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Publication Number:  FHWA-HRT-21-002    Date:  Winter 2021
Publication Number: FHWA-HRT-21-002
Issue No: Vol. 84 No. 4
Date: Winter 2021


Evaluating Ultrasonic Techniques to Detect Bridge Weld Flaws

by Hoda Azari and Russell Kok

FHWA researchers examined phased array ultrasonic testing for performing bridge weld inspections and compared the method to the historically used technique of radiographic inspection.

The welds between steel plates are critical components of steel girders in bridges. They must be welded and inspected carefully, and to ensure the integrity of the girder, inspectors use nondestructive evaluation (NDE) methods. However, the high levels of radiation needed for the historic approach, radiographic inspection, can present a safety and work area disruption concern for fabricators. Ultrasonic inspection presents an alternative NDE method.

Steel bridge girders on a bridge under construction. © Randy Hergenrether / Shutterstock.com.
The welds between steel plates on bridge girders are critical to the strength and integrity of the bridge. Advances in nondestructive evaluation techniques like ultrasonic testing may help both manufacturers and field inspectors identify and address flaws safely and accurately.

The American Association of State Highway and Transportation Officials/American Welding Society (AWS) D1.5M/D1.5 Bridge Welding Code establishes the rules for weld inspection, including which NDE methods can be used, what types and sizes of flaws need to be repaired, and more. Steel bridge girders comprise fabricated assemblies of built-up steel plates welded together in fabrication shops. For the evaluation of potential subsurface flaws in the welds, two NDE methods are used: radiographic inspection and ultrasonic inspection.

Why Use Ultrasonic Inspection?

Historically, fabricators have used radiographic methods to inspect welds. This technology dates back to the 1930s, and over time has become the baseline approach to inspect bridge welds. Whereas medical radiology uses a low-dose radiation source to expose a film or digital imaging plate to get an image of a broken bone, the thick steel welds require much higher energy radiation levels and doses to penetrate the weld and produce an image. These high radiation levels present a safety and productivity problem for fabricators.

Ultrasonic inspection methods have been available since the 1960s; however, many bridge owners do not yet accept them with confidence. While ultrasonic methods can be used for some less critical welds, other welds still require the cumbersome and dangerous radiography. Certified inspectors perform conventional ultrasonic testing by manipulating a handheld single-element probe around the weld and use their training and experience to detect and evaluate any weld flaws. This can be a subjective process that varies between inspectors, and is one reason bridge owners and code-writing authorities hesitate to fully accept the method. Also, bridge owners have more confidence in radiography because they can see the weld images themselves, whereas with conventional ultrasonic testing, they only get a documented report without images or other data.

A worker performs an ultrasonic inspection of a weld in a steel fabrication shop. © High Steel.
A worker performs an ultrasonic inspection of a weld in a steel fabrication shop.

Just as medical ultrasonic imaging has evolved with faster computing power and the resulting higher resolution images, industrial ultrasonic technology has followed with similar improvements. Phased array ultrasonic testing (PAUT) uses probe arrays in lieu of single-element probes and position encoders to capture the scan data and produce images of the inspection results, similar to a radiographic image. The probe array produces a fan of sound rather than the single sound beam used in conventional ultrasonic testing. This results in a more productive inspection because more weld volume can be inspected at one time, and, in conjunction with the stored scan data and encoder information, an inspector can produce an image of the scan, which adds confidence in the inspection results.

The latest development in PAUT testing is a more advanced image processing method called full matrix capture. This innovation can use the same basic instrument and probe, but offers new software processes to create a higher resolution image of any flaws.

Barriers and radiation warning signs in a steel fabrication shop. © High Steel.
Radiation boundaries in a fabrication shop help keep workers safe from the high levels of radiation needed to radiographically inspect welds.

The Federal Highway Administration's Advanced Sensing Technology (FAST) NDE Laboratory has been conducting research activities to explore PAUT and full matrix capture NDE techniques. The NDE laboratory is part of FHWA's Turner-Fairbank Highway Research Center in McLean, VA. The lab's PAUT research directly supports the overarching goal to support the implementation of ultrasonic techniques in lieu of radiographic techniques for AASHTO/AWS D1.5 Bridge Welding Code inspection of full penetration bridge fabrication welds.

When Can PAUT Be Used?

The 2015 D1.5 Bridge Welding Code added an annex, "Advanced Ultrasonic Examination," as a means to integrate PAUT into inspection workflow as a substitute for radiographic testing, pending approval of the substitution by the engineer. The annex provides requirements for the use of PAUT to inspect bridge fabrication welds in lieu of the historic single-transducer, manual ultrasonic testing allowed by D1.5 for decades.

