Meng Hou

Advances in fusion bonding techniques for joining thermoplastic matrix composites: a review

Abstract Joining composite materials is an issue because traditional joining technologies are not directly transferable to composite structures. Fusion bonding and the use of thermoplastic films as hot melt adhesives offer an alternative to mechanical fastening and thermosetting adhesive bonding. Fusion bonding technology which originated from the thermoplastic polymer industry has gain a new interest with the introduction of thermoplastic matrix composites (TPC) which are currently regarded as candidates for primary structures. The improvement of thermoplastic polymer matrices, with the introduction of recent chemistries such as PEEK, PEI and PEKEKK. exhibiting increased mechanical performance, service temperature and solvent resistance (for the semi-crystalline systems) also supported the growth of interest for fusion bonding. This review looks at the state of the art of fusion bonding technology and focuses particularly on the three most promising fusion bonding techniques: ultrasonic welding, induction welding and resistance welding. Physical mechanisms involved in the fusion bonding process for modelling purposes are discussed including heat transfer, consolidation and crystallinity aspects. Finally, the application of fusion bonding to joining dissimilar materials, namely thermosetting composites (TSC)/TPC and metal/TPC joints, is reviewed.

Experimental investigation of the resistance welding for thermoplastic-matrix composites. Part I: heating element and heat transfer

A comprehensive experimental investigation of the resistance welding of carbon-fibre and glass-fibre reinforced polyetherimide (PEI) laminates is presented. Lap-shear and double-cantilever-beam specimens were resistance welded by using fabric and unidirectional heating elements. The statistical distribution of the resistance of the heating elements was characterised, and the effects of the temperature on the heating element resistance were evaluated. The influence of the mechanical contact pressure on the contact resistance was also investigated. One of the main process parameters in resistance welding, i.e. the power density, was studied in detail. Heating uniformity in the heating elements was assessed through thermal imaging, allowing for a comparison between fabric and unidirectional heating elements. Temperature histories were measured and compared to those simulated by a three-dimensional transient heat transfer finite-element model. Factors limiting the size of the welded joint, i.e. temperature non-uniformity within the welding area and current leaking to the laminate, were investigated. Proper electrical insulation using a glass-fibre/PEI interlayer between the heating element and the laminate when joining carbon-fibre/PEI laminates effectively eliminated current leaking and enabled large-scale resistance welding.

An Experimental Study of Resistance Welding of Carbon Fibre Fabric Reinforced Polyetherimide (CF Fabric/PEI) Composite Material

This paper presents an experimental investigation of resistance welding of carbon fibre fabric reinforced polyetherimide composites (CF fabric/PEI). The heating elements were made from a single CF fabric/PEI prepreg sheet with neat PEI films co-moulded to both surfaces. Welding was conducted on a welding area of 25 mm × 12 mm. The quality of the welded specimens was studied using a non-destructive evaluation technique (C-scan) as well as a mechanical test (lap shear strength). The relationships between input power, input energy, welding time and consolidation pressure were studied. Experimental results indicated that sufficient joining was obtained at a power level from 80 to 160 kW/m2, under an initial welding pressure of 0.15 to 0.40 MPa. The maximum lap shear strength achieved through resistance welding was equivalent to that of the compression moulded benchmark. The fracture surfaces of welded specimens showed mostly cohesive-failure or intralaminar failure. An optimum processing window is proposed for the resistance welding of CF fabric/PEI composite system.

Experimental investigation of the resistance welding of thermoplastic-matrix composites. Part II: optimum processing window and mechanical performance

Abstract An experimental investigation of the resistance welding of carbon-fibre and glass-fibre reinforced polyetherimide laminates is presented. The optimum resistance welding time based on a criterion of maximum lap shear strength was determined. The time required to achieve intimate contact predicted by a three-dimensional transient finite-element model featuring heat transfer and consolidation correlated well with the optimal welding time. The influence of the welding pressure on lap shear strength was investigated, and the consolidation quality obtained in the welded joint was related to the processing conditions. The extent of flow occurring during welding, or the reduction of thickness of the welded joints, was shown to be related to lap shear strength. Four failure mechanisms, leading to different values of lap shear strength, were identified including interfacial failure, cohesive failure of the heating element, tearing of the heating element and tearing of the laminate. Experimental and numerical processing windows were constructed and correlated well to each other. A comparison between fabric and unidirectional heating elements, in terms of lap shear strength and the interlaminar fracture toughness, GIc, was performed. It was demonstrated that large-scale lap-shear coupons and double cantilever beam specimens can be resistance welded providing that current leaking to the laminate is avoided.

