High strain rate testing of kidney stones

Sections of struvite kidney stones were tested in compression at high strain rates ( approximately 3000s(-1)) using a Kolsky bar and at low strain rates ( < 0.001 s(-1)) using an Instron testing machine. The peak stress in both cases appeared to be similar. At high strain rates the values of flow stress measured were between 40 and 65 MPa and at low strain rates they were between 37 and 58 MPa. However, the morphology of the damage was dramatically different. Stones tested at low strain rates formed a small number of cracks but otherwise remained intact at the end of the test. In comparison, stones tested at high strain rates were reduced to a powder. Kidney stones are a two-phase material consisting of a crystalline ceramic phase and an organic binder. We speculate that in the high strain rate tests the large difference in the sound speed between the matrix and the crystalline grains leads to shear stresses that destroy the stone. These data indicate that shear stress induced by the internal structure may be a mechanism by which shock waves comminute kidney stones in lithotripsy.