Metal Fatigue Part 10

Tham khảo tài liệu 'metal fatigue part 10', kỹ thuật - công nghệ, cơ khí - chế tạo máy phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | 254 Chapter 14 I 100pm I Axial direction Figure Non-propagating crack in an unnotched specimen length 480 pm 9 . Table Size of the largest non-propagating cracks observed in unnotched specimens in rotating bending and in reversed torsion Loading Material S45C S10C Rotating bending 50 pm 100 pm Cyclic torsion 500 pm 1000 gm shear stress. Thus the existence of such long cracks in unnotched specimen is related to the rolled microstructure which consists of ferrite and pearlite layers. Table shows the maximum lengths of non-propagating cracks in unnotched specimens of c and c steels at the fatigue limit for rotating bending 17 19 and reversed torsion 9 18 Considering that both the c and c steel are produced by rolling the correspondence between the maximum nonpropagating crack sizes in torsion and rotating bending in both steels may be for the same reason. The maximum non-propagating crack sizes in torsion are too large to consider that the fatigue limit is determined by von Mises yield criterion and it is reasonable to regard the fatigue limit as the threshold condition for crack propagation. There has been some discussion on these points 20 . Recent detailed observations by Murakami and Takahashi 12 shows that even in torsion fatigue of unnotched specimen the fatigue limit is determined by the threshold condition of the cracks branching from axial cracks as in Fig. 9 . This means that the fatigue limit in torsional fatigue is essentially determined by non-propagating cracks of the same mode Mode I as in rotating bending or tension-compression. Figs. 9 show the state of artificial holes at the fatigue limit under reversed torsion. Since shear stress in torsion is equivalent to tension at 45 and compression at 45 non-propagating cracks are observed to emanate from the hole edge at 45 for d 500 im and 200 im. However there is no non-propagating crack at the hole for d 50 im. This is because the stress condition for this