Ultrasonic testing is a family of non-destructive testing techniques based in the propagation
of ultrasonic waves in the object or material tested. In most common UT applications, very
short ultrasonic pulse-waves with center frequencies ranging from 0.1-15 MHz, and occasionally
up to 50 MHz, are transmitted into materials to detect internal flaws or to characterize
materials. A common example is ultrasonic thickness measurement, which tests the thickness
of the test object, for example, to monitor pipework corrosion.
Ultrasonic testing is often performed on steel and other metals and alloys, though it can
also be used on concrete, wood and composites, albeit with less resolution. It is used in
many industries including steel and aluminium construction, metallurgy, manufacturing, aerospace,
automotive and other transportation sectors.
History On May 27, 1940, U.S. researcher Dr. Floyd
Firestone of the University of Michigan applies for a U.S. invention patent for the first
practical ultrasonic testing method. The patent is granted on April 21, 1942 as U.S. Patent
No. 2,280,226, titled "Flaw Detecting Device and Measuring Instrument". Extracts from the
first two paragraphs of the patent for this entirely new nondestructive testing method
succinctly describe the basics of such ultrasonic testing. "My invention pertains to a device
for detecting the presence of inhomogeneities of density or elasticity in materials. For
instance if a casting has a hole or a crack within it, my device allows the presence of
the flaw to be detected and its position located, even though the flaw lies entirely within
the casting and no portion of it extends out to the surface. ... The general principle
of my device consists of sending high frequency vibrations into the part to be inspected,
and the determination of the time intervals of arrival of the direct and reflected vibrations
at one or more stations on the surface of the part."
James F. McNulty of Automation Industries, Inc., then, in El Segundo, California, an
early improver of the many foibles and limits of this and other nondestructive testing methods,
teaches in further detail on ultrasonic testing in his U.S. Patent 3,260,105that “Basically
ultrasonic testing is performed by applying to a piezoelectric crystal transducer periodic
electrical pulses of ultrasonic frequency. The crystal vibrates at the ultrasonic frequency
and is mechanically coupled to the surface of the specimen to be tested. This coupling
may be effected by immersion of both the transducer and the specimen in a body of liquid or by
actual contact through a thin film of liquid such as oil. The ultrasonic vibrations pass
through the specimen and are reflected by any discontinuities which may be encountered.
The echo pulses that are reflected are received by the same or by a different transducer and
are converted into electrical signals which indicate the presence of the defect.”
How it works
In ultrasonic testing, an ultrasound transducer connected to a diagnostic machine is passed
over the object being inspected. The transducer is typically separated from the test object
by a couplant or by water, as in immersion testing. However, when ultrasonic testing
is conducted with an Electromagnetic Acoustic Transducer the use of couplant is not required.
There are two methods of receiving the ultrasound waveform: reflection and attenuation. In reflection
mode, the transducer performs both the sending and the receiving of the pulsed waves as the
"sound" is reflected back to the device. Reflected ultrasound comes from an interface, such as
the back wall of the object or from an imperfection within the object. The diagnostic machine
displays these results in the form of a signal with an amplitude representing the intensity
of the reflection and the distance, representing the arrival time of the reflection. In attenuation
mode, a transmitter sends ultrasound through one surface, and a separate receiver detects
the amount that has reached it on another surface after traveling through the medium.
Imperfections or other conditions in the space between the transmitter and receiver reduce
the amount of sound transmitted, thus revealing their presence. Using the couplant increases
the efficiency of the process by reducing the losses in the ultrasonic wave energy due
to separation between the surfaces. Features
Advantages High penetrating power, which allows the detection
of flaws deep in the part. High sensitivity, permitting the detection
of extremely small flaws. Only one surface needs to be accessible.
Greater accuracy than other nondestructive methods in determining the depth of internal
flaws and the thickness of parts with parallel surfaces.
Some capability of estimating the size, orientation, shape and nature of defects.
Non hazardous to operations or to nearby personnel and has no effect on equipment and materials
in the vicinity. Capable of portable or highly automated operation.
Disadvantages Manual operation requires careful attention
by experienced technicians. The transducers alert to both normal structure of some materials,
tolerable anomalies of other specimens and to faults therein severe enough to compromise
specimen integrity. These signals must be distinguished by a skilled technician, possibly,
after follow up with other nondestructive testing methods.
Extensive technical knowledge is required for the development of inspection procedures.
Parts that are rough, irregular in shape, very small or thin, or not homogeneous are
difficult to inspect. Surface must be prepared by cleaning and removing
loose scale, paint, etc., although paint that is properly bonded to a surface need not be
removed. Couplants are needed to provide effective
transfer of ultrasonic wave energy between transducers and parts being inspected unless
a non-contact technique is used. Non-contact techniques include Laser and Electro Magnetic
Acoustic Transducers. Inspected items must be water resistant, when
using water based couplants that do not contain rust inhibitors.
Standards International Organization for Standardization
ISO 7963, Non-destructive testing - Ultrasonic testing - Specification for calibration block
No. 2 ISO/DIS 11666, Non-destructive testing of
welds - Ultrasonic testing of welded joints - Acceptance levels
ISO/DIS 17640, Non-destructive testing of welds - Ultrasonic testing of welded joints
ISO 22825, Non-destructive testing of welds - Ultrasonic testing - Testing of welds in
austenitic steels and nickel-based alloys European Committee for Standardization
EN 583, Non-destructive testing - Ultrasonic examination
EN 1330-4, Non destructive testing - Terminology - Part 4: Terms used in ultrasonic testing
EN 1712, Non-destructive testing of welds - Ultrasonic testing of welded joints - Acceptance
levels EN 1713, Non-destructive testing of welds
- Ultrasonic testing - Characterization of indications in welds
EN 1714, Non-destructive testing of welds - Ultrasonic testing of welded joints
EN 12223, Non-destructive testing - Ultrasonic examination - Specification for calibration
block No. 1 is replaced by the EN ISO 2400:2012 "Non-destructive testing - Ultrasonic testing
- Specification for calibration block No. 1"
EN 12668-1, Non-destructive testing - Characterization and verification of ultrasonic examination
equipment - Part 1: Instruments EN 12668-2, Non-destructive testing - Characterization
and verification of ultrasonic examination equipment - Part 2: Probes
EN 12668-3, Non-destructive testing - Characterization and verification of ultrasonic examination
equipment - Part 3: Combined equipment EN 12680, Founding - Ultrasonic examination
EN 14127, Non-destructive testing - Ultrasonic thickness measurement
See also Non-Contact Ultrasound
Phased array ultrasonics Time-of-flight diffraction ultrasonics
Time-of-flight ultrasonic determination of 3D elastic constants
Internal rotary inspection system ultrasonics for tubes
EMAT Electromagnetic Acoustic Transducer ART
References
Further reading
External links Ultrasonic testing
Film about Russian ultrasonic testing unit ASK-132 in Kalinin NPP
Ultrasonic Testing on NDTWiki.com Video on ultrasonic testing, Karlsruhe University
of Applied Sciences [1]