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 size [1].
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 part [2]–[4] since,
according to the procedure, the hole surface is subjected to different
thermo-mechanical loads that can induce differences in the hole surface
integrity [5].
Field and Kahles [6] 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 Mondelin [7], 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 life [11] as 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 propagation [12]–[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
strength [15], [16]. These surface integrity parameters have
different impacts in the fatigue failure: some influence crack
initiation and some influence crack propagation (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 Gaier [18], 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. Koster [19] 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 Atlati
[20] 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 Atlati [20].
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 lives [21],
[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 evacuation [23].
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.