Time–temperature equivalence in the tack and dynamic stiffness of polymer prepreg and its application to automated composites manufacturing

A recently developed peel test designed to simulate the automated tape lay-up (ATL) process was used to measure tack and dynamic stiffness of newly developed ATL prepregs. Resin was extracted from the prepreg process before impregnation of the fibres. Isothermal small amplitude frequency sweeps were carried out in shear rheology to determine time–temperature superposition parameters in the form of Williams–Landel–Ferry equation. Gel permeation chromatography and differential scanning calorimetry demonstrated that the resin was not significantly changed during the prepregging process. The WLF parameters were used to transpose isothermal tack and dynamic stiffness results with excellent agreement found. This relationship offers manufacturers using composite prepreg a method to maximise and maintain tack levels at different feed rates by appropriate changes in temperature. This is of significant importance in improving the reliability of automated composite lay-up processes such as AFP and ATL, whose feed rate must vary to accommodate lay-up operations.     

A stochastic approach to model material variation determining tow impregnation in out-of-autoclave prepreg consolidation

Material variations are always present even though out-of-autoclave prepregs are machine-made. They strongly determine the consolidation and may eventually lead to voids within the final part, depending on applied process conditions. To capture any contingencies, stochastic differential equations are derived to describe various interacting phenomena in OoA consolidation. In a second step the probabilistic space is discretized using the Karhunen–Loève truncation and the Probabilistic Collocation method is applied in order to use deterministic solvers for flow and compaction problems. The initial degree of impregnation is represented by an Ornstein–Uhlenbeck process and calibrated with CT-images.     

Simulating tape resin infiltration during thermoset pultrusion process

The thermoset tape pultrusion is a widely adopted manufacturing process to produce long, constant cross-section composite structural parts. For high volume production, low cost can be achieved by maximizing the production rate which is a function of the material and process parameters, more specifically the rate of resin infiltration and resin cure. During resin infiltration, the resin saturates the dry reinforcement either under positive pressure in the pressure chamber, or, by the action of capillary and surface forces, within the resin bath. In either case, the saturation must be completed as the tape is squeezed into the final cross-sectional form at the entrance of the heated mold where the resin will be cured to form the composite part.This paper models the resin infiltration process during pultrusion, by modifying the pre-existing simulation tool for liquid molding processes. The formulated capability can be used not only to optimize the impregnation dynamics within the pressure chamber, but can also be used to predict the required forces for the selected pulling rate. The proposed model does allow one to handle a variety of tape cross-sections, not just rectangular prisms.     

A composite cost model for the aeronautical industry: Methodology and case study

This paper presents a novel composite production cost estimation model. The strength of the model is its modular construction, allowing for easy implementation of different production methods and case studies. The cost model is exemplified by evaluating the costs of a generic aeronautical wing, consisting of skin, stiffeners and rib feet. Several common aeronautical manufacturing methods are studied. For studied structure, hand layup is the most cost-effective method for annual volumes of less than 150 structures per year. For higher production volumes automatic tape layup (ATL) followed by hot drape forming (HDF) is the most cost-effective choice.     

Evaluation of eddy current testing for quality assurance and process monitoring of automated fiber placement

Standards in energy and cost efficiency are higher the ever especially in the aerospace industry. While structures made from carbon-fiber reinforced plastics (CFRP) show significant advantages in regards to specific strength and lightweight design, further improvements in their production processes are essential in order for CFRP to be competitive in the future. The authors present eddy current (EC) testing as a means for quality assurance (QA) and process monitoring for CFRP parts produced by automatic fiber placement (AFP), which is one the most prevalent production methods in aerospace industry. Eddy current testing shows the potential for highly automated process monitoring that can reduce error correction and cycle time in AFP.     

Chemical and thermal shrinkage in thermosetting prepreg

A methodology for predicting residual cure deformation and stresses in composite laminates during cure is proposed. The technique employs an unbalanced cross-ply strip denoted as a “bi-lamina” strip to measure the in situ development of chemical and thermal shrinkage deformation during a specified thermal cycle. The constitutive model of the composite material was developed based on self-consistent micro-mechanical homogenization with variable resin thermo-mechanical material properties during the cure cycle. The resin properties were determined as a function of cure and temperature using different experimental techniques, including differential scanning calorimetry, digital image correlation, rheometry and dynamic mechanical analysis. The predicted bending deflection profiles of the strip agreed closely with experimental observations. The proposed methodology can be used to validate the material model of the resin and composite during the cure cycle.     

