Influence of ply-drop location on the fatigue behaviour of tapered composites laminates

Abstract

The influence of ply-drop position in thickness direction under fatigue loading (R = -1) has been studied for a highly oriented composite laminate dropping from 20 to 12 plies. Compressive and tensile strengths have been determined for several configurations of ply-drop locations. Fatigue tests at a load ratio of R=-1 have been performed up to rupture. The first damages clearly identified are delaminations close to the drop-offs. Their initial locations and propagations kinetics before final failure were observed. Finite element simulations were performed to find out initiation spots of delamination. An interlaminar stress criterion has been proposed to predict initiation of delaminations. Effects of ply-drops configuration on fatigue are discussed.

Full text

This is a publisher-deposited version published in: http://oatao.univ-toulouse.fr/
Eprints ID: 4094
To cite this document: WEISS Ambrosius, TRABELSI Walid, MICHEL Laurent,
BARRAU Jean-Jacques, MAHDI Stéphane. Influence of ply-drop location on the
fatigue behaviour of tapered composites laminates. 10th International Fatigue
Congress, 06-11 June 2010, Prague, Czech Republik.
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Fatigue 2010
Influence of ply-drop location on the fatigue behaviour of tapered
composites laminates
A. Weiss1, W. Trabelsi1, L. Michel1*, J.J. Barrau1 and S. Mahdi2
1Université de Toulouse ; INSA,UPS, Mines Albi, ISAE, ICA (Institut Clément Ader),
10 avenue Edouard Belin -BP 54032 - F-31055 Toulouse cedex 4
2 Airbus Operations SAS, 316, Route de Bayonne, F-31060 Toulouse Cedex 03
Abstract
The influence of ply-drop position in thickness direction under fatigue loading (R = -1) has been studied for a highly oriented
composite laminate dropping from 20 to 12 plies. Compressive and tensile strengths have been determined for several
configurations of ply-drop locations. Fatigue tests at a load ratio of R=-1 have been performed up to rupture. The first damages
clearly identified are delaminations close to the drop-offs. Their initial locations and propagations kinetics before final failure
were observed. Finite element simulations were performed to find out initiation spots of delamination. An interlaminar stress
criterion has been proposed to predict initiation of delaminations. Effects of ply-drops configuration on fatigue are discussed.
Keywords: ply-drop, fatigue, damage modes, numerical simulation, composite
1. Introduction
Modern aeronautical structures are being made of laminated composites panels, i.e. several Carbon-Fibre
Reinforced Plastic plies stacked together. In order to optimise the structure weight, the thickness of the panel can be
tailored to the local stress distribution. These thickness variations may be efficiently produced by so-called ply drop-
offs. In these zones, however, out of plane stress concentrations are susceptible to initiate delamination failures. This
being a critical failure mode, several studies have been carried out about damage and delamination propagation in
these ply drop areas [1,3,4,6]. Design guidelines have been proposed to avoid, or minimise, damage initiation for
simple specimen geometry, i.e. with one or two ply drops [2,5]. These studies give basic ideas about the influence of
ply-drops on static and fatigue load behaviours. The present study goes further by addressing the fatigue behaviour
of specimens with several ply-drop offs configurations defined to meet the aeronautical industry design guidelines.
The objective of the study is then to evaluate the effect of variations in ply-drop positions on the fatigue
resistance of representative specimens. To do so, several ply-drop configurations have been defined and tested under
* Corresponding author. Tel.: +0-33561339141; fax: +0-33561339095.
E-mail address: laurent.michel@isae.fr.

2 A.Weiss et al. / Procedia Engineering 00 (2010) 000–000
static (tension and compression) and monotonic fatigue loading. For fatigue, the load ratio was fixed to R=-1, that is
considered to be the most critical one for composite specimens. Damage initiations, as well as damage evolutions
before final failure, were observed during fatigue mechanical tests in order to explain the differences in fatigue
lifetime of the different configurations. 3D Finite Element Models were developed to estimate the interlaminar
stresses close to the ply-drop locations. The locations of delamination initiation, observed experimentally in fatigue,
are analysed with the help of these numerical models. An interlaminar stress criterion is evaluated to estimate the
static results obtained in compression. The SN curves are then presented and ply-drop configuration effects are
discussed.
