Recent Developments and Applications of the ACFM Inspection Method and ACSM Stress Measurement Method
Alternating Current Field Measurement (ACFM) is an electromagnetic inspection method which is now widely accepted as an alternative to magnetic particle inspection in the Oil and Gas Industry, both above and below water. Although developed initially for routine inspection of structural welds, the technique has now been developed to cover broader applications across a range of industries. Increases in inspection speeds, application to non-planar crack morphologies and extension of sizing models to accommodate different crack types have all been achieved. These have required developments in both hardware and software and generally utilise array technology. This paper will describe how the use of the ACFM technique has been extended over the past few years, including examples of specific developments in a range of industries including power generation, railways and shipping. The paper will also consider one of the spin-off technologies now being developed, known as the Alternating Current Stress Measurement (ACSM) technique. ACSM is an electromagnetic method for non-contacting stress and load measurement. Examples will be given of early applications of the technique, including offshore mooring load measurement and tunnel lining stress monitoring. The Alternating Current Field Measurement (ACFM) technique is an electromagnetic inspection method for surface crack detection and sizing. The technique does not require coating removal and was originally developed in the late 1980s for use in the Oil Industry, where there was a requirement for improving the reliability and sizing capability of subsea inspection. The ACFM technique differs significantly from conventional eddy currents in using a locally uni-directional and constant electric field in the sample, which provides a number of benefits including the ability to mathematically model the perturbations in this field produced by a simple defect. The technique therefore allows crack depth sizing to be carried out by comparing measured field disturbances to predicted disturbances around pre-defined defects. The technique was originally developed for the detection and sizing of fatigue cracks, which tend to be single planar defects compatible with the model. When used to inspect more complex cracking, e.g. Stress Corrosion Cracking (SCC), the modelling becomes complex. Defect detection is not generally affected but the ability to determine the depth of complex cracks is limited by the theoretical modelling constraints. It has been found however that in certain situations, where the cracking is of a specific nature, the modelling of simple cracks can be extended empirically to provide reasonable estimates of crack severity. ECNDT 2006 - Tu.3.6.5
A Fuzzy Learning Approach for Identification of Arbitrary Crack Profiles Using ACFM Technique
This paper presents a new inversion approach to estimate the depth profile of a fatigue crack from the output signal of an alternating current field measurements (ACFM) probe. We first show that the mapping between the ACFM signals and corresponding crack depth profiles can be expressed by an affine transformation, then we utilize a generalized version of the fuzzy alignment algorithm (FAA) to find this mapping more efficiently. The proposed method is then extended using the Dave's objective function previously proposed for the fuzzy clustering. To evaluate the accuracy and robustness of the proposed inversion technique we reconstruct cracks of arbitrary shapes from both simulated and measured ACFM signals. The results show that the proposed approach is more robust and accurate than existing methods and gives reasonable results even in the absence of sufficient training data.
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