The D1.5 code permits ultrasonic testing inspection for some full penetration bridge welds, but still requires that welds subjected to tensile or reversal stresses be inspected by radiographic testing. Butt welds in fracture critical members, welds made by the electroslag welding narrow gap process, and electrogas welding also require both radiographic and ultrasonic testing.

A worker performs a phased array ultrasonic inspection in a steel fabrication shop. © High Steel.
PAUT enables inspectors to capture data and produce images similar to a radiographic image.

While the D1.5 code approves some applications of PAUT, there are still applications that require further investigation and validation of the performance of PAUT to potentially replace the cumbersome and costly radiographic testing method. The overall objective of the continuing PAUT work at the FAST NDE Laboratory is to evaluate the state of PAUT technology and its potential application as an alternative to all radiographic testing in D1.5.

Implementation of a PAUT program is a big step beyond manual ultrasonic testing. The benefits of moving to PAUT can result in more efficient and reliable inspections. While the general approach to ultrasonic testing of a weld is similar, PAUT is a more complex inspection process that requires an upfront investment in equipment and software, as well as the need for additional inspector training.

Exploring PAUT in the Lab

The FAST NDE Laboratory has been investigating PAUT technology for several years, including prior to the publication of the "Advanced Ultrasonic Examination" annex. The first phase of these investigations involved the fabrication of suitable test specimens and development of preliminary procedures. Bridge fabricators prepared welded test specimens to ensure that they represented commonly applied bridge fabrication welding techniques. They fabricated full penetration butt joints and transition butt joint specimens. The welders intentionally implanted weld defects such as porosity, cracks, lack of fusion, and slag in the test specimens.

The initial comparisons of the results from PAUT, conventional ultrasonic testing, and radiographic images provided a broad understanding of the effectiveness of PAUT. The results from PAUT generally agreed with the radiographic results on each of the test specimens, supporting the decision to continue the PAUT investigations.

The next phase of the study was an effort to continue to create PAUT requirements and support the development of PAUT acceptance criteria for proposed incorporation into the D1.5 specification. FHWA presented highlights of this early work to the AWS committees, along with contributions from many other participants supporting the committee, which resulted in the 2015 adoption of the annex PAUT requirements in D1.5.

A lab test plate with rulers to show the size. Source: FHWA.
FHWA fabricated specimens like this test plate to use in comparing PAUT and radiographic techniques. The test plate shown here has a submerged arc weld, and the section lies at the end of the plate where the runoff tab has been sawed off. These flaws do not necessarily represent the flaws along the entire length of the weld.

Researchers selected commonly available off-the-shelf PAUT equipment and transducers for the data collection and analysis. FHWA used a 64-element, 2.25 MHz linear array probe. While the AWS D1.5 requirements do not limit the probe frequency to the 2.25 MHz range as they do for manual ultrasonic inspection, researchers chose 2.25 MHz to better compare the results to manual inspection results.

FHWA performed the PAUT calibration in accordance with D1.5. The calibration block includes a series of 1/16-inch (1.6 millimeter) diameter side drilled holes at different depths to set a uniform inspection sensitivity throughout the thickness of the weld to be examined. This is a very different approach to calibration compared to the manual ultrasonic testing inspection requirements in D1.5, where a single side drilled hole is used. With manual D1.5 ultrasonic testing, the correction of inspection sensitivity as the sound attenuates through the material is calculated using an assumed sound attenuation rate. Currently, the AWS committee is discussing which approach is better for calibration because both have their pros and cons.

The research team fabricated test specimens to be representative of the materials, weld joint designs, thicknesses, and weld processes typical in bridge fabrication. To ensure that the welding would represent production bridge welding practices, the specimens were manufactured by two steel bridge fabricators using electroslag welding narrow gap and submerged arc welding processes. The fabricators attempted to implant natural defects in the specimens that could typically be encountered during the manufacturing process, such as cracks, lack of fusion, lack of penetration, porosity, and slag. FHWA fabricated a total of 10 butt joint specimens.

A bar chart with two sets of two bars. The first two bars indicate phased array ultrasonic testing results for detection and rejection of flaws, and the second two bars indicate radiographic testing and rejection results. The PAUT method detected 1 crack, 26 lack of fusion flaws, and 3 slag flaws, and rejected 1 crack, 24 lack of fusion flaws, and 3 slag flaws. The radiographic testing detected 1 crack, 25 lack of fusion flaws, 1 porosity flaw, and 4 slag flaws, and rejected 1 crack, 23 lack of fusion flaws, 1 porosity flaw, and 4 slag flaws. Source: FHWA.
This chart illustrates the good comparability of detection and rejection of flaws using PAUT and radiographic testing in the lab. This set of test plates had 39 flaws, including cracks, lack of fusion (marked LOF on the chart), porosity, and slag. The detection and rejection results for the two methods are nearly identical for this particular set of test plates.

