Takeshi Hanji

Weld Repair for Fatigue-Cracked Joints in Steel Bridges by Applying Low Temperature Transformation Welding Wire

When repairing fatigue damage by welding, the fatigue strength of cracked joints needs to be restored and also improved to prevent the reoccurrence of cracks. In this study, a new welding wire called low temperature transformation (LTT) welding wire, which enables the introduction of compressive residual stresses around a weld bead and the reduction of weld deformation, was applied to the weld repair. Its applicability was investigated by fatigue tests with compact tension specimens and a plate girder specimen, The compact tension specimens revealed that compressive residual stresses introduced by LTT welding wire have positive effects on crack initiation and propagation behaviour. The plate girder specimen indicated that the fatigue lives of cracked joints are restored and also improved by weld repair with LTT welding wire. Therefore, weld repair with LTT welding wire makes it possible to improve the fatigue strength of cracked joints.

Improvement of Extremely Low Cycle Fatigue Strength of Welded Joints by Toe Finishing

The improvement of extremely low cycle fatigue strength of welded joints by TIG dressing and burr grinding was investigated. The weld toes of T-shape welded joints were improved by TIG dressing or burr grinding and tested under displacement control conditions. The crack initiation life of finished specimens was at least threefold longer than that of as-welded specimens. There was no clear difference in improvement between TIG dressing and burr grinding. We also analysed the local strain around the cracking point by finite element analysis and performed local strain-based fatigue assessment. The results indicated that extremely low cycle fatigue strength of the joints improved by TIG dressing and burr grinding can be assessed by evaluating the local strain and referring to the extremely low cycle fatigue strength curve.

APPLICATION OF IMAGE ANALYSIS TO STEEL STRUCTURAL ENGINEERING

We introduce three applications of the image analysis technique into steel structural engineering. The first is the surface shape measurement of corroded steel plates using digital stereography. The second and third are strain measurements by digital image analysis. In all cases, the image analysis technique works well and provides us with innovative information on mechanical behaviors of steel structures.

Low-and High-Cycle Fatigue Behaviour of Load-Carrying Cruciform Joints Containing Incomplete Penetration and Strength Mismatch

This study investigated the low- and high-cycle fatigue strength of load-carrying cruciform joints containing incomplete penetration and strength mismatch between base metal and weld deposit, and proposed an assessment method based on the local approach. Low- and high-cycle fatigue tests were performed on specimens with varying mismatch and sizes of incomplete penetration. The test results revealed that the crack initiation life was extremely small for this type of joint. In fractographs, dimples observed around the crack initiation site provided evidence of ductile crack propagation. Also, it was found that strength mismatch had a negligible effect on the high-cycle fatigue strength but that under-matching reduced the low-cycle fatigue region. However, a unique relationship was found between local strain around a weld root tip calculated by elasto-plastic finite element analysis and both the low- and high-cycle fatigue life of the specimen.

Fatigue crack growth prediction of welded joints in low-cycle fatigue region

Low-cycle fatigue is one of the failure modes in steel structures during earthquakes. With a focus on crack growth in the low-cycle fatigue region, this study developed a fatigue crack growth curve and verified its applicability to crack growth prediction in welded joints. Fatigue crack growth tests under highly plastic conditions were performed using compact tension specimens with side grooves. The results indicate that the crack growth rate in the low-cycle fatigue region correlates with the cyclic J-integral range, which can be calculated using finite element analysis. Additionally, the results for both plain steel and weld metal were distributed in the same region within a narrow band. Based on the results, a formula for the fatigue crack growth rate under large cyclic strains was proposed. Low-cycle fatigue tests were then performed on welded joints to clarify their crack growth behavior, and the crack growth observed in these tests was compared with the crack growth calculated using the proposed formula. The calculation results were found to be in relatively good agreement with the experimental results, verifying the applicability of the formula.