MWM eddy current sensors for monitoring of crack initiation and growth during fatigue tests and in service

A new surface-mountable (conformable foil) eddy-current sensor called the Meandering Winding Magnetometer-Array (MWM™-Array) has the capability to monitor crack initiation and growth in fatigue test coupons. Fatigue tests with the MWM-Array mounted on the surface inside a 6.4-mm hole in an Al 2024-T3 tension–tension fatigue specimen, demonstrated the capability to detect cracks with l<50 μm (a<25 μm). This provides a new capability to monitor fatigue tests of coupons and components to determine the number of cycles to crack initiation and to study ‘short crack’ growth. Also, MWM conductivity measurements correlate with crack length, permitting crack length monitoring. Ongoing research is focused on depth monitoring as well. Numerous applications are under evaluation for permanently mounting MWM-Arrays in difficult-to-access locations on commercial and military aircraft. The motivation for permanently mounting MWM eddy current sensors is either (1) to replace an existing inspection that requires substantial disassembly and surface preparation (e.g. inside the fuel tank of an aircraft), or (2) to take advantage of early detection and apply less invasive life-extension repairs, as well as reduce interruption of service when flaws are detected. Implementation of permanently mounted MWM-Arrays and sensors is expected to improve fleet management practices and modify damage tolerance assumptions.