Experimental Determination of the Transient Uniaxial Stress in a Bar by Dynamic Photoplasticity

The transient compressive stress in a slender bar composed of a rate-dependent photoplastic material, subjected to uniaxial impact, has been determined by dynamic photoplasticity and compared to a finite-difference solution of the uniaxial wave equation. A mechanical constitutive equation based upon tests on this substance, a styrene-polyester copolymer, over eight decades of strain rate was employed in this analysis. The experimental arrangement provided for the impact of a pneumatically driven aluminum rod on a pressure bar which had an 8-in. bar of the copolymer in contact at its distal end. The stress pattern, reaching amplitudes of 7000 lb/in.², was observed by means of filtered polarized light emanating from a Xenon flash tube and was recorded by a high-speed framing camera. The experimental results and the predicted numerical values showed close correlation when the impact velocities were properly matched. Comparisons could be effected only during the loading process since constitutive relations for the material were not obtained for unloading. The dynamic photoplastic data exhibited an oscillation behind the wave front which was attributed to lateral inertia effects.