The principle of fatigue life assessment consists in comparing parameters believed to govern the physical phenomenon with their critical values leading to failure after a given number of cycles. Basically two approaches can be distinguished. The S-N approaches are based on stress analysis of the structure, assuming that fatigue failure occurs at the same stress level and cycles number as that of experimental tests. The fracture mechanics approach assumes an existing crack in the structure growing up to the failure under specific loading conditions. Thus, it rationally accounts for the influence of flaws on fatigue life, as it measures cracks, i.e. a physical effect, rather than damage/failure, a conceptual parameter. Moreover, it can be used in conjunction with NDT (non-destructive testing) information, and it can be joined with advanced stress analysis techniques. In this work, a sensitivity analysis is applied to a reliability model for fatigue life calculations for typical welded joints based on linear fracture mechanics. In the linear elastic fracture mechanics, a linear differential equation relates the stress intensity factor range and the crack growth rate in the range of interest of fatigue analyses. Integration of this differential equation leads to the relationship between the strength vs. the loads in time domain, i.e., crack size vs. number of load cycles. The reliability framework applied to fracture mechanics explicitly quantifies the uncertainties of all the parameters involved in the failure function as well as the ones deriving from inspection's updating (i.e. time of inspection and probability of crack detection). The safety margin (failure probability before a certain time) is expressed as the final crack size at failure. It is commonly assumed and recommended in rules of classification societies, that the long-term stress range is Weibull distributed. Hence, the stress effect is defined as the m-th statistical moment of the distribution. Inspections provide additional information that can be used to update the reliability model, as they provide an estimate of the size of the cracks in a structure with all cracks being smaller than the detectable one in ideal cases. Mathematically, the reliability of an inspection is defined by the probability of detection (POD) of fatigue cracks, which is commonly expressed by an exponential distribution, the mean detectable crack size being the only distribution parameter. Updating is based on the concept of conditional probability, i.e., the failure of a structural components given the inspection outcome. The reliability model for fatigue assessment has been applied to three typical structural details, a butt joint of two adjacent plates, a T-butt (fillet) joint of two perpendicular plates, and a bracket toe (longitudinal attachment). The main parameters for the fatigue model for all three details are found to be the load parameters (Weibull distribution) and the material parameter in the crack growth law. The model is more affected by load uncertainties than by fatigue strength uncertainties. After updating, the inspection quality has a dominant importance, which, however, rapidly decreases in time while load parameters correspondently increase their importance. No significant change in the failure probability is noted when the time of the inspection event is uncertain. Only the last inspection results significantly contribute to the updating model, whereas the effect of previous inspections is negligible.


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    Titel :

    Application of reliability analysis to the fatigue of typical welded joints of ships


    Weitere Titelangaben:

    Zuverlässigkeitsanalyse zur Vorhersage der Ermüdungslebensdauer der Schweißverbindungen von Schiffen


    Beteiligte:

    Erschienen in:

    Erscheinungsdatum :

    2007


    Format / Umfang :

    12 Seiten, 10 Bilder, 1 Tabelle, 36 Quellen



    Medientyp :

    Aufsatz (Zeitschrift)


    Format :

    Print


    Sprache :

    Englisch





    Application of Reliability Analysis to the Fatigue of Typical Welded Joints of Ships

    Rizzo, Cesare Mario / University of Genoa | Taylor & Francis Verlag | 2007




    Typical ships

    Engineering Index Backfile | 1913