VISCOELASTIC EFFECTS RELATED TO RESIDUAL THERMAL-STRESSES IN BISMALEIMIDE CARBON FIBER-COMPOSITE LAMINATES/

This paper studies the effects of the residual stresses which originat e in the laminates made of composite materials owing to the curing pro cess, and an stresses affected by time and temperature. In order to su stain high operating temperatures, the polymer composites' laminates a re made of high performance matrices that usually require processing c ycles at high temperatures. The strong thermal variation due to tile s ubsequent cooling process induces residual stresses, owing to the ther moelastic orthotropy of the material. For non-symmetrical stacking seq uences, these stresses notably modify the shape of the laminate and re duce both Its static strength and buckling load, as a consequence of t he latent content of elastic energy stored by the pre-loading. The pol ymeric composites show a viscoelastic behavior, which implies a consti tutive law dependent on both time and temperature, For this reason, th e analysis of the phenomenon should account for the creep and the rela xation mechanisms, The configuration of asymmetric laminate is evaluat ed by both analytical and numerical finite element (FEM) methods. The actual shape of laminates of different geometries is also experimental ly determined, On the basis of the analysis of the shape of laminates measured after different thermal cycles, the evolution of residual str esses with time and temperature is analyzed.