Classical mold filling simulations for Resin Transfer Molding are typically based on incompressible single-phase Darcy flow combined with volume-of-fluid techniques to track the advancing flow front. While these approaches accurately predict macroscopic flow front propagation, they fail to capture persistent air entrapment, as flow-induced voids artificially vanish over time. To overcome this limitation, a pressure-consistent compressible fluid mixture model is proposed. The model consists of volume-averaged continuity equations for the air and liquid phases and a momentum equation including Darcy viscous losses, coupled through a closed-form equation of state. Air is modeled as an ideal gas and the liquid as a weakly compressible fluid, enabling the build-up of counter-pressure in entrapped regions without introducing additional empirical parameters compared to classical Darcy flow models. The model is implemented in the open-source simulation tool LCMsim and validated against a radial flow experiment with deliberately induced air entrapment. In contrast to conventional Darcy-based approaches, the proposed model predicts persistent air pockets and captures their compression during flow front convergence. Additional test cases demonstrate its capability to identify critical regions and assess the influence of venting strategies. The results show improved physical realism while maintaining computational efficiency suitable for industrial applications.

Environmental concerns and regulations have augmented the interest in bio-based composites. However, their industrial uptake faces multiple challenges compared to fossil counterparts, including limited availability, manufacturing difficulties, and heterogenous material properties, which influence also environmental impacts. Previous Life Cycle Assessment has demonstrated that partially bio-based systems exhibit superior environmental performance than fully bio-based composites, due to high energy demand from longer curing cycles and inferior composite properties in the latter. This paper addresses this gap in environmental and material performance by analyzing and comparing four different resin systems based on epoxidized linseed oil and itaconic anhydride, incorporating various modifications and catalysts, with a partially bio-based Bisphenol A-based epoxy system. Vacuum-assisted resin infusion process was used to fabricate flax fiber-reinforced composites. The incorporation of a zinc-based catalyst enhanced the crosslinking reaction, reducing curing time from 20 to 3 h compared to the non-modified system, resulting in an estimated 77 % decrease in energy consumption, derived via linear extrapolation of steady-state holding data as a laboratory-scale limitation. This novel fully bio-based composite can achieve manufacturing parity with partially bio-based composite. Mechanical testing revealed that the fully bio-based composite achieved a tensile strength of 114 MPa, thus exceeding the tensile strength of the partially bio-based benchmark composite by 3.6 %. On the other hand, the flexural strength and interlaminar shear strength were found to be 82.3 % and 58.5 % of the benchmark materials, respectively. These results show that material and process optimization can increase manufacturing efficiency and competitiveness, and highlights future developments for the viability of functional fully bio-based composites.

Traditional composites often rely on synthetic fibers, raising environmental concerns due to their non-biodegradability and resource-intensive production. Consequently, sustainable composites have emerged as a potential substitute for these composites owing to their renewability, eco-friendliness, and biodegradability. Plant fibers are the key choice as reinforcement material for fabricating these composites. Researchers have explored the use of plant fibers to reinforce polymers, as a potential replacement for synthetic fibers, owing to the minimal environmental impact and contribution to carbon neutrality, lower greenhouse gas emissions, reduced energy consumption, and less dependence on fossil fuels. Flax, hemp, jute, ramie, etc., are some common examples of plant fibers with the potential to be used in various applications. However, several critical challenges persist in the processing of plant fiber composites, hindering widespread industrial adoption. The paper at hand provides a comprehensive overview and critically discusses challenges inherent to processing plant fiber-based polymer composites. It begins with an overview of common composite fabrication techniques, provides insight into the plant fiber structure, alongwith their mechanical properties and architectural configurations as reinforcements. A major challenge with these composites is the inherent hydrophilicity of plant fibers, leading to moisture absorption and swelling. The review explores absorption kinetics, including Fickian and non-Fickian models such as the Dual-Stage Fick's Law and the Carter-Kibler two-phase (Langmuir) model, and also discusses the corresponding swelling kinetics. Another critical focus of this review is on the compaction and impregnation behavior of plant fiber preforms, highlighting issues such as compressibility, non-uniform resin flow, and mold filling inconsistencies. Thermal stability during processing is also discussed, particularly focusing on the thermal degradation thresholds of plant fibers. Each section concludes with a focused discussion on the underlying mechanisms, current mitigation strategies, and knowledge gaps. By consolidating insights across these domains, this review provides a foundational understanding of the interplay between plant fiber characteristics and processing phenomena.

