# 3D Microcracking networks of stitched multiaxial laminates under thermal loadings

Quynh Nguyen Thi Thuy, François Valdivieso, Alain Vautrin

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Quynh Nguyen Thi Thuy, François Valdivieso, Alain Vautrin. 3D Microcracking networks of stitched multiaxial laminates under thermal loadings. 18th International Conference on Composite Materials (ICCM18), Aug 2011, Jeju Island, South Korea. [hal-02530473](https://hal.science/hal-02530473)

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<figure><img src="image_1.png" /></figure>

Distributed under a Creative Commons Attribution 4.0 International License
3D MICROCRACKING NETWORKS OF STITCHED
MULTIAXIAL LAMINATES UNDER THERMAL LOADINGS

Q. Nguyen Thi Thuy, F. Valdivieso, A. Vautrin
LCG, École Nationale Supérieure des Mines de Saint-Étienne
158, cours Fauriel, F-42023 Saint-Étienne Cedex2
Email: nguyen@emse.fr

**Keywords: Stitched laminates, Microcracking, X-ray Microtomography, Thermal fatigue**

**1. Introduction**

Until recently, structural composites used in the aeronautical industry were still manufactured by the traditional process based on prepregs autoclaving. But occurrence of stitched multiaxial laminates NCF (Non Crimp Fabric) conducts to a new type of high performance preforms for manufacturing complex composite structures by RTM (Resin Transfer Moulding) or infusion process. However, they have a particular morphology as well as a spatial heterogeneity with resin-rich regions inside the material. In this study, a specific 3D cracking network was found out within the resin regions after the material has been subjected to thermal fatigue. The aim of this work is to focus the resin-rich regions due to stitching yarn section, which are the cause of the specific crack network under thermal cyclical loading, and not the resin-fibre heterogeneity at general composite micro-scale. In order to characterise the microcracking and determine the effect of resin-rich regions on crack initiation and propagation, a cyclical thermal loading is defined and is applied. A refined analysis relying on X-ray microtomography is carried out on test specimens, periodically.

**2. Materials**

The material studied is a family of composites reinforced by NC2® (Non Crimp New Concept), which is a special type of graphite fiber quadriaxial [135/0/45/90]s NCF multiaxial laminates manufactured by HEXCEL Company [9]. This laminate is composed of two unidirectional layers that consists of Toho Tenax 12K HTS 5631 fiber and HexFlow® RTM6 resin; each one has its own stitching yarn system, namely PET 76 dTex, which binds four 0.25 mm thick and 268g/m² plies together. Constituent properties are detailed in the Tab. 1. The only aim of using stitching yarn is to make easier the implementation and to handle the layers. The effects due to fiber disturbance are still complex to be controlled [9, 10, 11, 12]. This leads to

the concentrated areas of resin not only at the stitching holes but also in the inter-layers. The material is then a repeatedly heterogeneous structure due to the stitching location. The pattern is reproduced along longitudinal and transversal direction every 5 mm. Since each layer has its own stitching system, the material does not have a perfectly symmetric morphology [9]. More details will be showed in paragraph 4.

**3. Methods**

**3.1 Loading conditions**

The hygrothermal effect is an important parameter in aeronautics since the composite material is designed to operate under different environmental conditions. The supposed lifetime of European Supersonic Civil Transport aircraft corresponds to about 20 000 thermal cycles between -55°C and 120°C with a total length of the plateau at 120°C of 80 000 h (10 years) [13]. There is then a need of accelerated aging cycles in order to estimate the durability of materials in laboratory tests. To identify the parameters which control the microcracking of the material [1,2], 400 pure thermal fatigue cycles [Fig. 1] have been applied throughout this study [9]. The range of temperatures goes from -55°C up to 80°C. The dry air injection is maintained to achieve the zero humidity during conditioning.

**3.2 X-ray tomography**

The main reasons that makes X-ray tomography a real need for characterization of certain composite microstructures are explained clearly in the literature [3-7], the principle of the method is presented in details as well [3-8].

