Continuous resistance welding of thermoplastic composites: Modelling of heat generation and heat transfer

A process model composed of electrical and heat transfer models was developed to simulate continuous resistance welding of thermoplastic composites. Glass fabric reinforced polyphenylenesulfide welded in a lap-shear configuration with a stainless steel mesh as the heating element was considered for modelling and experimental validation of the numerical results. The welding temperatures predicted by the model showed good agreement with the experimental results. Welding input power and welding speed were found to be the two most important parameters influencing the welding temperature. The contact quality between the electrical connectors and the heating element was found to influence the distribution of the welding temperature transverse to the welding direction. Moreover, the size of the electrical connectors was found to influence the achievable welding speed and required power input for a certain welding temperature.     

Modelling the Young''s modulus of platelet reinforced thermoplastic sheet composites

Particulate reinforced polymers is a mature field and many models are available to predict the Young's modulus of such composites. However, most existing models have a common flaw; they all predict that the composite modulus equals that of the reinforcing agent when the polymer content approaches zero. This implies, in this limit, a monolithic reinforcement whereas, in fact, it is composed of discrete particles with very little interaction. This is a serious drawback and therefore this study focussed on deriving an improved model for the prediction of the Young's modulus. The porosity of the present samples was correlated with the volume fraction binder and the maximum packing density of the pure reinforcement. A theoretical model for Young's modulus was derived along the lines of the Padawer and Beecher modified Cox model. However, it includes the effect of composite porosity on the composite's mechanical properties. In contrast to other available models, it correctly predicts the loss of material stiffness and strength in the limit of zero binder content. Good agreement was found between the predictions of this model and experimental measurements.     

Interstrand void content evolution in compression moulding of randomly oriented strands (ROS) of thermoplastic composites

Compression moulding of randomly oriented strands (ROS) of thermoplastic composite is a process that enables the forming of complex shapes with keeping final properties close to that of continuous fibre composites. During forming several deformation mechanisms occur. In this paper we focus on the interstrand void content (ISVC) reduction: the squeezing of each single strand during compression enables filling of the gaps between strands. A modelling of this deformation mechanism was developed. The compaction is ruled by an ordinary differential equation that was solved numerically. The model was validated experimentally using an instrumented hot press with Carbon-PEEK prepreg strands. The model accurately predicted ISVC in four characteristic cases. Using the proposed model, the influence of several process and material parameters were investigated. Finally, a design chart giving the final ISVC for a wide range of pressure, strand geometry and part thickness, was constructed.     

Modelling microscopic flow in woven fabric reinforcements and its application in dual-scale resin infusion modelling

A physical unit cell impregnation model is proposed for the micro-scale flow in plain woven reinforcements. The modelling results show a characteristic relationship between tow impregnation speed, the surrounding local macro-scale resin pressure and the tow saturation within the unit cell. This relationship has been formulated into a mathematical algorithm which can be directly incorporated into a continuum dual-scale model to predict the ‘sink’ term. The results using the dual-scale model show a sharp resin front in inter-tow-pore spaces and a partially saturated front region in intra-tow-pore spaces. This demonstrates that the impregnation of fibre tows lags behind the resin front in the macro pore spaces. The modelling results are in agreement with two reported experimental observations. It has been shown that the unsaturated region at the flow front could increase or have a fixed length under different circumstances. These differences are due to the variation in tow impregnation speed (or the time required for the tow to become fully impregnated), the weave architecture and the nesting and packing of plies. The modelling results have also demonstrated the drooping of the inlet pressure when flow is carried out under constant injection rates. The implementation of the algorithm into a dual-scale model shows coherence with a single-scale unsaturated model, but demonstrates an advantage in flexibility, precision and convenience in application.     

On avoiding thermal degradation during welding of high-performance thermoplastic composites to thermoset composites

One of the major constraints in welding thermoplastic and thermoset composites is thermal degradation of the thermoset resin under the high temperatures required to achieve fusion bonding of the thermoplastic resin. This paper presents a procedure to successfully prevent thermal degradation of the thermoset resin during high-temperature welding of thermoplastic to thermoset composites. The procedure is based on reducing the heating time to fractions of a second during the welding process. In order to achieve such short heating times, which are much too short for commercial welding techniques such as resistance or induction welding, ultrasonic welding is used in this work. A particularly challenging scenario is analysed by considering welding of carbon-fibre reinforced poly-ether-ether-ketone, with a melting temperature of 340 °C, to carbon-fibre reinforced epoxy with a glass transition temperature of 157 °C.     