Automated tape placement is capable of manufacturing high-performance composites. The degree of bonding and lay-up accuracy are critical quality criteria for achieving defect-free high-quality parts. Gaps and overlaps reduce the mechanical performance, which indicates the need for a precise definition of the processing parameter influences on the consolidated tape width. The influences of a flashlamp automated tape placement system on the tape width after consolidation for carbon fiber reinforced thermoplastic tapes are investigated. The process is analysed using a Taguchi design of experiments approach. Emphasis is placed on the variation within the tape width and the total tape width change. Special consideration is placed on the flashlamp heating parameters; voltage, frequency and pulse width. Temperature, consolidation pressure and width measurements are used to investigate the influences of those parameters. The results show that the consolidation force and heating system power/temperature followed by the consolidation roller hardness and lay-up speed have the highest influence on the tape width after consolidation and its variability. The voltage of the heating system has no significant influence on the tape width change. The frequency of the system has a higher influence compared to the pulse width on the tape width change. A high tool temperature leads to a reduction in tape width after consolidation. This research provides a comprehensive understanding of the factors influencing the tape width in flashlamp automated tape placement, offering valuable insights for optimizing manufacturing parameters to produce high-performance composites.

Bio-based composites offer potential environmental benefits over fossil-based materials, but limited research exists on manufacturing processes with varying material combinations. This study performs a cradle-to-grave Life Cycle Assessment of five composite types to evaluate the role of fully and partially bio-based composites, focusing on the manufacturing stage. The composite materials include glass or flax fiber-based reinforcements embedded in polymer matrices based on a fossil epoxy, a partially bio-based epoxy, or epoxidized linseed oil, fabricated using vacuum-assisted resin infusion. Flax fibers in a partially bio-based epoxy achieve the lowest environmental impacts in most categories when assessed at equal geometry. Glass fiber composites exhibit a higher fiber volume content and material properties and thus demonstrate competitive environmental performance at equal absolute and normalized tensile strength. Composites using epoxidized linseed oil are the least advantageous, with the manufacturing stage contributing a majority of the environmental impacts due to their comparatively long curing times. These results are based on methodological choices and technical constraints which are discussed together with benchmarking against previous studies. While partially bio-based materials can provide a middle ground for enhancing composite environmental performance, the further optimization of bio-based material functionality regarding material properties and processability is pivotal to exploit the full potential of bio-based composites.

The utilization of bio-based materials for the manufacturing of fiber-reinforced polymer composites is gaining importance under the sustainability paradigm. The identification of suitable process parameters and limited process reproducibility remain among the major challenges to enhance the industrial application potential of bio-based composites. This is especially relevant, as the manufacturing process influences composite quality, economic performance and environmental impacts. This study compares Resin Transfer Molding and Vacuum Assisted Resin Infusion for two sets of process parameters in order to manufacture a composite plate consisting of a flax-fiber textile impregnated with a partially bio-based epoxy matrix. Process quality is described through statistical analysis of processing and composite properties, and performance in terms of process replicability and reliability using performance estimates. Processing parameters were selected to depict a range of manufacturing scenarios that were suitable for the selected bio-based material system from curing for 180 min at 60 °C to curing for 30 min at 100 °C. For an identical set of process conditions, Resin Transfer Molding outperforms Vacuum Assisted Resin Infusion in terms of tensile and flexural characteristics. Conversely, the latter shows the strongest fiber-matrix adhesion and the most homogeneous impregnation. Whereas manufacturing at lower temperature leads to positive effects on composite quality, higher processing temperature with shorter curing cycles achieve highest process performance in terms of Pp and Ppk indices. An additional annealing at 120 °C neither increases composite quality nor reduces manufacturing-induced variability. Results depend on processing differences and indicators to determine process performance, as well as methodological choices.

