Modeling thermal residual stresses in composite patch repairs during multitemperature bonding cycles

Generally, thermal residual stresses in composite patch repairs can be reduced by lowering the cure temperature of the adhesive used. However, lowering the cure temperature requires a longer curing time to achieve the fully developed performance of the repair. The total curing time can be shortened by using a multistep cure-temperature cycle. In modeling of the cure cycle, a cure kinetic model was established first based on the result of differential scanning calorimetry tests that were performed to estimate the development of the degree of cure of the adhesive. Because stress relaxation depends on the degree of cure of the adhesive, this kinetic model was used in conjunction with the Maxwell viscous model to estimate stress relaxation in the adhesive during the curing cycle. Thus, the model has the capability of estimating the thermal residual stresses in a composite patch repair undergoing a multistep curing schedule. Thermal residual stresses induced by a cure cycle were quantified by delta-T(eff) defined as the temperature difference between the stressfree temperature and room temperature. This model was used to establish a guide for selecting an efficient two-step cure cycle, which yields a delta-T(eff) lower than that of the cure cycle recommended by the adhesive manufacturer.