Composites design and manufacturing

The growing concerns about the environment and energy have driven the widespread adoption of new lightweight materials such as polymer composites in many industrial sectors, crucial issues being performance, cost efficiency, and multifunctionality (Krawczak, 2019). Among them, thermosetting matrix composites offer not only many advantages such as high specific stiffness and strength and their consolidated use in primary aircraft structures but also technical issues such as long processing times, limited fracture toughness, reduction of mechanical properties in the presence of water and solvents, and difficulty of end-of-life treatment (Krawczak, 2011). A key cultural problem is associated with the need of a chemical reaction during part fabrication performed in mechanical/aerospace/naval industries.

Although the use of thermoplastic matrix composites in aeronautic structures has been proposed since the second half of the 80s (Nguyen and Ishida, 1987; Maffezzoli et al., 1989), their first application was at the beginning of the third millennium with the wing leading edge of Airbus A380 (Offringa, 2005). Their demand is continuously increasing (JEC Observer, 2020) since they offer many advantages such as high toughness, long storage time, easy repairing and recycling, higher performances in hot-wet conditions, and ability to be thermoformed and heat-welded. However, manufacturing issues such as high processing temperature, fiber impregnation, and warpage still need a deeper knowledge (Advani and Hsiao, 2012). Thermoplastic composites can play a key role in the 21st century industry as new materials are emerging day by day, as manufacturing processes are evolving to meet the severe industrial requirements of performance, production, and multifunctionality (Advani and Hsiao, 2012; Boisse, 2015), and as sophisticated numerical simulation tools are making virtual prototyping and engineering more reliable than ever (Binétruy et al., 2015; Boisse, 2015). In that field, innovation is multifaceted and may come either from the constitutive materials (high-performance engineering thermoplastic matrices, commingled semipreg of fibers/thermoplastics, self-healing, bio-based or biodegradable matrices, natural or thermoplastic fibers, 3D textile reinforcement…) or from the manufacturing processes (automated out-of-autoclave processes, hybridization/coupling of technologies, additive manufacturing, hybrid welding…). Moreover, thermoplastic composites allow for automated, high-speed mass production with reduced need for secondary operations (Mafeld, 2014), creating increased interest in the industry for various applications: aerospace and defense, automotive and rail transportation, construction and building, energy, oil and gas, sporting goods, drones, etc. Also, thermoplastic composites offer an interesting recycling potential using mechanical, thermal, or chemical processes (Oliveux et al., 2015). As thermoplastics, it is quite easy to recycle them by regrinding, compounding/blending, and reprocessing; additionally, they are at the forefront of current industrial scale-up attempts of chemical recycling, making it possible to recover not only fibers but also feedstock or original monomers which can eventually be re-polymerized to produce new polymers.
This Research Topic focuses on the latest advances and developments of fiber-reinforced thermoplastics, mainly referring to continuous fiber-reinforced composites and of related processing technologies. A collection of seven articles addresses some challenging issues related to advanced manufacturing processes, such as consolidation by compression molding (Ayadi et al.), injection-overmolding (Akkerman et al.), reactive pultrusion (Zoller et al.), joining by ultrasonic (Villegas) or induction (Scarselli et al.) welding, as well as solid-state drawing (Walker et al.) or electrospinning (Sessini et al.) of fibers. Numerical modeling and simulation, and performance assessment through advanced characterization techniques, correlating microstructure and properties, are also addressed. These papers highlight how thermoplastic matrix composites can be processed, adopting a completely different paradigm in comparison to thermosetting matrix ones, the former not requiring a reaction to set the properties but complex thermal cycles, often performed in a short time span.
The Guest Editorial team is confident that this collection of papers will benefit a large community in research and application of advanced thermoplastic composite materials so as to further advance along the pathway toward sustainability, performance, and competitiveness.

ABSTRACT: Accurate measurement of permeability is critical for fluid flow modeling in porous media. Various experimental methods devised to measure permeability as a porous material property in composites are reviewed. Liquid flow and gas flow methods of permeability measurement for in-plane and transverse directions specifically for fiber-reinforced composites are discussed, as well as issues related to these methods and some associated permeability models. Alternative methods of permeability determination based on cross transport phenomenon are reviewed as well.