NDE Lab Results

The research team found that PAUT and radiographic testing had comparable rates of detection and rejection of flaws. Out of the total 39 discontinuities in the data set, radiographic testing rejected 5 flaws accepted by ultrasonic testing, and ultrasonic testing rejected 4 flaws accepted by radiographic testing.

The largest flaw rejected by PAUT and missed by radiographic testing was a 5-inch (13-centimeter)-long defect suspected to be caused by lack of fusion. The largest flaw rejected by radiographic testing but accepted by PAUT was a 1.88-inch (4.78-centimeter)-long defect caused by lack of fusion. PAUT detected this lack of fusion, but at a signal amplitude below the threshold requiring evaluation per AWS D1.5.

While missing a lack of fusion almost 2 inches (5 centimeters) long may raise questions about the reporting threshold in AWS D1.5, welds can exhibit low amplitude discontinuities of this length. In NDE, thresholds are always needed to avoid the unnecessary evaluation of every small signal detected, and the AWS approach is based on the fact that low-amplitude signals have not been found historically to be detrimental. The radiographic testing process missing a 5-inch (13-centimeter)-long lack of fusion raises greater questions about the reliability of radiographic testing to detect that flaw. The NDE community understands the issue as a generally known weakness of radiographic testing.

The overall goal of FHWA's ongoing research effort is to establish whether PAUT is a viable alternative to the use of radiographic testing. The initial results—indicating a good correlation of the comparative inspection results between PAUT and radiographic testing—support PAUT as an alternative. However, there is a need to develop a more comprehensive set of weld flaws to ensure that researchers can evaluate a fully representative flaw set. Consequently, the FAST NDE Lab is currently using ultrasonic simulation software to supplement the test plate data with a virtual database of simulated flaws. The researchers will also model and use physical test plate flaws to validate the modeling results.

Advances in PAUT Techniques

Although D1.5 added the annex with PAUT requirements in 2015, the AWS committees continue to receive feedback to improve the requirements, incorporate evolving technology, and address lessons learned from early adopters of PAUT in the bridge fabrication community.

The latest advances in industrial ultrasound use full matrix capture and total focusing method PAUT techniques. These techniques employ the same basic PAUT instruments and probe arrays used for conventional PAUT, but they process the image data in ways not previously possible because of limitations in computer processing power.

To illustrate the capabilities of the evaluation and imaging techniques of various methods, FHWA conducted activities using a steel block with a series of closely spaced holes of 1 millimeter in diameter drilled in the sides, in three groups of three holes. The location of the holes presents a challenge to traditional ultrasonic testing methods using a single-element transducer, which cannot produce an image of high enough resolution to indicate all the holes.

A steel block with three sets of three tiny holes set in diagonal patterns. Source: FHWA. The graphic output of an ultrasonic scan show a single peak at the far left, representing part of the first set of holes, and a double peak in the center, representing two of the three holes in the middle set. Source: FHWA.
To illustrate the difference in the visual display capabilities of ultrasonic testing methods, FHWA's NDE Lab created this steel block with three sets of three holes with 1-millimeter diameters, and scanned it using different techniques. Scanning the steel block using historic methods of ultrasonic testing with a single-element probe results in a display like this one. The peaks represent the holes; the two peaks in the center of the display are two of the three holes in the middle set.

Using conventional PAUT testing with an unfocused sound beam—used when an entire weld is being inspected without knowledge of any existing flaws—creates an image indicating the three groupings. Rescanning with a focused sound beam—used for greater detail once the location and depth of a flaw are discovered—provides an even clearer image. Finally, using the more advanced PAUT full matrix capture technique—which does not require focusing at a known depth—creates the image with the greatest clarity showing the three groupings of three holes each.

The research team collected all of the PAUT and PAUT full matrix capture data with a 60-element straight beam 2.25 MHz probe in order to highlight the differences in imaging technology. Different frequency and different size probes will change the resolution of an image.

The ultrasonic sound beam path analysis carried out to develop the PAUT scan plans indicates that there is a need to conduct a minimum of two scans along each side of the weld at different probe index point offsets from the weld centerline to ensure complete volumetric coverage of these relatively thick welds. This scanning approach is applicable to both straight thickness butt welds and transition thickness butt welds.

