The acceleration of fatigue crack growth under periodic overstressing was studied with centernotched plate specimens of low-carbon steel under push-pull loading. Tests were carried out in moist air, dry air, nitrogen, and vacuum. A very small number of cycles of overstress applied intermittently during a very large number of cyclings of undersress below the theshold stress intensity range ΔK_th caused a significant acceleration of crack growth in the range of relatively large understress ΔK₁(ΔK_th/ΔK₁>0.5) in moist air, dry air, and nitrogen. In the region of relatively small understress, however, acceleration of carck growth in moist air was lesss than that in dry air and nitrogen. It was found by means of Auger spectroscopy that the maximum thickness of excess oxide within the cracks enhanced crack closure and reduced the effective stress intensity range of understress, (ΔK₁)_eff, resulting in less acceleration of crack growth in moist air. Accelaration of crack growth in vacuum, on the other hand, was lesss than that in other environments in all understress levels. Fractographic study suggested that rewelding in vacuum resulted in the less acceleration. There was no sequential effect of overload on the acceleration under two-level variable stress. The acceleration under multi-level variable stresses was predicted by a method using a linear cumulative concept on the basis of test data under simpler two-level variable stresses.

抄録全体を表示

This paper presents a new scheme for the prediction of unstable ductile fracture of statically indeterminate structures. In our previous studies, the method called `statically indeterminate fracture mechanics' was proposed for such structures to evaluate the stress intensity factor and to predict brittle fracture. In this paper, the above method is, at first, extended to evaluate the J-integral of an axially cracked cylinder under body force and internal pressure where a complimentary energy concept based on the deformation theory of plasticity is used to evaluate the J-integral and the deformation of both cracked and uncracked members of structure. Next, the proposed method is applied to the ductile fracture prediction of an axially cracked spinning cylinder, and the following results are obtained. Comparison with round-robin analysis shows the accuracy and the applicable limitation of the proposed method. Discussion on the stress distribution of the cracked section during the present analysis shows the new suggestion of J-controlled crack growth conditions of unstable ductile fracture analysis.

抄録全体を表示

This paper deals with the two-dimensional contact problem of a rolling rigid cylinder on an elastic half-space containing two surface breaking cracks located close to each other. The problem is solved using complex-variable techniques and is reduced to a pair of singular integral equations which are solved numerically. Numerical results of stress intensity factors are obtained for the case of two parallel cracks. The interference effects on the stress intensity factors with the distance between two cracks, and the effects of frictional coefficient and the ratio of the cracks on the results, are considered.

抄録全体を表示

The fatigue crack initiation and propagation behaviors were investigated on the Ti-6Al-4V alloy specimens which were hydrogen charged in 3% sodium chloride or lN sulfuric acid aqueous solution. The number of cycles to crack initiation obtained for notched tpecimens, N_i, was smaller in the specimens tested under hydrogen charging than in those tested in air when the range of the apparent stress intensity factor ΔK_a was relatively large, while when ΔK_a was small, N_i was rather large for the charged specimens. The crack propagation velocity da/dN was smaller in the specimens tested under hydrogen charging than those tested in air. The decrease in da/dN of the former specimens corresponded to the increase in crack closure : i. e., the relation between the range of effective stress intensity factor ΔK_eff and N_t for two conditions was expressed by the same line. According to an X-ray analysis, the main reason for the enhancement of crack closure in the hydrogen charged specimens seemed to be the expansion due to the hydride formation near the specimen surface but not on the crack propagation plane (a kind of phase-transformation-induced crack closure).

抄録全体を表示

In order to investigate the stress relaxation behaviors at the specimen surface during the corrosion fatigue process caused by the initiation and propagation of many cracks, rotary bending fatigue tests were carried out in a sodium chloride aqueous solution by using specimens of 6 and 12mm diameters. Stress decreases of about 25∼50 % were observed during the corrosion fatigue process performed at low stress amplitude. These stress relaxation behaviors are well evaluated by calculating the compliance increase of the specimen. Computer simulations of the corrosion fatigue process of unnotched specimens were conducted. The results showed good agreement with the experimental results. In the simulations, probabilistic crack initiation behaviors during the corrosion fatigue process, the stress relaxation effect at the cracked parts and crack coalescence behaviors among many distributed cracks are taken into consideration.

