Damage mechanisms induced by cyclic ply-stresses in carbon–epoxy laminates: Environmental effects

International Journal of Fatigue 28 (2006) 1202–1216

International Journalof Fatigue www.elsevier.com/locate/ijfatigue

Damage mechanisms induced by cyclic ply-stresses in carbon–epoxy laminates: Environmental effects M.C. Lafarie-Frenot

*

LMPM, UMR CNRS 6617, ENSMA, Teleport 2, 1 Ave Clement Ader – BP 40109, F-86961 Futuroscope-Chasseneuil Cedex, France Available online 22 March 2006

Abstract This paper aims to identify and to describe the damage mechanisms of CFRP composite laminates subjected to thermal cycling. Thermal cycling tests are performed in different atmospheres, more or less oxidative (air or oxygen) or neutral (nitrogen). Microscopic observations and weight measurements of cross-ply laminates samples, put in light a significant oxidation of the matrix when tests are carried out in oxidative environment. In such thermal loading, there exists a coupling between two degradation processes: oxidation and fatigue, which dramatically accelerates the damage build-up. In a second part of this paper, the results of some iso- and cyclic-thermal tests, on virgin or damaged specimens, are analysed in order to emphasise the coupling between the degradation mechanisms.  2006 Elsevier Ltd. All rights reserved. Keywords: Durability; Fatigue; Oxidation; Matrix cracking; Thermo-mechanical properties

1. Introduction The objective of this paper is to review the numerous experimental results obtained and analyses developed in our laboratory during the last five years. They mainly concerned the damage processes in CFRP laminates subjected to thermal cycling. Most of the results presented here have been obtained in the framework of the French ‘‘Aeronautical Supersonic Research’’ program. This program encourages fundamental research in fields where scientific and technological projections will be essential for the design of new generation of supersonic aircraft. Compared to Concorde, the weight saving required on the aircraft structure is estimated at about 30%. This requirement makes necessary the use of organic composite materials for the manufacturing of most of the structure (Concorde was primarily made of aluminium alloys). This new future supersonic aircraft, designed to fly at Mach 2, will have a lifetime from 3 to 4 times longer than that of the Concorde, corresponding to a minimum of 80,000 h, i.e., approximately 20,000 flights. *

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0142-1123/$ - see front matter  2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.ijfatigue.2006.02.014

In these aeronautical applications, structural parts are subjected to both cyclic mechanical loading and temperature variations (between 50 and about 130 C) depending on fly stages, the high temperatures being sustained during the supersonic flight. Because of the greatly anisotropic thermo-mechanical behaviour of each unidirectional ply of a CFRP composite laminate, these two types of ‘‘loading’’ induce cyclic stresses in each layer of a laminate. Those cyclic ply-stresses may induce ‘‘fatigue’’ damage development, mainly consisting of early matrix cracking. In this context, the knowledge of the long-term performances of organic matrix composites is essential and includes combined effects of thermo-mechanical cyclic loading and long-term ageing. Within the framework of the American program ‘‘High Civil Speed Transport’’, project at Mach 2.4, Arendt et al. [1] described a campaign of tests meant to estimate the durability of some composite materials, candidates to this type of application. Averaged cyclic thermo-mechanical conditions are simulated, consisting of the superposition of temperature cycles ( 36, 200 C) and of stress cycles, the period of which is the mean duration of a flight. This approach leads to very long tests, is very expensive, and requires making a priori the choice of definite materials.

M.C. Lafarie-Frenot / International Journal of Fatigue 28 (2006) 1202–1216

On the other hand, Gates [2] proposed another procedure, consisting of the definition of accelerated tests by increasing some critical values of parameters characteristic of physical ageing and chemical degradation of the materials considered. He insists on the need of understanding and characterising the coupling that can appear between these parameters, under the combined effects of a mechanical loading, of the temperature and of the environment. When composite laminates with long continuous fibres are subjected to temperature variations, the mismatch of thermal expansion coefficients of fibres and matrix as well as the difference of ply orientation in the lay-up, are such that local stresses appear, which can take part in the degradation of the laminate. When these thermal variations are cyclic, they induce, at the ply level, cyclic stress variations that can be compared, at this scale, to a fatigue phenomenon. Various types of damage similar to those observed in mechanical fatigue result from these cyclic stresses, like transverse matrix cracking, fibre/matrix debonding and delamination [3,4]. When these temperature variations occur in the presence of an oxidative environment, other damaging phenomena appear due to matrix oxidation. Concerning thermoset epoxy polymers, the oxidation phenomenon that occurs during high temperature exposure in oxidative atmospheres was studied through isothermal ageing experiments by Madhukar et al. [5], and Colin et al. [6]. Studies carried out on epoxy–resins [7,8], emphasise the superficial character of oxidation, which results in the creation of a low thickness oxidised layer on the free edges of the samples subjected to ageing. During ageing, the thickness of this layer grows quickly towards an asymptotic value depending on the studied material and on the temperature of ageing. These authors have shown that the oxidation of an epoxy–resin involves a weight loss and a density increase and as a consequence, some shrinkage of the skin layer. A kinetic model of radical chain oxidation coupled with the equation of oxygen diffusion predicts the concentration profile of oxidation products, the weight loss and the shrinkage profiles in a thick part of neat resin [8]. In composite materials, the oxidation mechanism strongly depends on temperature and oxygen pressure [9], specimen geometry, anisotropy [10–12] and matrix–fibre bonding [13]. The detection and the characterisation of this oxidation were also completed by micro hardness Vickers tests [14] on various composite laminates with carbon fibres and different thermoset or thermoplastic matrices. Parvatareddy et al. have shown that the oxidised layer has a smaller Vickers hardness compared with that of the virgin material and that this reduction in hardness depends on the atmosphere oxygen concentration. Few works deal with the long-term behaviour of composites subjected to high temperature under oxidative atmosphere. Bowles [15] studied the ageing of T650-35/PMR15 composites at various temperatures and experimentally found a linear relation between the properties in compression of the laminates and the thickness of the oxidised layer.

