In order to understand nonlinear behaviors of stress-strain responses of metallic materials under static and cyclic conditions from the viewpoint of their microscopic structures, we theoretically investigate an analytical model by applying some mathematical techniques such as the soliton theory and the theory of nonlinear systems which develop greatly in recent years. The analytical model is based on the typical atomic chain model which includes topological defects and consists of the thermal effect, the interactions of atoms, the friction from the environment and the external force. In addition, the analytical model is developed from the typical atomic chain model by including the effects which can relate with work hardening and internal friction. In the static case, we show that inelastic behavior is displayed by the analytical model. In the cyclic case, by using the analytical model, we obtain the nonlinear behavior of the hysteresis loop which qualitatively corresponds to well-known experimental data better than that of the typical model.
Molecular dynamics simulations of the shape-memory effect are carried out to investigate the atomistic behavior during deformation and shape-recovery processes. The embedded-atom-method potential function and parameters for Ni-Al alloy are applied. The initial configurations of atoms are set on the lattice points of the martensite structure, in which the distribution of the variant orientation is limited to the two-dimensional direction for simplicity. When the shear load is imposed toward the x direction, parallel to the variant interface, the deformation of the variants occurs, and finally, all variants settle into the uniform orientation. The deformed state is maintained after the load is released, and the original shape is recovered through heating and cooling processes because of phase transformation to bcc and martensite. In the loading process, the stress-strain curve exhibits a zigzag shape consisting of repeated stress increase and abrupt release. The interval of the stress peaks is revealed to be smaller as the model size becomes larger. Deformation observed in variant layers seems to occur at the same time at every points in the layer for a small model. However, the simulation with a large model indicates a nucleation and propagation behavior in each layer.
Si3N4/SiC ceramics were hot-pressed to investigate the effect of stress frequency on the crack-healing behavior under cyclic stress. The specimens having pre-crack of 100µ m were subjected to crack-healing under constant or cyclic bending stresses of 300MPa at temperatures between 800 and 1200°C. The resultant bending strength of the crack-healed specimen was investigated. At a healing temperature of 900°C, the pre-cracks had been completely healed both under constant stress and cyclic stress of 0.5Hz. However, the pre-cracks had not completely healed under a cyclic stress of 5 and 10Hz. Thus, the stress frequency affects crack-healing behaviors at 900°C. On the other hand, when the healing temperature was higher than 1000°C, the pre-cracks were healed completely under both constant stress and cyclic stress up to 10Hz. The effects of stress frequency were dependent on the healing temperature. The reasons for this were discussed from the viewpoint of competitive mechanism of fatigue crack growth and crack-healing.
Bioactive ceramics, β-tricalcium phosphate (β-TCP), particles reinforced bioabsorbable plastics poly-L lactide (PLLA) composites have been expected to apply for the fracture fixations which have more biocompatibility than monolithic PLLA. In this study, β-TCP/PLLA composites containing three different β-TCP contents (4.8, 9.5, 14.3wt%) were prepared by injection molding. The results of bending tests show bending strength decreases with increasing β-TCP contents. On the other hand, bending modulus increases with increasing β-TCP contents. After immersion tests in PBS at 37°C up to 8 weeks, the mechanical properties were hardly degraded in all specimens. The results of fracture surface observation by scanning electron microscopy indicated that microscopic damage such as debonding between β-TCP and PLLA initiates at β-TCP agglomeration and grows with increasing loading. Analytical predictions of the relationship between stress and strain based on micromechanics considering the progress of debonding between β-TCP and PLLA were in good agreement with experimental results.
This paper deals with fracture process of a ceramic-metal functionally graded material (FGM) under three-point-bending. The used material was fabricated by powder metallurgy using partially stabilized zirconia (PSZ) and stainless steel (SUS 304), and has a functionally graded surface layer (FGM layer) on a SUS 304 substrate. In order to investigate the fracture process of the FGM, three-point-bending tests of rectangular specimens and numerical analysis are carried out. During the three-point-bending tests, crack initiation and unstable crack growth occur in the FGM layer, and the crack is arrested at the interface between the FGM layer and the substrate. Then, the crack branches and both crack tips grow stably along the interface with increasing deformation. After some amount of crack growth, both crack tips are arrested, and a new crack is initiated and grows into the SUS 304 substrate ahead of the initial cracking of the FGM layer. The finite element analysis taking account of gradation of material composition and plasticity of SUS 304 phase is carried out for each stage of fracture process. Based on the numerical results of the stress intensity factor, plastic zone and stress distribution, the fracture behavior of the FGM is discussed in detail.
