Various types of interatomic potential functions have been proposed for atomic simulations, such as molecular dynamics and Monte Carlo calculation. Those functions are constructed based on the properties at equilibrium states of single crystal. Reliability of the potential functions under the condition far from such states, which is called transferability, is an important factor in the simulations of materials with nanoscopic complex structure or under high stress condition. However, it is not sufficiently investigated because it is difficult to get precise experimental data in such conditions. In this paper, simulations are conducted for aluminum bulk under high strain, grain boundary of aluminum and atomic chain using the potential function of the effective medium theory (EMT) as well as ab initio calculation in order to clarify its validity. In the case of single crystal under tensile strain, the results obtained from the EMT potential agree well with those obtained by ab initio analysis. The EMT gives also good correspondence in the grain boundary structure. However, EMT cannot be applied to the atomic chain because distribution of charge density eminently differs from that in the bulk.
The quantitative identification of the backstress evolution is carried out in a Cr-Ni-Mo-Al-Ti maraging steel under thermomechanical loading conditions. The evolution depends on the direction of the applied stress; tensile, compressive or shear. The iso-volume fraction lines (the iso-ξ lines) are determined between the transformation-start line and the transformation-finish line on the applied hold stress-temperature plane. The iso-ξ lines are composed of a tensile-branch and a compressive-branch, each of which is parallel to the martensite-start line. The cross point of the tensile- and compressive-branches of the iso-ξ line shifts to the direction of the backstress evolution. The almost hold stress-independent martensite-finish line is proved to be attributed to the backstress evolution.
In order to investigate a constitutive law associated with shear band in amorphous glassy polymers, polycarbonate specimens were tested under forward and backward loadings in simple shear at room temperature. It was shown that (1) the shape of shear stress-strain curve was decided by the formation, lengthening and widening of the shear band at both loading paths, (2) the stress drop at yielding was very sensitive to the lengthening of shear band and (3) once the shear band formed, the yield drop did not occur at backward loading.
Fatigue tests were conducted using talc-reinforced Polypropylene copolymers. Then, high-speed tensile tests were conducted using the fatigue-damaged samples to reveal the effect of fatigue damage on the impact energy-absorption. The fatigue damage progression mechanism was investigated due to fracture surface observation, density measurement, and local elastic property. The following conclusions have been drawn from the experimental results: 1) The residual impact resistance decreases significantly with an increase in the lifetime ratio. 2) The fracture surfaces in a high-speed tensile test show prominent talc exposure, differing greatly from the surfaces of undamaged materials or creep-damaged materials. This state may have arisen due to the separation of talc portions from the matrix as a result of fatigue loading. 3) From the variation of density and local elastic modulus, we clarified that fatigue damage differs from creep-induced damage, which is caused by the large accumulation of voids, and is in fact caused by the talc separation described above. 4) The local elastic property measurement was shown to be an appropriate non-destructive method, capable of detecting reductions in the residual impact resistance of working products.
In this paper, the solution of semi-infinite plane with one circular hole is presented for analysis of stresses around a shallow circular tunnel with a constant inclined surface. This solution is induced by repeating to superpose the solution of infinite plane with one circular hole and that of semi-infinite plane without holes to cancel out arising stresses on both boundaries. And this procedure is carried out until the stresses arising on both boundaries converge. This method does not need complicated procedure to calculate as the method using stress functions defined in bipolar coordinate system. Some numerical results are shown by the graphical representation.
This paper describes the stress ratio dependence of fatigue behaviour in a cast aluminium alloy, AC4CH. Axial fatigue tests have been conducted using smooth specimens at three different stress ratios, R, of -1, 0.05 and 0.4. Particular attention has been paid to crack initiation and small crack growth behaviour. The Goodman relationship gave fairly good or slightly conservative estimation of fatigue strength at 107 cycles. Cracks initiated from a casting porosity or Sb-segregated defect and the latter defect acted more predominantly as crack initiation site. Regardless of stress ratio, small cracks initially grew faster than the corresponding large cracks characterized in terms of the effective stress intensity factor range, ΔKeff, then gradually approached the da/dN-ΔKeff relationship for large cracks and coincided with it at a ΔK value of 2-3MPa√m. Based on detailed fractographic analyses, shear-type growth region following the crack initiation was clearly discerned, the size of which showed remarkable stress ratio dependence, with increasing crack size with decreasing stress ratio, particularly very large at R=-1. This shear-type crack growth was responsible for the acceleration of small cracks because of a different growth mechanism from large cracks.
