and bottom surfaces 334 336 the specimens 312 are comprised of rock or stone material from larger rock specimens of the type found in egs reservoirs oil or gas wells they are machined or core drilled to shape and sized to fit within the test chamber 308 in the example of fig1 a blind hole 340 mimics a stimulation well during testing the hole 340 is filled with working fluid f and is subject to cavitation by controlling the opening frequency and duration of the control valve s in the example of fig1 a series of artificial flaws 342 are included in the side surface 338 here artificial flaws 342 such as cracks or fissures are introduced into the side surface 342 with a band saw a water jet or other cutting device to simulate an existing crack structure and or to assist the initiation of crack stimulation in the examples of fig1 15 a shell body 344 is defined by a generally circular top surface 334 a circular bottom surface 336 and a convex side surface 338 joining the top and bottom surfaces 334 336 an aperture 346 having an interior surface 348 is disposed through the shell body 344 and is defined by a circular opening in the top and bottom surfaces 334 336 a separate core body 350 is defined by a.