Fig. 9 shows that fatigue failure of one high-strength bolt (S2-1)occurred in the first engaged thread between bolt and sphere, as a typicalform of failure in BSJs observed during the fatigue tests. All thebolt specimens exhibited a fatigue failure similar to that of S2-1.The fatigue fractographic analysis of one high-strength bolt (S5-1)was conducted using an electron microscope (TESCAN Mira3, LMH), asshown in Fig. 10. The fracture surface is mainly divided into three parts:fatigue source zone, crack propagation region and transient fractureregion. The crack initiation generally occurs at the surface of boltthread root and the fatigue source zone is relatively smooth and in deepcolor, showing river-like patterns toward the inside of the bolt. Themain feature of the fatigue propagation zone is the fatigue striations,and secondary cracks can be observed. The striations are approximatelyparallel, and extend toward the same direction of the secondary cracksand are perpendicular to the crack propagation direction. As the cracksextend, the effective bearing area of the specimens decreased because ofmaterial damage, the stress in the residual section gradually increaseduntil a sudden rupture occurred and resulted in the transient fracturezone, where the mixed morphology of fatigue fringes and dimples canbe observed. The transient fracture region is relatively rough andcontained a number of coarse particles. The presence of a large amountof dimples is the main microscopic features of plastic fracture of metal