# Carbon fibres/RTM6 vitrimer and non-dynamic epoxy thermoset composites: comparative study of manufacturing, mechanical characteristics, water absorption, impact resistance, repairing and compression after impact

Vincent Schenk, Philippe Olivier, Karine Labastie, Mathias Destarac, Marc Guerre

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Vincent Schenk, Philippe Olivier, Karine Labastie, Mathias Destarac, Marc Guerre. Carbon fibres/RTM6 vitrimer and non-dynamic epoxy thermoset composites: comparative study of manufacturing, mechanical characteristics, water absorption, impact resistance, repairing and compression after impact. Composites Part B: Engineering, 2025, 304, pp.112674. 10.1016/j.compositesb.2025.112674. hal-05100943

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# Carbon fibres/RTM6 vitrimer and carbon fibres/RTM6 epoxy composites.
Comparative study of manufacturing, mechanical characteristics, water absorption, impact resistance, repairing and compression after impact.

Vincent Schenk<sup>a,b,c</sup>, Philippe Olivier<sup>b,*</sup>, Karine Labastie<sup>a</sup>, Mathias Destarac<sup>a,c</sup>, Marc Guerre<sup>c*</sup>

**Keywords:** Vitrimer Matrix, Carbon fibre, Resin Transfer Moulding, Impact, Repairing

## Abstract

In aerospace, thermoset materials are prized for durability but face limitations in recycling, prompting exploration into alternatives. This study compares the mechanical behaviour of an aerospace-grade two-component epoxy system (RTM6-2) to that of a vitrimer counterpart with a disulfide hardener (RTM6-V), both reinforced by unidirectional carbon fibres composites and manufactured by resin transfer moulding. Conformity and quality of laminated plates are rigorously verified using non-destructive and destructive methods. Mechanical properties evaluation, including interlaminar shear stress tests, highlight the efficacy of RTM6-V in achieving good impregnation and fibre-matrix cohesion. Moreover, the study investigates water uptake and assesses the reprocessing and repair capabilities of CFR RTM6-V laminates. Effectively, series of carbon/RTM6-2 and carbon/RTM6-V laminates were submitted to low velocity, low energy impact testing campaigns and subsequent repair evaluations demonstrate a limited yet partial repairing of matrix cracks and delamination in impacted laminates. Overall, this work provides insights into the performance and applicability of RTM6-V in aerospace composite applications, shedding light on its strengths and limitations.

<footnote><sup>a</sup> IRT Saint Exupéry, bâtiment B612 3 rue Tarfaya, 31405 Toulouse cedex 4, France</footnote>

<footnote><sup>b</sup> ICA, Université de Toulouse, UT3, CNRS UMR 5312, Espace C. Ader, 3 Rue Caroline Aigle, 3140 Toulouse France</footnote>

<footnote><sup>c</sup> Laboratoire SOFTMAT, Université de Toulouse, CNRS UMR 5623, Université Paul Sabatier, 118 route de Narbonne, 31062 Toulouse Cedex 9, France</footnote>

<footnote>* Corresponding author</footnote>
## 1 Introduction

Aeronautics uses more and more carbon fibres reinforced thermoset matrix composites for high performance applications (e.g. structural parts such as fuselage frames or wings ribs, ...) owing to their exceptional durability and structural integrity [1]. One of the limitations in the use of this kind of materials is linked to their difficult and complex recycling. Current recycling processes use thermolysis [2,3], solvolysis [4,5] or other methods [6,7] to degrade the thermosetting matrix from a composite part, so that the reinforcements can be reclaimed and reused, but most of the time with altered mechanical properties [8]. Therefore, these fibres cannot be easily recycled several times and used for the same applications as they were initially intended for [5,9,10].

In the last few years, the scientific community has developed new approaches to overcome the difficulty of recycling thermosets matrices by either introducing cleavable bonds into the polymer matrix (but with often detrimental loss of properties) or by replacing the permanent bonds with dynamic analogues. This later strategy involving dynamic bonds endows these polymer materials with new properties such as reprocessability, healability and recyclability. Among the large research topic dealing with dynamic materials, vitrimers appear as a promising alternative gathering the advantages of thermosets with good mechanical characteristics and viscous flow at high temperature. Vitrimers properties are induced by an associative exchange mechanism in the polymer network which maintains the crosslink density constant during thermal activation with an unchanged number of chemical bonds through the reprocessing procedure [11]. This confers to vitrimers characteristics similar to those of vitreous glass at high temperature (i.e. Arrhenius dependence of viscosity with temperature) facilitating healing and reprocessing without the need of precise temperature control.

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Since then, a strong enthusiasm has emerged from the composite community to implement the recently developed dynamic chemistries for industrially relevant composites parts. For a more exhaustive report on vitrimer composites, the reader is directed to our recent review [12] and some other related reports [13,14]. Among the many different chemistries implemented or purposely developed by chemists [11,15–17] for vitrimers, only few of them were used in the context of composites: transesterification [18,19], transamination of vinylogous urethanes [20,21], disulfide exchanges [22,23], and imine exchanges [24,25], to name the most popular ones. This poor diversity is strongly linked to the limited large-scale commercial availability of dynamic chemical precursors preventing the use of sophisticated chemical platforms. Besides simple availability considerations, their implementation in practical industrial applications is not straightforward, especially for aeronautics where materials qualification process is very strict and lengthy. For some aeronautical applications, a high glass transition temperature $T_g$ (>200 °C) is one of the most important characteristics to consider for the matrix. However, only few recent reports fulfil this criterion [26,27] with most of the reports exhibiting $T_g$ below 170 °C [28–30]. Ruiz de Luzuriaga and co-workers recently developed a high $T_g$ epoxy vitrimer based on commercial resins TGMDA (MY21) and DGBF (PY306) and compared its properties to those of aerospace-certified resin Hexcel HexFlow® RTM6 which is widely used in industry for a few decades and especially designed for resin transfer moulding (RTM) process [26,27]. However, the resulting vitrimer resin afforded a glass transition of 176 °C still well below the $T_g$ of 220 °C of RTM6-based thermosets. Zhang and co-workers used one of the RTM6 matrix constituents (tetraglycidyl methylene dianiline, TGDDM) which they crosslinked by epoxy/anhydride reactions affording materials with high mechanical performance and high $T_g$ (>200 °C) [26,27]. However, the modification of the crosslinking strategy from epoxy/amine towards epoxy/anhydride considerably modified the crosslinking procedure and network composition with the formation of hydrolytically sensitive ester bonds. A full network
degradation was achieved in pure water without addition of catalyst. Although being an interesting property for certain applications, it becomes crippling in the context of aeronautical specifications. From a composite manufacturing process point of view, although a few papers reported vitrimer composites produced by RTM [26,31–33], none of them studied completely the crosslinking kinetics and reactivity with a time-temperature-transformation (TTT) diagram and the optimisation of the process parameters and curing cycles. TTT diagrams [34,35] were developed in the mid 80’s and widely used for thermosets, but were never reported yet for vitrimer materials, to the best of our knowledge. Lastly, it appears that optimising RTM manufacturing process parameters when producing a vitrimer matrix composite has not been investigated yet.