Throughout this non-destructive testing, the X-ray monochromatic beam is sent on a sample and the transmitted beam is recorded on a CCD detector of about 2284 pixel × 2176 pixel. The resulting image is a superimposed information (projection) of a volume in a 2D plane and the reconstructed volume is obtained with 3000 2D-images by rotating the
sample every 0.12° between 0° and 360°. The
perceptibility of an anomaly part in the object
depends on 3 factors: enlargement coefficient, image
resolution and contrast. The achievable resolution
(image sharpness) is at first influenced by X-ray
source dimension. It reached the tubes filled with
Nanofocus™ technology values below 1 micron.
The contrast is due to different absorption behaviors
in different areas of a given sample. The factors
affecting the absorption differences are changes in
thickness or material properties and energy in this
case of monochromatic source. In this study, the
phase contrast mode [3] was employed because of
the low density difference between the polymer resin
(1.14g/cm³) and graphite fiber (1.76g/cm³), atomic
numbers as well as in the absorption coefficients of
cracks and resin [3,4,6]. A tension of about 80 kV
and an intensity of about 40 mA are required in
order to have a better gray level histogram with a
good distinction of peak corresponding to cracks,
resin and composite. A spatial resolution of 2.57 µm
is obtained with a compromise between the
resolution and the specimen size. The specimen
dimensions are about 5.5 mm × 6.5 mm × 2 mm, that
can cover the material «REV (Representative
Elementary Volume)» (5 mm × 5 mm × 2 mm).

4. Results and discussion

4.1 Material morphology

After 400 thermal fatigue cycles, special specimens
were cut out from the tested material to be observed.
The complexity of the material morphology
characterized by X-ray microtomography is shown
in Figure 2.a. The resin-rich regions with cracks, the
stitching yarn's filaments were also visualized in 3D
direction [Fig. 3]. By using threshold segmentation,
the spatial resin-rich regions were extracted, Figure
2.b shows transverse channels formed inside the
plies and large interlayer gap between layers due to
the stitching yarn section. All these resin-rich
regions constitute a continuous resin network where
cracks are found lying within.

4.2. 3D microcracking network

A 3D microcrack network was then reconstructed
and visualized. The key result from the observations
is the occurrence of cracks in more or less every
resin-rich region [Fig. 4]. The most important cracks
seem to be located in surface ply resin-rich regions
(135° plies) and interlayer gaps (90° plies). On the
whole of observed volume, we found that cracks are
mainly oriented along the length of the resin-rich
regions, a small number of them however may go

through the width of those regions [Fig. 5]. This
crack morphology variety can lead to a high scatter
in the crack counting by 2D classical observation on
the specimen sides [2]. Virtual cuts of the 3D
microcrack network may show it well. At the
interface between two contiguous plies, the
contiguous crack connection goes through the
common resin area [Fig. 6]. Delamination was not
detected throughout these tests.

5. Conclusion and perspectives

The global resin-rich region geometries were
reconstructed and the representative 3D crack
network of the quadriaxial reinforced by NC2®
subjected to thermal fatigue 400 cycles was
revealed. A quantification of resin-rich regions
should be carried out so that the real fiber volume
fraction outside the disturbance regions would be
determined. The spatial location of crack network
inside the resin-rich regions should be picked up.
Future fatigue testing will be performed to know
how those cracks may propagate with cycle numbers
and especially whether they propagate outside the
resin-rich regions through the composite plies. The
damage criterion would be then taken care.

Acknowledgment

This research would not have been possible and the
results achieved would not have been outstanding
without the cooperation and the competence of X-
ray tomography group at SPIN (Sciences des
Processus Industriels et Naturels).

The authors would also like to thank the HEXCEL
Company for the material knowledge as well as the
original equipment.

<figure><img src="image_2.png" /><figcaption>Fig. 1. Cyclical purely thermal loading.</figcaption></figure>
<figure><img src="image_3.png" /><figcaption>a)</figcaption></figure>

<figure><img src="image_4.png" /><figcaption>b)</figcaption></figure>

<figure><img src="image_5.png" /><figcaption>Fig. 4. Microcracking network of a quadriaxial laminate</figcaption></figure>

<figure><img src="image_6.png" /><figcaption>135° Ply</figcaption></figure>

<caption>0° Ply</caption>

<caption>90° Ply</caption>

<caption>Fig. 6. Crack connection at two contiguous plies interface.</caption>

<figure><img src="image_7.png" /><figcaption>Fig. 3. Cuts from a 3D resin-rich region morphology.</figcaption></figure>

