Thermoplastic Matrix Composites Research Articles

Damage and fracture mechanisms in thermoplastic-matrix composites in relation to processing and structural parameters

The aim of this work is the study of damage and microstructural fracture mechanisms in injectionmoulded polypropylene/short-glass-fibre composites. We have also tried to elucidate the relationships between these mechanisms and some technological parameters such as: respective compositions of the matrix and of the size on glass fibres; injection conditions such as screw and barrel parameters, mould temperature and design; microstructures existing in these injection-moulded composites; and the testing conditions, i.e. speed and temperature.

Shear controlled orientation technology for the management of reinforcing fibres in moulded and extruded composite materials

The controlled application of macroscopic shears to solidifying melts containing alignable constituents provides for the management of fibres in, and the production of components to close tolerances. Shear controlled orientation technology (SCORTEC) may be applied to the thermoplastic and thermoset matrix composites, and to ceramic and metal matrix composites processed with the aid of sacrificial polymer binders. The principles of SCORTEC as applied to moulding and extrusion processes will be described, together with examples of typical property enhancement possible, and potential fields of application.

5471367 Compoosite structure for heat transfer and radiation

Key static mechanical properties and laminating processes of thermoplastic matrix continuous filament composites reinforced with Kevlar® aramid or graphite fibers are presented and discussed. A tow of Kevlar or graphite impregnated with a thermoplastic matrix from a proprietary melt-coating process involving injection of a melted thermoplastic polymer can be consolidated into a sound, void-free composite laminate by compression molding. The unidirectional composites of Kevlar 49 or graphite from the melt-coated tows possess mechanical properties superior to those of similar composites prepared by other processes such as yarn cowinding and film laminating. The unidirectional composites of Kevlar 49 with thermoplastic matrices prepared from the melt-coated tows are equivalent or superior to those using Epon® epoxies or polyvinylester in flexural, shear and compressive strengths. Using J-polymer, a polyamide copolymer and a proprietary Du Pont resin candidate as a thermoplastic matrix model, a static mechanical test data base has been developed for graphite/thermoplastic matrix composites showing significant advantages in damage tolerance, compression after impact, and interlaminar fracture toughness. Although the unidirectional compressive strength of graphite/epoxy composites has been shown to correlate with matrix modulus, the thermoplastic matrix composites show no such correlation.

An Overview of Recycling Issues for Composite Materials

A review of the major issues surrounding the postuse disposal of polymer composite materials is presented. Composites are compared to unreinforced plastics in terms of production volume and the technologies available for reprocessing. Not surprisingly, composites comprise a very small fraction of the plastics waste stream. In highvolume applications such as automobiles, however, there are reasons to be concerned about composites recycling. The fundamental categories of dealing with waste (burning, burying, reusing, or using less in the first place) are described in relation to plastics and composites. Reprocessing technologies with applications for composites are reviewed. For thermosetmatrix composites, processes involving regrinding waste materials and using them as filler for other polymer processes show promise. For thermoplasticmatrix composites, processes involving grinding and remolding by injection or compression molding are promising. Applications for recycled composites, without which reprocessing is futile, are also reviewed.

Processing of Thermoplastic Composites Using a Powder Slurry Technique. I. Impregnation and Preheating

Abstract The success of thermoplastic matrix composites depends upon the development of economical methods of impregnation. The purpose of this work was to develop and understand an economical impregnation procedure using a slurry based powder technology. An impregnation and preheating line consisting of a fiber tensioner, a slurry bath, a drying heater, a coating heater, and a fiber winder was used to make resin coated fibers. The influence of the process parameters on the impregnation, preheating, coating, and consolidation were studied. Part I of this paper presents the experimental investigation of the impregnation and preheating stages of the process together with a preheating model. Luikov's coupled equations of heat and mass transfer were used to model the heating and drying of the tow in the preheater. The predictions of the preheating and drying model are compared to the experimentally obtained results.