In a previous paper, a new high-performance epoxy vitrimer system coined RTM6-V (V stands for vitrimer) and with the highest glass transition temperature $T_g$ of 233 °C ever reported for vitrimers was formulated, characterised and compared to the aeronautical epoxy reference RTM6-2 [36]. RTM6-2 is the HexFlow® RTM6 system with its two-component resin (or prepolymer) and hardener (or curing agent) separated, while the RTM6 is delivered with resin and hardener already mixed. A comprehensive cure kinetics study was conducted on both thermoset RTM6-2 and RTM6-V matrices enabling the construction of the first time-temperature-transformation (TTT) diagram for a vitrimer. This TTT diagram enabled the production of unreinforced vitrimer plates by RTM process, and their reprocessability / reshapability properties and limitations were especially investigated. Although important limitations were clearly identified, reshaping by thermoforming opened some opportunities to use this RTM6-V vitrimer matrix in carbon fibre reinforced composites (CFRC).

Therein we report the use of RTM6-V matrix to manufacture unidirectional carbon fibre reinforced composites (UD-CFRC) by RTM according to the parameters previously determined
thanks to the TTT diagram [36]. Then, the conformity and quality of CFRC plates produced
were verified using different non-destructive and destructive testing methods.

Afterwards, the mechanical properties of UD-CFRC were determined owing several standard
mechanical tests including interlaminar shear stress (ILSS) test. The water uptake of the
composites and the correlation between results reported in our previous paper on unreinforced
RTM6-V [36] is also presented. Finally, the repairing abilities of CFRC RTM6-V are studied
and discussed. Effectively, low speed and low energy impact tests are carried out (according to
ASTM D7136) to create impact damages in standard specimens and to assess the repairability
of these CFR RTM6-V laminates with the aim of repairing matrix cracks and delamination
stemming from the impacts. Repair quality is controlled owing to X-ray tomography and
Ultrasonic C-Scans. Lastly, once impacted, the laminates are repaired, controlled (C-Scan) and
then submitted to standard compression after impact tests (CAI) according to ASTM D7137.

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1. Experimental, manufacturing

89 1.1. *Materials*

90 HexFlow® RTM6-2 Part A (prepolymer / resin) and Part B (curing agent / hardener) was
91 purchased from Hexcel Composites France. The 4-aminophenyl disulfide (4-AFD) was
92 purchased from Molekula group (UK).

93 As regards the reinforcement, all unidirectional laminates [0°] were produced using an
94 unidirectional dry fibres tape UD T700G 320 g/m² purchased from Hexcel Composites France.
95 T700G carbon fibre has a density of 1.8, an elastic modulus of 240 GPa and an ultimate tensile
96 stress of 4.8 GPa.

97 The Q.I. (quasi-isotropic) 5 mm thick laminates were produced using an unidirectional dry
98 carbon fibre tape UD 600B C24K 650 g/m² purchased from Chomarat France. In that UD dry
fibres tape, the carbon fibres exhibit characteristics very similar to the ones of T700G with a
density of 1.8, an elastic modulus of 240 GPa and an ultimate tensile stress of 4.8 GPa.

In both unidirectional dry carbon fibre tapes (Hexcel and Chomarat), the fibres are kept in
position owing to some transverse or weft glass fibre threads (see Figure S1).

**1.2. Characterisation methods**

- Densities were obtained using the Density Determination Kit Sartorius YDK03 at room
temperature according ISO 1183. The reported results are averages of three measurements.

- Fibre volume fraction ($V_f\%$) was determined using the matrix dissolution method based on
the standard EN 2564, method A. Three samples of each system were taken from CFRC plates.

- Ultrasonic testing C-Scan was performed with an Omniscan equipment enabling the quality
of CFRC plates produced by RTM to be assessed.

- Optical microscopy was performed with an inverted optical microscope Zeiss Axio Observer
with an enlargement x 200 on CFRC samples enabling the fibre/matrix distribution and
porosities to be observed.

- Dynamic Mechanical Analyses (DMA) were performed on a DMA+ 100 Metravib under air
at a heating rate of 3°C from 25°C up to 280°C. Samples were tested in tensile mode with a
static strength of 2 N, a frequency of 1 Hz, a displacement of 15 µm (meaning that the material
remain in the linear viscoelastic region) on rectangular samples measuring 40 x 6.00 x 1.20
mm. The results reported here are average of three measurements.

- Tensile tests on composites were performed with a SCHENCK HYDROPULS PSB 100 with
a 100 kN force cell in accordance with ISO 527-5 standard. Digital Image Correlation (DIC)
on speckle patterns was used to determine the strain field of test coupons during the tensile
tests. The samples were tested in the 0° direction (according fibres length).
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- Three-point bending tests were performed with an MTS C45 with a 5 kN force cell, in accordance with ISO 14125 standard. Test specimens were tested in the 0° direction of the carbon fibres.

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- Interlaminar shear stress (ILSS) tests were performed on an MTS C45 with a 5 kN force cell, in accordance with ISO 14130 standard. Test specimens were tested in the 0° direction of the carbon fibres.

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- Water uptake tests were performed in an environmental chamber with a relative humidity of 85 (±5) % and a temperature of 70 ± 2 °C for a period of more than 2400 hours, in accordance with EN 3615. Samples dimensions were 40 x 6 x 2 mm in order to carry out DMA experiments after the water uptake, in accordance with EN 2823.

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- Impact tests were performed with an impact drop tower with single column impactor guide mechanism at 15 J in accordance with ASTM D7136. All the 5 mm thick impacted samples had a quasi-isotropic (Q.I.) stacking sequence. The impactor had a mass of 2.0 ± 0.25 kg with a smooth hemispherical striker tip with a diameter of 16 mm.