抄録全体を表示

Structural ceramics are attracting attention in the development of space ships, aircraft and nuclear fusion reactors because they have excellent wear-resistant and heat-resistant characteristics. However, in some applications it is anticipated that they will be exposed to very high temperatures of the order of thousands of degrees. Therefore, it is very important to investigate their thermal shock characteristics. In this report, the distributions of temperatures and thermal stresses of cylindrically shaped ceramics under laser beams are discussed using the finite element computer code (MARC) with arbitrary quadrilateral axisymmetric ring elements. The relationships between spot diameters of laser beams and maximum values of compressive thermal stresses are derived for various power densities. From these relationships, a critial fracture curve is obtained, and it is compared with the experimental result.

抄録全体を表示

A study was conducted on the effect of surface cracks on the tensile strength of ceramics used for the lower-quality fueled gas turbine or diesel engine. The former study already reported that the existence of artificial corrosive ash containing vanadium pentoxide and sodium sulphate on flaws significantly enhances the tensile strength of the partially stabilized zirconia at 650°C where the specimens were heated with an electric furnace. This report deals with the burner rig test results of ceramics having considerable surface defects. After 49-hour exposure in a 900°C corrosive gas stream, the tensile strength of the silicon nitride specimens which had a surface defect induced with the vickers indentation load of 30kgf considerably increased. This strength is the same or better then that of smooth specimens.

抄録全体を表示

This paper presents a numerical study on thermal ratchetting of a thick cylinder, which was subjected to an axially moving temperature distribution in order to verify the ratchetting mechanism. Recently, the need to understand this kind of problem arises in the design of structures which are operated at elevated temperatures, for example, fast breeder reactors, gas turbines, supersonic airplanes, and so on. An elastic-plastic analysis is presented using an axisymmetric finite element method. As a result, thermal stress is induced by axially moving temperature distribution. Then, the plastic deformation increases during each cycle of the thermal transient. It is shown that thermal ratchetting is a dominant factor of thermal loading and causes unacceptable changes in dimension.

抄録全体を表示

A solution is given for the transient thermal stresses in a circular disk with a circular hole due to a moving heat source. The heat source suddenly generates at a starting point and rotates around about the circular hole at a constant angular velocity. The transient temperature distribution is obtained by making use of Green's function and Laplace transformation. The associated transient thermal stresses are analyzed by using the thermoelastic potential and Neuber-Papkovich stress functions. Numerical calculations are carried out for the case of thermally insulated disks. The effects of angular velocity and hole size on the thermal stress distributions are considered.

抄録全体を表示

The stress intensity factors of a small crack have been analyzed when an infinite plate is step-functionally heated in a circular region near the crack tip. The stress solution can be obtained by superposing the stress disturbance by the crack on the transient thermal stresses due to the heat source in an infinite plate. The problem is reduced to a singular integral equation which satisfies the boundary condition on the crack face. The numerical results of stress intensity factors are obtained for the case of a uniformly distributed heat source. The effects of the location of the heat source on the variations of stress intensity factors are considered.

抄録全体を表示

In the impact 3-point bending test, it had been shown that a loss of contact between the specimen and the anvils occurred in the early stage of the impact, and then the specimen was subjected to the reaction force from the anvils. The boundary conditions usually assumed for the simply supported beam are invalid for such a case. In the present paper, a simple method of evaluating the dynamic stress intensity factor which takes into account such behavior of the specimen during impact is presented. The specimen was modelled as the beam with a crack and analyzed based on Timoshenko's beam theory. A numerical example concerning the experimental results performed by Bo^.. hme et aI^.. was presented and its results were compared with the measured values by Bo^.. hme. As a result, a good agreement was obtained between the values. Furthermore, the procedure to estimate the reaction force from the anvils was also proposed. By using this procedure, it became possible to evaluate the dynamic stress intensity factor without measuring the reaction force.

抄録全体を表示

A bending tests of high strength aluminum alloy are carried out under hydrostatic pressure and with backup compression provided mechanically, and the stress and strain histories in the processes are calculated by the elastic-plastic FEM. The experimental results and the calculated results are used to study the fracture criteria. From the results, it becomes clear that the slip type fracture which occurs in bending with high backup compression using a small radius punch can be predicted by the criterion of a maximum shear stress, and the Oyane's criterion is successful in prediction of the critical punch strokes in bending in which the normal type fracture occurs. But the constants of Oyane's equation in bending do not agree with that in tensile test and in compression test.