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The prediction of matrix cracking growth under thermal cyclic variations was not studied as much, as its mechanical fatigue loading counterpart. Favre et al. [16], studied the damage behaviour of various composite materials, with carbon fibres and thermoset matrices (epoxy, cyanate or bismaleimide), subjected to thermal cycling. These authors highlighted an interaction between the matrix micro-cracking of the laminate layers due to the cyclic thermal stresses and oxidation already observed under isothermal ageing. However, as these authors noted, the knowledge of the mechanisms of both crack accumulation and of oxidation is necessary before a relevant prediction can be made. In our laboratory, campaigns of thermal cycling tests have been performed on various carbon/epoxy laminates in more or less oxidative or neutral atmospheres (dry air, pure oxygen and nitrogen). In a first part of this paper, we will attempt to put in light the influence of the environment of the thermal cycling test on different damage mechanisms: • Superficial matrix shrinkage due to the oxidation process [17]. • Matrix cracking, quantified through microscopic observations of the edges of the samples and X-radiographs [18]. • Weight loss. In a second part, the work in progress, made in collaboration through the ‘‘Aeronautical Supersonic Research’’ program, will be briefly presented and discussed: • Use of a kinetic model of oxidation coupled with the equation of oxygen diffusion to predict the mass loss of the samples subjected to thermal cycling in oxidative environment [19]. • Experimental evidence of coupling between oxidation and matrix cracking by isothermal ageing of damaged specimens [20].

2. Experimental conditions All the specimens used in the present study are cut in composite plates that were provided and processed by CCR-EADS (Corporate Research Centre – France – of the European Aeronautic Defence and Space Company). The composite material is made of an epoxy/amine matrix (ref 977-2) and reinforced by continuous carbon fibres (ref type IM7). The plates have been elaborated in an autoclave, according to a specific cycle of polymerisation, optimised for the needs of a supersonic use, which requires a stable material without any effect of overheating or evolution of properties. This polymerisation cycle consists in a 3 h long phase of gelation at 150 C followed by a phase of polymerisation at 180 C, 2 h long with a 7 bars pressure. A post cure cycle, 2 h long at 210 C, was thus added in order to improve the characteristics of the material.

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Laminate coupons of stacking sequence {03/903}s have been used (cf. Fig. 1). The specimens dimensions were 35 · 25 · 1.68 mm3, the length direction (x) being referred to as the 0 axis. Before test, the edges of the samples have been smoothly polished and dried under vacuum at 80 C to reach a dry state of reference. A specific thermal cycling device has been developed in order to control both the environment and the temperature of the test. First, the polished and dried samples are placed vertically on racks so that the gas is circulated in a homogeneous way: all the faces of the samples undergo the same effects due to temperature and gas flow (Fig. 2a). Then, the racks are put into a specific enclosure of low volume and small inertia, in which the gas circulates thanks to a small overpressure (Fig. 2b). Finally, this enclosure is placed inside a thermal equipment allowing controlled temperature variations to be prescribed (Fig. 2c). The ‘‘thermal loading’’ consisted in triangular thermal cycles, with constant cooling and heating rates of 4 C/ mn. Two types of thermal cycling tests have been performed: one consisted in 500 cycles, the maximum and minimum temperatures being, respectively, 180 and 50 C and the other in 1000 cycles between 150 and 50 C. The prescribed temperature variation was controlled by a thermocouple located inside the test-tube. The maximum temperatures of the thermal cycles, 150 and 180 C, were selected in order to accelerate the damage processes: they are much higher than those supported by this material in real flight, but lower than the glass transition temperature that has been measured between 200 and 210 C.

Fig. 1. Specimen geometry.

In order to put in light the specific role of oxidation on damage development, the tests presented here were conducted in three atmospheres: neutral (pure dry nitrogen) and oxidative (dry air reconstituted by 22% O2 + 78% N2, and pure dry oxygen). Thermo-elastic calculations [21], give an idea of the level of thermal stresses present in each ply of the laminate for a given temperature. These stresses stem from the mismatch of thermal expansion coefficients of plies with different orientations. The lay up restrains differential thermal deformation of the plies and induces thermal stresses in each oriented layer. Moreover, the magnitude of these thermal stresses increases as the temperature of the material deviates from the stress-free temperature. In order to have a rough idea on the thermal stress level, the stress-free temperature has been taken equal to the polymerisation temperature, i.e., 180 C. As an example, in the case of the thermal cycling between 50 and 180 C, the transverse thermal stresses are equal in the different layers of the cross-ply laminate {03/903}s, and lie between 0 and 53.6 MPa. In Fig. 3, the transverse thermal stresses (r22) are put in correspondence with the variation of temperature for the two thermal cycles considered: the cyclic temperature variations induce cyclic transverse stress variations, the maximum being reached at 50 C. The maximum value of the ply-stress cycle is therefore the same for the two tests whereas the minimum varies slightly (7 MPa between 150 and 180 C). This cyclic ‘‘thermal loading’’ is equivalent to a sort of thermally driven ‘‘fatigue’’ with DT analogous to the change in stress as previously noted by Hancox [22]. During tests, the samples have been removed regularly from the thermal cycling oven to observe damage development: on the free edges of the specimens by optical microscopy and Scanning Electron Microscopy (SEM) and inside by means of penetrant enhanced X radiography. As a precaution the samples were not re-used after SEM observations or X radiographs.

Fig. 2. Environmentally controlled thermal cycling device.

M.C. Lafarie-Frenot / International Journal of Fatigue 28 (2006) 1202–1216 T°C

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T°C 180˚C 150˚C

σ22

-50˚C

σ22

7 MPa

0 MPa

a

-50˚C 53.6 MPa

53.6 MPa

b

Fig. 3. Schematic representation of the prescribed thermal cycles and corresponding transverse ply-stresses in each layer of {03/903}s laminates.