The large-grain microstructure observed in electroslag welds did not influence the propagation of ultrasonic waves to a point where it affected the detectability of the implanted flaws.

Two key benefits result from the use of full matrix capture and total focusing method technology. The first is better weld flaw detection and characterization producing an image much more representative of the actual flaw shape than what can be visualized with previous ultrasonic tests. The second is better flaw sizing, supporting more advanced engineering analyses to evaluate whether a flaw needs to be repaired, which is particularly applicable to in-service bridge inspections where costly repair decisions are required.

Pennsylvania DOT Perspective

In 2017, the Pennsylvania Department of Transportation (PennDOT) procured a PAUT instrument. The agency's primary focus to date is to determine the feasibility of substituting PAUT for radiography by conducting numerous assessments of capability and limitations. PennDOT is looking to potentially replace the current testing practice for full penetration welds in a fabrication setting.

"Analyzing the results with the current workmanship standards, the data [have] shown a promising comparison to that of the radiographic method in place today in terms of length and detection," says Nicholas Shrawder, a civil engineer at PennDOT. "While it is understood that we will never get a true one-to-one comparison, the advantages of evaluating the change from radiographically to ultrasonically testing welded splices include eliminating potential radiation hazards, ability to better detect flaw types deemed more critical, and retention of an image for a permanent record much like that of radiography."

"Full matrix capture/total focusing method is without question the next generation of PAUT for more accurate sizing and discontinuity characterization," says Shrawder. "PennDOT has been closely monitoring the technology as it progresses."

PennDOT is evaluating a pilot project to be launched in 2021 assessing another advanced technology feature of PAUT for data analysis via market-available software using advanced algorithms for interpretation and reporting.

USACE's Perspective

The U.S. Army Corps of Engineers (USACE) is responsible for the maintenance and operation of thousands of steel bridges and hydraulic steel structures. The structures are steel gates used to maintain navigation pools, operate locks, and maintain reservoir storage for flood risk management and hydropower operations. Maintaining these assets becomes an ever-increasing challenge given decreasing operating budgets and the increasing age of the infrastructure. Structural failures can result in loss of life, as well as negative economic impacts. USACE continually searches for methods and tools to extend the service life of these structures while maintaining safety and operability.

One approach is to evaluate a structure for fitness for service, which is defined as the ability to demonstrate the structural integrity of an in-service component containing a flaw or damage. For welded steel structures, this means detecting and quantifying flaws and evaluating acceptability through the application of fracture mechanics.

"Ultrasonic testing is a useful tool for sizing or quantifying embedded flaws and the extents of surface flaws," says Phillip Sauser, a structural engineer with USACE. "Traditionally, single-probe, pulse-echo ultrasonic testing has been used and has given acceptable results when used by highly skilled operators. The challenge has been finding and procuring operators of this skill level."

The visual output of an unfocused sound wave scan with three smudges indicated the three sets of holes in the steel block. The unfocused sound wave does not provide enough resolution to distinguish individual holes in each set. Sources: FHWA. The visual output of a focused sound wave scan shows the three sets of holes in greater detail than the scan with an unfocused sound wave. Sources: FHWA.
ABOVE LEFT: Using conventional PAUT methods with an unfocused sound beam—as an inspector would when doing an initial scan for flaws without knowing whether any exist or where they might be located—produces a display like this one. The three sets of holes in the steel block are visible, but are not distinct.
ABOVE RIGHT: Once an inspector discovers flaws using an unfocused sound beam, the scan is repeated using a focused beam. The greater resolution of the image produced provides more information about the number and size of holes in each set.
RIGHT: With advanced full matrix capture PAUT scanning, the display provides enough resolution to clearly show the three sets of three holes in the steel block.
The visual display produced by full matrix capture PAUT techniques show three groups of three holes. Sources: FHWA.

Recently, USACE has been investigating more advanced testing methods including PAUT and time-of-flight diffraction. Initial PAUT studies showed a wide variation of results, with some measurements within less than 1 percent of actual size and others as much as 300 percent greater than actual size. These studies found that the greatest accuracy often comes with scanning both faces of the member from both sides of the weld, conducting multiple scan offsets from each face and each side, and rastering to maximize signal return.

"Access for this amount of scanning is not always possible and thus results cannot be optimized," says Sauser. "Time-of-flight diffraction studies have shown good results except near the scanning surface where detectability is less reliable and where member geometry conflicts with the time-of-flight diffraction scanning equipment."

USACE is currently conducting research on advanced ultrasonic testing methods, PAUT, time-of-flight diffraction, total focusing method, and full matrix capture in order to evaluate these systems for detecting and sizing defects in welded steel joints. The first phase will evaluate the capabilities of the equipment to determine what can reliably be detected with ultrasonic testing.

