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
Experimental study of alternating current stress measurement under a uniaxial stress
The alternating current stress measurement (ACSM) which is developed from alternating current field measurement(ACFM) is a new and effective electromagnetic non-destructive testing method with greatly application potential for stress measurement. Using this technology to detect the magnetic field disturbance on the specimen caused by the stress can realize the assessment for the degree of stress concentration. In the present paper, the relationship of stress and induced magnetic field was studied by tensile experiments of 20# low carbon steel. Experiment results which are extremely valuable show when the tensile force is less than the yield strength of the specimen, the percentage change of testing signal in the vertical direction of stress can be up to 8% and higher than that in the parallel direction. When the tensile force increases from 0kN to 40 kN, which can exceed the yield strength, the percentage change of testing signal in the vertical direction increases dramatically and can be up to 26%, while that in the parallel direction can be up to 23% too, which can effectively detect the degree of stress concentration of the ferromagnetic specimen. Meanwhile, a mathematical model of the relationship between stress and induced magnetic field was builded under the external magnetic field according to the principle of ferromagnetics, which can provide a reasonable interpretation for experiments.
Effects of stress on the magnetization of steel
The application of stress cycling to 1% manganese steel at low magnetic fields causes an irreversible shift in the magnetization towards the anhysteretic due to domain walls breaking away from their pinning sites under stress. The change in flux density ΔB caused by a single stress cycle of fixed amplitude \sigma_{\max} has been found to be proportional to the difference between the initial flux density B i and the anhysteretic flux density B an at the same field, H. The change in flux density as a function of stress has been found to be approximately proportional to σ.