The effects of dry grinding on X-ray diffraction pattern and electron spin resonance spectra of zinc oxide have studied. The grinding was carried out in a vibration mill with an alumina vessel, and the crystallite dimensions and lattice strains of the products were determined from their X-ray diffraction patterns by applying the Stokes collected Fourier analysis. The results obtained are as follows. (1) The electron spin resonance spectra of ground ZnO corresponded well with its lattice strains. (2) In the process of grinding, the dislocation density in ZnO crystallites increased and then decreased with decreasing internal strain of a-direction. (3) During the grinding process, the chemical adsorption density on the crystallite surface increased. This increase seemed to cause hardening effect.
The melt-sintered Al-Al2O3 dispersion-strengthened alloys have been fatigued in plane bending at strain amplitudes leading to fatigue life of about 106 cycles. Before testing, specimens were annealed at different temperatures in order to cause various states of microstructure. Optical, replication and transmission electron microscopy techniques were used to study the effects of microstructure on fatigue properties from a metallurgical point of view. It was found that the microstructure influenced the deformation mode and the nature of microcracking. In the specimens annealed at 373K subsequent to cold rolling, fatigue damage was characterized by microcracking along the boundaries of cells (less than 1μm in average diameter) introduced during the heat treatment. No dislocation cell structures produced by cyclic deformation were observed even in the vicinity of the fatigue crack. Only in some cells, many small dislocation loops were observed as a dislocation structure characteristic to cyclic stressing, although total dislocation density was not so high as those estimated for other metals. The fatigue deformation concentrated on the cell boundaries preferentially. In the specimens annealed at 773K, where recrystallization occurs, ill-defined cell structures were formed on a finer scale around fatigue cracks. Microcracking occurred in slip bands. The dispersed Al2O3 particles appear to improve fatigue strength due to the decrease in cell size and to their interaction with dislocations within cells. These results are discussed in comparison with those of pure aluminium.
As a study of the deformation of visco-elasto-plastic materials under complex loadings, the deformation behaviour of a thin-walled tubular specimen of celluloid softened by heating was investigated. Creep tests and continuous loading tests under combined axial and torsional stresses were carried out along orthogonal bi-linear stress trajectories which are related each other as a mirror transformation in the deviatoric stress vector space. The results obtained are summarized as follows: (1) The resistance to deformation of the material became remarkably small after the corner of the stress trajectory. (2) This decrease of the resistance to deformation appeared remarkably when the stress trajectory turned to the direction of small resistance to deformation. (3) The resistance to deformation in the direction of post-corner of stress trajectory was smaller than that in the direction of pre-stress. (4) After the corner, the directions of creep and inelastic strain-increment vectors did not coincide with that of stress vector. The difference of the directions of these two vectors became small with the lapse of time. This difference was influenced little by the location of stress trajectory.
By applying the combined stress state of axial tensile stress and torsional one to a thin-walled tubular specimen, the deformation behaviour of softened celluloid by heating was observed. These data were represented by a viscous-viscoelastic model proposed by Findley et al.. In this model, the strain is resolved into five components: linearly elastic, time-independent plastic, time-dependent nonrecoverable viscous (positive), time-dependent nonrecoverable viscous (negative) and time-dependent recoverable viscoelastic. The comparison of these test data with the calculation by this model shows that deformation of the material, when stress is varied continuosly by a constant stress rate or is changed stepwise under cyclic loading, may be represented by the viscous-viscoelastic model by using the creep test of stepwise constant stress followed by a recovery test after removal of stress. It seems necessary to make further investigation in order to represent the decrease of deformation resistance under the reversal of stress quite well for the case of stress reversal.
This paper presents a simplified method of predicting the nonlinear stress-strain curves up to the tensile and compressive failures at elevated temperatures for an unidirectional and symmetric biaxial laminates. The analytical procedure is based on the classical Laminate Plate Theory (L.P.T.). By applying L.P.T. to the small stress or strain increments of the stress-strain curve, a nonlinear stress-strain curve was predicted for the laminate as a continuous line. Comparisons were made between the analytical predictions and the experimental results. A fair agreement shows that the nonlinear stress-strain curve can be estimated for various cases.