2. Experiments
2.1. Material, Stacking sequence and dimensions of the ply-drop specimens
Fig. 1. Geometry of ply-drop specimen
The material is a pre-impregnated carbon /epoxy composite from Hexcel (T700-M21, 268 g/m2), the thickness of the
unidirectional plies is 0.25 mm. The ply-drop zone is non symmetrical with an angle of 7°, as may be seen in Fig. 1.
In the tapered area, the plies are dropped, from 20 to 12 plies, by respecting the typical design guidelines for ply-
drop-offs. The thick and thin sections of the laminates are strongly oriented, with 50% of the plies at 0°. Both
sections and the dropped plies are the same for all configurations (Fig. 2.). Six configurations of ply-drop offs were
defined to study the effect of ply-drop location in the thickness of the specimen on its mechanical behaviour. For
convenience and as the first two ply-drops close to the thin section were observed as being the most critical for
damage initiation, only the position of these ply-drops is presented in Fig. 3.. With these different configurations it
is possible to evaluate the effect of the location in the thickness of the 1st ply drop from bottom to top of the
specimen and also the effect of the orientation of this 1st ply drop either 45° or 0° ply. It shall be noted that a
different material batch was used for the configuration v5, and the comparison of fatigue performance will be
achieved with the v2 configuration of the same batch.
N° 8 7 6 5 4 3 2 1
20 90 90 90 90 90 90 90 90 90
19 45 45 45 45 45 45 45 45 45
18 0 0 x
17 0 0 0 0 0 0 0 0 0
16 -45 -45 -45 -45 -45 -45 x
15 0 0 0 0 0 0 0 0 0
14 45 45 45 45 x
13 0 0 0 0 0 0 0 0 x
12 -45 -45 -45 -45 -45 -45 -45 -45 -45
11 0 0 0 0 0 0 0 0 0
10 0 0 0 0 0 0 0 0 0
9 -45 -45 -45 -45 -45 -45 -45 -45 -45
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A.Weiss et al./ Procedia Engineering 00 (2010) 000–000 3
8 0 0 0 0 0 0 0 x
7 45 45 45 x
6 0 0 0 0 0 0 0 0 0
5 -45 -45 -45 -45 -45 x
4 0 0 0 0 0 0 0 0 0
3 0 x
2 45 45 45 45 45 45 45 45 45
1 90 90 90 90 90 90 90 90 90
Fig. 2. Example of a configuration (v2) with all the ply-drops in grey
v2 v13 v12 v4 v3 v5
90 90 90 90 90 90
45 90 45 90 45 90 45 90 45 90 45 90
0 45 0 45 0 45 0 45 0 45 0 45
0 0 0 0 0 0 0 0 0 0 0 0
0 0 0 0 0 0 0 0 0 0 45 0
-45 0 -45 0 -45 -45 -45 0 -45 -45 -45 45
0 -45 0 -45 0 0 0 -45 0 0 0 -45
0 0 0 0 0 0 0 0 0 0 0 0
-45 0 -45 0 -45 -45 -45 0 -45 -45 -45 0
0 -45 0 -45 0 0 0 -45 0 0 45 -45
0 0 0 0 0 0 0 0 0 0 0 0
0 0 0 0 0 0 0 0 0 0 0 0
45 45 45 45 45 45 45 45 45 45 45 45
90 90 90 90 90 90 90 90 90 90 90 90
No 2 1 2 1 2 1 2 1 2 1 2 1
Fig. 3. Location of 2nd and 1st ply-drops for the six configurations under study
2.2. Testing conditions and damage observation
Under load, the asymmetry in the specimen geometry creates a bending moment which causes an out of plane
deflection at the ply drop area. This may need to be controlled in order to be representative of the behaviour of an
aeronautical composite panel, where boundary conditions may limit large out-of-plane deflection. The specimens
were therefore clamped in an anti-deflection device and Teflon pads were inserted to minimise friction in the non-
clamped zone (Fig. 4 ).The tests have been stopped regularly and both sides observed with an optical microscope in
order to monitor the damages during fatigue loading,., Damage evolution has been filmed by a camera (60 images/s)
on one side of the specimen during the last cycles before final failure.