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- X-ray tomography measurements were performed with a high power and high-resolution X-ray tomography system RX Solutions EasyTom 230 with a voxel size of 22 µm.

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- Compression After Impact (CAI) tests were performed using a compressive machine SCHENCK PCS with a 450 kN cell force, on reference, impacted and repaired Q.I. CFRC samples according to ASTM D7137. Similarly, to tensile tests Digital Image Correlation (DIC) was used to determine the strain field during the CAI tests.

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## 1.3. Preparation of RTM6-2 and RTM6-V (vitrimer) unreinforced matrices
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The RTM6-2 reference epoxy network was prepared following the recommendations of Hexcel:

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the weight ratio between the RTM6-2 part A (resin) and part B (hardener) was fixed at 100:68.1.

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As regards the RTM6-V vitrimer network, the weight ratio of RTM6-2 part A (resin) and 4-AFD (dynamic hardener) was fixed at 100:54.5.

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**1.4. Manufacturing of carbon fibre-reinforced RTM6-2 and RTM6-V laminates**

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All the laminates were manufactured by VARTM. For unidirectional laminates [0°], the unidirectional dry carbon fibre tapes were aligned with the injection direction. A 2 mm thick 300 x 300 mm (inner in-plane dimensions) stainless steel frame was used as a spacer between top and bottom moulds to get 2 mm thick laminates. This corresponds to a theoretical $V_f$ of 48 % and this was experimentally verified. Two 300 x 300 x 2 mm laminated unidirectional plates for each matrix system (RTM6-V and RTM6-2) were produced.

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For Q.I. laminates devoted to impact tests, ASTM D7136 standard specifications were followed in terms of stacking sequence. A 5 mm thick spacer was thus used for the injection. This corresponds to a theoretical fibre volume fraction $V_f$ of 57.8 % and was verified experimentally after the composite manufacturing. The reader can get more information about the VARTM manufacturing steps in supporting information document section S1, Figures S2, S3 and Figure S4 for the injection parameters and cure cycles. It is important to note that curing conditions to produce CFRC plates were all based on the TTT diagrams [36].

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The last manufacturing step consisted in machining by water-jet cutting the laminated plates in order to obtain the mechanical tests specimens. However, the mechanical test results strongly depend on the CFRC plates quality obtained and test specimens must be exempt of defects e.g. porosities or dry zones. It was thus important to control the CFRC plates quality by using different methods such as ultrasonic testing (non-destructive) or cross-section optical microscopy.

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## 2. Results and discussion

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### 2.1. Assessment of CFR RTM6-2 and CFR RTM6-V laminated plates quality

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Ultrasonic C-Scan were performed onto CFR RTM6-V and CFR RTM6-2 laminated plates just after their manufacturing. This work is presented in supplementary information document section S2 in Figure S5 and Figure S6 respectively. These two figures include amplitude-based C-Scan, binarized C-Scan, depth-based C-Scan and photos of both faces of CFRC plates.

Yellow and blue arrows correspond to the injection and vacuum port respectively. Ultrasonic C-Scan of CFR RTM6-V plates reflects a consolidation of high quality with a surface consolidation rate of 80 %. This rate is calculated from the binarized image and is equal to the area respecting the acceptance criterion (green area) divided by the total area of the composite plate. Although the consolidation is satisfactory, a few flaws are present near the vacuum port (Figure S5.a). Some of these flaws could be observed visually on the upper face of a CFR RTM6-V plate (Figure S5.d). The depth of these defects could be determined from the depth-based C-Scan visible in blue corresponding to a depth of around 0.5 mm (Figure S5.c). Hence, mechanical test specimens had to be cut outside the area with these flaws to ensure the lowest possible variability of mechanical test results.

Similarly, CFR RTM6-2 plates analysed by ultrasonic C-Scan showed an 82 % surface consolidation rate attesting also the high quality of the plates manufactured. No defects could be observed in the C-Scan images hence facilitating the machining of mechanical test specimens in the plates. The same processing time was carried out for both systems, which led to few flaws for CFR RTM6-V plates. This could be explained by the slightly higher viscosity of RTM6-V at processing temperatures compared to RTM6-2, therefore decreasing the permeability of fibres. To overcome this problem, a longer injection time should be chosen for the RTM6-V system, although these flaws did not prevent the manufacturing of conform

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mechanical test specimens. Images of CFR RTM6-V and CFR RTM6-2 samples are presented in Figure 1.

<figure><img src="image_1.png" /><figcaption>Figure 1. Optical microscopy of a cross-section of a) CFR RTM6-V and b) CFR RTM6-2.</figcaption></figure>

Optical microscopy images show a satisfactory fibre/matrix distribution with almost no porosities. In addition, inter-plies are slightly visible for both systems and especially for CFR RTM6-V. Indeed, a few resin-rich areas between the plies can be observed with a zoom on two small porosities (Figure 1, right).

The CFRC samples densities and $V_f(\%)$ are reported in Table 1.

<table><caption>Table 1. Densities and fibres volume fraction ($V_f(\%)$) of CFR RTM6-V and CFR RTM6-2.</caption><thead><tr><th>Material</th><th>Density</th><th>V<sub>f</sub> (%)</th></tr></thead><tbody><tr><td>CFR RTM6-2</td><td>1.441 ± 0.013</td><td>46.67 ± 1.71</td></tr><tr><td>CFR RTM6-V</td><td>1.504 ± 0.006</td><td>46.41 ± 1.30</td></tr></tbody></table>

## 2.2. Physicochemical and mechanical properties

Once controlled by C-Scan CFRC laminated plates were machined using a high-pressure water jet to obtain test specimens. Different analyses were conducted to characterise the physicochemical and mechanical properties of both CFRC systems.
DMA tests were performed on samples cut in the transverse direction giving thus [90%] samples to obtain the mechanical manifestation of the glass transition $T_α$ and to get an overview of the mechanical stability of CFRC by observing the storage $E'$ and loss $E''$ moduli (Figure 2).