In the previous paper1), the author applied the Markov process to the deformation process of particulate materials and established a mechanical model of particulate materials. In the model the concepts of the potential barriers and the potential slip plane were introduced. These concepts represent the mechanical properties of particulate materials under deformation process at the particle scale. In this paper, the experimental consideration is described in order to examine the validity of the concepts of the potential barriers and the potential slip plane. The sandy soils were poured into a water tank and deposited in a soil tank which was set at different sediment angles, θ=0°, 30°, 60°and 90°. Then, the specimens with different fabrics were prepared by sampling from the soil tank. The drained triaxial compression tests were performed under a constant confining pressure by using the saturated specimens with different initial fabrics. The followings were made clear from the experimental results: the specimen with the sediment angle θ=0°showed the greatest resistance to the deformation, the greatest strength and the most violent dilatancy; the specimen with the sediment angle θ=60°showed the smallest resistance to the deformation, the smallest strength and the smallest dilatancy. These mechanical characteristics due to the different initial fabrics were qualitatively explainable by using the concepts of the potential barriers and the potential slip plane. This fact proves the validity of these concepts which were introduced in the mechanical model of particulate materials proposed by the author.
The expansive-concrete filled steel-pipe members have high tensile and compressive strengths due to the existence of the triaxial compressive prestress in the concrete induced by the expansion of concrete. In the present paper, the mechanism of the increase in strength of the members by the prestress was investigated on the basis of the elasto-plastic analysis by use of the flow theory of plasticity. The theoretical results agreed comparatively well with the experimental ones for the specimens having various initial expansions of the concrete and volume ratios of the steel-pipe to the concrete. The tensile strength of the members was nearly equal to the cumulative strength obtained by adding the failure strength of the concrete to the yield strength of the steel-pipe, and the compressive strength was higher than the cumulative one. The plastic strain induced by the first loading became to shakedown in the subsequent cyclic loadings.
The purpose of this paper was to make an experimental investigation on the influence of tufftriding time on tensile strength and deformability of the anti-tufftride steel. The anti-tufftride steel was examined by means of the structure observation, the tension and the bending tests at the tufftriding times from 5 minutes to 180 minutes. The experimental results are summarized as follows: (1) The compound layer and the diffusion layer in the anti-tufftride steel were initially formed in the solt bath owing to the porosity of film of coating. These formations, however, were not very significant and were not seriously influenced by the tufftriding time or the coating thickness. (2) The influence of tufftriding time on the tensile strength and ductility or deformability in the anti-tufftride steel was remarkably small when compared with the tufftrided steel. The tensile strength and deformability of the 180 minute anti-tufftride steel approximately corresponded to the tufftrided steel treated for 10 minutes.
The J-integral, an extension of the J-integral by Rice, is path-independent irrespective of the constitutive relation of material and is interpreted as the energy release rate associated with extension of a crack. In the present study, the J-integral for a preloaded tensile specimen with a center crack was analyzed on the basis of the flow theory of plasticity by the finite element method. The conclusions obtained are as follows: (1) Under monotonically increasing load, the value of J is in good agreement with that of J. (2) If the load is reduced from its current value, J decreases almost linearly and takes a negative value owing to the compressive residual stress produced by yielding at the crack tip. (3) In the case where the load in reloading stage is brought over its original value, the value of J agrees approximately with that of J calculated from the load versus load-point displacement curve with the simple formula by Rice, Paris and Merkle. (4) If the specimens unloaded from various load levels are reloaded until the crack tip opening displacement (CTOD) reaches a constant value, J also becomes a constant value.
Smooth specimens of 0.04% C annealed steel were fatigued under completely reversed plane bending. The results of microscopic observation of fatigue crack initiation and early propagation conducted both on the surface and in the interior of the specimen were discussed based on micromechanics. The followings are the summary of the study: (1) The initiation site of cracks was mainly along the grain boundary on the specimen surface and the path of most cracks changed from along the grain boundary to inside the grain at the depth about 1μm. The shape of small cracks was complicated by the grain boundary and orientation, and the crack length-depth ratio was scattered between 0.3 to 1.5. The average plane of microcracks was plane of the maximum shear stress resolved from the applied stress. (2) Above the fatigue limit, the relation between the stress amplitude, σa, and the initiation life, Nc, of the crack in the order of the grain size was expressed by the following equation derived from the Tanaka-Mura theory of fatigue crack initiation σa=148+3.83×104Nc-1/2 where 148MPa is the friction stress. The fracture surface energy used in the theory was 1.4× 105J/m2 for the present material. (3) Below the fatigue limit, non-propagating microcracks whose length were below the grain size were detected, and the aspect ratio of those cracks was around 0.6. The condition of microcrack growth at the fatigue limit was explained from the criterion of the critical microscopic stress intensity factor at the tip of slip bands emanating from microcracks. The critical value was determined to be 0.74MPa√m.