3. FINITE ELEMENT MODELLING
A global-local 3D finite element models approach has been developed, with the code Samcef©, in order to
calculate interlaminar stresses at ply drop areas. The local model is designed to correctly represent the free-edge and
ply drop-off effects on the interlaminar stress distribution.
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Fig. 4 Specimen clamped in the anti-buckling device
3.1. Geometry and boundary conditions of the global-local model
The global model represents the complete ply-drop zone plus the unclamped part of the laminates in the thick and
the thin section, including the Teflon part of the anti-buckling device. To simulate the clamping condition, all
degrees of freedom are restrained on both the right- and left-hand sides, except for the displacement in load direction
on the latter. The laminate is modeled ply by ply with 20 nodes cubic elements, with one element per ply thickness.
A contact condition without any friction has been imposed between the specimen and the anti-buckling device. The
meshing in the width direction is refined in order to get cubic finite elements at free edges where stress gradient is
the highest.
F
Interface elements
Interpolated displacements
Fig. 5. Global (top) & local (bottom) finite element models for interlaminar stresses calculation
The local model represents a zone around a ply-drop, with one ply on the top and one ply on the bottom Thus, as
many local models as ply-drops are analyzed. The displacements field obtained with the global model is imposed as
a loading boundary condition on the local model. Interface elements are used to calculate the interlaminar stresses
between consecutive plies. A mesh refinement study has shown that stress values are correctly represented with one
element per ply for the global model, and 2 elements per ply for the local model by keeping elements cubic at the
specimen edges. All material laws are linear elastic; the solver module is non-linear allowing for possible large
deflections.
3.2. Approach for calculation of representative values
There are two main problems when calculating numerical stresses around ply-drops: namely the stress singularity
at the dropped ply, and the edge-effect at the free edges. Convergence studies have shown that the average stress
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Fig. 7. Side 2 of v2 configuration tested at 50% and stopped at 10000 cycles with delaminations around ply-drop 1 and 2
Damage modes and locations have been identified by stopping the fatigue tests regularly and observing the two
sides of the specimens. An asymmetrical distribution of delamination on the specimen’s edges has been found, as
shown in Fig. 8. for the 2nd ply-drop of the v4. It is to be noted that the delamination is visible on one side only
(Side 2).
v4 side 1 v4 side 2
Fig. 8. Delamination at 2nd ply-drop of v4 tested at 50%, interrupted at 8000 cycles
The 3D global-local finite element model has been used to evaluate the interlaminar stresses close to the ply-
drops. It has been shown that the stress distribution through the specimen width is non-symmetrical; with stresses
being higher on one side than on the other.
Table 2. shows the frequency of delamination observed experimentally in fatigue tests, and the magnitude of the
calculated interlaminar stresses averaged over a squared zone of 0.25 x 0.25 mm with the global-local model.
Calculations were all performed for the compression load at static failure stress of the v2 configuration. The shear
stresses in yz direction are small when compared with the other values and, being considered as negligible, are not
presented.
