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.

Monitoring of thermoplastic carbon reinforced composites is critical for the safe operation and optimization of composite structures. Strain measurement during their manufacturing and operation is often achieved with embedded Fiber Bragg Gratings (FBG). However, conventional FBGs can only measure axial strain. To measure multiaxial strain in thermoplastic composites, FBGs inscribed in highly-birefringent side-hole elliptical core optical fiber (SH2 FBG) are proposed. SH2 FBG were used to monitor composite produced by automated tape laying process, first − to measure residual in-plane and out-of-plane strain during the manufacturing stage, second − in plane transverse strain during cyclic loading. It was demonstrated that the sensors can be used to measure multiaxial residual strain, observe changes in material stiffness, and calculate the area of the mechanical hysteresis loop. As a result − it was shown that embedding SH2 FBG can be an effective and robust method for monitoring the carbon fiber reinforced thermoplastic composite.

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Automated tape laying is a well-established composite manufacturing process for high performance industries. While the process has seen vast growth over the 30 years of its conception, in-process monitoring is the most relevant and researched topic for present and future process development. The presented review highlights the importance of developing in-process monitoring tools and provides a detailed overview of such techniques for defect detection. Commonly occurring manufacturing defects, their influence on part and structural performance are assessed and discussed. The evolution of defect detection technologies considering defects types, accuracy, and speed are detailed. A summary of most promising approaches as per defect detection capabilities is pre-sented. Defect rectification techniques are also discussed.

Despite significant advancements in bio-based natural-fiber-reinforced composites, the recyclability/reprocessing of thermoset composites remains a persistent challenge that needs to be addressed. In the present study, an effort is made to provide a justification for the recyclability/reprocessing assessment of sandwich composite panels made with 'recyclate' (i.e., recycled flax/bio-based epoxy composite) cores and (flax/bio-based epoxy) skins produced by liquid composite molding. Resin transfer molding and vacuum-assisted resin infusion processes were used to investigate the influence of production processes on mechanical properties. Two different recyclate sizes-4 mm and 10 mm-were used to fabricate sandwich composite panels to study the effect of size on the mechanical properties of the panels. This study aims to compare the qualities of sandwich panels to those of virgin composite panels in terms of their physical (density) and mechanical properties (tensile and flexural). Additionally, the recyclate packing was verified by employing digital microscopy. The results illustrated that the sandwich panels made with the 4 mm recyclates exhibited better mechanical properties compared to those made with the 10 mm recyclates. In comparison with virgin composite panels, the sandwich composite panels made of flax fiber and (flax/epoxy) recyclate exhibited significantly higher flexural moduli, which was attributed to their moments of inertia. This article emphasizes recycling/reprocessing and demonstrates an effective closed-loop approach. Thus, by preserving the structural integrity of recyclates, sandwich panels could be advantageous for semi-structural applications.

One-step injection molding compounding (IMC) is an innovative process to manufacture short-fiber-reinforced polymer composites. The aim of combining compounding and injection molding into one process is to enhance component quality and minimize environmental impacts. In this study, a screening Life Cycle Assessment (LCA) is conducted to evaluate and compare the environmental impacts of the IMC process with standard two-step manufacturing. Two scenarios for the IMC are considered, each differing in terms of machinery requirements, energy consumption, and material usage. Mechanically recycled polypropylene and glass fiber are used, and considered in the LCA employing a simple cut-off approach without awarding credits for substituting (primary) materials. The functional unit is the composite produced via the respective process, assuming equal functionality. Inventory data are obtained from initial experiments, literature, and the ecoinvent database. The impact assessment method selected is ReCiPe2016. Results indicate that the environmental performance improvement achieved by the IMC compared to the reference process is minimal in the conservative scenario where energy and material usage can be reduced but machinery usage is increased. However, in an optimistic scenario, the IMC can reduce the impacts of composite manufacturing by 34 %. The contributions at the midpoint level vary, and metal usage and energy consumption are the main contributors in all scenarios. A variation of the energy source for manufacturing shows the dependency of environmental impacts of components produced in both processes on the geographical location of production and its electricity supply. Methodological choices, such as the definition of the functional unit and modeling of recycled materials, have a large influence on LCA results, and alternative options are discussed.

