AN OPTICAL METHOD FOR DIRECT MEASUREMENT OF STRAIN IN A TORSIONAL HOPKINSON-BAR APPARATUS

A method has been developed to measure the relative rotation of the flanges of a thin-walled tubular specimen during a torsion test. The method, which is based on the Moir6-fringe technique, is capable of use at the high rates of strain encountered during a Hopkinson-bar test, as well as at low rates of strain. In the application described, the specimen gauge length is very short, but the method could be used for specimens of considerably longer gauge length. Direct calibration of the system is easily carried out at low angular velocities. The method can then be used to measure directly the specimen strain during a Hopkinson-bar test, and thus to check the value derived from measurements of torsional waves in the elastic bars. Results of such comparisons are given, and i t is found that the values given by the two methods agree we & the difference being attributable largely to inaccuracies in the torque measurement. The new method permits the determination of specimen deformation during the later stages of the test when multiple wave reflections render the wave analysis inaccurate. In particular, it has been found that the specimen may be subjected to reversed plastic straining, so that the total plastic strain cannot be determined from the permanent deformation at the end of the test. 1 ImRODUCTION The split Hopkinson-bar system has been widely used t