The first few millimeters under the surface are inspected using creeping wave probes (Figure
2A). Creeping waves can detect 0.5 mm high buried and surface-breaking defects in the first
4 mm of the weld’s depth. Figure 3A shows the responses from three surface-breaking notches
with heights of 0.5 mm, 1 mm, and 2 mm, respectively. The signal-to-noise ratio for the 0.5 mm
notch is higher than 18 dB, which demonstrates how easy it is to detect such defects.Starting
2.5 mm deep below the surface, the phased array probe mounted on the special wedge covers the
entire volume and fusion line of the weld, in addition to the heat affected zone (HAZ). Efficiency
is optimum up to 2 mm above the root (Figure 2B).