抄録全体を表示

This paper discusses the bending problem of a uniformly or concentrically loaded circular plate with mixed boundary conditions, i.e., a circular plate which has clamped and free portions of boundaries. The results calculated by the authors are compared with those by another investigator and are also compared with experimental results obtained for the case of a concentrated load.

抄録全体を表示

In order to prepare a mirror-quality end of an optical glass fiber available for light communication, some devices of the breaking procedure have been on the market and used in the field. However, a complete mirror quality end surface cannot always be produced by breaking. Hence, a compact method to judge the fracture surface of the fiber must be developed. In the present study, AE detection, suggested from the mutual relationship between the released fracture energy and acoustic emission (AE) pulse, was experimentally examined to be suitable as a compact method to judge the quality-end surface produced by breaking, The ratio of the mirror-size to the fiber diameter can be controlled by changing the size of the cutter-cracked defect. Then, the AE pulse emitted on breaking and the fractography of the fiber by SEM were compared with each other. Through this comparison for all fibers tested, it was found that the ratio of the mirror-area to fracture surface is mutually related to the peak voltage and frequency of the AE pulse.

抄録全体を表示

Hydrogen damage or attack is produced in steels exposed to a high-pressure hydrogen environment at high temperatures. This study was done to investigate the dynamic fracture processes of hydrogen-attacked steel. Some samples of high-carbon steel with differet hydrogen exposure times were prepared, and acoustic emission signals during fracture toughness testiness were detected, located and analyzed. The acoustic emissions showed the different characteristics according to the progress of the hydrogen attack. The fracture sources were quantitatively characterized in terms of crack size, crack formation speed and fracture mode.

抄録全体を表示

A functionally gradient material which decreases thermal stresses has been developed for structural components and/or mechanical elements in fields such as nuclear, aircraft and space engineering. Steady state thermal stresses in a plate made of the functionally gradient material are discussed. The main theme of this subject is how to distribute the component of the material in the functionally gradient material to decrease thermal stress. We discussed the general method to decrease thermal stress in laminated plates made of two and three layers. The plate of an optimally functionally gradient material was determined to decrease the steady thermal stresses.

抄録全体を表示

This paper deals with the stress concentration analysis of shoulder fillets in a round test specimen under torsion and in a flat plate under anti-plane shear. The stress fields induced by the ring force acting to the tangential direction in the infinite body and the anti-plane shear force in the infinite plate are used as fundamental solutions to solve those problems. The stress concentration factors are systematically calculated under verious geometrical conditions. Through the comparision of the present results with the previous research works, it is found that the results of Matthews-Hooke and of Tsuji et al. are in good agreement with the present results. The stress concentration factors are illustrated in tables and diagrams so they can be used easily in design or research.

抄録全体を表示

The cyclic behavior of recovery strain under constant maximum stress in TiNi shape memory alloy is investigated experimentally. The influence of initial strain and the heating temperature in the thermal cycle on cyclic characteristics of recovery strain is discussed. The main results are summarized as follows. The amount of strain variation under the thermal cycle is significant for initial strain in the region of stress-induced martensitic transformation (SIMT), but is slight for initial strain before and after the SIMT. For maximum strain in the region of SIMT, the recovery strain varies during the thermal cycle. The variation of recovery strain is significiant in the early cycles. Recovery strain under the thermal cycle increases with the heating temperature, but the increasing rate of recovery strain decreases. For initial strain in the region of SIMT, strain at low temperature increases to the finishing strain of SIMT during the thermal cycle. The transformation temperatures at which strain decreases or increases during heating or cooling process become higher with the number of cycles.

抄録全体を表示

The cyclic behavior of recovery stress under constant residual strain in a TiNi shape memory alloy is investigated experimentally. The influence of residual strain and the heating temperature in the thermal cycle on cyclic characteristics of recovery stress is discussed. The main results are summarized as follows. Stress at high temperature, stress at low temperature and recovery stress decrease in the early cycles but become almost constant after these cycles. Recovery stress under the thermal cycle decreases with residual strain. Stress at high temperature depends slightly on residual strain. Both recovery stress and stress at high temperature increase with the heating temperature but the increasing rate of the stresses decreases. The amount of decrease of stress at high temperature under the thermal cycle is smaller than that of yield stress of stress-induced martensitic transformation in the pseudoelastic region.

