Evaluating absolute specimen straining from clip gage extensometers in confining pressure varying tests

摘要

Tests in triaxial cells are usually performed on cylindrical specimens, for which axial and circumferential devices are required. A clip gage clips onto the specimen at distinct reference points and integrates the observed straining over a reference (gage) length. Since the clip gages are installed in-vessel and are subjected to the same confining pressure variations of the test procedure, their elastic response and other sensor element related deviations mask the displacement readouts. This paper proposes a model to compensate these effects in the test registers that can be implemented in the test machine controller, so that any controller action might benefit from real strain measures. The model consider each clip gage element separately to account for its mechanics. It includes a jacket (membrane) model to account for its effects on the circumferential strain readings. A procedure to evaluate the clip gage parameters is presented, as well as a fit example. The final model is versatile, as it may be applied to any test setup using a single set of clip gage parameters.