The Latest Development in EMAT Inspection of Welds Research Paper

Abstract

This article reviews the latest development in EMAT inspection of welds. EMAT technology has replaced the previous complex and expensive couplant delivery systems and immersion tanks, which used “piezoelectric transducer to perform ultrasonic inspection” (Lopez 24). EMAT technology uses guided waves for fast inspection of large-scale structures. It is versatile, durable, simple to use, and cost-effective.

Introduction

The best nondestructive method of examination should assist in controlling the quality of “the final product and offer valuable process control feedback to improve productivity, reduce cost, and increase the efficiency of the welding machine” (Lopez and Green 23). This has become critical in large-scale and uninterrupted production in which a slight mistake can result in remarkable losses. In the past few years, technological innovations have resulted in advanced ultrasonic electromagnetic acoustic transducer (EMAT). As a result, the EMAT has become the preferred choice for most applications.

Before the invention of EMAT, manufacturers relied on complex and expensive couplant delivery systems and immersion tanks that could facilitate ultrasonic inspection through piezoelectric transducer. However, the innovation of EMAT technology in 1970s transformed the process of inspection as a noncontact, dry inspection that provided an option to the piezoelectric transducers. During its developmental stages, users only applied EMAT in laboratories. However, the new EMAT has found application in a wide range of places that include high-end applications. It has become popular due to its new materials and increased speed.

EMAT Inspection for Welds by Using Guided Waves

The EMAT technology has “many benefits in inspection of welds than the conventional ultrasonic testing (UT) method” (Lopez and Green 24). Lopez noted that the most common method has been the “shear wave for ultrasonic weld inspection” (Lopez 24). There are shear horizontal (SH) and shear vertical (SV) for inspection. The conventional ultrasonic inspection has relied on the SV wave in which the angle ranges between “30 and 60 degrees from the normal beam” (Lopez and Green 24). It is necessary to keep the right position in order to get accurate inspection. The SV waves are unable to inspect the entire vertical volume of the unit.

In some cases, any issue may hinder the entire inspection. Conversely, the SH method has been effective in weld inspection by offering to solutions. First, the SH waves do no shift their direction during inspection or when they strike areas with comparable polarization directions. The ability to avoid such shift has been effective in austenitic welds and in particular steel materials with dendritic grains. Second, when the shear horizontal energy reaches a 90-degree position, it attains a guided wave capability and inspects the entire volume of the material.

The SH waves provide several advantages when deployed on weld inspection.

• SH waves have the ability to capture the entire volume of the material under inspection and allow for full inspection. They eliminate the need for ‘phased array’ of sensor and create a space-efficient system during inspection.
• The SH waves have both transmitter and receiver, which allow for normal processes, especially for self-calibration that provides system reliability.
• The system is not prone to changes in position when conducting inspection. This enhances automation and integration in production
• The system has ability to find minute defects

Benefits of using EMATs for Weld Inspection

The ability of EMAT technology to produce sound during inspection has provided many benefits over conventional ultrasonic ways of conducting automated weld inspection. The developments EMAT technology has introduced consistency and quality of reading alongside speed during inspection. This is possible due to the absence of couplant in the EMAT technology (Lopez 24).

The technology has proved resistant to “oil, dirt, water, and rust” (Lopez and Green 25). As a result, it is useful in such contaminated surfaces. EMAT can also explore and inspect “curved or uneven surfaces” (Lopez and Green 25). Therefore, engineers in chemical, petroleum, steel, nuclear, automotive, and automotive industries have applied EMAT in their processes because it can work in harsh environments through both manual and automated processes.

Lopez and Green observed that Innerspec Technologies has developed several types of EMAT systems, which can inspect different types of welds in a single day with precision in a vigorous industrial environment. Some of the Innerspec innovations in EMAT systems include the following:

Flash weld inspection in steel coils

This system has an integrated welding machine. It inspects the weld without several cycles before “reaching pickle line and cold mill” (Lopez and Green 25).

Laser weld inspection

It has both in-line and off-line inspection systems for inspecting “flat areas like tailor-welded blanks and tubular transmission parts” (Lopez and Green 25).

Girth weld inspection

This is a system for “automotive and oil industries that inspects tubes and shafts” (Lopez and Green 24).

Lap weld inspection

This EMAT technology is for inspecting mash seam welds.

Full Volume Inspection

The major advancement from the EMAT technology is the ability to perform a full volume inspection through guided waves (SH at 90 degrees). Guided waves have the ability to move at high-speed from the source. As a result, they are suitable for thick plates of up to 13 mm. The following are some of the advantages of guided waves.