抄録全体を表示

The influence of training and shape memory processing temperature on cyclic deformation of TiNi shape memory alloy helical springs was investigated. In the experiment, an offset crank shape memory alloy heat engine was used. The results are summarized as follows. For large strain, significant nonrecoverable elongation in the early cycles appears. The appearance of the nonrecoverable elongation can be avoided by training. The recovery force of the spring varies slightly, which shows the very stable cyclic characteristics of the force. The cyclic characteristics of deformation in the helical spring depend on the size of the region accompanied by stress-induced martensitic transformation in the cross section of the wire. In the case of low shape memory processing temperature, the nonrecoverable elongation is small, the axial force at high temperature is high and the number of cycles to failure is small.

抄録全体を表示

The resonance method of three-dimensional free vibrations with the cylindrical specimen, which had been developed by the authors, was applied to determine the elastic constants of polycrystals in a temperature range of 4-300 K. The temperature dependence of elastic constants of pure aluminum agreed well with the published data measured by the ultrasonic-pulse-echo technique over the temperature range of measurements. The sluggish change induced by the magnetic transformation could also be detected in niobium. In addition, the temperature dependence of elastic constants for superconducting Al-Si and Al-Ge supersaturated solid solutions was examined, and also the Debye temperature could be determined by extrapolating the elastic constants to the values at O K.

抄録全体を表示

Two-dimensional stress waves in an elastic circular cylinder with a thermal gradient were analyzed numerically when an axisymmetric impulsive load was applied to the end face of the cylinder heated (or cooled) to produce a thermal distribution. For numerical analysis, the finite difference method based on integration along bicharacteristics was employed. By obtaining the numerical results for the cases of various temperature profiles and impact loading conditions, it is clearly demonstrated that stress wave propagation is affected by the thermal gradient, and that when the thermal gradient in the pressure bars of the split Hopkinson bar becomes steeper, experimental results by the conventional method contain more noticeable errors.

抄録全体を表示

In the first report, a new expression of a constitutive law for elastic-plastic material was proposed using two transition parameters which describe the stress increment dependency. These parameters denote the magnitude and the direction of plastic strain increments. In this report, these are determinded by the biaxial loading test of aluminum specimens using testing machine constructed by trial and by the numerical simulation based on the KBW model. The results show that the stress increment dependency is the most considerable at the simple tension and decreases when the stres vector is turned toward the direction of the pure shear.

抄録全体を表示

Two-dimensional stress waves in an elastic/viscoplastic circular container filling with liquid were analyzed numerically, when axisymmetric impacts were applied to the end of the container. For this numerical analysis, the modified finite difference method based on integration along bicharacteristics was employed. By obtaining the numerical results for the cases of various sizes of the container, and of the impact loading conditions, it was clearly demonstrated that a large tensile stress arises at a solid-liquid interface. It is also found that, when viscoplastic deformation takes place, the maximum value of the caused tensile stress is smaller than that when only an elastic deformation takes place.

抄録全体を表示

The present paper proposes a constitutive equation for rubberlike materials from an elastic potential using the irrotational strain, which is obtained by integrating strain rate excluding rigid rotation effects from the stretching tensor. The obtained constitutive equation gives a nonlinear stress-strain relation involving higher-order terms of strain, and includes an elastic linear relation for metal alloys as a special case. Described are a theoretical treatment for perfect incompressible material and an expression in terms of principal values. A good agreement with the experimental results for typical loading problems, such as simple shear and simple elongation as well as uniform 2-dimensional extension cases, shows the validity of the present theory.

抄録全体を表示

The Brinell hardness H_B of elastic-linear-hardening materials with various yield stresses and hardening moduli is analyzed by FEM. The applicability of the empirical formula for the ratio of tensile strength σ_B(MPa) to H_B(kgf/mm²), σ_B/H_B ≌ 3.5 is lost for materials having large relative strain hardening modulus H/σ_S, where H is the strain hardening modulus in σ-ε curve and σ_S is the yield stress. The dimensionless stress∼strain curves (σ/σ_S∼ε) of elastic-linear-hardening materials and those of various actual metals give the crucial measure to determine the value of σ_B/H_B. The ratio of σ_B/H_B remains in the range 3.3∼3.8, if the relative strain hardening modulus H/σ_S is less than ∼2.5at large strain. Several other useful results such as indentation shapes, plastic zone size and residual stresses are also obtained.