Binder/tackifier materials are commonly used in preforming processes to preserve the structural integrity of the preform during processing. In the following resin infusion or injection process, this additional material will influence the resin flow. While the influence on fabric permeability is thoroughly examined in scientific literature, only few studies investigate the capillary behavior. By thermal activation of the binder, the material melts and spreads across the layer’s surface or is imbibed by the rovings. In this study, capillary rise experiments in planar direction with four different carbon fiber fabrics were performed. The tested stacks were activated at different temperature levels and compressed in a vacuum bag, one of them with additional external pressure in an autoclave. In case of no external pressure, the processing and testing conditions showed a larger influence than binder activation temperature, while autoclave-conditioned specimens showed a decreased capillary rise velocity for all levels of activation temperature. Digital microscopy images of the specimens show that molten binder can create a thin film between the layers, which prevents peripheral flow and thus forces the fluid to rise in the (angulated) capillary tubes inside the rovings.

An aged epoxy prepreg's glass transition temperature (Tg) is determined using Near Infrared Spectroscopy (NIRS). A miniaturized Near Infrared (NIR) spectrometer and a full-scaled FTNIR spectrometer are used for Tg determination and different measuring setups are tested for the miniaturized spectrometer. Partial least square (PLS) regression is used to develop calibration models by utilizing different spectral pretreatments. It is shown that with the miniaturized spectrometer and a suitable measurement setup, PLS model qualities similar to those of the FTNIR spectrometer can be achieved in cross-validation (CV), with a determination coefficient of CV (R2CV) of about 0.97 and a root mean square error of cross-validation (RMSECV) of approx. 0.7 degrees C in the measuring range between 25 degrees C and 42 degrees C. Cost-effective, miniaturized NIR spectrometers are thus a suitable technology for the quality assurance of prepregs.

Automated tape layup (ATL), is susceptible to a variety of manufacturing defects that affect productivity and mechanical performance of the final part. To increase process efficiency an inline monitoring tool is developed, which detects defects during manufacturing in a ply-by-ply manner. The presented work details the capability analysis of inline non-destructive testing (NDT) by means of thermography compared to industry approved NDT methods, namely, active infrared thermography and ultrasonic testing, which are performed subsequently. Two sets of samples were made, with foreign object and debris (FOD) and without any artificial defects. These in-situ consolidated samples were then checked for FOD and bond inhomogeneity via all three NDT methods. Quan-titative and qualitative analysis of research finding was performed. Inline thermography results conform to established industry standards in recognizing FOD defects. These defects can be localized and rectified imme-diately, increasing process reliability. Inline thermography can be used effectively for inline process control.

Amidst growing concerns about sustainability of composite materials, the renewed push towards adoption of bio-based polymer/natural fiber reinforced composites (Bio-composite) are gaining increasing demands for various applications which are being the environmental and eco-friendly alternative to synthetic composite materials. Hence, the bio based composite development should be integrated in the circular economy (CE) model to ensure a sustainable production that leads to the conception of closed loops in which resources are in the circulation of production and consumption. However, ironically, the environmental sustainability of composite materials itself is still a challenge, due to the difficulty of recycling and reusing its components when the products reach the end of their useful life. In this context, a holistic attainment of sustainability makes it imperative to adapt sustainable practices not only for raw materials but at every stage of the product. Hence, this work provides a detailed exploration of the appropriate processing of natural fiber reinforced bio-polymer composites and an insight on using recycled bio based composite constituents which could lead to a reduction in material waste and environmental footprints.

In this paper, the application of a fiber Bragg grating written in a highly birefringent side-hole elliptical core optical fiber for two-axial strain measurement is presented. Hybrid optical fiber structures achieved by combining large side-holes and elliptical core result in a very high birefringence of 1 × 10−3 and thus high initial Bragg peak spectral separation of 1.16 nm, as well as a very high transverse force sensitivity, of up to 650 pm/(N/mm) or even −1150 pm/(N/mm), depending on the fiber orientation with respect to the applied force. Due to the ~22 %m/m GeO2 concentration in the core the fiber being highly photosensitive, which significantly simplifies FBG fabrication by UV illumination without the need for prior hydrogen loading, which worsens thermal stability. Finally, the developed FBGs written in the highly birefringent side-hole elliptical core optical fiber were embedded in the square composite plates and applied for strain measurements. Tests of two-directional four-point bending have shown usability of such FBG for two-axial in-plane strain measurement with a single FBG in iso-thermal conditions.

Capillary-driven saturation of the reinforcing structure is an important aspect of liquid composite molding processes. Interacting with the flow-induced saturation, process speed must be optimized to guarantee full wetting while being time-efficient. The well-known Lucas-Washburn equation gives an estimate of the capillary imbibition in porous materials but lacks accuracy in case of textile structures. In this work, capillary rise experiments of different glass and carbon reinforcements are compared. An extension to the Lucas-Washburn equation is proposed based on regression-based fitting and validated with the experimental results. It considers peripheral flow in the textile as well as gravitational effects. Good conformity could be shown for textiles with many cross-sections. Unidirectional, low fiber volume content and irregular-shaped fabrics are more difficult to handle. The accurate determination of the capillary radius and the peripheral fluid flow are key factors.

