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Federal Highway Administration Research and Technology
Coordinating, Developing, and Delivering Highway Transportation Innovations

This report is an archived publication and may contain dated technical, contact, and link information
Publication Number: FHWA-HRT-04-124
Date: April 2005

Lab & Field Testing of AUT Systems for Steel Highway Bridges


The general goals of this study were to: (1) test and evaluate the AUT method of inspection in parallel with both RT and manual UT to determine possible differences in inspection results, (2) identify possible operational challenges in the fabrication shop environment, and (3) develop the appropriate procedures for implementing AUT as a replacement for RT. The following conclusions are based on the work performed as part of this project:

  1. AUT is a viable inspection technique for fabrication inspection of butt welded steel plates.
  2. AUT creates projection images of the defects in the weld along three axes. One of the three images (i.e., the C-scan) is analogous to a two-dimensional radiographic film. The other two images provide spatial information about the defect that RT does not provide. AUT images, like RT radiographic films, can be archived permanently as a hard copy or in an electronic format.
  3. AUT images provide more complete information than manual UT about the characteristics of the defects in the weld, including defect position, length, depth, and orientation. Note that RT only provides the x-position, y-position, and length of the defect.
  4. AUT findings were in full agreement with the RT laboratory results.
  5. AUT findings generally agreed with the RT field testing results. The slight lack of agreement is caused, in part, by the basic physics behind each inspection method. AUT operates based on an ultrasonic wave theory, while RT is based on differential absorption of penetrating radiation. The threshold levels prescribed by the code for UT and RT do not yield identical results. Therefore, certain defects that are rejectable by RT may be acceptable under the UT provisions. In this testing, it was found that AUT and UT could detect RT discontinuities; however, some of these discontinuities were acceptable according to the UT and AUT code requirements.
  6. AUT provides a relatively objective method of analysis. The decibel ratings yielded by AUT are determined directly from electronic measurements, without human interaction. With manual UT, the inspector must determine the decibel rating while simultaneously applying the transducer to the surface of the test specimen. This process includes maximizing the signals on the instrument screen and identifying the decibel level and transducer orientation when the signal matches the threshold value. For RT, the contrast observed on the radiographic film is subjectively analyzed by the inspector to identify discontinuities.
  7. The time required for setting up, calibrating, and performing an AUT inspection was generally greater than the time required by either RT or manual UT. However, AUT took less time than RT and manual UT combined.
  8. The P-scan system was not manufactured for use in a bridge fabrication shop environment. The system currently consists of seven separate modules with about eight interface cables. These modules should be interconnected using interface cables prior to performing an inspection at a fabrication shop. Assembling and transporting the P-scan system from one area of the shop to another was time-consuming and cumbersome. Thus, improved packaging by integrating the various modules and interface cables is recommended.
  9. Implementation of a fully automated (robotic) scanner instead of a manual scanner is recommended. This will improve the inspection rate and reduce human interaction.
  10. Currently, there are defects that can be accepted by RT and rejected by AUT or vice versa. Further research is recommended to explore the defect sensitivity of AUT in comparison to RT. This should provide a clearer understanding of the differences that exist in the acceptance‑rejection criteria between AUT/UT and RT.
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