A 6 inch diameter rod is being inspected for center-line cracks. The A scan presentation for one complete path through the rod is as shown in Figure 2. Where should there be an alarm gate?

A 6 inch diameter rod is being inspected for center-line cracks. The A scan presentation for one complete path through the rod is as shown in Figure 2. Where should there be an alarm gate?

• A. Alarm gate should be used between points B and D
• B. An alarm gate should be used at point D only
• C. Alarm gate should be used between points A and E
• D. An alarm gate could not be used for this application

Answer: (A)

In A-scan UT, you place an alarm gate in the time window where a real flaw would return a signal, not where surface echoes or noise obscure it. A center-line crack in a 6-inch rod will reflect energy after the initial near-surface echo but before the far-wall echo, so its reflection should appear in the middle portion of the trace. Placing the gate between the echoes that correspond to the near surface and the far surface ensures the flaw signal from a center-line crack will trigger the alarm, while avoiding premature or late signals from surface reflections. That window is the region shown between the two major surface echoes, which corresponds to the zone in the figure where a center-line crack would be detected.

In A-scan UT, you place an alarm gate in the time window where a real flaw would return a signal, not where surface echoes or noise obscure it. A center-line crack in a 6-inch rod will reflect energy after the initial near-surface echo but before the far-wall echo, so its reflection should appear in the middle portion of the trace. Placing the gate between the echoes that correspond to the near surface and the far surface ensures the flaw signal from a center-line crack will trigger the alarm, while avoiding premature or late signals from surface reflections. That window is the region shown between the two major surface echoes, which corresponds to the zone in the figure where a center-line crack would be detected.