3. Damage mechanisms in thermal cycling Prior to thermal cycling tests, samples were examined in order to ensure that there were no initial cracks or defects on the free surfaces, which were smoothly polished. Thermal cycling tests induce various types of damages, depending to a certain extent on the orientation and the thickness of the layers in the lay-up, but mainly on the environment of the specimen. In order to compare damages according to the location of the plies in the stacking sequence of the laminate, microscopic observations have been conducted on two perpendicular sides of the samples. Throughout thermal cycling tests, the observations of the free edges have shown three types of damages: permanent deformation of the matrix due to its shrinkage, debonding between fibres and matrix, and matrix cracking. 3.1. Matrix shrinkage In Fig. 4, SEM pictures of the free edge of a specimen that has experienced 500 thermal cycles ( 50 C/180 C) in air are shown as an example. One can see on these pictures some more or less important permanent deformation of the matrix, which is distributed on all the surface of the ply. This matrix ‘‘shrinkage’’ appears on the free edges of the sample as a difference of level between matrix and fibres and is visualised as dark and deep hollows, imprinted on the initial plane of the polished edge. Moreover, the wider the matrix areas concerned, the deeper the hollows. Such observations have been made throughout every thermal

cycling tests performed in oxidative atmosphere (air and oxygen). Moreover, in oxidative environment, numerous fibre/matrix debonding are observed in the circumference of the matrix holes, and short matrix cracks, which seem to be connected with those debonding (see Fig. 4). Such damage mechanisms do not exist when thermal cycling tests are performed in nitrogen as can be seen in Fig. 5, in which SEM pictures obtained after 500 thermal cycles in nitrogen, in air and in oxygen are compared. Throughout the tests, the matrix shrinkage was observable on all the surfaces of the specimens, but such observations did not allow specifying if its ‘‘depth’’ varies according to the time of ageing. In order to measure the depth of these residual deformations of the matrix, an optical profiler was used (WYKO NT1100 from Veeco). Thanks to this optical interferometer associated to a motorised stage for measurements over a larger field of view, quantitative results were obtained and processed statistically. For each experimental condition, about 150 measurements have been performed on three different specimens. As an example, all the data collected on specimens subjected, respectively, to 100, 300 and 500 thermal cycles ( 50 C/180 C) in air and in oxygen are gathered in the frequency distributions shown in Fig. 6. One can see in these figures that the shapes of the distributions are similar: they are not Gaussian, the lower depth values being quite more frequent than the greater ones. However, as the number of thermal cycles increases, the peaks of the distributions present a progressive shift towards higher depth intervals and the extent of the depth

Fig. 4. SEM observations of {03/903}s specimen free edge after 500 thermal cycles ( 50 C/180 C) in air; matrix shrinkage and fibre matrix debonding (sample titled at 45).

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Fig. 5. Comparison of SEM pictures (45 tilt) of the polished edges of {03/903}s laminates subjected to 500 thermal cycles ( 50 C/180 C) in different environments.

40

40

Air, 100 thermal cycles

Oxygen, 100 thermal cycles

Air, 300 thermal cycles

Oxygen, 300 thermal cycles

Air, 500 thermal cycles

Oxygen, 500 thermal cycles 30

Frequency (%)

Frequency (%)

30

20

20

10

10

0

0 0.4-0.5

0.5-1

1-1.5

1.5-2

2-2.5

2.5-3

3-3.5

3.5-4

4-4.5

4.5-5

5-5.5

5.5-6

6-6.5

6.5-7

7-7.5

0.4-0.5

7.5-8

0.5-1

1-1.5

1.5-2

2-2.5

2.5-3

3-3.5

3.5-4

4-4.5

4.5-5

5-5.5

5.5-6

6-6.5

6.5-7

7-7.5

7.5-8

Depth, (micrometer)

Depth, (micrometer)

Air

Oxygen

Fig. 6. {03/903}s laminates, frequency distribution of matrix shrinkage depth, for different numbers of thermal cycles ( 50 C/180 C) in air and oxygen.

values concerned increases. Moreover, one can see that the extent of the frequency distribution is larger in oxygen than in air. Measurements of matrix shrinkage depths have been done for specimens tested either in nitrogen, or in air and oxygen. The statistical results obtained from the depth distributions are collected in Table 1 for comparison. One can see in Table 1 that the mean ‘‘depth’’ value measured on flat surfaces of specimens submitted to 500 thermal cycles in nitrogen is very low (0.18 lm), the extent of the distribution being less than 0.4 lm. Such values can be considered equal to those obtained on the initially polished surfaces. Comparatively, the mean shrinkage depths measured on specimens tested in oxidative environments are much higher: after 500 cycles performed in air or in oxygen, the mean values are close to each other and more than ten times higher than in nitrogen (2.2 lm). Moreover, the mean depth values increase with the number of cycles, and are systematically higher in oxygen than in air. The same remarks can be done about the values of the standard deviation and extent of the frequency distribu-

tions, showing that this damage mechanism is only significant in oxidative atmospheres and all the more as the oxygen concentration is high. In accordance with the epoxy-matrix oxidation model presented elsewhere [6], these types of damages, absent when tests have been carried out in nitrogen, could be induced by the oxidation process which is very active above 120 C in air and in oxygen. When matrix shrinkage occurs (i.e., in oxidative environment), the high strain gradients present in matrix areas, close to fibres which have a very high stiffness, lead to high local stresses that can be favourable to debonding between fibres and matrix or/and to crack initiation as shown in Fig. 4. 3.2. Transverse matrix cracking 3.2.1. Crack initiation on the edges (microscopic observations) As the test duration increases, one can observe on the edges of the samples that short micro cracks of the matrix

Table 1 {03/903}s laminate, statistical values of matrix shrinkage depth according to the number of thermal cycles ( 50 C/180 C) and the test environment Number of thermal cycles

Nitrogen

Air

Oxygen

500

100

300

500

100

300

500

Average (lm) Standard deviation (lm) Extent (lm)

0.18 0.08 [0–0.4]

1.15 0.57 [0.4–4]

1.75 1.04 [0.4–5.5]

2.2 1.11 [0.4–6]

1.6 1.06 [0.4–7]

1.87 1.11 [0.4–7.5]

2.25 1.43 [0.4–8]

M.C. Lafarie-Frenot / International Journal of Fatigue 28 (2006) 1202–1216

grow through the layer thickness and develop as ‘‘transverse cracks’’: these cracks span the whole thickness of the inner or external layers and increase in number with the number of thermal cycles. In Fig. 7 the SEM aspects of transverse cracks present in two {03/903}s specimens after 500 thermal cycles ( 50 C/ 180 C), either in nitrogen or in oxygen are compared. Once more, important differences in relief of the specimen edges appear: they remain very flat in specimen tested in nitrogen, whereas they are quite uneven in specimen tested in oxygen. These pictures show that the crack path is more tortuous in nitrogen than in oxygen and that the crack opening is much larger in O2 than in N2. In oxygen, the cracks are wide, well opened, allowing observations of the cracked surface of the material: in that case, the fracture pattern appears very brittle, most of the fibres are naked with no traces of residual matrix on them (Fig. 7b). Comparatively the cracks propagated in nitrogen

