To predict fatigue life under variable loading accurately, it is important to understand the damage factors determining fatigue life. In this study, the damage factors of high-cycle fatigue were investigated for Type 316 stainless steel and a procedure for predicting the fatigue life of two-step tests was discussed. Fully-reversed axial fatigue tests were performed in ambient air at room temperature. In order to evaluate applicability of the linear damage accumulation rule, two-step tests were performed. The crack initiation and propagation were investigated by replica observations. It was found that the linear damage accumulation rule did not always evaluate the fatigue life conservatively due to stress dependence of crack size against normalized fatigue life. Furthermore, the fatigue limit decreased in the two-step test. In order to evaluate the role of cracking on the fatigue life, the surface layer of specimens was removed after the first step of the two-step tests. The fatigue lives of the surface-removed specimens were almost equivalent to that of undamaged specimens, although the fatigue limit was less than that of the undamaged specimens. It was revealed that the fatigue limit of surface-removed specimens decreased due to cyclic softening caused by the cyclic loading in the first step. The fatigue lives correlated with the strain amplitude rather than the stress amplitude and fatigue limit was almost the same for the surface-removed, undamaged, and undamaged specimens subjected to constant strain tests. It was concluded that the fatigue life of Type 316 stainless steel should be predicted based on the strain amplitude even for the high-cycle regime. Accuracy of the fatigue life prediction could be improved by considering the stress dependency of the crack size in addition to the cyclic softening and hardening material behavior.
The purpose of the present study is to evaluate the effect of σ-phase embrittlement on fatigue behavior in high-chromium ferritic stainless steels. When ferritic stainless steels are exposed to the temperature range of 700∼ 800°C, the precipitation of σ-phase occurs. The precipitation brings about the embrittlement of materials which is known as the σ-phase embrittlement. Vickers hardness of a high-chromium ferritic stainless steel, type 447 with a chromium content of 30.7% increased significantly by the aging at 750°C due to the precipitation of hard σ-phases which are Cr- and Mo-rich inter-metallic compounds. On the other hand, in type 444 whose chromium content was 18.7%, the precipitation of σ-phase and the increase of hardness were not recognized by the aging at 750°C. Axial fatigue tests were performed using type 447 aged at 750°C for two different aging periods of 150h and 300h, and the effect of σ-phase embrittlement on the fatigue behavior was discussed. The aged type 447 exhibited much lower fatigue strengths than the un-aged one, where fatigue strengths decreased with increasing aging time. The brittle fracture of σ-phase and softening of ferritic phase reduced both crack initiation and growth resistances of the aged specimens, resulting in the lower fatigue strengths.
Ultrasonic fatigue tests were conducted for medium strength steel, 2.25Cr-1Mo, under constant amplitude loading and repeated two-step variable amplitude loading conditions to evaluate the effect of loading conditions on fracture behavior. Most of the specimens were broken from an inclusion on the specimen surface. Cumulative damage value, D, calculated by Miner's rule, which ignored the damage by cyclic loading below fatigue limit, exhibited inappropriate prediction of fatigue life. The D values calculated by modified Miner's rule remarkably varied by the conditions of the repeated two-step variable amplitude loading. In some cases, specimen was not broken until D became larger than 60. Observations at fracture origin using replication technique revealed that stress amplitude below fatigue limit increased the resistance of fatigue crack initiation and/or growth. Mechanisms of these behaviors were discussed in detail.
