A series of fatigue tests had been carried out by using standard M9-10.9 bolts in order to show the influence of tightening bolts into yield by the angle-controlled tightening method on the fatigue strength of bolted joints. The tests were conducted with a 5 ton servo fatigue testing machine using a specially designed test fixture capable of subjecting alternating eccentric loads on bolted joints.
As the initial bolt prestress was increased to yield, the additional bolt stress transmitted by the equal external alternating load was decreased. For this reason, the endurance limit of the bolted joints tightened to 90kg/mm2 became 15% above that of the bolts tightened to 60kg/mm2 and 60% above that of the bolts tightened to 20kg/mm2.
On the other hand, the endurance limit of the bolt became less as the initial bolt prestress was increased. It could be interpreted through the fractographic examination using SEM that the above result was related to the influence of the initial bolt prestress into yield on the early stage of the fatigue crack propagation at the root of the thread groove.
