Alternating Current Field Measurement

Introduction

The ACFM method is an electromagnetic inspection technique which can be used to detect and size surface breaking (or in some cases near surface) defects in both magnetic and non-magnetic materials. ACFM is a current perturbation technique and is fundamentally different to conventional eddy current techniques.

Alternating current field measurement is an electromagnetic inspection technique that introduces an alternating current into the surface of a component to detect surface-breaking cracks.

The presence of a crack disturbs the electromagnetic field and the return signal is instantaneously converted by advanced mathematical techniques so that operators are alerted to the presence of defects.

Immediate defect sizing and recording is a major benefit compared to other NDT methods. Results from independent testing shows ACFM matches magnetic particle inspection (MPI) performances when inspecting underwater structural welds. The amount of missed and spurious signals is significantly lower with ACFM compared to MPI and conventional eddy current testing (ECT).

With ACFM's lower cleaning requirements and fewer false calls, inspections are significantly shorter, saving customers money.

Applications

The ACFM inspection technique is an electromagnetic inspection method, which is widely used in the oil and gas industry for the detection and sizing of surface-breaking cracks Stress Corrosion crack. It provides a number of advantages over conventional inspection methods, particularly in the ability to inspect without removing coatings or grease, and has recently been adapted for a number of specific rail applications.

• Detection and sizing of fatigue cracks and hydrogen cracking
• Inspection of fillet welds in mobile offshore drilling Units
• Inspection of fillet welds in highway bridges.
• Inspection of rail components
• Inspection of gear box, gear teeth, crank shafts, cylinder heads, turbines etc. in aerospace industries
• Detection of cracks and corrosion in vessels, storage tanks and piping in oil and gas and power generation industries
• Inspection of drilling tools
• Inspection of various components in automotive industries

Benefits

• The method both detects cracks and estimates their size and length.
• It can inspect any electrically conductive material.
• Data is recorded electronically for off-line evaluation if necessary and provides a permanent record of indications. Tests can be repeated and compared over time for ongoing monitoring.
• The method is non-invasive and can carry out inspection without removing any protective paint coating. With suitable probes, the method can be used on hot surfaces.