In this study, as a preventive maintenance method against root cracks at welded joints of cover plates, the tap bolt joint method with tap holes in both plates was proposed based on the concept of reducing the relative displacement between plates which is the cause of root crack initiation. As a fundamental study on the tap bolt joint, the fatigue strength of the joint and relative displacement of plates were evaluated by fatigue tests and FEA. It was revealed that the fatigue strengths were distributed around JSSC-E to F classes under cyclic tensile axial loads, JSSC-A class under cyclic compressive axial loads and much higher than JSSC-S class under cyclic shear loads.
