Ultrasonic testing (UT) comprises a range of non-destructive testing (NDT) techniques that send ultrasonic waves through an object or material. These high frequency sound waves are transmitted into materials to characterise the material or for flaw detecting. Most UT inspection applications use short pulse waves with frequencies ranging from 0.1-15 MHz, although frequencies up to 50 MHz can be used. One common application for this test method is ultrasonic thickness measurement, which is used to ascertain the thickness of an object such as when assessing pipework corrosion.

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How does it Work?

Ultrasonic inspection uses a piezoelectrictransducer connected to a flaw detector, which in its most basic form is a pulser-receiver and oscilloscope display. The transducer is passed over the object being inspected, which is typically coupled to the test object by gel, oil or water. This couplant is required to efficiently transmit the sound energy from the transducer into the part, however This couplant is not required when performing tests with non-contact techniques such as electromagnetic acoustic transducer (EMAT) or by laser excitation.

Pulse-echo and Through-transmission

Ultrasonic testing can be performed using two basic methods – pulse-echo and through-transmission.

With pulse echo testing, the same transducer emits and receives the sound wave energy. This method uses echo signals at an interface, such as the back of the object or an imperfection, to reflect the waves back to the probe. Results are shown as a line plot, with an amplitude on the y-axis representing the reflection’s intensity and distance or time on the x-axis, showing the depth of the signal through the material.

Through-transmission testing uses an emitter to send the ultrasound waves from one surface and a separate receiver to receive the sound energy that has reached the opposite side of the object. Imperfections in the material reduce the amount of sound that is received, allowing the location of flaws to be detected.

Contact and Immersion Testing

Ultrasonic testing can also be split into two main types: contact or immersion testing.

Contact ultrasonic testing is typically used for on-site inspections accessibility or portability. Contact ultrasonic inspection can be performed where only one side of a test specimen as reachable, or where the parts to be tested are large, irregular in shape or difficult to transport.

Immersion ultrasonic testing is a laboratory-based or factory-based non-destructive test that is best suited to curved components, complex geometries and for ultrasonic technique development. In this method, the component or material is submerged in a water, which acts as a couplant in place of the gels used for contact ultrasound. Immersion UT generally uses pulse-echo method, and robotic probe trajectories can be used to inspect complex surfaces which would be hard to cover with contact probes. Immersion UT can be used for a wide range of wall thickness and material types, making it a suitable testing method for a variety of applications and industries.

Why is it Used?

As a non-destructive testing method, ultrasonic testing is ideal for detecting flaws and defects without damaging the object or material being tested. Periodic ultrasonic inspections can also be used to check for corrosion or for growth of known flaws, and thus potentially prevent to a failure of a part, component or entire asset. It is used in a wide range of industries including aerospace, automotive, construction, medical, metallurgy, and manufacturing. 

What Materials Can Be Tested?

Ultrasonic testing is used in a wide range of industries due to its suitability for many different materials. UT is ideally used for inspection of dense, crystalline structures such as metals. Ceramics, plastics, composites and concrete can also be successfully inspected but with reduced resolution, since the attenuation in these materials is higher.

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Ultrasonic technology has been successfully employed in the medical sector for many decades, and is increasingly the preferred option for both routine diagnostic imaging and medical research because of the absence of ionising radiation.

Advantages

The advantages of ultrasonic testing include:

• High penetration power, allowing for flaw detection deep within a part
• High sensitivity, allowing for the detection of very small flaws
• Can be used to test when only one side of an object is accessible
• Greater accuracy, when compared to other non-destructive testing methods, for determining depth of internal flaws and the thickness of parts with parallel surfaces
• Able to estimate size, shape, orientation and nature of defects
• Able to estimate alloy structures of components with differing acoustic properties
• Non-hazardous to nearby personnel, equipment or materials
• Highly automated and portable operations possible
• Immediate results can be obtained, allowing for immediate decisions to be made

Limitations

There are, however, a few limitations to ultrasonic testing, as follows:

