Ultrasonic Examination Physical Principles
A high-frequency sonic (referred to as “ultrasonic”) wave is introduced into a solid material and is reflected by interfaces in the material. The reflected waves are analyzed to detect the existence of a flaw and determine its location. The interfaces that reflect the ultrasonic waves can be between the solid material and a gas, liquid, or other solids. Solid-gas interfaces tend to completely reflect the ultrasonic waves, whereas solid-liquid and solid-solid interfaces usually only partially reflect the ultrasonic waves. Flaws in metal pressure vessel shells normally act as either solidgas or solid-solid interfaces, and can, therefore, be either very good or very poor reflectors. The surfaces of the vessel shell are solid-gas interfaces that are normally very good reflectors.
Ultrasonic waves are mechanical waves that are created by displacement of particles (atoms or molecules) in an elastic medium from their equilibrium positions. The displaced particles are restored to their equilibrium positions by interatomic forces acting between the particles. However, the interatomic forces also induce displacement of adjacent particles that results in propagation of the wave through the medium. Two types of ultrasonic wave can be created, depending on the orientation of particle displacement relative to the direction of wave propagation. Longitudinal waves are developed when the particles are displaced parallel to the direction of propagation, whereas transverse (or shear) waves result when particles are displaced perpendicular to the direction of propagation, as illustrated in Figure 700-10. Shear waves will generally propagate only in solids, because the interatomic forces in liquids and gases are too weak for perpendicular displacements to induce displacement of adjacent particles.
A longitudinal ultrasonic wave propagating through one medium in a direction that is perpendicular to the interface with a second medium (i.e., angle of incidence = 0 degrees from vertical), will be both reflected back into the first medium and transmitted into the second medium without change in direction as a longitudinal wave. However, if the angle of incidence is not 0 degrees, the wave will be reflected at an angle equal to the angle of incidence. The portion of the wave transmitted into the second medium will be refracted at the interface, and will split into both longitudinal and shear waves that propagate into the second medium in directions that differ from the angle of incidence, as shown in Figure 700-11.
A longitudinal wave that is not perpendicular to the interface is refracted when it propagates into the second medium, because of the difference in the velocity of sound between the two media. Splitting of the incident longitudinal beam into refracted longitudinal and shear waves is known as “mode conversion,” and occurs because the displacement of particles at the interface parallel to the direction of the incident longitudinal wave is not parallel to the refracted longitudinal wave. The sine of the angles of refraction for the refracted longitudinal and refracted (mode converted) shear waves are proportional to the difference in their velocities of sound between the two media forming the interface. The refracted longitudinal wave has a greater angle of refraction than the mode converted shear wave, due to its higher velocity. Although the angles of refraction for both refracted longitudinal and mode converted shear waves vary with the angle of incidence, the angle between the longitudinal and shear waves will always be the same.
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