Detection and evaluation of damage in aircraft composites using electromagnetically coupled inductors

Abstract

The paper presents a quantitative damage evaluation of carbon-fibre reinforced polymer (CFRP) plates using a non-contact electromagnetic (EM) sensor. The EM sensor with coupled spiral inductors (CSI) is employed here to detect both impact induced and simulated damage leading to an accurate evaluation of the location, depth and width of sub-surface defects. The effect of inspection frequency, standoff distance and signal power are also investigated leading to the development of an engineering circuit design tool that relates the set up and calibration of the sensor to its detection performance. It is found that the dynamic range of the transmission coefficient is the limiting factor in the original Salski CSI sensor and this problem is addressed by adding ferrite layers to reduce the reluctance of the magnetic circuit, improving damage sensing by 22%. The study leads to a further development of utilising an open ferrite yoke with a pair of encircling coils, which shows a 57 % sensitivity improvement and clearer identification of air gaps (voids) and delamination in CFRP laminates. The proposed EM yoke design CSI sensor is low cost and could be assembled into an array for non-contact, in situ mechatronic scanning of aircraft composite wings.

Detection of Impact Damage in Carbon Fiber Composites Using an Electromagnetic Sensor

“…The capability of detecting the delamination induced by three-point bending has been reported in previous research [18]. Here, the effect of the air gap size (that could simulate delamination) on the signal response is further studied.…”

“…It should be noted that the standoff distance used will affect the penetration depth, as the CSI sensing is a kind of near-field detection technique. The sensitivity to the defects would decrease with increasing standoff distance, and the signal response asymptotes to the free space condition [18].…”

“…The apparent size of the impacted region is increased at a lower frequency, which suggests that the internal damage within the penetration depth can be gradually revealed by selecting an appropriate range of frequency (10-500 MHz). Accurate determination of the depth of the delamination can be done by well-prepared calibration, such as detection of some purposely produced defects with regular shapes [18]. 6(c), the profiler is made up of a microwave closed resonant cavity coupled to a tapered probing tip.…”

“…Furthermore, the authors optimised the CSI sensor for better impedance matching [17] and extended the applications to the detection of subsurface grooves [18], delamination caused by three-point bending [17] and impact damage [19].…”

Principles and Applications of Microwave Testing for Woven and Non-Woven Carbon Fibre-Reinforced Polymer Composites: a Topical Review

“…Safety precautions are usually not needed since the power of the signal used is relatively low (few milliwatts). It is noted that the microwave techniques should be differentiated from the lower frequency electromagnetic detection methods, like the eddy current testing (ECT) (100 Hz -10 MHz) [11][12][13] and coupled spiral inductors (CSI) (1-100 MHz) [14][15][16], where the field that exists only around the coil cannot propagate. In addition, the ECT and CSI do not work well for dielectric materials.…”