<figure><img src="image_2.png" /><figcaption>Figure 2. Dynamic mechanical analysis for a 3 °C/min ramp from $T_{room}$ to 260 °C for CFR RTM6-V and CFR RTM6-2. Samples tested in transverse direction [90%] in tension.</figcaption></figure>

Comparable storage modulus $E'$ of 4.04 GPa and 3.87 GPa (less than 5 % difference) were obtained for CFR RTM6-V and CFR RTM6-2 respectively. In addition, on the basis of results published on unreinforced matrices [36], it can be said that adding carbon fibres to RTM6-V and RTM6-2 to obtain composites affected positively their storage modulus $E'$ with an increase of approximately 50 % compared to unreinforced networks with values around 2.65 GPa. Obtained results are reported in Table 2.

<table><caption>Table 2. Properties obtained by DMA for CFR RTM6-V and CFR RTM6-2. $E'$ values at 40°C.</caption><thead><tr><td>Material</td><td>$T_α, E''$ (°C)</td><td>$T_α, tan δ$ (°C)</td><td>$E'$ (GPa)</td></tr></thead><tbody><tr><td>CFR RTM6-2</td><td>213.9</td><td>227.5</td><td>3.87</td></tr></tbody></table>
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Tensile tests stress-strain curves are plotted in Figure 3. The reported results in Table 3 are averages of five samples tested. This tensile test enabled to determine the ultimate tensile stress $\sigma_{ult}$, and the tensile strain at break $\varepsilon_{ult}$ as well as elastic modulus $E$.

<figure><img src="image_3.png" /></figure>

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<caption>Figure 3. Tensile stress as a function of strain for CFR RTM6-V compared to CFR RTM6-2. [0°] laminates.</caption>

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It cannot be noticed any significant differences (Figure 3) between CFR RTM6-V and CFR RTM6-2 regarding the elastic moduli $E$ and the ultimate tensile strains $\varepsilon_{ult}$. However, a higher ultimate tensile stress $\sigma_{ult}$ (+ 16 %) is observed for CFR RTM6-2 with a value of 1576 ± 30 MPa versus 1356 ± 63 MPa for CFR RTM6-V.

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Three-point bending tests were carried out and stress-strain curves recorded for each specimen tested and a representative curve for both systems was selected to compare them in section S3. Figure S7 of supporting information. The reported results in Table 3 are averages of five samples tested. This bending test enabled to determine ultimate bending stress $\sigma'_{ult}$, and
corresponding bending strain $\epsilon'_{ult}$ and bending modulus $E_f$. Firstly, the ultimate bending stress $\sigma'_{ult}$ obtained for CFR RTM6-V is higher (+ 16 %) with a value of 1434 ± 54 MPa compared to 1231 ± 33 MPa for CFR RTM6-2. Secondly, the vitrimer matrix composite breaks at a greater strain (+ 13 %) with $\epsilon'_{ult} = 1.58 ± 0.08$ % for CFR RTM6-V versus 1.40 ± 0.04 % for CFR RTM6-2. Thirdly, a higher bending modulus $E_f$ (+ 8 %) was obtained for CFR RTM6-V with a value of 97.5 ± 2.3 GPa compared to 90.4 ± 3.1 GPa for CFR RTM6-V.

Likewise the explanations given about the fibre orientation and $V_f(\%)$ which could explain the slight difference in bending properties, another aspect has to be considered. The bending properties not only depend on the carbon fibre properties as it was the case for tensile test in the 0° direction, but they also depend on the fibre/matrix adhesion and the matrix properties itself. Therefore, the higher stiffness of the unreinforced RTM6-V observed by Schenk et al. has an influence on the increase in the stiffness of CFR RTM6-V [36]. Moreover, these satisfactory bending properties attest of an excellent fibre/matrix adhesion for both systems and especially for CFR RTM6-V.

<table><caption>Table 3. Mechanical characteristics obtained of [0°] CFR RTM6-V and CFR RTM6-2 laminates.</caption><thead><tr><th>Test</th><th>Characteristics</th><th>CFR RTM6-2</th><th>CFR RTM6-V</th></tr></thead><tbody><tr><td rowspan="3">Tensile 0°</td><td>σ<sub>ult</sub> (MPa)</td><td>1576 ± 30</td><td>1356 ± 63</td></tr><tr><td>ε<sub>ult</sub> (%)</td><td>1.41 ± 0.11</td><td>1.28 ± 0.04</td></tr><tr><td>E (GPa)</td><td>119.8 ± 5.0</td><td>114.6 ± 1.1</td></tr><tr><td rowspan="3">3-point bending</td><td>Σ<sup>f</sup><sub>ult</sub> (MPa)</td><td>1231 ± 33</td><td>1434 ± 54</td></tr><tr><td>E<sup>f</sup><sub>ult</sub> (%)</td><td>1.40 ± 0.04</td><td>1.58 ± 0.08</td></tr><tr><td>E<sub>f</sub> (GPa)</td><td>90.4 ± 3.1</td><td>97.5 ± 2.3</td></tr><tr><td>ILSS</td><td>τ<sub>ult</sub> (MPa)</td><td>101.2 ± 3.8</td><td>103 ± 1.0</td></tr></tbody></table>

Interlaminar shear stress (ILSS) tests give an idea of fibres/matrix and interply adhesion. Several failure modes are possible such as single or multiple interlaminar shear, tension, compression, plastic shear and a mix of them. The only acceptable failure mode is the single or multiple interlaminar shear in order to calculate the apparent interlaminar shear strength $\tau$ (see
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standard ISO 14130). ILSS-displacement curves were recorded for each specimen tested and are reported in Figure 4, while average ultimate interlaminar shear strength $\tau_{ult}$ values are given in Table 3.

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<figure><img src="image_4.png" /></figure>

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<caption>Figure 4. ILSS as a function of time for CFR RTM6-V compared to CFR RTM6-2.</caption>

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Figure 5 clearly shows that when submitted to ILSS CFR, RTM6-2 and CFR RTM6-V do not exhibit the same behaviour. Effectively, even if the ultimate stress values are very close (see Table 3), CFR RTM6-2 coupons undergo a much larger displacement than CFR RTM6-V ones.

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According to ISO 14130, to be accepted after testing a sample must have a clear interlaminar shear failure mode as seen in Figure 6 for a CFR RTM6-V sample. For CFR RTM6-2 ones, they undergo a plastic deformation before any fracture (see supporting information document section S3, Figure S8). This invalidates the ILSS tests and means that rigorously interlaminar shear strength $\tau_{ult}$ values cannot be considered for [0°] CFR RTM6-2. However, it is important to see here that the RTM6-V matrix gives a very good interlaminar adhesion with high ultimate

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interlaminar shear strength $\tau_{ult}$ values (higher than 100 MPa) showing thus the viability of this
vitrimer matrix composite.

