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Theory And Applications of Ultrasonic Flaw Detection

Oct. 14, 2021

Of all the applications of industrial ultrasonic inspection, flaw detection is the most commonly used technique. Typically, high-frequency sound waves are reflected from a defect and produce a clear echo pattern. Portable instruments record and display these echo patterns. Ultrasonic inspection is a safety inspection method that is widely used in a variety of service industries and production processes, particularly in applications using welds and structural metals. This paper provides an overview of the theory, practice, and applications of ultrasonic flaw detection.

Basic theory

Sound waves are mechanical vibrations that pass through a medium such as a liquid, solid, or gas. These waves travel through the medium at a specific speed and in the desired direction. When these waves hit a boundary with a different medium, they are transmitted back. This is the principle of ultrasonic flaw detection.


Frequency, speed, and wavelength

Most ultrasonic flaw detection applications use frequencies between 500 kHz and 10 MHz per second. At frequencies in the megahertz range, sound energy travels easily through the most common materials and liquids but does not travel effectively through the air or similar gases. Furthermore, different types of sound waves propagate at different speeds.

HS 610e Digital Ultrasonic Flaw Detector

 HS 610e Digital Ultrasonic Flaw Detector

In addition, the wavelength is the distance between two consecutive points in a wave cycle as it passes through a medium. It is related to speed and frequency. In ultrasonic flaw detection and ultrasonic thickness measurement, the minimum detection limit is one-half wavelength and the minimum measurable thickness is one wavelength.


Mode of propagation

In solids, sound waves can exist in different modes of propagation, characterized by the type of motion involved. The modes commonly used for ultrasonic flaw detection are transverse and longitudinal waves.


Variables that limit sound transmission

Hard and homogeneous materials can reflect sound waves more effectively than soft, heterogeneous, or granular materials. Three factors, such as beam extension, attenuation and scattering, control the distance a sound wave can pass through a given medium.


Reflection at the boundary

The reflection coefficient or the amount of reflected energy is related to the relative acoustic resistance of the two materials. In ultrasonic flaw detection applications, it is common to find metal and air boundaries with a reflection coefficient of 100%. This is the basic principle of ultrasonic flaw detection.

 HS Q6 Ultrasonic Flaw Detector

 HS Q6 Ultrasonic Flaw Detector

Reflection and refraction angles

At ultrasonic frequencies, the acoustic energy is extremely directional and the sound beam used for flaw detection is well defined. According to Sneer's law of refraction, acoustic energy transmitted from one material to another will bend. A beam that travels in a straight line will travel in a straight line; however, a beam that strikes the boundary at an angle will bend.

HS Q6 Ultrasonic flaw detector is a lightweight, easy to carry, easy to operate, powerful hand-held flaw detector, which uses a true color highlight display, rocker type keys, SD card expansion storage, can be used in the oil and gas, aerospace, power generation and automobile industry in the most demanding testing environment for a long time.

Applications of Ultrasonic Flaw Detection

Ultrasonic flaw detection can potentially be applied to any standard engineering material to detect hidden cracks, voids, porosity, inclusions, and similar discontinuities. While most tests involve steel and other structural metals, flaw detectors can also be used on plastics, composites, fiberglass, and ceramics. Some common applications include


Weld inspection - by far the most important market for ultrasonic flaw detection

Primary metals - beams, billets, bars, ingots, forgings, tubular blanks

Infrastructure - bridges, bridge pins, railroad tracks, metal structures

Petrochemical - pipelines, storage tanks, support structures

In-service testing - railway wheels and axles, aircraft landing gear and engine mounts, crane arms, driveshafts, fuel tanks, and pressure vessels, bolt testing

Fabrication - spot welding, brazing, casting, bond testing

Composites - aerospace components, wind turbines, automotive composites, marine fiberglass.

If you want to get more information about the best ultrasonic flaw detector, welcome to contact us today or request a quote.  

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