Experimental characterization of the bending fatigue strength of threaded fasteners

• Threaded bolts were fatigue tested in bending using a novel test apparatus.
• We model the failure probability with a stress based weakest link model.
• The comparatively small stressed volume in bending results in high fatigue strength.
• The fatigue limit is 76% higher in bending than in axial tension.

The fatigue strength in bending of pre-stressed steel bolts is investigated and compared to the fatigue strength in axial tension. The strength is measured in terms of maximum engineering stress amplitude, neglecting any stress concentration in the threads. The experimental results reveal that the fatigue limit is 76% higher in bending than in axial tension. A finite element model is used to compute the stress state in the threaded region for both axial tension and bending. It allows fitting a volume based weakest link model to the experimentally observed failure probabilities. Based on the good fit of the weakest link model it is argued that randomly distributed defects in the highly stressed thread root determine the fatigue strength.

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