What is Ultrasonic Testing and How Does it Work? - TWI Global

Ultrasonic testing (UT) comprises a range of non-destructive testing (NDT) techniques that send ultrasonic waves through an object or material. These high frequency sound waves are transmitted into materials to characterise the material or for flaw detecting. Most UT inspection applications use short pulse waves with frequencies ranging from 0.1-15 MHz, although frequencies up to 50 MHz can be used. One common application for this test method is ultrasonic thickness measurement, which is used to ascertain the thickness of an object such as when assessing pipework corrosion.

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How does it Work?

Ultrasonic inspection uses a piezoelectrictransducer connected to a flaw detector, which in its most basic form is a pulser-receiver and oscilloscope display. The transducer is passed over the object being inspected, which is typically coupled to the test object by gel, oil or water. This couplant is required to efficiently transmit the sound energy from the transducer into the part, however This couplant is not required when performing tests with non-contact techniques such as electromagnetic acoustic transducer (EMAT) or by laser excitation.

Pulse-echo and Through-transmission

Ultrasonic testing can be performed using two basic methods &#; pulse-echo and through-transmission.

With pulse echo testing, the same transducer emits and receives the sound wave energy. This method uses echo signals at an interface, such as the back of the object or an imperfection, to reflect the waves back to the probe. Results are shown as a line plot, with an amplitude on the y-axis representing the reflection&#;s intensity and distance or time on the x-axis, showing the depth of the signal through the material.

Through-transmission testing uses an emitter to send the ultrasound waves from one surface and a separate receiver to receive the sound energy that has reached the opposite side of the object. Imperfections in the material reduce the amount of sound that is received, allowing the location of flaws to be detected.

Contact and Immersion Testing

Ultrasonic testing can also be split into two main types: contact or immersion testing.

Contact ultrasonic testing is typically used for on-site inspections accessibility or portability. Contact ultrasonic inspection can be performed where only one side of a test specimen as reachable, or where the parts to be tested are large, irregular in shape or difficult to transport.

Immersion ultrasonic testing is a laboratory-based or factory-based non-destructive test that is best suited to curved components, complex geometries and for ultrasonic technique development. In this method, the component or material is submerged in a water, which acts as a couplant in place of the gels used for contact ultrasound. Immersion UT generally uses pulse-echo method, and robotic probe trajectories can be used to inspect complex surfaces which would be hard to cover with contact probes. Immersion UT can be used for a wide range of wall thickness and material types, making it a suitable testing method for a variety of applications and industries.

Why is it Used?

As a non-destructive testing method, ultrasonic testing is ideal for detecting flaws and defects without damaging the object or material being tested. Periodic ultrasonic inspections can also be used to check for corrosion or for growth of known flaws, and thus potentially prevent to a failure of a part, component or entire asset. It is used in a wide range of industries including aerospace, automotive, construction, medical, metallurgy, and manufacturing. 

What Materials Can Be Tested?

Ultrasonic testing is used in a wide range of industries due to its suitability for many different materials. UT is ideally used for inspection of dense, crystalline structures such as metals. Ceramics, plastics, composites and concrete can also be successfully inspected but with reduced resolution, since the attenuation in these materials is higher.

Ultrasonic technology has been successfully employed in the medical sector for many decades, and is increasingly the preferred option for both routine diagnostic imaging and medical research because of the absence of ionising radiation.

Advantages

The advantages of ultrasonic testing include:

• High penetration power, allowing for flaw detection deep within a part
• High sensitivity, allowing for the detection of very small flaws
• Can be used to test when only one side of an object is accessible
• Greater accuracy, when compared to other non-destructive testing methods, for determining depth of internal flaws and the thickness of parts with parallel surfaces
• Able to estimate size, shape, orientation and nature of defects
• Able to estimate alloy structures of components with differing acoustic properties
• Non-hazardous to nearby personnel, equipment or materials
• Highly automated and portable operations possible
• Immediate results can be obtained, allowing for immediate decisions to be made

Limitations

There are, however, a few limitations to ultrasonic testing, as follows:

• Requires experienced technicians for inspection and for data interpretation
• False positive results, also known as spurious signals, may result from tolerable anomalies as well as the component geometry itself
• Objects that are rough, irregularly shaped, very small or thin, or not homogeneous are difficult to inspect
• Loose scale or paint will need to be removed before testing can commence, although clean, properly bonded paint can be left in place
• Couplants required for tests that use conventional UT
• UT may have reduced sensitivity for volumetric flaws, particularly metal inclusions, than radiographic testing

Applications

Ultrasonic testing has a variety of applications across industry, including testing the integrity of a material or component. This can include testing of welds to determine if there are any discontinuities present. This testing can be performed on both ferrous and non-ferrous materials as well as for thicker sections and those that are reachable from one side only. UT is also capable of detecting finer defects and planar flaws which may not be assessed as readily with radiographic testing.

