Issue 38

M. Kepka et alii, Frattura ed Integrità Strutturale, 38 (2016) 82-91; DOI: 10.3221/IGF-ESIS.38.11 88 Figure 7 : The relations between the effective stress amplitude σ a,ef , the limit stress amplitude σ a,BK and the load capacity factor a with respect to the S - N curve and the total number of cycles N d . k k j a i i BK d D R i n a N 1 , , , 1 1                           (2) k k j a i i BK d D R i n a N 1 , , , 1 1                           (3) k k j a i i BK d D R i n a N 1 , , , 1 1                           (4) tot BK BK BK BK BK a a a a a a 2 2 2 , , , , ,                (5) PROBABILITY APPROACH FOR EVALUATION OF COMBINED LOADING he methodology outlined above represents the deterministic approach. In a probability-oriented assessment, the random nature of the loading, as well as the random nature of materials properties must be taken into account. A load record of certain length represents a random portion of a vehicle’s service. Obviously, another measurement carried out at another time will be different due to the random nature of the load. If a process record of adequate length σ ( t ) is segmented appropriately, it can be substituted for repeated measurement runs. In-service loads will thus be represented by a set of records – process segments [5]. By this method, the segment length for a representative lifetime assessment and the variance of load spectra used in the lifetime calculation can be determined. In Fig. 8, this procedure is applied to the complex load capacity factor based on (5) which is calculated within a moving window of L size. In the figure, mean values and standard deviations for the set of values a tot obtained are plotted for the expanding interval L . The segment length, at which the value of a tot ceases to change significantly, is used for calculating the load capacity factor distribution function. In the present case, a distance of 21 km has been used. The random nature of materials properties is reflected in the tolerance limits of the S - N curve. The variability of the number of cycles to failure N is expressed through the standard deviation s(log N ) for the quantile d , depending on failure T