Evaluation of mixed-mode stress intensity factors under arbitrary distributed loadings

Kurzfassung

Due to the nature of cyclic mechanical and wave loads, ship and offshore structural components are at high risk of fatigue failures. To prevent the event of catastrophic failures, fatigue assessments of ship structures are carried out for both finite and infinite life evaluation. In this aspect, the fatigue crack propagation of welded joints takes the center of interest as crack initiations are likely to occur at locally high stress concentration areas. For a reliable FCP evaluation of welded joints, accurate estimation of stress intensity factors (SIFs) considering all relevant stress distributions plays an important role. To get the SIFs of welded joints, weight function method has been practiced in design process, and used in welding standards and regulations for fatigue assessments. It is a computationally efficient approach in the SIF evaluation without repeating fracture mechanics analysis for each geometry and loading condition. However, the established solutions are only available for some limited geometries and applied loadings, and no weighted integral solutions for cases with complicated crack face traction. In addition to that, only mode-I SIFs of simple cracked bodies were analyzed in previous studies. On the other hand, it is crucial to consider arbitrary loadings in getting the mixed-mode SIFs of welded joints as the combination of external cyclic loadings and welding residual stresses leads to arbitrary stress distributions on the initiated crack face. This study fills the research gap by developing an engineering formula for fracture parameter modification factors (µk) under arbitrary stress distributions, and a methodology to evaluate the MM-SIFs using estimated µk. Firstly, the MM-SIFs values of flat plate and welded joints are estimated based on the numerical influence function method which gives the solutions by superimposing the arbitrary applied loading with influence coefficients. After that, the µk equations are developed using the multivariate regression model. The proposed formula gives the approximate value of µk for a given surface crack in a T-joint under multi-axial loadings. The µk values estimated by the proposed equations and original values are only varying at a maximum of 2.59%. Then, the MM-SIFs of T-joints can be calculated using flat plate FE models and user-defined arbitrary loadings. Those given by the proposed methodology are validated using commercial finite element solvers, and considering welding residual stresses. The validations show good agreement in solutions with acceptable deviations. Therefore, the proposed µk method is efficient and accurate to determine the MM-SIFs of T-joints under arbitrary stress distributions. This study has some rooms to improve in the range of application for larger range of crack size and welded joint geometry, and to implement a user-friendly working scheme.