Electrical conduction in thermoplastic elastomer matrix composites containing catalytic chemical vapour deposited carbon whisker

Electrical conduction in carbon whisker/thermoplastic elastomer (TPE) composites was found to be a thermally activated process. The carbon whiskers used were obtained by a catalytic chemical vapour deposition (CCVD) technique conducted at 500 °C and the TPE was a styrene-ethylene butylene-styrene (S-EB-S) block copolymer. The resistivity, (ϱ), versus 1/T curves of the composites exhibited two regions with distinct slopes with an inflection at the glass transition temperature,Tg, of the elastomer, EB; Region I Tg. The thermally activated conduction mechanism of these composites is explained on the basis of electron transport in low-mobility solids with a large number of trap sites, Intra- and intermolecular motion of the polymer chains can result in the electron transport from such trap sites and were correlated to the observed activation energies. Intramolecular motion in region I, was related to the thermally assisted hopping with the activation energy, ΔEA, of 0.067 and 0.030 eV for 33% and 52% whisker volume fraction composites, respectively. Similarly, ΔEA due to the intermolecular segmental chain motions in region II for 33% and 52% whisker volume fraction composites was related to the equilibrium rate of trapping-detrapping of electrons from 0.240 and 0.138 eV deep traps.

Reaction injection pultrusion of thermoplastic and thermoset composites

AbstractCoventional pultrusion of thermoset composites is under increasing examination for emissions of harmful volatiles from the resin wetout tank. Even though the pultrusion of thermoplastic matrix composites produces no emissions, it is difficult to wet individual fibers due to their high melt viscosities. This paper addresses both the issues of volatiles and wetting with a process called Reaction Injection Pultrusion (RIP). A prototype RIP machine was used to make both thermoplastic polyurethane and thermoset polyisocyanurate matrix composites. The RIP process produces pultruded parts with low void content, good surface finish, and acceptable mechanical properties. The low viscosity constituents used in RIP help improve fiber impregnation, while the small volume of the impregnation bath reduces emissions. Processing parameters such as line speeds, catalyst levels, and die temperaures were varied to establish processing guidelines for sustained production.

Thermochemical and Mechanical Aspects of Composite Tape Laying

The overall objective of this investigation was to study the manufacture of organic matrix composites by the tape laying process. Specifically, the effects of the processing conditions on the quality of composite parts were investigated. Models were developed which relate the applied heating, applied pressure, tape laying speed, and mold material to the thermochemical and mechanical response of the material. On the basis of these models, computer codes were written to generate numerical results. With these computer codes, parametric studies were performed which indicated the influence of the various processing parameters on the temperature, degree of cure (for thermosetting matrix composites), crystallinity (for thermoplastic matrix composites), viscosity, and stresses inside the composite. From the numerical results, the processing conditions required for manufacturing good quality parts were identified.

A Model for the Consolidation of Aligned Thermoplastic Powder Impregnated Composites

This paper describes the role of surface energy effects, externally applied pressure, resin flow and fiber bed elasticity on the consolidation of thermoplastic-matrix composites manufactured by the powder impregnation route. Surface energy effects in the spreading of polymer droplets on fiber surfaces are discussed; then a model for the consolidation process is developed, relating the variables mentioned above. Consolidation experiments on powder-impregnated composites of the FIT type (Fibres Impregnees de Thermoplastique) were carried out using a mold attached to a servo-hydraulic testing machine. The model accurately predicts variations in void content during consolidation of carbon fiber/PEEK (CF/PEEK) and carbon fiber/PEI (CF/PEI) laminates. It was found that, at the pressures needed to achieve rapid consolidation, surface energy has a negligible influence on impregnation rate, but its effects on the void topology can be considerable. It was also shown that, when laminates of low void content are required, a minimum pressure is needed to overcome the effect of fiber bed elasticity.

Surface modification of ultra-high molecular weight polyethylene fibers by γ-radiation-induced grafting

A technique for grafting acrylic polymers on the surface of ultra-high molecular weight polyethylene (UHMWPE) fibers utilizing 60Co gamma radiation at low dose rates and low total dose has been developed. Unlike some of the more prevalent surface modification schemes, this technique achieves surface grafting with complete retention of the exceptional UHMWPE fiber mechanical properties. In particular, poly(butyl acrylate) and poly(cyclohexyl methacrylate) were successfully grafted onto UHMWPE fibers with no loss in tensile properties. The surface and tensile properties of the fibers were evaluated using Fourier transform infrared/photoacoustic spectroscopy (FTIR/PAS), X-ray photoelectron spectroscopy (XPS), and tensile tests. The reinforcement efficiency of untreated, polymer-grafted, and plasma-treated UHMWPE fibers in polystyrene and a poly(styrene-co-butyl acrylate-co-cyclohexyl methacrylate) statistical terpolymer was characterized using mechanical tensile tests. The thermoplastic matrix composites wer...

