The Influence of Uneven Temperature Distributions on Local Shear and Interlaminar Fracture Toughness Properties of a Thick Composite Laminate

Abstract

Abstract Because wind turbine blade length has increased over the years, so have the root and spar cap thicknesses. Temperature gradients and residual strains related to the manufacturing process of thick laminates are factors that determine the final laminate mechanical properties. The aim of this study is to investigate the manufacturing process influence on shear and fracture toughness properties for an epoxy glass fiber-reinforced polymer. Epoxy-infused laminates of 84 layers (55–60 mm) thick were manufactured. The laminate was manufactured using vacuum infusion on a single-sided mold. Thermocouples and strain gauges were embedded at different thickness positions to monitor temperature and residual strain during the curing process. The laminate was manufactured using the sublaminate technique: with the help of peel plies at different thickness positions, sublaminates were extracted and separately tested as standard coupons. Shear coupons were tested according to ASTM D7078-12, Standard Test Method for Shear Properties of Composite Materials by V-Notched Rail Shear Method, and fracture toughness double-cantilever beam coupons were tested in Mode I according to ASTM D5528-13, Standard Test Method for Mode I Interlaminar Fracture Toughness of Unidirectional Fiber-Reinforced Polymer Matrix Composites. Based on the extracted sublaminates, the relations between local curing cycles, residual strains, and the influence on the shear and fracture toughness were studied. This work reports temperature profiles and residual strain measurements that were monitored during thick laminate manufacturing. In addition, experimental data from shear and fracture toughness tests (static and fatigue) are correlated with the local residual strains and temperature measurements.