Uniformity of axial displacement in element testing

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.