Fig. 13 presents the typical macroscopic fractures of 6 bolts, the testloading stress ranges of them are 90 MPa, 110 MPa, 150 MPa, 160 MPa,190 MPa, 210 MPa, respectively. As can be observed from the Fig. 13,the initial cracks originated at the root surface of the thread, and eachfracture has two or more fatigue sources. Then, the cracks begin toextend toward the center of the bolt until the bolt failed. A fatigue step was formed which was caused by the intersection of multiple crackextensions. The crack propagation zone is relatively flat and smooth,and is in brighter color with shell-like lines can be observed. Thefracture surface begins to become rough at the transition from the extensionzone to the instant fracture zone. In the transient fracture region,the fracture surface is rugged and in irregular crystal shapes.