Background

Alternating current field measurement is an electromagnetic technique that uses induced uniform currents and magnetic flux density sensors to detect and size surface breaking discontinuities without calibration. The term uniform means that, at least in the area under the probe, current lines (components of electric field intensity E) in the absence of a discontinuity are parallel, unidirectional and equally spaced (Ex = Ez = 0 and Ey = constant ≠ 0). In the 1980s, there were moves to develop nondestructive test techniques for detecting and sizing fatigue cracks underwater in welded offshore structures.

The development focused on two existing techniques — eddy current testing for detection and alternating current potential drop testing for sizing.

Conventional eddy current techniques were not particularly effective because of the scanning patterns needed across the weld and because of the signals received from the weld itself. The developments therefore concentrated on producing eddy current probes that could test the whole weld while being scanned only parallel to the weld. This was achieved by using arrays of coils covering the weld. To assist detection, because the probe was no longer being scanned across the weld toe (and hence the discontinuity), coils of more than one orientation were used to obtain signals from the ends of a discontinuity as well as the center. Both the induction coils and sensing coils were relatively large to give wide coverage rather than high sensitivity because there was no need to detect discontinuities less than 1 mm (0.04 in.) deep in the types of welds found underwater. The development emphasized detection of discontinuities and suppression of lift off effects. Crack depth had to be sized with calibration slots.

Principle of Operation

The alternating current field measurement technique involves inducing a locally uniform current into a test object and measuring the magnetic flux density above the test object surface. The presence of a surface breaking discontinuity perturbs the induced current and the magnetic flux density. Relative, rather than absolute, amplitudes of components of the magnetic flux density are used to minimize variations caused by material properties, instrument calibration and other circumstances. These relative amplitudes are compared with values in sizing tables produced from a mathematical model to estimate discontinuity sizes without the need for calibration using artificial discontinuities such as slots. This feature of alternating current field measurement is useful because calibration on slots is prone to error for several reasons.