TWI Industrial Member Report Summary 982/2011
By Chris Worrall
This is an introduction on the topic of joining composite materials in aircraft structures with the focus on machining, mechanical fastening, and non-destructive inspection (NDI) of the joint, and has been prepared by carrying out a literature survey.
The use of composite materials has increased substantially over the last few decades, mainly due to the aerospace industry led demand for reduced component weight. This requirement has meant that composite material manufacturers have developed economically viable materials with performance advantages in a relatively short period of time.
Although the manufacturing processes of composite materials offer the potential to reduce the need for joining parts, composite structures must still contain some joints due to size, design, technological and logistic limitations, and to cater for in-service disassembly requirements for repair, maintenance, inspection and handling.
Mechanical fastening is still the most widely used technique in the aerospace sector, especially for primary structures (for joining both metals and composites). It is a well established technique that meets the requirement for easy disassembly, and when compared with adhesive bonding offers the further benefits of being insensitive to surface preparation, service temperature and humidity.
A prerequisite for mechanical fastening is the production of a hole to accommodate the fastener, and the technology of machining composites needs to be capable of providing cost effective holes of sufficient quality to realise maximum performance of the joint. Unfortunately, some of the processes currently employed to machine composite materials were originally developed for the woodworking or metalworking industries and are not ideal for composites. Aerospace composite materials are typically inhomogeneous, anisotropic and abrasive, and their machining behaviour is not well understood and can be problematic and costly.
Machining of composites has the potential to introduce defects into the material at the joint, which can not only cause a reduction in load carrying capacity of the structure, but also be detrimental to its long-term reliability. Furthermore, inappropriate design or incorrect assembly of mechanical joints can affect the structural integrity of an airframe. Clearly, detection and monitoring of critical defects is an important issue in the application of mechanically fastened composite joints, and non-destructive investigation techniques must be mature enough to guarantee the integrity of joints.
Since machining, mechanical fastening, and non-destructive inspection of composites are critical to the final performance and durability of aerospace structures, an investigation into the state-of-the-art and further developments in technology is required to if composite materials are to realise their full potential in the aerospace industry.
The objectives of the literature review were:
- Establish the state-of-the-art of machining processes for composites in the aerospace industry.
- Provide an overview of mechanical fastening systems and non-destructive inspection techniques currently used for composite materials in the aerospace industry.
- Signpost machining processes, mechanical fastening and NDI systems under development.
- Identify the challenges with current machining processes, mechanical fastening and NDI systems.
- Identify innovative technologies to improve the state-of-the-art of composites machining.