Ultrasonic Beam Characteristics

Ultrasonic Beam Characteristics
To properly identify discontinuities or defects, the location of the ultrasonic beam must be estimated. This estimation includes taking beam attenuation, beam spread, and beam centerline location into account.
Beam Attenuation
When sound waves travel through non-idealized (i.e., real) materials, there is a pronounced reduction in the signal strength. This phenomenon, known as attenuation, results primarily from two basic causes: diffraction and absorption.
Beam diffraction
When sound waves encounter a finite boundary, abrupt changes in the direction of propagation of the sound wave may occur. This is known as diffraction. Diffraction occurs when the sound beam encounters a boundary such as a crack tip or member edge. Diffraction also occurs continuously as the beam passes from each grain of material to the next. This important type of diffraction is commonly known as scattering. Scattering of the sound beam occurs as a result of the generally coarse-grained properties of metals. Each grain boundary is a small reflector that emits scattered and reflected signals. For very coarse-grained materials, this can actually lead to detectable echoes, which are commonly referred to as "grass," that typically present low-amplitude signals on an A-scan.
Beam absorption
The second cause of attenuation is known as absorption. In beam absorption, the sound energy passing through the test material is directly converted to heat. Absorption in crystalline metals can generally be thought of as a process of converting the signal energy to heat through friction. Describing the actual process of beam absorption is well beyond the scope of what is needed here.
Beam Spread (Beam Divergence)
Beam spreading occurs in all ultrasonic beams. By definition, beam spread occurs because the beam energy does not stay within the cross section of the transducer. Rather, the beam starts out as a cylinder and then, after some distance, spreads into a cone. This spreading reduces the intensity of the wave at each discrete point and, as a result, lowers the amount of energy that could be reflected at a defect. This phenomenon is combated through the use of Distance Amplitude Correction (DAC), which is described later. The angle of beam spread (β) can be approximated using equation 8. This equation gives the angle from the centerline of the beam to the perimeter of the central energy lobe.