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Preparatory Notes for ASNT NDT Level III Examination - Ultrasonic Testing, UT

Preparatory Notes for ASNT NDT Level III Examination - Ultrasonic Testing

Preparatory Notes for ASNT NDT Level III Examination - Ultrasonic Testing

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Crack height (a) is a function of the ultrasound velocity (v) in the material, the<br />

incident angle (Q2) and the difference in arrival times between the two signal<br />

(dt). Since the incident angle and the thickness of the material is the same in<br />

both measurements, two similar right triangle are <strong>for</strong>med such that one can<br />

be overlayed on the other. A third similar right triangle is made, which is<br />

comprised on the crack, the length dt and the angle Q2. The variable dt is<br />

really the difference in time but can easily be converted to a distance by<br />

dividing the time in half (to get the one-way travel time) and multiplying this<br />

value by the velocity of the sound in the material. Using trigonometry an<br />

equation <strong>for</strong> estimating crack height from these variables can be derived as<br />

shown below.

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