Viscoelastic effects on the interlaminar fracture behaviour of thermoplastic matrix composites: I. Rate and temperature dependence in unidirectional PEI/carbon-fibre laminates

The influence of the dependence of matrix toughness on deformation rate and temperature on the interlaminar fracture of polyetherimide (PEI)/carbon-fibre unidirectional laminates has been investigated. Fracture toughness versus crack speed data for the matrix and the composite, respectively, were determined by using the double torsion and the double cantilever beam tests. Tensile tests were also performed on the unreinforced resin. Displacement rates varying between 0.1 and 100 mm min were applied, and testing temperatures were varied between 23 and 170 °C. Fracture data reduction was achieved by means of time-temperature superposition, the shift factors being obtained from tensile tests. The toughness of PEI, by contrast with that of other thermoplastic polymers commonly used as matrices for composite materials (e.g. PEEK), showed a non-monotonic, but still moderate, sensitivity to the test conditions explored here. Interlaminar toughness of the corresponding composite increased steadily with crack speed and was always lower than that of the plain resin. This result indicates that the interlaminar structure of the composite material is not optimised as regards this particular property.

The Effects and Non-Destructive Evaluation of Defects in Thermoplastic Compression-Loaded Composite Cylinders

Measurements and predictions of the effects of voids and ply waviness on compression and interlaminar shear properties of thermoplasticmatrix composites are presented. Correlations between ultrasonic Cscan loss to voids and to compression strain-to-failure are described. The data are drawn from a database of seventy-six cylinders and cylinder sections fabricated by Du Pont Advanced Material Systems since 1988 using an insitu consolidation filament winding and tape laydown process. Composites with graphite (AS-4 and IM-7) or S-2 glass fibers in polyetheretherketone (PEEK) or polyetherketone ketone (PEKK) thermoplastic matrices are included. Property measurements are from various mechanical and external hydrostatic compression pressure tests on cylindrical shells intended for marine submersible applications and on autoclave or compressionmolded flat panels. Performance data show the negative impact of voids and ply waviness on mechanical properties and cylinder performance. NDE data show a correlation between ultrasonic loss and void content only over a broad void content range; over the important 0% to 2.0% range, no correlation is observable. Data directly relating C-scan loss with compression strain-to-failure show a continuous loss of performance with increasing void content.

Adhesive bonding of fibre-reinforced composites

The applicability of test methods for adhesive-bonded metal adherends to fibre-reinforced composites is considered. Bond strengths obtained by methods chosen as suitable are compared. The most critical factor in metal joints is the pretreatment of the adherend before bonding, as this controls the structure and chemistry of the oxide surface and hence durability in hot-humid conditions, often with little effect on initial strengths. For fibre-reinforced epoxy resin adherends, the critical factor is the initial bond. Initial bond strength, measured by any method, is related to the presence of contaminants on the adherend surface. Loss of strength in hot-humid conditions is related to water absorption by adhesive and matrix and not to reduced adhesion. Thermoplastic-matrix composites require pretreatments which increase their surface energy and wettability by adhesives.

Electrochemical process for preparing continuous graphite fibre-thermoplastic composites

An electrochemical process for preparing thermoplastic matrix-graphite fibre composites has been developed. Graphite fibre-poly(maleimide- co-styrene) matrix composites were prepared using unsized AS-4 graphite fibre preforms as the working electrode. Copolymers of a wide range of N-substituted maleimides and styrene were formed directly on the surface of the graphite fibres. The rate of electrocopolymerization varied with the electrocopolymerization parameters, such as the comonomer concentration, current density, supporting electrolyte concentration, electropolymerization time, nature of the solvent, solvent/dilute sulfuric acid ratio and the structure of the maleimide. Graphite fibre preforms preimpregnated with the electropolymerized thermoplastic matrices were processed into unidirectional composite laminates. The electrosynthesized thermoplastic matrix composites showed excellent physical and mechanical properties.

3-D stamp forming of thermoplastic matrix composites

In this investigation a mould with hemispherical cavity and 80 kN hydraulic press, allowing variable stamping speeds, are employed for experimentally studying of the 3-D stamp forming process of continuous fiber reinforced thermoplastic laminates. In particular, glass fiber (GF) reinforced polyetherimide (PEI) woven fabric made of sheath surrounded, polymer powder impregnated fiber bundles manufactured by Enichem, Italy, is used. Pre-consolidated laminates are heated by contact heating in an external heater up to about 120°C above the glass transition temperature (Tg) of the polymer matrix; they are then stamp formed in a cold matched metal tool. Typical cycle times (including preheating time of the preconsolidated laminates) are in the range of 3 min. Useful processing conditions, such as stamping temperature, stamping velocity and hold-down pressure required for stamp forming of this composite are determined. In addition the effect of die geometries (deformation radian) and original laminate dimensions are studied. The results describe the correlations between processing parameters and fiber buckling. Finally the thickness distribution in stamped parts are investigated in relation to different directions of fiber orientation.

