Processing Of Thermoplastic Composites Research Articles

A fractal Cantor set based description of interlaminar contact evolution during thermoplastic composites processing

Fusion bonding is the principal phenomenon governing the fabrication of layered thermoplastic-matrix composites using techniques such as tow placement and tape laying. An important step in fusion bonding is the intimate contact process, which refers to the development of interfacial contact area between thermoplastic plies, caused by the spreading of the surface asperities. The contacted areas, in turn, form sites for molecular interdiffusion which leads to interfacial bonding. The geometric complexity of the surface asperity profiles presents a fundamental challenge to an effective description of the intimate contact process. Toward addressing the challenge, this paper presents a model for interfacial contact area development utilizing the fractal properties of thermoplastic ply surfaces. Using a Cantor set representation of the surfaces, and a squeeze flow model to describe their spreading during the processing, the interfacial contact area evolution is related to the process parameters and the geometric parameters in the fractal surface description. The model predictions are shown to compare well with experimental data for several thermoplastic materials.

Fractal Description of Interlaminar Contact Development during Thermoplastic Composites Processing

Fractal Description of Interlaminar Contact Development during Thermoplastic Composites Processing

The Influence of Structure of Hybrid Yarns Glass/PA6 on Mechanical Properties of Composites

The fabrication process of thermoplastic composites using hybrid fabrics composed of yarns made of glass and PA6 fibres as prepregs is presented. The influence of the structure of hybrid yarns produced using commingled, texturising, twisting and friction spinning technology on the mechanical properties of composites is discussed. The results of the investigation indicate that the best tensile and bending properties of composites were achieved using fabrics made from texturing yarns produced under optimal conditions. These optimal conditions should ensure the minimum destruction of filaments during the blending process.

Variable frequency microwave processing of thermoplastic composites

The range of applications for variable frequency microwave (VFM) facilities (2–18 GHz) has been extended to thermoplastic composites. Five thermoplastic polymer matrix composites are processed and discussed, including 33 wt-% random carbon fibre reinforced polystyrene [PS–CF (33%)], and low density polyethylene [LDPE–CF (33%)]; 33 wt-% random glass fibre reinforced polystyrene [PS–GF (33%)], low density polyethylene [LDPE–GF (33%)]and Nylon 66 [Nylon 66–GF (33%)]. Bond strengths of lap joints were tested in shear and results were compared with those obtained using fixed frequency (2·45 GHz) microwave processing. The primer or coupling agent used was a 5 min, two part adhesive containing 100%liquid epoxy and 8% amine, which was more readily microwave reactive than the composites themselves. The VFM was operated under software control, which provided automatic data logging facilities. Results indicate that VFM can produce strong bonds for PS and LDPE.

A THERMOELASTIC MODEL FOR IN-SITU CONSOLIDATION OF THERMOPLASTIC COMPOSITES

A thermoelastic model for the calculation of stresses induced during the in-situ consolidation process for thermoplastic composites is developed. The model is composed of two infinitely long layers of orthotropic material moving at a constant velocity through a concentrated heat source. The two layers are continuously joined along the semi-infinite surface downstream of the consolidation point. The model is developed in three stages, consisting first of a thermoelastic particular solution and an isothermal complementary solution for each layer and finally a solution of the mixed boundary value problem defined by the contact between the two layers. The sum of the particular and complementary solutions satisfies the equilibrium and prescribed boundary conditions for each layer. The conditions of partial contact between the layers result in the existence of unknown tractions along the contact surface. An approximate numerical procedure for the solution of these tractions is outlined. Numerical examples are given that illustrate the results for a two-layer thermoelastic static contact case and for a two-layer quasi-static case simulating the thermal and transport effects of a laserassisted in-situ consolidation process.

Modeling and optimization of a thermoplastic composites stamping process

The potential for short process cycle times is one of the most important characteristics of thermoplastics, leading to rapid, low-cost production of composite structures. One area where thermoplastic composites processing may have the most potential is in high rate stamping. The challenge lies in developing the process to the extent that it exceeds the performance and reliability of its counterpart, steel stamping. To achieve this objective, a better understanding of the process is necessary. In this work, experimental investigations are conducted to determine the effects of composite stamping processing variables on part output which includes tensile strength and modulus. Results of experimentation are used to develop a process model for composite stamping via a supervised learning back propagation neural network.

