Non-destructive testing of FRP by d.c. and a.c. electrical methods

In situ stress and strain detection, together with health monitoring, would give improved durability and safety of composite structures. Different techniques for in situ observation and non-destructive testing have been used in the past. This paper gives an overview on investigations and possible applications of electrical methods such as d.c. and a.c. measurements on fibre reinforced polymers. In the case of d.c. measurements in CFRP the reinforcing carbon fibres themselves are used as sensors, functioning as electrical resistors. In GRP a conductive matrix filler such as carbon black can take on the role of a resistance sensor. In the case of a.c. measurements, i.e. capacitance and dissipation, the carbon fibres and their connecting points function as resistors, while the spaces between the fibres function as capacitors. It has been shown that it is possible to monitor strain and failure as well as to classify different failure mechanisms in static and dynamic load conditions using electrical methods.