Pulsed Eddy Current Detection of Cracks in Multilayer Aluminum Lap Joints

Abstract

Aging aircraft is susceptible to cyclic fatigue cracks around ferrous fasteners in multilayer aluminum wing lap joints, as found on Canadian Military aircraft CP-140 Aurora and CC-130 Hercules. Current methods of crack detection utilize bolt hole eddy current, requiring fastener removal, a time consuming process that also has the potential to result in collateral damage. In this paper, pulsed eddy currents (PECs) are induced in the aluminum wing structure by a probe centered over a ferrous fastener. The response signals are analyzed using a modified principal components analysis (PCA) and cluster analysis to show separation of groups of PCA scores from fasteners with and without notches in the bore hole. The Mahalanobis distance (MD), a cluster analysis method, is used to obtain a relative distance measurement between scores associated with cracks from those without cracks. The MD evaluation provides confidence intervals for detection of cracks, which range in extent from fastener holes from 0.9 to 5.5 mm, and a means of potentially sizing larger (>3 mm) second layer cracks. In this paper, 100% of the cracks in the sample were detectable. The results demonstrate PEC as a viable technique for second layer crack detection with ferrous fasteners present.Index Terms-Mahalanobis distance, multilayer aluminum, non-destructive testing, principal components analysis, pulsed eddy current.

Enhancing pulsed eddy current for inspection of P-3 Orion lap-joint structures

“…MPCA is preferable to conventional PCA [6] as it has less sensitivity to small amplifier offsets or shifts in the average response. The sum of the product of these vectors and coefficients can be used to reproduce the original data as follows [6]: ( 1 ) By using a relatively small number of vectors, the original signal can be reproduced, thereby reducing the data variation to a manageable number of scores [1]. These scores can then be used to determine the Mahalanobis Distance (MD) of each measurement [6].…”

“…This covariance matrix is then used to calculate the MD for all experimental data signals. The MD is then compared to a decision threshold in order to determine if the fastener hole has a notch present or not [1,7]. Figure 5 shows a 3-D view of MPCA scores and MD for data collected using the P-3 Orion sample.…”

“…The current method of crack detection uses bolt hole eddy current, which requires fastener removal. This process can be quite time consuming and there is an added risk of damage to components during disassembly [1]. Pulsed Eddy Current (PEC) is currently being examined as an alternative method of detecting cyclic fatigue cracking in multi-layered wing structure without fastener removal.…”

“…The PEC inspection technique is employed while the ferrous fasteners are present, and the fastener is used as a conduit for the magnetic flux created by the driving coil [2]. This allows for a deeper penetration of eddy currents, while sensing coils placed on the aluminum surface pick up resultant transient responses [1]. According to results obtained by Whalen [3], an increase in ferrite core diameter relative to the ferrous fastener head diameter will produce a greater depth of penetration.…”

Evaluation of machine learning tools for inspection of steam generator tube structures using pulsed eddy current

“…PEC signal response can be viewed as consisting of a spectrum of discrete frequencies, far greater in number than what is normally captured by multi-frequency eddy current testing, but with an associated complexity in signal response. In order to address the subtle variations and extract dominant features in the measured PEC signal, a modified principal components analysis (MPCA) has been applied [5][6][7]. MPCA is based on a minimization of sum square residuals interpretation of results instead of a variance minimization interpretation as used in conventional PCA for feature extraction [6].…”