Viscoelastic effects related to residual thermal stresses in bismaleimide/carbon fiber composite laminates

Abstract This paper studies the effects of the residual stresses which originate in the laminates made of composite materials owing to the curing process, and on stresses affected by time and temperature. In order to sustain high operating temperatures, the polymer composites' laminates are made of high performance matrices that usually require processing cycles at high temperatures. The strong thermal variation due to the subsequent cooling process induces residual stresses, owing to the thermoelastic orthotropy of the material. For non‐symmetrical stacking sequences, these stresses notably modify the shape of the laminate and reduce both its static strength and buckling load, as a consequence of the latent content of elastic energy stored by the pre‐loading. The polymeric composites show a viscoelastic behavior, which implies a constitutive law dependent on both time and temperature. For this reason, the analysis of the phenomenon should account for the creep and the relaxation mechanisms. The configuration of asymmetric laminate is evaluated by both analytical and numerical finite element (FEM) methods. The actual shape of laminates of different geometries is also experimentally determined. On the basis of the analysis of the shape of laminates measured after different thermal cycles, the evolution of residual stresses with time and temperature is analyzed.