Kenta Yamagiwa

Measurement of Three Dimensional Geometry of Creep Void and Grain Boundary With Combining 3D-EBSD Method and SEM Images

A-parameter, void area ratio and other methods about creep void are used to estimate creep damage resulting from creep voids. However, these methods are based on not three-dimensional but two-dimensional geometry, though creep voids are three-dimensional cavities. By combining the 3D-EBSD method with SEM images, we have observed the three-dimensional shape of creep voids and their geometrical relationship with grain boundaries at first. The method is applied to 1Cr-1Mo-0.25V turbine rotor steel subjected to a creep rupture test (580°C, 180MPa). Also, interrupted creep specimens are prepared to observe the progress of void growth. Forty sections with 0.5 μm interval and 100μm × 100μm area are measured by mechanical polishing in order to reconstruct the three-dimensional shapes. In the results, four types of creep void are observed. One is sphere type whose radius is approximately 1μm. It is observed in the specimen whose creep life fraction is 25%. In the specimens with 50% and 75% creep damage, prolate and oblate spheroid whose radius is approximately 2.5μm are observed. Finally, connected voids are located within ruptured specimen. As the creep damage is progressed, not only void growth but also void nucleation is observed. Especially, on prior austenite grain boundary which is three-dimensionally perpendicular to the stress direction, creep voids are nucleated and grow in a concentrated manner. However, such nucleated small voids do not affect the void volume fraction.Copyright

Estimation of Stress Ratio From Striation’s Three-Dimensional Geometry

Striation’s width is useful to estimate stress intensity factor range. To estimate maximum and minimum stress intensity factor, the relationship between stress ratio and aspect ratio of striation obtained by Furukawa, is used. However, accuracy of that method depends on the observer’s skill. In this paper, the parameter (P.S.sum), which relates to the striation height, is proposed and method for estimation of stress ratio, which is independent of observer’s skill, is also proposed. In order to calculate P.S.sum, the frequency analysis is applied to the three-dimensional geometry of striation obtained by laser microscope. The wavelength and the power spectrum are considered to indicate the striation width and height respectively. To consider the variation of striation, P.S.sum is calculated by the integration of power spectrum. The method is applied to the fatigue fracture surface of aluminum (A2024), steel (0.25% carbon) and high-tension steels. The condition of fatigue test was ΔK constant to keep the crack growth rate constant, in other words, to make the striations of which width were same. Applying the method, almost linear relationship between stress ratio and P.S.sum was obtained. Therefore, stress ratio can be quantitatively estimated by P.S.sum.Copyright