Date of Award
Master of Science in Civil Engineering (MSCE)
Committee Member 1
Committee Member 2
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 thesis 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. A parametric study was also conducted to investigate the variation of the stress concentration at cover plate terminations with respect to the cover plate and flange geometry. The stress concentration was analyzed using two and three-dimensional finite element analysis conducted with ABAQUS. The analysis indicated that the weld toe stress concentration at Category E’ cover plate details with unusually thick flanges and cover plates was comparable to more typically sized details. The severity of the stress concentration does not increase appreciably if the flange, cover plate, and fillet welds maintain similar proportions to one another. This finding suggests that if those geometric conditions are satisfied, these details would have comparable fatigue resistance as other Category E’ details.
Hui, Jonathan Fong-Ching, "Fatigue Life Improvement of Welded Girders with Ultrasonic Impact Treatment" (2017). Open Access Theses. 1287.