An erythrocyte and a spherocyte are subjected to aspiration pressure with a micropipette and analyzed by the finite element method (FEM). The comparison of the erythrocyte and the spherocyte indicates that the y-direction displacement of the erythrocyte was larger than that of the spherocyte under the same aspiration pressure. The x-direction stress distributions show that it is easier to change the shape of the erythrocyte model than that of the spherocyte model because a force in the opposite direction appears in the erythrocyte. This force seems to move the erythrocyte membrane to its center. The results indicate that the shape of the erythrocyte membrane changes partially under aspiration pressure at the point of contact with the micropipette and aspiration pressure. The results also indicate that the shape of the spherocyte membrane changes under aspiration pressure, but not only at a point of contact with the micropipette and the area subject to aspiration pressure; the entire spherocyte membrane seem to change the shape.
This report reveals gigacycle fatigue properties for a modified-ausformed V-added steel with the chemical composition of 0.3C-0.3Si-1.0Cr-0.7Mo-0.3V in mass %. Modified-ausformed and oil-quenched steels were prepared for fatigue tests, followed by tempering at 400°C and 600°C. The tensile strengths of the 600°C tempered steels were almost equal to those of the 400°C tempered versions because of secondary hardening due to fine precipitation of vanadium carbides. The fatigue properties of the 600°C tempered version of oil-quenched steel (QT600) showed little difference from the 400°C tempered version (QT400) in spite of the fine precipitation of vanadium carbides. The modified-ausformed steels (AF400 and AF600) revealed higher fatigue limits at 5× 109 cycles than the oil-quenched versions (QT400 and QT600), although the difference between AF400 and AF600 was small. The remarkable difference between AF400 and AF600 was fatigue strengths at around 106 cycles, i.e. the fatigue strength of AF600 at those cycles was higher than that of AF400. Based on the above results, the effect of the fine precipitation of vanadium carbides was small on the gigacycle fatigue properties, while modified-ausforming could improve those properties. On the other hand, the multiple effects of the fine precipitation and modified-ausforming was large on the fatigue strength at around 106 cycles.
Behavior of thermo-mechanical fatigue (TMF) of a single crystal Ni-base superalloy, CMSX-4, was studied, compared with that of isothermal low-cycle fatigue (LCF). Strain-controlled TMF and LCF tests of CMSX-4 were carried out under various test conditions, where the experimental variables were strain rates, strain ratio, temperature range, and strain/temperature phase angle. At first it was shown experimentally that the TMF and LCF failures took places, associated with some noteworthy characteristics which were rarely seen in the traditional polycrystalline heat resistant alloys. They could not be explained reasonably, based on the historical approaches. A new micromechanics model was proposed to predict the TMF and LCF lives, applying the Eshelby’s theory and the Mori-Tanaka’s averaging approximation. The model presented in this paper enabled us to successfully estimate not only the unique characteristics in the TMF and LCF failures but also the effect of γ’ geometry on the LCF lives.
Brown, C.M. and Mills, W.J. reported that Alloy 690, a nickel based high chromium alloy, exhibited a considerable reduction in fracture toughness below 149°C in hydrogenated water environments, and they proposed that the possible mechanism attributable to the toughness degradation was hydrogen-induced intergranular cracking. Based on experimental results of the compositional analysis of the fracture surface using energy dispersive x-ray spectrometry (EDS), Brown, C.M. et al. also reported that the cracks might have propagated in the matrix, very close to the grain boundary. The authors, however, had reported that intergranular cracking was confirmed in Alloy 690TT under a Slow Strain Rate Tensile (SSRT) test of a cathodically hydrogen-charged specimen. The objective of this study is to clarify the effect of temperature and hydrogen on the intergranular cracking of Alloy 690TT. Slow Strain Rate Tensile (SSRT) tests were performed at several temperatures in simulated Pressurized Water Reactor (PWR) primary water conditions and at room temperature in air. The appearance of the fracture surface after SSRT tests at about 50°C showed “ductile intergranular fracture” which is characterized by the appearance of intergranular cracks on the fracture surface accompanied by dimples. No great difference was observed between the maximum stress after a SSRT test at 320°C in simulated PWR primary water conditions and a tensile test at room temperature in air. In addition, no correlation was found between the appearance of the fracture surface after either the SSRT or tensile tests and the test conditions such as strain rate and the amount of dissolved hydrogen in the water. Brown, C.M. et al. observed hydrogen-induced intergranular cracking with a CT specimen where the crack penetrates through the high tri-axial stress field, while in this paper we observed ductile intergranular fracture with smooth SSRT specimens which represent a typical uni-axial stress state. Hydrogen, however, could be trapped in the high tri-axial stress state region where high stress, high strain and a high dislocation density coexist. Further experiments, including the high tri-axial stress of such cracks are needed in order to clarify in detail the mechanism for ductile intergranular fracture.
