Engineering guide
The E17N module is a supplementary input form for the simplified fatigue assessment of pressure vessels to EN 13445-3, Clause 17. It serves to define the stress factor of a component detail – that is, the factor by which the nominal stress range calculated from the pressure fluctuation is scaled up to the governing local stress at the design detail under consideration.
In addition, the module asks whether the butt weld under consideration is flush ground. This input affects the classification of the detail: welds ground flush with the plate surface and without misalignment achieve a more favorable fatigue class than welds in the as-welded condition, because notch effect and weld toe radius dominate the fatigue strength. The stress factor determined here then enters the actual load cycle calculation to Clause 17 (see the EN17 module), where the allowable number of load cycles follows from the pressure range, the correction factors and the fatigue curve.
Typical workflow
Identify the design detail
First, the fatigue-critical detail is identified – for example a circumferential weld, a nozzle attachment or the transition between head and cylinder. Each detail has its own stress factor and its own fatigue class.
Define the stress factor
The stress factor describes the ratio of the local stress at the detail to the nominal stress of the undisturbed shell. It is assigned to the detail under consideration in accordance with the provisions of Clause 17 of EN 13445-3 and is supported by the module's info function.
Assess the weld condition
The input on whether the butt weld is flush ground determines the classification of the weld class: ground welds without misalignment are rated more favorably than welds in the as-welded condition.
Hand over to the fatigue calculation
In the simplified fatigue assessment, the defined stress factor is multiplied by the pressure range and yields the pseudo-elastic stress range, from which the allowable number of load cycles is determined.
Frequently asked questions
What is the stress factor in the simplified fatigue assessment?
The simplified method of Clause 17 of EN 13445-3 does not work with locally resolved stresses from a detailed analysis, but scales the nominal stress of the shell with a detail-dependent stress factor. It covers the stress concentration caused by the geometry of the detail (nozzle, weld toe, head knuckle) in a lump-sum way. The larger the factor, the higher the stress range to be applied and the lower the allowable number of load cycles.
Why does grinding the butt weld flush improve the fatigue strength?
In welded components, the fatigue crack almost always starts at the weld toe, where notch effect, undercuts and tensile residual stresses coincide. If the weld is ground flush with the plate and misalignment and angular distortion are small, the geometric notch is largely eliminated – the detail may then be assigned to a higher fatigue class. The prerequisite is that grinding covers the full surface and is verified by inspection (e.g. surface crack testing).
When is the simplified assessment per Clause 17 no longer sufficient?
The simplified method applies to predominantly pressure-induced load cycles and is deliberately conservative. In the case of significant thermal stress cycles, superimposed external cyclic loads, or when the simplified verification does not confirm the required number of load cycles, the detailed fatigue analysis per Clause 18 of EN 13445-3 with local structural or notch stresses is required.
Technical content source: LV-Solved-Problems-Content 628d313247e7