13th International Conference on Fracture June 16–21, 2013, Beijing, China -8- case of α=0. For the specimens of α=αmiddle, the fracture position of bolt and the fracture surface are nearly same with the case of α=αsmall. 4.4. FEM analysis For the normal bolt and nut, it is known that the largest stress concentration appears at the No.1 thread of bolt. However, when α is introduced, the contact status between bolt and nut is changed, and therefore, the stress distribution along the bolt threads should be reconsidered. To analyze the stress status of the bolt threads, the finite element models are created by using FEM code MSC.Marc/Mentat 2007. Three models have the different pitch difference of α=0, α=αsmall and α=αmiddle in according with the experiment. As shown in Figure 9, the axisymmetric model of bolt and nut is created. The fixed component is assumed as a cylindrical clamped plate with an inner diameter 17.5mm, outer diameter 50mm and thickness 35mm. The Young’s modulus is 206GPa and the Poisson’s ratio is 0.3 for all the materials for bolt, nut and clamped plate. In the experiment, the bolt axial force F=(30±8.5)kN─(30±22.6)kN was studied. In the analysis, the bolt axial force F=30 ±14.1kN is considered as an example. The assumed amplitude Fa=14.1kN is corresponding to the stress amplitude σa=100 MPa at the minimum section of the bolt. The elastic calculation is performed as the first step, but the results show that the stress is over the yield stress σs=800 MPa of the material SCM435 for the wide region. Then, the elastic-plastic calculation is performed again. Based on the mean stress and the stress amplitude at each root of bolt thread, the endurance limit diagrams are obtained as shown in Figure 10 to Figure12. The fatigue limit σN of the material SCM435 is 420 MPa. The mean stress σm and stress amplitude σa are explained as following: max min m σ +σ σ = 2 , max min a σ -σ σ = 2 (2) where σmax is the maximum stress of each thread bottom under the maximum load of F=30+14.1KN and σmin is the maximum stress of each thread bottom under the minimum load of F=30-14.1KN. The endurance limit diagrams show the dangerous level of each bolt thread. From Figure 10, we can see that the No.1 thread bottom has the high stress amplitude and mean stress, which is corresponding to the fracture position in the experiment as shown in the section 4.3. In Figure 11, when a pitch difference of αsmall is introduced, the stress amplitude decreases at the No.1 thread bottom. On the other hand, plastic deformation happens mainly at the bottoms of No.7 and No.8 threads. However, the difference among the stress status at each thread is not very large compared Figure 9. Axisymmetric finite element model F=30±14.1kN Clamped plate Bolt Nut
RkJQdWJsaXNoZXIy MjM0NDE=