Fibre reinforced composite materials are increasingly used in industry; both monolithic and more complex sandwich structures. Their application has spread from high technology industries such as aerospace and defence to other sectors including chemical, offshore, marine, transport and civil applications. Each industry faces different challenges and the materials used and nature of the composite fabrications may differ.
A common feature is that defects and damage can occur, which reduce the strength and stiffness, and determine the safe working life of composite structures. These are complex, various and intricately related to a variety of service conditions and failure modes under many different circumstances. Defects may be introduced during manufacture, accidentally in-service or perhaps unavoidably in design because of the requirement to introduce discontinuities such as cut-outs, ply drops or structural connections.
The uptake of composites in industry has been limited by a perception that they are difficult to inspect. In some cases this has lead to over reliance on visual methods at the expense of component quality. In reality a number of common and established methods for non-destructive evaluation (NDE) are routinely available and there have been significant advances in NDE systems and technology. Newer methods such as laser shearography and transient thermography are now well accepted in aerospace and marine sectors and finding applications elsewhere; in many cases replacing traditional methods such as ultrasonic C-scanning. In ultrasonic systems the development of modern digital flaw detectors and improvements like wheel probes have opened up new avenues of data aquisition and analysis. Although relatively few NDE methods are practically used on composites by industry, there are an increasing number of newer and specialised methods such as microwaves, acoustography and vibro-thermography which are showing promise in specific applications.