Non Destructive Test -Ultrasonic Testing:
Ultrasonic testing is one of the
most widely used methods for nondestructive inspection. In plastics, the
primary application is the detection of discontinuities and measurement of thickness.
Ultrasonic techniques can also be used for determining the moisture content of
plastics, studying the joint integrity of a solvent-welded plastic pipe and fittings,
and testing welded seams in plastic plates.
The term ultrasonic, in a broad
sense, is applied to describe sound with a frequency above 20,000 cycles/sec.
Commercial ultrasonic testing equipment generally employs the testing frequency
in the range from 0.75 to 20 MHz. To provide a basis for understanding the
ultrasonic system and how it operates, it is necessary to introduce the
following terms:
Frequency Generator. A device that imposes a short
burst of high-frequency alternating voltage on a transducer.
Transducer. A transducer or a probe is a
device that emits a beam of ultrasonic waves when bursts of alternating voltage
are applied to it. An ultrasonic transducer is comprised of piezoelectric
material. Piezoelectric material is material that vibrates mechanically under a
varying electric potential and develops electrical potentials under mechanical strain,
thus transforming electrical energy into mechanical energy and vice versa. As the
name implies, an electrical charge is developed by a piezoelectric crystal when
pressure is applied to it and reverse is also true. The most commonly
encountered piezoelectric materials are quartz, lithium sulfate, and artificial
ceramic materials such as barium titanate.
Many different types of
ultrasonic transducers are available, differing in diameter of the probe,
frequency, and frequency bandwidth. Each transducer has a characteristic
resonant frequency at which ultrasonic waves are most effectively generated and
received. Narrow bandwidth transducers are capable of penetrating deep, as well
as detecting small flaws. However, these transducers do a poor job of
separating echos. Broad bandwidth transducers exhibit excellent echo separation
but poor flaw detection and penetration. Transducers of a frequency range of
2–5 MHz are most common. For plastic materials, transducers in the range of 1–2
MHz seem to yield the best results.
Couplants. Air, being one of the worst
transmitters of sound waves at high frequencies owing to a lack of impedance
matching between air and most solids, must be replaced by a suitable coupling
agent between the transducer and the material being tested. Many different
types of liquids have been used as coupling agents. Glycerine seems to have the
highest acoustic impedance. However, oil is the most commonly used couplant.
Grease, petroleum jelly, and pastes can also be used as couplants, although a
wetting agent must be added to increase wettability as well as viscosity. Some
couplants have a tendency to react with the test specimen material and
therefore chemical compatibility of the couplant should be studied prior to
application. Couplants that are difficult to remove from test specimens should
also be avoided. Any type of contamination between the test specimen and the
transducer can seriously affect the thickness measurements, especially in the
case of thin films. Therefore, it is absolutely necessary to remove these
contaminants before applying the couplant. The basic sequence of operation in any
ultrasonic measurement system is:
1. Generation of ultrasonic
frequency by means of a transducer.
2. Use of a coupling agent
(couplant), such as oil or water, to help transmit the ultrasonic waves into
the material.
3. Detection of the ultrasonic
energy after it has been modified by the material.
4. Displaying of the
energy by means of a recorder, cathode-ray tube, or other devices.
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