A crystal orientation is one of important factors that affect the mechanical properties of single- and poly-crystalline metals in deformation processes. X-ray diffraction method and electron back scatter diffraction method are the typical ways to identify the crystal orientation of grains. Although these methods are possible to give accurate results, they need relatively expensive instruments and operation trainings. The present study aims to show the possibility to use an indentation shape made on a grain's surface for the identification of it's crystal orientation in polycrystalline titanium, which is hexagonal close-packed crystal structure. The strong correlation between the indention shape and the crystal orientation was found for the grain of pure titanium. The results also show the possibility to evaluate the direction of c-axis to a fairly good accuracy by introduced procedures.

View full abstract

In order to investigate the effect of frequency on the crack growth behavior, ultrasonic fatigue tests were carried out for an extruded age-hardened Al alloy, 7075-T6, in ambient air and in N_2 gas, and the results were compared with those in rotating bending fatigue. In ultrasonic fatigue, the growth direction of a crack changed macroscopically from a tensile mode to a shear one which the direction was about 35 degrees against to the specimen axis, and fracture mechanism also changed from striations to transgranular facets with voids microscopically. Crack growth rate at the transition of crack growth direction was about 3×10^<-9>m/cycle. These results were explained from the time dependent environmental effect and texture of microstructure.

View full abstract

Thermal fatigue tests were conducted for austenitic stainless steel, SUS310S for boiler. The higher the heating temperature the smaller the number of heat cycles for thermal fatigue crack initiation is. The higher the heating temperature the faster the crack propagation rate is. The thermal fatigue crack propagation rate of SUS310S steel was slower than that of SB410 steel. Striation was predominantly observed on fracture surfaces of SUS310S steel. The striation spacing per cycle, S obtained from the measured striation spacing &lrtri;S versus &lrtri;K curve was well coincident with the da/dN〜&lrtri;K curve in relatively higher &lrtri;K region at all heated temperature. It can be concluded that thermal fatigue crack of boiler steel SUS310S propagate in association with striation.

View full abstract

This study was conducted to examine the effect of a hybrid surface modification on the fatigue strength of Ti-6Al-4V alloy. The hybrid surface modification was composed by plasma carburizing and WPC treatment. In the case of the carburized alloy, cracks were initiated from the compound layer at relatively low stress amplitude level, so that there was no improvement in the fatigue strength. However, when the alloy was further WPC-treated, the brittle compound layer was eliminated and compressive residual stress was introduced. As a result, the initiation site of cracks was moved to the inside under the hardened case and the fatigue strength was significantly improved. The results showed that the hybrid surface modification method estimated in this study can improve the fatigue strength of the titanium alloy as well as the wear resistance.

View full abstract

In this paper, inelastic deformation behavior of A7075 Aluminum alloy under tensile loading was experimentally investigated at various temperatures and strain rates. The constitutive equations were developed in the range from 0℃ to 120℃ on the basis of the Two-Factors overstress constitutive concept considering aging as well as viscosity. These equations are very useful for analysis of dynamic deformation of structures like automobiles and airplanes, and also can express relaxation behavior.

View full abstract

[in Japanese]

View full abstract

This study compares distributions of strain with those of martensitic volume fraction V_α' around fatigue cracks in SUS304 stainless steel. The specimens having random pattern on their surfaces were fatigued at room temperature (297K) in air. The photographs around fatigue crack were taken by advanced CCD cameras during fatigue tests. Distributions of strain around fatigue cracks were calculated by DIC, or digital image correlation method and those of volume fraction V_α' of α' martensitic phase induced around fatigue cracks were measured with ferrite scope. The higher the strain becomes, the more the α' phase was induced by plasticity-induced transformation around fatigue cracks. Distributions of V_α' closely resemble those of strain around fatigue cracks. These results imply that martensitic phase was transformed according to maximum strain value induced by fatigue loading.

View full abstract

Creep rupture behavior of a unidirectional T800H/Epoxy composite under constant load conditions at different temperatures is examined. Tensile creep rupture tests are performed at different stress levels on plain coupon specimens with various fiber orientations θ=0, 10, 30, 45, 90°. Transient creep behavior is dominant for all the fiber orientations, and ultimate creep rupture takes place along reinforcing fibers regardless of fiber orientation and test temperature. A creep damage mechanics model is then developed for describing anisotropic creep rupture behavior at different temperatures. The proposed model can adequately describe the directional and temperature-dependent off-axis creep rupture behavior of the unidirectional composite. Another creep rupture model that is formulated using the Larson and Miller parameter also succeeds in predicting the off-axis creep rupture behavior.

View full abstract