Surface film-bonded materials with resin interlayer are often used in electronic parts, fatigue properties of which have to be discussed in order to maintain the expected electronic function without fracture during operating. In this study, pure copper and commercial grade iron films with the thickness of 100μm were bonded to the surface of carbon steel base plate by epoxy resin adhesive or by diffusion. As a result of fatigue testing, there was a tendency for the epoxy-bonding layer to restrict fatigue crack propagation from the surface film to inner base plate. In addition, the epoxy-bonded iron film had a compressive residual stress and also caused smaller stress amplitude in the inner base plate between films, so that the fatigue life of the epoxy-bonded plate was longer for the iron film than for the copper film. The fatigue crack propagation rate, da/dN, for all the film-bonded plates was expressed by a power law of the measured crack opening displacement range, Δφ250, as well as for several metal base plates, although the fatigue crack on the epoxy-bonded film propagated at the low value of Δφ250 in which the fatigue crack did not propagate for the diffusion-bonded plate and the base plates.
Machines and systems applied shape memory alloys require the optimum design fused functions and structure strength. And so the reliability for functional and structural design is required to consider the scatters as well as the degradation behaviors of functional and mechanical properties of the alloys during practical service uses. In this paper, under considering the application of Ti-41.7Ni-8.5Cu (at%) to a reciprocating type heat engine, the increasing behaviors of irrecoverable strain with increasing of the number of thermo-mechanical cycles were clarified experimentally and the irrecoverable strain was used as an estimate parameter for predicting the behaviors of functional and structural strength properties. Thus, the formula-models concerning the behaviors of irrecoverable strain were made so as to estimate the functional properties such as transformation points and recovery stress from an irrecoverable strain. The scatters of irrecoverable strains in their behaviors were analyzed against the ratio of experimental data to the value estimated from model formula and were clarified to show Weibull distribution. The probabilistic estimation expression with inverse function of cumulative probability P was proposed for predicting probabilistically recovery energy and dispersion one by using an irrecoverable strain εir as an estimation parameter. The proposed model formula for degradation of functions and fatigue failure strength for Ti-41.7%Ni-8.5% Cu shape memory alloys under thermo-mechanical cyclic conditions can be available for the optimum design fused functions and structure strength against machines and systems such as a reciprocating type heat engine applied shape memory alloys.
The previous inelastic constitutive model developed by the authors for modified 9Cr-1Mo steel at 550°C was extended to apply to ratchetting deformation analysis in the temperature range between 200°C and 600°C. A kinematic hardening rule was changed from OW I to OW III model to improve accuracy of prediction of ratchetting deformation at lower temperatures, and evolutionary laws for cycle dependent variables were also modified to express cyclic hardening behavior observed at lower temperatures. Constants involved in the constitutive model were determined for each temperature based on the results of monotonic and cyclic loading tests. The extended constitutive model was applied to simulations of inelastic behavior under various loading conditions, including isothermal and anisothermal cyclic deformation and ratchetting deformation. It was demonstrated that the extended constitutive model is able to describe not only the ratchetting behavior but also the other inelastic behaviors including the anisothermal cyclic deformation with good accuracy.
New biaxial thermo-mechanical fatigue testing machine, which can perform thermo-mechanical fatigue (TMF) tests under any selected conditions of axial loading, torsional loading and temperature history with arbitrary amplitude and phase, was developed. TMF tests were performed under complicated axial/torsional loading and temperature waveform (Blade waveform) that simulates strain and temperature histories imposed in actual gas turbine blades, as well as fundamental in-phase and out-of-phase tests. Test temperature was selected between 450-850°C. In case of without strain hold, failure life of the out-of-phase is comparable to that of the Blade waveform, while failure life of the in-phase is longer than that of the out-of-phase and the Blade waveform due to occurrence of negative mean stress. On the other hand, the largest life reduction was observed in the in-phase condition by introducing 6 minutes strain hold at the maximum temperature. Thermo-mechanical fatigue lives under various conditions were well correlated to uniaxial isothermal fatigue data by the equivalent total strain range, Δεeq proposed in this study. Then, time dependent TMF life prediction model was proposed based on the nonlinear damage accumulation model incorporating with Δεeq and failure lives of the Blade waveform with strain hold were accurately predicted by the TMF life prediction model.
The remaining life evaluation is conducted on PP tested and CP tested 316LC steel by using the two procedures previously proposed for Mod.9Cr-1Mo steel by the authors. The creep-fatigue damage rules determined by the authors for 316LC steels are used, where the crack initiation period cannot be neglected in CP type straining as well as in PP type straining. As the results, it is found that both the proposed procedure 1 and 2 can estimate the remaining life, the material damage and the applied inelastic strain range with better accuracy in case of 316LC steel than in case of Mod.9Cr-1Mo steel. Among the two the procedure 1 is superior in prediction accuracy to the procedure 2, where the former needs the measured surface crack length at a given total number of cycles while the latter the measured surface crack growth rate. Especially satisfactory results are obtained when the procedure 1 is adopted and the measured surface crack length is 300μm or longer. That is, the ratio of the predicted value to the actual one is ranged below 1.1 for the remaining life and from 0.9 to 1.3 for both the material damage and the applied inelastic strain range.