To determine the friction coefficient of a press-fit pin in thin plating, both experiments and three-dimensional finite element analysis are carried out. The compliant press-fit pins are assembled into printed circuit boards with two types of plated through holes, one is Cu and Sn plated and the other only Cu plated, and the load-displacement relationships of the pin during assembly are recorded. Based on the load-displacement relationships of the pin obtained experimentally and the nodal reactions of the pin contacting with the plated hole, obtained from numerical analysis, performed assuming a fiction-less condition, the friction coefficients of the pin in plated holes during assembly are successfully determined. The friction coefficient of the pin in the Sn/Cu plated hole exhibits a higher value than that for the Cu plated hole during assembly, due to the adhesion in the contacting region. In an attempt to check the validity of the determined coefficients of friction, different press-fit assemblies are considered, and the load-displacement relationships of the pin are predicted. The simulations are found to be in good agreement with experimental measurements. The retention forces between the pin and the plated holes are also predicted.
The crack growth along the interface between a submicron-thick film (Cu) and a substrate (Si) under fatigue is experimentally investigated at the load-frequencies of 0.1Hz and 1Hz in a laboratory environment (45 ±5%RH). A modified four-point bend specimen, which has only one interface crack to facilitate the control of crack growth, is used for the tests. The results reveal that the clear interface crack between Cu and Si grows under the cyclic load. The crack growth rate per cycle, da/dN, is governed by the stress intensity factor range, ΔKi, at each frequency and the sigmoidal relationship consisting of three stages are observed in the da/dN-ΔKi curve; the threshold, the stable growth and the critical growth. da/dNgreatly increases as the frequency decreases in the stable growth region. The crack growth rate per time, da/dt, shows a good correlation with the maximum stress intensity factor, Kimax, independently of the loading frequency. This indicates that the environmental effect due to humidity in air plays a dominant role on the crack growth.
The surface aberration effect in the strain scanning method with a Ge analyzer was examined using high- energy X-rays from the undulator synchrotron source. The synchrotron X-rays from the undulator source had an enough intensity for the strain scanning method using a goniometer with the analyzer. The use of a Ge (111) analyzer showed remarkable reduction of the surface aberration effect. However, there still existed the surface aberration for the very-near surface region from the surface to the depth of 50µm. A correction method was proposed by taking into account of the effects of the divergence of the Ge analyzer, the mis-setting of the analyzer and the X-ray attenuation. The proposed correction method was very useful for eliminating the surface aberration effect. The correction method enables a high space-resolutive evaluation of the subsurface stress distribution. The method was successfully applied to the determination of the residual stress distribution of the shot-peened steel. A precise d0 value of the strain-free lattice spacing necessary was determined from the surface stress measured by the conventional sin2ψ method using Cr-Kα radiation.
A small in-plane bending fatigue testing machine for in-situ observation of small fatigue crack growth behavior by means of an atomic force microscope (AFM) was successfully developed. The multiple layer piezoelectric ceramics were adopted as an actuator in order to miniaturize the fatigue loading facility operating on the stage of an AFM. Small fatigue crack growth test under constant amplitude loading was then carried out on α-brass and successive observation of small fatigue crack growth behavior was performed by the AFM. The fatigue crack tended to grow along one slip direction with the highest Schmid factor, as the crack driving force of a small crack was not large enough to operate other slip directions with lower Schmid factors simultaneously. Frequent crack branching and deflection behavior were also observed during crack growth. It was considered that the constraint of slip deformation due to the cyclic strain hardening was mainly responsible for crack branching and deflection behavior. The intervals of branching or deflection were affected by the difference of mobility among slip planes.
