PULSE-ECHO ULTRASONIC TEST
After reading this section you will be able to do the following:
Your Turn - Try this normal beam test
A pulse-echo ultrasonic measurement can determine the location of a discontinuity with a part or structure by accurately measuring the time required for a short ultrasonic pulse generated by a transducer to travel through a thickness of the material. Then it reflects from the back or surface of a discontinuity and is returned to the transducer.
The applet below allows you to move the transducer on the surface of a stainless steel test block and see the reflected echoes as the would appear on an oscilloscope.
What the graphs tell us?
The ultrasonic tester graphs a peak of energy whenever the transducer receives a reflected wave. As you recall, sound is reflected any time a wave changes mediums. Thus, there will be a peak anytime the waves change mediums. Right when the initial pulse of energy is sent from the tester, some is reflected as the ultrasonic waves go from the transducer into the couplant. The first peak is therefore said to record the energy of the initial pulse. The next peak in a material with no defects is the backwall peak. This is the reflection from waves changing between the bottom of the test material and the material behind it, such as air or the table it is on. The backwall peak will not have as much energy as the first pulse, because some of the energy is absorbed by the test object and some into the material behind it.
The amount of distance between peaks on the graph can be used to locate the defects. If the graph has 10 divisions and the test object is 2 inches thick, each division represents 0.2 inches. If a defect peak occurs at the 8th division, we know the defect is located 1.6 (0.2 x 8) inches into the test object.
What if the thickness is unknown?
If the thickness of the object is unknown, it can be calculated using the amount of time it takes for the backwall peak to occur. The thickness of the object is traveled twice in that time, once to the backwall and once returning to the transducer. If we know the speed of our sound, then we can calculate the distance it traveled, which is the thickness of the object times two.
What happens when a defect is present?
If a defect is present, it will reflect energy sooner also. Another peak would then appear from the defect. Since it reflected energy sooner than the back wall, the defect's energy would be received sooner. This causes the defect peak to appear before the backwall peak. Since some of the energy is absorbed and reflected by the defect, less will reach the backwall. Thus the peak of the backwall will be lower than if had there been no defect interrupting the sound wave.
When the wave returns to the transducer, some of its energy bounces back into the test object and heads towards the back wall again. This second reflection will produce peaks similar to the first set of backwall peaks. Some of the energy, however, has been lost, so the height of all the peaks will be lower. These reflections, called multiples, will continue until all the sound energy has been absorbed or lost through transmission across the interfaces.