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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• Linearizing the stresses through the section thickness (path linearization in brick type finite element models)

• Geometric removal of the stress singularity by rounding the weld with an appropriate radius (CAB concept according to [11]); • Use of the stress at a position of 2.5 mm in front of the weld toe (Haibach concept) usually in brick type finite element models see [12].

Fig. 3. Methods for the determination of the structural hot-spot stress according to [4]

In [4] the recommendation is given, that where there are thermal loads generating highly non-linear through wall stresses, which could be combined with mechanical loads, the stress to be used for fatigue analysis could be determined by finite element modelling with solid elements, in association with surface extrapolation or with the CAB or Haibach method. In the case of a thermal stress distribution, both the non-linear and the equivalent linear stress distributions shall be determined for each stress component. For a more detailed discussion see [7]. Weld improvement techniques and their impact with regard of improving the fatigue performance of welded parts are addressed in a dedicated section. The assessment of unwelded components shall be based on the effective equivalent stress range of each cycle calculated using the components of the total stresses at the location under consideration. The total stresses should include the full effects of gross and local structural discontinuities (i.e. notches and stress concentrating features) from all type of loads including operating pressures, other mechanical loads and thermal transients. The total stresses can be obtained from a linear elastic finite element stress analysis using a solid model incorporating gross and local structural discontinuities. The total stresses can also be obtained from an analysis of the structural stresses amplified by a (effective) stress concentration factor for the effect of local discontinuities . Alternatively structural stresses may be used without a stress concentration factor in association with the Class 100 fatigue curve for welded components with the correction factors for welded components. This does not apply for sharply notched structures. Several correction factors are applicable taking account of surface finish (f S ), thickness (f e ), mean stress (f m ) and temperature (f T* ) are applicable. The applicable fatigue design curves for unwelded parts as a function of the ultimate tensile stress are shown in Figure 4. 4. Fatigue assessment of unwelded components