Hole quality assessment and tool wear evaluation in drilling of CFRP/CFRP and Al/CFRP stack laminates for aeronautical applications

Composite material parts are typically produced in the near-net-shape, i.e. very close to the finished product. However, additional machining processes are often required to meet dimensional and tolerance requirements. Particularly drilling stands out as the most widespread machining process of carbon fibre reinforced plastic (CFRP) composite parts, primarily in the aerospace industry, due to the widespread use of mechanical joints, such as rivets, rather than welded or bonded joints. CFRP drilling is noticeably challenging due to material abrasiveness, inhomogeneity and anisotropic properties; tool wear rates are inherently high leading to superior cutting forces and detrimental effects on part surface quality and material integrity. Damages such as delamination, cracks or matrix thermal degradation are often observed as the result of uncontrolled tool wear or improper machining conditions. The development of effective non-destructive control techniques, such as optical inspection for drilled hole quality assessment for process, tool and product quality evaluation is dealt with in this PhD thesis with the aim to contribute to the reduction of scraps and tool costs as well as to the improvement of process productivity in the drilling of CFRP composite material parts for aeronautical assembly. In this thesis work, the most effective parameters for hole quality evaluation in drilling of CFRP/CFRP and Al/CFRP stacks for aeronautical assembly were studied and selected. An automatic technique based hole image processing was developed for the evaluation of these quality parameters for holes drilled with a traditional tool and an innovative tool purposely developed. Tool wear was finally evaluated to verify its relationship with the hole quality