Norimitsu Koga

Proposal for an engineering definition of a fatigue crack initiation unit for evaluating the fatigue limit on the basis of crystallographic analysis of pearlitic steel

In this study, in order to define a fatigue crack initiation unit, the relationship between the fatigue crack initiation process and the crystal structure in the pearlitic steel used for railroad rails was examined and fatigue tests, focusing on crack initiation, were performed. The fracture surfaces were analyzed using a scanning electron microscope (SEM) and electron backscatter diffraction (EBSD). The crystal structure of the pearlitic steel is composed of “pearlitic colonies” that have the same lamellar structure direction and “pearlitic blocks” that have the same ferrite crystal direction. The SEM and EBSD results revealed that the crack initiation depends on the pearlitic colonies. Therefore, we defined the characteristic dimension for fatigue crack initiation as the pearlitic colony size. However, for safety purposes, the pearlitic block size should be considered the engineering definition of the fatigue crack initiation unit, since decreasing the pearlitic block size should cause an improvement in the fatigue limit of pearlitic steel.

Mechanical Stability and Deformation-Induced Transformation of Retained Austenite in TRIP Steels at Low Temperatures

The tensile properties and the stability of retained austenite in TRIP steels with different volume fraction of retained austenite have been studied at low temperature. The steels showed a good valance of strength and ductility at 193 K. Their work-hardening rates were decreased linearly and kept a high value in the high strain regime at 193 K. The retained austenite was mostly transformed into martensite less than 10% strain at 193 K.

Visualization of Strain Distribution on Deformed Steels Using Digital Image Correlation Method

Digital image correlation (DIC) method is a convenient strain analysis method calculating strain from the difference of images between before and after deformation and shows an advantage to apply to any deformation mode or materials as long as significant contrast. We reviewed basic principles of DIC method and then demonstrated strain distribution in tensile deformed ferritic steel and cyclic deformed tempered martensitic steel. Strain distribution in tensile deformed ferritic steel becomes inhomogeneous with lowering temperature due to restriction of slip systems at low temperature. Strain distribution around a fatigue crack in cyclic deformed tempered martensitic steel was visualized by DIC analysis for replica film and strain concentrated on crack tip same as previous report in DIC analysis for specimen surface, which suggests that strain distribution obtained from replica film has an enough reliability. From these results, it can be concluded that DIC analysis is effective method to investigate local deformation and relation between local deformation and fracture behavior in metal materials.

Effects of Slip Ratio on Damage and Microcracks in Carburized SCM420 Steel under Rolling Contact Fatigue

Damage and microcracks formed by rolling contact fatigue (RCF) were characterized for carburized SCM420 steel. A large number of microcracks were detected beneath the contact surfaces after RCF. The microcrack generation and strain distribution beneath the contact trail depended on the slip ratios of 0 %, -20% and -40 % in the roller pitting test. Such severe slip increased shear strain in the region higher than 160 µm in depth from the contact surface. Compressive stress also gave rise to strain in the region near the surface up to 100 µm in the depth. Those strain gradients may cause a strain incompatibility at the transition layer in which a crack branching was detected.

Temperature dependence on tensile properties of Cu-40mass%Fe dual phase alloy

The binary system of iron and copper shows low mutual solubility and cast Cu-Fe forms an iron (bcc) and copper (fcc) dual phase structure at room temperature. In this study, tensile properties, deformation and fracture behaviour of a rolled Cu-40mass%Fe alloy have been evaluated in order to reveal the temperature dependence on tensile properties in dual phase structures. The material formed a layer structure with ultra-fine grains of 1 μm in diameter. In both iron and copper grains, furthermore, many precipitates of copper or iron were revealed. The strength of this material increased at low temperatures, though the elongation was hardly changed, which suggests that fcc + bcc dual phase structure is effective to improve the tensile property at low temperature. Strain was inhomogeneously distributed at low temperature regardless of Cu and Fe region, and voids and cracks tended to form inside Cu layer. These results imply that the temperature dependence on tensile properties and deformation behaviour of each phase in dual phase structure is different from that of each single phase structure, and dual phase structure materials have a potential for becoming superior cryogenic structural materials.

Effects of cross-rolling on deformation texture evolution in unalloyed titanium

Unalloyed titanium was rolled with 20% reduction in each pass at 293 K using a cross rolling mill, where the upper and lower rolling axes were skewed each other at an angle of 0, 5 or 10 degree with parallel position. Multi-pass flat-rolling was carried out without any lubricants up to the true strain of 1, where two kinds of rolling directions such as tandem (uni-direction for all passes) and reverse (opposite direction in every passes) were adopted. The strain of specimens was increased proportionally as higher passes regardless of the rolling conditions. The transverse direction (TD) split deformation texture in titanium was generally developed under the cross angle of 0 degree. In the present strips of tandem, a main orientation was identified as (-12-18)[10-10]. In the case of tandem with the cross angle of 5 degree, a fiber texture was developed along (-12-18). That is the reason why a rotation in the rolling direction (RD) was overlapped. In the case of reverse with the cross angle of 5 degree, the main orientation was separated into [10-10] and [2-311] that were corresponded to TD and RD splits, respectively.