PSI - Issue 57

Magnus Andersson et al. / Procedia Structural Integrity 57 (2024) 307–315 M. Andersson et al. / Structural Integrity Procedia 00 (2023) 000–000

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Fig. 7. (a) Crack for di ff erent initial crack angles; (b) paths for interpolating the geometry factor for the bending stress.

4.2. Influence of membrane to bending stress ratio on the predicted fatigue life

In this example, the ratio of membrane to bending stress is 0.1, 0 and -0.1 for the three cases, see Fig 8. The initial crack angle is v = 0 ◦ . It may be seen that when the membrane stress decreases at constant bending stress, the crack angle increases. For decreasing membrane stress the fatigue life also increases.

Fig. 8. (a) Crack growth for di ff erent fractions of membrane stress; (b) paths for interpolating the geometry factor for the bending stress.

5. Real application examples

5.1. Comparing SFM with LEFM for a hauler A-stay

The A-stay is subjected to forces and moments in all directions (i. e. six load time signals) at the load position, see Fig. 9. The evaluated weld position (a double sided i2a5 fillet weld) is located near the bolted joint. For the SFM method the normal stress in local z ′ -direction is extracted along the local x ′ axis (from the weld root to the free weld surface) at the angle v ref = 45 ◦ for each of the six unit load cases. The structural stress is stored in σ FEA , i ( v ref ) for each unit load case i , see 3.1. In Fig. 9(c)(d) the stress intensity factors for mode I (for both SFM and LEFM) are shown. The normalized fatigue life is shown in Fig. 10(a). In the case of using linear elastic fracture mechanics (LEFM) analysis, stress intensity factors have been calculated at constant crack angles of v = 0 , 20 and 45 ◦ . For the SFM case with varying crack angle, the final crack angle is v = 0 ◦ . The crack angle is also held constant (at v = 0 , 20 and 45 ◦ ) for SFM. The fatigue life prediction follows the LEFM results well, but is circa 10% conservative. In Fig. 10(b) the stress time history for the dominating load case is shown. For most of the time the stress is negative (compressive). This means that if it is assumed that the weld residual stresses are zero or negative in the weld root, the fatigue life for both SFM and LEFM would be infinite.