In order to investigate the growth mechanism of a shear mode fatigue crack in an extruded bar of an age-hardened Al alloy 7075-T6, rotating bending fatigue tests were carried out using plain specimens of the alloy in environments of controlled relative humidity of 25%, 50%, 75% and 85%, distilled water and nitrogen gas. Fatigue strength was decreased by high humidity. The growth mechanism of a fatigue crack was different depending on environment and stress level. Although most of fractures occurred by the growth of a tensile mode crack, a crack propagated in a shear mode accompanying with slip planes and voids at high stress levels in high humidity and at all stress levels tested in nitrogen gas. The shear mode crack was related to the marked texture of the alloy. Growth rates of the shear mode crack were higher in high humidity than in nitrogen gas. Growth mechanisms of the shear mode crack were different between environments in nitrogen gas and in high humidity. That is, the reason for the growth of a shear mode crack in nitrogen gas was the suppression to growth of a tensile mode crack by reversible slip due to absence of oxide film. On the other hand, in high humidity, the growth of a shear mode crack was promoted by the formation and coalescence of voids, suggesting that the acceleration of the growth rate of a crack may be assisted by hydrogen accumulated around precipitated particles on glide planes.
The effect of heat treatment of S25C on the fatigue crack propagation rate enhanced by hydrogen was investigated under long term varying loading. The Vickers hardness was changed by heat treatment between 155 and 255. Stress hold at maximum stress was inserted in the 0.1Hz sinusoidal stress wave. The acceleration of crack propagation rate occurred irrespective of hold time in every material. However, the mean value of crack propagation rate decreased when hold time increased. The instantaneous crack propagation rate in time domain was dependent on the passage of time after the start of stress hold. The crack propagation rate was high at the beginning for several tens of second and it was reduced down to less than one hundredth after ten minutes. The time-dependent crack propagation was not maintained continuously in all materials whose Vickers hardness was between 155 and 255. As a consequence, the possibility of damage suffered by time-dependent crack growth is negligibly small even in the hardest material that can be reached by heat treatment on S25C.
In recent years, polymer-modified cement based materials composites (PMC) have been used for siding, panel, etc. produced by extrusion molding. The purpose of this study is to evaluate plasticity of the PMC using a water-soluble polymer, namely PEG (polyethylene glycol). The PMC were prepared with various water-powder ratios, polymer-cement ratios, replacing ratios of Mg-rich silicate mineral to silica, and conducted by compression test and penetration resistance test. After the compression test, the shape retention and the fluidity in the plasticity of the PMC were discussed by apparent elastic modulus obtained from the initial slope of stress-strain curve , and by work calculated from area under load-displacement curve, respectively. The results obtained are as follows : (1) The apparent elastic modulus of the PMC was increased with decreasing water-powder ratio, and in particular, provided a noticeable increase with increasing the polymer-cement ratio. (2) The work of the PMC was increased with raising the water-powder ratio, and decreased with an increase in the polymer-cement ratio. (3) The apparent elastic modulus and the work of the PMC were increased with an increase in replacing ratio of Mg-rich silicate mineral to silica. (4) There was a correlation between the apparent elastic modulus and the work of the PMC. (5) The plasticity of the PMC was improved with increasing the polymer-cement ratio.(6)The penetration resistance of the PMC was highly correlated with the apparent elastic modulus tested to the compression. (7) From these results, it was recognized that the plasticity of the PMC produced extrusion molding was able to evaluate by compression test.
The Strength properties, such as bending, shearing, and partial compression, were examined for full-size specimens (120 by 240 by 4000mm) of 10-ply laminae of the same thickness (24mm) of sugi (Cryptomeria japonica) and hinoki (Chamaecyparis obtusa) and 6-ply and 5-ply of hinoki laminae of the same thickness at the outer layer and different thicknesses of sugi elements (36 and 72mm). For each piece of laminated lumber, two laminae of hinoki were used on both sides of the outer layer. All strength properties in all types of laminated lumber showed higher values than those of solid sugi lumber. In addition, the coefficients of variation in the modulus of elasticity and modulus of rupture were lower in the laminated lumber than in the solid sugi lumber. On the other hand, no significant differences in strength properties, except for the modulus of shearing, were found among the three types of laminated lumber, suggesting that there was no decrease in strength properties in laminated lumbers made of thick elements instead of laminae. These results indicated that the strength properties of laminated lumbers composed of laminae and elements of different thickness without finger joints were almost the same as those of laminated lumbers composed of laminae of the same thickness.
