OS17-3-4 Detection of damage in CFRP laminates using the eddy current method

Abstract

Carbon fiber reinforced plastic (CFRP) laminates, used at the beginning in aerospace structure, are now widely used in various fields. It is, however, very sensitive to an impact which causes delaminations in the laminate. Since the delamination reduces the mechanical property of the laminate with difficulty of visual inspection, nondestructive inspection (NDI) is required to maintain the structural reliability of the CFRP structure. Eddy current method, which is often used for metallic materials, can be applied to CFRP laminate due to conductivity of carbon fibers. The advantages of the method are short scanning time, low cost and portability. The method has been studied to be applied to the CFRP laminate. Strong electric anisotropy and inhomogenity of the laminates, however, makes simple application of the method difficult. Since delamination is in-plane damage in the thin laminate, conventional eddy current method has problem for making current-loop which should be disturbed by the damage to be detected. In this paper, eddy current method with conventional probe was firstly applied to CFRP fabric and cross-ply laminate to clear the problem of the method. For the fabric laminate, the damage location was successfully detected. For the cross-ply laminates, it was difficult to detect the location. The large scatter of response signal was observed due to the strong anisotropy of cross-ply laminate. To resolve the problem, a new probe for electrically anisotropic CFRP laminate is introduced. The new probe is made of two coils put side-by-side and induces uniform electric current that is stronger than the electric current induced by a conventional probe. It is applied to the cross-ply laminate. The new probe is arranged perpendicular to the scanning direction and move straight along specimens with fiber breakages and a delamination; the applicability of the new probe is shown experimentally.