Purdue University: Fatigue Life Improvement of Welded Girders with Ultrasonic Impact Treatment

Author:  Jonathan F. Hui, Jason B. Lloyd, Robert J. Connor
Source:  Final Report ‐ April 2018
Doc ID:  2018050
Year of Publication:  2018
The fatigue life of welded connections can be improved by a variety of post‐weld treatment methods. One of the most effective methods is ultrasonic impact treatment (UIT). This technology may be applied during shop fabrication, but the greatest benefit comes from field retrofitting applications. Tensile cyclic stress ranges drive fatigue crack initiation and growth at the weld toe. This is made worse by tensile residual stresses at the weld toe that can reach the yield strength of the base metal resulting from differential cooling of the weld metal during fabrication. This concentration of tensile residual stress can have the effect of fully tensile cyclic stress ranges even in stress reversal zones of the bridge. UIT induces yield‐strength level compressive residual stresses by cold forming the material at the weld toe. Prior research has demonstrated the effectiveness of 27 kHz UIT systems for improving the fatigue life of welded bridge girders (Fisher and Roy, 2003). The AASHTO LRFD Bridge Construction Specification Commentary C11.9.1 suggests, but does not explicitly require, the use of 27 kHz systems. The existing language can be interpreted as a prohibition on other UIT systems. This report explores the effectiveness of a 20 kHz UIT system applied to transverse stiffener and cover plate termination welds. In this study, fourteen full‐scale girders with welded attachments were subjected to constant amplitude fatigue loading. The test matrix considered variables of stress range and minimum stress. Testing has shown that the 20 kHz UIT system provided equivalent effect to the 27 kHz UIT system. The treatment of the transverse stiffener welds improved the performance from Category C’ to at least Category B. The performance of the cover plate termination welds improved from Category E’ to at least Category C. The results demonstrated 20 kHz UIT as a viable option for enhancing the fatigue performance of welded bridge girders. This finding will expand the alternatives available to bridge owners seeking solutions for extending the life of their aging steel bridge inventory.

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