Issue 35

S. Xinhong et alii, Frattura ed Integrità Strutturale, 35 (2016) 441-448; DOI: 10.3221/IGF-ESIS.35.50 444 D ISCUSSION AND ANALYSIS Macroscopic fracture analysis he fractures of tension-torsion fatigue with mean stress are shown in Fig. 4 - Fig. 7. Meanwhile the measurement of crack initiation and early propagation plane orientation, and analysis of stress element are conducted. The macroscopic fracture and analysis of stress element is representative of each level of mean stress. The fatigue crack origin, propagation and final rupture region can be identified in the fracture appearance. As the stage of crack initiation and early propagation contributes to most of the fatigue life, only the effect of mean stress on crack plane orientation is investigated on the crack initiation region. The fracture surface, measurement of crack initiation and early propagation plane orientation, and stress element under tension-torsion loading for ,  x m =0MPa are shown in Fig.4. The regions of crack initiation, propagation and final rupture can be identified from the fracture surface. The crack surface in the initiation region is suffered repeated normal stress and shear stress, and then it shows gray and black caused by the friction between the surfaces. In the propagation region the fracture surface is bright. In the final rupture region there is a large area occupied by a rough surface. From the analysis of stress element, there are two maximum shear stress amplitude planes separated by 90 ° and the values of shear stress amplitude and normal stress amplitude on both planes are equal, respectively. Meanwhile the normal mean stresses on both planes are zero. One of the two planes is nearly parallel to the direction of specimen axial, but the other is nearly 80° apart with the direction of specimen axial. While the crack initiation and early propagation plane orientation is close to the front plane of maximum shear stress amplitude. The fracture surface, measurement of crack initiation and early propagation plane orientation, and stress element under tension-torsion loading for ,  x m =50MPa are shown in Fig.5. The regions of crack initiation, propagation and final rupture can be identified from the fracture surface, too. On the whole the entire fracture area has been reduce than the area for ,  x m =0MPa. It is still gray and black in crack initiation region, and bright in propagation region. The values of shear stress amplitude and normal stress amplitude on both maximum shear stress amplitude planes are equal, respectively. But the values of normal mean stress on them are not equal, and are both smaller than the values of normal stress amplitude. One of the two planes with larger normal mean stress has a larger maximum normal stress; the other plane with smaller normal mean stress has a smaller maximum normal stress. However, the crack initiation and early propagation plane orientation is close to the back plane. The fracture surface, measurement of crack initiation and early propagation plane orientation, and stress element under tension-torsion loading for ,  x m =100MPa are shown in Fig.6. On the whole the entire fracture area has been reduce than the area for ,  x m =50MPa. In crack initiation region the gray and black is significantly reduced, and the fracture surface in propagation region is flat. It can be noted that the characteristics of stresses on both maximum shear stress amplitude planes are nearly the same as ,  x m =50MPa, except that the values of normal mean stress on both planes have been increased. The value of normal mean stress is less than that of normal stress amplitude on the plane, which is nearly 80 ° apart with the direction of specimen axial; however it is approximately equal to the value of normal stress amplitude on the other plane. The crack initiation and early propagation plane orientation is different from the orientation for ,  x m =50MPa, but is closer to the maximum shear stress amplitude plane with larger normal stress. The fracture surface, measurement of crack initiation and early propagation plane orientation, and stress element under tension-torsion loading for ,  x m =150MPa are shown in Fig.7. There is a small amount of gray and black in crack initiation region, it becomes more flat in propagation region and the area of final rupture is further reduced. With the increasing of axial mean stress, the normal mean stress on both maximum shear stress amplitude planes is increasing. The value of normal mean stress is much smaller than that of normal stress amplitude on the maximum shear stress amplitude plane, which direction is nearly 80 ° apart with the direction of specimen axial. Meanwhile it is much greater than the value of normal stress amplitude on the other plane. Observation shows that crack initiation and early propagation plane orientation is closer to the maximum shear stress amplitude plane with larger normal stress. T