Evaluation of Hydrogen Cracking using Phased Array Ultrasonic Testing on Washington Avenue Bridge in Minneapolis, MN

The Washington Avenue Bridge over the Mississippi River in Minneapolis, MN was retrofitted to incorporate light rail as part of the Central Corridor Light Rail Transit (CCLRT) Project. During the retrofit construction, transverse cracks were found through magnetic particle testing in web-to-flange welds of the existing ASTM A514 “T-1” steel plate girders. Metallurgical evaluation of these cracks determined that they were consistent with hydrogen induced cracking. This type of cracking has historically been associated with A514 “T-1” steel due to the combination of high strength material and the insufficient control of hydrogen during fabrication welding. All cracks found during the initial testing program through visual, magnetic particle, and ultrasonic testing were removed, but further nondestructive testing (NDT) was requested to verify that no cracks remained. Fish & Associates, Inc. (FAI) reviewed previous nondestructive testing reports and procedures from initial inspections of this bridge. Phased array ultrasonic testing (PAUT) procedures were developed and implemented to provide enhanced measurement and characterization of the flaws compared to conventional ultrasonic testing (UT). PAUT is an advanced form of ultrasonic testing that utilizes multiple element transducers to send multiple signals at a range of angles in order to improve flaw detection and sizing. The flaw sizing measurements and signal responses were demonstrated through mockup fabrication and testing prior to field application. During testing, indications were noted at various depths and locations within the flange below the web-to-flange fillet welds. Conservative critical flaw size analysis was performed by other members of the project team to provide flaw acceptance criteria and for comparison against the PAUT inspection results. The improved NDT techniques involving PAUT to inspect for hydrogen cracking in web-to-flange welds during rehabilitation of an existing long span steel bridge for the new CCLRT Project in Minneapolis provided vital information. Working closely with design engineers, the evaluation techniques used in this project provided concise information to determine whether to accept or repair existing cracks in the bridge.

References

 [1] American Society for Nondestructive Testing, SNT-TC-1A: Personnel Qualification and Certification in Nondestructive Testing Recommended Practice No. SNT-TC-1A,  2011.

[2] American Welding Society, ANSI/AASHTO/AWS D1.5 - Bridge Welding and Inspection Code,  2010.

[3] American Welding Society, AWS D1.1 Structural Welding Code,  2010.

[4] National Highway Institute, (NHI 02-037) Fracture Critical Inspection for Steel Bridges FHWA Bridge Inspectors Training Manual (BIRM) , 2012.

[5] American Association of State and Highway Transportation Officials, Manual for Evaluation of Bridges , 2008.

[6] Federal Highway Adminstration, FHWA-HI-88-038 Inspection of Fracture Critical Members , 1988.

[7] Federal Highway Adminstration, FHWA-1P-86-26 Inspection of Fracture Critical Members , 1986.

[8] Olympus NDT, Introduction to Phased Array Ultrasonic Technology  Applications, 2007.

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