However the stress amplitudes are a measure of stress in the bolt. To conclude, bolts fail from fatigue failure when exposed to cyclic loading where the stress amplitude exceed the endurance limit. The endurance limit obtained will vary greatly depending on the Nominal size, material and preload of the bolt. If the working load applied to the bolted joint is lower than this endurance limit, the joint is perceived to have infinite life. This limit is called an endurance limit or fatigue limit. The resulting curve approaches a limit, showing that stress amplitudes below a certain value will not lead to fatigue failure. The cycles to failure is often represented with the log scale. S (Mpa) on the X axis and the Cycles to failure. The S-N curve is a diagram that shows the relationship between the stress amplitude. This is then repeated for a number of different stress amplitudes to obtain a S-N curve. Fatigue testing at a specific stress amplitude is repeated for 10 4 ~ 10 7 of cycles until fatigue failure occurs. The ISO global testing standard is specified by ISO 3800:1993. Axial load fatigue testing is performed by applying cyclic loading to a preloaded bolted joint. Bolted joints are evaluated with Axial load fatigue tests to understand the degree of cyclic loading required to fail a joint. Under what conditions do bolts fail from fatigue failure?īolts fail from fatigue failure when exposed to cyclic loading which exceeds a certain amplitude. Over 90% of bolt breakage cases are resultant from Fatigue failure. There are many reasons as to why bolts break. As mentioned on our “Factors influencing thread loosening” page, preload loss (bolt loosening) is one of the underlying causes of bolt breakage.
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