# Effect of water absorption on transverse cracking in thermoplastic and thermoset matrix carbon fibre laminated composites at cryogenic temperatures

Timothée Klein, Christophe Bois, Jean-Christophe Wahl, Caroline Petiot

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Timothée Klein, Christophe Bois, Jean-Christophe Wahl, Caroline Petiot. Effect of water absorption on transverse cracking in thermoplastic and thermoset matrix carbon fibre laminated composites at cryogenic temperatures. Composite Structures, 2025, 371, 10.1016/j.compstruct.2025.119445. hal-05504488

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# Effect of water absorption on transverse cracking in thermoplastic and thermoset matrix carbon fibre laminated composites at cryogenic temperatures

Timothée Klein<sup>a,b,c,*</sup>, Christophe Bois<sup>a,b</sup>, Jean-Christophe Wahl<sup>a,b</sup>, Caroline Petiot<sup>c</sup>

## ARTICLE INFO

### Keywords:

- Water absorption

- Transverse cracking

- Cryogenic temperature

- Thermoplastic matrix

## ABSTRACT

The development of linerless liquid hydrogen composite tanks is a major challenge for improving the performance and cost of zero-emission commercial aircraft. Since tightness must be provided by the composite itself, an in-depth understanding of damage mechanisms in laminated composites at cryogenic temperatures is required. It is well accepted that environmental conditioning can affect the performance of composite structures, in particular water ingress. It is unclear, however, especially at cryogenic temperatures, whether water absorbed during manufacture or operations leads to additional damage. The present study therefore aims to highlight and understand the effect of water absorption on transverse cracking in two laminated carbon fibre reinforced composite materials at cold and cryogenic temperatures. The results indicate that water absorption has a detrimental effect on transverse cracking in both materials and at all temperatures studied (–130 °C and –196 °C). The severity of these effects was seen to be linked to the affinity of the matrix and fibre/matrix interfaces with water, namely the amount of water absorbed by the system in a given environment. Finally, both reversible and irreversible absorption processes have been shown to contribute to the detrimental effects.

## 1. Introduction

The development of linerless liquid hydrogen composite tanks is a major challenge for improving the performance and cost of zero-emission commercial aircraft. Since tightness must be provided by the composite itself, an in-depth understanding of damage mechanisms in laminated composites at cryogenic temperatures is needed. When uni-directional laminated composites are subjected to thermomechanical loading, the predominant types of damage are transverse cracks running parallel to the fibres, followed by micro-delaminations at the crack-tip [1]. Spontaneous transverse crack generation and interaction between adjacent plies leads to the creation of crack networks which, if they coalesce in the thickness of the laminate, constitute leak paths [2–4]. Such behaviour continues to be observed down to cryogenic temperatures, but damage has proved to be exacerbated by the rigidification, decrease in ductility and embrittlement of the polymer matrix and the increased thermal residual stresses associated with the coefficient of thermal expansion (CTE) mismatch [5–7]. More specifically, it leads to a

decrease in transverse cracking initiation thresholds and an increase in cracking kinetics, resulting in higher transverse cracking rates [4,8,9]. These results were mostly obtained on thermoset epoxy-based composite systems. Although still little studied in the literature and particularly at cryogenic temperatures, some thermoplastic matrix composites have been shown to exhibit improved damage behaviours when compared to their thermoset counterparts, such as increased ductility and toughness [10–12]. For instance, Laurin et al. [10] observed diffuse damage consisting of short transverse cracks in a LMPAEK T700GC/TC1225 thermoplastic matrix composite at ambient temperature. Along with their high recyclability and weldability potential, thermoplastic matrix composites are thus increasingly being considered for the manufacturing of aeronautical structures.

Whether or not the cryogenic tank is expected to operate in a dry state, it will have to undergo maintenance inspections which may be more complex if the composite material has a high susceptibility to water absorption. In addition, variations in the time between curing and testing, seasonal fluctuations in ambient humidity and differences in

<footnote><sup>a</sup> Univ. Bordeaux, CNRS, INRAE, Bordeaux INP, I2M, UMR 5295, USC 1368, F-33400 Talence, France</footnote>

<footnote><sup>b</sup> Arts et Metiers Institute of Technology, CNRS, INRAE, Bordeaux INP, I2M, UMR 5295, F-33400 Talence, France</footnote>

<footnote><sup>c</sup> Airbus Central Research and Technology, Toulouse, France</footnote>

<footnote>* Corresponding author at: Univ. Bordeaux, CNRS, INRAE, Bordeaux INP, I2M, UMR 5295, USC 1368, F-33400 Talence, France.
E-mail address: timothee.klein@u-bordeaux.fr (T. Klein).</footnote>
preparation protocols between laboratories can all have a significant impact on the moisture absorption of composite samples during experimental test phases. Such effects could result in significant differences in the mechanical and damage behaviour between tested samples and the resulting end product.

Water uptake in composites and its influence on their mechanical behaviour at ambient temperatures has been widely studied across the literature. When exposed to a wet environment, polymeric resins (thermoset and thermoplastic) used as matrices absorb water through a diffusion process. Depending on the duration and amount of water absorbed, this leads to the formation of two degradation processes, one reversible (physical) and the other irreversible (chemical). Water is generally described as being in two states when diffused within a given polymer [13–15]. The first is an unbound state, where liquid water is present within the polymer’s free-volume voids. The second corresponds to a bound state where water molecules attach to the amorphous phase network via hydrophilic polar groups through hydrogen bonding. In both cases, water is free to move and diffuse within the polymer following concentration gradients. As water binds with the polymeric network, it disrupts interchain hydrogen bonding, leading to an increase in both macromolecular chain mobility and molecular network volume, or to the formation of micro-voids [13,16,17]. This physical and reversible effect, known as plasticization, leads to a decrease in the glass transition temperature $T_g$, a swelling of the matrix, a degradation of its mechanical properties and an increase in its CTE [13–15,17,18]. At higher temperatures and for prolonged immersion durations, a non-reversible chemical degradation can occur when water and certain groups in polymer molecules react with each other. Hydrolysis is a chain scission process resulting in a decrease in the thermoset crosslink density or in the molar mass in the amorphous phase of thermoplastic polymers. Depending on the nature and composition of the polymer, this reaction can lead to the formation of new molecules or a mass loss [19–21]. Both reversible and irreversible degradation processes result in a loss of strength and modulus in polymers exposed to water [22–24]. The kinetics of the absorption process are highly dependent on temperature, relative humidity, and the nature and state of the considered polymer [25–28]. For many polymers with few defects exposed to water below their glass transition temperature $T_g$, absorption as a first approximation usually follows a Fickian behaviour [22,29,30]. For longer durations, two-phase diffusion models such as Langmuir’s [31,32] generally account better for non-linear evolutions linked to the water’s bound and unbound states within the polymer.

