Theoretical and Experimental Studies on the Propagation of Longitudinal Elastic Strain Pulses in Wide Rectangular Bars

Citation

Jones, Orval Elmer (1961) Theoretical and Experimental Studies on the Propagation of Longitudinal Elastic Strain Pulses in Wide Rectangular Bars. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/KXTS-1Y59. https://resolver.caltech.edu/CaltechETD:etd-06142006-103619

Abstract

The method of birefringent coatings is considered as an experimental means of studying wave propagation phenomena in metals. Experiments using the Ellis ultra-high-speed camera and a rectangular aluminum bar having a birefringent coating of PhotoStress indicate that transient fringe patterns resulting from impact can be successfully photographed using existing film and light sources at exposure times of 0.05 microseconds. Pronounced fringe curvatures, indicating the warping of plane sections, were observed in the pictures during the passage of the strain pulse. The propagation of a longitudinal elastic strain pulse in a wide rectangular bar is considered on the basis of the approximate plane-stress equations of motion. Asymptotic expressions are obtained which, for large distances of travel, describe the propagation in a semi-infinite strip with stress free lateral edges, subject to the conditions that a uniform normal stress with a step-function time dependence is applied to the end and that the end undergoes no lateral extension. These expressions are shown to qualitatively predict the warping of plane sections observed in the high-speed pictures and in the dynamic photoelastic pictures obtained by other investigators. Measurements using conventional measuring techniques are described in which wide rectangular aluminum bars of several thicknesses were subjected to a step-function pressure loading produced by a shock tube. Comparisons show that the gross features of the experimental records for the head of the pulse are qualitatively predicted by the plane-stress theory. Both theory and experiment show that short-wavelength second mode disturbances arrive very early. Experimentally it is observed that these disturbances are accompanied by thickness mode activity which cannot be accounted for by the two-dimensional plane-stress theory.

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