Fatigue damage accumulation in steel welded joints, subject to (random) variable amplitude loading conditions

An improved fatigue-life model for naval ship structural design

Master thesis (2021)

Authors

M.L. Deul Mechanical Engineering

Contributors

J.H. den Besten Ship Hydromechanics and Structures - Mechanical, Maritime and Materials Engineering (supervisor 1)
J. W. van Bergen Ministry of Defence (supervisor 1)
C.L. Walters Ship Hydromechanics and Structures - Mechanical, Maritime and Materials Engineering (supervisor 2)
M. Veljkovic Steel & Composite Structures - CEG (supervisor 2)
G. Bufalari Ship Hydromechanics and Structures - Mechanical, Maritime and Materials Engineering (supervisor 2)
Faculty
Mechanical Engineering
Copyright: © 2021 Marije Deul
Effective notch stress Welded joints Steel marine structures Variable amplitude loading Generalised random fatigue limit Non-linear damage accumulation
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Published Date

28-07-2021

Language

English

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Faculty

Mechanical Engineering

Abstract

The current fatigue assessment methods for naval ships are intentionally conservative. This is in part due to uncertainties in the fatigue capacity (i.e. fatigue resistance) of the ship: the nominal and hot spot structural stress incorporate implicit conservatism due to the inherent data-scatter and the Linear Damage Accumulation Model (LDAM) by Palmgren and Miner is not sufficiently accurate to assess random variable amplitude loading histories. The safe-life engineering solution to this is to employ a safety factor. This typically reduces the critical damage from 1.0 to 0.5, or even 0.2, based on engineering judgement. The main flaw of the Linear Damage Accumulation Model is that it does not properly account for the effects of the loading history. To address this flaw the non-linear probabilistic fatigue damage accumulation model by Leonetti is researched and thoroughly validated with variable amplitude fatigue data from the literature. The combination between the effective notch stress and the model from Leonetti poses significant improvements in the quality of the fatigue life predictions. The effective notch stress is used to formulate a generalised fatigue resistance curve (reducing scatter due to different geometries, hot spot types and mean stress ratios), after which the non-linear probabilistic fatigue damage accumulation model is used to work towards a generalised damage accumulation model. For the considered variable amplitude fatigue databases from literature, the Effective Notch Stress Concept (ENSC) - Non-Linear Damage Accumulation Model (NLDAM) combination proves to reduce the lifetime ratio scatter index (1:1.13, instead of 1:1.28) the most in relation to the DNV method (Hot Spot Structural Stress concept with the LDAM), allowing to reduce the implicit conservatism in the fatigue-life model.