FATIGUE PROPERTIES OF 2024-T3 ALUMINIUM SPECIMENS
REINFORCED WITH COMPOSITE PATCHES
M.L. Pastor, X. Balandraud, J.L. Robert and M. Grédiac
Laboratoire de Mécanique et Ingénieries (LaMI)
Université Blaise Pascal – Institut Français de Mécanique Avancée
IUT de Montluçon – Avenue Aristide Briand – B.P. 2235
03100 MONTLUCON - FRANCE
ABSTRACT
An investigation of the fatigue behaviour of 2024-T3 aluminium specimens reinforced with composite patches is carried out.
The influence of milling and shape of the patch on the lifetime of the reinforced specimens is examined first. The fatigue
response of reinforced specimens subjected to different loading levels is then studied. The aim is to observe which of the
adhesive, the composite patch or the aluminium fails first. The fatigue tests are performed with a stress ratio equal to zero and
at a frequency of 30 Hz. The S-N curve plotted from plain aluminium specimens is used to compare lifetimes obtained with
reinforced specimens. It is observed that milling the composite patch has a significant influence on the fatigue strength of the
specimens, as the milling reduces the fatigue strength of the reinforced specimens. However, in all cases, the reinforcement
with composite patch enables to improve the specimen’s lifetime.
Introduction
Composite patches are widely used to repair metallic structures, especially in the aeronautical industry [1-2]. In fact, the
transportation industry has to face the challenge of removing and changing components on time, before improper working or
failure. It is an important economical issue as the cost of changing aeronautical components is often unacceptable. Fatigue
damage generates crack initiation which reduces the aircraft lifetime. So composite patches are bonded to avoid the growth of
fatigue cracks by reducing the stress levels existing within the critical part of structural components. The quality of the repair
mainly depends on the size [3], the number of plies [4], and the shape of the composite patch [5]… An alternative to repairing
aeronautical components is to reinforce them before crack initiation. In this case, the solution is to bond the composite patch
on the critical zone which is known a priori in order to delay crack initiation. The initial stress flow is deviated within the
composite patch and the critical zone is thus relieved [6]. As a low reduction of the stress level may significantly increase the
lifetime of the structure, the beneficial effect of the reinforcement is clearly visible.
In the present work an investigation of the fatigue behavior of 2024-T3 aluminium specimens reinforced with composite
patches is carried out. The first part of the study consists in determining the influence of the shape and of the type of
machining process. Some fatigue tests are performed on two types of composite patches. The shape and the cutting process
are different. The case leading to the best improvement of the residual lifetime is then considered. The aim of the second part
is to assess the lifetime improvement of reinforced specimens. So fatigue tests at different loading levels are performed and
results are plotted on the S-N curve obtained from plain aluminium specimens.
Experimental Procedure
Aluminium specimens are manufactured from 3 mm thick 2024-T3 aluminium sheets according to the specifications of ISO
1099 standards [7] (see Figure 1). The minimum cross-section exhibits a “dog-bone” shape. The specimens are first cut by
water jet and then milled in order to avoid any defect on the specimen edges.
Aluminium specimens are reinforced with a 3-ply composite patch bonded on each side of the specimens. The composite is a
914 T300 epoxy/carbon prepreg system and the adhesive is a bismaleimide Redux 312. The fibres are oriented along the
specimen axis. The composite patch is first bonded on the aluminium specimen and then the reinforced specimen is milled.
Two types of composite patches are tested (see Figure 2). The composite patch is rectangular for specimens B. Its width is
lower than the minimum width of the specimen cross section. So the final milling of the aluminium does not affect the
composite patch. The composite patch for specimens C exhibits the same shape as the aluminium specimens. It completely
covers its middle part. Aluminium samples and the composite patches are then milled together. Composite patches are
bonded with the same conditions in terms of temperature of polymerization for specimens B and C. Specimens A are
subjected to the same temperature cycle as specimens B and C so possible metallurgic transformations are the same for the
three types of specimens.
Specimen A
Figure 1. Plain aluminium specimens
Composite patch
Specimen C
Specimen B
Aluminium
Figure 2. Two types of reinforced aluminium specimens
Fatigue tests are performed on a 100 kN push-pull MTS servohydraulic machine. A suitable alignment system is used to avoid
any parasitic bending of the specimen due to a possible misalignment of the grips. Fatigue tests are carried out at 30 Hz with a
stress ratio equal to zero.
Influence of the composite patch milling in terms of fatigue lifetime
Six specimens of each type (A, B and C) are tested. The aim is to compare the different lifetimes obtained with reinforced and
unreinforced specimens. In order to investigate the improvement due to reinforcement, the same loading amplitude is used for
the three types of specimens. Hence the stress within aluminium is lower in reinforced specimens than in plain aluminium
specimens.
Figure 3 shows some specimens after failure. For specimens A, the failure occurs at the gauge section. For specimens B and
C, the composite patch separates from the aluminium substrate.
Specimen A
Specimen B
Specimen C
Figure 3. Specimens after failure
Table 1 shows the experimental lifetimes N1 and their theoretical counterparts N2 obtained from the S-N curve of plain
aluminium (obtained without heat treatment).
Specimen types
A
B
C
Maximum tensile
stress in
aluminium (MPa)
320
190
190
Experimental
lifetime N1 (cycles)
Standard deviation
(cycles)
41 000
385 780
292 118
4 400
150 346
35 745
Expected
lifetime N2
(cycles)
40 763
795 220
795 220
N2/N1
0.99
2.06
2.72
Table 1: Experimental and expected lifetimes results for the three types of specimens (force ratio R=0, maximum force
Fmax = 11 500 N)
N1 is nearly equal to N2 for specimens A. So the heat treatment of aluminium during polymerisation of the composite patch
does not affect its fatigue properties. On the contrary milling composite patch for specimens C significantly affects its fatigue
strength. Its experimental lifetime is 25% lower than the lifetime obtained for specimens B. However the life extension of both
types of reinforced specimens is important. This lifetime is 9 and 7 greater than the lifetime of plain aluminium specimens for
specimens B and C respectively.
