squeezing) led to the largest stresses in the cylindrical stones and the fewest shock waves to fracture. Reflection of the longitudinal wave from the back of the stone — spallation — and bubble‐jet impact on the proximal and distal faces of the stone produced lower stresses and required more shock waves to fracture stones, but cavitation stresses become comparable in small stone pieces. Surface cracks accelerated fragmentation when created near the location where the maximum stress was predicted.