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are more superficial and narrow (Fig. 7a). In that case, the crack opening is so small that the fracture pattern cannot be observed by SEM and therefore cannot be described from such observations. Equivalent observations of transverse cracks induced by thermal cycling in air have shown intermediate trends, between those of specimens tested in oxygen and in nitrogen. However, the transverse cracking features (large crack opening, brittle aspect of the fracture. . .) have been found closer to those of the cracks developed in oxygen: that observation emphasises the influence of the oxidative environment on the transverse cracking mechanisms. 3.2.2. Thermal cycling ( 50 C/180 C): cracking development on the edges according to the environment In Fig. 8, the transverse cracking rate values obtained from the counting of transverse cracks on the polished edges of the {03/903}s samples cycled in nitrogen, air and

Fig. 7. SEM observations of {03/903}s specimen free edge after 500 thermal cycles ( 50 C/180 C) in nitrogen and in oxygen; matrix cracks (sample tilted at 45).

Fig. 8. Edge transverse cracking rates vs. number of thermal cycles ( 50 C/180 C), {03/903}s, N2: dark grey, air: black thin, O2: black large; central layer: full lines, external layers: dotted lines.

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oxygen are plotted versus the number of thermal cycles ( 50 C/180 C). The transverse cracking rate values (1/q) are dimensionless and are calculated from the expression: 1/q = n · H, where n is the number of transverse cracks per millimeter (/mm) and H the cracked layer thickness (mm). In Fig. 8, the measurements are distinguished according to the position of the considered layer in the stacking sequence: either in the six ply central layer, or in the three ply external layer. In that figure, the full lines correspond to the average development of the crack densities in the central layer, and the dotted lines to that in the external layers. The crack density measurements have been repeated twice, on two different specimens, and for the external layer, each value of crack density plotted in this figure is the average of measurements made on both external layers. It can be seen in Fig. 8 that the scattering on the measurements is rather small, and low enough to consider the average curves as representative of the crack density growth throughout the thermal cycling test. Comparing the three environmental conditions, one can see in Fig. 8 that the transverse cracking development is much faster and more important in an oxidative atmosphere (oxygen and air) than in a neutral one (nitrogen). In nitrogen, cracks have been observed in the central layer only, there was no crack at all in the external layers, even after 500 thermal cycles. In that environment, cracks appeared in the inner layer between 200 and 300 cycles and they increased in number slightly, up to a cracking rate level of about 0.17 ± 0.06 at the end of the thermal cycling test. In air, the first cracking onset occurred much earlier than in nitrogen: after 100 thermal cycles, the cracking rate measured in the central layer is already equal to 0.25 ± 0.06. In that layer, the increase in crack number is fast, up to 300 cycles when saturation is reached with a value of 0.56 ± 0.06. In the external layers, the cracks ini-

tiated later, between 100 and 200 cycles; and their number regularly increased up to a level of 0.26 ± 0.01 at 500 cycles. In oxygen, the kinetics of transverse cracking due to thermal cycling is the fastest we have observed. After only 100 cycles, the cracking rate in the central layer is 0.46 ± 0.03, and quasi saturated, the saturation value being reached at 300 cycles. In this central layer, the saturation value (0.58 ± 0.03) can be considered as identical to the one measured in air (0.56 ± 0.06). If we examine the cracking development in external layers, we can assume that the cracks initiated before 100 cycles, since the cracking rate is equal to 0.06 as early as 100 cycles; then, the number of cracks increased very quickly, up to a cracking rate of 0.36 ± 0.02 at 500 cycles. The comparison of the edge transverse cracking development according to the environment of the thermal cycling test emphasises the accelerating effect of an oxidative atmosphere. Moreover, the higher the oxygen concentration, (oxygen compared to air), the more significant is the acceleration of the damage processes. 3.2.3. Thermal cycling in oxygen: comparison between ( 50 C/180 C) and ( 50 C/150 C) The averaged transverse cracking rate values, obtained during the two thermal cycling tests in oxygen, ( 50 C/ 180 C) and ( 50 C/150 C), are plotted in Fig. 9 against the number of cycles. The measurements procedure was the same that described in the preceding section as well as the remarks about the scattering of the data. This figure emphasises the significant influence of the maximum temperature of the thermal cycles on the matrix transverse cracking kinetics. Let us remind that the thermal ply-stresses induced by the two thermal cycling conditions are very close, as shown

Fig. 9. Edge transverse cracking rate vs. number of thermal cycles in oxygen, ( 50 C/180 C) and ( 50 C/150 C); {03/903}s; central layer (full lines), external layers (dotted lines).

M.C. Lafarie-Frenot / International Journal of Fatigue 28 (2006) 1202–1216

by the thermo-elastic calculations visualised in Fig. 3. The maximum stress values are the same, because they occur for the minimum temperature that is the same for the two thermal cycles, and the cyclic stress amplitudes are very close, the difference being of about 13%. However, it can be seen in Fig. 9 that the cracking development is much faster and more important when the upper temperature of the cycle is higher. During the ( 50 C/180 C) thermal cycling, the matrix cracking on the edge is already well developed in central and external layers after 100 cycles whereas, during the ( 50 C/150 C) thermal cycling, no crack has been observed in none ply before 300 cycles. As the number of cycles increases, the edge transverse cracking rate increases in both layers, but much faster with the ( 50 C/180 C) conditions. At the end of each test: 500 cycles ( 50 C/180 C) and 1000 cycles ( 50 C/ 150 C), the damage states are close but still lower with the ( 50 C/150 C) conditions. The ultimate values of the transverse matrix cracking rate are collected in Table 2: the cracking development appeared saturated in the inner central layer of the laminate, whereas, in the external layers, it is rather less in the specimen tested with the lower maximum temperature. 3.2.4. Crack propagation (X radiography) As the number of cycles increases, these transverse cracks increase in number and length. An example of such development of cracking is given in Fig. 10 in which four X-radiographs obtained after 300, 500, 750 and 1000 thermal cycles ( 50 C/150 C) in oxygen are shown. The cracks are seen as black lines, the horizontal ones corresponding to the cracks present in the two external layers, whereas the vertical ones visualise the cracks in the internal layer of the laminate. These pictures show that, in oxygen, some cracks are present in internal and external layers from 300 cycles, that after, they increase in number and length in every plies and that 1000 thermal cycles ( 50 C/150 C) induced a very Table 2 {03/903}s laminate, oxygen, ultimate values of edge transverse matrix cracking rate Oxygen

