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Meindert Anderson

Metrology

The Basics of Ultrasonic Flaw Detection

It’s easier than ever to use this valuable nondestructive testing tool

Published: Sunday, September 13, 2015 - 23:00

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Ultrasonic flaw detection is a powerful nondestructive testing (NDT) technology and a well-established test method in many industries. However it can seem complex to a person who has not worked with it.

Modern ultrasonic flaw detectors are small, portable, microprocessor-based instruments suitable for both shop and field use. They generate and display an ultrasonic waveform that is interpreted by a trained operator, often with the aid of analysis software, to locate and categorize flaws in test pieces. The detector will typically include an ultrasonic pulser/receiver, hardware and software for signal capture and analysis, a waveform display, and a data-logging module. Although some analog-based flaw detectors are still manufactured, most contemporary instruments use digital signal processing for improved stability and precision.

The pulser/receiver section is the ultrasonic front end of the flaw detector. It provides an excitation pulse to drive the transducer, and amplification and filtering for the returning echoes. Pulse amplitude, shape, and damping can be controlled to optimize transducer performance, and receiver gain and bandwidth can be adjusted to optimize signal-to-noise ratios.

Modern flaw detectors typically capture a waveform digitally and then perform various measurement and analysis function on it. A clock or timer will be used to synchronize transducer pulses and provide distance calibration. Signal processing may be as simple as generation of a waveform display that shows signal amplitude vs. time on a calibrated scale, or as complex as sophisticated digital processing algorithms that incorporate distance/amplitude correction and trigonometric calculations for angled sound paths. Alarm gates are often employed to monitor signal levels at selected points in the wave train to flag echoes from flaws.

The display may be a liquid crystal, an electroluminescent display, or, in older models, a CRT. The screen will typically be calibrated in units of depth or distance. Multicolor displays can be used to provide interpretive assistance.

Internal data loggers can be used to record full waveform and setup information associated with each test, if required for documentation purposes, or selected information such as echo amplitude, depth or distance readings, or presence or absence of alarm conditions.

Ultrasonic flaw detectors can be used in a variety of applications where nondestructive flaw detection and analysis is required. The type of tests performed varies depending on the application, but will be either straight-beam inspection or angle-beam inspection

Straight-beam tests

Straight-beam testing is generally employed to find cracks or delaminations parallel to the surface of a test piece, as well as voids and porosity, such as those found in plates, bars, forgings, castings, and so forth. It may use contact, delay line, dual element, or immersion transducers, all of which launch longitudinal waves on a straight path into the test piece. Straight-beam testing is also commonly employed in testing fiberglass and composites. An illustration of how straight-beam transducer works can be seen in figure 1.


Figure 1: Contact transducer used for straight-beam testing


Like all other ultrasonic flaw detection techniques, straight-beam testing utilizes the basic principle that sound energy traveling through a medium will continue to propagate until it either disperses or reflects off a boundary with another material, such as the air surrounding a far wall or the gap created by a crack or similar discontinuity. In this type of test, the operator couples the transducer to the test piece and identifies the echo returning from the far wall, as well as any fixed reflections originating from geometrical structures such as grooves or flanges. After noting the characteristic pattern of echoes derived from a good part, the operator then looks for any additional echoes that appear ahead of that back-wall echo in a test piece, discounting grain scatter noise if present. An acoustically significant echo that precedes the back-wall echo implies the presence of a laminar crack or void. Through further analysis, the depth, size, and shape of the structure producing the reflection can be determined. Figures 2 and 3 illustrate how an ultrasonic tester might show a test piece with no flaws, and one with flaws.



Figure 2: Straight-beam test with no flaws in test piece


Figure 3: Straight-beam test with a flaw in the test piece

Angle-beam inspection

Although straight-beam techniques can be highly effective at finding laminar flaws, they are not effective when testing many common welds, where discontinuities are typically not oriented parallel to the surface of the part. The combination of weld geometry, the orientation of flaws, and the presence of the weld crown or bead require inspection from the side of the weld using a beam generated at an angle. Angle-beam testing is by far the most commonly used technique in ultrasonic flaw detection. Angle-beam probes consist of a transducer and a wedge, which may be separate parts or built into a single housing. They use the principle of refraction and mode conversion at a boundary to produce refracted shear or longitudinal waves in a test piece. An example of an angle-beam transducer can be seen in figures 4 and 5.


