The three basic ultrasonic
measurement techniques most widely used today are:
1. Pulse echo
2. Transmission
3. Resonance
Pulse-Echo Technique
The pulse-echo technique is the
most popular of the three basic ultrasonic, nondestructive testing techniques.
The pulse-echo technique is very useful in detecting flaws and for thickness
measurement. The initial pulse of ultrasonic energy from a transducer is
introduced into the test specimen through the couplant. This sound wave travels
through the thickness of the specimen until a reflecting surface is
encountered, at which time the sound wave reflects back to the transducer. This
is called the back-wall echo. If the wave encounters a fl aw in its path, the
fl aw acts as a reflecting surface and the wave is reflected back to the
transducer. The echo in this case is referred to as a flaw echo. In both cases,
the reflected wave travels back to the transducer, causing the transducer
element to vibrate and induce an electrical energy that is normally amplified
and displayed onto a CRT or other such device. The echo wave coming from the
back wall of the specimen is marked by its transit time from the transducer to
the back wall and return. Similarly, the transit time for the fl aw echo can
also be determined by this technique. Since transit time corresponds to the
thickness of the specimen, it is quite possible to calculate the thickness of the
specimen using simple computer logic. One other technique, known as the
immersion test technique, has generated tremendous interest among the manufactures
that are in favor of automated inspection techniques. In the immersion technique,
the specimen is completely immersed in the liquid.
Transmission Technique
In this technique, the intensity
of ultrasound is measured after it has passed through the specimen. The
transmission technique requires two transducers, one to transmit the sound
waves and one to receive them. The transmission testing can be done either by
direct beams or reflected beams. In either case, the flaws are detected by
comparing the intensity of ultrasound transmitted through the test specimen with
the intensity transmitted through a reference standard made of the same
material. The best results are achieved by using the immersion technique since
this technique provides uniform and efficient coupling between transducers and
test specimen. The main application of the transmission technique is in detecting
flaws in laminated plastic sheets.
Resonance Technique
This method is primarily useful
for measuring the thickness of the specimen. This is accomplished by
determining the resonant frequencies of a test specimen.
APPLICATION OF ULTRASONIC NDT IN PLASTICS
Ultrasonic nondestructive testing
(NDT) has gained popularity in the past decade along with the growth of the
plastic industry and along with an increasing emphasis placed on automation and
material saving. Two major areas in which ultrasonic testing concepts are applied
extensively are fl aw detection and thickness measurement. The pulse-echo
technique is used to detect a flaw such as voids and bubbles in an extruded rod
of rather expensive materials such as Teflon and nylon. The flaw detection unit
and other auxiliary equipment can be programmed so that the specific portion of
the rod with a flaw is automatically cut off and discarded without disturbing
the continuous extrusion process. The transmission technique is commonly used
to detect flaws in laminates. Thickness measurement by ultrasonic equipment is
simple, reliable, and fast. This NDT technique simplifies the wall thickness
measurement of parts with hard-to-reach areas and complex part geometry.
Automated wall thickness measurement and control of large diameter extruded pipe
is accomplished by using the immersion technique. An ultrasonic sensing unit is
placed in a cooling tank to continuously monitor wall thickness. In the event
of an out of control condition, a closed-loop feedback control system is
activated and corrections are made to bring the wall thickness closer to the
set point. Many such systems are commercially available. The ultrasonic NDT
technique is used extensively by gas companies to examine the integrity of
plastic pipe socket joints after they have been solvent cemented together.
Ultrasonic measurements can also be used for determining the moisture content
of plastic. In materials like nylons, the attenuation and the acoustic velocity
change with the change in moisture content. The use of ultrasonics in testing
reinforced plastics and missiles and rockets has been discussed.