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<figure><img src="image_5.png" /><figcaption>Figure 5. ILSS test of a CFR RTM6-V specimen with a conform rupture mode in multiple interlaminar shear (according to ISO 14130). a) enlargement of the ILSS sample cross-section, b) enlargement of sample according its length.</figcaption></figure>

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## 2.3. Water absorption

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Water absorption experiments were conducted on CFR RTM6-V and CFR RTM6-2 samples
based on EN 3615. The standard EN 2823 was applied for DMA tests performed on specimens
after exposure to humid atmosphere and reaching the water saturation.

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CFR RTM6-V and CFR RTM6-2 samples were dried at 60°C prior to water absorption tests
showing that they lost around 0.4 % and 0.8 % per weight respectively. Then, dried samples
were exposed to a relative humidity of 85 (±5) % and a temperature of 70 ± 2 °C for more than
2400 hours (i.e. more than 3 months). CFR RTM6-V and CFR RTM6-2 samples were weighed
at regular intervals to obtain the water uptake as a function of time until saturation was reached
(Figure 6). The saturation is reached when samples mass remained constant. This corresponds
to mass gaps of maximum 0.5 mg per gram of samples obtained between three successive
weighs performed at intervals of 168 hours (i.e. 1 week). Figure 7 results are the average of the
values obtained on three samples per matrix system, while the water uptake (WU) is given by:

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$$WU = \frac{m_1 - m_0}{m_0} * 100 \quad (1)$$

With $m_0$ the initial mass of the dried sample and $m_1$ the mass of the sample after exposition to humid atmosphere.

<figure><img src="image_6.png" /><figcaption>Figure 6. Water uptake as a function of time in a climatic chamber at 70 °C and 85 % RH for CFR RTM6-V compared to CFR RTM6-2. Six plies U.D. samples.</figcaption></figure>

The water uptake of CFR RTM6-2 and CFR RTM6-V samples follows the same trend as previously reported for unreinforced samples [36]. The water uptake at saturation for CFR RTM6-V is higher than the one of CFR RTM6-2 with 3.90 % and 2.95 % respectively. These results are perfectly in accordance with the ones obtained for unreinforced RTM6-V with a value of 3.9 % and RTM6-2 with a value of 2.8 % [36]. Likewise, CFR RTM6-V is more sensitive to water uptake likely due to the presence of disulfide bonds in the matrix polymeric network, with also a potential impact on the physical and mechanical properties of the composites which are investigated in the next section.
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After water uptake saturation, CFRC DMA samples were removed from the humidity chamber for being submitted to DMA tests. EN 2823 specifies that samples which have reached water saturation have to be stored in a waterproof container during maximum 72 hours before performing the DMA experiments and have to be tested in the next 30 minutes after being removed from that waterproof container. Tensile DMA experiments on [90%] were conducted at 3 °C/min from $T_{room}$ up to 260 °C. The results reported in Table 4 are the average of three samples tested for the wet specimen. CFR RTM6-V vs CFR RTM6-2 wet DMA storage modulus $E'$, loss modulus $E''$ and $Tan \ \delta$ curves are presented in Figure 7.

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<figure><img src="image_7.png" /></figure>

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<caption>Figure 7. DMA for a 3°C/min ramp from $T_{room}$ to 260°C for CFR RTM6-V compared to CFR RTM6-2 after water uptake saturation. On [90%] samples in tension.</caption>

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Material behaviour shown by storage modulus $E'$ curves is similar for both systems up to 180 °C. Once at 180°C the storage modulus $E'$ of wet CFR RTM6-2 starts to decrease sharply while the one of CFR RTM6-V remains more stable over one ten of °C and this although CFR RTM6-V water uptake at saturation was larger. The temperature at main mechanical relaxation

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peak $T_α$ measured on loss modulus $E''$ curves remains slightly below 200 °C after water uptake
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saturation for both networks. Comparing Figures 8 and 3 enable the effects of water uptake to
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be assessed on carbon fibre reinforced vitrimer matrix composite. As expected water absorption
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induces a decrease in $T_{α,E''}$ for carbon/epoxy CFR RTM6-2, but the most remarkable fact is the
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significant decrease of -35°C of $T_{α,E''}$ for the CFR RTM6-V which is much more sensitive to
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the effects of water absorbed (Table 4). Nevertheless, even in the wet state $T_{α,E''}$ remains
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sufficiently high for airframe structures applications in terms of glass transition temperature.
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<table><caption>Table 4. Comparison of characteristics obtained by DMA for CFR RTM6-V and CFR RTM6-2 at dry and at wet states. $E'$ values were taken at 40 °C on [90%] samples in tension.</caption><thead><tr><th>Material</th><th><i>T</i><sub>α,E''</sub> (°C) <i>dry</i></th><th><i>T</i><sub>α,E''</sub> (°C) <i>wet</i></th><th><i>E'</i> (GPa) <i>dry</i></th><th><i>E'</i> (GPa) <i>wet</i></th></tr></thead><tbody><tr><td>CFR RTM6-2</td><td>213.9 ± 0.6</td><td>196.1 ± 0.7</td><td>3.87 ± 0.15</td><td>3.92 ± 0.18</td></tr><tr><td>CFR RTM6-V</td><td>232.6 ± 0.4</td><td>197.5 ± 0.3</td><td>4.04 ± 0.09</td><td>4.23 ± 0.12</td></tr></tbody></table>

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## 2.4. Impact tests on CFR RTM6-2 and CFR RTM6-V Q.I. laminates

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In order to investigate the repairing abilities of CFR RTM6-V it has been decided to work
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within a standardised frame. This is why quasi-isotropic 5 mm thick laminates CFR RTM6-V
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as well as CFR RTM6-2 (see supporting information document section S4, Figure S9) have
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been submitted to low energy (15 J), low velocity impact tests. For these laminates, their real
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fibre volume fractions were determined according matrix acid digestion EN 2564. The obtained
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average value $V_f = 60.03 \pm 2.89\%$ is slightly higher than the theoretical one previously
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calculated: 57.8 %.
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All impacted samples were controlled by US C-Scans to determine cumulated delaminated area
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through their thickness. After these C-Scans, three CFR RTM6-2 impacted coupons as well as
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three CFR RTM6-V ones were submitted to compression after impact (CAI) tests. The two
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remaining CFR RTM6-V impacted coupons were not submitted to CAI, but they were repaired
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with the aim to close all matrix cracks and delaminations and then submitted to CAI tests. Table 5 recalls the number of CAI tests and the kind of tested samples. Reference samples were not impacted and were directly submitted to CAI tests in order to obtain reference values. Lastly, note that some samples were controlled by X-ray tomography before and after repair to assess the efficiency of repairing procedure which will be described later in this paper.

