Continuous Fatigue Test of Welded Joint Taken from SBHS700 Girder Specimen

Steels for bridge high performance structures (SBHS) have high strength, good weldability and workability. SBHS700 of 780MPa class must particularly clarify with respect to fatigue problems. At the previous study, the fatigue test with the SBHS700 girder specimen was carried out and fatigue strength improvement methods to introduce compressive residual stress were used on its out of plane gusset welded joints. However, the previous study did not quantitatively show the fatigue strength improvement because these out of plane gussets have not failed.

In this study, in order to clarify the effects of the fatigue strength improvement methods, these out of plane gussets are taken from the girder by gas fusion cutting and made as small fatigue specimens. If there is enough compressive residual stress left at the weld toe, the fatigue tests are carried out.

Before the fatigue tests, residual stress at 2mm from treatment edge of centre gusset was measured by the X-ray diffraction method and compared with previous studies. The target of fatigue test was UIT (Ultrasonic impact treatment). At first, nominal stress range is 100MPa under the same condition of the previous study. If fatigue crack does not appear after 2 million cycles, nominal stress range is increased, such as 150MPa, 180MPa, 200MPa.

As a result, enough compressive residual stress was confirmed at the weld toe from the X-ray diffraction method. UIT specimens reached 2 million cycles in 200MPa stress range. Fatigue test results were considered based on an equation which modified Goodman's diagram applies to the weld toe due to estimate the fatigue limit. Therefore, high fatigue strength can be expected.