Continuous carbon fibre laminated composites generally follow a similar behaviour to the resin of which they are made, but with a slower diffusion rate. Indeed, it is widely accepted that, due to their non-polar nature, carbon fibres absorb minimal water (except within their pores) and are largely unaffected by it [14,30,33–36]. They do, however, act as obstacles to water diffusion in their transverse direction by generating a more tortuous path [30]. The laminate’s mechanical properties are thus affected in a similar way to that observed for polymer matrices, with decreased elastic moduli and strength, particularly in the shear and transverse directions at ambient temperature [15,22–24,37–39]. These are generally attributed to the plasticization of the polymer matrix, but the fibre-matrix interface also plays a role. Indeed, numerous studies have shown a decrease in the interlaminar shear strength and fracture toughness of the interface [14,17,24,37,38,40]. Hygroscopic swelling due to moisture uptake can account for fibre-matrix interface degradation. It is similar to thermal expansion, although in the opposite direction, and reduces radial compressive stresses at the interface and consequently the mechanical interlocking between the fibre and the matrix [24,29]. This directly impacts the laminated composites’ transverse cracking mechanisms. At room temperature, Humeau et al. [30] observed a decrease in transverse crack initiation threshold and an increase in crack density kinetics in ± 45° woven carbon/epoxy composites as water immersion duration increases. Nunemaker et al. [41] observed earlier damage onset and a decrease in the acoustic emission

energy in cross-ply fiberglass-epoxy composites immersed in water. Finally, Han and Nairn [42] observed a decrease in microcracking toughness in cross-ply carbon-fibre/Polyimide unidirectional laminated composite samples exposed to a wet environment. However, their results were obtained on dried samples after conditioning, and can be fully attributed to the hydrolysis of the polyimide thermoplastic matrix.

It is somewhat unclear whether the tendencies observed at ambient temperatures can be extrapolated to cold and cryogenic temperatures. At –2°C, LeBlanc et al. [37] noted similar results to those observed at ambient temperatures, namely a decrease in moduli, tensile and compressive strengths, and in the static mode I critical strain energy rate $G_{IC}$. Baschek et al. [43], however, show that observations performed at ambient temperatures do not necessarily extrapolate to cryogenic ones. Using a combination of a torsion pendulum, dilatometer and tensile tests down to 4.2 K, the team observed that the water-saturated thermoplastic specimens they studied were generally stiffer than dry ones at the same cryogenic temperature. Finally, Ju et al. [44] observed an increased microcrack density in woven carbon fibre / BMI (5250–4 RTM/IM7 4) laminated composites for pre-stressed samples (0.843 % in plane strain) subjected to thermal cycling (between –196 °C and 100 °C) under moist air conditions (100 %RH) compared to air with 50 %RH. They attributed this result to swelling and reduction in $T_g$ induced by water absorption in the matrix, thus reducing the interface strength between the fibre and the matrix. It should be noted that this last study performs thermal cycling up to 100 °C. It is therefore hard to determine whether the increased damage occurs at high or cold temperatures, or both.

While the effects of water absorption in carbon fibre laminated composites have been extensively studied at ambient temperatures, its impact on transverse cracking behaviour at cryogenic temperatures remains largely unexplored. Although some research has been conducted at moderately low temperatures, the interplay between moisture-induced swelling and contraction at cryogenic temperatures on these damage mechanisms remains unclear. A direct investigation into how pre-absorbed moisture affects the initiation and propagation of transverse cracks under cryogenic thermomechanical loading is still lacking. This study aims to address this gap by comparing the transverse cracking kinetics of two carbon fibre laminated composites: one with a thermoset matrix and the other with a thermoplastic matrix. The first section presents the materials and experimental protocols. This includes the determination of water absorption kinetics, and transverse cracking tests on crossed-ply laminates. The latter involves in situ observation of transverse cracks within plies oriented perpendicular to the loading direction, and is conducted at cold (–130 °C) and cryogenic (–196 °C) temperatures. The second section uses the measured water absorption kinetics to calibrate the conditioning of specimens prior to cracking tests, and to examine the potential irreversibility of water uptake. Finally, the influence of temperature and the presence or absence of water in the specimens is examined through analysis of the experimentally determined transverse cracking kinetics.

## 2. Materials and methods

### 2.1. Materials

Three types of materials are used in this study: one neat TP resin and two carbon fibre composites, with epoxy-based thermoset (CF/TS) and thermoplastic (CF/TP) matrices. The neat TP resin corresponds to that present in the CF/TP composite, and tensile specimens were manufactured by injection moulding into type 1A ISO 527–2 samples. For the CF/TS material, specimens were extracted from plates obtained from uni-directional prepreg material cured in autoclave. CF/TP plates were produced by Automated Fibre Placement (AFP), followed by heated press consolidation. Materials were chosen so that both have a similar thin unit ply thickness of approximately 70 µm. The manufactured samples were all checked by C-Scan and microscopic observation to guarantee the quality of their initial state and ply orientations. The void
content was below 0.5 % in the CF/TP material and 2 % in the CF/TS one, and the fibre volume fraction was close to 60 % in both composite materials.

## 2.2. Specimen definitions and experimental strategy

To study the effect of water absorption on the transverse cracking mechanisms of the considered materials at cryogenic temperatures, two types of tests were performed.

