A microscopic process of separating rock particles in drilling by means of a core boring bit is examined on the basis of fracture mechanics. When rock is drilled, concentrated loads are applied through the face stones of the core boring bit to the drilled surface of the rock. By modeling the microcracks on the drilled surface as edge cracks perpendicular to the surface of a semi-infinite elastic body, the problem of crack extension from the tip of the microcrack is analyzed by means of the method of continuous distribution of dislocations. The stress intensity factor at the tip of the edge crack is first discussed, and then numerical simulation of crack extension from the tip of the edge crack when the concentrated load approaches and passes the edge crack is presented.

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Laboratory hydraulic fracturing experiments were conducted on uniaxially loaded cubic specimens of anisotropic granite to investigate the effect of anisotropy on the hydraulic fracture, and to exploit a method for predicting the breakdown pressure based on uniaxial tensile strength. Based on uniaxial tensile strength data, the results of the hydraulic fracturing experiments were analyzed with respect to the fracture angle and breakdown pressure. It was shown that the concept of a critical tensile stress at a characteristic distance, modified to account for the anisotropy of tensile strength, was able to explain the observed extension behaviours of hydraulic fractures.

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An inverse problem to determine the crack size in nondestructive testing of closed cracks by ultrasonics is analyzed. Ultrasonic testing using a normal incident longitudinal wave is treated for sizing a crack which exists parallel to the measuring surface. Ultrasonic waves pass through the closed crack surfaces, and hence usual techniques applied to open cracks cannot be applied to size closed cracks. Reflection of the wave at the closed crack surface is analyzed based on dynamic elasticity by introducing a model of crack closure with unknown parameters. The unknown parameters are determined along with the unknown location of the crack tip by the inverse analysis, which compares the intensity of the reflected wave obtained through both theory and experiment. The ultrasonic measurements for a cracked specimen under several compressive loadings have shown that the present analysis offers a powerful tool for precise sizing of closed cracks.

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The theoretical study for quantitative evaluation of material quality by the backscatter of an ultrasonic Gaussian beam has been made. The density and location of flaws, such as microcracks and porosities can be estimated by the change of electrical power due to the backscatter. The power change is theoretically formulated based on the dynamic reciprocal theorem. Some kinematical postulations are used for simplicity to neglect the multiple scattering effect among the flaws; the characteristic dimension of a contained flaw is very small comparing to the incident wave length, and the flaw distribution is sparse. Numerical calculations of the analytical backscattering power have been carried out. The power is approximately proportional to the flaw density. Thus, the ultrasonic backscatter measurement can be made viable to estimate the flaw density.

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An experimental study was made on the extension behavior of the damaged region ahead of a notch tip in a continuous glass fiber-reinforced cross ply composite. The damage consists of a group of microcracks which extend in each ply of the composite. By applying the concept of fracture mechanics directly to these microcracks, the essential factor controlling the damage extension behavior is explained. In this study, elastic strain energy released by microcrack extension was measured by using a compliance method with quick unloading, which is separated from inelastically dissipated energy due to the viscoelastic character of epoxy resin. The experimental result shows that the released elastic strain energy is proportional to the total area of microcracks. This fact means that the damaged region extends under the condition of constant released elastic strain energy for unit area of microcracks.

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In order to investigate the effects of plate thickness and crack shape in cross section on the threshold condition for crack propagation under the mixed mode, a through or a surface precrack was made in a carbon steel plate with various thickness by a plane bending or a pulsating tension test. It was concluded that the magnitude of the frictional stress acting on the crack surface at the propagation threshold under cyclic torsion was remarkably affected by the plate thickness, so the accurate estimation of the threshold stress of crack propagation under the mixed mode with an optional plate thickness must be based on the strain energy density criterion with use of the plane bending and cyclic torsion tests of identical plate thickness.

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Effects of grain size on Intergranular crack propagation properties under low-cycle fatigue were investigated using austenitic SUS304 stainless steel at 973K in air. Wave shapes used were strain-controlled slow-slow and slow-fast triangular ones. Small crack propagation rate (SCPR) and large crack propagation rate (LCPR) were examined. The SCPR was lower in the specimens with a smaller grain diameter, and increased in proportion to about D^0.7 in the tests with slow-slow and slow-fast wave shapes. The LCPR increased with the grain size in proportion to about D^0.5 in the tests with slow-fast wave shape. In this type of wave shape, the main crack coalesced with the wedge-type cracks which were initiated at the front of the main crack. The dependence of LCPR on grain size was much the same as that of crack initiation life, as analysed by the authors.

