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Ultrasonic Signal Interpretation
With the use of six easily fabricated blocks, students studying ultrasonic testing can gain insight into transducer movement and indication behavior on the ultrasonic flaw detector screen. The student should have prior knowledge of screen range calibration and ultrasonic theory regarding angle of incidence and reflection.
The range of test blocks that can be used is nearly endless, but the following are some suggestions.
Sample 1: The slot is cut parallel to top surface, and far enough from the side of the block to allow a backwall reflect to be seen before the transducer is scanned over the top of the slot. Student should see the backwall reflection disappear and see multiple indications from the slot. The slot is also a little less than 1/2T so when the echo from the slot starts to appear the student should note a slight shift on the time base line as the back echo loses amplitude and the indication from the slot increases in amplitude.
Sample 2: A cavity is cut at an uneven angle to the top surface, and far enough from the side of the block to allow a backwall reflect to be seen before the transducer is scanned over the top of the cavity. Student should see the backwall reflection disappear and see an indication from the cavity appear. As the student scans left and right over the cavity, they should note the indication on the screen move to the right of the UT screen indicating an increase in depth from the reflector. Students should also note the shape of the signal. Instead of a nice straight up/down signal, the indication will have several small peaks included with an indication that occupies a large section of the time base line.
Sample 3: This sample has 2 drilled holes, one close to the top and the other close to the bottom. When the student scans over the top of the near side hole, the signal may show up in the dead zone, dependent upon transducer selection. If it does, have the student switch to a higher frequency transducer and emphasize that resolution is better with higher frequency and the dead zone is shortened. When scanning over the more distant hole the student should note that the indication from the hole drops off very quickly and the indication itself is a sharp indication that does not occupy a large area of the time base line.
Sample 4: The slot is cut 0.15 inches below the surface with a drilled hole underneath. As the student scans over the slot they will see multiple back echoes coming from the slot. The first back echo may be in the dead zone. Ask the student to study how the initial pulse changes when they scan over the top of the slot. Question the student as to where the drilled hole indication is on the screen. A lot of students do not remember that you are unable to see anything under the first reflecting surface. Have the student repeat the inspection from the opposite side of the block and compare and contrast the two inspections.
Sample 5: This slot is cut at an angle of approximately 15 degrees. As students scan over the top of the angled slot they will see loss of backecho and a low amplitude indication from the slot. The indication will occupy a large segment of the time base line. Have the student move the transducer across the top of the slot from right to left and notice the movement of the indication on the time base line. The indication moves to the right on the screen as the slot goes deeper into the part and left on the screen as the slot gets shallower in the part.
Sample 6: This block has a slot cut at an angle of approximately 30 degrees. When the students scan the block they should see total loss of back echo when over the slot. They may see an indication from the top edge of the slot if this refection does not fall in the dead zone. They should also see an indication that comes up beyond the location of the 1st backwall echo. This indication is the result of the sound reflecting from the slot, traveling to the upper left corner of the block and returning to the transducer along the same path. Emphasize that the angle of incidence is equal to the angle of reflection.