A study of the application of photoelasticity to the investigation of stress waves

NOTE: Text or symbols not renderable in plain ASCII are indicated by [...]. Abstract is included in .pdf document. The object of this investigation is to determine the suitability of photoelasticity for the purpose of quantitatively investigating stress waves in solids. Specifically, procedures for determining the dynamic mechanical and optical properties of a common photoelastic plastic, CR-39, were investigated, as well as the techniques for recording dynamic fringe lines. The dynamic mechanical properties of CR-39 were determined from the frequency and decay of free-free longitudinal vibrations of bars. It was found that CR-39 is a viscoelastic plastic whose wave speed, complex modulus, and damping depends on frequency. The stress-strain relationship for CR-39 was found to be slightly nonlinear. The birefringent properties of CR-39 were determined from impact tests in which the fringe order was detected by a phototube, and strain was measured by means of bonded wire strain gages. The results show that CR-39 has a strain-fringe constant of 3.42 x 10[superscript -4] in/fringe [plus or minus] 3%, which is the same as the static value within experimental error. Thus this constant is independent of the rate of loading. CR-39 is not stress-birefringent, which is the usual law quoted for photoelastic plastics. An approximate theory of longitudinal stress waves in elastic bars is derived, which shows that the Boussinesq theory, if used to calculate the stress-fringe constant from sharp impacts, leads to serious error. Using the Ellis camera, strain wave isochromatics can be photographed at framing rates up to 400, 000 per second. An RCA. 1P21 multiplier phototube was found to be extremely sensitive to optical retardation, by using it to detect the time duration of transient strains caused by cavitation. The tube has a frequency response of at least 18 megacycles