The authors conducted tests of RC slab specimens with UFC panels by using two types of different thickness under static loads. For two different types of thickness, the total thickness of the RC slab specimens were 110mm and 130mm had been prepared with 20mm of UFC panels. Moreover, the authors proposed the mechanical model for punching shear and load-carrying capacity equation based on failure conditions. The failure conditions of the slabs were that the RC slabs failed in punching shear, and the composite surface failed in delamination of on the UFC panel was subjected to single shear. Furthermore, the authors proposed the mechanical models for the punching shear of UFC panel using in the RC slab as following two cases : the composite surface of the UFC panel failed in shear, and the UFC panel failed in flexure as the panel was subjected to flexural tensile forces. The theoretical punching shear load-carrying capacity of UFC panel using in the RC slab was determined from the smaller one of the absolute values calculated in the two cases.
The purpose of this study is to grasp of controlling effect of alkali-silica reaction (ASR) by impregnation using hydrogen peroxide 5% aqueous solution (H5), hydrogen peroxide 5% with aluminium dihydrogenphosphate 3% aqueous solution (H5AP3) and lithium nitrite aqueous solution(L) as to impregnant. In this study, the expansion and compressive strength of impregnated and non-impregnated mortars were discussed. The mortars used for impregnation were subjected to various pre-curing conditions. The reactive aggregates used in the mortars were trachyte, glass cullet and andesite. The results obtained are as follows : (1) The reduction of ASR-expansion in the H5-impregnated mortars was influenced by total alkali content, pre-curing condition, impregnant loading and moist curing period of the mortar. (2) The expansion of the impregnant (H5, H5AP3 and L)-impregnated mortars was remarkably smaller than that of the non-impregnated mortar at moist curing eriod from 7 days to 3 years. Especially, the expansion of the H5AP3-impregnated mortar was decreased, as compared with L. (3) The compressive strength of the impregnant (H5, H5AP3 and L)-impregnated mortars was larger than that of the non-impregnated mortar at moist curing period of 3 years. Especially, the compressive strength by impregnation with H5AP3 was increased, as compared with L. (4) From these results, it was recognized that H5AP3 as to impregnant was possessed with controlling effect of ASR and high compressive strength developments, as compared with L.
Ceria-zirconia-yittria solid solution powders were synthesized by coprecipitation method. The products were characterized by XRD, colorimetric assessment and UV-Vis diffuse reflectance. A fluorite-type cubic phase was identified for all of the samples heated at 900°C, regardless of the amount of Zr content. Diffuse reflectance spectra of the solid solutions in the UV region show the broad shoulder-type band at 320-340nm, which is assigned to interband transitions in CeO2. The most absorption of visible light and the yellow color were observed in the range of x = 0.3-0.7 in ZrxCe0.8-xY0.2O1.9.
Several kinds of carbon steels and Invar alloys are applied to identify the dependence of transformation volume expansion on transformation plasticity for both diffusive and non-diffusive transformation. It is confirmed by the experiments that the notion of Leblond's model (transformation volume expansion over yield stress of mother phase and transformation plastic coefficient has proportional relationship) appears to be good for both ferrite and martensite transformation. In addition, the experimental results of Invar alloys also support the idea that if the transformation volume expansion is significantly small, the transformation plastic strain becomes small accordingly. It is commonly said that Magee effect (selective martensite variant) plays a main role during martensite transformation. However, for steel treated in this paper, Greenwood-Johnson effect (transformation volume expansion) is also significant for martensite transformation.
We propose a framework that can be used to study the local thermodynamic stability of materials at finite temperatures, by reconstructing the free energy surface based on metadynamics, constrained molecular dynamics, and local atomic deformation tensor analysis methods. We apply the proposed framework to FCC embedded-atom Cu models and estimate the activation energies, volumes, and critical local deformation tensor for a stacking-fault nucleation event in a Cu single crystal.