PSI - Issue 19

J. Rudolph et al. / Procedia Structural Integrity 19 (2019) 575–584 Author name / Structural Integrity Procedia 00 (2019) 000 – 000

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Fig. 4. Fatigue design curves for unwelded ferritic and austenitic forged and rolled steels (mean stress = 0)

Plasticity correction is realized by way of plastification factors k e .

5. Deviations from design shape

Discontinuities and departures from the intended shape of a vessel (i.e. "misalignments" as shown in Figure 5) will cause local increases in pressure-induced stresses in shells, as a result of secondary bending, and hence reduce fatigue life. Departures from intended shape include misalignment of abutting plates, an angle between abutting plates, roof topping where there is a flat at the end of each plate, weld peaking and ovality. In most cases these features cause local increases in the hoop stress in the shell but deviations from design shape associated with circumferential seams cause increases in the longitudinal stress.

Fig. 5. Deviations from design shape at seam welds

Clause 18 requires that the influence of misalignment be considered at the design stage. It impose to assume values for misalignment, to calculate the resulting secondary bending stresses, and to include them in the calculation of structural stress for the detail under consideration. When the fatigue life check fails, the clause imposes to tighten some or all of the tolerances to meet the required life or to change the detail geometry.