# Continuous microtomography of the hot press consolidation of high performance CF/PAEK layered composites

Adrien Le Reun, Arthur Lévy, Nicolas Lefevre, Henri-Alexandre Cayzac, Vincent Sobotka, Steven Le Corre

► To cite this version:

Adrien Le Reun, Arthur Lévy, Nicolas Lefevre, Henri-Alexandre Cayzac, Vincent Sobotka, et al. Continuous microtomography of the hot press consolidation of high performance CF/PAEK layered composites. 28th International ESAFORM Conference on Material Forming (ESAFORM 2025), May 2025, Paestum, Italy. pp.592-599, 10.21741/9781644903599-64. hal-05125012

HAL Id: hal-05125012

https://hal.science/hal-05125012v1

Submitted on 23 Jun 2025

HAL is a multi-disciplinary open access
archive for the deposit and dissemination of sci-
entific research documents, whether they are pub-
lished or not. The documents may come from
teaching and research institutions in France or
abroad, or from public or private research centers.

L'archive ouverte pluridisciplinaire HAL, est
destinée au dépôt et à la diffusion de documents
scientifiques de niveau recherche, publiés ou non,
émanant des établissements d'enseignement et de
recherche français ou étrangers, des laboratoires
publics ou privés.

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

Distributed under a Creative Commons Attribution 4.0 International License
# Continuous microtomography of the hot press consolidation of high performance CF/PAEK layered composites

Adrien LE REUN<sup>1,2,a*</sup>, Arthur LEVY<sup>1,b</sup>, Nicolas LEFEVRE<sup>1,c</sup>, Henri-Alexandre CAYZAC<sup>2,d</sup>, Vincent SOBOTKA<sup>1,e</sup>, Steven LE CORRE<sup>1,f</sup>

<sup>1</sup>Nantes Université, CNRS, Laboratoire de thermique et énergie de Nantes, LTeN, UMR 6607, F-44000 Nantes, France

<sup>2</sup>Arkema, Cerdato, Serquigny, France

<sup>a</sup>adrien.le-reun@univ-nantes.fr, <sup>b</sup>arthur.levy@univ-nantes.fr, <sup>c</sup>nicolas.lefevre@univ-nantes.fr,

<sup>d</sup>henri-alexandre.cayzac@arkema.com, <sup>e</sup>vincent.sobotka@univ-nantes.fr,

<sup>f</sup>steven.lecorre@univ-nantes.fr

**Keywords:** Thermoplastic Composites, Consolidation, Hot Press, Microtomography

**Abstract.** The consolidation of carbon fibre-reinforced thermoplastics is a challenge for the manufacturing of high performance composites, especially for the aeronautics field. Although the effects of pressure and temperature on the quality of consolidation have been widely demonstrated, the understanding of the phenomena occurring during consolidation remains partial. Based on previous developments, this work presents a hot press device specifically designed and fabricated for the in-situ observation of consolidation by X-Ray microtomography. A good correlation is found between the evolution of void content and the measure of the sample through-thickness strain obtained by LVDT sensors.

**Introduction**

Thanks to their physical properties such as the matrix transformation reversibility, the interest for thermoplastic composites increased for years. However, the understanding of the multi-step processing of such materials is not yet complete. Among all steps, the consolidation phase of thermoplastic composites consists of several phenomena [1] such as the establishment of intimate contact and healing of interfaces, closing of porosities, crystallization or residual stresses. The literature [2-5] has widely demonstrated the effect of consolidation conditions and processes on composites properties such as InterLaminar Shear Stresses (ILSS) or final void content. As well as for adhesion issues (interlaminar bonding) [6,7], pressure, temperature and time are the parameters governing the consolidation process.

The numerical and experimental approaches used in the understanding of consolidation in the manufacturing of thermoset composites [8,9] have been recently extended to thermoplastic composites. Indeed, experimental studies have been carried out to understand the governing mechanisms [10-12] of void evolution during thermo-mechanical cycles. Several studies [13,14] used original benches to measure in-situ macroscopic laminate properties such as thickness variation under various Vacuum Bag-Only (VBO) conditions. This approach led to noticeable results such as the role of the prepreg roughness [15] on the through-thickness strain during a same thermo-mechanical cycle.

