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Sound Reflection and Refraction - Seeing is Believing
To help students develop a mental picture of ultrasonic energy, light energy can be used for some simple demonstrations. In many ways, sound and light behave similarly. Both ultrasound and light are reflected from surfaces, refracted when going from one medium to another, and scattered by rough surfaces.
To help students understand reflection of energy, a simple mirror or piece of polished metal and flashlight can be used. Give one of the students the flashlight and ask him or her to direct it towards the flat reflector (mirror of polished metal). Hold the flat reflector so that it faces the student and reflects the light energy right back into the person with the light. Be sure to direct the energy right into their eyes and point out that a large percentage of the light is being reflected directly back at them as they can see. Explain that this setup is analogous to a normal beam ultrasonic inspection and that a large flat reflector that is planner in the direction normal to the sound beam will produce a large reflection.
Then begin to tilt the reflector off at an angle so the light energy is reflected away from the light source. Ask the person holding the light (the transmitter) whether any of the energy is returning to them. Point out that in ultrasonic testing it is possible for significant defects to go undetected if the test configuration is such that the sound energy is reflected away from the receiving transducer. Then ask a second person to act as a secondary receiver and to move to the area where the light energy is reflecting. Explain that in ultrasonic inspection it is sometimes necessary to use one transducer to send the sound and a second transducer to receive the reflected sound. The technique used depends on the orientation of the flaw with respect the the part surfaces.
Experimenting with various reflectors might also be useful. Ask the students to compare and contrast the amount of light energy reflected from a large rectangular reflector, a small rectangular reflector, a spherical reflector with smooth surface and a spherical reflector with an irregular surface. The reflectors for this exercise can be constructed by covering rectangular and spherical objects with aluminum foil.
In summary it should be explained that in ultrasonic testing it is very important to know as much about the flaw as possible. If the target defect is planer, such as a crack or delamination, knowing its orientation is critical so that a technique can be developed that assures reflected sound energy will make its way to a receiving transducer. While the orientation of volumetric flaws are not as critical because they reflect sound in multiple directions, they will reflect less energy in any one direction. Also, the rough, irregular surfaces of cavitation flaws will reflect less sound than the nice smooth surfaces of porosity. It is likely the instrument gain setting will need to higher when a volumetric defect is the subject of the inspection.