A woodpecker strikes its beak toward a tree repeatedly. But, the damage of brain or the brain concussion doesn’t occur by this action. Human cannot strike strongly the head without the damage of a brain. Therefore, it is predicted that the brain of a woodpecker is protected from the shock by some methods and that the woodpecker has the original mechanism to absorb a shock. In this study, the endoskeltal structure, especially head part structure of woodpecker is dissected and the impact-proof system is analyzed by FEM and model experiment. From the results, it is obvious that the woodpecker has the original impact-proof system as the unique states of hyoid bone, skull, tissue and brain. Moreover it is considered that woodpecker has the advanced impact-proof system relating with not only the head part but also with the whole body.
In this study, diffusion creep in Sn-37Pb as a low melting point alloy during a nanoindentation creep test was examined. The creep exponent, n, from the relationship between hardness and indenter dwell time decreases as a function of time and is saturated when n=1. From observations of the indented surface, plastic deformation due to the indenter takes place in the early stages. On the other hand, granular deformation takes place in the middle and last stages. Finite element analysis revealed that the reduction in Mises stress is faster than that of hydrostatic stress. Multiaxial stresses appear below the indenter, and axial components of stress remain after relaxation. Since the core hydrostatic stress causes a gradient in the chemical potential at grain boundaries, diffusion creep affects the behavior of indentation creep in the last stage. A transition from power-law creep to diffusion creep occurs below the indenter.
In this paper, a method for identifying of a crack in a plate that uses a genetic algorithm (GA) based on changes in natural frequencies is presented. To calculate the natural frequencies of the cracked plates, a FEM (Finite Element Method) program, which is based on the BFM (Bogner, Fox and Schmidt) model, is developed since the accuracy of the forward solver is important. In the analysis, two types of cracks, i.e., internal and edge cracks are considered. To identify the crack location and the depth from frequency measurements, the width and position of the crack in a plate are coded into a fixed-length binary digit string. Using GA, the square sum of residuals between the measured data and the calculated data is minimized in the identification process and thus the crack is identified. To avoid a high calculation cost, the response surface method (RSM) is also adopted in the minimizing process. The combination of GA and RSM makes the identification more effective and robust. The applicability of the proposed method is confirmed by the results of numerical simulation.
The thickness effect of a three-point-bend (3PB) specimen on dimple fracture behavior is studied experimentally and numerically. At first, fracture toughness tests were conducted using 3PB specimens of different thicknesses. Fracture toughness values and R-curves are obtained, and the thickness effect is discussed. Using scanning electron microscopy (SEM), dimple fracture surfaces are observed precisely. It is found that the thickness effect appears clearly in the void growth process. Finite element (FEM) analyses are conducted based on these experimental data. Using Gurson’s constitutive equation, the nucleation and growth of voids during the dimple fracture process are simulated. The distribution patterns of stress triaxiality and the crack growth process are obtained. The results show a good agreement with experimental ones qualitatively. The effects of specimen thickness on R-curves are explained well on the basis of these numerical simulations.
The inclusion element method with a simple grid model has been proposed as one of the analytical techniques of the mechanical behavior of textile composites, and the effectiveness of this method has been verified. The inclusion element method is applicable to the analysis of all types of textile composites because the element stiffness obtained by the inclusion method through cooperation with a fabric structure simulator is used. From the result of the analysis by the inclusion element method, it has been confirmed that the peculiar crimp-interchange of woven composites occurs and high tensile stress arises at the elements with fiber bundles oriented in the load direction. A comparison between the analyses using a real model and the inclusion element model has shown relatively good agreement. Although the analytical result is greatly dependent on the grid pattern, the inclusion element method can provide a sufficient accuracy of results even when the number of elements in the model is lower than that in the real model.
