Ultrasonic testing (UT) is a method of non-destructive testing based on ultrasonic waves being transmitted via a target material. Short ultrasonic waves (0.1-15MHz) are transmitted for the detection of internal flaws or defects.
What is ultrasonic testing (UT)?
In 1940, Dr. Firestone of the University of Michigan patented a study on ultrasonic testing, focusing on a device for detecting inhomogeneities of density or elasticity. For example, a manufactured casting could have a hidden crack or defect. This would highlight the presence of a flaw. The device could emit high frequency vibrations and establish the time intervals of the vibrations, indicating the status of the material.
Optimisation of the technique led to the application of a piezoelectric crystal transducer, electrical pulses of ultrasonic frequency. The crystal vibrates at the frequency and is mechanically coupled to the surface. This coupling may be effected by immersion of both the transducer and the specimen in a body of liquid e.g. oil. The ultrasonic vibrations pass through and are reflected by any discontinuities. The echo pulses that are reflected are received by the transducer and converted into electrical signals.
Micro-structural defect identification requires a more advanced nonlinear ultrasonic test. This method relies on a more intensive ultrasonic wave, which inevitably distorts as it faces micro damage. This intensity can be quantified by the acoustic nonlinearity parameter, β. β is related to first and second harmonic amplitudes which can be measured by decomposition of the ultrasonic signal via fast Fourier transformation or wavelet transformation.
Types of UT and how they work
In ultrasonic testing, a transducer is passed over the object being inspected. The transducer is typically separated from the object by a couplant e.g. oil or water. Ultrasonic testing with an Electromagnetic Acoustic Transducer (EMAT) doesn’t require a couplant.
Reflection and attenuation
In the first method, reflection (pulse-echo mode), the transducer sends and receives pulsed waves as the sound is reflected back to the device. The results are displayed as a signal with an amplitude representing the intensity of the reflection and the distance, illustrating the arrival time of the reflection. In an alternative technique, attenuation (through-transmission mode), ultrasound is passed through one surface, and a separate receiver detects the amount that has reached it on another surface after traveling via the medium. Imperfections in the space between the transmitter and receiver reduce the amount of sound transmitted, thus locating their presence. The couplant increases the efficiency by reducing the losses in the ultrasonic wave energy, due to separation between the surfaces.
Transducers for UT
Ultrasonic inspection becomes reliant upon the use of transducers, which have specific acoustic properties. The transducer applies an electric field across the active element producing mechanical deformation, generating ultrasonic vibrations. The transducer is critical for the reliability of inspection results. Ultrasonic transducers can be piezoelectric, electromagnetic, electrostatic or magnetostrictive. The four transducer types are straight or angle beam, delay line or twin crystal. Variation also occur in the active piezoelectric elements, their frequency and the bandwidth.
Advantages and benefits of ultrasonic testing
The key benefits are numerous including high penetrating power, which allows the deep detection. Very high sensitivity, resolves detection of small flaws. Generally, only one surface needs to be accessible and it is a very accurate test when working with parts with parallel surfaces. There is initial estimating of the size, orientation and shape of the defects and reasonable approximation of the structure of alloys with different acoustic properties. The technique is usually portable and is non-hazardous. One of the main advantages is that the results are instantaneous.
Disadvantages and limitations of ultrasonic testing
Manual operation requires very experienced operators. The transducers pick up significant noise. These signals must be distinguished. A secondary non destructive testing method may have to qualify the data. Irregular shapes, thin, or samples that are not homogeneous are very difficult to inspect. On all occasions, surface preparation is critical (cleaning and removing loose objects). Finally, couplants are necessary to provide effective transfer of ultrasonic wave energy.
Applications and materials to be tested via UT
Ultrasonic testing is conducted on steel and other metal alloys. Tests on non-metallics include concrete, wood and carbon composites. Industry sectors that use UT are construction, manufacturing, aerospace, automotive and the power sector.
Several key inspection methods are :
- Volumetric inspections of welds and castings
- Ambiguities in pipelines
- Industrial inspections of machines, engines and turbines
- Power stations including boilers, vessels and drums
- Inspections in the oil and gas industry including reactors, pipelines, storage tanks and heat exchangers
- Construction including bridges and highway infrastructure
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Ultrasonic testing (UT) is non-destructive testing method based on the propagation of ultrasonic waves in the material tested.
X-ray diffraction results from an electromagnetic wave interacting with a regular array of scatterers (arrangement of atoms within the crystal).
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