13th International Conference on Fracture June 16–21, 2013, Beijing, China -6- (b) Figure 4. Fracture surface (a) sample failed at 7.78x108 cycles (b) Crack initiation from the surface 3.3 Fatigue Crack Initiation It is well known that due to fatigue loading the cracks initiate from the localized plastic deformation [9]. For FCC materials, the local plasticity develops in the form of PSBs and twining and induces surface crack initiation. So far different conclusions have been made in the past for the surface based crack initiation phenomenon [8-11]. The changes in the microstructure, slip system, twin bands and surface defects have been attributed to the crack initiation in different materials. In this study, it was revealed that the mechanism of fatigue crack initiation at different fatigue loading cycles was different depending upon the stress levels. However, the initiation region was always found on the surface of the specimen. The surfaces of the fractured specimens, parallel to the loading direction, were investigated through the SEM and AFM. The specimens failed at higher stress levels up to 106 cycles showed smooth surface without having any slip markings. No damage or sign of local plastic deformation was observed. The crack initiation site was very localized region at the surface of the material with no nearby damage. This showed that the carbide particles at the grain boundaries, as found in earlier studies, acted as stress concentration regions for crack initiation [15]. However, the specimens failed beyond 106 cycles showed high density PSBs and deformations on the surface. Fig. 5 shows SEM image of the surface of specimen failed at 3x108 cycles. It can be seen that high density slip bands were all over the periphery of the specimen. The near fracture surface showed more number of PSBs. The majority of the dislocations were parallel to the loading directions which were wider and broader as compared to the others. However, some dislocations were observed transverse to the loading directions as well.
RkJQdWJsaXNoZXIy MjM0NDE=