THE EFFECT OF COOLING RATE ON DEFORMATION AND FRACTURE IN IM6/PEEK COMPOSITES

A series of tensile tests have been undertaken on IM6/PEEK (poly ether ether ketone) in order to assess the effect of varying the rate of cooling on its short- and long-term fracture properties. Tests on (90°) laminates have shown that below the glass transition temperature of the PEEK matrix the tensile mechanical properties of the composite are not influenced by the microstructure of the matrix. Above the matrix Tg, however, varying the percentage degree of crystallinity results in a change in both the short and long-term properties of the composite. Here, the slower-cooled laminate offered a superior ultimate stress and an improved creep resistance. Conversely, in the (+/-45°) composites significant cooling rate effects were observed only at lower temperatures. Optical micrographs taken from these specimens indicated that the fast-cooled laminate incurred considerably more internal cracking and delamination during the course of testing. It is believed that such volume damage results from the large thermal strains induced by the rapid cooling process. This information suggests that fast cooling, a procedure that is often encouraged in order to control the level of crystallinity, may be more detrimental to the mechanical properties than slow cooling. © 1990.