Fatigue Strength Evaluation of Defects Embedded in Large-Sized Stud Bolt of Marine Engine

Abstract

Millimeter-sized defects are often found during inspection near the centerline of the shank of large-sized stud bolts. Determination of allowable defect size is practically required in view of safety. An experimental investigation on the fatigue strength of the stud bolt has been carried out in order to evaluate the effect of defects embedded in the stud bolt on the fatigue strength. Fatigue testing was conducted for stress ratios of 0.0, 0.4 and 0.7 using plain specimens and defected specimens having a defect embedded inside to examine the fatigue limit according to the stress ratio. Macroscopic and microscopic analyses were also conducted in order to determine initial defect size, crack initiation site and propagation rate from beach marks on the fracture surface. Existing small defect evaluation methods currently available were used for the strength evaluation. Allowable internal defect size was determined based on the high mean and high cycle stress states of the bolt by linear elastic fracture mechanics.