A cohesive-zone approach is used to study the interaction between an approaching crack and a particle embedded in a matrix material as a function of the mismatch in elastic and fracture properties. Crack-particle interaction is a crucial issue governing fracture behavior of particle-dispersed materials. Special attention is given in the present work to the effect of the mismatch in fracture properties, namely fracture strength and energy, which has not been fully-explored in the literature. Based on extensive finite element simulations using cohesive elements, the basic fracture mechanisms governing the crack-particle interaction are identified, namely particle fracture, crack deflection and interface debonding. The details of the cracking sequences are elucidated and the role of secondary cracks is highlighted. The effect of pre-existing flaws on the fracture behavior is analyzed both for flaws inside the particle as well as flaws on the particle/matrix interface. Several flaw configurations in terms of size, orientation and location are considered. In addition, the effect of the mismatch between the matrix and the interface fracture properties is also considered for a wide range of adhesive characteristics. The results of the simulations are summarized in the form of several fracture maps for different configurations, whereby the main fracture mechanisms are identified in regions inside a two-dimensional space of strength and toughness mismatch between the particle and the matrix. It is observed that the mismatch in the fracture properties usually plays a more dominant role on the crack trajectory than the mismatch in elastic properties in a particle-dispersed system. Pre-existing flaws/defects in the particle and the interface are found to be one of the principal controlling factors that alter the crack propagation characteristics. These results can be used as a guideline for designing particulate composite system with a preferred fracture mechanism, namely matrix cracking, interface debonding or particle fracture.

A process for the impregnation of glass rovings with a polyamide melt is studied in which the glass rovings were led over spreader pins in a melt bath and then through a wipe-off die. A set-up was made to measure the pulling forces. The degree of impregnation of the bundle before it entered the die, the bundle size, the die size and the pulling speed were varied to determine their effect on the friction forces and the impregnation of the wipe-off die. With a dry bundle no impregnation is obtained in the die and the friction forces are low. If a partially impregnated bundle is led through the die, the impregnation is enhanced, but the friction forces are high.

A process for the impregnation of glass rovings with a polyamide melt is studied in which the glass rovings were led over spreader pins in a melt bath and then through a wipe-off die. A set-up was made to measure the pulling forces. The degree of impregnation of the bundle before it entered the die, the bundle size, the die size and the pulling speed were varied to determine their effect on the friction forces and the impregnation of the wipe-off die. With a dry bundle no impregnation is obtained in the die and the friction forces are low. If a partially impregnated bundle is led through the die, the impregnation is enhanced, but the friction forces are high.

A cohesive-zone approach is used to study the interaction between an approaching crack and a particle embedded in a matrix material as a function of the mismatch in elastic and fracture properties. Crack-particle interaction is a crucial issue governing fracture behavior of particle-dispersed materials. Special attention is given in the present work to the effect of the mismatch in fracture properties, namely fracture strength and energy, which has not been fully-explored in the literature. Based on extensive finite element simulations using cohesive elements, the basic fracture mechanisms governing the crack-particle interaction are identified, namely particle fracture, crack deflection and interface debonding. The details of the cracking sequences are elucidated and the role of secondary cracks is highlighted. The effect of pre-existing flaws on the fracture behavior is analyzed both for flaws inside the particle as well as flaws on the particle/matrix interface. Several flaw configurations in terms of size, orientation and location are considered. In addition, the effect of the mismatch between the matrix and the interface fracture properties is also considered for a wide range of adhesive characteristics. The results of the simulations are summarized in the form of several fracture maps for different configurations, whereby the main fracture mechanisms are identified in regions inside a two-dimensional space of strength and toughness mismatch between the particle and the matrix. It is observed that the mismatch in the fracture properties usually plays a more dominant role on the crack trajectory than the mismatch in elastic properties in a particle-dispersed system. Pre-existing flaws/defects in the particle and the interface are found to be one of the principal controlling factors that alter the crack propagation characteristics. These results can be used as a guideline for designing particulate composite system with a preferred fracture mechanism, namely matrix cracking, interface debonding or particle fracture.

A one-dimensional through-the-thickness simulation is developed in which heat conduction, kinetic, viscosity and flow (squeezed sponge model) equations are solved as a coupled system of equations. The control-volume method combined with the alternating direction explicit method is employed in the solution. Heat capacity, density, thermal conductivity and permeability of the composite are considered as functions of fibre volume fraction. It is shown that either temperature overshoot at the middle of the composite or incomplete through-the-thickness consolidation can put a restriction on the application of each cure cycle up to a certain thickness. Although the recommended cure cycles for a thick composite can satisfy the temperature conditions, complete through-the-thickness consolidation mostly cannot be achieved. Pressure effect, bleeding from the top and bottom and prebleeding are studied as possible solutions for incomplete consolidation. It seems that the prebleeding technique is the most promising method of fabrication for thick composites.