Table 2. Frequency of delaminations observed in fatigue tests and calculated interlaminar stresses for the 2 first ply drops
Critical location Side 1 (MPa) Side 2 (MPa)
PlyDrop Side Interface ızz ıxz ızz ıxz
1 1 0°/0° 34 98 33 57 v2
2 2 -45°/0° 38 24
61 103
1 2 0°/0° 32 73
33 129 v3
2 2 0°/0°
45 -123 30 -138
1 2 0°/0°
51 -168 36 -198 v4
2 2 0°/0° 27 81
29 134
v12 1 1 0°/0° 33 79
37 132
v13 2 2 0°/0° 25 74
29 117
v5 1 2 45°/-45° 56 47
66 -79
Side location of the first delamination
In bold Highest stress value (ızz ou ıxz )
It is important to notice that, for all the configurations, the side where delaminations initiate the most frequently
during fatigue tests corresponds to the side where interlaminar stresses are the highest. Furthermore, on the preferred
side of delamination initiation, the shear stress Vxz is always higher than the opening stress Vzz. This is observed for
Delamination
No damage
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A.Weiss et al./ Procedia Engineering 00 (2010) 000–000 7
all configurations except for v5 where the dropped ply is at 45° instead of 0°. For v5 configuration, shear stress and
opening stress are of the same intensity.
4.3. Criterion of delamination initiation
To establish a basic (qualitative) criterion enabling us to predict delamination initiation at the edges of specimens
some assumptions have been made. First of all, the material behavior is assumed to be linear without any damage.
The critical delamination locations leading to the quasi-instantaneous failure of the specimen in static loading are
supposed to be the same as the ones observed in fatigue.
The basic approach developed here is inspired from delamination onset criteria developed for free-edge
delamination specimens [7-9]. It has been chosen to work with a quadratic interlaminar stress criterion where W is
the resultant of the two shear components (Vxz and Vyz). To deal with problems of singularity at free edges and
close to ply-drops it has been chosen to average the stresses over a squared zone (due to the assumed isotropy of the
interface). Identification of the criterion requires finding out the critical opening and shear stresses and the size of
the zone used to average stresses.
1
2
2
¸
¸
¹
·
¨
¨
©
§

¸
¸
¹
·
¨
¨
©
§
rupturerupture
zz
zz
W
W
V
V
(1)
For each configuration, the interlaminar stresses fields were calculated for all the ply drop-offs at the compressive
failure load of the configuration. The identification of the 3 criterion parameters was then performed by maximizing
the number of critical points found out in a confidence interval, corresponding to the maximal standard deviation
observed for the static strengths. As can be seen in Fig. 9 , most of the critical areas where delamination initiations
lead to the brutal failure of the specimen under compressive static loading are correctly included inside the
confidence interval.
0
20
40
60
80
100
120
140
160
180
200
0 102030405060
ızz (MPa)
W
(MPa)
No delaminations
Delamination s pots
Identified criterion
Interval of confidence
Fig. 9 Delamination initial criterion under static loading for all configurations
The parameters identified are presented in Table 3 . The distance needed to average stresses represents 1.5 times
the elementary ply thickness which is consistent with literature data [7-8]. Interlaminar stresses are of correct order
of magnitude, but the critical shear stress is clearly not representative of a typically expected value. This is certainly
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8 A.Weiss et al. / Procedia Engineering 00 (2010) 000–000
due to the material linear assumption used to calculate the interlaminar stresses and this has to be addressed in the
future. The criterion herein defined is semi-empirical and may be regarded as qualitative only.
Table 3 Identified criterion parameters
a (mm) Vzz (MPa) W (MPa)
0.375 55 150
4.4. Damage evolution in fatigue
Fig.10 presents the variation of total elongation vs cycle ratio to failure for the specimens cycled at 50% load
level. For all configurations, the stiffness remains almost constant at the beginning of the fatigue life, up to a point
where it starts to decrease steadily and to finally, close to the end of life, drop brutally. Damage evolution for the v2
and v4 configurations presents the most progressive evolution of stiffness during fatigue life. To identify the damage
kinetics during fatigue one side of the specimen has been filmed. Observations show that there are three stages: a
cycling without apparent damage, then delamination initiations with a stable propagation, slow or fast depending
upon the configuration, and finally an unstable propagation very close to final failure.