The impact of air evacuation on the impregnation time of Out-of-Autoclave prepregs

Most Out-of-Autoclave prepregs (OoA) are only partially impregnated with resin. Their impregnation completes during the cure cycle, solely driven by the difference between atmospheric and vacuum pressure. Increased part length leads to an impregnation time gradient caused by the transient air flow inside the fibrous medium. In this work, a novel numerical approach capable of predicting the local impregnation time of a fibrous domain with resin, at isothermal conditions, under the influence of transient air flow, is proposed (delayed air evacuation). Sensitivity studies prove the robustness of the numerical scheme, for a large range of flow time-scales. The same approach is used to predict the local impregnation time of a commercial OoA prepreg tow, for a wide range of part lengths. It is demonstrated that for manufacturing long parts OoA, accurately capturing the influence of the air pressure on the local impregnation state of the tow, is important for quantifying the risk for residual tow porosity.     

A resin film infusion process for manufacture of advanced composite structures

This paper reports work conducted at the CRC-ACS for the manufacture of advanced composite structural demonstrators using a resin film infusion (RFI) process. The influences of a number of processing parameters on the quality of cured parts are discussed. A number of demonstrators are presented including an aileron skin panel, swaged wing rib and 3-bay aft-box structure. These demonstrators indicate the cost-effectiveness of the RFI process for manufacture of advanced composite structures.     

Thermal press curing of advanced thermoset composite laminate parts

An alternative process to autoclaving, called Thermal Press Curing (TPC), is proposed, whereby an uncured composite laminate is pressed between a heated curing mold and customized rubber-faced mold that are designed to provide uniform temperature and pressure conditions. TPC was demonstrated by designing a complex 3-D ‘benchmark’ part shape, applying a simple computational algorithm to derive the required tool shapes, and fabricating the tooling. A comparative study was performed involving the benchmark part made from four plies of woven carbon/epoxy prepreg material. Identical laminates were pre-formed by double diaphragm forming and then cured and consolidated by autoclaving, Quickstep, and TPC using standard industry practice. Results of the study indicate that the TPC part is of similar quality as compared to those made by autoclaving and Quickstep, but, more importantly, requiring significantly less energy and resource consumption, lower cost (capital and recurring), and less preparation and cycle time.     

Dynamic saturation curve measurement for resin flow in glass fibre reinforcement

A methodology is presented to determine the saturation curve of a resin/glass fabric system, during infiltration in a transparent mould under constant flow rate. Video acquisitions are transformed by image analysis into saturation level versus position and time, and coupled to inlet pressure measurements. A numerical multiphase flow model is then used to simulate the infiltration for various combinations of drainage curve parameters. The numerical parameters to describe the saturation and relative permeability are determined by response surface optimization. The drainage curve and relative permeability equations determined at one time are shown to adequately describe the entire injection process, and to be flow-rate dependent.     

Flow analysis during compression of partially impregnated fiber preform under controlled force

Compression resin transfer molding (CRTM) is an alternative solution to conventional resin transfer molding processes. It offers the capability to produce net shape composites with fast cycle times making it conducive for high volume production. The resin flow during this process can be separated into three phases: (i) metered amount of resin injection into a partially closed mold containing dry fiber preform, (ii) closure of the mold until it is in contact with the fiber preform displacing all the resin into the preform and (iii) further mold closure to the desired thickness of the part compacting the preform and redistributing the resin. Understanding the flow behavior in every phase is imperative for predictive process modeling that guarantees full preform saturation within a given time and under specified force constraints.     

