Crack Growth Monitoring with Structure-Bonded Thin and Flexible Coils

Abstract

Structural health monitoring with thin and flexible eddy-current coils is proposed for in situ detection and monitoring of fatigue cracks in metallic aircraft structures, providing a promising means of crack sizing. This approach is seen as an efficient replacement to periodic inspections, as it brings economic and safety benefits. As such, printed-circuit-board eddy-current coils are viable for in situ crack monitoring for multi-layer, electrically conductive structures. They are minimally invasive and could be attached to or embedded into the evaluated structure. This work focuses on the monitoring of fatigue crack growth from a fastener hole with structure-bonded, thin, and flexible spiral coils. Numerical simulations were used for optimization of the driving frequency and selection of crack-sensitive coil parameters. The article also demonstrates the fatigue crack detection capabilities using spiral coils attached to a 7075-T6 aluminum coupon.

Effect of Bonding Material on Survivability of Surface-Attached Eddy Current Sensors

“…The impedance through the coils is determined as the ratio of the voltage drop across them to the current passing to a series resistance. The relative change in the impedance of the crackmonitoring coil with respect to the reference one was found to be a sufficiently sensitive feature for further analysis [4]. The presence of a crack lowers the mutual coupling between the part and the monitoring coil, resulting in an increased coil impedance.…”

“…The crack nucleation is dependent on many macroscopic and microscopic variables and the latter ones are responsible the crack initiation. For the two instrumented coupons, the crack length at the end of the test is measured via surface eddy current scans [4] and the crack growth equation is used to determine when the crack started growing. Since the number of cycles necessary to have a nucleating crack could vary from specimen to specimen, depending on surface condition and grain boundaries arrangement, the crack growth curve of the instrumented specimens is assumed to have the same shape as the one obtained a-priori on the un-instrumented one, but translated by a constant value for the number of cycles for which the crack starts to grow.…”

Quantitative Detection for Fatigue Natural Crack in Aero-Aluminum Alloy Based on Pulsed Eddy Current Technique

“…Consequently, there is significant attentions paid to the initiation and propagation of cracks, leading to research efforts aimed at understanding the formation and expansion of cracks in engineering materials like aeroaluminum alloys [3][4][5][6]. On the other hand, some of the attentions are involved in seeking the effective detection of cracks to ensure the safety operation of equipment [7][8][9]. The ability to accurately identifying and assessing cracks is integral to maintaining the structural integrity and performance of engineering systems.…”