Manufacture of a carbon-fabric-reinforced polyetherimide (CF/PEI) composite material

Relationships between impregnation mechanisms, consolidation quality and the resulting mechanical properties of carbon-fibre-fabric reinforced polyetherimide (CF/ PEI) thermoplastic composites have been investigated. A compression-moulding procedure was applied to simulate the effects of different processing conditions (i.e. pressure, holding time and processing temperature) on the quality of finished samples. Microscopic studies of cross-sections, density measurements and flexural mechanical properties were used to examine the quality of impregnation and consolidation. A qualitative model to describe the impregnation and consolidation processes of this material was developed. It predicts the variations of void content during consolidation, as well as the holding time, moulding temperature and pressure required to reach full consolidation. Good agreement between theoretical predictions and experimental data indicates the success of the approach. Finally, aileron ribs for a civil aircraft were successfully manufactured from the CF/PEI material according to the suggested optimum processing conditions.

Characteristics of resistance welding of lap shear coupons.: Part II. Consolidation

Abstract A consolidation model based on transient three-dimensional heat transfer for the resistance welding of thermoplastic-matrix composite lap-shear specimens is established. The consolidation occurring in the resistance welding process was studied in terms of intimate contact and autohesion processes. Effects of power level on the time to achieve full intimate contact were determined. The influence of the consolidation pressure on the degree of intimate contact was investigated. Different welding configurations of lap-shear specimens were evaluated, i.e. APC-2 laminate/PEEK film, APC-2 laminate/PEI film and CF–PEI laminate/PEI film. The bonding time was compared with experimental electrified times, and close agreement was obtained. Local thermal degradation in the heating element was discussed for high power levels. A processing window for the CF–PEI/PEI configuration was established, which showed close agreement with that determined experimentally.

Manufacturing process and mechanical properties of thermoplastic composite components

Abstract This paper presents some current results of manufacturing advanced composite components for a civil aircraft by compression moulding process using a carbon fibre fabric reinforced polyetherimide composite (CF/PEI). Effects of processing conditions on the consolidation quality and mechanical properties of CF/PEI composite are studied. A qualitative model to describe the impregnation and consolidation processes of this material was developed, with which an optimum processing window was established. The spring-in effect was taken into consideration in the design of processing mould system. Thermoplastic composite demonstration parts, aileron ribs and high load hinges were successfully produced according to the optimum processing window.

Stamp forming of continuous carbon fibre/polypropylene composites

Abstract A right angle tool is employed for the experimental study of the stamp forming process of continuous fibre-reinforced thermoplastic laminates. In particular, a carbon fibre/polypropylene system is used. Unidirectional and quasi-isotropic preconsolidated laminates are heated by contact heating in an external heater above the melting temperature of the polymer matrix and are then stamp formed in a cold matched metal tool. Typical cycle times (including preheating time of the preconsolidated laminates) are about 2 min. Useful processing in conditions, such as stamping temperature, stamping time, stamping velocity and pressure required for stamp forming of this composite are determined. Fibre movement at bend range were qualitatively investigated with Cu tracer wires embedded in preconsolidated laminate samples. A process window for stamping is generated, which describes the correlations between processing parameters and final part thickness and final part angle respectively.

Resistance welding of carbon fibre reinforced thermoplastic composite using alternative heating element

The focus of this work is the use of a metal mesh as an alternative heating element for the joining of carbon fibre fabric reinforced polyetherimide composite laminate. A more homogeneous temperature distribution was generated by the metal mesh at the bonding surface. Glass fibre fabric reinforced PEI (GF/PEI) was used as an electrical insulator between the heating element and adherend laminates. Experimental results show that the GF/PEI prepreg could effectively prevent current leakage and enlarge the welding area. Welding parameters, such as input power level, welding time and pressure, were optimized according to the results of mechanical and microstructure characterization. Mechanical performance of composite specimens joined using metal mesh, in terms of lap shear strength and Mode I interlaminar fracture toughness, was equivalent to that of compression moulded benchmarks. Fracture surfaces of welded specimens showed mostly cohesive failure or intralaminar failure, indicating that good bonding between the PEI matrix and metal mesh was achieved.

Spring-in study of the aileron rib manufactured from advanced thermoplastic composite

Owing to high processing temperatures, the residual stresses in thermoplastic composites are inevitable. These residual stresses lead to a reduction in the enclosed angle in channel or angle components. This reduction in enclosed angle is referred to as spring-in. In addition, the residual stresses may cause defects such as delamination, fibre waviness, microcracking, etc. This paper investigates the use of a spring-in model to assist with the design of a mould system for an aileron rib. The aileron rib was manufactured from carbon fibre/polyetherimide (CF/PEI) thermoplastic composite material by compression moulding technique. Through microscopic examination and void content measurement, the necessary processing temperature range was determined. The spring-in angle was then predicted based on this processing temperature and an allowance for this was made during tool design. Experimental results showed that there was good agreement between theoretical predictions and the final part angles.