Nevertheless, this characterization is limited to a macroscopic measure of the laminate and does not offer any local or statistical information on void morphologies and distributions at several times of the cycle. Arquier et al. [16] performed a study on different samples which underwent increasing fractions of a similar reference thermo-mechanical cycle (heating ramp followed by a dwell) before cooling. For example, the first sample underwent a 5 minutes dwell at 380°C under 1 N whereas last sample was consolidated 2 additional hours under 45 N. A post-process

<figure><img src="image_2.png" /></figure>
characterization by optical microscopy followed to observe the microstructure. Despite remarkable results on the evolution of void content and inter-ply microstructure, this study relies on discrete measure performed on several samples and does not allow a continuous tracking of a same microstructure through one thermo-mechanical cycle.

In order to overcome this limitation, an original bench [17], named *in CODETO* (*in-situ* Composites DEconsolidation TOmography), has been developed for the observation of void evolution in the context of deconsolidation cycles by X-Ray Computed Tomography (XCT). In this work, a new version of the system is presented for the study of laminates consolidation from prepreg tapes.

## Experimental bench and material

The experimental bench consists of an instrumented hot press, updated from the aforesaid previous version. The *in-situ* Composites COnsolidation TOmography (*in*CO2TO) device aims at observing the consolidation of hand lay-up plies in hot press representative conditions by SCT measurement. The apparatus, described in Fig. 1, consists of heating and pressuring a 20 mm diameter circular sample composed of 12 plies oriented according to the desired laying architecture such as UniDirectional (UD), Cross-Ply (CP) or Quasi-Iso (QI). The prepregs are encapsulated in a 0.5 mm thickness aluminium capsule (N°14 in Fig. 1). A tight fit between the prepregs and the capsule is achieved to limit the matrix squeezing out.

<figure><img src="image_3.png" /><figcaption>Figure 1 – *in*CO2TO, an instrumented hot press device.</figcaption></figure>

An aluminium plate with the same thickness as the support base is placed on top of the sample. Two control thermocouples coupled to Eurotherm 3508 temperature controllers allow the sample to be heated using two copper platens into which 200 W heating cartridges are placed. A thermal insulator on both parts of the platens is used to avoid heat transfer through the system. In addition to the weight of the upper part of the device, a pneumatic actuator controlled by a proportional-pressure regulator Festo VPPM applies a compaction force on the sample up to a pressure of 10 bar.
Apart from the thermal measurement, the system is instrumented with a force sensor Kistler 4576A placed between the lower thermal insulator and the lower plate, which measures the compaction force. Two Linear Variable Differential Transformer (LVDT) RDP (D6/02500 and D5/3000) are used to obtain the variation of the sample thickness. The lower LVDT fixed on the frame is in contact with a brass disk whereas the upper LVDT relies on a quartz rod inserted in the thermal insulator. Two fans fixed to the tube frames are used to cool the sensors in addition to their optimized configuration chosen to limit their heating. Moreover, a fan is turned on when requiring a fast cooling of the sample.

Because consolidation is a dynamic process, the XCT observation is required to be fast. The system has been designed to perform continuous XCT measurement on the beamline ID19 at European Synchrotron Radiation Facility (ESRF) in Grenoble (France). The setup fits on the rotating stage that enables scans in one second. To ensure a smooth rotation of the system, every cable (control, measure, power supply) is directed to a slip ring.

The first step is dedicated to a thorough calibration of the bench which consists of a thermo-elastic model of the system with equivalent springs which length is sensitive to pressure and temperature. This phase enables to retrieve the temperature T and pressure P effect on the LVDT displacement measure in order to correct it and obtain the sample thickness (Eq. 1) at each time of the cycle such that:

$$h_s(T, P) = L_u(T, P) + L_l(T, P) - \Delta L_h(T, P) - (h_{sh} + h_d)(1 + \alpha_{alu}(T - T_{amb})) \quad (1)$$

where $L_u$ and $L_l$ are respectively the upper and lower LVDT value, $\Delta L_h$ the height of copper platens, $h_{sh}$ and $h_d$ the respective thickness of the aluminum support and disk with a coefficient of thermal expansion $\alpha_{alu}$ and $T_{amb}$ the ambient temperature. The sample deformation $\epsilon$, related to the through-thickness strain, is then determined using the initial sample thickness $h_{s,0}$ (Eq. 2):

$$\epsilon = \frac{h_s - h_{s,0}}{h_{s,0}}, \quad (2)$$

As the outcome of the calibration phase, an uncertainty of 0.03 mm and 0.1 bar is estimated in the sample thickness measurement and compaction pressure measurement, respectively.

