STUDY OF DYNAMIC BIREFRINGENCE AND STRAIN PRODUCED IN TRANSPARENT POLYMERS BY MECHANICAL IMPACT

摘要

TO obtain basic information about the mechanical and optical response of polymers, the stress-wave propagation in transparent plastics is being studied at the U. S. Army Natick Laboratories by observation of the dynamic strain and birefringence produced by mechanical impact. Instrumentation and techniques have been developed to achieve the following: synchronization;projectile align¬ment;intense, monochromatic illumination;and approximate one-dimensional wave propagation in the impacted specimen. Observations were made of the com¬parative wave shapes and velocities in two dissimilar polymers (CH.-39 and polyvinylbutyral). Changes in wave characteristics were noted during crossing of an interface between these materials.nIn this initial study, the dynamic strain-fringe constant of CR-39 plastic was studied over a range of apparent strain rates between 2, 300 and 30, 600 per¬cent strain per second by the simultaneous measurement of the strain and bire¬fringence occurring during the nearly linear leading edge of the strain pulse. Over this dynamic range no distinct dependence on the strain rate could be dis¬cerned. However, comparison of the average dynamic strain-fringe constant with the corresponding static value indicated a slight rate dependence over this larger total range of apparent strain rates.