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

Ultrasonic testing (UT) is a collection of non-destructive testing (NDT) techniques that utilize ultrasonic waves to analyze an object or material. These high-frequency sound waves are transmitted through materials to characterize them or detect flaws. Most UT inspection applications employ short pulse waves with frequencies ranging from 0.1 to 15 MHz, although frequencies up to 50 MHz are also applicable. A common use for this method is ultrasonic thickness measurement, which determines the thickness of an object, such as in assessments of pipework corrosion.

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How Does it Work?

Ultrasonic inspection relies on a piezoelectric transducer connected to a flaw detector, which, in its simplest form, consists of a pulser-receiver and an oscilloscope display. The transducer is applied to the object being inspected, typically coupled with a gel, oil, or water. This couplant is essential for efficiently transmitting sound energy from the transducer into the material. However, couplant is not necessary when using non-contact techniques, such as electromagnetic acoustic transducer (EMAT) or laser excitation.

Pulse-Echo and Through-Transmission

Ultrasonic testing is performed using two basic methods: pulse-echo and through-transmission.

In pulse-echo testing, the same transducer both emits and receives the sound wave energy. This method utilizes echo signals at an interface, such as the back of the object or a defect, reflecting the waves back to the probe. Results are displayed as a line plot, with amplitude on the y-axis representing the reflection's intensity and distance or time on the x-axis, indicating the signal's depth within the material.

Through-transmission testing uses one emitter to send ultrasound waves from one surface and a separate receiver to catch the sound energy that reaches the opposite side. Defects within the material reduce the sound received, allowing for the detection of flaws' locations.

Contact and Immersion Testing

Ultrasonic testing can be categorized into two primary types: contact and immersion testing.

Contact ultrasonic testing is often employed for on-site inspection due to its accessibility and portability. This method can be performed when only one side of a test specimen is reachable or when the parts under test are large, irregularly shaped, or difficult to transport.

Immersion ultrasonic testing occurs in laboratory or factory settings and is best suited for curved components and complex geometries, as well as for ultrasonic technique development. In this method, the component is submerged in water, which acts as a couplant in replacement of gels used in contact ultrasound. Immersion UT generally employs the pulse-echo method, and robotic probe trajectories can be utilized to inspect complex surfaces that are hard to access with contact probes. Immersion UT is versatile, adaptable to a wide range of wall thicknesses and material types, making it suitable for various applications in multiple industries.

Why is it Used?

As a non-destructive testing method, ultrasonic testing effectively detects flaws and defects without damaging the object being examined. Regular ultrasonic inspections can also monitor corrosion or the growth of known defects, potentially preventing the failure of a part, component, or entire asset. Its applications span a wide range of industries, including aerospace, automotive, construction, medical, metallurgy, and manufacturing.

What Materials Can Be Tested?

Ultrasonic testing is widely applicable due to its compatibility with various materials. UT is particularly effective for inspecting dense, crystalline structures like metals. Ceramics, plastics, composites, and concrete can also be inspected but often with reduced resolution due to higher material attenuation.

Ultrasonic technology has been successfully used in the medical field for decades and is increasingly favored for routine diagnostic imaging and research due to the absence of ionizing radiation.

Advantages

The benefits of ultrasonic testing include:

• High penetration power for flaw detection deep within a component
• High sensitivity for detecting even very small flaws
• Ability to examine objects accessible from only one side
• Greater accuracy in determining the depth of internal flaws and the thickness of components with parallel surfaces compared to other non-destructive methods
• Capability to estimate the size, shape, orientation, and nature of defects
• Ability to analyze alloy structures of components with varying acoustic properties
• Non-hazardous to nearby personnel, equipment, or materials
• Facilitation of highly automated and portable operations
• Immediate results for prompt decision-making

Limitations

However, ultrasonic testing has some limitations, including:

• Requirement for experienced technicians for inspection and data interpretation
• Potential for false positive results, known as spurious signals, due to tolerable anomalies and component geometry
• Challenges in inspecting rough, irregularly shaped, very small, or thin objects, or those that are not homogeneous
• Necessity to remove loose scale or paint before testing, although clean and properly bonded paint can remain
• Couplants are needed for tests using conventional UT
• Reduced sensitivity to volumetric flaws, especially metal inclusions, compared to radiographic testing

Applications

Ultrasonic testing is utilized in various applications across industries to test the integrity of materials or components. This includes assessing welds for discontinuities. UT can be performed on both ferrous and non-ferrous materials and is effective for thicker sections and objects accessible from one side. It is adept at detecting fine defects and planar flaws that might not be easily identified through radiographic methods.

