Fatigue behaviour of a box-welded joint under biaxial cyclic loading: effects of biaxial load range ratio and cyclic compressive loads in the lateral direction

The biaxial fatigue of a steel plate (JIS SM400B) having a box-welded (wrap-around) joint was experimentally studied. Special concerns were focused on the effects of the biaxial load range ratio and compressive cyclic loading in the lateral direction. The direction of fatigue crack propagation under biaxial cyclic tensile loading, which has a phase difference of pi, changed according to the biaxial load range ratio, R-xy=DeltaP(x)/DeltaP(y). When R-xy was less than 0.56, fatigue cracks propagated along the toe of the weld in the x-direction because the principal tensile stress range Deltasigma(y) at that location exceeded the orthogonal value Deltasigma(x) at the box-weld toe. The fatigue lives in biaxial tests related well to the data from uniaxial tests when invoking the Deltasigma(5) criterion. However, the location and direction of Deltasigma(5) should be chosen according to the R-xy value and the failure crack direction. An increase in Deltasigma(5) , as induced by the Poisson's ratio effect from either the out-of-phase tensile loading or the in-phase compressive loading in the y-direction, leads to an increase in fatigue damage (decrease in fatigue resistance or specifically a faster crack propagation rate), and this effect can be successfully estimated from uniaxial fatigue test data.