PSI - Issue 58

J.R. Steengaard et al. / Procedia Structural Integrity 58 (2024) 61–67 J.R. Steengaard et al. / Structural Integrity Procedia 00 (2024) 000–000

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Stress Range, x

FAT90 Back Bottom Front

Fatigue life, N f

Fig. 7. Section of the FAT 90 design S-N curve with points for the three welds. The shown section of the design S-N curve has a slope of 3.

results, it can be seen that the updated FE model accurately predicts the strains in the experimental cutterbar. Using the accurate updated FE model, fatigue assessments of various locations on the cutterbar is performed.

4. Fatigue assessment

Three types of welds of the cutterbar are known to be loaded more than the rest. Thus, these are of interest in the following case study. The locations of one of each type of weld are highlighted on Fig. 3. These are the welds on the back, the bottom, and the front. The FE model is modified in accordance with the IIW guidelines for determining the hot spot stresses (Hobbacher (2016)). The hot spot stresses are found by extrapolating the surface stresses from two reference points using Eq. 2 (Hobbacher (2016)), where σ hs is the hot spot stress, σ 0 . 4 t is the stress at a distance of 0.4 times the plate thickness from the hot spot, and σ 1 . 0 t is the stress at a distance equal to the plate thickness from the hot spot. This is done for both normal and shear stresses. σ hs = 1 . 67 · σ 0 . 4 t − 0 . 67 · σ 1 . 0 t (2) To predict the fatigue lives and obtain equivalent uniaxial stresses for the three hot spots, the modified Gough–Pollard fatigue criterion by IIW is used, Eq. 3 (Gough and Pollard (1935); Hobbacher (2016)), where ∆ σ x is the normal stress range normal to the weld, ∆ σ R is the resistance stress range for normal stress, ∆ τ xy is the shear stress range parallel to the weld, ∆ τ R is the resistance stress range for shear stress, and CV is a comparison value, which is equal to 1 for proportional loading. Only the relative fatigue lives are considered, as the applied load is much higher than observed during field tests. The fatigue lives and the corresponding equivalent stresses are plotted for the three welds on the FAT 90 design S-N curve, which is often used for hot spot stress evaluation, see Fig. 7. The weld points are on the first part of the S-N curve, where the slope is 3, because the applied deflection is large. ∆ σ x ∆ σ R 2 + ∆ τ xy ∆ τ R 2 ≤ CV (3) From Fig. 7 it can be concluded that the back weld is the most exposed, as it has the highest equivalent stress and the lowest relative fatigue life. The bottom weld is the second most exposed, and the front weld is the least exposed. These results fit well with prior experiments, where the back weld has been the location of highest fatigue damage. The relative fatigue lives are calculated as N rel = N i / N min , where N i is the fatigue life, and N min is the minimum fatigue life of the considered welds. The relative fatigue lives are 1.0, 1.3, and 5.3 for the back, bottom, and front weld, respectively.