This paper presents new methods for source location and fracture dynamics study of guided waves produced by the delayed fracture of a pre-stressed concrete (PC) steel wire. Utilizing the guided wave produced by laser ultrasonic system, the author first proposed a new source location method of guided waves in the PC wire of 7mm diameter. Source location in the transverse section was determined from the amplitude ratio of the maximum peak of flexural (F-) mode to that of first arrival longitudinal (L-) mode. Location in the axial direction was determined from the time difference between the first arrival L-mode and the peak arrival time of the F-mode. Estimated progression of micro-cracks agreed well with characteristic features observed on fracture surface. Dynamics of micro-cracks were estimated by the waveform matching of the first packet of the L-mode to the monitored one and found to be generated within 0.5 microseconds.
Corrosion tests were done for silver plates in gaseous environment that contained H2S. After corrosion tests, electric contact resistance ρ of silver plate increased with exposed period of time t by the corrosion product Ag2S and was correlated linearly with the loss of weight W of Ag plate, that was derived from the formulae proposed and verified by the authors. Simple and useful procedure of corrosion life prediction was established for connectors composed of Ag, in which the environmental assessment plays an important role.
When high and low temperature materials, such as heated water, LNG, LPG and frozen food etc. actually are stored temporally in openings excavated in rock mountain, as the quantity of these changes continually, the rock mass around openings will receive the effects of thermal hysteresis of high and low temperatures. Therefore, obtaining the strength and deformation characteristics of rocks after receiving thermal hysteresis becomes important for discussing the stability of the openings. In this study, these characteristics of rocks were examined. From the results of the tests, it was found that strength and deformation characteristics of rocks decrease with the increasing amount of thermal hysteresis. However, the ratio of the decrease also decreases. From these facts, it is supposed that the strength and deformation characteristics of rocks will converge to a constant value. Then using the results of the tests, thermal behavior of rock mass around openings were analyzed and discussed.
Fatigue characteristics of a metal diaphragm in a light touch switch are evaluated with experimental approach considering a cyclic load acting on it. The approach consumes much time and costs for evaluation. In previous paper, we have described that the mechanical properties of stainless steel have anisotropy and the fatigue characteristic must be discussed with considering the large scatter of material properties induced by the anisotropy. Therefore, the purpose of this study is to establish a simple method to evaluate the reliability of diaphragms with the consideration of anisotropy. The proposed approach consists of three procedures. Firstly, residual stress after press forming is evaluated by FEM, and feeling curves when the pushing load by is applied on the diaphragm are also analyzed. And, the stress amplitude and the mean stress at every point on the diaphragm can be calculated. Secondly, the experimental database about S-N curve and Goodman diagram considering the effect of anisotropy have been prepared. Finally, the fatigue lives considering failure probability on the diaphragm can be estimated by comparison of Goodman diagram and numerical results. From the obtained results, it is revealed that the proposed approach is very useful for the reliability evaluation of the diaphragm.
Recently, the position sensitive proportional counter (PSPC) has been becoming popular as a detector for X-ray stress measurement. However, little information is available in the literature regarding the effects of specimen mis-setting and/or collimator misalignment on the stress measurement. This is because many factors such as the stress and the X-ray diffraction broadening of specimen, the X-ray focus size, the dimensions of the collimator, the PSPC and the goniometer, et al., are complicatedly related to the effects of specimen mis-setting and/or collimator mis-alignment. The authors have presented a model of Ω assembly X-ray stress measurement using a PSPC as the detector enabling us to simulate the stress measurement under the various conditions including specimen mis-setting and/or collimator misalignment. And they have successfully reported on their effects to the stress measurement. In this paper, the incident beam methods with and without focusing are discussed using the model and the simulation method of the Ω assembly X-ray stress measurement from the view of the collimator intrinsic stress and the stress errors caused by specimen mis-setting and collimator/slit misalignment. The incident beams treated are point focus beam, collimated beam and parallel beam. As the conclusions obtained by this study, it was found that the parallel beam method gives smaller intrinsic stress than the other incident beams do. It was also found that it is possible to reduce the intrinsic stress using the focusing method for the point focused beam and collimated beam. It was demonstrated that the focusing method with the point focused beam and/or collimated beam makes the stress measurement sensitive to the specimen mis-setting but not to the collimator/slit misalignment. On the other hand, the defocusing method is sensitive to the collimator/slit misalignment but not to the specimen mis-setting.