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 Error Characterization Approach for Crack Depth Profile Estimation in Metallic Structures Through ACFM Data
The estimation of fatigue crack depth profile is one of the most important issues in nondestructive evaluation techniques. In this regard, providing a method which is able to determine the error rate of the depth profile estimation will be significant. Although several methods have been proposed to estimate the depth profile with reasonable accuracy, these mentioned methods are unable to determine the error rate of estimation process. In this paper, to eliminate mentioned drawbacks, a first and second fuzzy approximation methodology is proposed to estimate the surface crack depth profile of an arbitrary crack in metallic structures based on alternating current field measurement signals. In the proposed method, it is feasible to achieve a definite error rate to determine the surface crack depth profile under evaluation. Next, to achieve the most reasonable accuracy and the least computational complexity simultaneously, a solution will be proposed for the method presented in this paper. Finally, the results of several common methods are compared with the method presented in this paper in analytical and experimental forms.
Eddy‐current induction in a uniaxially anisotropic plate
A theoretical model is presented to describe the time‐harmonic electromagnetic induction of eddy currents in a uniaxially anisotropic plate. The model assumes that the plate is nonmagnetic and that the axis of uniaxial anisotropy is horizontal (i.e., lying in a plane parallel to the surface of the plate). The fields and currents induced by a general magnetic source are obtained by formulating the problem in terms of reduced electromagnetic potentials (Hertzian potentials) and making extensive use of integral transforms. Closed‐form expressions for the reduced potentials are given explicitly, enabling the induced fields to be calculated in all regions provided the free‐space source fields are known. The change in probe‐coil impedance ΔZ due to eddy‐current induction is considered in detail, and an explicit closed‐form expression for ΔZ is derived for the general case. This general theory is illustrated by calculating ΔZ for a cylindrical air‐cored coil when (i) the coil axis is normal to the surface of the...