PSI - Issue 53

Benjamin Möller et al. / Procedia Structural Integrity 53 (2024) 190–202 Author name / Structural Integrity Procedia 00 (2023) 000–000

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included, where 16 results show a fatigue fracture and three result in a run-out, marked with horizontal arrows added to the points in Fig. 5. The method of maximum likelihood including run-outs from fatigue tests was introduced by Spindel and Haibach (1978). As it recommended for aluminum / welded joints by Sonsino (2007), the slope after the knee point was set to k * = 22.0. With these assumptions, the Wöhler (S-N) curve is described by M a,k = 40.7 Nm, N k = 800,000, k = 3.1, k * = 22.0, and T  = 1 : 1.93.

Fig. 5. Load-based Wöhler ( S - N ) curve.

The failure criterion was derived from the fatigue test stop related exceedance of maximum displacement. Fatigue fracture was observed from the tensile stresses at the outer surface of the specimen as expected. However, the location in height for each specimen differs from test to test, depending on the local appearance of the surface, i. e. surface notch effect. A typical fatigue fracture after testing of specimen HB_2D is shown to the right of Fig. 6. Crack start could be identified from the notch of the critical weld bead root. The fracture surfaces show a brittle fracture without major internal defects, as the microscopic images of specimen HB_6U in Fig. 6 left visualize.

Fig. 6. Typical fatigue fracture and crack initiation location.