**1. Introduction**

Fibre metal laminates (FML) are hybrid materials comprised of alternating metal sheet and fibre composite layers, that can be bonded in distinct sequences. Unlike hybrid multi-material stacks, in which thicker composite and metal layers are simply stacked and fastened by means of rivets, adhesive joints or bolted connections, the FML layers' thickness is typically less than 1 mm and consolidated through hot-curing cycles [1,2]. The superior performance of the laminate combination when compared to the isolated composite and metallic materials is highly relevant in structural applications, such as the transportation sector, where high specific strength is required, while also maintaining good impact and bending resistance [3,4]. Moreover, when such materials are adopted in the aviation industry, energy savings of approximately 30% are achieved [5,6]. Effectively illustrating the FML expression in aircraft applications and its increasing adoption [7] is the excellent fatigue strength, damage tolerance and overall durability of these materials, due to fibres acting as a barriers, thus delaying metal crack propagation [8]. Furthermore, worth mentioning is the good thermal insulation, corrosion and flame resistance properties of such materials [7,9].

FML can be currently manufactured in near-net shape geometries, such as large fuselage panels and stringers in aeronautics and also complex-shaped floor assemblies in the automotive industry [10]. Nonetheless, the fastening of multiple components relies

**Citation:** Marques, F.; Silva, F.G.A.; Silva, T.E.F.; Rosa, P.A.R.; Marques, A.T.; de Jesus, A.M.P. Delamination of Fibre Metal Laminates Due to Drilling: Experimental Study and Fracture Mechanics-Based Modelling. *Metals* **2022**, *12*, 1262. https:// doi.org/10.3390/met12081262

Academic Editor: Francisco J. G. Silva

Received: 30 May 2022 Accepted: 23 July 2022 Published: 27 July 2022

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**Copyright:** © 2022 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

mostly on mechanical joints, such as rivets or bolts, which can amount up to 3 million in a commercial aircraft [11]. Hole-making is, therefore, intensively performed for parts assembly, enabling riveting of aircraft panels such as the fuselage, wings and stabilizers [12,13] and despite the existent non-conventional feasible alternatives, such as laser machining and water-jet cutting, drilling remains the most employed technique [14]. The heterogeneity of FML allied to the highly abrasive properties of fibre reinforcement make drilling operations a challenging task. Their success may be compromised by simultaneous occurrence of (i) the well-known entry (peel-up) and exit (push-out) delamination, matrix thermal damage, fibre pull out and formation of abrasive fibre particles in composite layers as well as (ii) strain-hardening, continuous chip formation and thermal softening in the metal layers [15–17].

The lack of research regarding drilling operations on FML is evidenced by Bonhin et al. [16], especially in what concerns aluminium alloy (AA) and carbon-fibrereinforced polymers (CFRP) configurations with thermoset matrices. To the authors' knowledge, no studies have been performed in AA-CFRP laminates with thermoplastic matrices. The enhanced sustainability of the thermoplastic polymers (promoted by their improved recyclability) has encouraged the increasing usage of thermoplastic-based FML in relevant sectors, such as the aeronautical industry. Ekici et al. [18] experimentally analysed hole quality and delamination on AA-CFRP material samples, using Physical Vapour Deposition (PVD)-coated and uncoated drills. Despite the little number of holes each drill performed, the authors found that the uncoated condition (carbide) outperforms (PVD) coated drills in terms of entry delamination and hole nominal size. Sridhar et al. [19] systematically analysed the influence of operative conditions on drilling performance indicators such as thrust force and roughness, being able to identify ideal cutting parameters for AA-CFRP laminates using a conventional drill geometry. Despite the more intricate process kinematics of helical milling when compared to drilling, Bolar et al. [20] report advantages of the former, concerning cutting load, thermal impact, chip evacuation and hole nominal size. The implementation of analytic/numerical models capable of delamination prediction is of key relevance for both FML processing/assembly and drill tool manufacturers. Although some work can be found for CFRP materials, no data are available regarding these novel AA-CFRP material configurations. Feito et al. [21] compare the predictability of both complete simulation of a drilling operation with a simplistic model, in which the drill acts as a punch that pierces the laminate. The latter yields very reasonable cost-effective results with slight overestimation of delamination factor, setting upper limits that are highly valuable as support decision techniques.
