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Refinement of Steel Bridge Ultrasonic Testing Techniques


NCHRP research project 14-35 discovered issues with ultrasonic testing (UT), including phased-array ultrasonic testing (PAUT) that need to be addressed. The issues relate to calibration and to how acceptance criteria should be related to the possibility of cracks.

Regarding calibration, 14-35 discovered that the current calibration methods are lacking because they sometimes fail to adequately account for sound velocity and attenuation. Current methods were developed in the 1960s and rely on calibration blocks made of A36 steel. Project 14-35 found differences in sound velocity and attenuation between A36 and A709 steels, particularly for steels made using thermo-mechanical controlled processing (TMCP). It makes sense to account for these differences in calibration, but it is not known how much correction is needed, such as whether calibration can be addressed by using a block made from the same grade of steel or if calibration to the heat or plate is needed.

Regarding acceptance criteria, 14-35 found that current methods may not find some cracks. From decades of practice, it is known that RT and UT as are effective in discovery of workmanship discontinuities, and work by Florida DOT demonstrated that PAUT acceptance criteria of D1.5 Annex K are comparable to RT and UT. However, project 14-35 discovered that current criteria will not reject cracks that are of a critical size. D1.5 does not allow any cracks, but when indications are found in welds, it is not always clear whether the indication is a crack. The 14-35 investigator proposed acceptance criteria intended to preclude cracks, but it was not within the scope of the 14-35 study to try these out in the shop, so it is not known how effective they would be. More work is needed to better correlate Code acceptance criteria for RT, UT and PAUT with critical crack sizes and establish how methods should be adapted to ensure such cracks are avoided.


The objectives of these study are

  • Discover how calibration should be improved for UT and PAUT to effectively account for velocity and attenuation differences in bridge steels
  • Discover how UT, PAUT, and RT acceptance criteria should be adapted to ensure successful performance of bridge welds, including the suitability of the performance criteria proposed in study 14-35.

This project will improve nondestructive evaluation of welds, improving the accuracy of test results and increasing confidence in NDE methods and bridge welds. Improved confidence will facilitate the replacement of RT and traditional UT with PAUT. PAUT is a better method for evaluating welds because

  • Compared to traditional UT, it facilitates evaluation at a broad variety of angles, improving the characterization of discontinuities. Further, with encoding, PAUT creates a permanent electronic file of the test results that can be reviewed at any time; by contrast, with traditional UT only the UT technician’s reports and notes are available.
  • PAUT does a better job of evaluating the weld than RT because RT can only provide a flat-plan view of the weld; this means that RT lacks information about discontinuity depth, and it is likely to miss thin planar discontinuities that are parallel or near parallel to the X-ray or gamma ray propagation such as incomplete fusion or tight straight cracks. These types of discontinuities are considered more detrimental to the welded member and cracks regardless of size are not permitted.* PAUT is less intrusive and time consuming in the shop than RT. Thus, the move to PAUT will make fabrication more efficient.
Related Research:

The following are found in the existing body of knowledge:

  1. As described above, the NCHRP studied the use of PAUT for bridge welds under project 14-35 at Purdue University. The study 1) provided recommendations for critical flaw sizes, including recommendations for addressing stress concentrations in thickness transitions; 2) raised concerns about the skill of PAUT technicians based on round robin testing and provided recommendations for addressing skills; 3) discovered calibration issues related to sound velocity and attenuation (described above); and 4) provided recommended changes for scanning to D1.5 Annex K (described above). The results of this study air published in NCHRP Report 908.
  2. Florida DOT conducted a comparative study of RT, UT and PAUT (described above) and found good agreement, including what they describe as promising results
  3. The FHWA studied four welds with built-in defects using PAUT. Calibration was performed on a calibration block made of A1018 steel. They found that scanning from two index points would probably be necessary to get weld coverage and plan a second phase to evaluate the 6-db drop method of evaluating welds (Development of Phased-Array Ultrasonic Testing Acceptability Criteria (Phase 1), FHWA Publication No. HRT-14-074).
  4. Work done by the Indian Institute of Technology found that weld crown ripples can compromise evaluation when using the third leg but that use a dual-matrix array with a transmit-receive longitudinal wave is very effective for far-side bevel evaluation without removing weld reinforcing.
  5. Minnesota DOT studied the use of PAUT for corrosion evaluation and found it to be very effective for mapping and loss-of-section evaluation but that rough surfaces make evaluation difficult.

Anticipated research tasks include

  • A literature review and light synthesis of current practice, including which fabricators and owners have used PAUT on bridge projects
  • Check the velocity and attenuation of a broad sample of the types of steel used in bridge fabrication, including the variety of processing. From this sampling, establish how much variation in velocity and attenuation can be expected in bridge steels.
  • Conduct round-robin testing using PAUT on bridge steels including both D1.5 Annex K and NCRHP Report 908 criteria as well as RT and UT. Then compare the results and assess the criteria for their effectiveness in evaluation bridge welds.* Develop proposed changes to D1.5 based on the results of the study.
  • Publish the report and recommendations.

The results of this study will be presented to the AASHTO/AWS Bridge Welding Code committee as well as AASHTO CBS committee T-17 for consideration of adoption into D1.5.

Sponsoring Committee:AKB20, Steel Bridges
Research Period:12 - 24 months
RNS Developer:Ronnie Medlock, Member AKB20
Source Info:Ronnie Medlock
VP Technical Services
High Steel Structures, LLC
1915 Old Philadelphia Pike
Lancaster, PA 17602
Date Posted:07/21/2021
Date Modified:07/21/2021
Index Terms:
Bridges and other structures

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