1. Introduction

Parts of aircraft structures are mostly assembled using fasteners (rivets or screws) introduced into holes machined by drilling. Assembling an aircraft may require several hundred thousand to several million drilling operations depending on the aircraft size1. These holes for fastening are critical areas where fatigue damage can be initiated because they are areas of high stress concentration. The drilling procedure used for the machining of the fastening holes can affect the fatigue life of the drilled part2–4 since, according to the procedure, the hole surface is subjected to different thermo-mechanical loads that can induce differences in the hole surface integrity5.
Field and Kahles6 were the first to introduce the concept of surface integrity in a technical sense by defining it as theinherent or enhanced condition of a surface produced in machining or other surface generation operation. Since then, this definition has been completed by adding the notion of surface functional performance. Thus, according to Mondelin7, the surface integrity can be considered as a set of characteristics allowing a surface to be qualified with regard to a given application . Regarding the fatigue performance of a surface, it is well known that the surface integrity includes the concepts of surface topography, residual stresses and the metallurgical condition of the material subsurface (microstructure and microhardness).
Extending the fatigue life of aircraft is one of the aircraft industry’s major concerns. This requires an understanding of the impact of the surface integrity on the fatigue strength. Thus, several works have been carried out in the past few decades to study the relationships between the machining process parameters, the resulting surface integrity and the associated fatigue life for various materials and machining processes. The reviews by Novovic et al., Saoubi et al. and Pramanik et al.8–10 synthesize many of these works. The main conclusions concerning the individual contributions of the surface integrity parameters for commonly used materials (aluminium alloys, titanium alloys, steel and nickel alloys) are the following. The lower the surface roughness, the longer the fatigue life11as the micro-notches of a machined surface induce stress concentration areas where a local plastic strain field can be generated when a stress is applied, thus defining a path for the crack. Compressive residual stresses improve the fatigue strength due to the crack closure effect, which slows crack propagation12–14. In contrast, tensile stresses reduce the fatigue strength through the crack opening effect, which facilitates crack propagation. The fatigue life increases with the work hardening of the surface, which increases the surface yield strength15,16. These surface integrity parameters have different impacts in the fatigue failure: some influence crack initiation and some influence crack propagation17 (Table 1).
However, a machining operation impacts all the surface integrity parameters mentioned above at the same time and some parameters depend on one another, so it seems difficult to judge which parameter has a predominant influence on the others. Moreover, depending on the level range considered for a parameter, its influence on the fatigue strength can vary. For instance, according to Siebel and Gaier18, a reduction in the fatigue endurance limit occurs only above a certain critical groove depth. In addition, the impact of the surface integrity parameters varies from one material to another. Koster19 showed that the endurance limit of steels was dependent on the surface roughness, whereas this was not the case for Ti 6-6-2 and Inconel 718.
Thus, the question of the relationship between the surface integrity and the fatigue strength seems very broad and complex and the conclusions of the works cited in the reviews mentioned above cannot be generalized to all machining processes and to all materials. In some fields, there are gaps in our understanding of these correlations. This is the case for the hole surfaces obtained by drilling processes in aluminium alloys, which are the most widely used alloys in the aircraft industry. A clear understanding of the impact of these surfaces’ integrity on the fatigue life of the drilled part is not yet available, although it is required by the aircraft industry to optimize the fatigue behaviour of their assemblies.
The lack of studies on the integrity of drilled aluminium surfaces may be explained by the fact that the thinness of the layer of material affected by the machining process makes the experimental characterization of the surface integrity difficult. A finite element study preliminary to the present study and simulating the lateral cutting of a drilling process in a 2024-T351 aluminium part showed that the depth of the subsurface material affected by plastic strain and residual stresses after machining was a few tens of microns (Fig. 1). The finite element model developed for this study was a mechanical model equivalent to the one developed by Atlati20 and it was developed with the Abaqus/explicit software. The mechanical laws of material behaviour and friction, and the simulation strategy used, were based on those employed by Atlati20.
This paper investigates the impact of the hole surface integrity on the fatigue life of a 2024-T351 aluminium drilled part. This alloy was chosen because it is commonly used in the aircraft industry due to its low density and its high fatigue performance. As axial and orbital drilling processes can lead to different fatigue lives21,22, these two processes were considered in the study in order to generate potential differences in hole surface integrity. Axial drilling is the most common process for the machining of a fastening hole in the aircraft industry and is usually named ‘conventional drilling’. Orbital drilling is a more recent process, corresponding to helical milling (Fig. 2), which has many advantages over the conventional process, such as avoiding burr formation and allowing better chip evacuation23.
To carry out the study, fatigue tests and surface integrity analysis (roughness measurements, hardness measurements, metallographic observations and residual stresses analysis) were performed in order to identify the parameters that control the fatigue life.