Flaw Gates

Figure 5-30. Angle Beam Contact Search Unit

TM-1-1500-335-23 Nondestructive Inspection Methods Manual
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T.O. 33B-1-1 5-28 5.2.1.5 Flaw Gates. A gate is an electronic feature that allows an inspector to monitor for discontinuities within specific zones of the test part. A gate appears on the display as a short horizontal sweep segment above the baseline. The gate can be adjusted so that any signal that appears within the limits of the gate will energize an audible or visual alarm, alerting the inspector to a possible flaw that needs to be investigated further. Information about signals within a gate also may be recorded or displayed in C-scan format on a video monitor, from which the image data subsequently may be saved on a hard or floppy disk. Controls for the gate on the display are as follows. 5.2.1.5.1 Gate Start. This control is similar to the Sweep Delay and is used to adjust the location of the leading edge of the gate on the display. 5.2.1.5.2 Gate Width/Length. This control is similar to Sweep Length/Range and is used to adjust the width of the gate or the location of the trailing edge of the gate. 5.2.1.5.3 Threshold/Alarm Level. This control adjusts the vertical position of the gate (accept/reject level). Only signals that exceed the level of the gate cause an alarm or a record to be made. 5.2.2 Search Units / Transducers. CAUTION Search units are fragile. They shall be handled with care. Sharp blows, caused by dropping a search unit or banging a search unit against a surface, can cause extensive damage. 5.2.2.1 General Search Unit Construction. Search units are available in a great variety of shapes and sizes. Figure 5-29 schematically shows the basic parts of a typical straight beam search unit used for contact inspection, while Figure 5-30 schematically shows an angle beam search unit. The backing material, shown in Figure 5-29, serves to damp the ringing of the transducer element after it is excited. This affects the resolution of an inspection as explained in paragraph 5.2.2.2.b. The plastic wedge, shown in Figure 5-30 serves to transmit longitudinal waves to the test part surface where mode conversion occurs. Refracted longitudinal, shear, or surface waves (depending on the angle of the plastic wedge) are generated in the test part.

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