|
A railcar comes off the tracks,
rupturing its tank and spewing toxic chemicals into a river.
An oil tanker runs aground, fouling the water and endangering
marine life. A truck overturns on a highway, dumping tons
of waste.
It may be impossible to eliminate such catastrophes, but
their effects can be minimized with proper testing. Gaging
the thickness of an oil tanker's hull or a railcar can best
be accomplished through ultrasonic thickness gaging.
Thickness gages cater to many different applications,
whether you're measuring the thickness of a petrochemical
pipe or the paint on a fire hydrant. Anytime you need to
measure the thickness of an object and can only reach it
from one side, an ultrasonic thickness gage is the answer.
Ultrasonic thickness gages have the ability to measure
thickness of materials and some coatings without destroying
the material being tested. Using the transmission of high-frequency
sound waves to detect imperfections or to measure the thickness
of a material allows for less guesswork and leaves the material
intact. Most engineered materials, including metals, plastic,
ceramics, composites, epoxies and glass, can be measured
ultrasonically.
The most commonly used ultrasonic testing technique is
pulse echo, which introduces sound into the test object,
and reflections from the far side are returned to a receiver.
Sound wavelengths travel best through a medium that's thicker
than air, so the test piece is either submerged in water,
or glycerin or oil is placed between the gage sensor and
the material to be measured. Corrosion and precision gages
usually operate at frequencies between 2 kHz and 25 MHz.
Higher frequencies mean a tighter dispersion of sound waves,
resulting in higher accuracy.
Ultrasonic thickness gaging has been around for about
60 years. The technology is related to sonar technology
developed in the 1940s for use during World War II. But
some say thickness measurement dates back even earlier.
"Thickness gaging has been around since the beginning
of mankind," says Joseph Walker, vice president of
Elcometer. "When the pyramids were built, every block
was cut to precise standards. Each block was checked to
see that it met those standards. Only then was it set in
place."
Obviously, thickness gaging has changed significantly
since the pyramids were built. Originally--and in some cases
today--thickness measuring was done with a caliper. But
when making measurements, both sides of the object had to
be accessible in order to gage the material's thickness.
Other techniques included tapping the material with a
hammer and listening to the resulting sound. This method
helped to distinguish a thick spot from a thin spot, but
it provided little accuracy.
Formulas were also used. Knowing the original thickness
of the material, maintaining it over the years and calculating
the wear factors would allow for a reasonable estimate of
its thickness.
Most of today's ultrasonic gages are hand-held instruments
with digital readouts that display the material's thickness.
It once took expensive equipment and highly paid technicians
to get the same quality readings that can be done today,
and the impact of this technology's improvement isn't lost
on those in the industry. "The world has definitely
become safer because we can electronically measure things
more accurately than we could 50 years ago," says Tim
Hasselbeck, president of Staveley Instruments.
Most commercial ultrasonic thickness gages fall into one
of two types: precision or corrosion gages. Determining
which is needed depends largely on the material to be tested.
A precision gage measures the thickness of curved or flat
surfaces, perhaps for measuring a critical flight component
in a jet engine or the thickness of a container. Measurement
accuracy can reach 0.004 in., and the gage usually operates
at frequencies between 5 kHz and 25 MHz. Other applications
include plastics, rubber tubing, fiberglass and ceramics.
Corrosion gages measure many types of material that hold
or transport corrosive substances. Marine transport, petrochemical
pipe and tubing, barges, and railcars are constantly monitored
to make sure the hulls don't get too thin from corrosion.
Another important difference between a precision gage and
corrosion gage, from an engineering standpoint, is the type
of transducer used on the gage.
Transducers--or probes, as they are more commonly known--generate
bursts of sound waves when excited by electrical pulses.
Which type of transducer is called for depends on the type
of material, its thickness and the accuracy required. There
are three types of transducers used in thickness gaging:
Contact transducers--Used when measuring the thickness
of an object that's not necessarily a very thin element.
These are accurate to about 0.02 in. They are single-element
contact transducers normally used for general measurements.
Delay line transducers--Used specifically for measurement,
they can measure on flat, concave or convex materials, or
materials in confined spaces. Measurements are most often
taken in a medium such as water.
Dual-element transducers--Used for monitoring corrosion.
They provide better penetration on rough or corroded parts
such as pipelines, storage tanks, pressure vessels, hulls
and steam lines. Dual-element transducers are very good
for finding pitting in material. In addition to measuring,
they also look for pitting or corrosion on the inside wall
of a pipe. Dual-element transducers use a separate transmitter
and receiver bonded to separate delay lines.