Rotating bending fatigue tests were performed on three high tensile-strength aluminum alloys, i. e. ZK41-T6 extruded, 2024-T6 extruded, and drawn Al alloy in various environments in order to investigate cup and cone fatigue fracture. Cup and cone fracture was only observed on ZK41-T6 extruded Al alloy tested in the environments of argon gas, nitrogen gas, or mineral oil. It is thus concluded that environment is one of the factors that control the cup and cone fracture of high tensil-strength Al alloys. In argon or nitrogen gas, fatigue strength was smaller than in air, due to the occurrance of cup and cone fracture. In the oil environment, fatigue strength also decreased below that in air, because oil wedge action against fatigue crack growth cannot be expected for cup and cone fracture. From the air-oil two stage fatigue tests, it was observed that the mechanism of rupture changed from tear to sliding rupture when the environment was changed, and that small cup and cone fracture appeared. However, the fatigue life increased as compared with that calculated from Miner's law.
In order to investigate the influence of surface layer on the corrosion fatigue properties of metal prestrained by pulling, high cycle fatigue tests were performed on aluminum alloy (2017) sheet specimens under completely reversed plane bending stresses in laboratory air and in salt water (3.0% NaCl). The distributions of crack length at various stages of fatigue lives were examined, and these distributions for the specimen with surface layer (Non-polished) were compared with those for the specimen without surface layer (Polished). The conclusions obtained are summarized as follows. (1) The crack length was found to be arranged by a single Weibull distribution in air and in the early stage of corrosion fatigue. In the latter half of corrosion fatigue, the distribution of crack length was made up of two types of crack groups. One is the crack which propagates individually, and the other is the crack which is accompanied with crack connection. (2) The distributions of crack length at a certain stress cycles was able to be explained by a statistical calculation, which takes into account both the variation of number of cracks during stress cycling and the scatter of crack propagation rate. (3) The influences of surface layer on the both crack initiation and propagation behaviors in corrosion fatigue were explained on the basis of Weibull distribution of crack length and the statistical calculation as mentioned above.
Recently, considerable attention has been paid to the strength problem of the adhesive-bonded joint because of its wide usage to many mechanical structures, e. g., automobile, airplane and so on. But there exists some uneasiness for use of this type of joint, for no reasonable design standard has been established yet. So, the authors planned and conducted a series of the experiments to research the effect of surface roughness of the adherend specimen on the adhesive strength under the torsional shear loading condition by using a thin wall cylindrical butt joint specimen, where an epoxy resin (Epikote 828) mixed with a hardner (TEPA) was used as an adhesive, structural steel (JIS. S45C) as an adherend and furthermore polysulfide (Thicol, LP-3) as a plasticizer. The rigidity of the adhesive and the absorbed energy of the adhesive-bonded specimen in impact tests were varied to some extent by controlling the post curing temperature or by adding the plasticizer, and thus obtained three sorts of the adhesive-bonded specimens having different mechanical properties were used in this experiment. From the results, it was observed that the surface roughness of the adherend specimen gave the influence on the shear strength in somewhat different manner depending on the mechanical properties of the adhesive used. To explain the strength behavior obtained in this study, discussions were tried based on fracture modes considered in connection with the adhesive/adherend interfacial feature of each adhesive-bonded butt joint specimen.
The 105hr rupture strength, which may be obtained by the creep rupture testing over the period of more than twelve years, will undoubtedly provide us a crucial keynote for the design of power generating facilities as well as chemical plants. In this respect, much effort has been made to predict the strength by use of several extrapolation methods. Unfortunately, however, few seem to have been successful. On the other hand, it is a standard and necessary practice for engineers particularly in electric power companies to evaluate, in short time, the creep properties of defective materials in case of troubles. Hence, they are in an urgent need to know how they can estimate and what is the best method for the prediction of creep rupture strength. To meet the aforementioned requirement, an attempt has been made in the present study to clarify to what extent each extrapolation method is successful for the prediction. Five extrapolation methods have been selected and applied to the creep data originally published by the National Research Institute for Metals. In fact, these data are outcomes of intensive and consecutive work of creep testing of more than several years. 3×104hr and 105hr strength of twentyeight heats of three different types of high temperature steels have been predicted by using these methods on the basis of a vertually limited size of creep data (i. e. partial aggregation of creep data). Secondly, the predicted strength was compared with the corresponding actual data with their coincidence being evaluated statistically. It was found that 3×104hr rupture strength can be predicted within fifteen percent error by using Larson-Miller equation with 3rd order regression on the basis of the creep rupture data up to 3×103hr.