Modified AASHTO design S – N curves for post-weld treated welded details

Abstract

Post-weld enhancement of the fatigue resistance of common attachment details such as transverse stiffeners, cover-plates, gusset plates, bulkheads and other welded details that experience crack growth from a weld toe is essential for efficient use of modern high- performance steels. Recently fatigue performance of treated welded joints was evaluated in large-scale rolled and built-up ferrite-steel beam specimens having yield strength of 345 to 690 MPa. The welded details consisted of transverse welds at the cover-plate terminus and at the transverse stiffener to tension flange joint. The welds were treated at the toe by ultrasonic impact treatment (UIT). In as-welded condition, these details are characterized as categories C′ and E′ in the AASHTO specification. Accelerated constant amplitude fatigue tests were conducted at various treatments of tensile minimum stress and stress range, resulting in positive stress ratios up to 0.6. The test results confirmed that the post-weld impact treatments substantially improved the fatigue performance of welded details. Analyses of the data in comparison to the current AASHTO design curves for as-welded details indicated that the treatments effectively elevated the fatigue limit without changing the slope of the S – N curve in the finite life region. The extent of enhancement was found to be dependent on both minimum stress and stress range. Within the range of available test results, the increase in fatigue strength appeared to be independent of the strength of material. Fatigue design curves based on two parameters, minimum stress and stress range, was developed for the post-weld treated details as a modification to the current AASHTO specification for as-welded details. Simplified fatigue design guidelines are proposed for post-weld treated details conforming to AASHTO category C′ and E′ details. The treatment provisions should be included in the specifications for enhancing performance of both new designs and existing construction.