Abstract
When bolted joints are subjected to cyclic loading conditions, fatigue strength of the bolts is critical for integrity of the joints. In the present study, two types of fatigue tests were applied for M8 bolts made of alloy steel, SNB7, used for flange joints of pipes in petro-chemical plants up to 400℃. The bolts were fixed with double nuts at the both ends and applied cyclic loadings at various stress ratios at 5Hz by a hydraulic fatigue testing machine. In the other fatigue test, the bolts were cut with a length having resonance frequency of 20Hz, and tested by an ultrasonic fatigue testing machine, which is suitable for evaluating very high cycle regime. Stress analyses for the bolt specimens were performed by finite element method (FEM) in order to obtain the stress concentration by the screw and to adjust the nominal stress values for the two types of the fatigue tests. Wide range of fatigue life of the bolts were determined by those tests and the effect of stress ratio on the fatigue strength were investigated.
