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example where a crack is parallel to the weld, the x-direction will be parallel to the crack edge.

Figure 6 – Schematic ACFM probe.

With no defect present and a uniform current flowing in the y-direction, the magnetic field is uniform in the x-direction perpendicular to the current flow, while the other two components, By and Bz, are zero. The presence of a defect diverts current away from the deepest part and concentrates it near the ends of a crack. The effect of this is to produce strong peaks and troughs in By and Bz (above the ends of a crack), while Bx shows a broad dip along the whole defect. A qualitative explanation of the signals is shown in Figure 7.

Figure 7 - Qualitative explanation of the nature of Bx and Bz above a notch.

ACFM probes generally measure Bx and Bz, the former being used to estimated crack depth and the latter giving an estimated of crack length. An example of the Bx and Bz signals above a crack is shown in the chart recorder plot on the left in Figure 8.

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