Bentonite consists of nanometer scale of smectitic clay minerals (mainly montmorillonite) and micrometer scale of macro-grains (mainly quatz). Properties of saturated bentonite are characterized by hydrated clay minerals. For a diffusion problem in bentonite we developed a new multiscale homogenization analysis (HA) procedure which relates microscale characteristics with a macroscopic behavior. We discuss diffusion properties of tritium water (HTO) compared with experimental data. Microscopic properties of the montmorillonite hydrate such as diffusivity of chemical species which are used in HA are determined by a molecular dynamics (MD) simulation method. We show the calculated results are consistent with macroscale experimental data.
The one-dimensional calculations of thermally induced residual stresses that are generated during the fabrication of multilayered functionally graded material plates (FGMs) because of a continuous macroscopic variation in the composition are presented. In this study, a laminated plate theory is used on the alumina/nickel FGM system to analyze the residual and thermal stresses in the multilayered FGM plates fabricated with a powder metallurgy method. Furthermore, the modeled ratios of thermo-mechanical parameters are employed to evaluate the residual thermal stress. The influences of residual thermal stress during thermal shock testing were also evaluated. The analytical results indicate that the residual thermal stress distributions can be controlled by adjusting the compositional gradient across the thickness of the FGM plates and the characteristics of thermal residual stress are more strongly dependent on the thermal expansion coefficients mismatch between ceramics and metal, in comparison with the mismatch of elastic moduli. It also indicates that during thermal shock process the thermal shock stress on the alumina surface does not change much by altering the compositional gradient in the FGM plate, and the release of thermal stress on the alumina surface is accomplished mainly by decreasing the tensile residual thermal stress.
For quasi-isotropic CFRP laminates, the elastic stress-strain behavior is expected to be the same in any loading direction and stacking sequence of laminate. But fracture and strength of them may strongly depend on the loading direction and stacking sequence of laminate. The laminate has structurally made up many interfaces and the lamina in the laminate is anisotropic due to embedded fiber. Therefore, once matrix cracking or delamination takes place in the loading laminate, the laminates change into anisotropic ones. In this paper, the tensile tests were carried out in the constant displacement velocity for the specimens, cut out in several directions from each laminate, to investigate the influence of loading direction and stacking sequence on the damage pattern and tensile strength. A FEM code was used to analyze the stresses in the specimens, and the comparison between the calculated stress-strain curves and experimental ones was attempted. In addition, using Tsai-Wu failure criterion, the strength of the laminates were predicted in the various loading directions. And the correlations between the predicted results and experimental one are discussed.
We investigated the effect of an epoxy surface layer to improve the impact damage resistance in carbon-fiber-reinforced-plastic (CFRP) laminate. The specimens were CFRP laminates covered on the impact face or the back face with a highly viscous epoxy resin. The thickness of the epoxy layer was 1.0mm or 2.0mm. Projectiles were launched from an air gun and impacted onto the laminates. The projectiles were made of silicone rubber or aluminum alloy. C-scan images obtained with a scanning acoustic microscope after the tests revealed that the damage mechanism for the CFRP laminates is independent of the surface-layer material and the material properties of the projectile. When we compared the contact area, the crack length on the back face and the delamination areas on each specimen, we found that the epoxy surface layer improved the impact damage resistance of the CFRP laminate. The epoxy surface layer on the impact face decreased the impact force transferred to the laminate, and that layer on the back face suppressed the generation and propagation of fiber-directional matrix cracking on the back face during impact.
The effects of bias voltage and discharge current on the mechanical properties of TiN films deposited on carbon steel JIS S45C by reactive do magnetron sputtering are investigated. The residual stress, hardness, toughness and adhesive strength are examined by X-ray diffraction method, nano-indentation test, indentation fracture method and scratch test, respectively. The films are revealed to exhibit high compressive residual stress that increases with the bias voltage and discharge current. The hardness also increases with the bias voltage and discharge current, whereas the adhesive strength decreases and the toughness increases only with increasing bias voltage. The variation of these properties correlates well with the variation in residual stress, regardless of changes in coating conditions. These properties are considered to depend mainly on the residual stress. The width of the X-ray diffraction peak also increased with bias voltage and discharge current, and correlated well with the change in residual stress. It was confirmed that the residual stress was generated by bombardment with high-energy ions during the coating process, and it was concluded that increasing the bias voltage and discharge current had the same effect to enhance the ion bombardment. Based on these findings, the ion bombardment is considered to be the dominant mechanism governing the mechanical properties of TiN films.
In recent years, the use of bioactive calcium phosphate coatings for biological fixation of load-bearing implants has been attracting much attention. Many techniques have been used to produce calcium phosphate, especially hydroxyapatite (HAP) coatings on metallic substrates. In this paper, a simple wet-chemical method, chemical bath deposition is reported to prepare calcium hydroxyapatite coatings on a Ti plate at relatively low temperature (95°C). The method is based on chelating calcium ions with a complex reagent and then increasing the temperature to dissociate the calcium ions, which induces the precipitation of HAP. EDTA is chosen as the complexion agent. At 60-95°C the Ca(EDTA)2- complex thermally dissociates and releases free calcium ions. Substrates used in this research are Ti plates with 0.2mm in thickness. The substrates were gritted by metallographic Al2O3 paper and then treated in a solution of 2mol/dm3 KOH at 95°C for 1 hour. The HAP coatings were performed in a chemical bath that was heated from 60°C to 95°C. XRD data indicated that the coatings are highly crystalline. FTIR and EDX analyses revealed that the coatings consist of Ca-deficient apatite. SEM micrographs of the coatings show that the coating is composed of evenly small crystal grains. The surface was uniform without apparent pores or agglomerates.
