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

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.