Flexible ECT probe with front-end differential setting for inspection of curved CFRP structures

Abstract

Carbon-fiber-reinforced plastic (CFRP) structures with curved surfaces are widely used. However, an efficient tool for curved CFRP inspection is still lacking. This paper proposes a flexible eddy current testing (ECT) probe with front-end differential setting designed specifically for curved CFRP structures inspection. The probe contains a pair of coils that are fabricated on a flexible printed circuit board . It can be bent to fit a curved surface without the need to redesign the probe. The two coils of the probe work simultaneously and highlight a defect's signal by the front-end differential configuration. It effectively suppresses common mode interference , which is generally evitable in the experiment of CFRP structures inspection using a high frequency ECT system. A finite element method (FEM) model is established to study the inspection principle in viewing of the anisotropic electrical conductivity of CFRP. A prototype probe is fabricated and tested, with which a flat and three curved CFRP samples with machined and natural impact cracks are imaged. A signal processing algorithm including detrending, gradient calculation and nonlinear transformation is developed to improve the probe's defect detectability . Experiment results show that a small defect located on the curved surface of a CFRP sample with dimensions: length 2 mm, width 1 mm and depth 0.4 mm can be detected by the probe. In addition, the effects of the operating frequency and orientation of the defect are discussed.