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Three-point bending and cyclic bending fatigue tests were conducted on two kinds of hot-pressed silicon nitride. To examine the relationship between the defect size 2c and the static and fatigue strength σ_F, σ_w respectively a semicircular crack with a radius of 70μm to 310μm long was introduced on each specimen surface with the aid of a Knoop indentation. The result shows that up to a critical value of 2c_c, both σ_F and σ_w were insensitive to 2c. For 2c > 2c_c, both σ_F and σ_w decreased with increasing 2c. Both materials had almost the same value of 2c_c. The fatigue strength of the materials with a small crack was found to be about 40% of the static bending strength.

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Mechanical structures are usually subjected to an atmospheric environment, where the temperature and the humidity vary depending on the place, the season and the time of day. In order to examine the effect of atmospheric conditions on the fatigue life distributions, statistical fatigue tests in rotating bending were carried out on carbon steel for machine structural use (JIS:S35C) under the following five conditions; 1) 35°C-WET, 2) 35°C-DRY, 3) 20°C-WET, 4) 20°C-DRY and 5) 3°C-Room Air. Based on the experimental results, it was found that the temperature was the predominant factor in the fatigue life distribution rather than the humudity. The temperature dependence of the fatigue life distribution was then analyzed, and an analytical procedure was developed to evaluate the effect of the atmospheric conditions on the fatigue life distribution.

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Fretting fatigue tests at elevated temperature were carried out using shot-peened specimens of 12Cr-Mo-W-V steam turbine steel. The shot peening process increased the fretting fatigue strengths by a factor of 1.8 at an elevated temperature (773K) as well as at room temperature. This is mainly due to the compressive residual stress induced by shot peening. In order to investigate the effectiveness of shot peening for long-term practical use, the fretting fatigue tests of the specimens shot-peened and subsequently exposed for long periods up to 30000hrs at 773K were carried out. Although the compressive residual stress was reduced by a factor of 3 after this process, the fretting fatigue strength at 773K increased by a factor of 1.8. The effectiveness of the shot peening was found for long-period practical use at 773K. Prediction of fretting fatigue lives was made on the basis of elastic-plastic fracture mechanics, where the compressive residual stress by shot peening and the frictional force between the fretting pad and the specimen were taken into consideration. The predicted lives were in good agreement with the experimental results.

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Although the AE method provides useful information on the operating fracture processes of materials, a synthesized evaluation of AE signals using several AE parameters is necessary for composite materials because AE signals arise from various fracture processes. In this paper, the relationships between several AE paramaters and AE original sources were investigated, and fracture mechanisms of CFRP are discussed in terms of observations of fracture surfaces and internal damage of the laminates by a SEM and a scanning acoustic microscope (SAM), respectively. Synthesized AE signal wave analyses including the distributions of AE amplitude, AE event duration, and counts/duration (corresponding to frequency), give us useful and powerful information to analyze the complicated fracture mechanisms of composite materials. The fracture in tensile tests and cyclic tensile tests of unidirectionally reinforced carbon/epoxy composites goes through fiber/matrix interface debond and fiber breaking.

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A new numerical method is presented for computing the transient waves in a composite laminated plate excited by a line impact load. The laminated plate is divided into N plate elements, and the principle of virtual work is applied to reduce the two-dimensional equation of motion to a one-dimensional one. The method of Fourier transforms in conjunction with modal analysis is used to determine the response of displacements. Two numerical examples are computed and discussed; one is for an isotropic plate and the another is for a hybrid composite laminated plate.

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One of the major objectives of this paper is to offer a practical tool for stochastic materials design of unidirectional composite laminates under the in-plane multiaxial load. Quadratic polynomial failure criteria and fist order reliability method (FORM) are applied to construct the evaluation model of probabilistic safety of composite laminates. From the numerical analysis of T300/5208, the relation between the applied stress vector and safety indices β is discussed. A new design diagram which corresponds to safety index or failure probability is proposed. The stochastic failure envelope diagrams, which are drawn in the in-plane strain space, enable one to evaluate the stochastic behavior of composite laminate with any lamination angle under multiaxial stress or strain condition.

