Full Field Deformation Measurements in Tensile Kolsky Bar Experiments: Studies and Detailed Analysis of the Early Time History

The very early stages of high rate tensile loading are important when attempting to characterize the response of materials during the transient loading time. To improve understanding of the conditions imposed on the specimen during the transient stage, a series of high rate loading experiments are performed using a Kolsky tensile bar system. Specimen forces and velocities during the high rate loading experiment are obtained by performing a thorough method of characteristics analysis of the system employed in the experiments. The in-situ full-field specimen displacements, velocities and accelerations during the loading process are quantified using modern ultra-high-speed imaging systems to provide detailed measurements of specimen response, with emphasis on the earliest stages of loading. Detailed analysis of the image-based measurements confirms that conditions are nominally consistent with those necessary for use of the one-dimensional wave equation within the relatively thin, dog-bone shaped tensile specimen. Specifically, measurements and use of the one-dimensional wave equation show clearly that the specimen has low inertial stresses in comparison to the applied transmitted force. Though the accelerations of the specimen continue for up to 50 μs, measurements show that the specimen is essentially in force equilibrium beginning a few microseconds after initial loading. These local measurements contrast with predictions based on comparison of the wave-based incident force measurements, which suggest that equilibrium occurs much later, on the order of 40–50 μs .