Effective Notch Stress Concept Investigations for Mode-III Loading & Response Conditions

Abstract

Fatigue strength is typically a governing limit state for marine structures. Predicting an accurate fatigue lifetime is important for structures (e.g. wind turbines) that are supposed to be out at sea for decades. It is valuable for all type of structures that encounter stochastic sea loadings to optimise the design so that fatigue failure is predicted for, or material is saved in preventing over conservative estimates. This research contributes to the investigation of multiaxial fatigue concepts by getting insight in the mode-III loading & response conditions for the effective notch stress concept. The mode-III loading & response will in this research be represented by a tubular welded joint loaded under torsion.
A semi-analytical formulation is developed to describe the weld notch shear stress distribution analytically. This formulation can be used as input for the effective notch stress concept. The formulation is based on three components, the notch stress, weld load carrying stress and far field stress component. Furthermore, the static force and moment equilibrium is satisfied. The weld load carrying stress component is based on a weld load carrying stress estimate. This coefficient is used because the weld geometry causes a local change in stiffness, a shift in neutral axis, meaning the weld becomes load carrying up to some extent. The distribution must be corrected for this phenomenon which is done by this estimated term.
The geometry is a tubular welded joint. This specimen can be loaded with torsion which in the case of the tubular joint cause mode-III shear in the through thickness direction at the weld notch level. The weld load carrying stress estimate is based on the geometry ratios of the assessed specimens. It is individually fitted for all 6,300 geometries by comparing the results to stress distribution obtained with 2D finite element analysis. The results of the individual fitted load carrying stress estimates are used as input of a multi variable polynomial regression analysis. The regression analysis led to a polynomial function that describe the load carrying stress estimate which uses geometry ratios as input.
The far field information and geometry dimensions are the input for the semi-analytical formulation to obtain the weld notch shear stress distributions analytically. Accurate results are obtained and therefore the formulation can be used as input for the effective notch stress concept.
The effective notch stress concept is also based on a material characteristic length, ρ*, over which the stress distribution can be integrated to obtain the effective notch stress. This parameter is obtained using a log likelihood regression analysis and led to a value of ~1 [mm]. Although this is a good result, the confidence regarding the value is low due to low variety in the experimental data set and little data in general.
The findings of this study will be used in further research regarding the 4D fatigue project and the effective notch stress concept in general at the Delft University of Technology.