In this paper, we examine the applicability of the passive electric potential CT (computed tomography) method to the quantitative identification of three-dimensional cracks in structures. In this method, a piezoelectric film is glued on the surface of structures. The electric potential values on the piezoelectric film change due to the strain distribution on the surface of the structures, when the structures are subjected to an external load. The strain distribution induces an electric potential distribution on the piezoelectric film. Then, this method does not require electric current application, and passively observed electric potential values on piezoelectric film can be used for crack identification. The electric potential distribution on piezoelectric film was investigated numerically and experimentally. It was found that the electric potential distribution shows a characteristic change corresponding to the shape of the surface crack. An inverse method based on the least residual method was applied to crack identification from the electric potential distribution. In this inverse method, the square sum of residuals is evaluated between the measured electric potential distributions and those computed from the electric potential distribution of the piezoelectric film. Three-dimensional surface cracks were identified from the measured electric potential distribution. It was found that the location and size of the crack can be quantitatively estimated using a two-dimensional distribution of electric potential.
The effectiveness of the Natural Strain theory for describing a large deformation is mentioned in this paper. The Natural Strain is obtained by integrating infinitesimal strain increment on an identical line element over the whole process of the deformation path. Consequently, the shearing strain becomes pure angular strain, which is obtained by removing the rigid body rotation from the rotating angle of a line element. Since the expression of the Natural Strain is different from the strain expression of ordinary rate type, the additive low of strain on an identical line element can be satisfied. In this paper, the finite deformation analyses of a pure elastic body concerning the three different types of deformation paths are discussed on the combined deformation of uni-axial tension and simple shear, and the Natural Strain proposed in this paper is compared with other strain expressions and the rationality of this strain expression is confirmed.
This paper describes the experimental results of ultrasonically welding ceramics and metals. In comparison to other methods, ultrasonic vibration is easier and quicker for welding ceramics, such as ZrO2, SiC, and Si3N4, and metals such as aluminum, magnesium, and copper. In this study, ceramics and Mg were welded under the following conditions: amplitude, 30µm; welding pressure, 10MPa; required duration, 1.0s. Ultrasonic welding of the ceramics with metals was possible when the condition E=KPn < f(P, E) (E: energy density P: welding pressure) was satisfied and the welding interface temperature was in the range of 300-400°C. When the ceramics were preheated, welding was possible within a short time and under low pressure, and the material had good weldability even at a high temperature (200°C). It is presumed that in this environment, oxide and organic films are efficiently removed from the bonded interfaces by the vibration of ultrasonic waves.
Fiber-matrix interfacial adhesion in composites is traditionally evaluated by means of a stress-based parameter. Recently, an interfacial energy parameter is suggested to be a valid alternative. However, the energy-based approaches overestimated the energy release rate to initiate the interfacial debonding (interfacial energy), since the plastic deformation in the vicinity of the debonding was neglected for simplicity. An effect of the plastic deformation on the interfacial energy of a fiber-reinforced polymer matrix composite is studied to evaluate the initiation of the interfacial debonding. The fragmentation tests with a model of glass fiber-reinforced vinylester matrix composite were performed, and the interfacial energy with the energy balance method taking into account an energy dissipation of the plastic deformation was calculated. The following results are confirmed; the plastic deformation has a significant influence on the interfacial energy, and the energy balance scheme taking into account the plastic energy dissipation leads to the constant interfacial energy without reference to the amount of the released potential energy. The differences between our model and the previous one are discussed.
Two-dimensional displacement measurement using digital image correlation with lens distortion correction is described in this paper. A single cross-grating is used as a calibration reference. Using two-dimensional Fourier transform, the phases of the grating pattern are analyzed and lens distortion distribution is obtained from the unwrapped phase maps. After detecting lens distortion, the coefficients of lens distortion are determined using the least-squares method. Then, the displacement distributions without the lens distortion are obtained. The effectiveness of the method is demonstrated by applying the proposed method to the rigid body translation test and the uniaxial tension test. The results show that the proposed distortion correction method removes the effect of lens distortion from the measured displacements. By the proposed method, accurate measurements can be performed even if images are deformed by lens distortion.
Scratchitti vandalism, a new type of graffiti vandalism, in public transits systems and city neighborhood is a serious problem. To solve this problem, an innovative approach was developed-controlled fire polishing, which incorporates a technique of localized softening and surface tension. Intensive heat is positioned near to the scratch marks on the glass panel. The heat melts a thin layer of glass into liquid, changing the glass’s viscosity to a formable state. The glass is melted to a level close to the depth of the scratch, and allowed to cool down naturally. During the cooling process, the surface tension of the melted glass will even out the scratching indent. After cooling, the glass will be as even and smooth as it was originally. The process will enable the reuse of the damaged window/door and eliminate the otherwise waste by replacement new glass.