13th International Conference on Fracture June 16–21, 2013, Beijing, China -5- too small, the expected anti-loosening performance cannot be realized. On the other hand, when α is too large, the clamping ability will decrease significantly because large deformation takes place on the bolt thread. Therefore, considering both the anti-loosening ability and the clamping ability, α=αmiddle can be selected as the most suitable pitch difference. From the loosening experiment results we can see that, a proper pitch difference has a positive effect on the anti-loosening performance of bolt and nut. Next, we will focus on the effect of pitch difference on fatigue strength of bolt, considering three types of pitch difference of α=0, α=αsmall and α=αmiddle. 4. Effect of pitch difference on fatigue strength of bolt 4.1. Fatigue experiment The M16 (JIS) blots and nuts, strength grade 8.8, are employed in the fatigue experiment. The specimens are subjected to a mean force of 30 kN. Since the cross sectional area AR of the bolt is 141 mm2, the corresponding mean stress of the specimens is 213 MPa. For normal M16 bolt, it is known that the fatigue strength is about 50 MPa. Here, the stress amplitude is ranged from 50 MPa to 160 MPa in this experiment. The 392kN (40tonf) Servo Fatigue Testing Machine is used with cycling frequency 8Hz. The assembly drawing is illustrated in Figure 5 and the experimental device is shown in Figure 6. Three groups of bolt joint specimens of α=0, α=αsmall and α=αmiddle are investigated. Figure 5. Schematic illustration of fatigue experiment [mm] Nut M16 Washer Φ 17×95×15t Washer Φ 19×95×15t Bolt M16×30 F=(30±8.5)kN─(30±22.6)kN
RkJQdWJsaXNoZXIy MjM0NDE=