抄録全体を表示

The contact stress distribution, the force ratio (the relationship between an increment of bolt axial force and an internal pressure) and the gasket properties (the gasket seating width and the moment arm) in a bolted joint of a cover of the pressure vessel with a raised face metallic gasket are analyzed as a four-body contact problem using an axisymmetrical theory of elasticity. In the analysis, a hub of the body is taken into consdieration, and a cover, a raised gasket, a body and a hub are replaced with a finite solid cylinder and finite hollow cylinders, respectively The effects of the stiffness and the thickness of gasket on the contact stress distribution and the gasket properties are obtained by numerical calculations. Experiments were carried out concerning the force ratio, the maximum stress produced in bolts and the stress producd on the hub. The analytical results are satisfactorily consistent with the experimental results.

抄録全体を表示

This paper reports the experimental data of our'research concerning the yielding of mild steel plates under tensile stress. Yielding was induced at a longitudinal section of the plate by frictional force which was applied to the surface of the plate under uniaxial compression. An analytical explanation of the results was attempted by proposing a new coefficient of friction which was a constant relationship between the frictional force and the strain on the surface of the plate. The analytical explanation is supported by the results of our experiments.

抄録全体を表示

This paper describes the accuracy of surface integrals in the indirect boundary integral method containing singular integrals. Here, the surface integral is carried out directly and numerically by the use of Simpson's and Legendre-Gauss's quadrature formulae. Also, the line integral, obtained by applying Gauss-Ostrogradskii's transformation to the surface integral, is examined. The accuracy is evaluated by the convergence behaviors of surface and line integrals and by the comparison with the analytically obtained integral values.

抄録全体を表示

The expert system which diagnoses the sensitivity of environmentally assisted cracking (EXENAC ; EXpert system for ENvironmentally Assisted Cracking) has been developed using the OPS83 programming language. This system is composed of three independent systems, which respectively diagnose the sensitivity of I ) stress corrosion cracking of austenitic stainless steels in a low- or high-concentration chloride solution and/or high-temperature water, II) hydrogen embrittlement of high-strength steels in various aqueous solutions and gaseous environments, and III) hydrogen attack of steel in highly pressurized high-temperature hydrogen gases. The knowledge base consists of production rules, which have certainty factors to represent the uncertainty (contribution) of each rule to environmentally assisted cracking. Applying the developed expert system to the case histories the diagnoses were accurately made. We also proposed the quantitative measure to evaluate the importance of each rule in the diagnosis through the use of the developed expert system.

抄録全体を表示

The Boundary element method (BEM) is a useful engineering tool for structural analysis but there are various strategies to obtain the accurate solutions. We developed an expert system for two-dimensional elastostatic BEM analysis on the engineering work station (EWS). This system supports the input of shapes and boundary condition, modelling, BEM analysis, error estimation, adaptive meshing and post-processing. This paper describes the development of the user interface and the expert system for BEM modelling and meshing, which are developed by using object-oriented programming and representing of knowledge. It is found that the object-oriented representation of knowledge is very useful in the design of user interface and expert system in this study.

抄録全体を表示

The development of an expert system for two-dimensional elastostatic BEM analysis has been presented in the 1st report. This paper describes a new efficient non-residual type error estimator by using the direct regular method (DRM) and an adaptive meshing method based on the estimated error. Numerical results on the stress concentration problem as well as on the crack problem are given and the good accuracy and convergence of this error estimator are presented. This a posteriori rror estimation and adaptive meshing method are implemented into the expert system for BEM analysis which generates the BEM mesh based on priori error estimation. In the system developed here, the user can estimate the error for the mesh generated by the expert system and refine the mesh automatically to obtain the optimal mesh.

抄録全体を表示

This paper presents a new inverse analysis method which combines knowledge engineering technology with mathematical programming procedures. First, the current status of mathematical programming technology is briefly surveyed, and several issues on the technology are defined. Then, our new version of D^-1-prolog, which performs heuristic controls over optimization algorithms, is introduced. The key concepts of heuristic controls are qualitative classification of design variables and generation of subregions based upon the landmark values of design variables. A visualization of global shapes of objective functions is also presented. Finally, the application of this method to the identification of geometric parameters for microstructures in Functionally Gradient Materials is shown.

抄録全体を表示

Acoustic emission (AE) source wave analysis is a new nondestructive technique for the investigation of the dynamic fracture process. We applied this technique to the quantitative characterization of crack sources in brittle fracture. Fracture toughness testings of S80C steel were carried out, using six transducers to detect, locate and analyze AE signals. The dynamic process of cleavage fracture was revealed through the AE activity and the source location. The fracture sources were quantitatively characterized in terms of crack sizes and crack formation speeds.

抄録全体を表示