The first type consists of transverse cracking tests (see section 2.4 for the detailed protocol) on the composite materials, carried out at two temperatures (−130 °C and −196 °C) on samples that were subject to different environmental conditionings. Two possible extreme cases are considered, namely dry and close to water saturation. More precisely, half of the samples were dried in a desiccator with silica gel (room temperature, RH < 10 % relative humidity) for at least two weeks until the mass stabilised, while the other half were immersed in deionised water at 30 °C for a duration of $t_{immersion}$. The temperature was selected to remain below both materials' $T_g$. The immersion time was based on the recommendations of standards PR-EN-3615 [45] and PR-EN-2823 [46], aiming to approach the plateau of the absorption curve and achieve substantial water uptake without reaching full saturation, in order to avoid irreversible chemical reactions with water. The exact immersion durations, the condition of samples during testing, and the assessment of possible irreversible effects are discussed in the following sections. For simplicity, samples that were placed within the desiccator will be referred to as “dry”, while those immersed in water will be referred to as “immersed”. In total, a set of 13 samples were tested. For each environmental conditioning (dry and immersed) and for each material (CF/TS and CF/TP), two specimens were tested at −130 °C and only one at −196 °C (except for the CF/TP under dry conditions where two samples were tested). In this type of test, provided the observed length is sufficiently large, the intrinsic variability of the material is naturally accounted for due to the large number of cracks propagating within the 90° ply. As a result, experimental scatter remains low [47–50], and the effects of water absorption and temperature can be assessed using a limited number of specimens. The configurations and number of samples tested are summarised in Table 1. Tests were carried out on 250 x 25 mm² rectangular laminate specimens fitted with tabs, with 0° along the 250 mm length. Each composite sample has a layup of [0/90/0/90°/2/0/90°]s, which enables three different ply thicknesses to be studied at each time: thin (90 alone), intermediate (90°) and thick (90°). However, for simplicity and as conclusions are similar for all ply thicknesses, only the thick (90°) plies with average thickness of 280 µm will be considered in the present study.

The second type of test is measurement of the water absorption and desorption kinetics. Three types of samples were used. The neat TP resin sample was cut into small plates of approximately 30 x 20 x 1.7 mm³.

<table><caption>**Table 1**
Summary of the configurations and number of samples tested.</caption><thead><tr><th>Type of Test</th><th>Material</th><th>Environmental conditioning</th><th>Temperature [°C]</th><th>Number of samples tested</th></tr></thead><tbody><tr><td rowspan="6">Transverse cracking</td><td rowspan="3">CF/TS</td><td>Dry</td><td>-130</td><td>2</td></tr><tr><td rowspan="2">Immersed</td><td>-196</td><td>1</td></tr><tr><td>-130</td><td>2</td></tr><tr><td rowspan="3">CF/TP</td><td>Dry</td><td>-196</td><td>1</td></tr><tr><td rowspan="2">Immersed</td><td>-130</td><td>2</td></tr><tr><td>-196</td><td>2</td></tr><tr><td rowspan="4">Water absorption / desorption kinetics</td><td>CF/TS</td><td>Dry</td><td>-130</td><td>2</td></tr><tr><td>CF/TP</td><td>Immersion& Drying</td><td>-196</td><td>1</td></tr><tr><td rowspan="2">Neat TP</td><td>Immersion& Drying</td><td>30</td><td>2</td></tr><tr><td>Immersion& Drying</td><td>30</td><td>2</td></tr></tbody></table>

Samples of 125 x 25 x 1.12 mm³ (with 0° along the 125 mm length) were extracted from the same CF/TS and CF/TP composite plates used for the transverse cracking tests. These dimensions are similar to those of the transverse crack test specimens in order to accurately predict their state during environmental conditioning. For each material, two samples were manufactured, resulting in a total of six samples. The number of tested samples is also summarised in Table 1.

## 2.3. Water uptake experimental protocol

All samples were first dried for several weeks in a desiccator at RH < 10 % and 30 °C until their mass stabilised. They were then immersed in 30 °C deionised water. After 45 days, half of the samples (one from each material) were placed in the desiccator (RH < 10 %, 30 °C) for desorption of water. This allows for the monitoring of the desorption kinetics and the presence of irreversible effects linked to immersion in water, indicated by a variation in the mass of the sample after drying. The other samples were kept immersed until their mass stabilised to determine their full absorption curve. In both water bath and desiccator, a temperature controller is used with a precision of ± 1 °C, and is monitored using thermocouples.

During absorption and desorption, the kinetics of water diffusion were studied by gravimetry, i.e. by successive weighing of the samples, where the resulting mass variation was monitored using a balance with a precision of 0.01 mg. The percentage mass gain $M_t$ at time $t$ is expressed using the equation [19,25,51]:

$$M_t = \frac{m_t - m_0}{m_0} \cdot 100 \qquad (1)$$

where $m_0$ [kg] and $m_t$ [kg] are respectively the initial mass and that at time $t$ of the sample. It is then possible to determine the maximum quantity of water contained in the sample $M_\infty$, the diffusion coefficient $D$ of the water molecules in the studied species, and the immersion time necessary for the stabilisation of the mass.

