Fatigue tests were performed on 90° V-shape bending-worked aluminum alloy specimens with various radii of curvature under deflection-controlled fully-reversed out-of-plane bending to clarify the effect of bending-work on fatigue strength of plates. Fatigue strength of the bent specimens was entirely lower than that of smooth flat one and decreased with reduction in radius of curvature. In order to investigate the cause of the reduction in fatigue strength of bent specimens, stress state in the vicinity of the bending-worked portion was analyzed employing a finite element method. Besides, crack initiation and growth behavior was observed and compared with those of smooth flat specimen. The crack initiation tends to be earlier with decreasing the radius of curvature because of the higher stress concentration around the center of inner surface of the bent portion. In addition, the crack growth rate in bent specimens was higher than that in flat specimen, especially in an early stage of short crack growth regime and increased with decreasing the radius of curvature. Since cyclic bending stress decreased with crack growth and accumulation of residual strains, the crack growth rate gradually lowered in latter stage of the life. Consequently, the reduction of fatigue strength in bending worked plate results from the earlier crack initiation and the acceleration of crack growth rate in early stage due to the stress concentration around the bent portion.
Effect of strength level on the coaxing behavior of high strength steels was studied. The materials used were high strength steels, SCM435, SNCM439 and SUJ2. Stress incremental tests were performed using cantilever-type rotary bending fatigue testing machines. Fatigue limits obtained by the conventional fatigue tests were 525MPa and 575MPa for SCM435 and SNCM439, respectively. On the other hand, since a step-wise S-N curve was seen in SUJ2, stress-incremental tests were started from a stress level just below the transition stress, 1275MPa, where failure mode was changed from surface to subsurface. It was found that SCM435 and SNCM439 indicated a marked coaxing effect and non-propagating cracks were not seen. In SUJ2, a noticeable coaxing effect was hardly appeared but subsurface fracture was found in failed specimens. Based on hardness and static strain aging ability measurements, it was suggested that the coaxing effect in SCM435 and SNCM439 was attributed to strain-aging, but not work hardening. The stress-incremental tests results were analyzed by Miner's rule. In SCM435 and SNCM439, the cumulative damage values in all stress-incremental test results are more than unity and increase linearly with increasing the number of cycles in a step. In SUJ2, the cumulative damage values are independent of the number of cycles in a step and are small compared to those of SCM435 and SNCM439.
In recent years, fatigue limit estimation based on dissipated energy is being introduced in various industries because of its time and cost effectiveness. However, the energy dissipation mechanism and the relationship between energy dissipation and fatigue damage have not been well investigated. In the 1st report, the present authors conducted fatigue limit estimation based on dissipated energy measurement and conventional fatigue test for JIS type 304 and 316L austenitic stainless steel. We found that the fatigue limit of the type 316L steel could be precisely estimated, although estimated fatigue limit of the type 304 steel was conservative values compared with that obtained by conventional fatigue tests. It was considered that the fatigue limit estimation method based on dissipated energy could not evaluate the microstructure change, such as dislocation and phase transformation during short cyclic loading. In this study, fatigue tests under the constant stress amplitude were conducted for austenitic stainless steel to investigate the relationship between dissipated energy and microstructure change during fatigue tests. It was found for the type 316L steel that tendency of the observed dissipated energy during the fatigue tests was similar as that of plastic strain energy. On the other hand, for the type 304 steel, the dissipated energy decreased in spite of increasing plastic strain energy around N = 105cycles, when the applied stress amplitude was close to the actual fatigue limit. It is considered from the observation of the change in martensitic volume fraction, the decrease of dissipated energy was caused by the martensitic transformation ; some amount of irreversible plastic strain energy was consumed for phase transformation. It was found from the experimental studies that dissipated energy reflects the microstructure changes, such as material softening, hardening and martensitic phase transformation. Therefore, the condition of the number of cycles in the fatigue limit estimation based on the dissipated energy should be changed depending on the behavior of microstructure change in material under cyclic loading.
Titanium alloy is one of the typical aerospace structural materials because of its lightness, high specific strength, and high corrosion resistance. However, it has extremely low thermal conductivity, too. This causes relatively high temperature rise due to the heat converted from plastic work, especially under impact loading. In this study, a series of compression tests for β-titanium alloy in the wide ranges of testing temperature (77K∼673K) and strain rate (3.3 × 10-4∼103s-1) was carried out to investigate the effect of the heat generation on the mechanical properties. The temperature rise of the specimen during impact compression tests was measured by an infrared radiation thermometer with a photoconductive element. The conversion ratio of plastic work into heat βh was also examined at temperatures of 201K∼473K. The principal results obtained are as follows: The temperature rise of specimen during plastic deformation at 201K is greater than that observed at room temperature (298K) and at 473K. The conversion ratio βh is not necessarily to be constant, i.e. βh is less than 0.1 in the early plastic deformation and gradually increases up to around 0.7 with the increase of plastic deformation at 298K. The increase of βh at lower temperature is more drastic.
