ANGLE BEAM TESTING
After reading this section you will be able to do the following:
- Explain why it is important to know about sound refraction and Snell's Law when performing an angle beam inspection.
- Explain what a shear wave is.
Often straight beam testing will not find a defect. For example, if the defect is vertical and thin enough, it will not reflect enough sound back to the transducer to let the tester know that it exists. In cases like this, another method of ultrasound testing must be used. The other method of ultrasound testing is angle beam testing. Angle beam testing uses an incidence of other than 90 degrees. In contact testing, an angled plastic block is place between the transducer and the object to create the desired angle. For angle beam testing in immersion systems, a plastic block is not needed because the transducer can simply be angled in the water.
If the angle of incidence is changed to be anything other than 90 degrees, longitudinal waves and a second type of sound wave are produced. These other waves are called shear waves. Because the wave entered at an angle, it does not all travel directly through the material. Molecules in the test object are attracted to each other because solids have strong molecular bonds. The molecules carrying the sound are attracted to their surrounding molecules. Because of the angle, those sound carrying molecules get pulled by attracting forces in a direction perpendicular to the direction of the wave. This produces shear waves, or waves whose molecules travel perpendicular to the direction of the wave.
Angle beam testing and a change in the angle of incidence also creates further complications. Remember that when a wave hits a surface at an angle, it will be refracted, or bent, when it enters the new medium. Thus, the shear waves and the longitudinal waves will be refracted in the test object. The amount of refraction depends on the speed of sound in the two mediums between which the wave is traveling. Since the speed of shear waves is slower than the speed of longitudinal waves, their angles of refraction will be different. By using Snells law, we can calculate the angle of refraction if we know the speed of sound in our material.
- An angle beam test cannot be performed unless the angle of refraction is calculted using Snell's law, and the speed of sound must be known too.
- Shear waves are produced when the angle of incidence is not 90 degrees.