Critical damage modes in high-performance drilling of carbon fibre reinforced epoxy composites

The growing number of fibre deposition methods for composite structure enables highly tailored, high-performance architectures from one design intent to another. This raises the question, “How much should damage tolerances differ for one composite part design to another?” This initiates a framework for further exploration of the boundaries of acceptability, using a quantitative approach to assess the influence of process parameters on in-service performance. The framework is illustrated through a case study of a feature machined into a composite structure, namely a hole for part assembly. Mechanical and thermal hole defects were deliberately induced during dry drilling of quasi-isotropic pre-impregnated laminates. In-process forces and temperatures in combination with post-machining microscopy, X-ray computed tomography and scanning electron microscopy revealed micro-damage leading to isolated, global defects of delamination and thermal damage types. Comparisons with healthy, undamaged holes in static mechanical tests enabled quantitative assessment of the impact of different damage types on in-service performance, in this case compressive strength, of a composite part. High-speed drilling (52% faster) reduced strength (3.5%) due to hole geometry errors. Low-speed drilling (11.5x slower) increased strength (2%). Delamination (40% faster) reduced strength (2.5%). Future research should focus on defect implications, virtual defects and testing thereof, and in-situ monitoring.