PSI - Issue 75

Ralf Glienke et al. / Procedia Structural Integrity 75 (2025) 474–488

477

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Ralf Glienke et al. / Structural Integrity Procedia 00 (2019) 000 – 000

(2024)). The IIW has meanwhile published many recommendations (Haagensen et al. (2013), Marquis et al. (2016)) describing the modification of the S-N curve, equipment technology and quality assurance.

1000

Improved detail category resulting from post treatment

100 Direct stress range D [N/mm²]

Detail category at 2x10 6 number of cycles of the untreated constructional detail

10

10 4

2×10 6

10 7

10 5

10 6

Fatigue life N [-]

Fig. 2. Modification of the S-N curve due to post-weld treatments (PWT)

2.3. Consideration of blast-cleaning on the fatigue resistance For the blast-cleaning process, which is closely related to shot peening, there are no standardised rules, although its positive effect on improving fatigue strength has been proven by various authors, see Hensel et al. (2019), Gericke et al. (2025), Ahola et al. (2023), Glienke et al. (2024a). Blast-cleaning with defined parameters and sharp-edged material (steel grit) to achieve the surface preparation grade Sa 3 induces compressive residual stresses close to the surface and leads to work hardening. At the same time, the surface roughness is increased to improve the adhesion of the corrosion protection, which potentially leads to a higher risk of crack initiation and decreased fatigue strength. A change in the seam geometry is also observed. It is therefore comparable to HFMI post-weld treatment. In the meantime, there are already further findings that the influence of the blast-cleaning company carrying out the work is negligible within the framework of defined process parameters, Hensel et al. (2019). Reliable methods are also available for quality assurance (e. g. Almen intensity, degree of coverage control), which make the targeted effect on the fatigue strength reproducible, (Hensel et al. (2019), Gericke et al. (2022)). The following application limit applies to nominal stress for HFMI post-weld treatment: -0.8 f y ≤  ≤ f y (Marquis et al. (2016)). Such a limitation is also expected for blast-cleaning treatment. 2.4. Recommendations for fatigue tests When carrying out fatigue tests, a number of specifications are available from various standards (ISO/TR 14345 (2012), DVS 2403 (2020), IIW Recommendations (2024)) regarding the number of tests, documentation of the test specimens, and statistical evaluation. To determine the detail category, FprEN 1993-1-9 (2024) requires at least n = 12 tests in the finite life region of the S-N curve, which are to be carried out downwards from the yield strength, whereby fracture events between N = 10 4 and N = 5 ∙ 10 6 are to be statistically evaluated. Regarding the fact that the detail categories according to the above-mentioned standards are to be applied independently of mean stresses, there is some difficulty in considering the influence of mean stresses and residual stresses, particularly for welded joints. Small scale specimens show lower residual stresses in comparison to real welded components (scale effect, Lotsberg (2016), Hobbacher (2010)). These residual stresses superimpose with acting stresses. The guidelines recommend that small scale specimens are either to be tested at high mean stresses (R = 0.5) or the determined fatigue strength is to be reduced by 20 %. Due to the high fatigue resistance of some constructional details and post-treated weld specimens, it is not possible to generate fracture events in the full range of the S-N curve when testing downwards from the yield strength and to derive a valid slope parameter m. Another possibility is described in Lotsberg (2016) and Pedersen (2024). To compensate the scale effect, it is recommended to conduct the tests with a constant upper stress  max = f y , so that fatigue testing is performed at variable R-ratios. This simulates the effect of real components with loading and residual stresses at the level of the yield strength. As a result of this test procedure, steeper S-N curves are obtained in