Defects and damage can occur during manufacture, installation and in-service, 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.
There are 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 acquisition and analysis. Several newer and specialised methods such as microwaves, acoustography and vibro-thermography which are also showing promise in specific applications.
The choice of NDE method will depend on a number of factors including access, defect type, thickness, surface condition and material quality. In-service inspection is more difficult than in manufacture.
Whilst NDE techniques are well established there are some key challenges still facing the industry:
- Thickness of composites – particularly in aerospace applications
- Accessibility for inspection
- Coupling and surface condition – can be quite rough
- Positive materials identification
- Signal attenuation and scattering
- Inhomogeneous and anisotropic structure
- Lack of adequate standards – but the situation is improving
- Interpretation of results – analysing the data obtained, what does it mean/represent?
- Unfamiliarity of inspectors with non-metallic structures
- Increased reliance on operator experience
- Inspection of repairs on composite structures
Interactive knowledge base (IKB) on NDE of composites:
HOIS Good Practice Guide: www.hois2000.com
Materials Solutions: www.materialssolutions.info
British Institute of NDT (BINDT): www.ndt.net
NCN Best Practice Guide on NDE Techniques for Composites