Fig.10 . Elongation variation for all configurations at 50% load level
-5
0
5
10
15
20
25
30
35
0 102030405060708090100
lifetime in %
Elongation varia tion (%)
v2 5 0%
v3 5 0%
v4 5 0%
v12 50%
v13 50%
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A.Weiss et al./ Procedia Engineering 00 (2010) 000–000 9
Close to the final failure event, the films show that all the configurations are separated into three sub-laminates,
resulting from the propagation of delaminations that initiated around 1st and 2nd ply-drops, propagating towards the
thicker part of specimen. The number of continuous plies at 0° contained in these sub-laminates directly depends
upon the 1st and 2nd ply-drop locations in the specimen thickness (see Table 4).
For all configurations, where the 1st and 2nd ply-drops are at 0°, stress/life curves comparison (Fig.11 ) shows the
effect of ply-drop location has a fair effect on the fatigue behaviour. But there is no clear tendency concerning a
precise effect of these locations. However, when comparing the effect of the orientation of 1st & 2nd ply drop on
stress/life curves, (Fig. 12 ) it is seen that it may be beneficial for fatigue performance to first stop disoriented plies
at 45°, rather than plies at 0°.
100
150
200
250
300
350
400
450
500
550
600
1,E+00 1,E+01 1,E+02 1,E+03 1,E+04 1,E+05 1,E+06
Number of fatigue cycles
Peak Stress (MPa)
v2 v3 v4 v12
Fig.11 Stress/life curves for configurations with 1st & 2nd ply drops at 0°
0
50
100
150
200
250
300
350
400
450
500
550
600
1 10 100 1000 10000 100000 1000000
Numbe r of cycles to fa ilure
Peak stress (MPa)
1st & 2nd Plydrop at 0°
1st & 2nd plydrops at 45°
Fig. 12 Comparison of Stress/life curves for configuration with 1st & 2nd ply drops at 0° (v2) and at 45° (v5)
It was observed that the location in the thickness of the 1st and 2nd ply-drops at 0°, where delaminations initiate
first, determines the number of plies at 0° in the sub-laminates created by the delaminations propagation. And this
parting of laminates seems to have an effect on the fatigue performance. The larger the number of 0° plies per sub-
laminate, the longer the lifetime (see Table 4).
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10 A.Weiss et al. / Procedia Engineering 00 (2010) 000–000
Table 4 Number of plies at 0° per sublaminate compared to Nf at 50% of load level for each configuration
N° of Sub-laminate v2 v13 v3 v12 v4
1 2 1 4 1 0
2 2 3 1 4 6
3 2 1 1 1 0
Average Nf (at 50%) 31600 50000 111000 190000 207260
5. Conclusions and perspectives
A specific tapered highly oriented composite laminate has been defined with 8 ply-drops. Several specimen
configurations with different positions of ply-drops in the thickness direction have been tested, under static and
fatigue loading at R=-1.
Static tests have shown that compressive loading is much more critical than tension. Catastrophic failure does not
allow the identification of the static damage process. However, observations during cyclic loading have shown that
delaminations firstly initiate around the 1st and 2nd ply-drops the closest to the thin side part of specimen.
Furthermore, delamination initiates preferably on one side of specimens depending upon the configuration and the
ply-drop. A 3D global-local FE model has been developed to calculate the interlaminar stresses due to both free-
edge effects and ply-drop effects. Comparing the experimental observations with the numerical values shows that
the side with the highest frequency of delamination damage corresponds to the side with the highest interlaminar
stresses. A semi-empirical interlaminar stress criterion has then been evaluated to predict the delamination initiation
leading to the static failure of the different configurations.
Damage evolution during fatigue was observed and follows a three stage process: namely, a cycling without
damage, then delamination initiations with a stable propagation, slow or fast depending upon the configuration, and
finally an unstable propagation very close to final failure. Stress/life curves have shown that ply drops design
configurations have a relatively effect on the fatigue life. For instance, it may be beneficial to drop the ply with a
45° orientation first. Furthermore, it has been observed that the propagation of initial delaminations separate the
laminate into three sub-laminates depending on the 1st and 2nd ply-drops location. The number of 0° plies inside
these sub-laminates presents a clear relation with the fatigue life, giving an idea of the key role played by ply-drop
locations on fatigue behaviour of tapered laminate.
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