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Home > News > What Is the Principle of Ultrasonic Flaw Detection?

What Is the Principle of Ultrasonic Flaw Detection?

Dec. 20, 2022

Ultrasonic testing or ultrasonic flaw detection is a non-destructive test that uses high frequency ultrasound to perform inspections and measurements. Ultrasonic testing is commonly used to find and evaluate defects, measure dimensions, characterise materials and much more.


At atmospheric pressure, sound waves with a frequency greater than 20kHz and a wavelength of 1.9cm or less are known as ultrasound. More simply put, ultrasound waves are sound waves with frequencies greater than the limits of human hearing.


Ultrasonic flaw detection uses these ultrasonic waves to detect defects within a solid metal or concrete object without causing any damage to its original structure.


In most conventional ultrasound applications, pulses of ultrasound with a centre frequency of 0.1 to 15 MHz and a boundary frequency of up to 50 MHz are transmitted into the material to find internal defects or to characterise the material. A typical example is ultrasonic thickness measurement, which measures the thickness of a test object by monitoring the corrosion of a pipe.


Digital Ultrasonic Flaw Detector

 Digital Ultrasonic Flaw Detector

Ultrasonic flaw detection is most typically performed on steel and other metals and alloys, although it may also be used on concrete, wood and composites with less resolution. Steel and aluminium construction, metallurgy, manufacturing, aviation, vehicles and other forms of transport are all industries where it is used.


Of all the applications of industrial ultrasonic testing, flaw detection is the most commonly used technique. Generally, high frequency sound waves are reflected from a defect and produce a clear echo pattern.


Portable instruments record and display these echo patterns. Ultrasonic testing is a safe testing method and is used in a wide variety of service industries and production processes, particularly in applications using welding and metal construction. This paper provides an overview of the theory of ultrasonic flaw detection.


HS PA20-Ex Multi-function Phased Array Ultrasonic Flaw Detector

 HS PA20-Ex Multi-function Phased Array Ultrasonic Flaw Detector

The theory behind ultrasonic flaw detection

Ultrasonic flaw detection is based on the principle that sound waves produce mechanical vibrations in different media such as liquids, solids or gases. The common modes of propagation of sound waves in solids are shear and longitudinal waves. This technique utilises the flow of ultrasound waves through solid metal, concrete or wooden objects.


When any sound wave passes through a medium, it has a specific frequency, wavelength and speed. If the sound wave encounters the edge of a different medium (i.e. a defect) it is immediately transmitted back to the same end where it can be detected. This is the basic theory of ultrasonic flow detection.


 HS PA30-E Ultrasonic Phased Array Detector

HS PA30-E Ultrasonic Phased Array Detector

Principles of ultrasonic flaw detection

Ultrasonic flaw detection is based on three main principles, namely frequency, velocity and wavelength.


Frequency (f)

In most ultrasonic flaw detection applications, the frequency is between 500 kHz and 10 MHz per second. At higher frequencies, the sound energy flows efficiently through most common solids and liquids.


Wavelength (λ)

It is the distance between two consecutive points that form a wave period as the wave propagates through a medium. The minimum detection limit for ultrasonic flaw detection techniques is 1/2 wavelength.


Velocity (c)

The speed of a sound wave depends on the medium through which it travels and is influenced by the density and elasticity of the medium. Sound waves move at different speeds. The exact speed of the ultrasound waves is detected in ultrasonic flaw detection.

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