In this paper, a study on high performance CF/PAEK composites consolidation is presented in two steps. First, in-lab tests are conducted under various conditions on different samples. Based on the obtained results, a series of experiments is designed and performed at ESRF for in-situ XCT measurement of void evolution.

## In-lab tests

A large number of in-lab tests has been performed to evaluate the response of the sample to the thermo-mechanical cycle conditions. The studied parameters are: heating and cooling ramp, dwell temperature and time, pressure, layup and pre-conditioning of the sample. The nominal thermal cycle is plotted in Fig. 2. It corresponds to an initial dwell at 30°C followed by three heating ramps with decreasing heating rate (20, 15 and 5 °C/min) until 200, 300 and 370°C (dwell temperature), optimized after several tests to ensure a proper temperature control and an appropriate heating time for the speeds of deformation and a limited test length. After a nominal dwell time of 30 min, the cooling rate is set at 5°C/min until 150°C (below the glass transition temperature) by free convection and then maximized by the effect of the fan without counter-heating action. The nominal compaction pressure is 0.3 bar which corresponds to the weight of the upper part of the structure. Finally, the nominal layup is a symmetrical CP configuration [0,90]$_{2s}$.
<figure><img src="image_4.png" /><figcaption>Figure 2 – Nominal thermal cycle with setpoint, upper and lower platen temperatures.</figcaption></figure>

<figure><img src="image_5.png" /><figcaption>Figure 3 – Evolution of sample thickness as a function of temperature at low and high pressure
(a) on the whole cycle, (b) after 330°C until cooling start.</figcaption></figure>

The evolution of sample thickness is plotted in Fig. 3(a) with respect to its temperature during a similar thermo-mechanical cycle at low (0.3 bar) and high (5.0 bar) compaction pressures. First point of the curve corresponds to the beginning of the heating ramp once pressure has been applied. As a result, the initial sample thickness is noticeably lower for the high pressure test showing the effect of a cold compaction (debulking). The pressure effect leads to a difference of 0.42 mm between the two samples, thickness is equivalent of two ply.

First, a linear decrease of the thickness is observed which is attributed to a slow compaction counteracting thermal expansion. This phenomenon is mainly explained by small slips at plies interfaces and with the capsule edges. Then, a steep drop happens at a temperature of 170°C corresponding to the matrix glass transition which enables a mobility of the polymer chains. This change is followed by a stabilization of the sample thickness where compaction and thermal expansion balance. The samples behavior does not evolve until the temperature gets closer to the melting point (335°C). Indeed, the sample at low pressure exhibits a quick and noticeable increase of thickness at 340°C. This deconsolidation peak is not observed for the high pressure sample where the compaction force is sufficient to prevent this phenomenon [13, 17]. As a result, the consolidation begins earlier in the case of higher pressure as highlighted in Fig. 3(b). This phase after melting is characterized by a quick compaction in the very first moment which slows down until an asymptote is reached during dwell. Finally, a linear smooth decrease of sample thickness occurs at the beginning of cooling when compaction and thermal shrinkage add up. For both samples, an inflection point is observed at 290°C which is identified as the crystallization temperature for a cooling rate of 5°C/min by differential scanning calorimetry. At this point, the
sample thickness decreases faster by the additional effect of transformation shrinkage induced by a partial crystallization of the matrix. Following this phenomenon, the sample thickness decreases at a lower rate until reaching its final value. However, a very slow increase after the glass transition is observed for the 5 bar sample.

The representative macroscopic behavior of a sample processed by the inCO2TO hot press is summarized in Fig. 4 with a sequence of the phenomena whose magnitude depends on the time-temperature-pressure conditions:

* I. Cold compaction before heating under pressure.

* II. Slow compaction / Interfaces slip.

* III. Glass transition: the polymer chains mobility is freed.

* IV. Deformation plateau before melting.

* V. Deconsolidation peak in case of low compaction pressure.

* VI. Consolidation after melting.

* VII. Thermal contraction.

* VIII. Crystallization shrinkage.

Although the evolution of the through-thickness strain gives substantial information about the macroscopic behaviour, a lack in the understanding of void content evolution during the cycle remains. The following section addresses the question of voids by in-situ SCT measurement.