On the characterisation of transverse tensile properties of molten unidirectional thermoplastic composite tapes for thermoforming simulations

The development of Finite Element (FE) thermoforming simulations of tailored thermoplastic blanks, i.e. blanks composed of unidirectional pre-impregnated tapes, requires the characterisation of the composite tape under the same environmental conditions as forming occurs. This paper presents a novel approach for the characterisation of transverse tensile properties of unidirectional thermoplastic tapes using a Dynamic Mechanical Analysis (DMA) system in a quasi-static manner. The relevance of the presented method is assessed by testing, under the same environmental conditions, a control material with both a universal testing machine and a DMA system. For simulation purposes, a unidirectional thermoplastic tape is characterised under environmental forming conditions using the presented test method. Experimental results, which include stress–strain behaviour and transverse viscosity, are eventually used to identify, via an inverse approach, simulation parameters of a thermo-visco-elastic composite material model (MAT 140, PAM-Form, ESI Group). Comparisons between simulated and experimental results show good agreement.     

In-situ toughened CFRP composites by shear-calender orientation and fiber-bundle filtration of PA microparticles at prepreg interlayer

Epoxy matrix toughened by polyethersulfone (PES) and polyamide (PA) microparticles was designed and the in-situ interlaminar toughened carbon fiber/epoxy composites were fabricated. Synergistic toughening effect of PES and PA on epoxy matrix was achieved due to semi-IPN structure of PES toughened matrix and uniform dispersion of PA microparticles. Shear-calender orientation of PA microparticles was found during prepreg processing and the microparticles remained on the surface of prepreg due to fiber-bundle filtration. The in-situ formed toughening interlayer of PA microparticles and interfacial bonding between PA and epoxy matrix were detected, which resulted in enhanced fracture toughness, CAI, and transverse flexural strength of the composite based on the PES/PA synergistically toughened matrix. SEM images of fracture morphology of the composite showed evidence of enhanced plastic deformation created by PES and PA, and crack deflection and bridging by PA microparticles.     

The experimental determination of prepreg tack and dynamic stiffness

A new peel test has been developed which quantifies the tack and dynamic stiffness of uncured prepreg. The test is designed to simulate the automated tape lay-up (ATL) and automated fibre placement (AFP) processes. It includes a pressure controlled application stage, where contact time is inversely proportional to peel rate. The use of a thin film allows stiffness to be isolated from peel resistance in a continuous two stage test. A repeatability study revealed consistent results with 16% standard deviation. Tack and stiffness variability has been observed across roll width and between faces in commercial hand lay-up prepregs. The overall tack and stiffness values for commercial hand lay-up prepregs were found to be inconsistent with the levels specified by manufacturers.A temperature increase revealed inconsistent effects on tack between materials. The contradictory results were rationalised by observing failure modes. The two failure modes observed appeared equivalent to those found in pressure sensitive adhesive (PSA) peel. The shear storage modulus of the prepreg resin was compared to the PSA Dahlquist criterion and found to support the principle of contact efficiency. However, the actual value for the criterion is expected to be a function of prepreg specific conditions such as resin content, fibre distribution and surface pattern.     

Cure path dependency of mode I fracture toughness in thermoplastic particle interleaf toughened prepreg laminates

The effect of cure cycle on fracture behaviour of a commercial thermoplastic particle interleaved prepreg system was investigated. Laminates were manufactured at 700 kPa in an autoclave using eight different thermal cycles that included both raising the cure temperature above the standard 180 °C cure cycle and incorporating an intermediate dwell stage between 150 and 170 °C prior to reaching the 180 °C cure temperature. Double cantilever beam tests were conducted on specimens from the cured laminates. The stick–slip crack behaviour, observed in samples manufactured using the standard cure cycle, changed to stable crack growth when processing deviated by 10 °C. The mode I fracture toughness values were reduced by 11–22% when incorporating an intermediate dwell stage before the final cure temperature. Scanning electron microscopy inspection of the fracture surfaces showed differences between samples made by standard cure cycles and those made using process deviations.     

