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Technical report | Discrimination between Fatigue Cracking and Mechanical Damage in Aircraft Fastener Holes by Eddy-Current Phase Rotation


Fatigue cracking in metallic aircraft structure often originates at fastener holes and can be detected using the nondestructive inspection technique of bolt-hole eddy-current (BHEC) testing. BHEC testing is based on detection of a disturbance in the induced current or magnetic field caused by a surface-breaking crack in the bore of the hole. However, if the surface of the hole contains benign mechanical damage or superficial scoring, false positives can be produced leading to potentially unnecessary maintenance on a hole that is otherwise sound. In principle, analysis of the phase of the eddy-current response can assist in distinguishing fatigue cracking from such mechanical damage. In the present work, a systematic BHEC and fractographic study has been conducted by inspecting several hundred fastener holes in ex-service F/A-18 aircraft bulkheads, following fatigue testing. The results demonstrate that measurement of the relative phase of the eddy-current response provides additional information which can assist in discrimination between genuine cracks and mechanical damage.

Executive Summary

Bolt-Hole Eddy Current (BHEC) testing is a nondestructive inspection (NDI) technique used for the detection of surface-breaking cracks occurring in fastener holes in metallic aircraft structure. The technique involves the removal of fasteners and the insertion of a rotating spindle containing an eddy-current coil into the hole. The eddy-current coil scans the bore surface of the hole as the spindle rotates. The coil is excited by an alternating current with frequencies typically 100 kHz – 2 MHz and cracks within the hole are detected via the induced currents and fields produced by coil. If a defect is detected, the hole is typically ‘oversized’ via machining with a larger diameter drill to remove the discontinuity. A matching fastener is then inserted into the larger-diameter hole. BHEC has been in active use for several decades and finds significant application in the Royal Australian Air Force (RAAF) fleet.

A difficulty can arise if the hole is free of cracking but instead contains mechanical damage such as gouges, heavy scoring or surface roughness. These discontinuities may be benign in terms of structural integrity, but disturb the induced field and currents, resulting in a change in coil impedance that is erroneously assumed to be cracking. These false positives can lead to unnecessary oversizing in the cases where the mechanical damage does not pose a structural risk to the aircraft. The presence of mechanical damage also places an artificial limit on the defect reporting threshold and consequently may reduce the achievable inspection reliability. A given hole may only be oversized a number of times, due to geometry limitations such as the distance from other holes or component edges, and this unscheduled maintenance also adds a logistics, cost and time burden, so it is desirable to avoid unnecessary maintenance by attempting to discriminate between fatigue cracks and mechanical damage.

Analysis of the phase rotation of the eddy-current defect signal has been suggested as a method for discriminating between cracking and mechanical damage. This technique is employed informally by NDI technicians, and in some cases is referred to explicitly in aircraft maintenance standards. However, it has not been the subject of any formal studies published in the open literature. The absence of such published studies has prompted the program of work reported here.

In the present study, a systematic BHEC and fractographic study was conducted by inspecting several hundred fastener holes in ex-service F/A-18 aircraft bulkheads, following fatigue testing. During the course of the work a total of 365 eddy-current defect indications were recorded and the eddy-current phase rotation was used to provisionally classify the defects as either possible cracking or possible mechanical damage. A large sample of the inspected holes were subsequently broken open and 165 of the defects were examined using fractography to determine the nature of the defect: whether mechanical damage or cracking. The results demonstrate that measurement of the relative phase of the eddy-current response provides additional information which can assist in discrimination between the BHEC responses from genuine cracks and mechanical damage. The data also illustrate the burden imposed on the BHEC inspection process by mechanical damage, with of the order of 30% of defect indications arising solely from damage in this case.

The use of BHEC phase rotation has the potential to improve inspection reliability, increase accuracy of estimates of the fatigue crack population in an aircraft fleet, and reduce unnecessary over-sizing of holes in aircraft structure. There is however a clear risk to structural integrity if a defect is incorrectly identified and a crack is mistakenly assumed to be benign mechanical damage and left in place. Assessment of this risk and formulation of a framework for aircraft structural management which incorporates both imperfect defect detection and imperfect defect classification, together with associated confidence levels, would require a formal probability-of-detection trial and NDI reliability study.

BHEC phase discrimination is clearly a useful tool and we recommend that the significance of the technique and its limitations should be introduced to RAAF eddy current inspector training to assist in defect classification and to broaden the understanding of the BHEC techniques.

Key information


M E Ibrahim; G D Surtees; G R Hugo; H J Morton and S K Burke

Publication number


Publication type

Technical report

Publish Date

August 2016


Unclassified - public release


Nondestructive Testing; Eddy Current Testing; Aircraft Structures; Crack Detection and Sizing