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.