Analysis and computations have been performed by the Utah CSM group to support experimental investigations of unvalidated assumptions in plasticity theory. The primary untested assumption is that of a regular flow rule in which it is often assumed that the direction of the inelastic strain increment is unaffected by the total strain increment itself. To support laboratory testing of this hypothesis, the general equations of classical plasticity theory were simplified for the case of axisymmetric loading to provide experimentalists with two-parameter control of the axial and lateral stress increments corresponding to a specified loading trajectory in stress space. Loading programs involving changes in loading directions were designed. New methods for analyzing the data via a moving least squares fit to tensor-valued input-output data were used to quantitatively infer the apparent plastic tangent modulus matrix and thereby detect violations of the regular flow rule. Loading programs were designed for validating isotropic cap hardening models by directly measuring the effect of shear loading on the hydrostatic elastic limit.

**UofU Contributors/collaborators:**

Michael Braginski (postdoc, Mech. Engr., UofU)

Jeff Burghardt (PhD student, Mech. Engr., UofU)

**External collaborators/mentors:**

Stephen Bauer (Manager, Sandia National Labs geomechanics testing lab)

David Bronowski (Sandia geomechanics lab technician)

Erik Strack (Manager, Sandia Labs Computational Physics)

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This post needs eye-catching graphics — perhaps add some images from the Sandia LDRD project on this topic?