• Requires experienced technicians for inspection and for data interpretation
• False positive results, also known as spurious signals, may result from tolerable anomalies as well as the component geometry itself
• Objects that are rough, irregularly shaped, very small or thin, or not homogeneous are difficult to inspect
• Loose scale or paint will need to be removed before testing can commence, although clean, properly bonded paint can be left in place
• Couplants required for tests that use conventional UT
• UT may have reduced sensitivity for volumetric flaws, particularly metal inclusions, than radiographic testing

Applications

Ultrasonic testing has a variety of applications across industry, including testing the integrity of a material or component. This can include testing of welds to determine if there are any discontinuities present. This testing can be performed on both ferrous and non-ferrous materials as well as for thicker sections and those that are reachable from one side only. UT is also capable of detecting finer defects and planar flaws which may not be assessed as readily with radiographic testing.

Applications for UT include those within the aerospace, automotive, construction, rail, medical and oil and gas industries.

TWI Services and Courses

TWI provides a number of ultrasonic testing services to our Industrial Members as well as a range of non-destructive testing training courses for those wishing to learn about the techniques involved.

We can provide a full range of testing services and expertise, including in methods such as phased array ultrasonic testing (PAUT), laser ultrasonic testing and manual ultrasonic testing.

Ultrasonic flaw detection

What is NDT? 

Usually, pulsed beams of ultrasound are used and in the simplest instruments a single probe, hand-held, is placed on the specimen surface. An oscilloscope display with a time-base shows the time that it takes for an ultrasonic pulse to travel to a reflector (a flaw, the back surface, or other free surface) in terms of distance across the oscilloscope screen – the so-called A-scan display. The height of the reflected pulse is related to the flaw size as seen from the transmitter probe. The relationships of flaw size, flaw distance and flaw reflectivity are complex and considerable skill is required to interpret the display.

Complex multiprobe systems are also used with mechanical probe movement and digitisation of the signals, followed by computer storage; methods of computer interpretation are developing rapidly.

There are several forms of mechanical vibration, depending on the direction of particle movement in the wave motion, and so there are several forms of ultrasonic waves, the most widely used in NDT being compressional and transverse (shear) waves.

By suitable design of probe, ultrasonic beams can be introduced into solid material at almost any angle.

Compressional waves will also travel through liquids and a common technique is to immerse the specimen in a large tank and use a stand-off probe with a mechanised movement. With such equipment, alternative methods of displaying the signals are possible and a two-dimensional ultrasonic image can be produced (B-scan and C-scan displays).

Generally, a single probe acts as both transmitter and receiver, so that inspection can be done from one side only of the specimen. Large-grain materials such as austenitic steel welding, copper castings etc produce severe attenuation and scattering and are at present difficult to inspect with ultrasound, but large thicknesses of fine-grain material such as forged steel can be tested without difficulty.

Because the usual indication of a flaw is a pulse on an oscilloscope trace, flaws must be characterised and also sized. New techniques such as time-of-flight diffraction, TOFD, have been developed to assist this technique.

Ultrasonic attenuation and ultrasonic velocity measurements are used to study various material properties.

The use of ultrasonics for sizing flaws
Once flaws have been detected it is often desirable to determine their size. For flaws smaller than the ultrasonic beam width, a pseudo-sizing can be obtained by comparing the flaw signal amplitude with that of a reference reflector (flat-bottomed or side-drilled hole) at the same range. When the flaw size is greater than the ultrasonic beam width, conventional probe movement sizing techniques can often be used to provide an estimate of flaw size. The maximum amplitude technique uses a measure of the probe movement between the maximised signals from flaw extremities to size flaws. The 6 dB and 20 dB drop techniques use the reduction in the signal amplitude from the flaw as the probe passes over the edge of the flaw as an indicator of flaw dimensions. However, the interaction between the ultrasonic beam and flaw, depending as it does on flaw nature and orientation, limits the effectiveness of these techniques, when dealing with complex and mis-orientated flaws.

Techniques which make use of the diffracted signal from the flaw extremities to locate and size flaws are most effective in sizing planar flaws. The time-of-flight diffraction (TOFD) technique uses the ultrasonic transit time between probe(s) and flaw extremities to locate and size flaws. Flaw sizing accuracies of better than ±2 mm can be achieved with optimised techniques (see also Ultrasonic advanced methods).

 

For more information, please visit Ultrasonic Flaw Detection.