One of the outcomes of this phase will be guidance on developing test procedures to optimize equipment capabilities and to define the limits of what can be reliably detected and sized with these methods. The second phase will evaluate the reliability of operators by conducting round-robin testing of flawed samples. The outcome of this phase will be the development of operator qualification requirements, such as performance qualification procedures or other means that increase the reliability of results. The overall outcome of the research will be to quantify the reliability of the system, equipment, and operator and to incorporate that into the evaluation process for fitness for service.

The Bridge Fabricator's Perspective

PAUT offers significant advantages in weld inspection. Compared to radiographic testing, it is far less disruptive in the shop, is much safer, and is better at locating flaws because PAUT provides flaw depth information that radiographic testing does not. Further, it provides inspection at a wide range of angles, improving discovery of planar discontinuities, and, when defects are discovered, it provides more accurate defect location. Compared to ultrasonic testing, PAUT generally takes longer to use, but it is less operator-sensitive, and when encoded, it offers the advantage of providing a permanent record of the data that can be reexamined or verified at any time.

A worker welds steel plates in a fabrication shop. © High Steel.
The welds between steel plates used in fabricating bridge girders can develop flaws both during manufacturing and after construction.

"Each of the three of the volumetric methods in D1.5 presents some advantage over the others, but, on balance, PAUT offers the best overall quality verification," says Ronald Medlock, the vice president for Technical Services at a steel manufacturer. "Given this and its improved safety, PAUT represents a logical and superior replacement for radiographic testing in a steel bridge fabrication shop."

The Future of Advanced Methodologies

All of the major manufacturers of PAUT equipment have systems capable of full matrix capture and total focusing method either already on the market or in the process of development for marketing. That all the major manufacturers have pursued this relatively new system capability is a strong indicator that these methods are the new direction for ultrasonic inspection and parallels how PAUT was developed and implemented. As recently as 5 or 10 years ago, only a few major NDE equipment manufacturers marketed PAUT equipment. Today, there are more than 10 major manufacturers marketing PAUT systems. FHWA expects that full matrix capture and total focusing method will follow the same path. In fact, the capability of full matrix capture and total focusing method systems has emerged much faster than PAUT, with at least five major suppliers already providing the equipment.

FHWA Future Work

FHWA's research plans include the fabrication of additional weld specimens to ensure that researchers can address a more complete test bed of representative flaw types, joint configurations, and plate thicknesses. The new specimen flaw types will be established after getting additional input from bridge owners, fabricators, and other industry stakeholders. The expected weld flaws include longitudinal cracks, transverse hydrogen-related cracks, incomplete fusion, incomplete penetration, slag, and porosity. The vertical fusion faces in electroslag welding narrow gap and electrogas welding processes require special manual pitch-catch type ultrasonic scans when a discontinuity is noted in weld metal-base metal fusion interface. The feasibility of using PAUT techniques to perform these historic manual pitch-catch type scans needs to be developed and evaluated as a potential AWS D1.5 annex requirement.

FHWA will also evaluate advanced ultrasonic flaw modeling software to supplement the ultrasonic validation data. Through modeling, the ultrasonic response from flaws with various types, sizes, lengths, and orientations can be simulated and evaluated according to the AWS D1.5 criteria. Using modeling should limit the number of additional weld specimens that need to be fabricated and inspected. A small set of actual implanted flaws will be manufactured and used to physically validate the modeling and provide additional confidence in the modeling results.

FHWA is also evaluating PAUT full matrix capture, time of-flight diffraction, two-dimensional PAUT arrays, and other advanced ultrasonic testing techniques to evaluate potential improvements in flaw detection and flaw sizing. The current AWS D1.5 manual ultrasonic acceptance criteria require the use of probe motion-based techniques to identify flaws with significant flaw-through-wall height. The use of advanced techniques will support the potential addition of a quantifiable acceptance criterion for this condition. AWS is considering the adoption of new acceptance criteria that use the flaw-through-wall height, as there is a general industry trend toward more of a fitness for service approach to ultrasonic acceptance criteria in lieu of the historic workmanship flaw signal amplitude-based criteria.

Hoda Azari is the manager of the NDE Research Program and FAST NDE Laboratory at FHWA's Turner-Fairbank Highway Research Center. She holds a Ph.D. in civil engineering from the University of Texas at El Paso.

Russell Kok is a research engineer working as a contractor in the FAST NDE Laboratory. He holds a B.S. in mechanical engineering from the State University of New York at Buffalo.



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