Experimental evaluation of the effect of preload on the fatigue life of bolts

The majority of bolts are subjected to fluctuating loads, leading to the well-known fatigue phenomenon which is responsible for most of the premature failure in bolts. The effect of preload on the fatigue life of bolts is investigated by producing a series of S-N curves for different levels of preload. For this reason a specific fixture is designed and manufactured for performing the tests. The experimental results shows that the preload reduces the endurance limit of bolts by an amount proportional to the increase in the mean stress produced by the preload. However, beyond a significant mean stress, the endurance limit increases too. The fatigue test results are compared with the BSI standard fatigue life formulas and conventional fatigue life prediction method. FE modeling of the problem reveals that the amount of the axial load applied to the bolt does not affect the stress concentration factor.