Experimental Study of the Interrelation between the Theory of Dislocations in Polycrystalline Media and Finite Amplitude Wave Propagation in Solids

Abstract

By use of experimental data from a large number of free flight impact tests involving diffraction grating measurement of strain, a detailed and very close experimental verification is given for the G. I. Taylor theory of dislocations in polycrystalline annealed aluminum. Applying the finite amplitude wave theory, with a theoretical parabolic stress-strain curve obtained from dislocation theory, experimental and theoretical correlations within approximately 1% are obtained for the velocities of strain propagation, maximum strain, largest distance of penetration of maximum strain, dynamic stress levels in the first diameter, time of contact, coefficient of restitution, mushrooming in the first diameter, and an energy balance before and after impact. Variations in the plastic wave development and dynamic stress behavior below the Karman critical velocity are considered theoretically and experimentally in terms of a difference in the initial elastic reflection behavior below the critical velocity. Plastic deformation is found to be independent of strain rate, even at experimentally determined values of 2000 in./in./sec. Initial shock fronts exceeding the dilatational velocity are experimentally identified near the impact face. The subsequent development of dispersive plastic wave fronts following the passage of the initial shock, also are found to be in accordance with predictions based upon dislocation theory. The equipartition of energy in the vicinity of the impact face has been found to be related to the reflection behavior of these initial shock fronts at the lateral surface of the rod.