Fick's law is largely used across the literature to study water diffusion through polymers [19,25,51]. By assuming a constant diffusion coefficient $D$ and a one-dimension system (i.e. infinite plate), water concentration $C$ diffusing through the polymer thickness is provided by:

$$\frac{\partial C}{\partial t} = D \cdot \frac{\partial^2 C}{\partial x^2} \qquad (2)$$

where, in the case of an infinite plate of thickness $h$, Crank proposed in 1975 an analytical solution to this differential equation [52]:

$$\left(\frac{M_t}{M_\infty}\right)_{\text{Fick}} = 1 - \frac{8}{\pi^2} \sum_{n=0}^{\infty} \frac{1}{(2n + 1)^2} \exp\left(-\frac{D(2n + 1)^2 \pi^2 t}{h^2}\right) \qquad (3)$$

Due to the presence of both bound and unbound states of water, some materials are reported to have a non-Fickian behaviour, especially for long ageing durations. A famous model used is Langmuir's, developed by Carter and Kibler [31]. Here, the model distinguishes two states of water: one where the water molecules are free to move and one where they are bonded to the polymer network due to reversible chemical reactions. At any given time, there is a probability $\delta$ that a free water molecule becomes bonded, and a probability $\gamma$ that a bonded one becomes free. When $\gamma$ and $\delta$ are both small compared to $\pi^2 D / h^2$, one has:

$$\left(\frac{M_t}{M_\infty}\right)_{\text{Langmuir}} = \frac{\gamma}{\gamma - \delta} e^{-\delta t} \cdot \left(\frac{M_t}{M_\infty}\right)_{\text{Fick}} + \frac{\gamma}{\gamma - \delta} \cdot (e^{-\gamma t} - e^{-\delta t}) + (1 - e^{-\gamma t}) \qquad (4)$$

## 2.4. Transverse cracking test protocol

Once the desired immersion or drying time has been reached, the test specimen is removed from its environment and a transverse cracking test is performed. In order to preserve the specimen's dry/immersed state,
the test is carried out without interruption, for a maximum duration of 1 office day. The amount of water absorbed or lost during the test remains small and will be discussed in the next section.

As illustrated in Fig. 1, a transverse cracking test consists in applying a stepwise static thermomechanical loading on a cross-ply ([0/90/0]) laminate using a tensile machine in order to initiate and accumulate damage in the 90° ply [1,8–10]. At each loading step, the specimen temperature is first lowered progressively to the target temperature. It is then subjected to a uniaxial mechanical loading in quasi-static regime (0.5 mm/min) up to a targeted deformation value εmax, leading to the initiation or accumulation of damage. The specimen is then unloaded and the temperature raised back to 20 °C. The edge of the sample, previously polished using a 3 µm mat, is then inspected for the presence of transverse cracks using a digital microscope. In order to open the cracks and facilitate their observation yet not generate additional damage, deformation is maintained at εobs = 0.8 • εmax. The temperature is then reduced again before the specimen is subjected to a deformation greater than that of the previous step. Damage density in the specimen can be quantified at each loading step by counting the number of transverse cracks present in the studied 90° ply over the observed length Lobs = 80mm. In the case of partial cracks not propagating through the entire thickness of the ply, the crack’s relative length Lc = Lc/hply is considered, being the ratio of the crack length Lc [mm] to that of the ply thicknesses hply [mm]. As proposed in several studies [9,10], in order to compare plies of different materials and thicknesses, the reduced crack

density $\bar{\rho}[-]$ at a given longitudinal strain εx is obtained by the following equation:

$$ \bar{\rho} = \frac{\sum L_c}{L_{obs}} \cdot h_{ply} \qquad (5) $$

The presence and proportion of partial cracks relative to full-thickness cracks will be discussed in section 3.2. Uniaxial mechanical loading was performed using a 150kN capacity electromechanical tensile testing machine. The deformation of the specimens is measured using strain gauges, and their temperature is monitored using thermocouples placed on their surface.

Although the liquid hydrogen tank is expected to operate at –253 °C, liquid nitrogen (LN2, –196 °C) is commonly used as a coolant to approach that temperature during experimental testing, thanks to its relative ease of use and affordable cost [8,9]. LN2 can be used in two ways: it can be pulverised in order to reach intermediate temperatures, or samples can be directly immersed in it. In the present study, two cryogenic chambers are used to provide thermal loading depending on the target temperature. The first is an environmental chamber fitted around the tensile testing machine jaws (see Fig. 1a), where pulverised LN2 cools the specimen, enabling temperature control down to –130 °C. The second is an immersion chamber attached to the bottom jaw of the tensile testing machine (see Fig. 1b), in which the sample is attached to an additional jaw in the chamber. Cooling is achieved by initially injecting cold nitrogen gas, followed by small quantities of LN2 until the

<figure><img src="image_3.png" /><figcaption>Fig. 1. Diagrams of the experimental setup used for measurements at –130 °C (a) and –196 °C (b). Red arrows point at transverse cracks. Experimental setup and sample used for the liquid nitrogen immersion transverse cracking test (c).</figcaption></figure>
sample is fully immersed, allowing tests at -196 °C to be carried out. To prevent damage from thermal shock, the temperature during each thermal loading cycle was decreased at a maximum rate of -30 °C/min. There is a 5 min stabilisation period before mechanical loading to ensure temperature uniformity along the entire length of the specimen. In the case of measurement at -130 °C, the thermal chamber has a window through which the specimen edge can be observed (see Fig. 1a). As there is no glass in the immersion chamber, the specimen has to be completely unloaded and placed in a second tensile machine, on which the optical microscope is placed (see Fig. 1b). An image of the experimental setup along with a flat sample is provided in Fig. 1c.

To assess the presence of potential deleterious irreversible effects of water absorption, the specimens tested at -196 °C (dry and immersed) were dried in the desiccator after testing, for several weeks. They were then subjected to a final mechanical loading at -196 °C and approximately 80 % of the maximal strains reached during the first testing phase. The number of additional cracks present in the sample were then monitored according to the aforementioned protocol.

# 3. Results and discussions

## 3.1. Water absorption and desorption

Fig. 2 presents the absorption and desorption curves determined experimentally on all six samples. For each material, Sample 1 (S.1) refers to the samples that remained immersed in water and Sample 2 (S.2) refers to those that were dried after being immersed for 45 days. By numerically fitting the experimental data with Fick's law (Eq. (3)), we can identify the diffusion coefficients $D_i^j$ and ultimate moisture uptakes as time tends to infinity $M_{\infty,i}^j = M_{t,i}^j(t \to \infty)$ for each material $j = \{CF/TS: CF/TP: TP\}$ in the absorption or desorption phases $i = \{abs: des\}$.

