The high strain rate (-104/s) mechanical properties of a 152μm grain size OFHC copper were determined from the analysis of ballistic test specimens subjected to the Taylor Test. Deformation was analyzed by two independent techniques, one an analytical method and the second a finite element numerical model. The results from these complementary analyses were combined to construct a high strain rate dynamic stress-strain curve for this OFHC copper. This dynamic stress-strain curve is then compared with conventional quasi-static mechanical test data. When deformed at these high strain rates, the OFHC copper exhibits an increase in the yield strength, the strain-hardening rate and the ultimate dynamic strength relative to their conventional quasi-static values. The OFHC copper results are then compared with high strain rate measurements for high purity 1100 aluminum. This reveals distinct differences of the high strain rate deformation of these two metals. It is suggested that the differences may relate to the ability of the OFHC copper to deform by twinning, whereas the aluminum does not exhibit a twinning mode of deformation.
Peculiar deformation accompanied by Lüders band having 46% Lüders strain was observed in the  oriented Fe-30%Cr alloy single crystal. Specimen was again deformed after aging at 473K for 1.8ks at the 23.5% strain. The first Lüders band can not move due to locking by aging and second Lüders band started from the opposite side of the chuck. The specimen having  tensile axis is thought to begin to slip ideally by four slip directions. In the present case, this specimen deformed by slip of  and  directions. Thus, the slip system of Lüders band could be determined. From the surface observation of the side surface of the specimen, the propagation speed, width and the strain-rate in the Lüders band were determined. The mobile dislocation density in the Lüders band was also estimated to be about 2×108cm/cm3. The effective and internal stresses were determined during the straining in order to analyze the mechanism of Lüders deformation.
To analyze the rate-controlling mechanism of Lüders deformation found in the  oriented Fe-30%Cr alloy single crystals, the activation volume and the activation enthalpy were obtained by a strain-rate and temperature change method during the deformation from 293K to 511K and were found to be 94-176b3 and 0.8-1.4eV, respectively. Aging at 473K was made after extension by 23.5%. The Lüders band didn't move on further straining and the second Lüders band started from opposite side of the chuck. It was considered that the Lüders front was aged and locked by some solute atoms. The line intensity of carbon atom fairly increased around the Lüders front after aging. The interaction of moving dislocations and carbon atom is considered to be the most likely rate-controlling mechanism for the deformation. The activation distance was calculated to be 4.1-6.2 times larger than carbon diameter and would be reasonable value considering the moving dislocations overcome carbon atoms.
Eutectoid steel is one of the most important industrial materials. In addition, these materials are usually pre-strained before use during production or machining process. In this study, fatigue test has been performed to research the effect of pre-strain on fatigue properties of eutectoid steel with different plastic pre-strain ratio at εp=0, 2%, 4% and 6%, respectively. At the same time, microscopic behavior of the crack initiation and propagation are also investigated by the successively taken replica method during the pre-strain and fatigue test. The results of the research indicate pre-strain directly affect the fatigue strength and crack initiation and propagation behavior. All fatigue limits of specimens with different pre-strain ratio are lower than that of specimens without pre-strain (εp=0) and the fatigue limits of pre-strained specimens have no obvious change from εp=2% to 6%. Fatigue cracks initiate from slip lines or micro-cracks that are generated in the process of plastic pre-strain. However, for non-pre-strained one, fatigue cracks initiate from boundary or inter-lamellar of the pearlite. The mechanism for this difference is discussed in this paper.