Solid state bonding between superplastic duplex stainless steel with carbon steel under low pressures is studied for the objectives to understand bonding behaviors and to determine mechanisms controlling bonding through the activation energy analysis. Experiments were carried out by combining several bonding parameters such as bonding temperatures, bonding pressures and surface roughness. Bond quality was evaluated by its tensile strength. Bonding strength increased with temperature, time and pressure but decreased with surface roughness. Sound bonds comparable to that of the parent metal were obtained at considerably short bonding time and low pressure. The best bonding condition obtained in this study was 1373K temperature, 3MPa pressure, 0.32μm Rmax and 220s bonding time, producing the parent metal strength with around 1% of deformation ratio. Changes in microstructures and hardness were observed across the bonding interface, due to diffusion of atoms, mainly C, from the carbon steel side to the duplex stainless steel side. A hard carburized layer formed at the DSS side and a soft decarburized layer at the carbon steel side. From the value of activation energy and experimental data, bonding was controlled mainly by two mechanisms, which were the superplastic deformation at the early stage and the diffusion of carbon at the second stage.
The risk of fracture does exist even in the carefully designed ceramic components under excess stress unexpected in design procedure such as high stresses caused by restraining the deformation at the contact region of the component. To assess the safety and the reliability of ceramics components under such circumstances, it is indispensable to take the damage tolerance of the material into consideration. Ceramics is thought to be perfectly brittle and have no damage tolerance. But, though very little, it has the damage tolerance. The important problems open to us now are to estimate the extent of the damage tolerance of ceramics quantitatively and to clear the relation between the damage tolerance of the material and the reliability of the component. In this paper, we demonstrate the existence of the damage tolerance in the ceramics by tests using specimens of a porous cordierite. The nonlinear stress-strain curve of the brittle material is thought to be a reflection of the damage tolerance of the material and it is shown that the nonlinear stress-strain curve obtained can be simulated by the distributed micro cracks model developed here. The relation between the damage tolerance and the reliability is also discussed through several simulation works. These simulations show that when the damage tolerance of the material is getting larger, the reliability of the component becomes higher. This conclusion is supported by the fracture test results using notched specimens of a porous cordierite where the notch sensitivity is shown to be very small in this material. This means that this material has the high reliability under the stress concentration.
The transformation temperatures of shape memory alloy vary with the composition and processing and manufacturing conditions of the alloy. The purpose of the present study is to clarify the effects of cold working and heat treatment on various transformation temperatures, and to estimate probabilistically the transformation temperatures by means of a model. The shape memory alloy used was Ti-41.7Ni-8.5Cu (at%), and the transformation temperatures were measured by the differential scanning calorimeter method (DSC method). The effects of processing and heat treatment on the transformation temperatures were investigated by changing the cold working ratio (10-40%) and the heat treatment temperature (623-773K), respectively. As a result, the transformation temperatures were found to decrease with increasing cold working ratio. The transformation temperatures were also found to increase with increasing heat treatment temperature. Therefore, the transformation temperatures were modeled by considering quantitatively the effects of the cold working ratio and the heat treatment temperature. A probabilistic prediction formula was proposed so that the reliability of the prediction may be estimated quantitatively.
Machines and systems that use shape memory alloys require the optimum design for combining functional properties and structural strength. In the reliability study of functional and structural design, the scatter as well as the behaviors of the functional and mechanical properties of the alloys during practical use should be considered. In this study, considering the application of Ti-41.7Ni-8.5Cu (at%) to a reciprocating-type heat engine, the increase of the irrecoverable strain with increasing number of thermo-mechanical cycles was clarified experimentally and the irrecoverable strain was used as a parameter for predicting the behaviors of functional and structural properties. Thus, models of the behaviors of irrecoverable strain were made to estimate functional properties such as transformation points and recovery stress as a function of irrecoverable strain. To analyze the scatter of irrecoverable strain, the ratio of experimental data to the value estimated from the model was considered and found to show the Weibull distribution. The probabilistic expression estimated as an inverse function of cumulative probability P was proposed to predict probabilistically the recovery energy and the dispersion energy by using irrecoverable strain εir as an estimation parameter. The proposed model for the behavior of the functional degradation and fatigue strength properties of Ti-41.7%Ni-8.5%Cu shape memory alloys under thermo-mechanical cyclic conditions can be used for the optimum design that combines the functional properties and structural strength of machines and systems such as a reciprocating-type heat engine that uses shape memory alloys.
Creep-fatigue remaining life of 316LC steel subjected to PP-type testing and CP-type testing is evaluated by use of two procedures that the authors proposed in the previous work for Mod.9Cr-1Mo steel. A new creep-fatigue damage rules determined by the authors for 316LC steels are used, where crack initiation life cannot be neglected in CP-type straining or in PP type straining. The results show that both proposed procedures yield more accurate estimations of remaining life, material damage and applied inelastic strain range for 316LC steel than for Mod.9Cr-1Mo steel. Among the two procedures, procedure 1 yields superior prediction accuracy over procedure 2, where the former requires measurement of surface crack length at a given total number of cycles, whereas the latter requires measurement of surface crack growth rate. Especially satisfactory results are obtained when procedure 1 is adopted and the measured surface crack length is 300μm or longer. That is, the ratio of the predicted value of remaining life to the actual value is below 1.1 and the corresponding ratios for material damage and applied inelastic strain range are from 0.9 to 1.3.