Waves of the Future

Guided-wave technology that effectively inspects and monitors large structures is finding its way into numerous industries and applications

By Hegeon Kwun, Ph.D.

Dr. Hegeon Kwun, a staff scientist in the Sensor Systems and Nondestructive Evaluation Technology Department, has pioneered the use of magnetostrictive sensors for detecting corrosion and cracking in pipes and other structures. Thus far, 14 patents have been awarded to SwRI for this technology, with several others pending. Kwun joined SwRI in 1980 after obtaining a doctorate in physics from Brown University.

From time to time, we hear or read about catastrophic accidents that were caused by the failure of large structures, such as transmission gas pipelines, piping networks in refineries, aboveground storage tanks, highway bridges, ships and aircraft, that result in injuries or deaths, severe financial losses and disruption of everyday lives in nearby communities. To prevent structural failure and to protect public safety and the environment, inspections are regularly performed. Visual inspection is the most common method used for spotting obvious signs of defects, distress or failure - much as we visually check for flat tires before driving our car. While simple and inexpensive, visual inspection can't detect hidden or internal defects that are obstructed from view by insulation, paint and other barriers. Other inspection methods and tools, such as X-ray radiography, ultrasonics and eddy current, overcome the shortcomings of visual inspection; however, these methods examine small areas at a time, require direct access for inspection and are cost-prohibitive on large structures.

Since the early 1990s, Southwest Research Institute (SwRI) has pioneered the use of magnetostrictive sensor (MsS^®) technology, an emerging inspection method that can be used to rapidly and cost-effectively inspect large structures and monitor their structural health.

SwRI developed a guided-wave system that uses magnetostrictive sensor technology for rapid and cost- effective inspection of large structures, such as this buried gas transmission pipeline. Here, engineers are permanently installing sensors for periodic structural health monitoring, which is another application for MsS technology. A laptop computer attaches to the permanent system shown in the inset. Comparisons of data taken at regular intervals give a quick and accurate assessment of changes in the physical condition of the pipeline.

Background

MsS technology uses structure-borne elastic waves, called guided waves, which propagate along the structure confined and guided by its geometric boundaries. Guided waves in relatively low frequencies (under a few hundred kilohertz) can propagate a long distance along the structure at speeds of more than three miles per second. A good example of long-distance travelling guided waves is the sound of the train wheels we can hear from miles away by pressing our ears against a railroad track.

The MsS is a device that generates and detects guided waves electromagnetically in ferromagnetic materials. With MsS technology, a pulse of relatively low frequency (typically under 200 kHz) guided waves of a certain wave mode (for example, longitudinal or torsional in piping) is launched along a structure from a fixed test location. When the propagating guided-wave pulse encounters welds or defects, such as corrosion wall loss and cracks, some of the waves are reflected back to the original test location where they can be detected by the same sensor and analyzed for
structural conditions.

Because these guided waves propagate at a high speed, MsS technology can rapidly inspect a long length of the structure from a single test location and provide comprehensive structural condition information - ideal for such industries as the petrochemical industry, which maintains hundreds of miles of piping. The guided wave inspection technique is also useful for inspecting areas that are difficult to access, such as those at high elevations, behind walls or under insulation, from a remote accessible location. This saves the time and money that would otherwise be used for scaffolding, insulation removal or excavation.

The MsS system launches a pulse of guided waves that travel two to three miles per second along structures. When the waves encounter corrosion or defects, some of them reflect back to the sensor, creating a signal for defect detection. The portable system comprises the MsS instrument, probes for plate-type and piping structures, and a laptop computer that acquires, stores and analyzes data.

Under the SwRI internal research program, the principle of magnetostrictive sensors was first proven in 1992 in an effort to find an efficient method for inspecting steel cables in highway suspension bridges. Since then, through subsequent research and development efforts supported by SwRI and various industrial companies, MsS technology has made tremendous progress, reaching the point of commercialization in some applications, notably for piping inspections in refineries and chemical plants.

