Assembly technologies
Project leader
Dr.ir. Harald Bersee
PhD. student
MSc. Darko Stavrov
Researcher
Ing. Bert Weteringe
Ing. Marc van Dongen
Contactperson
Dr.ir. Harald Bersee
Summary
Joining is a critical step in the manufacturing of composite products. As advanced composite parts become larger and more complex, there is a corresponding need for improved methods of assembly. In most of the composite structures the joining techniques of metals have been mimicked. These traditional joining techniques are mechanical fastening (riveting and bolting) and adhesion.
The difference between orthotropic materials, like composites, and isotropic materials is that damage from drilling and machining makes orthotropic materials susceptible to interlaminar shear, delamination and peeling. The holes also weaken the structure and introduce stress concentrations. These stress concentrations are of concern because composites lack the plasticity that allows stress redistribution like in metals. In addition there is the possibility of galvanic corrosion between the metal fasteners and carbon fibres. The joint is not waterproof requiring a sealant. Another disadvantage is the weight increase due to the fasteners and local reinforcement to counteract the stress concentrations.
Adhesive is a good alternative, since no stress concentrations or weakening of the material occurs. On the other hand, this method needs extensive surface preparations, clean room conditions and long curing cycles. Therefore, the total processing cycle is very long and ultrasonic inspection is needed to verify the bond quality. Another disadvantage is the large amount of waste during production.
A promising new assembly method is offered by the processing characteristics of thermoplastics, welding. Welding is already extensively being used for short fibre reinforced thermoplastics; however, the technique is quite new for continuous fibre reinforced thermoplastics. Fokker Aerostructures is the first to use the welding technology (resistance welding) for the manufacturing of the Airbus A340-600 J-nose (see figure 1).
Welding is a joining method that uses the property of thermoplastic matrices to flow when heated above their glass transition temperature Tg (for amorphous polymers) or the crystalline melting point Tm (for semi-crystalline polymers). Known also as fusion bonding, it can generally be described as joining of two parts by fusing their contact interfaces, followed by cooling (consolidating) under pressure, which enables the bond to be made. It appears to overcome all of the above-mentioned problems connected to the traditional techniques. Additionally, it offers no foreign material at the interface. In spite of the existence of several drawbacks, like the possibility of inducing residual stresses in the bond, uneven heating, delamination and distortion of the material, welding is widely considered as the ideal bonding technique for thermoplastic composites.
The used type of heating usually classifies fusion-bonding techniques. Vast number of means can provide heat to the interface. Hot plates, hot gas, spinning, ultrasonic and radio signal, microwaves, joule effect in a resistor, laser and induction is some of them. From the variety of means, three have emerged as the most promising: induction, ultrasonic and resistance welding.
Both resistance- as ultrasonic welding are being investigated at the disciplinary group Design and Production of Composite Structures.
Resistance welding, known also as resistive implant welding, electrical-resistance fusion or electro-fusion is one of the most attractive welding techniques for thermoplastics today. It is rather simple method that uses resistive heating element sandwiched between the bonding surfaces of the laminates. When current flows through the heating element, generated heat follows the Joule's Law, where the energy dissipated from the resistor is proportional to the resistance, current and elapsed time:
A large difficulty is the occurrence of current leakage in case of welding of carbon fibre reinforced thermoplastics. The research on thermoplastic composites is focussed on the design and manufacturing of large primary aircraft structures. These structures will be made off carbon fibre composites; therefore, development of resistance welding technology for carbon fibre reinforced thermoplastics is one of the main objectives of this research project.
Ultrasonic welding is a well-known technique in almost all major industries in which thermoplastics are used. High frequency vibrations are introduced through a horn, attached to a transducer, causing rapid local heating of the thermoplastic. Primary advantages are short cycle times, moderate capital cost and suitability for mass production. Next to that tooling can be changed quickly and the process is suitable for automation. Drawbacks for ultrasonic welding are the controllability, the need for a supporting backside and locating tool and special surface preparations in the form of an energy director. An energy director is a thermoplastic spike or ridge to direct the vibrations over the complete weld area.
In this research project, the ultrasonic welding of large welds in continuous fibre reinforced thermoplastics will be investigated.