All curves in Fig. 2 are normalised by the same factor, namely the maximal moisture uptake reached by the CF/TP composite sample 1 (S.1) $M_{\infty,abs}^{CF/TP} = M_{t,abs}^{CF/TP}(t \to \infty)$ when immersed in water such that:

$$ \bar{M}_t = \frac{M_t}{M_{\infty,abs}^{CF/TP}} \qquad (6) $$

Table 2 synthesises the normalised diffusion and ultimate moisture uptakes for each material identified for the absorption and desorption phases. Diffusion coefficients were normalised with that of the CF/TP material during the absorption phase such that:

$$ \bar{D}_{\text{abs}} = \frac{D_{\text{abs}}}{D_{\text{abs}}^{\text{CF/TP}}} \quad \text{and} \quad \bar{D}_{\text{des}} = \frac{D_{\text{des}}}{D_{\text{abs}}^{\text{CF/TP}}} \qquad (7) $$

Note the good reproducibility of the absorption curves, where all sam-

<figure><img src="image_4.png" /><figcaption>Fig. 2. Experimental measurements (dots) of normalised percentage moisture uptake $\bar{M}_t$ (see Eq. (6) in neat TP resin, CF/TS and CF/TP samples when immersed in water (samples 1 – S.1) and dried (samples 2 – S.2). Fick's law (Eq. (3), dotted line).</figcaption></figure>

<caption>Table 2</caption>

Normalised (Eq. (6) and Eq. (7)) diffusion and ultimate moisture uptake coefficients for neat TP resin, CF/TS and CF/TP samples during absorption (samples 1 – S.1) and desorption (samples 2 – S.2) obtained with Fick's law (Eq. (3)).

<table><thead><tr><th>Material</th><th>D<sub>abs</sub></th><th>D<sub>des</sub></th><th>D<sub>des</sub>/D<sub>abs</sub></th><th>M<sub>∞, abs</sub></th><th>M<sub>∞, des</sub></th></tr></thead><tbody><tr><td>Neat TP</td><td>3.77</td><td>4.84</td><td>1.28</td><td>3.15</td><td>-0.12</td></tr><tr><td>CF/TS</td><td>0.58</td><td>1.95</td><td>3.39</td><td>1.69</td><td>0.08</td></tr><tr><td>CF/TP</td><td>1</td><td>1.45</td><td>1.45</td><td>1</td><td>-0.28</td></tr></tbody></table>

ples of the same material follow a similar trend with an average relative difference lower than 5 % for the composite samples and 10 % for the neat TP resin samples. It can be seen that all samples follow a Fickian behaviour, except for the CF/TS material where the mass keeps increasing and starts diverging from the Fickian fit after 65 days. This indicates the presence of non-linear diffusion in this material. Using Langmuir's two-phase diffusion model (Eq. (4)), we can see in Fig. 3a that the experimental observations are much better reproduced. Korkees et al. [16] also observed a two-stage diffusion process in neat epoxy and CF/Epoxy composite samples. After a thorough study they were able to narrow this behaviour down to irreversible structural changes in the network, where relaxations in the resin seem to provide additional pockets of free volume for the water molecules to diffuse into. This type of behaviour, in which absorbed water induces the formation of micro voids, was also noted by Ulus et al. [17]. Using Fick's model, however, it is possible to reproduce the absorption phase of all our materials with sufficient accuracy, and to estimate the mass of water absorbed at full saturation. This model will therefore be used for the rest of the study.

Significant differences between the two composite materials can be noted, where the kinetics of the CF/TP material are faster (diffusion coefficient approximately 1.7 times that of the CF/TS material) yet absorb approximately 1.7 times less water at saturation. The diffusion rate in the neat TP resin is faster than in its composite counterpart, by a factor of almost 4. This is explained by the presence of fibres that represent obstacles to water diffusion in their transverse direction [30,53]. In the hypothesis that carbon fibres do not absorb or deteriorate in the presence of water [14,30,33–36], the maximum moisture absorption capacity of the CF/TP composite sample $M_{\infty}^{CF/TP}$ should be related to that of the neat TP resin $M_{\infty}^{TP}$ with the Resin Mass Fraction (RMF) in the composite such that [30,53]:

$$ M_{\infty}^{CF/TP} = RMF \cdot M_{\infty}^{TP} \qquad (8) $$

Fig. 4 presents the neat TP resin experimental measurements multiplied by RMF = 0.315 (see Table 2). This is coherent with the manufacturer's specifications of 30.6 ± 1.7%. The faster kinetics in the neat TP resin samples can be well appreciated, along with the fact that both curves tend towards the same asymptote during the absorption phase, in accordance with Eq. (8).

The second set of samples (S.2) were used to determine the diffusion coefficients for desorption (see Table 2). In accordance with other results reported in the literature, desorption diffusion coefficients are higher for all materials than their absorption counterpart. A possible explanation is that during the absorption process, water molecules need to overcome the obstruction of polymeric chains which is not the case during desorption [16,32,54,55]. Since both absorption and desorption were performed at 30 °C, the mass of the samples should tend towards a value of $M_t = 0$ if only reversible effects occurred during the first absorption part. As shown in Fig. 2 and Table 2, the Fickian fit of CF/TS sample 2 (S.2) tends towards a small yet positive value. The experimental measurements still follow a decreasing trend, however, indicating that the mass is converging towards the initial mass of the sample for longer drying durations. The same decreasing trend is indeed followed by Langmuir's model, as shown in Fig. 3b. For the neat TP and CF/TP samples, on the contrary, a mass loss is observed following the drying phase. This could imply the presence of a chemical reaction between the
<figure><img src="image_5.png" /><figcaption>Fig. 3. Experimental measurements (dots) of normalised percentage moisture uptake $\bar{M}_t$ (see Eq. (6)) of CF/TS samples when immersed in water (sample 1 – S.1) and dried (samples 2 – S.2). Comparison of fits using Fick's law (Eq. (3)), dotted line) and Langmuir's model (Eq. (4)), solid line).</figcaption></figure>

<figure><img src="image_6.png" /><figcaption>Fig. 4. Experimental measurements (dots) of normalised percentage moisture uptake $\bar{M}_t$ (see Eq. (6)) of CF/TP samples and of neat TP resin multiplied by RMF = 0.315, when immersed in water (samples 1 – S.1) and dried (samples 2 – S.2).</figcaption></figure>

TP and water. Fig. 4 highlights this effect and shows that when looking specifically at the resin in the CF/TP sample, more mass is lost proportionally than for the pure TP resin sample. This suggests that irreversible effects occur in the TP resin, which are further amplified by the presence of fibre/matrix interfaces. Indeed, it has been shown that this region could be a privileged one for water. The larger proportion of pores, hydrophilic sites (polymer end-chains in the case of thermoplastics, or un-cured thermoset prepolymers), as well as thermal residual compressive shear stresses in that region, are conducive to capillary flow, faster diffusion and increased binding of water molecules [56–58].