( 50 C/180 C)

( 50 C/150 C)

Number of thermal cycles Transverse cracking rate central layers Transverse cracking rate external layers

500 0.58 ± 0.03

1000 0.56 ± 0.03

0. 36 ± 0.02

0.21 ± 0.02

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developed matrix cracking in every layer of the {03/903}s cross-ply laminate. At the end of the test, transverse cracks are well distributed in the length and width of the coupon. The cracks present in the external layers span the whole sample length, while, in the inner layer, the crack length does not exceed half the specimen width. Moreover it can be noted on those X-radiographs that, in the inner central layer, the cracks initiate on the free edges of the sample, are first very short and propagate from the edges towards the core of the specimen. On the contrary, in the external layers, the cracks initiate preferentially in the centre of the ply and when they appear, are longer than those in the central layer. In Fig. 11, the ultimate X-ray pictures obtained either after 500 thermal cycles ( 50 C/180 C), or after 1000 thermal cycles ( 50 C/150 C), are compared according to the environment of the test. In nitrogen (Fig. 11a), the damage built up during the two thermal tests is quite small. After 500 cycles ( 50 C/180 C), very few short cracks (whose length is less than the quarter width of the layer) are seen in the central layer whereas no crack is revealed in the external layers, in agreement with microscopic observations. Compared to that low damage state, the crack pattern observed in the sample that has experienced 1000 cycles ( 50 C/150 C) is a little bit more developed. The cracks present in the internal layer are longer and there are some cracks in the external layers. This observation is reasonable according to the small difference in stress amplitude and the great difference in number of cycles of the two thermal cycling tests. In oxygen (Fig. 11b), the matrix transverse cracking is very developed and one can observe that the samples are uniformly damaged, the crack distribution appearing very regular. Moreover, at that step of thermal cycling, some small grey areas can be seen on X-ray pictures, surrounding some matrix cracks and corresponding to partial delamination between central and external layers. In oxygen, the ultimate damage patterns are similar, in agreement with the microscopic observations of the edges. Those observations permit to conclude that, in oxygen, 500 thermal cycles ( 50 C/180 C) are, at least, as damaging as 1000 thermal cycles ( 50 C/150 C). 3.3. Weight loss In agreement with the literature, when our composite samples are subjected to thermal cycling or isothermal ageing under oxidising atmospheres, the high temperatures

Fig. 10. Cracking development in thermal cycling ( 50 C/150 C) in oxygen; X-ray pictures of {03/903}s samples.

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Fig. 11. Comparison of {03/903}s samples X radiographs according to the environment and to the amplitude of the thermal cycling test.

involve a thermo-oxidation of the matrix. This phenomenon of thermo-oxidation affects primarily the external surfaces of the samples and induces a weight loss of the specimens. This variation of mass is, for a long time, taken into account like an indicator of the degradation of the composite material. In this section, we present gravimetric follow-ups of the samples throughout thermal cycling tests. The specimens have been weighed at each interruption of the different tests and the obtained relative mass loss values (DM/M) are gathered in Fig. 12, against the number of thermal cycles. In this figure one can observe a more or less important decrease in weigh occurring during the first thermal cycles. This mass variation corresponds probably to a phase of residual drying. Indeed, we have noted that the duration of this phase depended on that of the preliminary drying made in vacuum before tests: the longer the initial drying, the lower the number of thermal cycles until the mass loss rate appeared constant. One can see in Fig. 12 that, from around 100 cycles, the weight of the specimen tested in thermal cycling ( 50 C/150 C) in nitrogen does not vary any more. For the three other test conditions, the mass of the samples decreased regularly, the variation appearing linear with time of ageing. In the following, in

dM/M 0

order to exclude the part due to the residual drying from the total weight loss, we have considered only the weight loss rate of the linear part of these curves as representative of the effect of the thermal ageing. One can see in Fig. 12 that the thermal cycling ( 50 C/ 180 C) in nitrogen induced a significant loss of mass, contrarily to the ( 50 C/150 C) thermal cycling test in the same neutral environment. In agreement with some authors [23,24], this weak but continuous mass loss observed in nitrogen could be due to a thermolysis of the matrix occurring at the highest temperatures of the thermal cycles, above 150 C. In order to compare the oxidation level of the samples according to the maximum temperature of the thermal cycling imposed, it appeared necessary to exclude the mass loss due to both the residual drying and the thermolysis phenomenon. In Table 3 are given the rates of the relative mass variations due to the sole oxidation, obtained from the differences of mass variations measured during tests in oxygen and in neutral environments, in the linear part of their evolution. One can see in that table that the mass loss rate due to oxidation is about four times higher when the maximum temperature of the thermal cycle increases

cycles 200

400

600

800

1000

0,0E+00

[-50˚C/150˚C]

5,0E-04 1,0E-03 1,5E-03

[-50˚C/180˚C]

2,0E-03

N2

2,5E-03

O2

3,0E-03

Fig. 12. Experimental relative weight loss of {03/903}s specimens subjected to thermal cycling, ( 50 C/180 C) and ( 50 C/150 C), in nitrogen and in oxygen.

