a. What is Ultrasound?
Sound generated above the human hearing range (typically 20 kHz) is called
ultrasound. However, the frequency range normally employed in ultrasonic
nondestructive testing and thickness gaging is 100 kHz to 50 MHz. Although
ultrasound behaves in a similar manner to audible sound, it has a much
shorter wavelength. This means it can be reflected off very small surfaces
such as defects inside materials. It is this property that makes ultrasound
useful for nondestructive testing of materials.
b. Frequency, Period and Wavelength
Ultrasonic vibrations travel in the form of a wave, similar to the way
light travels. However, unlike light waves, which can travel in a vacuum
(empty space), ultrasound requires an elastic medium such as a liquid or a
solid.
c. Velocity of Ultrasound and Wavelength
The velocity of ultrasound (c) in a perfectly elastic material at a given
temperature and pressure is constant. The relation between c, f, l and T is
given by
l = Wavelength
c = Material Sound Velocity
f = Frequency
T = Period of time
l=c/f
l=cT
d. Wave Propagation and Particle Motion
The most common methods of ultrasonic examination utilize either
longitudinal waves or shear waves. Other forms of sound propagation exist,
including surface waves and Lamb waves.
• A longitudinal wave is a compressional wave in which the particle
motion is in the same direction as the propagation of the wave.
• A shear wave is a wave motion in which the particle motion is
perpendicular to the direction of the propagation.
• Surface (Rayleigh) waves have an elliptical particle motion
and travel across the surface of a material. Their velocity is
approximately 90% of the shear wave velocity of the material
and their depth of penetration is approximately equal to one
wavelength.
• Plate (Lamb) waves have a complex vibration occurring in
materials where thickness is less than the wavelength of ultrasound
introduced into it.
e. Applying Ultrasound
Ultrasonic nondestructive testing introduces high frequency sound waves
into a test object to obtain information about the object without altering
or damaging it in any way. Two basic quantities are measured in ultrasonic
testing; they are time of flight or the amount of time for the sound to travel
through the sample, and the amplitude of the received signal. Based on
velocity and round trip time of flight through the material the material,
thickness can be calculated as follows:
T = Material Thickness
c = Material Sound Velocity
t = Time of Flight
T=ct/2
f. Sensitivity and Resolution
• Sensitivity is the ability of an ultrasonic system to detect reflectors
(or defects) at a given depth in a test material. The greater the
signal that is received from a given reflector, the more sensitive the
transducer system.
• Axial resolution is the ability of an ultrasonic system to produce
simultaneous and distinct indications from reflectors Iocated at
nearly the same position with respect to the sound beam.
• Near surface resolution is the ability of the ultrasonic system to
detect reflectors located close to the surface of the test piece.
