The European Pressure Vessel Standard EN 13445 provides in its part 3 (Design) a simplified method for fatigue assessment. Use of the simplified method is limited to cases where the cyclic load is mainly given by pressure fluctuations. The stress ranges needed for the evaluation of the allowable number of cycles are calculated only on the basis of the pressure range and of the maximum allowable pressure of each vessel component, obtained from the normal design formulae. Suitable tables are provided in order to establish, for each vessel component, a relationship between the stress range and the pressure range. Other tables are provided in order to define a suitable fatigue curve on the basis of the so called “Weld Class”: curves with higher values of stress ranges are given for full penetration welds having better NDT testing, while curves with lower stress ranges are given for not fully penetrated welds with poor or no NDT testing. Since only the plain Design by Formulae is needed for the use of the method, no FEM calculations are required. The simplified method may therefore give a suitable simple and economic solution in many common cases of cyclic loading. In this paper two worked examples have been provided in order to compare the results of fatigue calculations made with the simplified method with the results obtained by means of a complete F.E.M. stress analysis.

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