During filament winding of thick cylinders, fibre wrinkling often occurs which severely decreases compressive strength. To eliminate fibre wrinkling, appropriate processing conditions must be found. Fibre migration and stress relaxation due to resin flow are generally considered the most important factors affecting fibre buckling. Therefore, the effect of stress relaxation on fibre wrinkling during the filament winding process was investigated. To study the stress development during filament winding of thick cylinders, experiments were carried out using graphite/epoxy prepreg tows as well as dry graphite fibre. Cylinders of approximately 12 mm thickness were hoop wound on a 50.8 mm diameter aluminium mandrel. Winding tensions ranged from 13 to 34 N and winding speed was constant. A foil-type pressure sensor was applied on the mandrel to monitor the interface pressure throughout winding and storage of the cylinder. Significant stress relaxation was found to occur during winding with prepreg tow. Mandrel pressure increased over the winding of the first eight layers or so. However, between the winding of one layer and the next. mandrel pressure dropped quickly. Also, it began to decrease after reaching a maximum value. A stress relaxation analysis was carried out to determine the stress in the cylinders during winding. Several parameters were not known a priori and had to be inferred from the data. Stress distributions following winding were calculated for each case. The radial stress in prepreg wound cylinders was found to relax nearly to zero in the inner part of the tubes. Compressive circumferential stresses occurred throughout each of the cylinders. However, they reached greater magnitudes in the dry wound cylinders due to very low radial moduli. No fibre wrinkling was evident in any of the wound cylinders.

Powder impregnation techniques have been developed to increase design and manufacturing flexibility with thermoplastic composites. The effect of pressure, temperature, mould closing rate and time on the consolidation of poly(ether ketone ketone) (PEKK) powder impregnated glass fibre towpregs in compression moulding were studied. A design of experiments approach was used. Isothermal pultrusion experiments using nylon 11 and PEKK powder impregnated glass fibre tows were conducted. These experiments were repeated at different temperatures and pulling speeds. The microstructural changes during coating and consolidation of the powder impregnated tows were studied. Temperature and mould closing rate were observed to be the significant parameters affecting consolidation in compression moulding. Superposition of the pulling force-pulling speed data at different processing temperatures was demonstrated in pultrusion.

The technique for strengthening structures such as reinforced concrete beam, column and slab by method of externally bonded Fibre Reinforced Polymer (FRP) system is successfully being promoted to the Malaysian construction players over the past few years. Deteriorated concrete structures require repair and maintenance or sometimes need strengthening to extend their service life, therefore, the long-term durability of the bonded system is the key element in ensuring that a material system used is able to maintain its integrity throughout its service life. From the study, it shows that the specimens subjected to sustained load for the duration of six and twelve months and exposed to tropical outdoor condition produced a sign of deterioration on the overall bond performances.

The resin transfer moulding (RTM) process involves the loading of dry reinforcement into a mould. After the mould is closed, resin is flowed into the mould cavity and cured. The RTM process has traditionally been used to produce low fibre volume fraction composites. There is now increasing interest in using the process to manufacture high fibre volume fraction composites for structural applications. Experiments have been conducted to monitor the force required to compress a typical plain-woven glass fibre reinforcement. The load displacement curves for monotonic loading, and for relaxation after repeated reloading cycles to a maximum load are presented. The loading cycle responses for the fabric have been fitted to power-law relationships, and the relaxation cycles have been fitted to exponential decay functions.

A model for the in-plane deflections of a system of discrete axially loaded fibres in a viscous matrix has been developed to study the evolution of fibre waviness during forming of aligned thermoplastic composites. A simplified numerical solution approach to the theoretical model is implemented, allowing for the analysis of a large number of fibres with a reasonable computational effort. The results of the model show the maximum fibre deflections to be consistent with the linearized beam-column theory used in the model development, but that the time predicted to attain the maximum deflections is comparable to typical forming times. Non-uniform fibre loadings are modelled and the resulting distributions of fibre deflections are analysed. The interactions between neighbouring plies and between neighbouring fibres within a single ply are found to have significant effects on the distributions of fibre deflections and on the response times of the system.