Integrated processing of thermoplastic composites

A novel flexible and cost-effective processing concept for composites and polymers has been developed in order to open new market opportunities for thermoplastic materials. The integration of polymer and composite preforms is performed during a single operation in a machine equipped with different processing techniques. Interfacial properties are optimised by using non-isothermal bonding conditions and by promoting fibre bridging between the adherends of an integrated component. Consequently, a final component can offer complex shapes, optimum distribution of mechanical properties, and integration of performance and functions.

Processing models and characterization of thermoplastic composite wound parts

AbstractSqueezing flow was adopted to model the processing of thermoplastic composites in filament winding. It is found that the resin flow is facilitated as a result of the slip condition, thus reducing the process times. An apparatus with a constant dead weight was employed to characterize the rheology of the thermoplastic composite in filament winding. The system is carbon fiber (AS4) reinforced with the polyetherketone‐ketone (PEKK) thermoplastic matrix. It is found that resin flow is dominant in the transverse fiber direction and there is insignificant flow along the fiber direction. An empirical equation of viscosity for AS4/PEKK as a function of temperature was obtained based on the model. For mechanical properties, either in a 4‐point bending test or a short beam shear test, most of the hoop wound rings failed at the inside radius or on the compression side. Insufficiently applied heat and a cold mandrel in the beginning of the winding process may result in low degree of consolidation on the compression side. The average void content of the hoop wound ring is lower than that of the preimpregnated tow because of void compression through resin flow during the winding process.

Rheology of Layered Thermoplastic Matrix Composites during Compression Molding

Abstract In view of optimizing the industrial compression molding process of thermoplastic composites, the rheological and microstructural behavior of a polypropylene/glass fiber composite is investigated in model squeeze-flow geometries. The overall stress / strain behavior of the material at various compression rates is recorded and the induced orientation of the fibers is investigated by means of a special electron microscopic characterization method. By contrast to pure polypropylene, it is shown that in the high speed range, the macroscopic flow process is controlled by both the viscous extension and the relative sliding of parallel fibrous layers, the latter becoming unstable when the flow undergoes a rapid transition from divergent to convergent. Under high pressure, voids are dissolved in the polymer melt. In the case of non-isothermal compression, the rheology of the composite is not significantly affected by the cooling of the surface layers. The multilayer plug-flow model based on the sliding mechanism of viscous layers is found to reproduce correctly the experimental stress/strain behavior.

Thermal simulation of quartz tube infra-red heaters used in the processing of thermoplastic composites

The use of infra-red heaters in the processing of thermoplastic composites is increasing due to the potential for quick process cycle times. Accurate modelling of the heating characteristics will help processors predict heat-up times for their processes and avoid the disadvantages associated with infra-red heating. This paper deals with the modelling of quartz tube infra-red heaters, taking into account both the characteristics of the emitted radiation and the absorption properties of the materials involved. Comparison of the model and experimental results shows that the model successfully predicts the heat-up rate and temperature distribution though different materials when heated using quartz tube infra-red heaters.

Electric field modeling for electrostatic powder coating of a continuous fiber bundle

The application of electrostatic powder coating technology to the processing of thermoplastic composites is discussed. An electrostatic powder spray gun is used to deposit polymer powder on a continuously moving glass fiber bundle. The glass fiber bundle is locally spread in the region of powder deposition to expose the filaments. A three-dimensional finite element model is presented for predicting the electric field strength due to the applied corona electrode. The model provides the spatial variation of the electric field vector for the specific spray process geometry. Corona voltage, corona electrode height, fiber bundle spreading, and space charge density were varied in the model to predict their effect on the electric field strength. The finite element results display the “wrap-around” effect of the electric field to the bottom surface of the fibers. Experimental results substantiate the electric field distribution along the length of the glass fiber bundle.