## In-situ SCT measurement

The ESRF configuration is adapted to in-situ SCT measurement with parallel incident X-ray beams enabling, with the use of fine resolution detector, the study of microscopic and mesoscopic voids. The acquisition configuration consists of a complete rotation of the device at constant speed during which 2000 radiographies are obtained for a scan duration of 1 second. The images are then used by NOVITOM (Grenoble) to compute a 3D reconstruction of the volume. The gap between air and carbon/polymer X-ray absorption coefficients leads to a sufficient contrast to detect porosities. An amount of approximatively 30 reconstructions per sample is selected to analyse a 7.74 x 7.74 x 4.99 mm³ volume centred on the core of the sample, at different temperatures of the thermo-mechanical cycle. The volumes are characterized by a voxel size of 3.84 µm. An example of a cross-section reconstruction before consolidation phase is presented in Fig. 5 with the underline of interlaminar and intralaminar voids.

<figure><img src="image_6.png" /><figcaption>Figure 4 – Deformation-temperature curve with a sequence of distinct phenomena for a sample processed at 1 bar.</figcaption></figure>
<figure><img src="image_7.png" /><figcaption>Figure 5 – Cross-section XCT reconstruction of a 12 plies samples before consolidation phase.</figcaption></figure>

The void analysis method needs to be rigorously defined to precisely characterize the void evolution during the consolidation. To do so, a three steps processing methodology is used. First, pre-processing operations are applied to the raw reconstructed volume to crop the region of sample and enhance contrast between porosities and composites. Then, the extraction of porosities is performed by segmentation. Among several available methods, two strategies have been explored: thresholding segmentation and pixel classification by deep learning solutions. Only the thresholding segmentation results are presented in this paper. Finally, post-processing operations are used to extract global, local and statistical void features from the binary volumes.

The void content of the previous sample estimated after a manual thresholding segmentation is presented in Fig. 6. A high void content of 12% is initially measured and reduced to 6% by a cold compaction. After a smooth void reduction until the glass transition, a stabilization is observed as well as the deformation. Whereas the deformation exhibits no deconsolidation peak, a slight increase of porosities is observed from 5.4 to 5.6%. Then, the consolidation occurs with a steep decrease of void content which reaches 2.7% at the end of the dwell. During the beginning of cooling phase, the void content keeps on decreasing and finally slightly varies around 2.3%.

<figure><img src="image_8.png" /><figcaption>Figure 6 – Evolution of void content for a sample processed at 1 bar after manual thresholding segmentation.</figcaption></figure>

## Conclusion

In-situ measurement of composite features during a thermo-mechanical cycle is a key stage to understand precisely the mechanisms and phenomena of consolidation. To this end, an original hot
press device is designed for the in-situ measurement of through-thickness strain and void evolution with the use of embedded LVDT sensors and X-Ray tomography tests. This on-going work highlighted a standard macroscopic consolidation scenario correlated with the void content evolution. The perspectives include a comparative study of the aforesaid segmentation methods with the effect on void features. Moreover, a post-processing analysis of the voids distribution, morphology and position will follow.

## Acknowledgments

This work is funded by Arkema Inc with the financial support of Agence National de la Recherche Technologique.

## References

[1] Arquier, R., Iliopoulos, I., Régnier, G., Miquelard-Garnier, G., 2022. Consolidation of continuous-carbon-fiber-reinforced PAEK composites: a review. Materials Today Communications 32, 104036. https://doi.org/10.1016/j.mtcomm.2022.104036

[2] Hoang V.-T., Kwon, B.-S., Sung, J.-W., Choe, H.-S., Oh, S.-W., Lee, S.-M., Kweon, J.-H., Nam, Y.-W., 2020. Postprocessing method-induced mechanical properties of carbon fiber-reinforced thermoplastic composites. Journal of Thermoplastic Composite Materials 36, 089270572094537. https://doi.org/10.1177/0892705720945376

[3] Saenz-Castillo, D., Martín, M.I., Calvo, S., Rodriguez-Lence, F., Güemes, A., 2019. Effect of processing parameters and void content on mechanical properties and NDI of thermoplastic composites. Composites Part A: Applied Science and Manufacturing 121, 308-320. https://doi.org/10.1016/j.compositesa.2019.03.035

