PSI - Issue 19

Helen Bartsch et al. / Procedia Structural Integrity 19 (2019) 395–404 Helen Bartsch, Benno Hoffmeister, Markus Feldmann / Structural Integrity Procedia 00 (2019) 000 – 000

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the weld is modelled with a reference radius. The calculated notch stresses are compared to a single reference material-dependent S-N curve, in the frame of the presented investigations with the fatigue strength of 225 MPa and the inverse gradient m = 3 (Hobbacher, 2008).

2.2. Fatigue of prestressed bolts in end plate connections As a highly notched component, the bolt can only withstand very low fatigue loadings. The elastic flexibility of the end plate in connections also has a negative effect on the fatigue behavior of the high-strength bolts: In addition to the bending of the bolts that then occurs, additional support forces arise, which increase the stress of the bolt (Lacher, 1987). In (Eurocode 3-1-9, 2005) the bolt is categorised in FAT class 50 (Detail 14 in Table 8.1) and it is pointed out that bending and tension due to supporting forces as well as other bending stresses must be taken into account in the fatigue check. However, there are no exact specifications for the arising stresses of bolts under such loads. Some calculation models exist to determine the nonlinear bolt force function in relation to an external tensile force, but they are not applicable in all design cases (Petersen, 2013) (Schmidt, et al., 1997) (VDI 2230, 2015). Stress measurements on bolts in end plate connections show that the choice of geometry and unintended imperfections have a significant influence on the bolt stress and must be taken into account in the design (Schaumann, et al., 1999). The fatigue strength of 10.9 grade high-strength prestressed bolts in bolted end plate connections has already been tested and theoretically investigated in several additional studies (Hedenkamp, 1992) (Lacher, et al., 1984). In practice, it is usual to idealize the essential tensile area of the connection by means of a symmetric T-model (small-scale) and to transfer the knowledge gained there to the end plate connection (large- scale). In several reported studies, this transferability could not be confirmed and will hence also be examined during the present study. 2.3. Fatigue of welds in end plate connections After premature weld failures occurred during fatigue tests of high-strength bolts (Schaumann, et al., 1999), the fatigue strength of the welds has been determined in further investigations as the second element at risk of fatigue failure next to the bolts in the end plate connection. Welds must realize a force flow that is as undisturbed as possible. According to current regulations (Eurocode 3-1-9, 2005), the fatigue strength for T-joints and cross-joints is defined equally. However, the force flow in the T-joint differs from that of the cross-joint: In the cross joint, the force flow passes directly through the welds. Only a slight change of direction occurs due to material thickening in the area of the welds. With the T-joint, on the other hand, the change in direction by 90° deflects the force flow in the area of the welds, thus relieving the area of the weld root (Fig. 1) (Schaumann, et al., 1999).

Fig. 1. Force flow in cross- and T-joints

According to (Eurocode 3-1-9, 2005), the T-joint is classified in Table 8.5 "Welded joints", depending on the type of failure to be verified: The root crack for fillet or non-fully penetrated butt welds is assigned to FAT class 36* or 40 (Detail 3). For the verification against cracking at the weld toe in butt welds and all non-fully penetrated welds (Detail 1 and 2), the fatigue class is depending on the geometry. The highest fatigue class available is FAT 80. For Detail 1, with increasing expansion of the geometry, the FAT class decreases. This also applies to Detail 2, covering