Abstract

The objective of the program discussed in this paper was to develop a method, using photoelasticity and low-modulus materials, for studying dynamic stress distributions. Early in the program a number of low-modulus materials were studied and Hysol 8705 (a urethane-rubber compound) was selected as the most promising. A complete study of its mechanical and optical properties was made under static and dynamic loadings. It was established, that Poisson’s ratio ν is independent of rate of loading, the stress fringe value fσ is independent of rate of loading for strain rates greater than 8 in/in/ sec, and both the modulus of elasticity E and the strain fringe value fϵ were dependent oil the rate of loading. The specific energy loss for the material was about 10 per cent for the stress ranges associated with photoelastic determinations. Experimental observations of photoelastic fringe patterns in a rectangular strut subjected to axial impact were made to illustrate the potential of the method. Three different end conditions were imposed on the unloaded end of the strut: (a) A free end normal to the axis; (b) a fixed end normal to the axis; and (c) a free end inclined 45 deg to the axis. For the cases where the ends were normal to the axis it was found that the fringes followed the same law of reflection as the law for stresses given by elementary wave theory.

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