Multiaxial fatigue life prediction of tubular K-joints using an alternative structural stress approach

Abstract

Tubular K-joints, frequently employed in offshore structures, are subjected to multiaxial stresses. It is difficult to accurately assess fatigue strength of tubular K-joints using the uniaxial fatigue theory. In this paper, a multiaxial fatigue life prediction method for tubular K-joints is proposed, which is based on an improved Zero Point Structure Stress (ZPSS) approach that accounts for the stress gradient in the thickness direction to quantify the structure stress at the weld toe of a tubular K-joint. The multiaxial fatigue life is predicted making use of the Modified Wöhler Curve Method (MWCM) as the multiaxial damage criterion. It is found that the multiaxial fatigue estimation using the proposed method predicts the fatigue critical point well. Meanwhile, the estimated multiaxial fatigue lives are much closer to experimental lives, in comparison with the uniaxial fatigue estimation using the CIDECT specification.