Short- and long-term degradation of polymer-based composites

The thermal stability of high-performance thermoplastic matrix composites is studied by thermogravimetric analysis. The study is focused on the degradation behavior, under different environments, of continuous carbon-fiber-reinforced composites with poly(ether-ether ketone) (PEEK) matrix. The experimental results are analyzed by two different methods. In one method a phenomenological kinetic model is developed and compared with the degradation behavior measured in isothermal and constant heating conditions. In the second method, a simple correlation is presented to describe the onset of the degradation at long times and under normal service conditions. Both approaches give similar results and confirm the good thermal stability of PEEK matrix/carbon fiber composites at high service temperatures.

Evaluation of fiber surfaces treatment and sizing on the shear and transverse tensile strengths of carbon fiber‐reinforced thermoset and thermoplastic matrix composites

AbstractThe mechanical performance of advanced composite materials depends to a large extent on the adhesion between the fiber and matrix. This is especially true for maximizing the strength of unidirectional composites in off‐axis directions. The materials of interest in this study were PAN‐based carbon fibers (XA and A4) used in combination with a thermoset (EPON 828 epoxy) and a thermoplastic (liquid crystal poymer) matrix. The effect of surface treatment and sizing were evaluated by measuring the short‐beam shear (SBS) and transverse flexural (TF) tensile strengths of unidirectional composites. Results indicated that fiber surface treatment improves the shear and trasverse tensile strengths for both thermosetting and thermoplastic matrix/carbon fiber‐reinforced unidirectional composites. A small additional improvement in strengths was observed as the result of sizing treated fibers for the epoxy composites. Scanning electron microscope photomicrographs were used to determine the location of composite failure, relative to the fiber‐matrix interface. Finally, the epoxy composites SBS and TF strengths appear to be limited to the maximum transeverse tensile strength of the epoxy matrix, while the thermoplastic composite SBS and TF strengths are limited by the LCP matrix shear and transverse tensile strengths, respectively.

Interlaminar Fatigue Crack Propagation in Mode I of Carbon Fiber/PEEK Composites

A study of the mode I interlaminar fracture toughness of a unidirectional carbon fibre reinforced thermoplastic matrix composite has been made using Double Cantilever Beam, DCB, specimens. Delamination growth per fatigue cycle, da/dN, was related with the maximum applied cyclic strain energy release rate, GIMAX, using a power law.

Modeling of the dynamic mechanical properties of semicrystalline thermoplastic matrix composites

AbstractThe modeling of the viscoelastic properties of semicrystalline polymeric matrix composites is discussed. Different mathematical models are applied to describe the development of the storage modulus, measured by dynamic mechanical analysis, as a function of the crystalline content during isothermal crystallization experiments. Best results are obtained with an empirical power law model developed in this work. The application of the Halpin‐Tsai equation and of a theoretical model, based on a combination of parallel‐series arrangements of viscoelastic elements representing the crystalline and amorphous phases, is also discussed. The main objective of this research is the comparison between experimental results obtained by differential scanning calorimetry and dynamic mechanical analysis during crystallization processes.

A New Approach to the Forming of Thermoplastic-Matrix Continuous-Fiber Composites — Part 2: Experiments and Model

"Die-less forming" is a new concept, intended to manufacture long compo nents of continuous-fiber thermoplastic-matrix composites. The concepts behind die-less forming have been tested in experiments on a two-roller demonstration machine. The experiments focused on finding the effects of important forming parameters on the shape. Laminates with special lay-up were also made and formed to study the effect of lay-up on shape. Quasi-isotropic laminates showed an unexpected "excess bending." An elastic/visco- plastic model has been developed to explain and predict this excess bending during die- less forming. The model is rate dependent and captures the shape dependency on strain rate, transverse feed magnitude and the lay-up. The model is based on the application of the principle of virtual displacements. The model predicts the net bend angle well for quasi-isotropic laminates.