抄録
In the history of the study on the fatigue behavior of metallic materials, the fracture mechanics approach on the fatigue crack has given us many important results required to the safety design of mechanical structures. Macroscopic crack propagation behavior has been examined from a view point of Paris' law which gives the relationship between stress intensity factor range ΔK and fatigue crack propagation rate da/dN. But fatigue crack initiation behavior and crack growth behavior at early stage remain unsolved due to the difficulty of the direct observation and influence of the material microstructures. In order to observe the crack initiation behavior, we developed a fatigue-testing machine combined with SEM. By using this machine, fatigue tests were carried out in pulsating tension for 70/30 brass and S 35 C carbon steel. Thus crack initiation behavior was carefully observed together with the microscopic deformation preceding the crack initiation. Based on the SEM observations, the authors have the following findings ; (1) in the case of 70/30 brass, fatigue cracks take place as the slip band crack and the pencil glide crack. (2) in the case of S 35 C carbon steel, fatigue cracks tend to occur along the grain boundaries and then they tend to grow into the crystalline grains. (3) ΔK-da/dN relationship indicates a distinct fluctuation in the small crack region which gives the low stress intensity factor range of ΔK＜1 MPa・m1/2.
