An experimental campaign was conducted in order to study the effect produced by the shot-peening process on fretting fatigue. All fretting-fatigue tests were carried out using an ad hoc test device. A scheme for this test device, which is similar to that described in [
28], is shown in
Figure 1a. In this test machine, the first two cylindrical contact pads were pressed against a dog–bone-type test specimen by means of normal constant load
N. Then, a cyclic (harmonic) axial load with an amplitude of
P was applied to the dog–bone test specimen, and due to the stiffness of the system and the friction between the contacting surfaces, a cyclic and in-phase tangential load
Q was developed. Under this load configuration, fretting cracks always initiate at the slip zone near the contact trailing edge; see a sample fretting scar obtained from these tests in
Figure 1b.
Due to the moving supports, it is possible to modify the stiffness of the device, and thus obtain different values of tangential load amplitude Q without varying axial load amplitude P. This fact makes multiple fretting-load combinations (P, Q, N) achievable with the present device. In this type of fretting-fatigue test, it is important to monitor all fretting loads, so the test device was instrumented with load cells able to measure these forces. The signals from these load cells were transferred to a signal conditioner, then passed via a data-acquisition card to a PC in order to display and record fretting loads P, Q, and N in real-time.
In the present work, the material of both parts, dog–bone test specimens and contact pads, was the aluminum alloy 7075-T651, which is a widely used material in the aerospace industry to manufacture wing skins, panels, covers [
29], seat rear legs, and seat spreaders [
30]. In
Table 1 and
Table 2, the chemical composition for this aluminum alloy [
31] and its leading mechanical properties [
32] are shown, respectively. The main geometric features for the contact pads and fretting-fatigue test specimens are shown in
Figure 2. On the contact zone, the fretting-fatigue test specimens have a rectangular cross section of 8 × 10 mm
2; the contact is produced on the 8 mm side. The pads have a cylindrical contact surface with an
R = 100 mm radius. This radius in conjunction with the test-specimen width (8 mm) led us to obtain a wide range of contact-stress values using achievable fretting loads (
P,
Q,
N) by the present test device. In addition, the geometry of the contact pair, and assuming that a great part of it was under plain strain conditions, allowed us to assume that the behavior of both contacting bodies (test specimens and contact pads) was as half-planes [
33]. Under these hypotheses, analytical formulae are available for a first estimation of the contact stress and strain fields and contact areas [
34,
35]. These data were very useful in order to determine the range of fretting-fatigue loads to be applied. In
Table 3, all the load combinations used in the experimental campaign are shown. In addition, this table shows obtained contact parameters from contact Hertzian theory. The expressions for these parameters are the following [
34]:
where
a is the contact semi-width,
c is the stick-zone semi-width,
e is the eccentricity of the stick zone,
p0 is the maximum normal pressure,
σ is the axial stress amplitude due to
P, Δσ
xx is the range of the direct stress at the contact trailing edge, and
N* is the normal load per unit length (
N* =
N/8 N/mm in the present case).
In order to analyze the effect produced by the shot-peening process in the above fretting tests, dog–bone-type test specimens were shot-peened. The parameters describing the shot-peening treatment according to the AMS Standard [
36] are 9A 230-H 90°. The test specimens were shot-peened along all their surfaces, even in the threaded zones (to improve fatigue performance, and thus avoiding an undesirable fatigue failure at those zones). A cross section of a shot-peened test specimen is shown in
Figure 3. In this figure, the highly deformed layer typical of the shot peening process and the characteristic surface with valleys and peaks are easily observable. In addition, the formation of cracks due to the severe deformation produced by the ball’s bombardment during the treatment is noticeable. Even when these cracks clearly have a negative effect on fatigue behavior, previous works [
16,
37,
38] showed that, in a fretting-fatigue situation, the beneficial effect produced by residual stress and cold working due to shot peening makes the detrimental influence of these cracks vanish.
The residual-stress field produced on the test specimens by the above shot-peening treatment was measured by means of two different techniques: the hole-drilling and X-ray diffraction (XRD) methods. Both residual-stress measurement techniques are widely used and their results are reliable. Regarding the hole-drilling method, residual-stress distribution was established using the integral method [
39,
40], which for the hole-drilling technique, is the most suitable method to measure residual-stress distribution with a steep gradient. Regarding the hole-drilling device, an MTS 3000 instrument from the manufacturer HBM™ was used for both drilling and reading strain values during the drilling process. On the other hand, measurements obtained via XRD were carried out according to the EN 15305:2008 standard [
41], and the in-depth correction of the measurements via the method described by Moore and Evans [
42] was used. The equipment used was a portable residual stress analyzer iXRD from the manufacturer PROTO. All measurements were done on the midthickness test specimens (
xy plane), where both methods, the integral method and that of Moore and Evans, are applicable.
Figure 4 shows the residual-stress distributions measured with the above methods. In both cases, the hole-drilling and XRD methods, only the residual stress in the longitudinal direction of the test specimen, σ
xx, was plotted, which is thought to be more important from a fatigue point of view. In any case, the residual-stress measurements showed that in the
xz plane, principal residual stresses were very similar, indicating that the residual-stress values were almost independent of the considered direction. In
Figure 4, the residual-stress distributions lie in a series of bands that indicate the different values obtained among all measurements. Both measurement methods produced residual-stress distributions with a similar shape, although values are notably different. These differences can mainly be attributed to the different methods (integral and Moore and Evans) used to consider the redistribution in the stresses produced by the removed material during the drilling (hole-drilling) or electropolishing processes (XRD). In any case, both methods produced maximum values and residual-stress distributions that were similar to those obtained in a previous study for the same material and test specimens, but slightly different shot-peening process parameters [
16].