According to the results presented in the last column of Table 1, it can be however observed that reinforced specimens fail
before the expected lifetime. A possible conclusion is that adhesive fails before aluminium. When the composite patch
separates from the substrate (which corresponds to the adhesive failure), the aluminium minimum cross section sustains the
entire loading, thus leading to actual load amplitude which is greater than in the case of reinforced specimens.
Because of the detrimental effect of the composite patch milling, only specimens B are prepared and tested in the following.
Fatigue tests performed on specimens B
19 specimens B are prepared and tested. Four different loading levels are applied: σ=160, 230, 287 and 307 MPa. These
stresses are calculated at the aluminium gauge section. The case σ=160 MPa is just under the fatigue strength at 5 X 105
cycles, σ=230 MPa is above the fatigue strength at 5 X 105 cycles, σ=287 MPa lies in the middle of the S-N curve and σ=307
MPa corresponds to the top of the S-N curve. One of the aims of this study is to determine which of three materials involved
(aluminium, adhesive, composite) fails at first. The important shear stress concentration in the adhesive at the free edges of
the composite patch may be responsible for the fatigue failure [6].
Experimental lifetimes
Maximum stress (MPa)
Experimental results are plotted in Figure 4, where the vertical axis corresponds to the maximum stress existing within the
aluminium substrate under the composite patch.
405
Plain aluminum
specimens
355
Unfailed plain aluminum
specimen
305
255
Reinforced specimen
failure
205
155
100
1 000
10 000
100 000
1 000 000
Lifetime (cycles)
Figure 4. Fatigue life of patched specimens against plain aluminium S-N curve
The lifetime obtained from the reinforced specimen is lower than the lifetime of plain aluminium specimen subjected to the
same stress amplitude. The difference between the two lifetimes increases as the load amplitude increases. This phenomenon
is due to the fact that adhesive fails always earlier compared to aluminium failure. Nevertheless the reinforcement enables to
increase the lifetimes in all cases (see Table 2). It is multiplied by 2.35 and 1.40 at σ= 160 and 230 MPa respectively.
Maximum
tensile stress
in aluminium
(MPa)
Experimental
lifetime N2
(cycles)
Standard
deviation
(cycles)
160
230
287
307
200 422
25 897
5 025
517
36 319
4 810
693
45
Expected
lifetime N1
(cycles)
N1/N2
Not defined on the S-N curve
117 840
4.55
48 388
9.63
36 674
70.94
Stress in
aluminium with
unstuck
composite
(MPa)
249
358
448
476
Expected
lifetime N3
(cycles)
N2/N3
85 300
2.35
18 631
1.39
Not defined on the S-N
curve
Table 2: Life extension due to the reinforcement by composite patches
Benefit of the reinforcement
The beneficial effect due to the composite patches is clearly observed on Figure 5 where the experimental data corresponding
to the stress in aluminium after unstuck of the composite patch is reported. The black round points report the experimental life
results by stating the stress level of the aluminium substrate when the composite patch is unstuck. They are located on the
right of the material S-N curve. It clearly shows that the composite patch improves the lifetime of the single aluminium
substrate.
Maximum stress (MPa)
455
405
plain aluminium
specimens
355
aluminium stresses with
bonded composite patch
305
aluminium stresses with
unstuck composite patch
255
Unfailed plain aluminium
specimen"
205
155
1 000
10 000
100 000
1 000 000
Lifetime (cycles)
Figure 5. Fatigue life of reinforced and not reinforced aluminium specimens
Fatigue fractures
Two types of failure of the reinforced specimens are observed (see Figure 6). Either the composite patches remain bonded to
the same part of the specimen or it separates from each of its two remaining parts.
Figure 6. Failure of composite patch
Another particular phenomenon is observed (see Figure 7). As the maximum load increases, the failure angle θ observed on
the lateral edge of the specimen increases.
160 MPa
230 MPa
θ
287 MPa
θ
θ
Figure 7. Reinforced aluminium fracture orientation
The same feature was observed for plain aluminium specimens. So the composite patch improves the specimen’s lifetime
because it reduces the aluminium stress level but it does not modify the fracture mechanism.
Conclusion
This paper examines the fatigue behaviour of the 2024-T3 aluminium reinforced with composite patches. The shape and the
cutting process of the composite patch significantly influence the lifetime of the specimens. As a result, the milling of the
composite patch has a negative effect from the fatigue point of view as it reduces the fatigue life of the reinforced specimen
with respect to the non milled reinforced specimen.
Reinforcement by composite patches enables to increase the fatigue lifetime. The improvement in terms of lifetime ratio
depends on the stress level applied to the aluminium substrate. For the fatigue tests that were carried out, the specimens fail
before the expected lifetime under the assumption that the adhesive and the composite patch do not fail. So the adhesive is
probably the cause of the anticipated failure. Nevertheless it can be noted that all fatigue tests performed in this study are
performed on specimens initially free of damage. When the aluminium specimens are initially fatigue damaged, the adhesive
could not be the first cause of the failure of the reinforced specimens.
Further investigations are in progress in order to carry out some fatigue tests on specimens initially damaged and then
reinforced with composite patches.
Acknowledgments
The “Atelier Industriel de l’Aéronautique de Clermont-Ferrand” is gratefully acknowledged for providing the specimens
reinforced by composite patch.
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