M.C. Lafarie-Frenot / International Journal of Fatigue 28 (2006) 1202–1216 Table 3 {03/903}s laminate, oxygen, mass loss rate according to the thermal cycle amplitude Oxygen ( 50 C/150 C) ( 50 C/180 C)

Mass loss rate cycle 6.2 · 10 25.0 · 10

1

7 7

from 150 to 180 C! As a consequence, and because the thermal ply-stresses are not significantly different in both tests, it appears that the preceding observation of a faster and a higher damaging process for the ( 50 C/180 C) thermal cycling is essentially due to the oxidation of the matrix which occurs in the upper part of the cycle. 3.4. Conclusion The comparison of these damage parameters according to the environment and the amplitude of the thermal cycling test emphasises the accelerating effect of an oxidative atmosphere. The comparison between tests conducted in air and in oxygen has shown that the higher the oxygen concentration, the more significant is the acceleration of the damage processes. Two different amplitudes of thermal cycling have been considered, which differed only by the value of the maximum temperature imposed. These experiments highlighted a significant influence of the matrix oxidation on the degradation rate of the composite laminate. All these results show that, in such type of loading, there exists a coupling between matrix cracking and oxidation processes, the degradation rates as well as the ultimate damage states depending on both the ply-stresses (essentially determined by the minimum temperature of the cycle and the laminate stacking sequence) and the thermal ageing conditions (maximum temperature of the cycle and oxygen concentration). 4. Experimental evidence of coupling between oxidation and matrix cracking 4.1. ‘‘Equivalent’’ isothermal ageing [19] Following the preceding conclusions, the aim of this experimental study was to separate the oxidation and mechanical stresses effects that are both combined in thermal cycling. Following that goal, we used a kinetic model of radical chain oxidation coupled with the equation of oxygen diffusion, which was developed and identified for the neat resin matrix and the UD composite material used in this study, in the framework of the ‘‘Supersonic research program’’. This model was used to predict successfully the weight loss of UD composite materials exposed to isothermal ageing [25,26]. Because in isothermal conditions, the weight loss does not vary in a monotonous way for short exposure time, the total weight loss of a sample subjected to a thermal cycling test ( 50 C/180 C) in air has been calculated for 500 cycles. These 500 cycles (that corresponds to a total duration of 1000 h) have been represented

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by a set of isothermal stages for which an oxidation time is attributed corresponding to the temperature variation of 4 C/min. With such a procedure, the total weight loss variation predicted by the model was found equal to 5.7 · 10 4 which is equivalent to the predicted weight loss of the same specimen which would experienced an isothermal ageing in air at 150 C during 500 h. From those calculations, we have considered that the isothermal test: 500 h, air, 150 C, would be ‘‘equivalent’’ to the thermal cycling one: 500 cycles ( 50 C/180 C) in air, in terms of oxidation. Such an isothermal test was performed and the obtained results where compared to those of the cyclic test to put in light the part of oxidation in the damage build-up. For comparison, a similar experiment consisting in 800 h at 150 C has been performed in vacuum. 4.1.1. Weight loss due to oxidation For this study, we have followed the same experimental procedure that is detailed in the preceding section concerning the weight measurements and the data analysis. First, we have noted that, except a slight decrease of weight observed at the very beginning of the test, probably due to a residual drying of the sample, no variation in weight has been measured at the end of that test in vacuum. To compare the oxidation level induced by the two types of tests performed in air, the differences of mass variations measured during tests in air and in neutral environments are plotted in Fig. 13 against either the number of cycles for the thermal cycling test, or the number of hours for the isothermal ageing. This representation corresponds to the equivalence of the two oxidation times predicted by the model. One can see in Fig. 13 that the relative mass variation due to the single oxidation decreases in a similar manner during the two types of tests and, in Table 4, that the rates of that decrease are quite similar. Moreover these experimental values are found close to that predicted by the model (Table 4). From these results, the equivalence in oxidation of both tests, which was predicted by the simulation, is checked in term of weight loss rate. 4.1.2. Matrix shrinkage SEM and optical microscopic observations of initially polished edges of the specimens have shown that a significant shrinkage of the matrix exists only in specimens tested in air whatever the type of test, isothermal or cyclic. However, important differences have been observed between the series of isothermal ageing and that corresponding to 500 thermal cycles in air. In Table 5, the averaged depth values of matrix shrinkage measured on specimens maintained 400 and 600 h at 150 C in air are close (1.5 lm) but quite lower than those measured on samples submitted to 500 thermal cycles (2.2 lm). In thermal cycling, the frequency distribution of shrinkage depth is overall shifted towards greater depth values, with other important peaks reaching up to 4 lm depth.

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Fig. 13. Experimental relative weight loss of {03/903}s specimens due to oxidation in air: thermal cycling ( 50 C/180 C) and 150 C isothermal ageing.

Table 4 Weight loss rates of {03/903}s samples in air: in 150 C isothermal ageing, thermal cycling ( 50 C/180 C), and according to the model Isothermal test 150 C, air

Thermal cycling ( 50 C/180 C), air

Simulation

1.2 ± 0.5 · 10 6/h

0.9 ±0.55 · 10 6/cycle

1.1 · 10 6/h

Table 5 {03/903}s laminate, statistical values of matrix shrinkage depth according to the type of test in air Air

Isothermal ageing 400 h, 150 C

Thermal cycling 500 cycles ( 50 C/180 C)

Isothermal ageing 600 h, 150 C

Average (lm) Standard deviation (lm) Extent (lm)

1.34 0.89

2.2 1.11

1.58 1.15

[0.4–7.0]

[0.4–6]

[0.4–7.5]

These observations and measurements confirmed that the matrix shrinkage can be associated to the phenomenon of oxidation. However, even if we have shown that 500 h at 150 C in air induced an equivalent mass loss than 500 thermal cycles in air, the matrix shrinkage phenomenon is more pronounced in thermal cycling. Is there another phenomenon than oxidation that leads to so deep superficial deformations in thermal cycling? Up to now, it is supposed that the presence of cyclic-thermal stresses at a microscopic level, between fibres and matrix, would enhance the oxygen diffusion and accelerate the oxidation process. Work is in progress to verify such an interpretation. 4.1.3. Transverse matrix cracking The matrix crack accumulation and growth are very different according to the gaseous environment of the test: None crack or very few are observed in samples tested in vacuum or nitrogen, whereas they are numerous when the tests are performed in air, in every layer of the laminate coupon, whatever the type of thermal test (isothermal or

