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Home > News > What Are Ultrasonic Flaw Detectors And How Do Them Work?

What Are Ultrasonic Flaw Detectors And How Do Them Work?

Oct. 24, 2022

Of all the applications of industrial ultrasonic inspection, flaw detection is the oldest and most common. Since the 1940s, the laws of physics that control the propagation of sound waves through solid materials have been used to detect hidden cracks, voids, pores, and other internal discontinuities in metals, composites, plastics, and ceramics. High-frequency sound waves are reflected from defects in a predictable manner, producing a unique echo pattern that can be displayed and recorded by portable instruments. Ultrasonic inspection is completely non-destructive and safe, and it is a well-established inspection method in many basic manufacturing, fabrication, and service industries, particularly in applications involving welds and structural metals.

 

What are ultrasonic flaw detectors?

Modern ultrasonic flaw detectors are small, portable, microprocessor-based instruments suitable for use in the workshop and in the field. They generate and display ultrasonic waveforms interpreted by a trained operator, often with the aid of analysis software, to locate and classify defects in the test piece. They typically include an ultrasonic pulse generator/receiver, hardware and software for signal capture and analysis, waveform display, and data logging modules. Although some analog-based flaw detectors are still being manufactured, most modern instruments use digital signal processing to improve stability and accuracy.

Kw-4 Digital Ultrasonic Flaw Detector

 KW-4 Digital Ultrasonic Flaw Detector

The pulse generator/receiver section is the ultrasonic front end of the flaw detector. It provides the excitation pulses to drive the transducer and amplifies and filters the returned echoes. Pulse amplitude, shape, and damping can be controlled to optimize transducer performance, and receiver gain and bandwidth can be adjusted to optimize the signal-to-noise ratio.

 

Modern flaw detectors typically capture waveforms digitally and then perform various measurement and analysis functions on them. A clock or timer will be used to synchronize the sensor pulses and provide distance calibration. Signal processing can be as simple as generating a waveform display showing signal amplitude versus time over a calibrated range, or as complex as a digital processing algorithm that combines distance/amplitude correction and triangulation of angled sound paths. Alarm gates are typically used to monitor the signal level at selected points in the wave train to flag echoes from defects.

 

The display may be a CRT, LCD, or ELD. Screens are usually calibrated in terms of depth or distance. A multi-color display can be used to provide interpretation assistance.

 

An internal data logger can be used to record the complete 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 the presence or absence of alarm conditions.

 HS Q7 Digital Ultrasonic Flaw Detector

 HS Q7 Digital Ultrasonic Flaw Detector

The procedure of Ultrasonic Flaw Detectors

Ultrasonic flaw detection is essentially a comparative technique. Using appropriate reference standards as well as knowledge of acoustic wave propagation and generally accepted test procedures, a trained operator can identify specific echo patterns corresponding to the echo response from good parts and representative defects. The echo pattern from the test part can then be compared to the pattern from these calibration standards to determine its condition.

 

  • - Straight beam testing - Straight beam testing using contact, delay line, bicrystal, or immersion transducers is typically used to find cracks or delamination parallel to the surface of the test piece, as well as voids and porosity. It utilizes the basic principle that sound energy propagating through a medium will continue to travel until it is dispersed or reflected in the boundary of another material, such as the air around a distant wall or air found within a crack.

 

In this type of test, the operator couples the transducer to the test piece and locates the return from the far wall of the test piece, then looks for any echoes that arrive before that back wall echo and disregards particle scattering noise if it is present. Acoustically significant echoes prior to the back wall echo imply the presence of laminar cracks or voids. Further analysis allows the depth, size, and shape of the structure producing the reflection to be determined.


In some special cases, the test is carried out in a straight-through mode, where the acoustic energy propagates between two transducers placed on opposite sides of the test piece. If there are large defects in the acoustic path, the beam will be blocked and the acoustic pulse will not reach the receiver.

HS 612e Digital Ultrasonic Flaw Detector for Pillar Porcelain Insulator and Porcelain Sleeve

 HS 612e Digital Ultrasonic Flaw Detector for Pillar Porcelain Insulator and Porcelain Sleeve

  • - Angled Beam Testing - Cracks or other discontinuities perpendicular to or inclined relative to the surface of the test piece that is not normally visible using the straight beam testing technique due to their orientation relative to the acoustic beam. Such defects can be found in welds, structural metal parts, and many other critical components. To find them, the angled beam technique is used, employing either a common angled beam (wedge) transducer assembly or an aligned immersion transducer in order to direct the acoustic energy into the test piece at a selected angle. The use of angled beam inspection is particularly common in weld seam inspection.

 

A typical angle beam assembly uses mode conversion and Sneer's law to generate shear waves in the test piece at selected angles (most commonly 30, 45, 60, or 70 degrees). As the angle of the incident longitudinal wave increases with respect to the surface, more and more of the acoustic energy is converted to transverse waves in the second material, and if the angle is high enough, all the energy in the second material will be in the form of shear waves. There are two advantages to designing ordinary angular beams to take advantage of this mode conversion phenomenon. Firstly, energy transfer is more effective at the angle of incidence where shear waves are generated in steel and similar materials.


Second, minimum flaw size resolution is improved through the use of shear waves, since at a given frequency, the wavelength of a shear wave is approximately 60% of the wavelength of a comparable longitudinal wave.


For further technical information talk with our specialist or visit our website https://www.ndtzkcx.com. We can help you to identify the best solution for your application.

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