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Lab & Field Testing of AUT Systems for Steel Highway Bridges

1. INTRODUCTION

This report documents efforts to evaluate the effectiveness of automated ultrasonic testing (AUT) as a replacement for radiographic testing (RT) for fabrication inspection of welds in steel bridges. An AUT system was tested in parallel with both RT and manual ultrasonic testing (UT) to determine the differences in the inspection results, to identify operational challenges that may be faced while implementing AUT in a fabrication shop environment, and to develop the appropriate procedures for implementing AUT as a replacement for RT.

Fabrication inspection of welds is intended to ensure the quality of workmanship during the fabrication process. Welding processes have the potential for introducing internal defects or discontinuities that are not observable by visual inspection. These discontinuities generally can be described as either volumetric defects that are three-dimensional (e.g., slag inclusions and porosity) or planar defects that are essentially two-dimensional (e.g., cracks or lack of fusion). Internal discontinuities can reduce the strength of the weld and provide stress risers that act as fatigue crack initiation sites. An inspection is intended to identify internal discontinuities and determine their severity so that significant discontinuities can be repaired during the fabrication process. Two nondestructive evaluation (NDE) methods are available in the American Association of State Highway and Transportation Officials (AASHTO)/American Welding Society (AWS) D1.5M/D1.5: 2002 Bridge Welding Code(1) to detect internal defects in welds, UT and RT.

The most commonly used inspection method is RT, which provides a radiographic image of the weld being inspected. This method is required by code for complete joint penetration (CJP) welds (e.g., groove welds) that are subject to tensile stresses or stress reversal. RT is preferred by bridge owners because the radiograph can be retained for future reference, and it provides quantitative confirmation of the inspection process through the visibility of image quality indicators (IQIs) in the radiograph. Also, the extent of the area inspected is observable on the radiograph, providing evidence that a given weld has been fully inspected. Furthermore, volumetric and planar defects can be observed on the radiograph, providing information on the spatial location and the extent of the discontinuity.

In general, RT is less likely to detect planar defects than volumetric defects. (See references 2, 3, 4, 5, and 6.) For example, if the crack is parallel to the x-ray photon path or is tightly closed, the crack may not be detected. Also, no information on the through-thickness depth of the crack is available on the radiograph.^(2,3)

A significant disadvantage of RT is the health risks associated with the use of radioactive materials within the fabrication shop. As a result of the health risks, RT inspections frequently are conducted in isolated locations within the shop or behind protective shielding. Inspections are sometimes conducted during overnight shifts when a shop is less populated. The relocation of the welded assemblies to conduct RT is common. The procedures required to protect workers from the health hazards of RT increase the time required and the costs associated with bridge fabrication.

UT reports provide substantially less information than radiographs relative to the nature of any internal discontinuities and the inspection process. UT reports typically consist of tabulated data indicating the amplitude of the ultrasonic response and the corresponding spatial data as recorded by the technician. The ultrasonic signals are not retained as part of the inspection report. The UT technician is relied on to ensure that a given weld has been inspected fully and effectively. The UT method has an inherent reliance on the capabilities of the inspector. The variability between UT inspectors is well known.(2,3) Additionally, UT provides no information on the characteristics of a given discontinuity and, as such, all are analyzed as if they were cracks. As a result of these factors, bridge owners traditionally have preferred RT. However, UT has the advantage of minimal health hazards associated with the inspection process and can be conducted at any appropriate location within a fabrication shop. UT is also more sensitive to planar defects than RT. The effect of planar defects can be more severe than volumetric defects because of the high stress concentrations associated with their crack-like shape.

AUT provides significantly more information to the bridge owner than does UT. Images created by AUT systems can provide evidence that a given weld has been fully inspected, the ultrasonic signals can be retained for future reference, and the spatial location and extent of the defects can be displayed in images created by the system. In addition, the depth of a defect can be determined and used, making for efficient repairs. The differences in the detection of volumetric and planar defects that exist between RT and UT also exist between RT and AUT. The effects of these differences are explored in this research. AUT typically requires more sophisticated instrumentation, additional training for technicians, and may require additional time to complete scanning relative to UT or RT. These implications and the ability of AUT to perform in a shop environment are also explored as part of this project.

PROJECT SCOPE AND OBJECTIVE

The objective of this research is to determine if AUT can be used as a replacement for RT for fabrication inspection of steel bridges. There are many issues that need to be addressed to determine if such a replacement is possible and beneficial. These issues include the following:

• Does AUT provide equivalent or improved inspection results relative to RT?
• Is it possible to apply the manual UT procedures described in the code to an AUT system?
• Is it feasible to use AUT in and around a shop environment?
• Does AUT provide an effective quality control tool that would satisfy the needs of bridge owners?
• Is the cost or training requirements of AUT systems prohibitive?

To achieve the project objectives, an AUT system was identified and procured. Later, the AUT test results were compared to manual UT and RT test results. The tasks to be conducted were:

• Literature search to identify previous efforts to compare the three commonly used nondestructive weld inspection techniques (manual UT, AUT, and RT) in industries other than bridge fabrication.
• Market survey of the available sources for AUT systems.
• Laboratory proof-of-concept study comparing AUT and RT inspection of butt welds containing common defects.
• Field-testing studies to assess the feasibility, accuracy, reliability, and defect sensitivity of AUT relative to RT in a bridge fabrication shop during the fabrication process.

The identified tasks were executed between August 2000 and February 2003. Updates were provided to the National Steel Bridge Alliance (NSBA), a unified industry organization of businesses and agencies dedicated to the development of cost-effective steel bridges. This organization provides a forum for communicating research progress with both bridge owners and steel bridge fabricators.