Ultrasonic testing is most commonly performed on steel and other metal alloys.
UT is used in many industries including structural steel, aluminium, construction, metallurgy, manufacturing, aerospace, piping and ship building.
This method of non-destructive testing (NDT) is used to detect cracks and internal defects in welds and materials. Especially where the welded joint is critical. Ultrasonic testing is also used to measure wall thickness of a material (thickness testing).
How does ultrasonic testing work
Ultrasonic testing passes high frequency sound waves through the weld and heat affected zone.
This is done using different probes angles to ensure the entire weld has been examined. Imperfections are evaluated to a specific standard. The results of the test are then recorded.
When using ultrasonic testing, a transducer (or probe) is connected to a diagnostic machine (flaw detector). Ultrasound cannot pass through air. Therefore, a couplant is used to create an air tight seal between the probe and the test surface. The couplant is usually a liquid gel which forms a perfect seal between probe and test surface. The probe is then passed over the object being inspected.
Types of ultrasonic testing: attenuation and reflection
There are two methods of receiving the ultrasound waveform, attenuation and reflection.
Attenuation: In attenuation a transmitter probe directs ultrasound waves through one surface to the other, to a receiver probe. The receiver detects the amount of sound that has reached it on the other surface.
Imperfections between the transmitter and receiver reduce the amount of sound transmitted, revealing their presence. The use of couplant increases the effectiveness and efficiency of the process. This enables a reduction in the loss of ultrasonic wave energy.
Reflection, also known as pulse-echo mode, the transducer does both the sending and receiving of the ultrasound.
Pulsed sound waves are reflected back to the ultrasonic testing machine. Reflected ultrasound comes from an interface, such as the back wall of the object or from an imperfection within the object.
The diagnostic machine displays these results in the form of a signal. An amplitude representing the strength of the reflection and the distance is then displayed. The returning wave representing the arrival time of the reflection. Pulse echo is most the commonly used form of ultrasound for weld testing.
Welded joints can have different edge preparations of varying beveled angles. Most welds are usually made with manual or semi-automatic processes.
This means there is a potential for many different defect types to exist in the weldment. Thorough examination of the weld using ultrasonic testing, requires the use of different probe angles.
As ultrasound relies on perpendicular reflections to reveal defects, at least two different probe angles are normally required. Depending on the thickness of the test material, often three angles are used to provide a thorough test.
Ultrasonic Testing Advantages
Advantages of ultrasonic testing include:
High accuracy in determining the depth of internal flaws and the thickness of parts with parallel surfaces
High penetrating power allowing the detection of flaws deep in the test piece
Instantaneous results. Allowing immediate decisions to be made
It is sensitive to both surface and subsurface discontinuities, permitting the detection of extremely small flaws
In many cases only one surface needs to be accessible
Capability of assessing the size, orientation, shape and nature of defects
Non hazardous to technicians and nearby people
here is no effect on equipment and materials in the vicinity
Ultrasonic testing is non destructive.
Ultrasonic Testing Limitations
Limitations of ultrasonic testing include:
Ultrasonic testing requires experienced and knowledgeable technicians
Extensive technical and general knowledge is required for the development of ultrasonic testing procedures
Parts that are lumpy, unbalanced in shape, very small or thin are harder to be tested
Reference standards are needed, both for calibrating the equipment and for characterizing flaws
Cast iron and other coarse grained materials are difficult to inspect due to low sound transmission and high signal noise
Linear defects oriented parallel to the sound beam may go undetected.
All Weldtest New Zealand ultrasonic reports provide the Principal or Designer with the information in understandable terms as to the compliance or non compliance of the production piece being tested.
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