Polyaxial Yielding of Granular Rock

Results from true triaxial testing on a granular dense marble are analyzed with emphasis on the incremental relations during strain-hardening (initial yield to peak strength) at constant intermediate and minor stresses. The strong dependency of the behavior on the value of intermediate principal stress is confirmed. Consistency of experimental flow rule was revealed for the examined yield states corresponding to initial yield, to a given value of plastic strain invariant, and to peak strength. A stress invariant and the corresponding one of plastic strain increment were almost equal, postulating a simple stress strain relation. The vector of plastic strain increment and the normal vector on the yield surface at the relative point formed an angle ranging from 10 to 23 degrees. On the basis of the preceding observations, a work balance equation, the resulting flow rule, and yield equation were extended to model three-dimensional behavior. Reasonable agreement was obtained between the predicted and experimental stress-strain relations.