Abstract
In this study we propose a simple computing procedure to determine in probabilistic form the material constants that enter fatigue-life calculation, namely fatigue strength (ductility) coefficient σ’_f (ε’_f) and fatigue strength (ductility) exponent b (c). The procedure, which assumes log-normal distribution of fatigue life on the S-N curve, is based on the shift of the regression line (representing the fatigue-life curve) to the desired value of the probability of occurrence, and yields a set of material constants σ’_f, b, ε’_f, c ranging in probability of occurrence from 1 % to 99 %. Suitable application of this procedure to the life estimation model under random loading conditions yields the fatigue-life distribution function taking into consideration the scatter of cyclic material properties.
The application of this method to evaluate fatigue reliability of a vehicle’s construction is also presented. Based on the proposed methodology, the maximum permissible value of the scatter of a welded joint’s cyclic properties was determined to ensure that the probability of premature fatigue failure occurrence would not exceed the allowable value.

Key words
material’s cyclic properties, fatigue-life distribution function, probabilistic analysis, welded joint

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