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Uniform axial deformation is a basic assumption in element testing, where axial strain typically is determined from displacement measurements. In applying force through rigid platens, however, some rotation may occur such that the fundamental stress field is perturbed. Thus, nonuniformity among measures of axial displacement may be present, and the response may consist of a component due to uniform deformation and a component due to rotation. To estimate this nonuniformity for cylindrical specimens, at least three sensors are needed, and in general, the axial strain cannot be estimated without error from only two sensors. For equiangular placement of three sensors, it is shown that the mean of the displacement readings is equal to the displacement from the uniform component. The rotation does not affect the average value, but the rotation must be calculated to estimate the nonuniformity. The ratio of the measured maximum and minimum displacement does not provide an objective evaluation.

The constitutive model used to describe deformation of crushed salt is presented in this paper.Two mechanisms -dislocation creep and grain boundary diffusional pressure solutioning -are combined to form the basis for the constitutive model governing deformation of crushed salt. The constitutive model is generalized to represent three-dimensional states of stress. Recently completed creep consolidation tests are combined with an existing database that includes hydrostatic consolidation and shear consolidation tests conducted on Waste Isolation Pilot Plant (WIPP) and southeastern New Mexico salt to determine material parameters for the constitutive model. Nonlinear least-squares model fitting to data from shear consolidation tests and a combination of shear and hydrostatic tests produces two sets of material parameter values for the model. Changes in material parameter values from test group to test group indicate the empirical nature of the model but show significant improvement over earlier work. To demonstrate the ' predictive capability of the model, each parameter value set was used to predict each of the tests in the database. Based on fitting statistics and ability of the model to predict test data, the model appears to capture the creep consolidation behavior of crushed salt quite well..

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