Under joint support from more than 10 industrial companies in the United States and abroad, staff in the Sensor Systems and Nondestructive Evaluation Technology Department in the SwRI Applied Physics Division in 1998 developed a field MsS system (Model MsSR^® 1000) for piping inspection. The system received an R&D 100 award from R&D Magazine for being one of the 100 most significant technical accomplishments of that year. Since then, the MsS instrument has been upgraded to Model MsSR 2020D (pulse-echo operation and digital output). In addition to axially long structures such as piping, rods and cables, the MsS system can be used to inspect plate-type structures. Also, the thin ferromagnetic strip approach developed from 2001 to 2002 made the MsS technology easier and more versatile to use and further expanded its applications to include non-ferrous structures and long-term structural health monitoring (SHM) with permanently installed sensors.

In addition to the long-range guided wave inspection and monitoring applications, the MsS system is an excellent and versatile tool for experimental studies of guided wave properties in various structural geometries and configurations. The system is currently available for purchase for laboratory R&D or commercial inspection purposes, with the latter requiring a licensing agreement with SwRI.

The areas where the MsS technology has been applied successfully are further described below, along with the ongoing and future developmental efforts.

Piping Inspection and Monitoring

Thus far, MsS technology has been most used for long-range piping inspection applications, operated primarily in torsional guided wave mode based on the thin ferromagnetic strip approach. For commercial inspection use, SwRI has developed special system software that allows the inspector to complete the data analysis and generate an inspection report within minutes. SwRI has licensed the MsS technology to a number of companies that provide piping inspection services.

Because of the low cost for permanently installing MsS and the ability to obtain comprehensive structural information over long segments of piping, MsS technology now is also used for long-term online SHM of high-risk piping networks in refineries and chemical plants. The permanent MsS installation involves adhesively bonding a thin ferromagnetic strip, such as nickel, around a pipe, winding 10 or so turns of coils over the strip, covering the coils and strip with a protective tape to guard against long-term exposure to the operating environment and terminating the ends of the coils at an accessible location. If the test location is easily accessible to allow placement of MsS coils, only the strip needs to be installed and protected. Long-term structural health is monitored by periodically acquiring data by connecting the MsS system to the installed sensor and comparing new data with previously measured data. This allows accurate and quick tracking of structural condition changes so that suitable maintenance decisions can be made very economically.

Efforts are ongoing to apply the MsS for the long-term SHM of buried high-pressure gas transmission pipelines.

SwRI engineers used the MsS system to inspect a 24-inch outside diameter (OD) buried gas transmission line. Although the system typically can be used from an easily accessible location, underground pipelines require small areas to be excavated for access. The data sample at right illustrates the defects found in a 4.5-inch OD pipe filled with water.

Suspension Bridge Cable Inspection

As mentioned earlier, the initial proof-of-concept of the MsS was aimed at applying it to highway bridge cables. The United States has more than 30 highway suspension bridges with spans of 700 feet or longer that were built before 1950. Their increasing age and heavier and more frequent traffic loads are pushing the original design limits of these bridges. Considering their importance to the national transportation infrastructure and high replacement costs, maintaining their structural integrity for safe and prolonged operation is of serious concern.

Under sponsorship from the Federal Highway Administration and the Port Authority of New York and New Jersey with collaboration from the Parsons Transportation Group, SwRI scientists applied MsS technology to the inspection of suspenders on the George Washington Bridge (GWB). The bridge, almost a mile long with a span of approximately 3,500 feet over the Hudson River, was opened in 1931. It has two roadway levels with 14 traffic lanes that are supported by four main cables, each approximately 3 feet in diameter consisting of 26,474 individual wires. The bridge and traffic loads are carried by vertical suspender ropes, each approximately 2.85 inches in diameter and composed of 283 wires. Twice as large as the Golden Gate Bridge in San Francisco, the GWB is a key infrastructure in the northeastern United States and has been undergoing rehabilitation.

Using 10-kHz longitudinal guided waves, the SwRI system could inspect the entire length of a suspender (up to 330 feet long) from a test location near the upper level bridge deck and provide data that could be used for grading the physical condition of suspenders to help prioritize the rehabilitation schedule.

With some additional developments in the inspection procedures and system software for data analysis and reporting, MsS technology could also be transferred to commercial companies for bridge cable inspection.