Influence of preforming technology on the out-of-plane impregnation behavior of textiles

Various Liquid Composite Molding processes provide out-of-plane impregnation. Thereby, the emerging pressure distribution leads to a hydrodynamic compaction of the fiber structure. A prediction of the behavior of the fiber structure under these conditions is highly complex, due to strong interdependencies between pressure distribution, compaction behavior, and impregnation behavior. In this study possibilities to influence the hydrodynamic behavior of textiles by preforming technology are investigated, by comparing an untreated glass fiber woven textile with sewed, bindered, sheared, and pre-compacted samples of the same material. A novel measurement apparatus, reproducing the conditions during out-of-plane impregnation, is applied for this purpose. It enables out-of-plane permeability determination simultaneous to online compaction monitoring. Sewing accelerates out-of-plane impregnation, while inactivated binder can preserve the available permeability at high injection pressures. The increasing superficial density and the geometrical changes caused by shearing decrease permeability.     

Modeling of heat transfer and unsaturated flow in woven fiber reinforcements during direct injection-pultrusion process of thermoplastic composites

This paper provides a methodology for the modeling of heat transfer and polymer flow during direct thermoplastic injection pultrusion process. Pultrusion was initially developed with thermosets which have low viscosity. But the impregnation becomes a critical point with thermoplastics which exhibit higher viscosity. There are very few reported works on direct thermoplastic impregnation with injection within the die. In addition, the rare studies have not adequately addressed the issue of unsaturated flow in woven fiber reinforcements. The solution proposed here, models the polymer flow through dual-scale porous media. A heat transfer model is coupled to a flow model enriched with a sink term. Specific changes of variables are made so as to model the steady state solution of unsaturation along a continuous process. The sink term, added to the continuity equation, represents the absorption rate of polymer by the bundles. Data were measured on a pultrusion line and micrographs confirmed the modeling strategy with an unsaturated flow approach. The flow modeling coupled to heat transfer of the thermoplastic pultrusion process aims at determining the saturation evolution through the die so as to manufacture pultruded profiles with the lowest residual porosity.     

In situ process monitoring of hierarchical micro-/nano-composites using percolated carbon nanotube networks

We report a method to monitor the manufacturing process of hierarchical micro-/nano-composites that uses integrated and percolated multi-walled carbon nanotube (MWCNT) networks with the aim of reducing part-to-part variability. Composites were fabricated by VARTM. Fiber textiles were spray-coated with MWCNTs, electrodes were embedded prior to vacuum bagging. In situ process monitoring was achieved by measurement of the electrical resistance of electrode pairs. The effects of MWCNT density and length on the ability to monitor the manufacturing process were evaluated. Experiments showed that monitoring the changes in resistance between electrode pairs on the conductive MWCNT network allowed various events during the manufacturing process, including part infusion, onset of crosslinking, and gel point of the resin, which are necessary for accurate evaluation of part quality. Our simple yet effective method to monitor the manufacturing processes and predict the final-part quality of multiscale composites can be integrated into existing processes with minimal modifications.     

Experimental investigation and flow visualisation of the resin transfer mould filling process for non-woven hemp reinforced phenolic composites

Resin transfer moulding (RTM) of glass fibre reinforced polymeric composites offers the advantages of automation, low cost and versatile design of fibre reinforcement. A replacement of glass fibres with natural plant fibres as reinforcement in polymeric composites provides additional technological, economical, ecological and environmental benefits. The resin transfer mould filling process has significant effects on different aspects, such as fibre wetting out and impregnation, injection gate design, “dry patch” and void formation. Flow visualisation experiments were carried out using a transparent RTM mould to develop a better understanding of the mould filling process for hemp mat reinforced phenolic composites. The mould filling of unreinforced phenolics was characterised by a “quasi-one-dimensional steady state” flow. In the case of hemp non-woven reinforced system, the mould filling process can be considered as the flow of fluids through porous media. “Fibre washing” was a typical problem encountered during the injection process, leading to poor property uniformity. In addition, a preferential flow path was usually created near the edges and corners of the mould. The path exhibited low flow resistance and caused the resin flow front to advance much faster in these regions. The edge flow disturbed the steady flow, leading to difficulties in venting arrangement and “dry patch” formation. The edge flow and fibre washing were alleviated by reinforcement manipulation so steady state flow could be achieved. The relationships between the filling time and injection pressure and between filling time and different fibre weight fractions have been established for certain specific injection strategies.