Laser-assisted automated tape placement systems are currently the state of the art regarding thermoplastic tape placement. Flashlamp heating systems are rather new in this field of application and offer high energy density with low safety requirements and moderate costs compared to laser-assisted automated tape placement systems. In this study, the effect of processing parameters on interlaminar bonding of carbon fiber-reinforced polyamide 6 tapes is investigated using a flashlamp heating system. The temperature during placement is monitored using an infrared camera, and the bonding strength is characterized by a wedge peel test. The bonding quality of the tapes placed between 210 °C and 330 °C at a lay-up speed of 50 mm/s is investigated. Thermogravimetric analysis, differential scanning calorimetry, and micrographs are used to investigate the material properties and effects of the processing conditions on the thermophysical properties and geometric properties of the tape. No significant changes in the thermophysical or geometric properties were found. Moisture within the tapes and staining of the quartz guides of the flashlamp system have significant influence on the bonding strength. The highest wedge peel strength of dried tapes was found at around 330 °C.

Increasing global concerns regarding environmental issues have driven significant advancements in the development of bio-based fiber reinforced polymer composites. Despite extensive research on bio-composites, there remains a noticeable gap in studies specifically addressing the challenges of repairing bio-composites for circular economy adoption. Traditional repair techniques for impacted composites, such as patching or scarf methods, are not only time-consuming but also require highly skilled personnel. This paper aims to highlight cost-effective repair strategies for the restoration of damaged composites, featuring flax fiber as the primary reinforcement material and distinct matrix systems, namely bio-based epoxy and bio-based vitrimer matrix. Glass fiber was used as a secondary material to validate the bio-based vitrimer matrix. The damage caused specifically by low impact is detrimental to the structural integrity of the composites. Therefore, the impact resistance of the two composite materials is evaluated using instrumented drop tower tests at various energy levels, while thermography observations are employed to assess damage evolution. Two distinct repair approaches were studied: the resin infiltration repair method, employing bio-based epoxy, and the reconsolidation (self-healing) repair method, utilizing the bio-based vitrimer matrix. The efficiency of these repair methods was assessed through active thermography and compression after impact tests. The repair outcomes demonstrate successful restoration and the maintenance of ultimate strength at an efficiency of 90% for the re-infiltration repair method and 92% for the reconsolidation repair method.

Near infrared (NIR) spectroscopy is employed to monitor the degree of cure of a composite material directly during the curing phase in the resin-transfer-molding (RTM) process. The composite material used consists of natural fibers and an epoxy/amine resin. The quantitative determination of the degree of cure, α, from the NIR spectra is realized by partial least square (PLS) regression in conjunction with various pre-treatments of the spectral data. As a reference, the degree of cure is also determined by isothermal DSC experiments. The best PLS model that could be obtained for α is characterized by a determination coefficient of prediction (R²P) of 0.980 and root-mean-square error of prediction (RMSEP) of 4.4 %. While for every RTM trial the spectra during curing change corresponding to literature, major differences can be observed between the spectra of different RTM trials. The reasons for this are discussed in detail. The findings show the potential of inline NIR spectroscopy for monitoring α in the RTM process.

The integration of natural fibre thermoplastic composites, particularly those combining flax fibres with polypropylene, offers a promising alternative to traditional synthetic composites, emphasising sustainability in composite materials. This study investigates the mechanical properties of flax/polypropylene composites manufactured using flashlamp automated tape placement and press consolidation, individually and in combination. Tensile, compression, three-point bending, and double cantilever beam tests are utilised for comparing these manufacturing processes and the mechanical performance of the resulting composites. The microstructure of the tapes is investigated using cross-sectional microscopy, and the thermophysical behaviour is analysed utilising thermogravimetric analysis and differential scanning calorimetry. The temperature during placement is monitored using an infrared camera, and the pressure is mapped with pressure-sensitive films. The natural fibre tapes show a good aptitude for being manufactured with automated tape placement. The tensile performance of tapes manufactured with automated tape placement is close to that of press consolidated samples. Compression, flexural properties, and the mode I fracture toughness critical energy release rate all benefit from a second consolidation step.