Selective reinforcement of LLDPE components produced by rotational molding with thermoplastic matrix pultruded profiles

This work is aimed to study the use of pultruded profiles for the selective reinforcement of linear low density polyethylene (LLDPE) parts produced by rotational molding. A preliminary screening on different types of pultruded profiles was performed, highlighting the relevance of adhesion to LLDPE in order to prevent debonding of the reinforcing pultruded profiles. As expected, high density polyethylene (HDPE) matrix pultruded tapes are characterized by a very high adhesion to rotomolded LLDPE. Therefore, HDPE matrix pultruded tapes, fastened on the inner surface of the mold, are incorporated into LLDPE during rotomolding. Plate bending tests performed on reinforced rotomolded plates and pressurization tests performed on the box shaped prototypes showed a significant increase of the stiffness with a negligible amount of reinforcement and increase of the weight of the component.     

Strength development versus process data in ultrasonic welding of thermoplastic composites with flat energy directors and its application to the definition of optimum processing parameters

Ultrasonic welding of thermoplastic composites is a very interesting joining technique as a result of good quality joints, very short welding times and the fact that no foreign material, e.g. a metal mesh, is required at the welding interface in any case. This paper describes one further advantage, the ability to relate weld strength to the welding process data, namely dissipated power and displacement of the sonotrode, in ultrasonic welding of thermoplastic composite parts with flat energy directors. This relationship, combined with displacement-controlled welding, allows for fast definition of optimum welding parameters which consistently result in high-strength welded joints.     

Deconsolidation in glass mat thermoplastic composites: Analysis of the mechanisms

During reheating and post-processing of thermoplastic-based composites, deconsolidation is often observed: the volume fraction fibre decreases and the void content increases. In this article, the phenomena leading to deconsolidation are investigated, with particular emphasis on the elastic release of stress in the preform, also called springback effect. A model is proposed to simulate the evolution of the specimen thickness with time. A comparison with model experiments consisting in relaxation of glass mats in polyethylene-glycol is provided. This result, together with reheating experiments of Glass Mat reinforced Thermoplastics GMT parts, showed that deconsolidation is mainly governed by the elastic behaviour of the fibre preform. It is also observed that the air initially dissolved in the matrix tends to coalesce during reheating due to diffusion, but also to tensile forces induced by the springback effect.     

A displacement-detection based approach for process monitoring and processing window definition of resistance welding of thermoplastic composites

The evolution of weld displacement, or the thickness of welding stack, with welding time during resistance welding of thermoplastic composites was characterised, and based on this the possibility of using displacement data for process monitoring and processing window definition was investigated. Resistance welding of glass fabric reinforced polyetherimide using a metal mesh as the heating element was studied, and weld displacement was detected using a laser sensor. The effect of welding parameters on the displacement curve was studied. Welding defects, such as voids and squeeze flow, could be detected by monitoring the weld displacement. Fast definition of the welding processing window was found to be possible using displacement curves, and the predicted processing window showed good agreement with the processing window determined from mechanical tests.     

Induced forming modes in a pre-consolidated woven polypropylene composite during stretch forming process at room temperature: I. Experimental studies

In the current study, rectangular specimens of pre-consolidated woven Self-Reinforced Polypropylene (SRPP) possessing different fibre orientations and aspect ratios were stretch formed in an open die. Induced displacements were recorded by an in-situ 3D photogrammetric measurement system. Resultant principal strains were investigated to clarify the role of different deformation modes during stamp forming. The dependency of induced deformation modes to the specimens’ geometries was studied. A novel path/deformation dependent failure criterion was established to distinguish between safe and failed regions of SRPP in a stamping process and to elucidate the dependency between failure and induced forming modes in a woven composite. The experimental results highlighted the suitability of consolidated SRPP to be formed into complex doubly curved geometries by the stamp forming process at room temperature. It was found that required forming depths could be achieved if a proper combination of specimen size, boundary condition, and fibre orientation was selected.     