It can be postulated that the enhanced moisture uptake exhibited by the CF/TS material may result in increased effects of water absorption on its mechanical and damage behaviour compared to the CF/TP material in their immersed conditions. After drying, irreversible effects of water in the CF/TS are not expected as remnant mass is very small and keeps decreasing. The dried CF/TP samples after water immersion may, on the other hand, exhibit some further damage as irreversible chemical reaction at the fibre/matrix interface was highlighted.

## 3.2. Transverse cracking tests

Based on the absorption kinetics identified in the previous section, the duration of water immersion $t_{immersion}$ can be determined and discussed. Standards PR-EN-3615 [45] and PR-EN-2823 [46] stipulate that a specimen can be considered saturated when its mass variation remains below 0.05 % over the last 336 h, i.e., two weeks. In our case, this

corresponds to approximately 37 days for the CF/TP samples and 56 days for the CF/TS ones. In practice, to accommodate the scheduling constraints of the experimental equipment, the CF/TP specimens were tested after approximately 40 days of immersion, while the CF/TS specimens were tested after around 50 days.

The diffusion coefficients determined earlier also allow for a more precise assessment of the saturation state of the specimens, both before and after testing. After approximately 40 days immersion, the CF/TP samples were at 95 ± 1 % of their full saturation. Similarly, CF/TS samples were at approximately 90 ± 2 % of their full saturation before testing, after being immersed for a minimum of 50 days. As transverse cracking tests can take up to one work day, it is necessary to ensure that conditioning of both dry and wet samples does not vary significantly. The mass variation of dry composite samples placed in a climatic chamber (20 °C, 50 %RH) for 7 h was less than 0.05 % of their respective $M_\infty$. This small variation is considered as acceptable and should not affect the study's conclusions. From the diffusion coefficients identified in the desorption phase, it is estimated that the immersed CF/TP and CF/TS samples would be, at worst, at 80 ± 1 % and 77 ± 2 % of their respective full saturation by the end of the test. This is considered as acceptable, as the desorption kinetics were identified on samples dried in a desiccator at 30 °C and RH < 10 %, which is more severe than the actual environment of the test closer to 20 °C and 50 %RH. Note also that this should affect the studied 280 µm ply less as it is located at the centre of the laminate.

Prior to testing, and after a first decrease in temperature with no
mechanical loading, all specimen edges were examined under a micro-scope to determine if specimen cutting and handling, environmental conditioning or cryogenic temperature caused any damage. None was observed in any specimen prior to mechanical loading.

Using the protocol described above, the effect of temperature, water absorption and the nature of the material on the transverse cracking behaviour can all be assessed. As in the previous section, all results will be normalised. In order to still allow comparison between results and facilitate discussion, axial strains $ε_x$ for each material and temperature considered will be normalised with the same value $ε_{norm}$ such that:

$$ \bar{\epsilon}_x = \frac{\epsilon_x}{\epsilon_{norm}} \qquad (9) $$

where $ε_{norm}$ is arbitrarily defined and is the average strain at which most transverse cracking tests were stopped.

### 3.2.1. Effect of temperature

Fig. 5 and Fig. 6 present the reduced crack densities of the thick (280 µm) plies at -130 °C and -196 °C under dry and immersed conditions for the CF/TS and CF/TP materials, along with microscopic observations.

For the two materials studied under dry and immersed conditions, lower temperatures lead to reduced transverse crack initiation thresholds. At -130 °C, no transverse cracks were visible in the dry CF/TS samples, despite reaching their failure for a normalised strain of $\bar{\epsilon}_x \approx 1.13$ (see Eq. (9)). At -196 °C, on the contrary, many transverse cracks develop within the ply. This is also the case for the immersed samples, where the decrease in temperature leads to a shift of the reduced crack density to lower loading levels. This can be explained by matrix embrittlement as the temperature drops, as well as an increase in thermal residual stresses [4,5,7-9].

Observations for the CF/TP material show similar trends. Notably, except for the dry sample tested at -130 °C, this material exhibits fewer transverse cracks and more diffuse damage than the CF/TS material. Micrographs in Fig. 6 reveal that, near failure at -130 °C, both dry and immersed samples show only a small number of partial cracks across the observed area. These cracks do not fully penetrate the ply thickness and typically propagate gradually with continued loading (see Fig. 6). In fact, only two partial cracks were observed in the dry sample, while the immersed samples showed a total of ten partial cracks near failure ($\bar{\epsilon}_x = 1$, see Eq. (9)). Multiple steps of edge abrasion and polishing confirmed that these partial cracks extend into the bulk of the material. Although Eq. (5) accounts for them based on their relative length ($L_c$), the

resulting reduced crack density remains very small at this temperature. Such behaviour was previously reported by Laurin et al. [10] at ambient temperature in a LMPAEK T700GC/TC1225 thermoplastic composite, where short transverse cracks also formed. This could be attributed to the high fracture toughness of the matrix or plastic deformation at the crack-tip. Indeed, un-toughened epoxy resins typically exhibit critical energy release rates between 70–300 J/m² [59,60], whereas some thermoplastics can see theirs in the ranges of the [$kJ/m²$], even down to cryogenic temperatures [12,61]. At -196 °C, the increased stiffness, reduced ductility and embrittlement of the polymer matrix led to the formation of fully developed transverse cracks in both dry and immersed CF/TP samples (see Fig. 6). Partial cracks are still observed but are rare (less than 5%), and therefore contribute minimally to the reduced crack density at this temperature (see Eq. (5)).