Figure 4: Angle-beam transducer used to detect a flaw in a weld


Figure 5: Angle-beam transducer in use

Most modern ultrasonic flaw detectors incorporate digital rather than analog signal processing. Although some older analog instruments are still in use, microprocessor-based digital processing is now the industry standard for portable flaw detectors. Its major advantages include:
• Precision and repeatability of test setups, which can be stored for quick recall
• Stability of horizontal and vertical linearity (no drift with time or temperature)
• Digital precision in measurement of amplitude and depth/distance, including trigonometric calculations in angle-beam testing
• Digital filtering to improve near-surface resolution and signal-to-noise in high-gain applications
• Waveform freeze, peak memory, and zoom functions for easier analysis
• Fast and reliable implementation of sizing techniques such as DAC, TVG, and DGS
• Datalogger software for storage of screen displays and measurements in internal memory or on removable storage cards
• USB interface for offloading test data to computers for storage and further analysis

The potential advantage of analog processing in some cases is faster screen refresh and/or data acquisition rates.

Transducer selection

In many cases, the type of transducer used in a particular test will be determined by established inspection code or procedural requirements that the inspector needs to follow. Codes such as AWS D1.1 and ASTM E-164 detail recommended transducers and wedges, and in some cases transducer selection will be governed by past practices in the same test. If there is no code or procedure in place, then the inspector must select an appropriate transducer based on the specific test requirements, and using his or her knowledge of common test practices and general ultrasonic theory. In some cases this will involve experimentation with several different types of transducers on reference standards representing the part to be tested, to determine which one provides the best response.

Once a transducer type has been selected, other important factors affecting performance are frequency, diameter, and bandwidth.  Optimizing these factors in a given test often requires balancing advantages and disadvantages.
Frequency. Higher frequency transducers can resolve smaller flaws due to their shorter wavelength, while lower frequency transducers will penetrate farther in a given material because attenuation decreases with frequency.
Diameter. Larger diameter transducers can scan a given area more quickly, while smaller diameters will have better response to small reflectors and couple more efficiently into curved surfaces.
Bandwidth. Narrowband transducers have greater penetration but reduced near surface resolution, while broadband transducers have better near-surface resolution but less penetration.

This article has focused on conventional flaw detection using single-element and dual-element transducers. But as ultrasonic NDT technology develops, phased-array systems that use sophisticated array probes with as many as 256 or more elements are seeing increasing use in industrial settings to provide new levels of information and visualization in common ultrasonic tests that include weld inspection, bond testing, thickness profiling, and in-service crack detection.

The benefits of phased-array technology over conventional ultrasonic testing come from its ability to use multiple elements to steer, focus, and scan beams with a single transducer assembly. Beam steering, commonly referred to as sectorial scanning, can be used for mapping components at appropriate angles. This can greatly simplify the inspection of components with complex geometries.

The small footprint of the transducer and the ability to sweep the beam without moving the probe also aids inspection of such components in situations where there is limited access for mechanical scanning. Sectorial scanning is also typically used for weld inspection. The ability to test welds with multiple angles from a single probe greatly increases the probability of detection of anomalies. Electronic focusing permits optimizing the beam shape and size at the expected defect location, thus further optimizing probability of detection. The ability to focus at multiple depths also improves the ability for sizing critical defects for volumetric inspections. Focusing can significantly improve signal-to-noise ratio in challenging applications, and electronic scanning across many groups of elements allows for C-Scan images to be produced very rapidly.

An example of a phased-array readout can be seen in figure 6.

Phased array imaging
Figure 6: Phased-array readout

Despite the seeming complexity of the tool and the physics behind it, ultrasonic flaw testers are a relatively easy tool to learn how to use. Advances in digital signal processing and improved software analysis algorithms have made ultrasonic tester readouts easier to interpret than ever before, making ultrasonic testers an important tool for nondestructive testing in the field.

Meindert Anderson is the corporate director of marketing communications at Olympus Scientific Solutions Americas, a Quality Digest content partner.

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About The Author

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Meindert Anderson

Meindert Anderson is the corporate director of marketing communications at Olympus Scientific Solutions Americas in Waltham, Massachusetts. He has been working in the field of marketing since 1987. Olympus manufactures and markets ultrsonic flaw detectors, eddy current and eddy current array flaw detectors as well as thickness gauges, videoscopes, borescopes, microscopes, in-line and advanced nondestructive testing systems, and XRF and XRD analyzers.