PSI - Issue 19

Okan Yılmaz et al. / Procedia Structural Integrity 19 (2019) 302 – 311 Yılmaz et al. / Structural Integrity Procedia 00 (2019) 000–000

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Fig. 3. Two di ff erent bolt head orientations used in this study. Flat side orientation (a) is the default orientation while corner point orientation (b) is denoted as the alternative bolt head orientation. Arrows specify the loading direction.

3.2. Pre-tension and surface condition

The bolt pre-tension is the force applied to the bolt during the tightening processes that compresses together the components in the connection. The pre-tension produces compressive stresses around the hole area and creates fric tional forces between mating surfaces that carry shear loads. It is believed that the compressive stress state around the hole may arrest the propagation of fatigue cracks [16], leading to an increase in the fatigue performance of the bolted connection. However, the increased contact pressure from a larger pre-tension might shift the contact state from gross sliding to partial slip, which would be a detrimental factor for fretting fatigue failure mode. To evaluate the e ff ect of pre-tension in HSS bolted connections, shear lap bolted joint tests are performed under three pre-tension levels. The first two of the pre-tension level are defined by M16 grade 10.9 bolts loaded up to approximately 75% and 130% the design preload force specified in the Eurocode standard for slip-resistant connections [10]. The highest preload level was achieved using M16 grade 12.9 bolts, not considered in [10], tightened up to yielding. In addition, same pre-tension levels are utilized for four-point bending loading tests at the previously defined force levels to analyze out-of-plane bending loading mode. The surface condition of bolted components also directly a ff ects the slip behavior of the bolted joint, therefore the sti ff ness of the hole structure. Surface treatments can induce compressive residual stresses that impede the nucleation and propagation of fatigue cracks. The e ff ects of primer and paint coatings and shot-peened layers are currently being analyzed in an ongoing study and will be reported in depth in [13] for the same pre-tension and stress amplitude levels. The bolt flange configuration has a significant influence on the fatigue performance of a bolted connection. We use an M16 grade 10.9 flange bolt (loaded up to 130% the design preload force) for the base case of out-of-plane loading mode. In addition, we consider di ff erent bolt head orientations illustrated in Fig. 3 and the e ff ect of having a shot-peened layer covered by a cataphoretic primer coating. It must be noted that unless stated otherwise, flat side orientation is used and corner point orientation is denoted as the alternative bolt head orientation. The tests are carried out only for S700MC grade at the force level F fpb , max = 10 kN. Pre-tension is always applied from the side that will remain in the compressive face. 3.3. Flange configuration

4. Finite-element simulation

To obtain additional information about the failure process, a finite-element model of the four-point bending test is built using the commercial software Abaqus [17]. The model, illustrated in Fig. 4, consists of a plate, the simplified geometry of a flanged bolt-nut assembly and rigid rollers. Making use of the symmetry, only a quarter of the geometry is modeled. Linear brick elements with reduced integration (C3D8R) are used. Since we aim for a qualitative estima tion of failure location, a linear-elastic material model is used and a displacement of 1-mm in negative y-direction is applied only to have stresses close to the yield stress of the material. Mesh size is determined by following the mesh