Numéro : 2612 - Year : 2011
A new probability level for rule fatigue loads evaluation
Quentin DERBANNE, BUREAU VERITAS – Division Marine
Guillaume de HAUTECLOCQUE, BUREAU VERITAS – Division Marine
This paper presents a comprehensive analysis of the contribution to fatigue damage of the different probability levels of the stress distribution, modeled by a Weibull distribution. It shows that the 10-2 probability level is the most appropriate for the evaluation of the rule fatigue loads., because it minimizes the influence of the Weibull shape factor.
A sensitivity analysis has been done on the choice of the reference level 10-p of the design loads. At 10-4 or 10-5 the total fatigue damage is very sensitive to the choice of the shape parameter of the Weibull distribution. However if the reference value for the design loads is chosen at 10-2, the shape parameter has hardly no influence on the total damage. Thus it is proposed to define directly the rule design loads at 10-2, and to forget about the shape parameter. The rule fatigue damage is then computed from the 10-2 stress range, the mean cycle period, and a shape factor of 1.0. The advantage of this method is to minimize the number of rule formulae, and to reduce the total error.
Some comparisons have been done with some stress histograms coming from direct calculations (in linear frequency domain) on several ships, which show the accuracy of this method. This method is currently applied in the IACS Harmonized Structural Rules.
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