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A boundary element technique for elastodynamic contact problems is suggested by using the penalty function method. We consider an impingement between two elastic bodies which must satisfy contact conditions of the displacements and tractions on the interface boundary corresponding to contact states such as open, sticking and slipping states. A functional of the virtual work is adopted and the contact conditions at each time step are assembled in the functional by using the penalty function method. An integral representation which provides the displacement field at each time step in terms of boundary values of displacements and tractions is derived with the aid of the method of weighted residuals. This formulation does not need to expand or decompose coefficient matrices in a final system of algebraic equations to satisfy the contact conditions. Results of impinging problems between long bars and between circular disks are also described.

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The boundary element method (BEM) has been used in various fields because BEM is convenient compared to FEM. BEM does not need an area integral in two-dimensional elastic problems without body forces, but in unsteady two-dimensional thermoelastic problems, the area integral is usually necessary. A method of transforming the area integral in BEM for two-dimensional unsteady thermoelasticity to a boundary integral has been recently reported [1]. However, a cell was used in a numerical example, and the time integral was carried out numerically. This paper shows that the time integral can be carried out analytically. The values of the stress discontinuities on the boundary are also clarified. In conclusion, it is shown that the same equation can be used in both the internal area and on the boundary.

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A three-dimensional coupled thermal stress analysis for an infinite elastic slab is developed in this paper, and an analytical solution is given under the thermal condition such that the nonsymmetrically distributed local heats are supplied from both surfaces of the slab. In the analytical process, we divided the problem into two parts, one of which corresponds to the case of symmetrical surface heat supply and the other, the case of an antisymmetric supply. Then, we analyzed these cases individually and obtained the required solutions. As an illustration, numerical calculations are carried out for the case of local heating from one surface of the slab. The temperature distributions and the associated thermal stress distributions are represented in figures, and the influence of the thermomechaical coupling effect on these numerical data is examined briefly.

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Residual-stress redistribution and the resulting deformation during the formation of a full-depth hole in a plate with residual-stress distribution along the width and the thickness were elastically calculated using the three-dimensional finite-element method. The initial residual stress was introduced into each element through the intermediary of the most suitable eigen-strain distribution in such a way that its redistribution due to the surface removal accorded with the X-ray residual stresses on the surface removed successively by electropolishing. The experimental results of the residual-stress redistribution and the resulting deformation due to hole cutting on the electric discharge machine were in good agreement with the analytical results. It was also found in the analysis that the shape of the hole was influenced by the residual stress released in the process of creating the hole.

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Solutions to a state of plane stress and to a state of generalized plane stress of transversely isotropic, moderately thick plates in rectangular Cartesian coordinates are obtained. The solutions are derived from the generalized Elliott solution which includes five independent potential functions. Expressions for components of displacement and stress are concretely presented in series form, with reference to transversely isotropic, moderately thick rectangular plates. As applications of the solutions, in-plane stretching and pure bending of a transversely isotropic, rectangular thick plate are analyzed.

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An a.c. potential drop technique is applied for measurement of the stress-intensity factor, K₁. When a conductive material with a surface crack is loaded under a flow of a.c., a change occurs in electrical potential difference between two points located on either side of the surface separated by the crack. By measuring the change for different loadings, it is found that the change shows a linear relationship with the change in K₁. Also, the relationship is revealed to be independent of the crack depth. A.c. in the amount of 1 ampere with a frequency of 10kHz is used. Finally, a procedure for applying the present result to an in situ measurement of K₁ is proposed and verified.

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Plastic constraint and local strain at the notch root for notched plates and bars were examined in the elastic-plastic and full plastic regions by FEM. By introducing the mean equivalent stress, σ(Mises), and the strain, ε, at the minimum cross section, and the equivalent stress concentration factors, K_t, the results obtained in this study are summarized as follows: (1) The plastic constraint factor k defined by σ/σ_n was nearly constant under the wide range of deformations, and independent of the material constants. (2) The general yield stress σ_GY was derived based on the idea that the mean equivalent strain in the elastic state is almost equal to that in the plastic state. (3) The relationship between σ and ε for σ≥σ_GY was approximately in accord with the constitutive equation of the materials and was ε=Bσ^m for σ≤σ_GY. (4) The equivalent strain, ε, at the notch root was simply estimated from ε=K_t ε.

