Abstract
Biaxial fatigue behaviour of a box-welded (wrap-around) joint in a JIS SM 400 B steel (12 mm thick plate) was studied using a multiaxial fatigue test facility. For the specimen, two stiffeners were attached to a main plate by a CO2 semi-automatic welding procedure. Residual stress measurements and finite element analyses (FEA) were also done. Fatigue tests were performed under both uniaxial and biaxial cyclic loads, and the results were comparatively examined. In the biaxial fatigue tests, special concerns were focused on effects of biaxial load range ratio and compressive cyclic loads in the lateral direction.
It was found that the direction of fatigue crack propagation under biaxial cyclic tensile loads, which have a phase difference of π, changes according to the biaxial load range ratio, Rxy=ΔPx/ΔPy, and the crack direction was estimated from the value of the stress range at a distance of 5 mm from the weld toe, Δσ5, calculated by the FEA. In the load cases where Rxy was less than 0.56, fatigue cracks propagated along the weld toe in the x-direction because Δσy at that location exceeded Δσx at the boxing weld toe.
As a general result in this study, the crack initiation life, Nc, and the failure life, Nf, in the biaxial tests were well related to those in the uniaxial tests by using Δσ5, whereas the location and direction of Δσ5 should be properly chosen according to the Rxy value and the failure form (crack direction). And it was concluded that an increase in Δσ5, which is induced by the Poisson's effect from the out-of -phase tensile loads or in-phase compressive loads in the y-direction, leads to an increase in fatigue damage, and the damage increase can be successfully estimated from the uniaxial fatigue test data.
