Effect of Single-Step Change in Stress on Fatigue Life of Plastic Laminates Reinforced with Unwoven 'E ' Glass Fibers

摘要

The fatigue life of five types of plastic laminates, made of Scotchply epoxy resin and unwoven E glass fibers, was measured at two stress levels after damage had been programed at either higher or lower stress levels. In general the results show that: (1) If these laminates are subjected to one or three cycles at the 80 percent stress level and then cycled to failure at the 40 or 60 percent stress level, the first cyclic stress does not affect the fatigue life significantly at the second level. Exceptions occur and are reported. (2) If these laminates are subjected to a cycle life where about 10 percent failures would probably occur at the 80 percent stress level and then cycled to failure at 40 or 60 percent stress level, this first cyclic stress significantly damaged 30 percent of groups tested. (3) If a relatively few number of cycles are programed at the 40 percent stress level and then cycled to failure at the 60 percent stress level, the first cyclic stress significantly extended the life at the second stress level for three of the five materials. (4) If, however, the laminates were subjected to a cyclic life which would cause about 10 percent failures at the 40 percent level and then run out at a higher stress level, the first cyclic stresses caused no significant difference in median life for four out of five materials.