Fatigue performance characterisation of resistance-welded thermoplastic composites

This study investigates the fatigue performance of resistance-welded thermoplastic composites. Lap shear specimens consisting of unidirectional carbon fibre/poly-ether-imide (CF/PEI), unidirectional carbon fibre/poly-ether-ketone-ketone (CF/PEKK) and 8-harness satin weave fabric glass fibre/poly-ether-imide (GF/PEI) composites were resistance-welded using a metal mesh heating element. The specimens were fatigue-tested at various percentages of their static lap shear strengths at a load ratio R = 0.1 and frequency f = 5 Hz. The fatigue performances of resistance-welded semi-crystalline (PEKK) and amorphous (PEI) composites were compared and the failure modes of the specimens were described. The stiffness degradation was monitored during the tests in order to evaluate the damage accumulation in the specimens. Linear stress-life (S–N) curves were obtained for all three materials when plotted on a semi-log scale. Interlaminar failure modes, involving tearing of the heating element and damage to the adherends were observed. The indefinite fatigue lives of CF/PEKK and CF/PEI welded specimens were obtained at 25% of their static lap shear strengths. The indefinite fatigue life of the GF/PEI welded specimens was obtained at 20% of the static lap shear strength.     

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.     

Development of slate fiber reinforced high density polyethylene composites for injection molding

During the last decade the use of fiber reinforced composite materials has consolidated as an attracting alternative to traditional materials due to an excellent balance between mechanical properties and lightweight. One drawback related to the use of inorganic fibers such as those derived from siliceous materials is the relative low compatibility with conventional organic polymer matrices. Surface treatments with coupling agents and the use of copolymers allow increasing fiber–matrix interactions which has a positive effect on overall properties of composites. In this research work we report the use of slate fiber treated with different coupling agents as reinforcement for high density polyethylene from sugarcane. A silane (propyltrimethoxy silane; PTMS) and a graft copolymer (polyethylene-graft-maleic anhydride; PE-g-MA) were used to improve fiber–matrix interactions on HDPE-slate fiber. The effect of the different compatibilizing systems and slate fiber content were evaluated by scanning electron microscopy (SEM), dynamic thermomechanical analysis (DTMA) as well as mechanical properties (tensile, flexural and impact). The results show that the use of silane coupling agents leads to higher fiber–matrix interactions which has a positive effect on overall mechanical properties. Interesting results are obtained for composites containing 30 wt.% slate fiber previously treated with propyltrimethoxy silane (PTMS) with an increase in tensile and flexural strength of about 16% and 18% respectively.     

Eliminating volatile-induced surface porosity during resin transfer molding of a benzoxazine/epoxy blend

We studied the mechanism of volatile-induced surface porosity formation during the resin transfer molding (RTM) of aerospace composites using a blended benzoxazine/epoxy resin, and identified reduction strategies based on material and processing parameters. First, the influence of viscosity and pressure on resin volatilization were determined. Then, in situ data was collected during molding using a lab-scale RTM system for different cure cycles and catalyst concentrations. Finally, the surface quality of molded samples was evaluated. The results show that surface porosity occurs when cure shrinkage causes a sufficient decrease in cavity pressure prior to resin vitrification. The combination of thermal gradients and rapid gelation can generate large spatial variations in viscosity, rendering the coldest regions of a mold susceptible to porosity formation. However, material and cure cycle modifications can alter the resin cure kinetics, making it possible to delay the pressure drop until higher viscosities are attained to minimize porosity formation.     

A thermo-viscoelastic approach for the characterization and modeling of the bending behavior of thermoplastic composites

Currently, there is no established and cost-effective method for the bending characterization of continuous fiber reinforced thermoplastic composites. Isothermal mechanical testing techniques are time and labor-intensive and deliver information only about distinct points of the temperature-dependent property curves. In this study, Dynamic Mechanical Analysis (DMA) as well as novel rheometer-based bending experiments were performed to assess temperature-dependent and viscoelastic behavior. On the basis of the experimental results a new method was defined and validated for the efficient characterization of temperature-dependent elastic bending behavior via DMA. Furthermore a linear viscoelastic material model was derived from DMA experiments by means of time–temperature superposition. As the material behavior proved to be of a highly viscoelastic nature, a method was developed to calibrate a material model, the parallel rheological framework, implemented in Abaqus.     

