Detectability of delamination in laminated CFRPs with diverse stacking sequences using eddy current method with T-R pancake coil

Abstract

A T-R probe composed of two non-coaxial pancake-based coils is used to identify delamination at different depths of laminated CFRPs with diverse stacking sequences. This type of probe is capable of inspecting delamination, since an interactional in-plane currents across the adjacent and non-adjacent layers of CFRPs stacked by different fibers can be induced using pancake-type coil. For optimizing the probe performance, a frequency sweep method is conducted to obtain the appropriate working frequency. To fabricate the imperfect samples containing the delamination-based damage, a tailored Teflon film is inserted into the appointed interfaces of 16-layer CFRPs. The physical mechanism of delamination detection using pancake coil is analyzed by using FE method. After then, a set of ECT experiments for various CFRPs are performed to verify the sensitivity to delamination detected by pancake coil. Correspondingly, both shape and location of the delamination are compared with those obtained from a well-established UT device. Experimental results indicate that the sensitivity to delamination is high at the interface formed by different stacked fibers, but it lacks sensitivity to delamination in unidirectional one due to the same aligned fibers, which are qualitatively consistent with the numerical results. Unexpectedly, delamination at the interfaces between adjacent layers stacked by the same fibers of the other two CFRPs can still be distinguished from the non-damage region, while this phenomenon fails to explain by the numerical analysis. Quantitatively, the visualized damage region agrees with the UT C-scan results.