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<caption>Table 5. Number of CAI test samples for both systems. Reference: samples not impacted. Impacted: samples impacted at 15 J. Repaired: samples repaired after impact at 15 J.</caption>

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<table><thead><tr><th></th><th>Reference</th><th>Impacted</th><th>Repaired</th></tr></thead><tbody><tr><td>QI CFR RTM6-2</td><td>2</td><td>3</td><td>Not possible</td></tr><tr><td>QI CFR RTM6-V</td><td>2</td><td>3</td><td>2</td></tr></tbody></table>

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Generally speaking, 15 J impacts on Q.I. carbon/epoxy laminates are considered as capable of generating barely visible impact damage (BVID) and of inducing resin microcracks and interply delamination without affecting integrity of carbon fibres (i.e. no carbon fibres fracture). The ICA drop tower used for these is equipped with sensors enabling impactor velocity (impact energy calculation) and impact force to be measured. Obtained values are given in Table 6, while in supporting information document section S4, Figure S10 shows the recorded impact forces versus time for RTM6-2 and RTM6-V quasi-isotropic composites. The maximum impact force is lower for QI CFR RTM6-V with a value of 6.01 ± 0.33 kN versus 7.07 ± 0.37 kN for QI CFR RTM6-2. These differences can be explained by the higher stiffness of RTM6-V matrix, but as we will see with US C-Scans, this could also have a negative effect on its impact resistance since this higher stiffness goes with a higher brittleness.

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<table><caption>Table 6. Impact energy and maximum force recorded during the impact tests on QI CFRC laminated plates.</caption><thead><tr><th></th><th>Impact energy (J)</th><th>Impact force (kN)</th></tr></thead><tbody><tr><td>QI CFR RTM6-2</td><td>14.59 ± 0.12</td><td>7.07 ± 0.37</td></tr><tr><td>QI CFR RTM6-V</td><td>14.58 ± 0.19</td><td>6.01 ± 0.33</td></tr></tbody></table>

After the impact tests, all Q.I. CFR plates were visually analysed (supporting information document, section S4, Figure S11) showing a barely visible impact damage (BVID). No other defects could be visually observed after the impact tests. The delaminated area which directly impair the mechanical strength of composites was determined – as already said – by US C-Scans immediately after impact (see Figure 8).

<figure><img src="image_8.png" /><figcaption>Figure 8. US C-Scan after impact of: a) QI CFR RTM6-2 and b) QI CFR RTM6-V.</figcaption></figure>

The red ellipses (Figure 8) correspond to the delaminated area after the impacts. When comparing delaminated areas it can be seen (Figure 9) that the one of QI CFR RTM6-V laminates is 62% higher than the one of QI CFR RTM6-V2 laminates. Even if ILSS tests results show that CFR RTM6-2 and CFR RTM6-V [0°] have an average $\tau_{ult}$ (MPa) of the same order of magnitude (Table 3), here impact tests with the resulting delaminated areas demonstrate that the RTM6-V matrix is more brittle than RTM6-2 one. Delaminated areas and indent depth are given in Table 7 where it can be read that the delaminated area after the 15 J impact is of 6186 ± 130 mm² for QI CFR RTM6-V samples versus 3819 ± 552 mm² for the QI CFR RTM6-2.
In Figure 9, the first delamination (dark blue) is located at a depth of around 0.6 mm from the laminate impacted side, which corresponds to the interface between the 90° and the -45° plies and seems to be oriented in the 90° direction. The second delamination (light blue) is located at a depth of around 1.3 mm, which corresponds to the interface between the -45° and the 0° plies and seems to be oriented in the -45° direction. The third delamination (cyan) is located at a depth of around 1.9 mm, which corresponds to the interface between the 0° and 45° plies and seems to be oriented in the 0° direction. Lastly, the fourth delamination observable on the depth-based C-Scan is located at a depth of around 2.5 mm, which corresponds to the interface between the two 45° plies and is logically oriented in the 45° direction. The impact energy of 15 J is not high enough to generate damages through the whole thickness of the laminates.

Depending on the matrix behaviour, the indent remaining after impact can be more or less visible. Several methods exist to determine the dent depth. In this study digital image correlation (DIC) was used for the deformation mapping during CAI test, but also for the calculation of the dent depth. A speckle pattern was applied onto the surface of the sample studied. DIC was used with 2 cameras allowing to obtain the deformation mapping in 3 dimensions (3D). The cameras are located at 2 different angles and record images of the sample during the trial. VIC 3D software enabled to analyse images before the CAI tests with the reconstruction of the surface mapping in three-dimensions. The 3D surface mapping is represented in different view angles for QI CFR RTM6-2 (Figure S12, supporting information document, section S4) and QI CFR RTM6-V (Figure 9).
<figure><img src="image_9.png" /><figcaption>Figure 9. 3D surface mapping with different view angles, a) 3D, b) xy-plane, c) yz-plane, for QI CFR RTM6-V impacted samples. For QI CFR RTM6-V laminate please see figure S12.</figcaption></figure>

These 3D surface images give an accurate view of the flatness of the sample studied and in addition, show the exact location and dimensions of the impact. As expected, the dent depth of QI CFR RTM6-V is lower with a value of 0.17 ± 0.02 mm compared to 0.25 ± 0.02 mm for QI CFR RTM6-2 (Figure S12, supporting information document, section S4). Dent depth and delaminated area results are reported in Table 7.

