Ultrasonic testing

• Ultrasonic testing of materials utilises high frequency sound waves to detect flaws
and make measurements.
• Ultrasonic inspection can be used for flaw detection/evaluation, dimensional
measurements, material characterization, and more.
• Basic principles of sound propagation
• Any mechanical wave is composed of oscillations of discrete particles
of material.
• All particles in the first plane are then forced to oscillate with the
same amplitude (width of oscillation) and frequency (number of
oscillations per second) as the external source.
• The elastic forces transmit the oscillations to the particles in the
second plane and so forth.
• The figure shows the instantaneous picture of a section of the model
in which a wave travelling from left to right has not yet reached the
right-hand edge.
• It can be seen that the phase shift of the oscillations creates zones
where the particles approach each other particularly closely.
• These compression zones alternate with rarified zones.
• The chronological pattern of the wave shows that these zones are
constantly recreated on the excitation side and that they travel in the
body at constant velocity and uniform intervals towards the right. This
represents an elastic wave.
• The only thing that travels in the wave is its state, in the case of elastic
waves the state of compression and rarefaction.
• The particles themselves remain in place and merely oscillate about
their positions of rest.
Types of sound waves

• Types of Waves Based on Propagation Mode

1.Longitudinal Waves: These waves feature particle oscillations parallel to the wave’s
direction of travel. They are prevalent in fluids (both gases and liquids) and can also
propagate through solids. Longitudinal waves are characterized by alternating
compressions and rarefactions of the medium.
2.Transverse Waves: In these waves, particle motion is perpendicular to the wave’s
direction of propagation. Transverse waves are exclusive to solid media, where shear
deformation is supported. They are significant in understanding the Earth’s interior and
the mechanical properties of materials.
3.Surface Waves: Occurring at the boundary between two distinct mediums, surface
waves combine aspects of both longitudinal and transverse waves. They diminish in
amplitude with depth in the medium and are essential in applications like seismology
and surface acoustic wave (SAW) devices.
• Types of Waves Based on Frequency
1.Audible Sound Waves: Falling within the human hearing spectrum of 20
Hz to 20 kHz, these waves encompass the sounds of daily life, from speech
and music to environmental sounds.
2.Infrasound Waves: With frequencies below 20 Hz, infrasound waves are
imperceptible to humans but can carry over long distances and through
various mediums. They are used in studying natural phenomena and
monitoring environmental conditions.
3.Ultrasound Waves: Frequencies above 20 kHz, beyond human hearing, are
utilized in numerous applications, from medical diagnostics (e.g.,
ultrasonography) to industrial cleaning and materials testing.
• Standing waves : Standing waves are a unique phenomenon
resulting from the interference of two waves traveling in
opposite directions with the same frequency.
• They are characterized by nodes (points of no movement)
and antinodes (points of maximum oscillation).
• Standing waves are fundamental in the study of musical
instruments, architectural acoustics, and the design of
resonant cavities for various applications.
• Understanding how standing waves form and their
properties helps in the precise control and manipulation of
sound in spaces and devices.
• Principle of UT
• A typical UT inspection system consists of several functional units, such as the
pulser/receiver, transducer, and display devices.
• A pulser/receiver is an electronic device that can produce high-voltage electrical
pulses.
• Driven by the pulser, the transducer generates high-frequency ultrasonic energy.
The sound energy is introduced and propagates through the materials in the form
of waves. When there is a discontinuity (such as a crack) in the wave path, part of
the energy will be reflected back from the flaw surface.
• The reflected wave signal is transformed into an electrical signal by
the transducer and is displayed on a screen.
• In the applet below, the reflected signal strength is displayed versus
the time from signal generation to when a echo was received.
• Signal travel time can be directly related to the distance that the
signal traveled. From the signal, information about the reflector
location, size, orientation and other features can sometimes be
gained.
• Advantages of Ultrasonic inspection
• It is sensitive to both surface and subsurface discontinuities.
• The depth of penetration for flaw detection or measurement is superior to
other NDT methods
• Only single-sided access is needed when the pulse-echo technique is used.
• It is highly accurate in determining reflector position and estimating size and shape.
• Minimal part preparation is required.
• Electronic equipment provides instantaneous results.
• Detailed images can be produced with automated systems.
• It has other uses, such as thickness measurement, in addition to flaw detection.
• Disdvantages of Ultrasonic inspection
• Surface must be accessible to transmit ultrasound.
• Skill and training is more extensive than with some other methods.
• It normally requires a coupling medium to promote the transfer
of sound energy into the test specimen.
• Materials that are rough, irregular in shape, very small, exceptionally thin or
not homogeneous are difficult to inspect.
• Cast iron and other coarse grained materials are difficult to inspect due to
low sound transmission and high signal noise.
• Linear defects oriented parallel to the sound beam may go undetected.
• Reference standards are required for both equipment calibration and the
characterization of flaws.