Applications for UT include those within the aerospace, automotive, construction, rail, medical and oil and gas industries.

TWI Services and Courses

TWI provides a number of ultrasonic testing services to our Industrial Members as well as a range of non-destructive testing training courses for those wishing to learn about the techniques involved.

We can provide a full range of testing services and expertise, including in methods such as phased array ultrasonic testing (PAUT), laser ultrasonic testing and manual ultrasonic testing.

Flaw detection is the most commonly used technique among all the applications of industrial ultrasonic testing. Generally, sound waves of high frequency are reflected from flaws and generate clear echo patterns.

Portable instruments record and display these echo patterns. Ultrasonic testing is a safe testing method that is widely used in various service industries and production process, particularly in applications where welds and structural metals are used. The paper gives an overview of the theory, practice and application of ultrasonic flaw detection.

Fundamental Theory

Sound waves are mechanical vibrations that pass through a medium such as liquid, solid or gas. These waves pass through a medium at a particular velocity in an expected direction. When these waves bump into a boundary having a different medium, they are transmitted back. This is the principle behind ultrasonic flaw detection.

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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, sound energy travels easily via most common materials and liquids, but does not pass efficiently via air or similar gasses. Also, sound waves of different types travel at different rate of velocities.

Additionally, wavelength refers to the distance between two subsequent points in the wave cycle as it passes via a medium. It is related to velocity and frequency. In ultrasonic flaw detection and ultrasonic thickness gaging, the minimum limit of detection is one-half wavelength and the minimum measurable thickness is one wavelength, respectively.

Modes of Propagation

In solids, sound waves can be present in different modes of propagation that are characterized by the type of motion involved. The common modes used in ultrasonic flaw detection are shear waves and longitudinal waves.

Variables that Limit Sound Transmission

When compared to soft, heterogeneous or granular materials, hard and homogeneous materials are able to reflect sound waves more efficiently. Three factors, such as beam spreading, attenuation and scattering, control the distance a sound wave will pass in a particular medium.

Reflection at a Boundary

The amount of reflection coefficient or energy reflected is associated with the relative acoustic hindrance of the two materials. In ultrasonic flaw detection applications, metal and air boundaries are commonly seen, wherein the reflection coefficient reaches 100%. This is the basic principle involved in ultrasonic flaw detection.

Angle of Reflection and Refraction

At ultrasonic frequencies, sound energy is extremely directional and the sound beams employed for flaw detection are clearly defined. As per the Snell's Law of refraction, sound energy transmitting from one material to another will bend. A beam that is traveling straight will travel in a straight direction; however, a beam that hits 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.

Figure 1. Cross section of typical contact transducer

In flaw detection applications, five types of ultrasonic transducers are usually employed. They include contact transducers, immersion transducers, delay line transducers, angle beam transducers, and dual element transducers.

Advanced Ultrasonic Flaw Detectors

Panametrics-NDT Epoch series are ultrasonic flaw detectors that are compact and portable instruments based on microprocessor. They are ideal for shop and field applications and display an ultrasonic waveform that is easily understood by a trained operator, who detects and classifies the flaws in test pieces. The series comprises a waveform display, an ultrasonic pulser/receiver, a data logging module, and software and hardware for signal capture and analysis. In order to optimize the performance of transducer, pulse amplitude, damping and shape can be controlled. Likewise, in order to signal-to-noise ratios, receiver gain and bandwidth can be modified.

Procedure of Ultrasonic Flaw Detection

A trained operator can identify particular echo patterns related to the echo response from representative flaws and good parts. This can be done by utilizing correct reference standards and accepted test procedures along with a good knowledge of sound wave propagation. Two calibration standards such as straight beam testing and angle beam testing are used in ultrasonic flaw detection. The latter technique is commonly used in weld inspection.

Figure 2. Typical angle beam assembly

Conclusion

Ultrasonic flaw detection is a comparative method. Although some analog-based flaw detectors are still being produced, most modern instruments employ digital signal processing to promote enhanced stability and accuracy.

This information has been sourced, reviewed and adapted from materials provided by Evident Corporation.

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