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How Do Ultrasonic Flaw Detectors Work?

May. 09, 2022

Flaw detection is the most common technique used in all applications of industrial ultrasonic inspection. 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 widely used in various service industries and production processes, especially in applications using welds and structural metals. This paper provides an overview of the theory, practice, and application 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 that has a different medium, they are transmitted back. This is the principle behind ultrasonic flaw detection.

Multi-function Phased Array Ultrasonic Flaw Detector


Multi-function Phased Array Ultrasonic Flaw Detector      

Frequency, velocity, and wavelength

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


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

Mode of propagation

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


Variables that limit sound propagation

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

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

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

Boundary reflection

The magnitude of the reflection coefficient or 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 where the reflection coefficient reaches 100%. This is the basic principle involved in ultrasonic flaw detection.


Reflection and refraction angles

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


Ultrasonic Transducers

A transducer is an instrument that is capable of converting energy from one state to another. Ultrasonic transducers can transform electrical energy into sound energy and vice versa.


For ultrasonic flaw detection, standard transducers employ an active element that is made of either a polymer, composite, or piezoelectric ceramic. When an electrical pulse of high voltage is applied to this element, it vibrates through a particular spectrum of frequencies and produces sound waves. When an incoming sound wave vibrates this element, it produces an electrical pulse.

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