Recent studies of carbon fiber and carbon/glass hybrid laminates have reported compression strengths and failure strains which are borderline for wind turbine blade designs, depending upon the reinforcement architecture, matrix resin, and environment. Compressive strength is known to be sensitive to the straightness of the fibers, with even relatively small degrees of waviness or misalignment causing significant decreases in compression properties. The effects of fiber waviness, induced by infusion processes and inherent in fabric architectures, on compressive strength, have been investigated. Structural details such as ply drops and ply joints can cause significant levels of fiber misalignment, depending on parameters such as ply thickness, fraction of plies dropped, ply drop location, ply joint gap, and mold geometry and pressure. These parameters have been varied in the study reported in this paper, with compressive properties determined in each case. The results show that prepreg laminates containing ply drops and joints can provide adequate compressive strength, but that severe knockdowns can occur for geometries where large misalignments are induced.

Delamination at ply drops in composites with thickness tapering has been a major concern in aerospace applications of carbon fibers, where the plies are typically very thin. This study explored the resistance to delamination in fatigue of hybrid carbon fiber and glass fiber prepreg laminates containing various ply drop geometries, and using thicker plies typical of wind turbine blades. Strain levels to produce significant delamination at both carbon and glass fiber ply drops were determined and compared in terms of a simple delamination model. The carbon fiber laminates with ply drops, while performing reasonably well under static loads, delaminated in fatigue at low maximum strain levels except for the thinnest ply drops. The lower elastic modulus and higher interlaminar toughness of the glass fiber prepreg resulted in much higher strains to produce delamination at equivalent ply drops, compared with carbon fiber prepreg using the same resin system. The results indicate that the thickness of ply drops with carbon fibers should be much less than those commonly used for glass fibers.