Moving backward and forward, like a pendulum; vibration. A cycle
is an interval of time in which a certain succession of events
or phenomena is completed, and then returns again and again, uniformly
and continually in the same order.
A backward and forward movement, like a pendulum or vibration.
Ultrasonic testing is based on time-varying deformations or vibrations
in materials, which is generally referred to as acoustics. All
material substances are comprised of atoms, which may be forced
motion about their equilibrium positions. Many different patterns
of vibrational motion exist at the atomic level, however, most
are irrelevant to acoustics and ultrasonic testing. Acoustics
is focused on particles that contain many atoms that move in unison
to produce a mechanical wave. When a material is not stressed
in tension or compression beyond its elastic limit, its individual
particles perform elastic oscillations. When the particles of
a medium are displaced from their equilibrium positions, internal
(electrostatic) restoration forces arise. It is these elastic
restoring forces between particles, combined with inertia of the
particles, that leads to the oscillatory
motions of the medium.
In solids, sound waves can propagate in four principle modes
that are based on the way the particles oscillate. Sound can propagate
as longitudinal waves, shear waves, surface waves, and in thin
materials as plate waves. Longitudinal and shear waves are the
two modes of propagation most widely used in ultrasonic testing.
The particle movement responsible for the propagation of longitudinal
and shear waves is illustrated below.
waves, the oscillations occur in the longitudinal direction
or the direction of wave propagation. Since compressional and
dilational forces are active in these waves, they are also called
pressure or compressional waves. They are also sometimes called
density waves because their particle density fluctuates as they
move. Compression waves can be generated in liquids, as well as
solids because the energy travels through the atomic structure
by a series of compressions and expansion (rarefaction) movements.
In the transverse
or shear wave, the particles oscillate at a right angle or
transverse to the direction of propagation. Shear waves require
an acoustically solid material for effective propagation, and
therefore, are not effectively propagated in materials such as
liquids or gasses. Shear waves are relatively weak when compared
to longitudinal waves. In fact, shear waves are usually generated
in materials using some of the energy from longitudinal waves.