Once data are gathered, the information can be transmitted
and stored. Thickness gages can be configured with or without
data logging capability. In addition, they can store and
replay data through software packages that allow review
of information on computer displays. Usually, about 10,000
thickness readings can be stored, and there are several
software programs available that allow you to capture thickness
readings and store the data.
Sometimes gages must be used in explosive atmospheres,
which means the packaging of a gage becomes very important.
Additionally, in harsh environments, a gage's ability to
withstand extreme temperatures is critical. The different
types of packaging range from general-purpose to very application-
and environment-specific.
Temperature changes in the materials may also require
inspectors to closely monitor changes in material velocities
and equipment calibrations. Each material has its own rate
of velocity for sound. When the temperature of that material
changes, the rate at which the sound wavelengths resonate
also changes. Standard velocities are known for specific
materials, but each piece must be tested on an individual
basis to ensure more accurate readings. Steel, for example,
can minutely change after heat-treatment or coating, which
can have an effect great enough to affect accuracy due to
velocity change. Common variations in the manufacturing
process from batch to batch can affect cure characteristics
in coatings. This can cause changes in their ultrasonic
properties, calling for precise measurement of specific
materials on specific substrates.
Thickness gages have digital readouts that range from
simple one-button operations to sophisticated readouts that
provide a visual representation of the sound path and eliminate
false indicators. But in critical inspections--often for
safety concerns--there's no room for a false reading. Such
circumstances call for gages equipped with A-scans. These
gages can "see" the sound as it goes through the
part, which means a user can watch the sound path as the
sound enters the part and bounces back.
As manufactured parts become more complex, thickness gages
must evolve to keep up.
"How powerful, sophisticated and versatile can you
make an instrument and still keep it simple to operate?"
contemplates Hasselbeck. "Performance and simplicity
of operation are big issues when it comes to gages. We're
obviously trying to make technology simpler while maintaining
powerful performance."
This evolution can be seen in the new products thickness
gage suppliers are developing. Here's what a few suppliers
have in the works:
Stavely Instruments has been working on sensor recognition
for its instruments, i.e., linking the sensor with the instrument
so there's an enhanced communication line. "We now
make an instrument that can recognize the transducer you
plug into it," says Hasselbeck. Sensor-instrument recognition
simplifies the instrument use because it automatically knows
which test is about to be performed and sets itself up for
that type of thickness measurement. Certain controls can
be automatically shut off when the full versatility of the
instrument isn't needed.
"We're also making more serviceable products,"
he continues. "We're designing instruments with a unique
modularity, so downtime is kept to a minimum."
Defelsko has used ultrasonics to measure the thickness of
coatings over nonmetals.
"Prior to reliable ultrasonic coating thickness measurement,
parts were often destructively tested by cutting them and
examining them under a microscope," recalls David Beamish,
general manager of Defelsko Corp. "Alternatively, some
manufacturers were able to substitute a metal test panel
in the coating process that could be measured with a conventional
magnetic coating thickness gage."
Elcometer has found a way to test coating powder or dry
paint and use that test to calculate the cured thickness
of the powder before the part enters the curing process.
The company has developed a thickness tester that works
without relying on a medium. "This is an expensive
gage, but it's the only thing in the world that will do
what it does," says Walker.
BETA LaserMike has put ultrasonic thickness gaging to use
in the telecommunications industry. It provides online measurement
solutions for applications with ultrasonic measurement of
wall thickness and concentricity of materials. Because of
a drop in the bandwidth market, BETA LaserMike has had to
look elsewhere.
"That's the beauty of the ultrasonic product,"
says Tom Riley, marketing communications manager. "Rather
than using things like a laser scanner or other technologies
that were used to measure thickness and diameter in the
past, we're able to use ultrasonic technologies and broaden
our applications into plastics."
This article is only a basic overview of nondestructive
ultrasonic thickness gaging, its history and new developments.
From the pyramids of Egypt to measuring bandwidth in telecommunications,
thickness gaging has progressed and diversified. One of
the most visible types of thickness gaging focuses on minimizing
the effects of accidents and promoting safety by measuring
corrosion in the petrochemical industry. But this is just
one area of application for ultrasonic testing. For more
information on the subject, visit the e-Journal of Nondestructive
Testing at www.ndt.net or
the American Society for Nondestructive Testing at www.asnt.org.
Hallie Gorman is an editorial assistant at Quality Digest.
Letters to the editor regarding this article can be sent
to letters@qualitydigest.com.
|
