An Influence of Tightening Torque Stored in Tightening Process on Fatigue Strength of Aluminum Bolts

Abstract

Fatigue strength of bolts has been generally investigated loading only axial cyclic loads on the bolts. However since the bolt is used to be tightened into a internal thread part or a nut, the bolt receives twisting torque due to friction torque between thread torque during tightening. Therefore an influence of the thread torque on the fatigue strength shoud be investigated. In this study, transverse fatigue tests have been conducted because the test can perform the fatigue tests in situations a bolt is tightened. And the transverse fatigue tests loosening the nut slightly to release the thread torque after tightening have been also conducted. Test bolts and nuts were hexagon head bolts M8 and the bolts and the nuts were made of aluminum alloy A5056. The bolt/nut assemblies were lubricated by Molybdenum disulfide grease in the transverse fatigue tests. Results showed that the transverse fatigue strength of bolt/nut assemblies for aluminum alloy bolt decreased by influence of the thread torque. Results also showd that the transverse fatigue strength of bolt/nut assemblies for aluminum alloy bolt could improve 15 % by means of releasing the thread torque.