In the development of functional ceramics, the study on the relation between the macroscopic properties and the microstructures has been a critical issue. However, it has not been made clear by many experimental works. On the other hand, this paper takes the computational mechanics approach using the homogenization method. To model the very complex microstructure architecture three-dimensionally, the image-based modeling technique is employed. A porous alumina with needle-like random pores with 3.1% porosity ratio is studied in this paper. The effect of the location of extracting the unit cell and its size on the homogenized elastic constants is investigated. Then, the predicted values were compared with the measured ones. As a result, a unit cell region expressed by one million voxel elements with 2µm resolution provides very accurate prediction regardless of the location of the unit cell in the random microstructures. The error between the numerical prediction and the measurement is only 1%.
A multi-scale computational method using the homogenization method is applied to the stress analysis of ceramics considering the random and complex microstructure three-dimensionally. The main purpose of this paper is to propose a numerical method to understand quantitatively the microscopic stress distribution in the random microstructure especially for porous ceramics. The voxel mesh is used to model the microstructure automatically with the help of the image-based modeling technique. In the analysis of porous alumina with needle-like random pores, the distance from the nearest pore is measured for all the voxel elements, which is plotted with the histogram of the microscopic stress distribution. This is of great help to study the relation between the stress concentration and the pore geometry and dispersion.
In general, the structural shape optimization method based on fully stressed design gives us a solution in which the shape complexity may cause difficulty in manufacture. The present paper proposes a novel approach to the determination of moderately complex shapes that satisfy the requirement for light weight, high stiffness and sufficient strength. There are two original ideas in this method. One is for the clear definition of shape complexity: A new parameter CD (curvature deviation) which represents the degree of structural shape complexity is proposed. The second is a method for deriving moderately complex shapes that satisfy the requirement for light weight, high stiffness and strength. A smoother shape is derived by an evolutional change of the CD, while a structural shape is obtained by an iterative change of an allowable stress region. The proposed method has been verified by applying it to the design of a hook.
This paper presents the finite element stress analysis of a spent nuclear fuel disposal canister to provide basic information for dimensioning the canister and configuration of canister components and consequently to suggest the structural analysis methodology for the disposal canister in a deep geological repository which is nowadays very important in the environmental waste treatment technology. Because of big differences in the pressurized water reactor (PWR) and the Canadian deuterium and uranium reactor (CANDU) fuel properties, two types of canisters are conceived. For manufacturing, operational reasons and standardization, however, both canisters have the same outer diameter and length. The construction type of canisters introduced here is a solid structure with a cast insert and a corrosion resistant overpack. The structural stress analysis is carried out using a finite element analysis code, NISA, and focused on the structural strength of the canister against the expected external pressures due to the swelling of the bentonite buffer and the hydrostatic head. The canister must withstand these large pressure loads. Consequently, canisters presented here contain 4 PWR fuel assemblies and 33×9 CANDU fuel bundles. The outside diameter of the canister for both fuels is 122cm and the cast insert diameter is 112cm. The total length of the canister is 483cm with the lid/bottom and the outer shell of 5cm.
This paper addresses the basic concept of MDO methodology and the structural analysis that should be performed in the design process of a mechanical heart valve prosthesis with flat leaflet using MDO methodology. In the structural design of the mechanical heart valve (MHV) prosthesis, the fluid mechanics analysis is executed for the blood flow passing through the leaflets of a mechanical heart valve prosthesis. Thereafter, the rigid body dynamics analysis of the leaflet motion is performed to obtain the structural condition for the structural mechanics analysis of the deformed leaflet. Then the structural mechanics analysis of the deformed leaflet follows to confirm the minimum thickness of the leaflet for the structural durability of the mechanical heart valve prosthesis. This paper shows that the minimum leaflet thickness can be evaluated to be 0.6mm among the suggested thicknesses.
This paper describes a constitutive model for solder alloys and a method to determine the material parameters of the model. First, a constitutive model that divides the inelastic deformation into plastic and creep parts is proposed. Next, a numerical method for the determination of the material parameters used in the model is developed. In this method, the material parameters for the creep part are determined first by pure tensile tests at different strain rates, and the creep strain is calculated next. Subtracting the calculated creep strain from the pure tensile deformation, the stress-strain relation for the time independent strain is obtained. Then, the material parameters for the time independent strain, such as the elastic and plastic strain, are simply determined from the obtained stress-strain relations. Simulations of the stress relaxation after several different preloadings were carried out to verify the accuracy of the proposed model and numerical method.