RTMsim is a robust, easy-to-use and simple-to-extend software tool, implemented in Julia (Bezanson et al., 2017) for resin transfer moulding (RTM) filling simulations. A shell mesh, injection pressure, resin viscosity and the parameters describing the preform are required input. The software was validated with results from literature (Advani et al., 1994) and compared with results from well-established RTM filling simulation tools (Gion Barandun et al., 2012),(ANSYS, 2021).

This paper presents a novel testing method for evaluating the compaction behaviour of textile reinforcements in the context of liquid composite moulding processes. The existing testing approach utilizing pre-saturated samples (ex-ante) fails to accurately represent the unsaturated state of samples during vacuum infusion or resin transfer moulding (RTM) processes, leading to unreliable results and potential discrepancies with simulation. To address this limitation, a newly designed test-rig is introduced in this study, enabling compressibility testing based on real process specifications. The proposed method allows for the measurement of both dry and wet compression characteristics using a single specimen through in-situ impregnation of the materials under compressive load. Moreover, the test-rig enables tests according to ex-ante specifications, facilitating direct comparison with the proposed in-situ method. Finally, the test-rig allows for compressibility tests at elevated temperatures up to 200 C-degrees. This is of particular relevance for studying the compaction behaviour of bindered technical fabrics. Preliminary comparative tests demonstrate excellent agreement between the results obtained using the ex-ante method under the 2020 international benchmark exercise and the novel in-situ impregnation method. This confirms the validity and reliability of the results obtained through the proposed testing method. By providing a more realistic representation of the compaction behaviour of textile reinforcements, the novel approach presented in this study offers valuable insights for optimizing liquid composite moulding processes and improving the accuracy of simulation models.

Resin Transfer Molding (RTM) is a manufacturing process for fiber reinforced polymer composites where dry fibers are placed inside a mold and resin is injected under pressure. During mold design, filling simulations can study different manufacturing concepts (i.e. placement of injection gates and vents) to guarantee complete filling of the part and avoid air entrapment where flow fronts converge. In this work, a novel software tool LCMsim, which was implemented by the authors, is benchmarked against other tools and real-world flow experiments. Its development was driven by two ideas: Easy-of-use for the mold engineer and maximum flexibility for the researcher. Two experiments were used for validation. In the first, zones with different preform properties were present and in the second, race-tracking was enforced. Flow fronts from LCMsim and experiment agree with 7% error and simulated flow fronts from LCMsim and the commercially available software PAM-RTM agree with 3% error. [GRAPHICAL ABSTRACT]

Fiber reinforced polymers (FRP) provide favorable properties such as weight-specific strength and stiffness that are central for certain industries, such as aerospace or automotive manufacturing. Liquid composite molding (LCM) is a family of often employed, inexpensive, out-of-autoclave manufacturing techniques. Among them, resin transfer molding (RTM), offers a high degree of automation. Herein, textile preforms are saturated by a fluid polymer matrix in a closed mold.Both impregnation quality and level of fiber volume content are of crucial importance for the final part quality. We propose to simultaneously learn three major textile properties (fiber volume content and permeability in X and Y direction) presented as a three-dimensional map based on a sequence of camera images acquired in flow experiments and compare CNNs, ConvLSTMs, and Transformers. Moreover, we show how simulation-to-real transfer learning can improve a digital twin in FRP manufacturing, compared to simulation-only models and models based on sparse real data. The overall best metrics are: IOU 0.5031 and Accuracy 95.929 %, obtained by pretrained transformer models.

