The characteristics of mechanical behavior are investigated for Ti-6Al-4V alloy. Four kinds of specimens are prepared under different heat treatments in order to produce different microstructures. In the present investigations, impact, tensile and fatigue crack growth tests are performed for each test specimen. The results obtained through the investigations are compared. Additionally, fractal dimensions of crack path are obtained using the box counting method. The results are: 1) the microstructures show as equiaxed, bimodal and lamellar microstructures respectively, 2) the impact energy and elongation are superior for the bimodal microstructure, and the hardness and tensile strength are superior for the lamellar microstructure, 3) the fatigue crack growth rate is similar for all microstructures in the low ΔK region while that of equiaxed microstructure is the largest, and that of lamellar microstructure is the lowest in the high ΔK region respectively, 4) the fractal dimension, D of lamellar microstructure shows higher value than that of the equiaxed and bimodal microstructures under 200 magnification view of the SEM micrographs.
The aged degradation of material is observed when heat-resisting steel is exposed for long periods of time at high temperatures. In the present study, the degraded 1Cr-0.5Mo steel that is used for long periods of time at high temperature(about 515°C) and artificially reheat-treated materials are prepared. These materials were used to study the effect of aged degradation on fretting fatigue behavior. Through this experiment, it is found that the fretting fatigue strength of reheat-treated 1Cr-0.5Mo steel is approximately 46 percent lower than that of the plain fatigue strength of the
same material. Furthermore, the fretting fatigue strength of degraded 1Cr-0.5Mo steel was less than 53 percent of the same material™s plain fatigue strength. The maximum value of fatigue strength difference is observed as 57 percent between the fretting fatigue of degraded material and plain fatigue of reheat-treated material. These results can be used as basic data in a structural integrity evaluation of heat-resisting steel considering aged degradation effects.
Fretting is a potential degradation mechanism of structural components and equipments exposed to various environments and loading conditions. It is well known that the fatigue life under fretting condition decreases approximately 50-70% compared with that under non-fretting fatigue condition. The specific gravity of titanium alloy is 4.5 which is lighter than steel, however, its specific strength, heat and corrosion resistance are superior to steel. Ti-6Al-4V alloy is a kind of a+b phase titanium alloy, and mechanical properties are changed by alloy elements, shapes and distributions of microstructures. In this study, three different kinds of specimens are prepared under different heat treatments in order to produce different microstructures. Through various kinds of mechanical tests, the following conclusions are observed: 1) The microstructures are observed as equiaxed, bimodal and lamellar microstructures respectively. 2) The elongation percentage is superior for the equiaxed microstructure, and the hardness and tensile strength are superior for the lamellar microstructure. 3) The plain fatigue limit of lamellar structure shows higher value than that of the equiaxed and bimodal structures. 4) The fretting fatigue limit considerably decreases compared with the plain fatigue limit
for all materials. 5) The fretting damage of contact surface increases with an increase of cyclic loading amplitude under the constant contact pressure.
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