• They have the ability to cover a wide area with a single transducer
• They can reach inaccessible areas
• They can detect extremely small defects that other systems cannot detect

The system for inspecting pipelines can deliver all these three advantages simultaneously. It can detect cracks, eroded walls due to corrosion, and pitting in the pipelines (Varma et al. 2). The system can operate in two ways. First, it uses axial scanning that relies on a pitch-catch sensor system with a design to remit ultrasonic guided waves in the circumference of the material under inspection. Any defect in the material remits attenuation of sound to the system. This system has the ability to deliver up to 100 percent of volumetric inspection at high-speed as the sensor advances axially throughout the pipe. The variations in waves provide the type of defect that the system has scanned.

Second, there is also circumferential scanning. This system has sensors with a design of a pitch-catch within the pipe, which remit and receive ultrasound at height of three feet (one meter) in either direction. The method has been effective in inspection of pipes under air-to-ground line method in order to sense corroded wall, pits, and any cracks.

Still, MISTRAS has developed practical and inexpensive EMAT for inspection. MISTRAS notes that its systems can detect defects in “buried, hidden, or not easily accessible long run pipelines at low costs” (MISTRAS Group, Inc. 2). The company can design its systems to conduct inspections at high-speed testing, sizing, and inaccessible inspection of the material. Moreover, MISTRA EMAT technology is not only for pipelines, but also for other engineering areas.

EMAT technology for welds inspection is versatile. It has the ability to perform testing at various angles, thickness, guided waves, and phased array as it supports designs for “Pulse Echo or Through Transmission with small or large transducers both for rapid, high-speed screening and correct sizing” (MISTRAS Group, Inc. 2). Moreover, this technology does not depend on liquid couplant in order to operate.

Ribichini and fellow researchers analysed various form of EMAT technology and noted differences in performances (Ribichini et al. 2581). They noted that EMAT without any additional magnetostrictive layer produced weak signal amplitude, which relied on the correct magneto-mechanical features of the material. However, designers have the ability to transform this configuration through joining another layer with extreme “magnetostrictive material between the transducer and the plate” (Ribichini et al., 2581).

As a result, the system can achieve high order of signal amplitudes by making the transducer a contact instead of a noncontact. However, this transformation required additional preparation in order to produce effective mechanical bond. The periodic permanent magnet (PPM) EMATs generated transitional amplitudes, which were noncontact. It was also easy to configure, and it is insensitive to different properties of materials under inspection.

Conclusion

Before the invention of the EMAT technology, engineers relied on “complex and expensive couplant delivery systems and immersion tanks, which used piezoelectric transducer for inspection” (Lopez 24). The EMAT technology has created a noncontact and dry alternative method of inspection that goes beyond laboratories to other areas of manufacturing and engineering. It relies on guided waves for inspection. Moreover, its imperviousness nature on features of the material and integration capabilities have made EMAT technology to be the most preferred form of automated inspection, which is simple to use, has high-speed, and reliable at low-costs.

Works Cited

Lopez, Borja. “Tube Inspection and Measurement with Ultrasonic EMAT.” Inspection Trends (2006): 24-26. Print.

Lopez, Borja AND Pam Green. “Ultrasonic EMATs for Weld Inspection.” Inspection Trends, Summer (2004): 23-25. Print.

MISTRAS Group, Inc. EMAT (Electro-Magnetic Acoustic Transducer) Inspection Services. 2013. Web.

Ribichini, Remo, Frederic Cegla, Peter Nagy and Peter Cawley. “Study and Comparison of Different EMAT Configurations for SH Wave Inspection.” IEEE Transactions on Ultrasonics, Ferroelectrics, and Frequency Control 58.12 (2011): 2571-2581. Print.

Varma, Venugopal, Raymond W. Tucker,Jr., and Austin P. Albright. EMAT-Based Inspection of Natural Gas Pipelines for Stress Corrosion Cracks. Oak Ridge, Tennessee: Oak Ridge National Laboratory, 2004. Print.

Glossary

• Couplant – oil or water that enhances the flow of ultrasound waves.
• EMAT – electromagnetic acoustic transducer.
• Guided waves – latest approach in non-destructive inspection that uses waves.
• Weld inspection – non-destructive inspection.
• Ultrasonic – reflection of acoustic waves by internal interfaces and distant surfaces.
• Versatile – of having multiple applications.