Low-cycle fatigue behavior of a newly developed cold-work tool steel containing 0.8C-8Cr-2Mo was investigated. The aim of this study is to examine the fatigue behavior in connection with its microstructure and hardness of the specimens tempered at several temperatures. The specimens were tempered at 473K, 793K and 823K, and then were tested under fully reversed stress amplitudes of 1568MPa and 1176MPa. The reasons of choosing three kinds of tempering temperatures were follows. Firstly, the secondary hardening was remarkable at 793K and the hardness at this was 62HRC while the hardness of specimens tempered at 473K and 823K was almost same of 60HRC. Secondly, amount of retained austenite (γR) were 14vol% and 2vol% for specimens tempered at 473K and at 823K, respectively. At the higher stress amplitude of 1568MPa, the fatigue lives of the specimens tempered at 793K and 823K were almost same, while the life of the specimen tempered at 473K was the shortest. In this case, the fatigue lives were attributed to the time till carbide cracking. Thus, the crack initiation process was influenced by the strength of steels. On the other hand, at the medium stress amplitude of 1176MPa at which the fatigue life was 104-105 cycles corresponding the actual tool lives, fatigue lives of the specimens tempered at 473K were the longest, then with increasing tempering temperature from 793K to 823K, the fatigue lives decreased. Fisheyes were observed on the fracture surface of the specimens tempered at 473K and 793K. The fatigue lives in this fracture morphology were attributed to the crack propagation process until it reached to KfC. Additionally, it was found that γR contributes the increase of the resistance for crack propagation.
The effects of prestraining on high-cycle fatigue strength of newly developed low alloy TRIP-aided steels with different matrix structures and different retained austenite characteristics were investigated for automotive applications. Prestraining to 10% in tension increased the fatigue limit of the TRIP-aided steels, especially in steel with a polygonal ferrite matrix. It was considered that the polygonal ferrite matrix brought on high a fatigue limit mainly due to TRIP of the retained austenite and high compressive internal stress in the matrix resulting from a hard second phase on prestraining. On the other hand, the steel with bainitic ferrite lath matrix exhibited only a small increase in the fatigue limit after prestraining. This was expected to be mainly due to “strain-induced martensite hardening” on prestraining, with small contributions of TRIP of compressive internal stress in the matrix. In addition, a very interesting finding was obtained that the internal stress is the most effective parameter among some parameters to increase the fatigue strength in low alloy TRIP-aided steels.
Epoxy resin systems with different amounts of hardener were prepared. Bisphenol A type epoxy resin prepolymer was used and the hardener was m-xylylenediamine. Specimens were immersed in water, 10wt% sulfuric acid solution and 10wt% sodium hydroxide solution at 80°C. The epoxy resins showed high corrosion resistance to water and sodium hydroxide solution regardless of the amount of hardener. On the other hand, color changed layers were observed on the both surfaces of specimen uniformly in sulfuric acid solution. Sulfuric acid did not penetrate into color unchanged layer. From the results of weight change in sulfuric acid solution, it was suggested that the larger the amount of hardener, the faster was the penetration of sulfuric acid solution. Especially for specimen cured with the largest amount hardener, cracks and defects were observed. The flexural strength, which was measured immediately after taking from sulfuric acid solution, decreased as immersion time increased. The flexural strength recovered by drying. However, the specimen with the largest amount of hardener indicated irreversible degradation due to formation of cracks. The results suggested that excess hardener promote the penetration of sulfuric acid solution. The amount of hardener must be determined carefully in order to avoid corrosion.
2D-C/C composite is one of the promising materials as a next-generation core material in gas-cooled reactors. Effect of air-oxidation on the thermal diffusivity of the 2D-C/C composite was investigated in this study. Tested composite consists of 6K plain-woven fabrics with PAN-based carbon fiber and graphite matrix. Final heat-treatment of around 3073K was applied to the composite. The C/C composite specimens for measurement of thermal diffusivity were oxidized from 1 to 11% weight loss in air at 823K. Oxidation loss of the composite preferentially occurred at matrix part near the fiber bundles, and then occurred at fiber bundles. This composite exhibited large anisotropy in thermal diffusivity, higher value for parallel to lamina direction and lower value for perpendicular, e. g. thermal diffusivity of 1.1cm2/s for parallel to lamina and 0.2cm2/s for perpendicular at room temperature. Thermal diffusivity at room temperature declined 10-20% for parallel to lamina direction and 5-9% for that of perpendicular within 11% weight loss by oxidation. Thermal diffusivity tended to decrease gradually as the increase of oxidation loss in parallel to lamina, however, it decreased in the beginning of oxidation pretty much and not so changed by further oxidation loss in perpendicular to lamina. The different behavior due to air-oxidation on the thermal diffusivity in two directions was discussed from the fiber and/or matrix texture changes due to air-oxidation. Change in thermal conductivity under oxidation condition was also estimated from the obtained thermal diffusivity.