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
Inspection of nuclear storage tanks using remotely deployed ACFMT
Stainless steel nuclear storage vessels offer a challenge to NDT techniques when inspection is required to detect cracking in vessel walls and floors. Medium to high levels of radiation are often a barrier to manually deployed methods and access generally can be difficult. TSC Inspection Systems have developed an advanced underwater alternating current field measurement technique (ACFMT) array probe for Cete Apave to inspect the welds between plates of these types of vessel for surface breaking cracks. The array probe is deployed by a crawler and can gain access to areas where the clearance between the vessel and obstructions is only 35 mm. This enables inspection even under the fuel rod crates. The ACFMT array covers a width of 50 mm, the whole weld cap and toes, with each pass and the data is displayed in real-time and stored for future reference. The system offers both detection and sizing of defects for length and depth using its mathematical algorithms without the need for prior calibration by the operator.
Magnetoelastic Effect of 3.25%Si–Fe Single Crystal under Low Magnetic Field
The effect of stress on magnetization and the dependence of magnetization on crystallographic orientation were investigated using single-crystalline and polycrystalline specimens. It was found that the magnetization curve shifts toward the high-magnetic-field side with the application of stresses, and this shift is proportional to the stress and has a strong dependence on crystallographic orientation. A model was proposed in which, as the domain observation testified, magnetization took place in such a manner that a certain domain structure satisfied the minimum free-energy condition. It explained the experimental results well. It may be concluded that magnetization under low magnetic field is attained with a balance between magnetoelastic and magnetization energies without increasing magnetostatic energy.