Enhanced properties of lignin-based biodegradable polymer composites using injection moulding process

Composites from polybutylene succinate (PBS) and lignin-based natural material were fabricated using a melt mixing process. The effects of lignin material and polymeric methylene diphenyl diisocyanate (PMDI) compatibilizer on the properties of composites were investigated. Incorporation of 65% lignin material into PBS was achieved with an improvement in the tensile and flexural properties of composites. Incorporation of 1% PMDI in 50% lignin filled composites enhanced the tensile, flexural and impact strength simultaneously. Heat deflection temperature (HDT) of the virgin plastic also increased with lignin and PMDI incorporation. Improved interfacial adhesion was observed from SEM micrographs of the compatibilized composites.     

Numerical prediction of saturation in dual scale fibrous reinforcements during Liquid Composite Molding

This paper presents a fractional flow model based on two-phase flow, resin and air, through a porous medium to simulate numerically Liquid Composites Molding (LCM) processes. It allows predicting the formation, transport and compression of voids in the modeling of LCM. The equations are derived by combining Darcy’s law and mass conservation for each phase (resin/air). In the model, the relative permeability and capillary pressure depend on saturation. The resin is incompressible and the air slightly compressible. Introducing some simplifications, the fractional flow model consists of a saturation equation coupled with a pressure/velocity equation including the effects of air solubility and compressibility. The introduction of air compressibility in the pressure equation allows for the numerical prediction of the experimental behavior at low constant resin injection flow rate. A good agreement was obtained between the numerical prediction of saturation in a glass fiber reinforcement and the experimental observations during the filling of a test mold by Resin Transfer Molding (RTM).     

Electrical properties of short sisal fiber reinforced polyester composites fabricated by resin transfer molding

The electrical properties of sisal fiber reinforced polyester composites fabricated by resin transfer molding (RTM) have been studied with special reference to fiber loading, frequency and temperature. The dielectric constant (ε′), loss factor (ε″), dissipation factor (tan δ) and conductivity increases with fiber content for the entire range of frequencies. The values are high for the composites having fiber content of 50 vol.%. This increment is high at low frequencies, low at medium frequencies, and very small at high frequencies. The volume resistivity varies with fiber loading at lower frequency and merges together at higher frequency. When temperature increases the dielectric constant values increases followed by a decrease after the glass transition temperature. This variation depends upon the fiber content. Finally an attempt is made to correlate the experimental value of the dielectric constant with theoretical predictions.     

Resin transfer moulding of anionically polymerised polyamide 12

Liquid composite moulding of Lactam 12 monomer and activating system (APLC12) into satin weave carbon fabrics is investigated, with emphasis on minimising the void content in the final part. The main sources for void formation are identified. The solidification shrinkage is quantified to account for at most 9% in the matrix. Optimal flow conditions are determined to minimize void content during liquid moulding. Finally, as the monomer is kept under Nitrogen prior to processing, diffusion and solubility of Nitrogen in the monomer are characterized, to indicate that Nitrogen coalescence during injection is a major cause of voids in the final part. The average void content is reduced from initially 15% to below 1% in polyamide 12 based composite plates with optimised process conditions.     

Shear characterisation of uni-directional fibre reinforced thermoplastic melts by means of torsion

Intra-ply shear appears during the forming process of hot thermoplastic laminates with a uni-directional fibre reinforcement. This paper proposes a torsion bar test to characterise the longitudinal shear mechanism, which can be performed with a standard rheometer. Sensitivity analyses showed that most reliable shear property measurements can be obtained by using torsion bar specimens with a close to square cross section. The method is implemented in practise and critically evaluated. Storage and loss moduli were determined for carbon UD/PEEK specimens at high temperatures. Non-linear material behaviour was found for relatively small shear strains. The linear regime was focussed on subsequently, where the characteristics were found to be similar to that of a visco-elastic solid or weak gel, confirmed by a dominant storage modulus and a weak frequency dependency. Future work is recommended to be focussed on the large strain regime, for which this paper provides a found basis.