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.

Automated tape layup (ATL) has been used extensively for manufacturing composites laminates using unidirectional prepregs for high-performance industries like aerospace. Residual stress is one of the defects that adversely affect the layup quality. These stresses affect geometrical tolerances in the form of distortion of the final product and are found to have a detrimental impact on the mechanical properties. In-line monitoring of such defects will help in productivity increase and achieving a reliable process control. The aim of the present work is to demonstrate the feasibility of fiber Bragg grating sensors for monitoring residual strains. FBGs are embedded inside a thermoplastic UD laminate. Temperature and strain discrimination is performed to recognize intermittent residual strains during the layup. Finally, intermittent residual strain is used to develop an understanding of the global residual strain. Effect of selective process parameter on residual stress formation and the evolution of the same is also analyzed.

Liquid composite molding (LCM) is a widely used group of various different processing techniques allowing to produce small, medium or even very big sized components from prototype level up to series production. During the infiltration it is necessary to run the process in a way preventing void formation. The typically used textile reinforcing structure results in a dual-scale impregnation consisting of micro impregnation within the constituent yarns of the textile structure and a macro impregnation between the yarns. Capillary rise experiments on flat textile samples are used and the well-known Lucas-Washburn equation has been extended to cover the special configuration. A porous capillary wall is assumed to better represent the three-dimensional nature of capillary networks within reinforcing textiles. An according test rig is presented. Accurate experimental results are gained and capillary radii are computed simple and fast via curve regression.

Automated tape layup, is predisposed to positioning defects like gaps and overlaps which are detrimental to the structural integrity of the part. This work investigates the details of the design of inline width control concept, a technique for inline control of consolidated tape width to eliminate gaps. Continuous inline width variation is achieved by online control of the compaction force, during the layup process. Successful calibration and benchmarking tests using standard pressure measurement sensors are presented. Performance assessment tests are performed which demonstrate online variation of force from 50 N to 1000 N, having a standard deviation of 10 N. Correlation models for width control are obtained and it is shown that a width increase of 5-50 % of initial width is achievable using the prototype. An example of linearly varying force resulting in a trapezium shaped consolidated tape is also shown as a part of the feasibility study.

For the winding of dry (e.g., not impregnated) fibers on convex structures, an adhesive force between surface and roving has to be established. Providing friction is necessary to prevent the roving from slipping. Usually, rovings impregnated or coated with (epoxy-) binder material are used. The investigation of the tack performance of these rovings differs from established studies on pressure-sensitive adhesives and prepregs. It was found that the common probe test is not suitable for bindered rovings. Instead, shear tests were performed on a heated, flat plate with a robotic winding setup. The horizontal force to pull the roving from the plate was measured. Variation of temperature was in focus. Compaction force, pulling speed, and twisting of the roving were also investigated. It could be seen that compaction and thorough heating are the key factors. Due to the small surface, twisting had no significant effect on the tack behavior.

Near-infrared spectroscopy (NIR) was implemented in the resin transfer molding (RTM) process to inline monitor the degree of curing of a bio-based epoxy resin, which consists of epoxidized linseed oil (resin) and citric acid (hardener), respectively. A NIR micro-spectrometer was used for the development of robust calibration models using partial least squares (PLS) regression. Since the micro-spectrometer offers a smaller wavelength range compared with conventional NIR devices, and typical absorbance peaks are not directly involved in the captured data range, the results show new insights for the utilization of this technology. Different pre-treatments of the spectroscopic data have been tested, starting with different reference spectra, i.e., uncured resin and polytetrafluorethylene (PTFE), and followed by chemometrical algorithms. As a reference method for the degree of curing, direct current (DC) supported by differential scanning calorimetry (DSC) was used. The results show the potential of these cost-efficient and compact NIR micro-spectrometers for the intended inline monitoring purpose to gain relevant information feedback during the process.

In this study, the empirical modeling of the compaction response of different fibrous reinforcements with a focus on natural fiber-based textiles is investigated. The main objective is the improvement of an empirical modeling approach in terms of a constant high accuracy for different compaction behaviors and varying test conditions. Therefore, a new two-parameter model function is derived based on the characteristics of the compaction work progression. Five different plant fiber textiles (flax and hemp) with different fiber architectures (nonwoven, woven, and unidirectional noncrimp fabric) and a woven glass fiber reinforcement are tested within compaction experiments, considering the variation of the stack height, the compaction rate and repetitive load-unload cycles. The new empirical model shows an overall better accuracy with smaller deviations in quality compared to alternative, commonly used empirical model functions based on the power and exponential law, which are considered for comparison purposes.