How can composites be monitored?
As with all materials, defects and damage can occur, and with fibre-reinforced polymer composites (FRPs) this usually manifests itself as a reduction in strength and/or stiffness. Dependent upon the materials used to create the FRP, damage can be complex, varying 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 or structural connections.
There is a perception that FRPs are difficult to inspect, and in some cases this has led 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 (sometimes known as non-destructive testing – NDE or NDT) are routinely available and there have been significant advances in this technology over recent years. 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. 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.
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
British Institute of NDT (BINDT): www.ndt.net