<table><caption>Table 7. Impact depth and delaminated area obtained for QI CFRC impacted samples.</caption><thead><tr><td></td><td>Dent depth (mm)</td><td>Delaminated area (mm²)</td></tr></thead><tbody><tr><td>QI CFR RTM6-2</td><td>0.25 ± 0.02</td><td>3819 ± 552</td></tr><tr><td>QI CFR RTM6-V</td><td>0.17 ± 0.02</td><td>6186 ± 130</td></tr></tbody></table>

These impact results show that QI CFR RTM6-2 can absorb a fraction of the impact energy by plastic deformation visible in the dent depth (Figure 10). On the opposite, QI CFR RTM6-V
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exhibit a lower dent depth and a larger delaminated area. However, to carry on this comparative
work between ‘conventional’ RTM6-2 matrix composite and RTM6-V vitrimer matrix ones we
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are now going to try to close the damages generated by the impacts on CFR RTM6-V laminate
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by using the vitrimer properties. As shown below in Figures 13 and 14 impacts induce interply
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delamination and matrix cracks.
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**2.5. Repairing impacted CFR RTM6-V Q.I. laminates**

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Although impact tests highlighted a lower impact strength for QI CFR RTM6-V samples, their repairing ability could be an advantage compared to QI CFR RTM6-2 samples. To do so, a repairing procedure was applied to the damaged QI CFR RTM6-V samples involving the use of a parallel heating plates press. The impacted laminates were inserted into the press at 245 °C. This temperature – lower than the thermal degradation of RTM6-V matrix – corresponds to the optimal temperature previously determined from stress relaxation experiments carried out on unreinforced RTM6-V matrix [36]. Before applying any compressive stress (in the z-axis direction perpendicular to the lamination plane) on the damaged laminates, the press was closed and a stabilisation of 10 minutes was enforced to enable the samples to reach the repairing temperature. Afterwards, a 4 MPa pressure was applied for 15 minutes. Finally, the pressure was removed, and the sample freely cooled down to room temperature. The QI CFR RTM6-V sample thus repaired was first controlled by US C-Scan mappings see Figure 10.
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<figure><img src="image_10.png" /></figure>

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<caption>Figure 10. UT C-Scan of QI CFR RTM6-V sample: a) before repairing, b) after repairing.</caption>
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US C-Scan mapping shows a reduction of the delaminated area by approximately 23 %, with a value of 4777 ± 235 mm² for QI CFR RTM6-V repaired samples. In addition, 3D surface mapping of the same samples obtained by 3D DIC before CAI clear show that once repaired the laminates no longer show any impact dent (Figure 11).

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<figure><img src="image_11.png" /></figure>

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<caption>Figure 11. 3D surface mapping with different view angles: a) 3D, b) xy-plane, c) yz-plane, for QI CFR RTM6-V repaired.</caption>

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In addition, comparing Figures 10 and 12 shows that even the laminate flatness is corrected by the repairing procedure and this without inducing any changes in QI CFR RTM6-V thickness. Even if these results are promising, only the CAI test will attest of the efficiency of repairing procedure enabling or not the recovering of the initial mechanical properties.

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To go further in analysing the laminates structural changes generated by the repairing procedure, X-ray tomography CT-scans were carried out. In this work, a representative delaminated area of 1800 mm² around the impact dent was observed by X-ray tomography CT-
Scan. This corresponds to a square area of 42.4 x 42.4 mm. X-ray tomography CT-Scan experiments were conducted on QI CFR RTM6-V plates before and after repairing. Figure S13, supporting information, section S4, shows the area analysed by X-ray tomography CT-Scan on an impacted plate. Avizo software was used to process reconstructed 3D images. The exact same area of a QI CFR RTM6-V laminate before and after repairing was analysed with identical parameters in order to be sure that we observe the influence of the repairing procedure and no other artefact. The volume of porosities and delaminations were extracted from sample reconstructed volume with Avizo and are shown in Figure 13. They are represented before (in red) and after (in yellow) repairing (see Figure 12).

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<figure><img src="image_12.png" /><figcaption>Figure 12. a) QI CFR RTM6-V sample reconstructed volume. Extracted volume of porosities and delamination: b) before repairing (represented in red), c) after repairing (represented in yellow) of QI CFR RTM6-V.</figcaption></figure>

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Comparing the two images before and after repairing shows that some delamination planes have been repaired. From these images, a volume of porosities/delamination can be obtained. Indeed, the volume of porosities/delamination is of 3.93 % after impact without repair, versus 3.46 % after repairing. This corresponds to a repairing efficiency of 12 %. A low fraction of porosities

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501 is stemming from the RTM manufacturing process itself. These process-induced porosities are
502 mainly located at the interface between the glass fibre weft threads and the matrix.

503 It may be somewhat hard to connect the 3.93% of damaged volume in the QI CFR RTM6-V
504 laminates with what it shown in Figure 13 b) and c). This is why it has been decided to analyse
505 the differences in the microstructure by optical microscopy through a cross-section of the
506 damaged and repaired QI CFR RTM6-V laminates. Figure 14 shows the microstructure of a
507 laminate in the xz-plane at the middle of the impact dent before (Figure 14.a) and after repairing
508 (Figure 14.b). The impact dent is marked with a red arrow (Figure 13.a) and the delaminated
509 planes and resin cracks arose from this impact. In addition, the images show the glass fibres
510 transverse weft threads. After the repairing procedure, the impact dent has completely
511 disappeared together with some of the delaminated planes and resin cracks. This is highlighted
512 in Figure 13.c by the overlay of the two images before and after repairing where the repaired
513 areas are visible in red.

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<figure><img src="image_13.png" /></figure>

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<caption>Figure 13. xz-plane image of QI CFR RTM6-V: a) before repairing, b) after repairing and c) overlay (plotted in red) of the differences between the two images.</caption>

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As shown by Figure 14.b, after repairing most of the remaining (non-closed) resin cracks are perpendicular or exhibit a 45° angle to the lamination plane. It is reminded that beyond the temperature the repairing procedure consisted in applying a 4 MPa compression perpendicularly to the lamination plane (i.e. according z axis). So, this means that in addition to the reprocessing parameters (time, temperature and pressure), the direction of compression to repair the defects is also of importance. In fact, these repairing tests show that it is necessary to get a fine knowledge of the damages orientations in order to design a specific repairing procedure, because here it is demonstrated that simply putting a damaged vitrimer matrix laminate under a pressure perpendicular to its lamination plane at the adequate temperature is not sufficient to enable the healing of matrix cracks. To illustrate that a basic F.E. simulation of repairing a laminate after impact has performed with Abaqus. In this simulation two defects are assumed in the CFR RTM6-V Q.I. laminate: a 0.2 mm thick delamination and a 0.2 mm thick matrix crack oriented at 45°. While the whole laminate is placed at 245°C, a 4 MPa pressure is applied on the laminate top surface while the bottom surface is simply supported. Laminate's behaviour assumption and characteristics used for that numerical simulation are given in supporting information document in section S4. Figure 14 results show that these 'repairing conditions' cannot close the 45° matrix crack. It is thus clear that specific ways to put the damaged laminate under repairing stresses must be found for each case and for each laminated part. This requires a fine knowledge of the damages (i.e. kinds, sizes, orientations) in order to design right repairing conditions.

