PEC Frequency Band Selection for Locating Defects in Two-Layer Aircraft Structures With Air Gap Variations

Multilayer structures are widely used in aircraft fuselage. Because of the interlayer air gap caused by deformation or disbonding, conventional single-frequency eddy current cannot discriminate between second-layer defect signals and gap signals. In this paper, several defects at varied locations (i.e., first-layer surface, first-layer subsurface, second-layer surface, and second-layer subsurface) are manufactured into two-layer Al-Mn 3003 alloy specimen with various air gaps. Pulsed eddy current (PEC) is investigated in combination with principal component analysis (PCA) to classify and locate defects in the specimen. The new feature named differential frequency to zero is proposed, and the frequency responses of selected frequency band are processed through PCA. The principal components are used for locating defects. The experimental results show that first-layer surface defects, first-layer subsurface defects, second-layer surface defects, and second-layer subsurface defects can be classified when air gap is varied from 0 to 1.4 mm through the proposed methods. In conclusion, PEC testing with the help of PCA can eliminate the interlayer air gap and liftoff effect, which has potential for defect characterization in multilayer aircraft structures.