6-1. General. Ultrasonic is the name given to the study and application of sound wave frequencies higher than those to
which the human ear can respond, 20,000 Hz (hertz or cycles per second). In contact ultrasonic testing the most
commonly used frequencies range is from 2.25 to 10 MHz (megahertz or million cycles per second). Frequencies below
this range and up to about 25 MHz are also used on occasion.
6-2. Advantages. Ultrasonic detection equipment has made it possible to locate defects in all types of parts without
damaging the part being inspected. Minute cracks, checks, and voids, too small to be seen by X-ray, are located by
ultrasonic inspections. Access to only one surface of the part is necessary.
6-3. Disadvantages. The sound beam is not a straight sided projection of the face of the search unit having uniform
intensity. The sound beam spreads out beyond the face of the search unit and varies in intensity. This causes dead
zones and other problems. Refer to TM 55-1500-335-23.
6-4. Basic Testing Methods. Contact and immersion methods are used for ultrasonic inspections. In the contact
method of ultrasonic inspection, the search unit is placed directly on the test part surface using a thin film of couplant,
such as oil, to transmit sound into the test part. In the immersion method, the test part is immersed in a fluid, usually
water, and sound is transmitted through the water to the test part.
6-5. Nature of Ultrasonic Waves. Ultrasonic sound beams have properties similar to light beams. For example, when
an ultrasonic beam strikes an interrupting object, sound beam energy is reflected from the surface of the interrupting
object. The angle of incidence is equal to the angle of reflection. The reflected energy may be picked up by a search
unit. This search unit is usually the same search unit used to generate the sound beam, but may be a second search
unit. The search unit transforms the received ultrasonic energy into electrical energy. The ultrasonic instrument
amplifies this electrical energy and presents it as a vertical deflection on a cathode ray tube.
6-6. Modes of Vibration. Sound energy is propagated in an object by the vibration of particles in the object. Ultrasonic
energy is transmitted from one atom to another The direction in which the particles (atoms) vibrate in relation to the
direction of the ultrasonic beam propagation is dependent on the mode of vibration.
Longitudinal Waves. The longitudinal or compressional wave mode is characterized by particle movement
parallel to the direction of sound beam propagation.
Transverse Waves. The transverse wave mode is characterized by particle movement perpendicular to the
direction of the sound beam propagation. Tranverse waves travel at approximately one half the speed of longitudinal
waves. Transverse waves are introduced into a test part by using an angle beam search unit. This type of search unit
consists of a transducer element mounted on a plastic wedge, so that ultrasonic waves enter the test part at an angle
Rayleigh Waves. Surface waves, or Rayleigh waves, are a special type of shear wave in which the motion of the
particles is confined to a thin layer on the free boundary of a solid.
Lamb Waves. Lamb wave propagation occurs when ultrasonic waves travel along a test part with thickness less
than the wavelength. There are two general classes of Lamb waves: symmetrical and asymmetrical waves. An infinity
of modes of each class of vibrations are possible in a given test part Theory shows that the velocity of the lamb waves is
dependent on the mode and can exhibit many different velocities.
6-7. Transmission Characteristics. Reflection, refraction and mode conversion, and beam divergence are explained
by the following paragraphs.
Reflection, When an ultrasonic beam strikes a boundary between two different objects, part of the energy is
transmitted to the second medium and part is reflected. The percentage of sound transmitted and reflected is related to
the acoustic impedances of the two materials. Acoustic impedance, z, is the product of density, p or rho, and velocity, v,