Anchor Rod Inspection

Many thousands of miles of high-voltage electrical transmission power lines are supported by guyed tower structures. The guys are typically stabilized with steel anchor rods, ranging from approximately 0.5 to 2 inches in diameter, with lengths ranging from 10 to 20 feet and embedded in soil or concrete. These anchor rods are subject to corrosion and, when the corrosion progresses to unsafe levels, could result in catastrophic failure of the tower structure. Until recently, the only reliable means for inspecting anchor rods for underground corrosion was visual inspection, which first requires a time-consuming and costly excavation with the potential of damaging the coating used to protect the steel and potentially accelerating future corrosion.

With collaboration from EDM International Inc. and the Electric Power Research Institute, SwRI scientists successfully applied MsS technology to anchor rods to detect underground corrosion. Using longitudinal guided waves at approximately 32 kHz, hundreds of anchor rods in a variety of operating environments across the United States were inspected, successfully proving the effectiveness of the MsS system for assessing degrees of corrosion.

EDM International is currently licensing the technology to offer commercial anchor rod inspection.

Inspection and Monitoring of Plates

In addition to elongated structures such as piping, steel cables and anchor rods, MsS technology has also shown applicability to guided-wave inspection and long-term SHM of plate-type structures, such as large aboveground storage tanks, steel liners in the containment buildings of nuclear power plants, pressure vessels, ship hulls and steel I-beams. Instead of the encircling coil MsS used for elongated structures, a slender rectangular-shaped coil MsS is used for plates. The plate MsS coil is typically 8 to 10 inches long and 1-inch-wide and could be made of a flexible printed circuit board. Much like the searchlight of a lighthouse, the sensor launches a beam of guided waves perpendicular to its lengthwise direction and detects signals reflected back by defects. Large areas of plate-type structures are rapidly examined by scanning along a pre-planned path. From a test position, the MsS can inspect and monitor approximately 20- to 40-foot-long areas in either direction of the sensor.

Automobile Crash Sensing

Airbag systems are commonly used in automobiles to protect the driver and passengers during a crash. The systems sense an impact and determine within a split second whether or not to deploy the airbag. Keeping the sensing time as short as possible while accurately and reliably discriminating between crash and non-crash situations is critical for proper operation of the airbag systems.

Reducing the sensing time is particularly important for side-impacts where the crash endangers the occupant faster than in a frontal impact. The automotive industry has found that the accelerometers used to detect frontal crashes are not as effective in side-impact crashes.

The MsS has shown excellent applicability for detecting and discriminating the crash impacts for airbag systems. The MsS detects structure-borne elastic waves generated by impacts that spread quickly along the car frames and skins. With collaboration from Key Safety Systems Inc., which has licensed the MsS technology for automotive sensing applications, the MsS has been subjected to various crash tests by the automotive industry. Thus far, magnetostrictive sensors have passed most industry requirements, including those for side-impact detection, and have been shown to be superior to accelerometers in sensing time and discrimination of crash and non-crash events. Onboard MsS applications for airbag systems are expected in the very near future.

Ongoing and Future R&D

MsS technology is a highly versatile and economical tool for inspecting and monitoring the condition of large structures. It can help reduce operation and maintenance costs while improving structural reliability. For these reasons, the technology is also being examined for use in SHM of aircraft and spacecraft structures; for example, fastener structures for corrosion and cracks, aluminum skin structures for corrosion, and composite repair patches for identifying patch disbondment and defect growth.

SwRI scientists are also developing a modeling approach for simulating guided wave signals from arbitrary-shaped defects in pipe and plate structures for use in the development of defect characterization techniques and algorithms. A preliminary version of the model has already shown encouraging results, with efforts continuing to further refine and validate the model.

Other ongoing MsS research and development activities include developing a system for heat exchanger tube inspection and designing a high-power system for piping to extend the range of inspection, particularly on underground bitumen-coated, high-pressure gas transmission pipelines, which are highly attenuative. The latter work is supported by the U.S. Department of Transportation (Office of Pipeline Safety, Research and Special Programs Administration), Pipeline Research Council International, SOCAL and Gulf South Pipeline Company.

Southwest Research Institute will continue research and development of magnetostrictive sensors to improve performance and capabilities and to expand practical applications.

Comments about this article, including inquiries on MsS system purchases and licenses? Contact Glenn M. Light, Ph.D., at (210) 522-2218 or glight@swri.org.

Published in the Fall 2003 issue of Technology Today®, published by Southwest Research Institute. For more information, contact Joe Fohn.

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