<table><caption>Tab. 1. Constituent properties of the composite reinforced by NC2® (longitudinal / transversal).</caption><thead><tr><th>Constituents</th><th>Nomenclature</th><th>Filament count</th><th>Filament Φ (μm)</th><th>Twist (t/m)</th><th>Density (g/cm<sup>3</sup>)</th><th>Linear density (tex)</th></tr></thead><tbody><tr><td>Fibre</td><td>Toho Tenax HTS 5631</td><td>12000</td><td>7</td><td>0</td><td>1.76</td><td>0.068</td></tr><tr><td>Resin</td><td>RTM6</td><td>-</td><td>-</td><td>-</td><td>1.14</td><td>-</td></tr><tr><td>Stitching yarn</td><td>PET (Polyester)</td><td>24</td><td>18</td><td>-</td><td>-</td><td>7.6</td></tr></tbody></table><table><thead><tr><th>Nomenclature</th><th>E (GPa)</th><th>ν</th><th>σ<sub> tens ultime</sub> (Mpa)</th><th>Elongation (%)</th><th>CTE (10-6/K)</th><th>Service temperatures</th></tr></thead><tbody><tr><td>Toho Tenax HTS 5631</td><td>238/25</td><td>0.27/0.028</td><td>4300</td><td>1.8</td><td>-0.1</td><td>[, 160°C]</td></tr><tr><td>RTM6</td><td>2.89</td><td>0.38</td><td>75</td><td>3.4</td><td>50</td><td>[-60°C, 180°C]</td></tr><tr><td>PET (Polyester)</td><td>8.4/[2.8, 4.1]</td><td></td><td>[55, 75]</td><td>21</td><td>[5, 50]/[40, 100]</td><td>[, 163°C]</td></tr></tbody></table>

## References

[1] PJ. Liotier, A. Vautrin, C. Delisée "Characterization of 3D morphology and microcracks in composites reinforced by multi-axial multi-ply stitched preforms". *Composites: Part A*, 41, pp 653-662, 2010.

[2] PJ. Liotier, A. Vautrin, JM. Béraud "Microcracking of composites reinforced by stitched multiaxials subjected to cyclical hygrothermal loadings". *Composites: Part A, In Press*, 2011.

[3] F. Cosmi, A. Bernasconi, N. Sodini "Phase contrast micro-tomography and morphological analysis of a
short carbon fibre reinforced polyamide”. *Composites Science and Technology*, 71, pp 23-30, 2011.

[4] A. Bernasconi, F. Cosmi, D. Dreossi “Local anisotropy analysis of injection moulded fibre reinforced polymer composites”. *Composites Science and Technology*, 68, pp 2574-2581, 2008.

[5] P. Badel, E. Vidal-Sallé, E. Maire, P. Boisse “Simulation and tomography analysis of textile composite reinforcement deformation at the mesoscopic scale”. *Composites Science and Technology*, 68, pp 2433-2440, 2008.

[6] J.S.U. Schell, M. Renggli, G.H. van Lenthe, R. Müller, P. Ermanni “Micro-computed tomography determination of glass fibre reinforced polymer meso-structure”. *Composites Science and Technology*, 66, pp 2016-2022, 2006.

[7] PJ. Schilling, BR. Karedla, AK. Tatiparthi, MA. Verges, PD. Herrington “X-ray computed microtomography of internal damage in fiber reinforced polymer matrix composites”. *Composites Science and Technology*, 65, pp 2071-2078, 2005.

[8] L. Salvo, P. Cloetens, E. Maire, S. Zabler, J.J. Blandin, J.Y. Buffière, W. Ludwig, E. Boller, D. Bellet, C. Josserond “X-ray micro-tomography an attractive characterisation technique in materials science”. *NIM B Beam interactions with Materials & Atoms*, 200, pp 273-286, 2003.

[9] Q. Nguyen Thi Thuy, PJ. Liotier, JM. Béraud, A. Vautrin “Microcracking of new composites based on multiaxial multiply stitched performs under cyclical thermal loadings”. *JNC 17, Poitiers, France*, pp 1-9, 2011.

[10] D.S. Mikhaluk, T.C. Truong, A.I. Borovkov, S.V. Lomov, I. Verpoest “Experimental observations and finite element modelling of damage initiation and evolution in carbon/epoxy non-crimp fabric composites”. *Engineering Fracture Mechanics*, 75, pp 2751-2766, 2008.

[11] S.V. Lomov, E.B. Belov, T. Bischoff, S.B. Ghosh, T. truong chi, I. verpoest “Carbon composites based on multiaxial multiply stitched preforms. Part 1. Geometry of the preform”. *Composites: Part A*, 33, pp 1171-1183, 2002.

[12] H. Heb, Y.C. Roth, N. Himmel “Elastic constants estimation of stitched NCF CFRP laminates based on a finite element unit-cell model”. *Composites Science and Technology*, 67, pp 1081-1095, 2007.

[13] V. Bellenger, J. Decelle, N. Huet “Ageing of a carbon epoxy composite for aeronautic applications”, *Composites: Part B*, 36, pp 189-194, 2000.