### 3.2.2. Effect of water absorption

For both materials and temperatures studied, water absorption negatively affects transverse cracking behaviour, although the extent of this effect varies depending on the material.

As shown in Fig. 5, transverse cracks appear at lower strains in the immersed CF/TS samples compared to dry ones, at both temperatures tested. Notably, at -196 °C, the reduced crack density curves for immersed and dry samples are shifted nearly parallel to each other towards lower loading levels (by approximately 12%, or $\Delta\bar{\epsilon} \approx 0.08$), with little change of the slope. Interestingly, no transverse cracks were observed in the dry CF/TS samples tested at -130 °C, even at strain levels above $\bar{\epsilon} > 0.8$, suggesting that the effect of water absorption are more pronounced at -130 °C than at -196 °C. As discussed in section 1, water absorption in polymer matrices typically reduces the glass transition temperature ($T_g$), induces swelling, and decreases the elastic modulus [13-15,17,18]. These effects collectively lead to a reduction of thermal residual stresses at microscopic and mesoscopic scales. It should consequently result in higher transverse crack initiation thresholds and a change in the reduced crack kinetics. Sepasdar et al. [62] demonstrated this behaviour using a micromechanical model of a 185 µm thick CF/ Epoxy composite in a cross-ply ([0/90/0]) configuration. The nearly parallel shift of reduced crack density curves observed at -196 °C in Fig. 5 is therefore more likely attributable to the degradation of both the matrix and the fibre-matrix interface following water absorption. Reduced radial stresses at the interface due to matrix swelling [24,29], combined with decreased matrix and interface toughness caused by micro-void formation [14,17,24,37,40], may lead to a decrease in the fibre/matrix interface fracture toughness and strength. The model proposed by Sepasdar et al. [62] supports this, showing that such

<figure><img src="image_7.png" /><figcaption>Fig. 5. Reduced crack density as a function of normalised axial strains (see Eq. (9)) applied to the laminate of the thick (280 µm) ply at -130 °C and -196 °C for the CF/TS material under dry and immersed conditions. Microscopic observations of dry CF/TS samples close to failure ($\bar{\epsilon}_x = 1$) during a cracking test at -130 °C and -196 °C are shown on the right. Red arrows point at transverse cracks.</figcaption></figure>
<figure><img src="image_8.png" /><figcaption>**Fig. 6.** Reduced crack density as a function of normalised axial strains (see Eq. (9)) applied to the laminate of the thick (280 µm) ply at -130 °C and -196 °C for the CF/TP material under dry and immersed conditions. Microscopic observations of dry CF/TP samples at increasing axial strains during a cracking test at -130 °C and -196 °C showing partial and full-thickness cracks are provided on the right.</figcaption></figure>

degradations cause a shift in cracking kinetics similar to what is observed in Fig. 5.

In the CF/TP material, although more damage is observed in the immersed samples at -130 °C, the difference compared to dry samples is not substantial, and no fully developed transverse cracks appear, unlike in the immersed CF/TS samples. At -196 °C, both dry and immersed CF/TP specimens exhibit similar transverse crack initiation thresholds, though the immersed sample shows an increased reduced crack density kinetics, leading to approximately twice as many cracks. This suggests that, in addition to the fibre/matrix interface, the thermomechanical properties of the TP matrix itself might be affected by water absorption. Compared to the CF/TS material, the CF/TP appears, however, less affected by water absorption. This may be due to its lower overall uptake, as shown in Fig. 2: it absorbs about 1.7 times less water than the CF/TS material under the same immersion conditions. Previous studies have also shown that the extent of mechanical degradation increases with the amount of absorbed moisture [22,24,30].

### 3.2.3. Estimation of irreversible effects

The transverse cracking test samples were immersed in water for an average of 45 days at 30 °C. The aim was for a substantial amount of water to be absorbed, but without promoting irreversible chemical reactions (hydrolysis and oxidation). However, the diffusion kinetics identified in section 3.1 highlight irreversible mass loss in the CF/TP immersed sample, and indicate a reversible absorption/desorption process in the CF/TS sample (Fig. 2, Table 2). In order to investigate whether irreversible degradation of the matrix and fibre/matrix interfaces occurred during water immersion, one sample of each material and conditioning (dry and immersed) that was tested at -196 °C up to $\bar{\epsilon}_{max}$ was dried in the desiccator for several weeks. These samples were then subjected to an additional mechanical loading $\bar{\epsilon}_{add}$ at -196 °C, lower than the maximal strains reached during the first testing phase $\bar{\epsilon}_{max}$ as shown in Table 3. The reduced crack density was measured before (i.e. $\bar{\rho}(\bar{\epsilon}_{max})$) and after the additional loading step (i.e. $\bar{\rho}(\bar{\epsilon}_{add})$), and is represented for each sample with stars and dashed lines in Fig. 7.

<caption>**Table 3**</caption>

Strains and resulting reduced crack densities within samples after drying and application of an additional loading step at -196 °C.

<table><thead><tr><th>Sample</th><th>ε<sub>max</sub></th><th>ε<sub>add</sub></th><th>ε<sub>add</sub>/ε<sub>max</sub></th><th>ρ̄(ε<sub>add</sub>)/ρ̄(ε<sub>max</sub>)</th></tr></thead><tbody><tr><td>CF/TS; Dry</td><td>1</td><td>0.75</td><td>0.75</td><td>1</td></tr><tr><td>CF/TS; Immersed</td><td>0.92</td><td>0.85</td><td>0.92</td><td>1.05</td></tr><tr><td>CF/TP; Dry</td><td>1</td><td>0.81</td><td>0.81</td><td>1.02</td></tr><tr><td>CF/TP; Immersed</td><td>0.85</td><td>0.69</td><td>0.82</td><td>1.51</td></tr></tbody></table>

<figure><img src="image_9.png" /><figcaption>**Fig. 7.** Reduced crack density as a function of normalised axial strains (see Eq. (9)) applied to the laminate of the thick (280 µm) ply at -196 °C for the CF/TS and CF/TP materials under dry and immersed conditions. The additional loading step (dashed line, star) at -196 °C is represented.</figcaption></figure>

The increase in reduced crack densities following this additional loading step is also given in Table 3.