Application of Taguchi method for process enhancement of on-line consolidation technique

During laser assisted processing of thermoplastic composites, the individual effects of laser power, tape speed and consolidation pressure on the interply bond strength cannot be easily separated unless a large number of experiments is carried out. Because the properties of an end product depend upon the selection of processing conditions, it is essential that the conditions under which a well consolidated part is obtained should be defined. To investigate the effects of individual process parameters from a minimum number of tests, the Taguchi method is applied. For this investigation, PEEK/carbon fibre (APC-2) thermoplastic composite rings were manufactured by a thermoplastic tape winding process at selected conditions, and the influence of dominant process parameters on interply bond strength was evaluated. The influence of three dominant factors, namely laser power, consolidation pressure and tape speed, on bond strength were investigated. The experimental design involved using L 9 orthogonal arrays, and the percentage contribution of each factor to the quality of bonding was estimated by the ANOVA technique.

Reduction of infra-red heating cycle time in processing of thermoplastic composites using computer modelling

Abstract This paper deals with increasing the speed of the infra-red (IR) heating cycle in the processing of thermoplastic composites. A constraint on the heating process is that all parts of the material must be within the recommended processing temperature range before forming can start. A mathematical model is used to predict the transient temperature distribution through the thickness of flat consolidated panels of continuous carbon fibre-reinforced poly(ether ether ketone) (APC-2) during heating. The model includes (i) natural convection, (ii) medium and long wave radiation and (iii) one-dimensional conduction through the material. Experimental validation of the model is conducted using an IR test rig. The following process parameters were varied to obtain optimum process conditions: (i) heater power: (ii) heater-to-composite distance: (iii) composite thickness; (iv) degree of oversizing of heater area compared with surface area of composite: and (v) one- or two-sided heating. Results presented show that reduction of the heater-to-composite distance from 100 to 50mm increases the steady-state temperature of the composite by 88%, whereas almost doubling the heater power density from 25.6 to 47.3 kWm 2 - increases the composite temperature by only 17%. Using one-sided heating, experimental results show that upward-facing heaters produce a more even temperature distribution across a panel surface than downward-facing heaters. Model results showing 1 R heating times for composite panels of thickness 0.5 to 9.5 mm are also presented.

Processing of thermoplastic composites from powder/sheath-fibre bundles

The relationship between impregnation process, consolidation quality and resulting mechanical properties of GF/PP and CF/PEEK thermoplastic composites manufactured from powder/sheath-fibre bundles have been investigated. A small processing mould was used to simulate the different processing conditions (i.e. applied pressure, holding time and processing temperature). The consolidation quality of finished samples were characterized mainly by microscopic studies of material structure, density measurements and characterisation of mechanical properties through a small transverse flexure testing facility. A model for qualitatively describing the consolidation process of powder/sheath-fibre bundles was developed, which predicts variations of void content during consolidation as well as the time and pressure required to reach full consolidation. Good correlations between predictions and the experimental data indicate the success of the approach. Based on a desired, minimum level of void content (e.g. XV=5% for GF/PP and XV=2% for CF/PEEK composites), optimum processing windows for manufacturing of composite parts from powder/sheath-fibre preforms are suggested.

Effects of Bundle geometry of Fit® Material Pre-Forms on Consolidation process of Thermoplastic Composites

Effects of variations in bundle geometry on consolidation behavior of FIT® material pre-forms are discussed. The results for CF/PEEK composites indicate that increasing in the shape-ratio of the FIT® material is one way to improve the impregnation process and consolidation quality in manufacturing thermoplastic composite parts.

Micromechanics and acoustic emission analysis of the failure process of thermoplastic composites

The finite element method (FEM) and acoustic emission technique (AE) were applied to the micromechanics analysis of the failure process of composites with thermoplastic matrix materials. FEM calculations to local stress-strain distribution and the influence of very different intermediate layer properties are interpreted with regard to microscopic failure mechanisms in composite materials. The strongly differing AE behaviour of both chalk-filled Polyvinylchloride and high density polyethylene and short-glass-fibre reinforced polypropylene, polyamide, PBTP, SAN and ABS in tensile test experiments is demonstrated. Representative loading limits are derived from the nature and extent of the dominating failure mechanisms by comparison of theoretical and experimental results. The influence of critical strain, shear strength and fracture toughness properties of the modified matrix as well as the composite morphology and phase adhesion on significant deformation and failure stages is discussed. Finally some conclusions are drawn about a possible critical long-term strain of composites.

