A fatigue assessment method based on weld stress

This paper presents a method for extending the structural stress method as defined in design guidance documents for fatigue assessment of welded plate-type structures. The conventional structural stress approach is not sensitive to plate thickness and an empirical thickness correction factor is recommended. Also, the traditional method specifies alternate fatigue design curves depending on whether the welded joint is generally classified as load carrying or as non-load carrying. The proposed modified structural stress improves both of these limitations by defining a multi-linear stress distribution through the plate thickness beneath the weld toe. The linear structural stress is modified by taking into account the nominal stress in the weld. This modified structural stress better captures the local effect of the weld size and weld stress. Simple equations are presented for determining the proposed stress distribution which, depending on the geometry of the structure, is composed of two or three linear segments. The proposed modified structural stress distribution is easily determined based on the through-thickness stresses computed using finite element analysis. The method is presented and illustrated in the case of a symmetric welded cover plate subject to both axial loading and bending loading. The method has been verified using both fracture mechanic calculations and published experimental results for cruciform joints with varying weld sizes, base plate thicknesses and attachment sizes.