After drying and prior to the additional loading, few to no new cracks were observed for all samples. This shows that the single drying process does not create further damage. It is the same for the two initially dry CF/TS and CF/TP samples, where few to no cracks formed after the additional loading. This is in accordance with the selected protocol where damage should not progress at 80 % of the maximum strains applied to the sample [9,10].

The immersed CF/TS sample shows a 5 % increase in crack density at approximately 90 % of the maximal applied load (see Table 3). Since both dry and immersed CF/TS samples show little variation in reduced crack density after drying, the observations made in Fig. 5 and conclusions drawn in section 3.2.2 can be attributed to the reversible plasticization of the matrix.

Conversely, the immersed CF/TP sample shows a 50 % increase in reduced crack density when subjected to the additional loading. This supports the observations described earlier, where irreversible degradation of the TP matrix and interfaces may have occurred during water immersion. Whether the observations performed in section 3.2.2 on the reduced crack density curve of the immersed CF/TP sample are due to reversible (plasticization) or irreversible effects (chemical reactions) is
unclear. As the ply studied is located at the centre of the layup, the irreversible effects could have continued to develop during the drying phase. However, the results could also indicate that the plasticization of the TP matrix is capable of counteracting part of the degradation generated by the irreversible effects.

## 4. Conclusion and perspectives

The present study proposes a method and results aiming at highlighting and understanding the effect of water absorption on the transverse cracking behaviour of laminated carbon fibre composites down to cryogenic temperatures. Two materials based on unidirectional carbon fibres with thermoset (CF/TS) and thermoplastic (CF/TP) matrices were considered. After being either dried using a desiccator at room temperature, or immersed in 30 °C deionised water until the mass stabilised, mechanical tests under thermomechanical loading on cross-ply laminates with microscopic monitoring were set up to analyse the evolution of the transverse cracking process at very low temperatures (–130 °C and –196 °C). The influence of temperature and water absorption on the kinetics and cracking mechanisms of the two materials was investigated for 280 µm thick 90° plies.

Both immersed and dried samples of the two materials considered exhibited a similar behaviour regarding the effect of temperature to what is widely observed and accepted across the literature, namely that lower temperatures lead to a decrease in the crack initiation threshold. Whether CF/TP samples were dried or exposed to a wet environment, partial cracks developed at all temperatures considered, unlike the CF/TS material. The reduced crack density of the CF/TP material is also lower than that of the CF/TS material for all conditionings (dry and immersed) and at all temperatures considered. This could be attributed to a higher fracture toughness of the TP matrix or plasticity at the crack-tip, associated with a larger fracture process zone.

Water absorption proved to have a detrimental effect on the transverse cracking behaviour of both materials and at all temperatures studied (–130 °C and –196 °C). This will therefore impact industrial operations during manufacturing, installation and maintenance operations or alternatively imply a more restricted composite material specification on water uptake. Immersed CF/TS samples have indeed shown to have a 12 % decrease in transverse crack initiation threshold at –196 °C with similar transverse cracking kinetics when compared to the dry samples. At –130 °C, this effect was more pronounced, as no transverse cracks were observed in the dry specimens, whereas a significant number propagated during loading in the immersed samples.

The CF/TP material, by contrast, appears to be slightly less affected by water absorption: the transverse crack initiation threshold in the immersed samples remains unchanged, yet the cracking kinetics are approximately doubled. The difference in the severity of the effect of water saturation on the transverse cracking behaviour can be attributed to a lower sensitivity to water absorption when exposed to a wet environment. Indeed, the CF/TS material has proved to absorb approximately 1.7 times more water than the CF/TP material.

The detrimental effect of water may be imputed to the degradation of both the matrix and fibre–matrix interface caused by reversible plasticization and irreversible chemical reactions following water absorption. The reversibility of the degradation process was investigated by measuring absorption/desorption kinetics in CF/TS, CF/TP and neat TP resin samples, and by performing an additional cryogenic thermomechanical loading step after drying the tested specimens. Water absorption after 45 days was found to be fully reversible in the CF/TS material, while the CF/TP material exhibited an irreversible mass loss. These results suggest that the detrimental effect on the damage behaviour of the CF/TS material is primarily driven by reversible mechanisms, such as plasticisation, swelling and micro-void formation. In contrast, irreversible processes appear to be the main contributors to the degradation observed in the CF/TP material. However, since reversible and irreversible effects could not be entirely decoupled in the CF/TP

material, the interpretation of their respective roles remains less definitive.

Future work will focus on intermediate and less severe environmental conditionings such as 50 %RH or 80 %RH, with more emphasis on the dissociation of reversible and irreversible effects.

### CRediT authorship contribution statement

**Timothée Klein:** Writing – original draft, Validation, Software, Methodology, Investigation, Formal analysis, Conceptualization.

**Christophe Bois:** Writing – review & editing, Supervision, Project administration, Methodology, Funding acquisition, Conceptualization.

**Jean-Christophe Wahl:** Writing – review & editing, Supervision, Project administration, Methodology, Conceptualization. **Caroline Petiot:** Writing – review & editing, Supervision, Methodology, Funding acquisition, Conceptualization.

### Declaration of competing interest

The authors declare the following financial interests/personal relationships which may be considered as potential competing interests: Christophe Bois reports financial support was provided by Airbus SAS. KLEIN reports a relationship with Airbus SAS that includes: employment. Petiot reports a relationship with Airbus SAS that includes: employment. If there are other authors, they declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

### Acknowledgments

The authors would like to thank Airbus SAS for its financial support.

### Data availability

Data will be made available on request.

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