In this paper, a method to measure the stress intensity factor of a two-dimensional crack existing in a member of real structure is presented. The proposed method uses the following properties of piezoelectric materials; (1) isotropic piezoelectric material induces an electric charge which is proportional to the sum of stresses (σx+σy) on the structural member's surface; (2) the electric charge is also proportional to the integral of (σx+σy) in the area where the piezoelectric element is adhered to the structure. In order to obtain the stress intensity factors of Mode I and Mode II separately, two small pieces of piezoelectric elements are adhered near the crack tip so that the two piezoelectric elements are placed close to each other with the crack tip positioned between the piezoelectric elements. The electric current from the piezoelectric elements are integrated by the integration circuits and the output voltages which are proportional to the electric charge induced in the piezoelectric elements are measured. The stress intensity factors of Mode I and Mode II of the crack are estimated by the use of the measured output voltages.
This paper examines fatigue crack initiation and growth behavior in a ferritic stainless steel, SUS444. Fatigue tests were conducted in laboratory air and in 3%NaCl aqueous solution using an electro-hydraulic fatigue testing machine. Specimens with two different orientations, L and T directions, were evaluated. Cracks initiated from slip bands within ferritic grains regardless of direction and the effect of orientation on fatigue life could not be observed in either environment. An environmental effect could be identified, i. e. fatigue lives were shorter in the 3%NaCl aqueous solution than in laboratory air. In early crack growth regions, no discernible differences could be seen in either direction, but crack growth rates in the 3%NaCl aqueous solution were faster than in laboratory air. In long crack growth behavior, anisotropy could only be observed at low ΔK region, where the crack growth rates in L-T orientation were faster than in the T-L orientation. After allowing for crack closure, the difference between both orientations disappeared. An environmental effect could also be seen in the region of ΔK≥15MPam1/2 for both orientations, where crack growth was enhanced in 3%NaCl aqueous solution.
This paper deals with the surface crack growth path description and the fatigue life prediction due to repeated rolling/sliding contact on the elastic half-space, accompanied by frictional heat generation and crack-face pressure. The stress intensity factors are analyzed for the surface crack which is kinked in multiple times from the inclined initial main crack. The rolling/sliding contact is simulated as a Hertzian contact pressure and a frictional load with heat generation, moving with constant velocity over the surface of the half-space. Applying the maximum energy release rate criterion to each kinked angle, the crack growth path can be described, and employing a mixed mode fatigue crack growth law, the associated fatigue life also can be predicted. The effects of frictional coefficient, slide/roll ratio and crack-face pressure on the crack growth path and associated life are considered for a high carbon-chromium bearing steel (AISI52100).
Residual stresses of plasma sprayed coatings, NiCr alloy, CoNiCrAlY alloy, Al2O3 ceramics and ZrO2-8%Y2O3 ceramics, were measured in this study. SACHS method with a strain gauge was used to measure the residual stress. Type 304 stainless steel and mild steel were selected for the substrates. The results obtained were summarized as follows: 1) Tensile residual stresses were generated in plasma sprayed coatings. 2) The values of residual stress in ceramics coatings were relatively small and the values took lower than 40MPa. 3) Tensile residual stress values of metallic alloy coatings on mild steel substrates were higher than those on type 304 stainless steel substrates. 4) Residual stresses in Low Pressure Plasma Sprayed (LPPS)-coatings were larger than those in Atmospheric Plasma Sprayed (APS)-coatings. 5) Pre-heating of a substrate will effectively reduce tensile residual stress in a coating.
Propagation of cracks within a corrugated interface of a layered composite was investigated by calculating energetic conditions for a crack loaded in a mixed mode of opening and shear to be deflected around the saw-tooth corner of the interface profile. It was predicted that in case of a defect-free interface, in which a crack is deflected in consequence of competition between penetration into the bulk layer and deflection around the saw-tooth shaped corner, deflection is possible only when the interface is impractically weak. However, when the corner defect is embedded within the interlayer, coalescence of the primary crack into the defect to form an L-shaped crack is energetically favoured, while the interface fracture energy as high as that of the planar crack-deflecting interfaces in an equivalent composite laminate is allowed. Therefore, it is proposed that by incorporating defects in a controlled manner, it is possible to produce layered composites with corrugated interfaces for improved shear resistance without sacrificing the interface strength properties.
