Post-repair residual stresses and microstructural defects in wind turbine blades: Computational modelling

Wind turbine blades require regular maintenance and repair. The post-repair reliability and lifetime depend on the quality and method of repair. Residual stresses present in the repaired sections of the blades are one of the reasons for reduced lifetime and premature failure of repaired blades. Residual stresses occur due to the thermal cycle used to cure the composite repair patch or adhesive and chemical shrinkage during curing. The presence of voids further aggravates the magnitude of the residual stresses. To investigate these effects, analytical models are used to study the cure kinetics and void evolution while the development of residual stresses during the post-gelation period of curing and the effect of the voids are obtained using coupled temperature-displacement analysis in ABAQUS. User subroutines (UEXPAN, HETVAL and UMAT) are used to define the chemical shrinkage, heat generation due to curing and cure-dependent modulus. Both hard and soft patch repair models are considered. It is found that the temperature cycle used affects both the time taken for complete cure and the residual stresses developed in the adhesive. Increasing the first dwell temperature decreases the time to cure but increases the residual stresses. Also, the presence of voids is found to cause stress concentration regions, with stresses increasing up to 2 to 2.5 times the average stresses. Therefore, a balance is needed to minimise the time taken for the repair process while maintaining residual stresses as low as possible, especially in wind turbine repair applications.