Out-of-plane wrinkling in carbon–fibre composites: A comprehensive review for propeller-blade design and inspection

Amid the aviation industry's drive to reduce emissions and improve efficiency, carbon–fibre reinforced polymers (CFRPs) are essential in next-generation turboprop and hybrid-electric propulsion systems. However, out-of-plane wrinkling, or fibre waviness, remains a critical defect in CFRP propeller blades and other aerospace structures. Even moderate wrinkles can reduce compressive strength by 30 % to 75 %, initiate delamination, and reduce fatigue life by up to 50 %. These defects typically arise from excess fibre length, poor draping, or imprecise consolidation, such as during manual lay-up or automated fibre placement. This review examines the mechanisms driving wrinkle formation and their impact on mechanical performance, with particular emphasis on recent advances in detection and characterisation. Non-destructive testing methods, including phased-array ultrasonics, total focusing method (TFM), and synchrotron computed tomography (CT), have improved geometric resolution. Complementary techniques such as X-ray diffraction, nanoindentation, and digital volume correlation allow micromechanical analysis and support finite-element models using progressive-damage criteria. Despite these advances, key research gaps remain. Wrinkle-specific acceptance criteria are undefined, and high-fidelity pipelines to convert inspection data into predictive models are not yet standardised. The effects of residual stress and multiaxial loading on wrinkle-induced failures also remain poorly quantified. Moving forward, integrating real-time monitoring, machine learning, and defect-tolerant design will be critical to ensure safe, cost-effective, and certifiable CFRP structures. This review consolidates current knowledge and outlines a forward-looking research agenda to support the deployment of high-performance composite propeller blades in low-emission aviation.