PSI - Issue 53

R.D.F.S. Costa et al. / Procedia Structural Integrity 53 (2024) 376–385 Costa, R. D. F. S./ Structural Integrity Procedia 00 (2023) 000 – 000

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Keywords: Fiber metal laminates; drilling; delamination; tool wear

1. Introduction Composite materials were first introduced in the industrial sector to overcome the problems felt with the use of metals, particularly in weight reduction, with the aim to save fuel for transportation industries, such as the aeronautical and, later, automotive (Muhammad, Rahman, Baini, & Bin Bakri, 2020). Nevertheless, these materials also lacked some essential properties, such as structural integrity, damage tolerance and thermal conductivity, reason why a different solution had to be found. Fibre metal laminates, such as carbon fibre reinforced aluminium laminates (CFRP/Al), also known as CARALL, constitute an improvement in the composites field, each time being more prominent due to the combination of both the materials ’ properties. The higher strength and stiffness of metals allied to the lightweight, resistance to corrosion, fracture and fatigue of composites, among other characteristics, lead to the development of multi-material solutions (Rúben D.F.S. Costa, Sales-Contini, Silva, Sebbe, & Jesus, 2023; Lakshmi Kala & Prahlada Rao, 2022). Drilling is a fundamental stage in product manufacturing for components which will be used in the assembly of parts, primarily fastened by rivets and bolts, thus requiring the hole-making procedure (Zhang, Liu, Tian, & Liao, 2015). The drilling process of fibre metal laminates is more difficult than the typical materials, as it presents significant challenges in achieving damage-free holes with precise tolerances, and one of the reasons for that stands in the necessity of cutting through two different materials at the same time, causing an excessive tool wear, mainly at their interface (Zhong et al., 2022). One method to reduce this wear is to use coated tools, but a proper coating has to be chosen, as it behaves differently depending on the material to be machined (R. D. F. S. Costa, Jesus, Simões, & Barbosa, 2023). The geometry of drills also needs a special attention, as the cutting edges disposition and angles usually cause a significant impact on the cutting performance. Additionally, the machining parameters, such as cutting speed and feed, have to be properly selected, to reduce the probability of issues (for example unattained dimensional accuracy, uncut fibres and low surface quality) to appear. Due to CFRP ’s anisotropic and heterogeneous nature, several defects may arise in the holes, for example delamination. The fibres of the composite are torn out during the drilling process due to the rotating action of the tool, sometimes leaving the surrounding area negatively affected (Xu, Li, Mi, An, & Chen, 2018). The aeronautical industry has a demand for high quality in its produced parts, which requires that delamination cannot exceed determined values, the maximum admissible delamination limits. Based on this, drill bit manufacturing companies have suggested guidelines on hole quality requirements (Sandvik Coromant, n.d.), so that it could be possible to quantify the caused damage with a better interpretation. The cited guideline states that the delamination factor considering the hole diameter (F d ) should not exceed 1.125; however, this concept is not applicable for the F da approach, a more complete delamination factor considering both the diameter and area of the defect area. Several researchers adopted a standardized criteria based on previous experimental works which consisted of three levels of delamination, considering the damage levels. The first one, from 1.000 to 1.100, shows acceptable hole quality with minimum delamination and the second, still agreeable, is represented by 1.101 to 1.200. Finally, the third level, above 1.201, states the point where the damage is considered to be severe and unacceptable (Sobri et al., 2020). Besides this, the common demands for hole requirements usually determine that no delamination or chipping must exist in the hole exit (Sandvik Coromant, n.d.). Considering surface roughness, another essential aspect to be assessed after drilling, in the aeronautical industry, CFRP panels have a maximum permissible value of arithmetic average roughness (Ra) of 3.2 µm, while for aluminum and titanium panels this value is 1.6 µm (Franz, Vantomme, & Hassan, 2022). Although other defects may appear on the holes, such as burr, a raised portion at the entry and exit of the drilled hole which appears particularly in aluminium (Devitte, Souza, Souza, & Tita, 2021), the tool wear is a significant factor to be evaluated in every machining process, since it is responsible for a large part of hole size issues (D’Orazio, El Mehtedi, Forcellese, Nardinocchi, & Simoncini, 2017) . Several forms of wear can be noticed on the cutting tool after drilling both composites and metallic materials, being the two most frequent the flank wear and, to a lesser extent, crater wear. Abrasive wear and material adhesion to the tool’s surface, usually leading to a built-up