[4] Rosselli, F., Santare, M.H., Güçeri, S.I., 1997. Effects of processing on laser assisted thermoplastic tape consolidation. Composites Part A: Applied Science and Manufacturing 28, 1023-1033. https://doi.org/10.1016/S1359-835X(97)00072-9

[5] Pourahmadi, E., Shadmehri, F., Ganesan, R., 2024. Interlaminar shear strength of Carbon/PEEK thermoplastic composite laminate: Effects of in-situ consolidation by automated fiber placement and autoclave re-consolidation. Composites Part B: Engineering 269, 111104. https://doi.org/10.1016/j.compositesb.2023.111104

[6] Butler, C.A., McCullough, R.L., Pitchumani, R., Gillespie, J.W., 1998. An Analysis of Mechanisms Governing Fusion Bonding of Thermoplastic Composites. Journal of Thermoplastic Composite Materials 11, 338-363. https://doi.org/10.1177/089270579801100404

[7] Avenet, J., Levy, A., Bailleul, J.-L., Le Corre, S., Delmas, J., 2020. Adhesion of high performance thermoplastic composites: Development of a bench and procedure for kinetics identification. Composites Part A: Applied Science and Manufacturing 138, 106054. https://doi.org/10.1016/j.compositesa.2020.106054

[8] Anderson, J. p., Altan, M. c., 2015. Formation of voids in composite laminates: Coupled effect of moisture content and processing pressure. Polymer Composites 36, 376-384. https://doi.org/10.1002/pc.22952

[9] De Parscau du Plessix, B., Le Corre, S., Jacquemin, F., Lefebure, P., Sobotka, V., 2016. Improved simplified approach for the prediction of porosity growth during the curing of composites parts. Composites Part A: Applied Science and Manufacturing 90, 549-558. https://doi.org/10.1016/j.compositesa.2016.08.024
[10] Zhang, D., Heider, D., Gillespie, J.W., 2018. Characterization of interlayer air permeability of thermoplastic prepreg stacks. Journal of Composite Materials 52, 731-745.
https://doi.org/10.1177/0021998317715089

[11] Swamy, J.N., Grouve, W.J., Wijskamp, S., Akkerman, R., 2023. An experimental study on the role of different void removal mechanisms in VBO processing of advanced thermoplastic composites. Journal of Reinforced Plastics and Composites 07316844231159134.
https://doi.org/10.1177/07316844231159134

[12] Swamy, J.N., Grouve, W.J., Wijskamp, S., Akkerman, R., 2023. Vacuum-bag-only consolidation of C/PEKK fiber placed preforms with engineered gas evacuation channels. Journal of Thermoplastic Composite Materials https://doi.org/10.1177/08927057231177571

[13] Amedewovo, L., Levy, A., De Parscau Du Plessix, B., Aubril, J., Arrive, A., Orgéas, L., Le Corre, S., 2023. A methodology for online characterization of the deconsolidation of fiber-reinforced thermoplastic composite laminates. Composites Part A: Applied Science and Manufacturing 167, 107412. https://doi.org/10.1016/j.compositesa.2022.107412

[14] Saffar, F., Sonnenfeld, C., Beauchêne, P., Park, C.H., 2020. In-situ Monitoring of the Out-Of-Autoclave Consolidation of Carbon/Poly-Ether-Ketone-Ketone Prepreg Laminate. Frontiers in Materials 7. https://doi.org/10.3389/fmats.2020.00195

[15] Saffar, F., Beauchêne, P., Sonnenfeld, C., Park, C.H., 2022. Influence of prepreg parameters on the interlaminar consolidation of fiber reinforced thermoplastic laminates manufactured by vacuum-bag-only process. Frontiers in Materials 9. https://doi.org/10.3389/fmats.2022.1035427

[16] Arquier, R., Sabatier, H., Iliopoulos, I., Régnier, G., Miquelard-Garnier, G., 2024. Role of the inter-ply microstructure in the consolidation quality of high-performance thermoplastic composites. Polymer Composites 45, 1218-1227. https://doi.org/10.1002/pc.27847

[17] Amedewovo, L., Orgéas, L., de Parscau du Plessix, B., Lefevre, N., Levy, A., Le Corre, S., 2023. Deconsolidation of carbon fiber-reinforced PEKK laminates: 3D real-time in situ observation with synchrotron X-ray microtomography. Composites Part A: Applied Science and Manufacturing 107917. https://doi.org/10.1016/j.compositesa.2023.107917