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Boundary conditions. Bottom surface: fully supported.
Top surface: 4 MPa pressure perpendicular to surface

<figure><img src="image_14.png" /></figure>

<figure><img src="image_15.png" /></figure>

<figure><img src="image_16.png" /></figure>

<figure><img src="image_17.png" /></figure>

<figure><img src="image_18.png" /></figure>

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541 Figure 1. Numerical simulation of the repairing procedure applied to FRC RTM6-V impacted

542 composites with a 4 MPa pressure at 245°C. a) optical microscopy image of damages after

543 impact. b) $\sigma_{zz}$ stresses (in MPa) generated by the 4 MPa pressure around the delamination. c)

544 $\sigma_{zz}$ stresses generated by the 4 MPa pressure around the matrix crack.

545

## 2.6. Compression after impact (CAI)

547 In aeronautics, compression after impact (CAI) test is widely used to evaluate the impact
548 strength of primary composite structures. CAI test were performed according to ASTM D7137.

549 During the test, various parameters are monitored and recorded, including the applied force,
550 crosshead displacement, and strain in the specimen. The strain was determined using 3D DIC
551 enabling to obtain several information such as the 3D surface mapping of the sample, in addition
552 to the strain field recorded. CAI test set up with DIC is presented in supporting information,
553 section S4, ** ERROR ! Source du renvoi introuvable.**

554 After the CAI test, the failure mode of the specimen is carefully assessed. Data collected during
555 the CAI test, including load-displacement curves and strain data, are analysed to determine the
556 compressive strength of the composite material. This information is valuable for engineers and
designers, as it helps them assess the structural integrity of composite components after
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experiencing impact damage. CAI data for reference, impacted and repaired QI CFRC samples
558
are shown in Figure 14. It is reminded here that QI CFR RTM6-2 ref and QI CFR RTM6-V ref
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samples were not impacted prior CAI tests.
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<figure><img src="image_19.png" /><figcaption>Figure 14. Compressive stress as a function of strain for QI CFRC samples.</figcaption></figure>

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As shown in Figure 14, the compressive strength of QI CFR RTM6-V ref laminates is 26% higher than the one of QI CFR RTM6-2 ref laminates with values of 256.8 MPa and 203.8 ± 10.2 MPa respectively. These values will be kept as reference values for their respective systems i.e. RTM6-V and RTM6-2. Regarding the mechanical properties after impact, the compressive strength of QI CFR RTM6-2 impacted was reduced by 26 % compared to the reference. For QI CFR RTM6-V impacted, the compressive strength was reduced by 59 %. These results are in accordance with the related delaminated area determined by US testing, which is 62 % larger or QI CFR RTM6-V compared to QI CFR RTM6-2. It is therefore logical
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to have more important compressive strength reduction for the RTM6-V system than for the RTM6-2 one.

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Lastly, the repairing efficiency based on the compressive strength measured during these CAI tests was around 14 %, with an augmentation of the compressive strength at 120.8 ± 14.3 MPa after repair against 106.1 ± 5.2 MPa for the impacted system. This repairing efficiency is comparable to the value obtained with X-ray tomography (12 %). However, the repaired compressive strength of the repaired laminates QI FRC RTM6-V repaired remains lower than the reference compressive strength, with a reduction of 53 %. The compressive strengths of all QI CFRC samples are gathered in Table 8.

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<caption>Table 8. Compressive strength at break for reference, impacted and repaired QI CFRC samples.</caption>

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<table><thead><tr><th></th><th>QI CFR RTM6-2</th><th>QI CFR RTM6-V</th></tr></thead><tbody><tr><td>σ<sub>max</sub> (MPa) Ref (no impact)</td><td>203.8 ± 10.2</td><td>256.8</td></tr><tr><td>σ<sub>max</sub> (MPa) Impacted</td><td>151.8 ± 1.8</td><td>106.1 ± 5.2</td></tr><tr><td>σ<sub>max</sub> (MPa) Repaired</td><td>impossible</td><td>120.8 ± 4.3</td></tr></tbody></table>

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### 3. Conclusion

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Our previous work on unreinforced RTM6-V [36] was used to manufacture unidirectional carbon fibre reinforced RTM6-V through RTM. CFRC plates' quality was verified with the use of ultrasonic testing and optical microscopy confirming their satisfactory production quality. CFRC plates with V<sub>f</sub> of around 46 % were machined using a water jet cutting to obtain several mechanical test specimens. Mechanical tests such as tensile, 3-point bending, or DMA were performed according to ISO standards and CFR RTM6-V presented similar properties as the CFR RTM6-2 reference. More importantly, interlaminar shear stress tests were carried out showing comparable ILSS maximal stresses between the two networks, with value of 103.3

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MPa for CFR RTM6-V indicating an excellent fibre/matrix adhesion, equivalent to high
performance thermoset composites.

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Then, water uptake experiments were conducted on DMA composite samples in a humid
atmosphere to reach water saturation. DMA experiments revealed an increase of water uptake
for CFR RTM6-V compared to CFR RTM6-2 with also a lower T<sub>a</sub> but still remaining near 200
°C. The same conclusions were made for the unreinforced systems [36].

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Finally, low energy impact tests at 15 J were conducted on quasi-isotropic CFRC samples to
study the impact strength of QI CFR RTM6-V as a comparison to QI CFR RTM6-2, but also
to evaluate the repairing efficiency of the vitrimer system. Reference, impacted and repaired QI
CFRC plates were analysed by US C-Scans, X-ray tomography, digital image correlation and
compression after impact test. From these tests, similar conclusions were established. QI CFR
RTM6-V has a lower impact strength compared to the reference RTM6-2. The repairing
efficiency is also limited. Firstly, this is due to the fact that applying a compressive pressure on
a laminated plate does not generate enough stress for closing matrix cracks oriented at 45° with
respect to the lamination plane while all delaminations were effectively repaired. Secondly, this
is also due to the narrow reprocessing temperature and time frames, with values ranging
between 12 and 23 %. This insufficient repairing efficiency highlights once more the severe
limitations of this RTM6-V system, corresponding to the limits of such high T<sub>g</sub> systems using
disulfide chemistry.

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