thermal cycling). In Fig. 14, as an example, we have plotted the crack density values of the inner layer, measured on the edges of samples tested in air, against the number of cycles for thermal cycling and against the number of hours for isothermal test, in accordance with the equivalence given by the oxidation model. We can see in this figure an earlier crack initiation and a higher crack growth rate in thermal cycling than in isothermal test. Indeed, in isothermal ageing, the first cracks appear only after 300 h of test, whereas in thermal cycling the cracks are already numerous at 100 cycles. Thereafter, in isothermal test, the crack density increases up to a value of 5.5 f/cm measured after 500 h; whereas in the same interval of ‘‘equivalent’’ time, in thermal cycling in air, the density of cracks had already reached a ‘‘saturated’’ value of 7.4 f/cm. Moreover, X-ray pictures of the different specimens have been taken at the end of the tests. In Fig. 15a, the X-ray picture corresponding to the sample submitted to 500 thermal cycles ( 50 C/180 C) shows numerous cracks present in both layers, and well developed in the core of the sample. On the contrary, Fig. 15b shows that the damage is very superficial and concerns only a few millimeters depth of the specimen isothermally aged in air: in that case the crack length is shorter than the spatial resolution of the X-ray technique, so that none of the numerous cracks counted on the edge of the specimen is visible on the X-radiograph. These results show that thermal cycling in air induces a combination of ageing, due to oxidation and fatigue that accelerates dramatically the damage process. From the physicochemical point of view, isothermal ageing results in an oxidation of the matrix on the surfaces directly exposed to the atmosphere. In these conditions, the cracks would appear when locally, the stress induced by the matrix shrinkage exceeds a critical value (e.g., the fracture stress of the oxidised matrix). On the other hand, the transverse matrix cracks induced by 500 thermal cycles are more

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Fig. 14. {03/903}s laminates, transverse crack densities for isothermal (150C) and thermal cycling ( 50 C/180 C) tests in air.

Fig. 15. {03/903}s laminate, air, X-ray pictures at the end of the different thermal tests: (a) 500 thermal cycles( 50 C/180 C); (b) 600 h 150 C ageing.

numerous and especially much longer. This fast propagation of the matrix cracks might be related to the cyclic transverse stresses which develop in the plies as shown in the first section, and which induce a ‘‘fatigue’’ phenomenon. The fatigue process coupled with the matrix oxidation accelerates not only the crack initiation on the specimen surfaces but also their propagation towards the core of the laminate.

Fig. 16 presents the relative variations of mass against the ageing time: the predicted weight loss for a virgin specimen is represented by a full line and must be compared to the experimental measurements that concern the initially damaged specimens. One can see in that figure that the experimental values obtained for the three specimens are very close from each other till around 1500 h. Moreover, they correspond to the values predicted by the model for a virgin specimen: this correspondence might be explained by the low sensitivity of this global degradation parameter, inadequate to detect the small mass variations that occur during the first hours of isothermal ageing. Beyond 1500 h of ageing, the decrease in specimen weight varies like the initial damage state: the higher the initial cracking, the faster the degradation process. In that range, the relative mass variations of the damaged specimens deviate

4.2. Isothermal ageing of damaged specimens [20] The experimental conditions of this study have been designed in order to put in light the coupling between cracking and oxidation mechanisms of epoxy matrix composite laminates subjected to thermal cycling or isothermal ageing tests. Following that goal we used the specimens damaged by 500 thermal cycles ( 50 C/180 C) in different atmospheres, whose the damage states have been characterised in the first section of this paper. The samples have been exposed in isothermal conditions at 150 C under 3 bars of air. The test continued 4250 h, and the weight variations of the specimens were measured. In order to evaluate the loss of weight of a virgin specimen subjected to such an isothermal ageing, we used the oxidation kinetic model presented elsewhere [8,26] and validated above for our composite material [19].

Fig. 16. Relative mass variations of {03/903}s laminates initially damaged by 500 thermal cycles ( 50 C/180 C) in different atmospheres and exposed 4250 h at 150 C under 3 bars of air [20].

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increasingly with time from the values expected for a virgin sample. At the end of the test, after 4250 h of isothermal ageing, the relative weight loss of the specimen initially subjected to 500 thermal cycles in oxygen is more than 3 times higher than that predicted for an undamaged sample. X-radiographs of the three specimens have been taken at the end of the isothermal ageing test (after 2450 h) and are compared in Fig. 17 to the initial X-ray pictures obtained at the end of the 500 thermal cycles. One can see in that figure that the isothermal ageing in air induced an important increase of damages. Because the initial damages were very small in the sample first tested in nitrogen, the X-ray picture shown in Fig. 17a is typical of the damages induced by an isothermal ageing: in the internal layers we observe very numerous short cracks and local delaminated areas close to the edges of the sample whereas cracks appear evenly distributed in the external layers. Moreover, one can see some internal cracks that developed from the longest external cracks, probably open enough to permit the diffusion of air. The pictures shown in Fig. 17b and c put in light a correspondence between the initial cracking pattern and the ultimate one. The initial crack array can be recognised, the cracks appearing longer and wider open,

and surrounded by very numerous short cracks distributed all through the plies. The differences in cracking density mentioned after the thermal cycling between specimens tested in air and in oxygen persist; the last samples appear as the most damaged. This comparison between the initial damage states and the ultimate ones clearly shows that there is a coupling between the matrix cracking and the oxidation processes: each cracked surface is a new site for oxidation which favours the cracking process and so on. . .it leads to a damage state all the more developed as the specimen was previously damaged. These observations roughly explain the comments we made about the specimen weight measurements: because of this coupling, the degradation state of a specimen, (characterised in that study by the crack distribution and by the relative weight loss of the sample), strongly depends on the history of its ageing. 5. Conclusion These studies aimed to better understand the damage mechanisms of carbon/epoxy laminates subjected to cyclic variations of temperature. The main result obtained is that

Fig. 17. X-ray pictures of [03/903]s. coupons LEFT: after 500 thermal cycles ( 50 C/180 C): (a) nitrogen; (b) air; (c) oxygen RIGHT: PLUS 4250 h of isothermal ageing in air, (150 C, 3 bars).

