Alternating Current Field Measurement (ACFM)
· Alternating Current Field Measurement testing (ACFM) is a well-established electromagnetic nondestructive testing technique for the in service inspection of welds. It offers many advantages over other conventional testing methods such as Eddy Current and Magnetic Particle testing by avoiding the need for extensive cleaning before testing.
· ACFM was developed in order to reliably detect and size fatigue cracks. In weld structures the fatigue damage tends to occur at the weld toe, where the stress concentration is at their highest. Fatigue cracks tend to be essentially planar and individual cracks can grow to form larger cracks.
· Recent applications have been by inspection of lap and fillet welds inside above ground steel storage tanks. ACFM has been applied for detection and sizing of surface breaking cracks in metals
· Surface roughness or large coating thicknesses cause less of a problem than in conventional Eddy Current testing. With ACFM, it is possible to detect defects through 5mm (0.2 in.) or more of non-conductive coating material. Typical non-conductive coatings that the technique has been used through include: epoxy coating, standard paints, and passive fire protection material.
· ACFM has also been successfully used on some metallic coatings such as: zinc coating, copper coated threads, and flame sprayed aluminum.