This paper presents time-temperature parameter (TTP) methods for predicting the slow crack growth (SCG)-type creep rupture life in ceramics. First, a probabilistic delayed-fracture (PDF) model was proposed on the basis of the Arrhenius-type SCG concept in conjunction with the two-parameter Weibull distribution. Larson-Miller parameter (LMP) and Orr-Sherby-Dorn parameter (OSDP) were then derived from the PDF model, respectively. As a result, it was theoretically demonstrated that LMP could not be then related with the SCG concept , but OSDP could be related with that. Therefore, OSDP has more clear physical meaning than LMP for ceramics. Second, the material parameters associated with the PDF model were determined using the experimental creep data for silicon nitride. The validity of the model was confirmed through the comparison of LMP plots and OSDP plots using the experimental creep data and the material parameters. In addition, it was found that SCG mechanism changes at a transition temperature where the crack propagation index drastically drops.
Rice husk contains 13∼29mass% inorganic constituents. The main component is silica which constitutes 87∼97mass% of them. A lot of rice husk ash is discharged as a by-product of thermal power generation. The ash contains a high proportion of SiO2 (94.3mass%) with a small percent of carbon. The SiO2 fired at a low temperature is amorphous, water-soluble and high-reactive. The ash in the present study is produced by combustion at a low temperature around 800°C in thermal power plant. The ash is not effectively utilized for recycling and disposed as soil. By the way, zeolite is attracting attention as an environment improvement material, and high uniformity synthetic zeolite are in demand. For the effective use of the ash, in the present study, bulky consolidated zeolite A has been synthesized using the ash. Bulky consolidated zeolite A was synthesized with the ash and sodium aluminate. The consolidated zeolite A obtained at a heat-treatment temperature of 70°C. The microstructure of consolidated zeolite A was controlled by foam formation technique, because zeolite A has several functions depending on pore size in the consolidated body. In addition to intrinsic pore in the consolidated zeolite body, pores with diameter from tens to hundreds (m were uniformly formed in the zeolite body by adding Si and surface acting agent during preparation of the precursor. The consolidated zeolite A exhibited good humidity conditioning characteristics.
The tribological properties of the new developed Si-particle dispersed Al sintering materials were investigated. The materials impregnated lubricant oil, semi-solid state Vaseline with solid lubricant and solid state Vaseline were experimented by using two disc end sliding contact testing machine. (1) The impregnated semi-solid state Vaseline with MoS2 type solid lubricant has the effects on improvement of friction, wear and scuffing limit compared with the impregnated lubricant oil. The scuffing limit (PV value, P : contact pressure, V : sliding velocity) was improved about 45%. (2) The impregnated solid state Vaseline with MoS2 is effective in improvement of the tribological properties for sliding bearing materials. The scuffing limit was improved about 52% compared with the impregnated material of lubricant oil.
It is important to clarify the variation of particle contact angle of fresh concrete with shear deformation for understanding its deformation resistance and mechanical behaviors. In this study, using X-ray CT imaging, the authors investigated mean particle contact angles (θ) of fresh mortar before and after sheared in a ring shear device, which was composed of alumina powder, sand, and water, etc. As a result, the mean particle contact angle increases with increasing the shear deformation, but when reaching a peak, it decreases with the shear deformation. The increase and the decrease of θ are respectively the characteristics before and in shear failure state. Also, the greater the vertical stress on the shear plane, the smaller the increase or decrease of θ with the shear deformation.
A variety of methods have been developed and applied to ground improvements, depending on the purpose. For example, surface ground improvement or columnar improvement methods are generally frequently applied to improvements of soft ground. The chemical injection method is commonly selected for the prevention of the collapse of natural ground. Furthermore, in case of ground improvement to solve the problem of liquefaction, forced precipitation or subgrade reinforcement under an existing building, especially when the cavity under the building should be filled in, the injection of urethane (organic), one of the chemical injection methods, is considered. With the injection methods, filling materials can be categorized into mainly two types : chemicals and non-chemicals, and selection should be appropriately made according to the condition of the site. In this study we focused on water-swelling urethanes with a view to applying them to ground improvement, and performed a test for evaluation of their basic characteristics by creating simulated mixing hardening materials of various combinations of water-swelling urethanes and soil materials. As a result, it was discovered that water-swelling urethanes mixed with various soil materials showed a tendency to have a constant swelling rate for both fresh water and artificial sea-water. Also it was discovered that the water-swelling urethanes with larger water contents and smaller particle sizes tended to have a quicker increase of viscosity, and that the strength of hardening materials was related to the particle diameters of soil materials. In this way, the basic characteristics of water-swelling urethanes can be understood in relation with soil materials. These characteristics need to be known before the injection of such filling materials. Therefore, there is a possibility that the water-swelling urethanes are highly applicable to ground improvements.