Ultrasonic Examination – Hydrogen Blisters

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The use of longitudinal wave UT for detecting, locating, and determining the size of hydrogen blisters is illustrated in Figure 700-17. The blisters are internal flaws that have a reflecting surface at a depth from the front surface that is less than the distance to the back surface (thickness) of the shell component. Therefore, the blisters will cause reflected peaks to appear in the oscilloscope display at a distance less than the thickness of the shell.

Detecting Hydrogen Blisters with Longitudinal Wave UT

The peaks attributable to the blisters will usually be lower than the DAC, because the surface of the blisters will not normally be as good a reflector as the back surface (similar to the effect of a corroded surface). Furthermore, very thin blisters may have sufficient contact between their opposite surfaces to permit propagation of some of the ultrasonic wave through the blister. This situation is depicted by the position of Transducer B, in which case a small reflection from the back surface will appear in the oscilloscope display. However, no portion of the ultrasonic wave can propagate through blisters with more widely separated opposite surfaces, and no reflection from the back surface will be observed, as depicted by the position of Transducer C.

The distance of the peaks on the oscilloscope display resulting from the blisters indicates their depths below the surface, and the size of each blister can be estimated by moving the transducer along the surface until the reflection attributable to the blister disappears.

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