Issues in diaphragm forming of continuous fiber reinforced thermoplastic composites

AbstractA literature survey is presented on sheet‐forming methods and mechanisms for continuous fiber reinforced thermoplastics. The diaphragm forming process is shown to be one of the more promising fabrication routes for complex‐curvature structures. The primary deformation mechanisms involved in the sheet‐forming processes are identified and discussed. Earlier approaches to develop mathematical models either have been kinematically based or have treated only one of the primary deformation mechanisms. A promising constitutive model for the highly anisotropic behavior of the composite at forming temperature is examined. The composite sheet is assumed to behave as a transversely isotropic Newtonian fluid that is both incompressible and inextensible in the fiber direction. The second section of the paper treats the experimental development of the polymeric diaphragm forming process for thermoplastic composites. The viscoelastic properties of the diaphragm material are characterized by dynamic mechanical analysis. The rate sensitivity of the phenomenon of shear‐buckling during forming of certain cross‐ply and quasi‐isotropic composite laminates is investigated, using a shallow female mold. Finally, the interface condition between the diaphragm and the composite during forming is examined.

The Effect of Thermoplastic Composite Processing on the Performance of Embedded Nitinol Actuators

Intelligent materials are a class of material systems currently being devel oped for numerous applications. They usually consist of a composite or hybrid material system with fibrous or distributed actuators, various sensors and control modules The nitinol or shape memory alloy wire actuator is currently being developed for use in intelli gent material systems. In order for the nitinol actuator to be a reliable part of the adaptive hybrid system, the effect of the composite manufacturing on the nitinol performance must be determined. The results of a study investigating the effects of a "high temperature" ther moplastic composite processing cycle on the nitinol actuator are presented. The actuators were exposed to a number of thermoplastic processing cycle simulations at 400°C. Criti cal parameters, such as processing time, training of the actuators, and type of nitinol actu ators, were varied to determine their effect on the performance of the nitinol actuator. With processing times from 5 to 20 minutes, the nitinol actuator demonstrated a useable recovery stress (σ ur) between 173-265 MPa. The σ ur of unprocessed actuators, thermoset processed actuators and actuators embedded in an actual APC-2 composite material are compared to the data from the simulation tests. The performance of the embedded nitinol actuators agree well with the simulation.

Laps and gaps in thermoplastic composites processing

During tape lay-up, laps and gaps result from machine and human inaccuracies, as well as the inability of the tape to conform to complex geometries. This paper examines whether these flaws can be removed through processing by transverse flow during consolidation. Treating unidirectional thermoplastic composites as macroscopically homogeneous materials, it is demonstrated that transverse flow can be described by the equations governing the flow of a power law fluid. This theory is then applied to gaps (with good agreement), and laps (with poor agreement). The differences between theory and experiments are explained, and it is shown that under correct conditions, laps and gaps, an order of magnitude larger than what is currently accepted, can be removed through processing.

Solidification in anisotropic thermoplastic composites

AbstractHeat transfer analysis in thermoplastic composites manufacturing offers several challenges. The melting/solidification process results in the appearance of a molving solid‐fluid interface or two‐phase zone. The presence of reinforcing fibers with thermal conductivities substantially different than those of matrix meterials causes the composite material to behave anisotropically. Furthermore, an analysis should be adaptable to complex geometries if it is to be useful for practical applications. In this paper, a numerical approach is presented to develop an analysis tool for the processing of thermoplastic‐matrix composites. A numerical grid generation technique is employed to determine the temperature distribution within the part while accounting for the heat absorption and liberation during the solidification stage. The influence of anisotropy in the domain is accounted for by considering thermal conductivity as a second‐order tensor which varies continuously throughout the domain. Sample part configurations are used to demonstrate the applicability of this technique to thermoplastic composites processing.