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the test atmosphere: neutral (nitrogen) or oxidative (air or oxygen) has a very significant influence on the damage processes and degradation rates of the composite material. Observations and measurements made during thermal cycling tests in different environments have shown that the exposure of a sample in an oxidative atmosphere produces, at the highest temperatures of the cycle, a significant oxidation of the epoxy–resin matrix. This first superficial oxidation induces some shrinkage of the matrix on the edges of the specimens, which is often associated to numerous and deep debonding between fibres and matrix. These zones, weaken by the oxidation, are favourable sites for crack onset. As a consequence, these experiments have shown matrix cracking much more developed in oxidative environment than in nitrogen, and cracking growth rates as high as the maximum temperature of the thermal cycle. In such cyclic-thermal loading, there exists a coupling between the two material degradation mechanisms incriminated, which are oxidation and fatigue. So, the degradation rates as well as the ultimate damage states have been found depending on both the ply-stresses (essentially determined by the minimum temperature of the cycle and the laminate stacking sequence) and the thermal ageing conditions (maximum temperature of the cycle and oxygen concentration). In a second step, some experiments have been carried out in order to put in light the magnitude of that coupling and to discriminate the part of each mechanism on the damage build-up. The use of a model of oxidation has permitted to determine the conditions of an isothermal ageing test, ‘‘equivalent’’ in terms of oxidation. The comparison of the damage states induced by that isothermal test and by the thermal cycling experiment has shown that, in thermal cycling, the cyclic transverse stresses which develop in the plies and which induce a ‘‘fatigue’’ phenomenon, accelerates the crack initiation on the specimen surfaces and above all favours their growth towards the core of the laminate. Finally, samples presenting different initial damage states have been exposed to a long isothermal ageing in air. At the end of the test, the damage build-up was found all the more important as the specimen was previously damaged. All these observations permit to conclude that, in thermal cycling, because of the coupling between oxidation and fatigue processes, the degradation state of a carbon–epoxy laminate strongly depends on the history of its ageing. Acknowledgements The author like to acknowledge the financial support of the French Research Department and the French Transport Department, E.A.D.S. (CCR Suresnes, France) for supplying the composite plates and samples, P. Ladeveze, G. Lubineau (LMT, ENS Cachan), J. Verdu and V. Bellenger (LTVP ENSAM Paris) for our fruitful collabora-

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tion, and S. Rouquie and N.Q. Ho, my Ph.D. students for their passionate stake in this research. References [1] Arendt C, Brunner M, Wang J, Cebeci T. Long term durability testing of polymer composite materials. In: Massard T, Vautrin A, editors, 12th International conference on composite materials (ICCM12), Paris, France, 5–9 July; 1999. [2] Gates TS. Durability assessment of polymeric composites for high speed civil transport. In: Cardon AH, Fukuda H, Reifsnider KL, Verchery G, editors. Proceedings of the DURACOSYS 99, Brussels, Belgium, July 11–14, ‘Recent developments in durability analysis of composite systems’, Balkema; 1999. [3] He´naff-Gardin C, Lafarie-Frenot MC, Desmeuzes JL. Modelling of crack evolution under cyclic thermal loading in cross-ply laminates. In: Poursartip A, Street K, editors. 10th International conference on composite materials, (ICCM10), Whistler, Canada, August 14–18. Woodhead Publishing Limited; 1995. p. 447–54. [4] He´naff-Gardin C, Goupillaud I, Lafarie-Frenot MC. Evolution of matrix cracking in cross-ply CFRP laminates: differences between mechanical and thermal loadings. In: Cardon AH, Fukuda H, Reifsnider KL, Verchery G, editors. DURACOSYS 99, Brussels, Belgium, July 11–14, 1999, ‘Recent developments in durability analysis of composite systems’, Balkema; 1999. p. 69–76. [5] Madhukar MS, Bowles K, Papadopoulos DS. Thermo-oxidative stability and fiber surface modification effects on the in plane shear properties of graphite/PMR 15 composites. J Compos Mater 1997;31(6):596–618. [6] Colin X, Marais C, Favre JP. Damage/weight loss relationship of polymer matrix composites under thermal ageing. In: Massard T, Vautrin A, editors, 12th International conference on composite materials (ICCM12), Paris, France, 5–9 July; 1999. [7] Colin X, Marais C, Cochon JL. Kinetic modelling of weight changes during the isothermal oxidative ageing of bismaleimide matrix. In: Cardon AH, Fukuda H, Reifsnider KL, Verchery G, editors. DURACOSYS 99, Brussels, Belgium, July 11–14, ‘Recent developments in durability analysis of composite systems’, Balkema; 1999. p. 49–55. [8] Decelle J, Huet N, Bellenger V. Oxidation induced shrinkage for thermally aged epoxy networks. Polym Degrad Stabil 2003;81:239–48. [9] Tsotsis TK, Keller S. Aging of polymeric composite specimens for 5000 hours at elevated pressure and temperature. Compos Sci Technol 2001;61(1):75–86. [10] Bowles KJ, Meyers A. Specimen geometry effects on graphite/PMR 15 composites during thermo-oxidative ageing. In: 31st International SAMPE symposium; 1986. p. 1285–99. [11] Nam JD, Seferis JC. Anisotropic thermo-oxidative stability of carbon fibre reinforced polymeric composites. SAMPE Quart 1992;24(1):10–8. [12] Salin IM, Seferis JC. Anisotropic effects in thermogravimetry of polymeric composites. J Polym Sci B Polym Phys 1993;31:1019–27. [13] Bowles KJ, Madhukar MS, Papadopoulos DS, Inghram L, Me Corkle L. The effects of fiber surface modification and thermal aging on composite toughness and its measurement. J Compos Mater 1997;31(6):552–79. [14] Parvatareddy H, Wang JZ. An evaluation of chemical aging/ oxidation in high performance composites using the Vickers mircoindentation technique. J Compos Mater 1996;30(2):210–30. [15] Bowles KJ. Predicting the durability of PMR-15 composites aged at elevated temperature. In: Cardon AH, Fukuda H, Reifsnider KL, Verchery G, editors. DURACOSYS 99, Brussels, Belgium, July 11– 14, ‘Recent developments in durability analysis of composite systems’, Balkema; 1999. p. 441–7. [16] Favre JP, Levadoux H, Ochin T, Cinquin J. Vieillissement des composites a` matrice organique aux tempe´ratures moyennes: Un premier bilan. In: Baptiste D, Vautrin A, editors. 10e`mes Journe´es Nationales sur les Composites JNC10, Paris, France. AMAC; 1996. p. 205–14.

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