NONDESTRUCTIVE INSPECTION OF CARBON FIBER-REINFORCED PLASTICS USING EDDY-CURRENT METHODS

The possibilities and limitations of eddy current methods for the inspection of carbon fibre-reinforced plastics have been investigated. The electrical properties of these composites lead to modifications of current methods applied to metals; in particular the required frequencies are much higher, 1-30 MHz for weave and 10-500 MHz for unidirectional reinforcements. A proper design of the probes is essential. Single turn coils are adequate, and it is shown how a higher sensitivity and suppression of the lift-off effect can be obtained. Fibre orientations can be detected conveniently with dedicated probes using a polar scan technique. The eddy currents are insensitive to delaminations. Only for unidirectional reinforcements and extensive delamination, over 20%, is the effect large enough to show up in C-scan images. In contrast, fibre fracture is readily detected for unidirectional and weave reinforcements, with a lower limit of about 8% fracture. As a result, eddy current methods are useful to establish the type of defect when a composite is damaged. This is confirmed by comparing eddy current and ultrasonic measurements on composites with impact damage.