Effect of Pre-Cycle Stresses on Fatigue Life of Plastic Laminates Reinforced with Unwoven 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. The laminates were made of three types of resin and two orientations of fibers: =5 , and at 0 and 90 degrees to the principal axis. S-N curves are presented for these laminates from 1/4 cycle to 10 million cycles. Five replications of tests are presented to establish median fatigue lives at specific stress levels. The three specific stress levels were at approximately 40, 60, and 80 percent of their ultimate strength. In general, the results show that if these laminates are subjected to one or three cycles at the 80 percent level and then cycled to failure at the 40 or 60 percent level, the first cyclic stress does not significantly affect the fatigue life at the second stress level. Exceptions appear in the report. The results also show, for three of the five materials, that if these laminates are subjected to a program of cycles at the 40 percent stress level (equal to a small portion of their normal life) and then cycled to failure at the 60 percent stress level, the first cyclic stress significantly extends the fatigue life at the second stress level. For the other two of the five materials, the first cycles produced some damage but not a significant amount. (Author)