The studies of probabilistic distribution of fatigue crack growth life (or the fatigue fracture probability) have been reported. However, some studies have concluded that each crack grows independently, without crack coalescence; while others indicate the defects should be treated as 2-dimensional cracks. In this study, a method to predict the probabilistic distribution of fatigue crack growth life that considers crack coalescence of the surface crack at the welded line is presented, and the influence of several parameters on the life distribution is discussed.
Railroad wheel brittle fractures normally occur from the tips of thermal cracks originating in the tread or flange. Tensile stress that contributes to the fracture is produced by abnormal brake heating. In the US, there used to be a comparable number of derailments due to such brittle wheel fractures. In Japan, brake conditions have not been so severe, and maintenance has been moderate. Therefore, such fractures have not been a major concern in wheel damage. However, conditions such as load and speed of railroad vehicles have become more severe in recent years. Therefore, more precise evaluation of wheel strength and safety is necessary. On the other hand, recent developments in fracture mechanics and elastic-plastic finite element analysis enables us to understand the causes and processes of mechanical failure such as brittle fracture. This paper reports on the research on strength estimation and safety improvements, which are required to prevent the railroad wheel fractures.
Lamb waves generally consist of many dispersive modes, which makes mode identification difficult. This study describes the extraction of a single mode from multiple modes. The single mode extraction is based on two-dimensional Fast Fourier Transform (2D FFT) in time and space, signal filtering, and 2D inverse FFT. An air-coupled ultrasonic technique was adopted for non-contact fast measurements. Using this technique, the A0 and S0 mode were clearly detected in preliminary tests of intact plates. Furthermore, multiple reflections of the A0 mode, which were shown only in the computer simulation, were experimentally confirmed in reflection tests of plates with a rectangular notch. In the reflection tests, two reflected waves were extracted in the intervals corresponding to notch widths.
Most failures of ductile materials in metal forming processes occurred due to material damage evolution-void nucleation, growth and coalescence. In this paper, modified version of Gurson-Tvergaard's yield function in conjunction with the Hosford's non-quadratic anisotropic yield criterion is studied to clarify the plastic deformation characteristic of voided anisotropic sheet metals. The void growth of an anisotropic sheet under biaxial tensile loading and damage effect of void growth on forming limits of sheet metals are investigated. Also the characteristic length defining the neck geometry is introduced in M-K model to incorporate the effect of triaxial stress in necked region on forming limits. The forming limits theoretically predicted are compared with some experimental data. Satisfactory agreement was obtained between the predictions and experimental data.
In-service degradation mechanisms of over-aluminized MCrAlY duplex coatings have been determined by evaluating two long-term service-run blades. Cyclic oxidation tests were also conducted to characterize the degradation of the duplex coatings, GT29+ and GT33+. The results show that degradation of duplex coatings is manifested by the formation of oxide scale, the transformation of β-phase into γ, the coarsening of β-phase particles in the MCrAlY coating, and the enlargement of the interdiffusion zone. Considering all of the degradation mechanisms observed in these service-run coated buckets, a coating life prediction model, developed under an EPRI-funded program, was used to estimate the life of GT29+ and GT33+ coatings. A comparison of the model calculations and experimental data indicated that, conservatively, the useful life of duplex coatings can be predicted solely based on the Al content in the top aluminide coating. The predicted value of the coating life is increased by a factor of 1.4 to 2 if MCrAlY is included in the lifetime estimation.
Wood sawdust and chips, as carpentry processes remains, are introduced as alternatives for filling mediums in energy mitigation systems. The natural, economical and environment friendly material, wood, is modeled and its interaction with the collapse of tubes, typical energy absorption elements, has been considered. Compression tests on wood sawdust were used to extract its mechanical properties and the results of which have been used in the validation of the analytical models. The collapse of PVC tubes, filled with wood sawdust of different grades and densifications, has been analyzed considering multi-lobe mode, concertina mode and mixed mode. In these models, the interaction of wood sawdust filler on the final mode of collapse of PVC tubes has been incorporated and the final values of the mean crushing load show favourable results.