RESIDUAL THERMAL-STRESSES IN BISMALEIMIDE CARBON FIBER-COMPOSITE LAMINATES/

This is a study of residual thermal stresses in composite laminates, d ue to curing cycles. An extended formulation of Classical Lamination T heory (CLT) is adopted, which is able to take into account geometrical nonlinear effects owed to finite displacements. The approach is appli ed to square asymmetric laminates that have different dimensions and t hicknesses. Final laminate shapes are evaluated after complete curing cycles; they can be cylindrical or saddle-like, and their equilibrium configuration stable or unstable depending on thickness vs. side lengt h ratio. The same laminates are stacked, press-cured and the radii of curvature experimentally measured. In addition, they are modeled by me ans of finite element method (FEM) codes, using both linear and nonlin ear techniques. Except for the thick laminates, the results obtained u sing nonlinear theoretical and numerical approaches show good agreemen t with experiment. Thermal residual strains are computed from non-mech anical strains by subtracting laminae free thermal deformations; the c orresponding stresses are evaluated through layer stiffnesses. Residua l stresses are evaluated both theoretically and experimentally. For th icker laminates the disagreement is mainly due to a mixed viscous phen omenon which takes place in resin interlaminar layers and matrix intra laminae. The share of relaxed stresses is evaluated and methods that i nclude optimal cooling path techniques are suggested.