Material as well as process variations in the composites industry are reasons to develop methods for in-line monitoring, which would increase reproducibility of the manufacturing process and the final composite products. Fiber Bragg Gratings (FBGs) have shown to be useful for monitoring liquid-composite molding processes, e.g., in terms of online gel point detection. Existing works however, focus on in-plane strain measurements while out-of-plane residual strain prevails. In order to measure out-of-plane strain, FBG inscribed in highly birefringent fiber (HB FBG) can be used. The purpose of this research is the cure stage detection with (a) FBG inscribed in single mode and (b) FBG inscribed in highly-birefringent side-hole fiber in comparison to the reference gel point detected with an in-mold DC sensor. Results reveal that the curing process is better traceable with HB FBG than with regular FBG. Thus, the use of HB FBG can be a good method for the gel point estimation in the RTM process.

The major advantage of products made from composite materials is given by their superior weight-specific mechanical properties. These can be weakened by defects induced in the manufacturing process. Therefore, online detection and analysis of the processed fiber bundle geometry is a key factor for the quality assurance of the final part. In this article, the instrumentation and data evaluation for determining the surface geometry of fiber bundles by means of light sectioning was examined. Bundles of glass and carbon fibers were measured continuously on an inspection test rig. Different background materials have been used in order to validate the applicability of the approach. By utilization of a polynomial fitting algorithm, data segmentation of object and baseline was robustly achieved. By means of cross correlation, the data alignment could be evaluated faster and more reliable compared to a method previously presented by us. The information could then be used for the determination of the fiber bundle width, centerline, spatial changes, and oscillations. In addition, unwanted defects as well as lateral movement of the fiber bundles were reliably detected. The information revealed by the proposed algorithm provides the basis for robust online monitoring of fiber bundle geometry in highly automated composite manufacturing processes.

This research examines the compression, as well as short- and long-term relaxation behaviour of bindered textiles at elevated temperature levels. Experiments were conducted on a carbon fibre non-crimp fabric with epoxy resin binder in a specifically designed compressibility test rig. Expanding past research activities at room temperature [1, 2] it was found in series of loading-relaxation-unloading tests, that the test temperature level significantly influences the maximum compaction pressure during the loading stage as well as the pressure characteristics during the relaxation stage [3]. Furthermore, a significant change in the compression behaviour, well below the specified processing temperature of the binder, was found. Also, a proof-of-concept demonstrates the “in-situ”-injection capability of a novel test-rig, reproducing RTM-like conditions in a controlled laboratory environment. The findings of this work are intended to support optimizing preforming and preform handling steps for liquid composite moulding processes.

Carbon fiber reinforced polymers (CFRP) offer highly desirable properties such as weight-specific strength and stiffness. Liquid composite moulding (LCM) processes are prominent, economically efficient, out-of-autoclave manufacturing techniques and, in particular, resin transfer moulding (RTM), allows for a high level of automation. There, fibrous preforms are impregnated by a viscous polymer matrix in a closed mould. Impregnation quality is of crucial importance for the final part quality and is dominated by preform permeability. We propose to learn a map of permeability deviations based on a sequence of camera images acquired in flow experiments. Several ML models are investigated for this task, among which ConvLSTM networks achieve an accuracy of up to 96.56%, showing better performance than the Transformer or pure CNNs. Finally, we demonstrate that models, trained purely on simulated data, achieve qualitatively good results on real data.

The major advantage of products made from composite materials, such as carbon fiber reinforced polymers (CFRP), is given by their superior weight-specific, mechanical properties such as strength and stiffness. These properties can be weakened by defects induced in the manufacturing process. In dry fiber filament winding, online detection and analysis of the processed fiber bundle geometry is a key factor for the quality assurance of the final part. In this work, the instrumentation and data evaluation for determining the surface geometry of fiber bundles by means of a light sectioning sensor was examined. Profiles of glass fibers were measured continuously in a specifically developed inspection test rig. By application of an interactive polynomial fitting algorithm, data segmentation of object and base line was robustly achieved on varying background conditions. In addition, unwanted defects as well as lateral movement of the fiber bundles were reliably detected. The information revealed by the proposed algorithm provides the basis for robust online monitoring of fiber bundle geometry in highly automated composite manufacturing processes.