Applications of UT span the aerospace, automotive, construction, rail, medical, and oil and gas industries.

TWI Services and Courses

TWI offers a variety of ultrasonic testing services to our Industrial Members, as well as an array of non-destructive testing training courses for those who want to learn about these techniques.

We provide comprehensive testing services and expertise in methods such as phased array ultrasonic testing (PAUT), laser ultrasonic testing, and manual ultrasonic testing.

Flaw detection is the most commonly employed technique within the spectrum of industrial ultrasonic testing applications. Typically, high-frequency sound waves are reflected from flaws, generating clear echo patterns.

Portable instruments record and display these echo patterns. Ultrasonic testing is recognized as a safe testing method across various service industries and production processes, particularly where welds and structural metals are present. This paper provides an overview of the theory, practice, and application of ultrasonic flaw detection.

Fundamental Theory

Sound waves are mechanical vibrations that travel through a medium such as liquid, solid, or gas. These waves propagate through a medium at a specified velocity in a defined direction. Upon encountering a boundary with differing media, they are reflected back. This principle underpins ultrasonic flaw detection.

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Frequency, Velocity, and Wavelength

Most ultrasonic flaw detection applications utilize frequencies between 500 KHz and 10 MHz. At these megahertz frequencies, sound energy traverses most common materials and liquids effectively but struggles to pass through air or similar gases. Additionally, different types of sound waves travel at varying velocities.

Furthermore, wavelength denotes the distance between two successive points in the wave cycle as it propagates through a medium. It relates to velocity and frequency. In ultrasonic flaw detection and ultrasonic thickness gauging, the minimum detection limit is one-half wavelength, while the minimum measurable thickness is one wavelength.

Modes of Propagation

In solids, sound waves can exist in different modes of propagation characterized by their motion type. Common modes used in ultrasonic flaw detection include shear waves and longitudinal waves.

Variables that Limit Sound Transmission

Compared to soft, heterogeneous, or granular materials, hard and homogeneous materials reflect sound waves more efficiently. Three factors—beam spreading, attenuation, and scattering—determine the distance sound waves can travel within a specific medium.

Reflection at a Boundary

The reflection coefficient or energy reflected is linked to the relative acoustic impedance of the two materials in ultrasonic flaw detection applications. Metal and air boundaries are frequently encountered, where the reflection coefficient can reach 100%. This principle constitutes the basis of ultrasonic flaw detection.

Angle of Reflection and Refraction

At ultrasonic frequencies, sound energy is highly directional, and the sound beams used for flaw detection are distinctly defined. According to Snell's Law of refraction, sound energy transitioning from one material to another will bend. A beam traveling straight continues in a straight path; however, a beam approaching a boundary at an angle will refract.

Ultrasonic Transducers

A transducer is a device that converts energy from one form to another. Ultrasonic transducers can transform electrical energy into sound energy and vice versa.

In ultrasonic flaw detection, standard transducers feature an active element made from polymer, composite, or piezoelectric ceramic. When a high-voltage electrical pulse is applied to this element, it vibrates across a spectrum of frequencies, generating sound waves. Conversely, when an incoming sound wave vibrates this element, it produces an electrical pulse.

Figure 1. Cross section of a typical contact transducer

In flaw detection, five types of ultrasonic transducers are commonly employed: contact transducers, immersion transducers, delay line transducers, angle beam transducers, and dual element transducers.

Advanced Ultrasonic Flaw Detectors

Panametrics-NDT Epoch series are compact, portable ultrasonic flaw detectors based on microprocessor technology. Ideal for shop and field applications, they present an ultrasonic waveform that is easily interpretable by trained operators, who classify detected flaws in tested samples. This series includes a waveform display, an ultrasonic pulser/receiver, a data logging module, and software and hardware for signal capture and analysis. To optimize transducer performance, pulse amplitude, damping, and shape can be adjusted. Receiver gain and bandwidth can also be modified to enhance signal-to-noise ratios.

Procedure of Ultrasonic Flaw Detection

A trained operator can identify specific echo patterns associated with echo responses from representative flaws and sound materials. This process involves utilizing proper reference standards and accepted testing methods, alongside a comprehensive understanding of sound wave propagation. Two calibration standards, namely straight beam testing and angle beam testing, are fundamental within ultrasonic flaw detection. The latter technique is especially prevalent in weld inspection.

Figure 2. Typical angle beam assembly

Conclusion

Ultrasonic flaw detection functions as a comparative method. Though some analog-based flaw detectors are still manufactured, most contemporary instruments utilize digital signal processing to ensure improved stability and accuracy.

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

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