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In the case that the plastic strain path is not proportional and has a sharp corner, the subsequent yield surfaces show remarkable rotation, distortion, and translation behaviour. In order to describe such complicated behaviour in the subsequent yield surfaces for nonproportional loading, both the modified yield function of the 4th-order anisotropic moduli tensors and the kinematic hardening rule, which includes the nonproportional loading function, are proposed. Some simulated results based on the modified theory are also shown for particular loading processes in a mild steel and they are compared with the experimental results.

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The three-point bending deformation of a beam under a constant deflection-rate has been analyzed using the constitutive equation of superplasticity. Then the bend test and a tension test of a yttria-stabilized tetragonal ZrO₂ polycrystal (Y-TZP) in which the occurrence of superplasticity has been confirmed have been carried out at various deflection-rates and strain-rates at different temperatures. Results of the bend test have been compared with those of the tension test. Outer-fiber stress (σ_xc)_s and strain-rate (ε_xc)_s at the center of the beam have been calculated from measured values of bending force P_c, deflection-rate υ_c and deflection-rate sensitivity index m' defined as δln P_c/δln υ_c The strain-rate sensitivity index m obtained from the slope of the log-log plot of (σ_xc)_s and (ε_xc)_s agrees well with the m-value obtained from the tension test within the range of the present experiment. An activation energy of the deformation Q obtained from ln P_C^1/m' vs. 1/T or ln υ_C vs. 1/T plot in the bend test corresponds well with the Q-value obtained from the tension test, where T is temperature. A relatively rapid decrease in Young's modulus and an abrupt increase in deflection to fracture with an elevation in temperature are thought to be an indication of the onset of superplasticity. We can conclude from these results that the three-point bend test is a good method to evaluate superplastic properties in ceramics of which tension test is generally not so easy.

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The lumbar spine is an important region of the spine, often involved in trauma, degeneration and disease. A clarification of the mechanical causes of low-back pain requres a knowledge of the states of stress and strain throughout the lumbo-sacral spine. Since a purely experimental approach cannnot provide this information, analytical model studies to supplement measurements are neccessary. In this paper, computational simulations of whole human lumbar spine including the sacrum under flexion were carried out using the finite element method. The finite element models proposed are based on in-plane stress analysis considering the thickness effect of different material layers in an element. As a result, in flexion at constant moment, the stress distributions of five intervertebral discs in the lumbar spine are almost the same. Oh the other hand, at shear force, the stresses of discs in the lower region are higher.

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An efficient simulation method for predicting the width-spread in caliber rolling by the rigid-plastic finite element method is proposed. In this method, the three-dimensional deformation of the workpiece between the caliber rolls is simplified to the generalized plane-strain deformation where the strain rate is assumed to be uniform in the rolling direction over a cross section. Rolling by shaped rolls is treated as equivalent to compression with shaped dies, and the axial stress due to friction is taken into account. The deformed shape of a rectangular bar after caliber rolling is predicted and the calculated results of the width-spread are shown to agree with the experimental results. The deformed shape, the distribution of equivalent strain in the workpiece and the computing time are compared with those of the full three-dimesional simulation. The proposed simplified simulation method is found to provide sufficiently accurate results in terms of width-spread with a very short computing time.

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The inverse problem under consideration deals with a problem where a structural component includes unknown internal defects, and the boundary conditions on whole boundary are prescribed. It is assumed that some additional information is available concerning the displacement responses at several selected points on the outer boundary. In order to analyze this inverse problem use is made of the so-called method of multiple force applications previously proposed by the authors. Numerical experiment is carried out for two-dimensional problems to identify an elliptical defect equivalent to two circular defects.

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An intelligent structure concept which can freely change its geometric configuration as well as its physical properties is developed to meet various space mission requirements. In this paper, the optimal geometry of an intelligent truss structure with some of its member's lengths actively controlled is investigated. Structural analysis is performed by means of a matrix method and the optimization of the truss geometry is carried out by using the multiplier method. It is shown that for a load applied from a given direction, the optimal truss geometry obtained by adjusting the lengths of the active members provides a higher structural strength than a fixed truss structure.

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