Material degradation and its mechanism in a plain woven GFRP under biaxial (combined tension-torsion) cyclic loading were studied. A plain woven glass fabric composite using polyester resin was selected for this study. Residual strength and stiffness reduction were observed to estimate the degree of fatigue damage. Based on the cumulative damage mechanics, a damage model to predict the stiffness reduction under biaxial cyclic loading was proposed. An attempt to predict biaxial fatigue life using the proposed model was also made, wherein the concept of the critical damage level was introduced. Experimental results confirmed that the model could predict both stiffness reduction and fatigue life reasonably.

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Surface damage of type 304 stainless steel, which is one of the candidates for the first-wall structural material in a fusion reactor, at cyclic plasma disruption loadings is simulated by a high heat flux Neutron Beam Injection. Influences of the cyclic thermal loads on the residual fatigue life are studied. The results obtained are summarized as follows. (1) The maximum tensile residual stresses near the damaged surface increased with increasing thermal loading cycles from 1 to 5. (2)The fatigue strength and the fatigue limit of the specimen subjected to 5 cycles of thermal loading and those for 1 cycle are the same. (3) A critical crack length, 2b_e to vanish from sight caused by thermal loading is important for the evaluation of the residual fatigue life. If the defect length, 2b < 2b_e, residual life after the second thermal loading and that before thermal loading are the same. If 2b > 2b_e, fatigue crack growth occurs due to the cyclic thermal load.

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Structural components operating at high temperatures in power plants are subjected to interaction of thermal fatigue and creep which results in creep-fatigue damage. In evaluating the life of those components, it is important to understand microscopic damage evolution under creep-fatigue conditions. In this study, static creep and creep-fatigue tests with tensile holdtime were conducted on SUS304 stainless steel by using a high-temperature fatigue machine combined with a scanning electron microscope (SEM), and cavity growth and coalescence behaviors on surface grain boundaries were observed continuously by the SEM. Quantitative analysis of creep cavity growth based on the observation was made for comparison with theoretical growth models. As a result, it was found that grain boundary cavities nucleate at random and grow preferentially on grain boundaries in a direction almost normal to the stress axis. Under the creep condition, the cavities grow monotonously on grain boundaries while they remain the elliptical shape. On the other hand, under the creep-fatigue condition the cavities grow with an effect of local strain distribution around the grain boundary due to cyclic loading and the micro cracks of one grain-boundary length were formed by coalescence of the cavities. Also, cavity nucleation and growth rates for creep-fatigue were more rapid than those for static creep and the constrained cavity growth model coincided well with the experimental data for creep.

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Model I delamination fatigue crack growth behavior was investigated with unidirectional laminates made from ICI APC-2 (AS 4/PEEK). Tests were carried out by using double cantilever beam specimens. Fatigue tests under the constant maximum energy release rate indicated that the crack growth rate decreases with increasing crack length from the initial defect. Hence, the conventional load-shedding method gives higher threshold values corresponding to the increment of crack length. The most conservative growth law and the threshold value were obtained by using the growth rate at the zero increment of the crack length from the starter film. Here, the existence of the growth threshold was shown by nonpropagating cracks. Since this method requires a computer controlled test system and a number of specimens, another simplified method was proposed. This newly developed method is the iteration of several load-shedding tests. In this method, the threshold value converged after only several tests. This threshold value agreed well with that obtained by the tests under the constant maximum energy release rate. Moreover, the threshold value for this method was lower than that determined by the delamination growth onset method.

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Growth behavior of mode I interlaminar cracks in two different carbon/epoxy laminates, Toray T800H/#3631 and T300/#3601, was studied under cyclic loading. Under a constant value of the applied stress intensity range, ΔK_{I ap}, the crack propagation rate decreased with crack extension for T800H/#3631 laminates, while the crack propagation rate was constant for T300/#3601 laminates. The deceleration of crack growth in the former material is caused by fiber bridging. For each laminate, the crack propagation rate was expressed by a power function of the crack-tip value of the stress intensity range, ΔK_{I tip}. The formation of fiber bridging strongly depends on the morphology of resin-rich layers between laminae. In T800H/#3631 laminates, the many fiber-rich regions disconnecting resin-rich layers are the origin of fiber bridging, while there is no such region in T300/# 3601 laminates having continuous resin-rich layers.

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Growth behavior of interlaminar cracks in unidirectional carbon/epoxy laminates, Toray T800H/# 3631, was studied under mode II static and cyclic loadings. The end notched flexure (ENF)method and the end loaded split (ELS) method were used for crack growth tests. In static tests, the valtle of mode II interlaminar fracture toughness, K_IIc, was constant and independent of the crack extension, Δa. Under mode II shear loading, there was no increase in crack growth resistance due to fiber bridging, in contrast to the case of mode I crack growth. In cyclic tests by keeping the stress intensity range, ΔK_II, constant, the crack propagation rate was independent of the crack extension, Δa, as in static tests. At the stress ratio between 0. 2 and 0. 6, the crack propagation rate, da/dN, was given by a power function of the stress intensity range, ΔK_II, in the region of rates above 10^-9 m/cycle. The exponent of the power function became larger with increasing stress ratio. Below this region, there exists a threshold value of the stress intensity range, ΔK_IIth, for fatigue crack propagation. The value of ΔK_IIth is independent of the stress ratio.

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It is widely understood that the fatigue life of nickel-base superalloys is greatly increased as a consequence of the inclusion elimination in the melting process. Electron beam cold hearth remelting is a recently developed technique that offers the most dramatic improvements in the cleanliness of superalloys. However, the relation between fatigue characteristics and cleanliness has not been well investigated. In this work, fatigue crack growth rate tests were conducted at ambient temperature on commercial grade IN 718 and electron beam melted clean IN 718. The fatigue crack growth rate test shows that small surface cracks in the clean materials propagate considerably slower than those in the conventionally processed materials. In contrast to the behavior of small cracks, long through cracks propagate faster in the clean materials than in the conventionally processed materials. Since the small crack initiation and propagation period is the major portion of the fatigue life, the behavior of small cracks has a more important effect on the fatigue life than the behavior of long cracks.

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Dynamic fracture behavior of silicon nitride ceramics is directly observed using a dynamic fracture testing apparatus of caustics combined with an ultrahigh-speed camera, Imacon 790, at room and elevated temperatures over 1000°C. Consequently, it is confirmed that dynamic fracture toughness K_Id is almost the same as the value of static fracture toughness K_Ic in this temperature range. These values of K_Id and K_Ic are slightly increased from room temperature (R. T.) to 800°C and they are decreased at temperatures above this. Crack-propagation fracture toughness K_ID at R. T. is nearly constant at K_ID=6.0 MN/m^3/2 in the crack velocity range up to 3800 m/s, and this is the same tendency at 1000°C. The effect of the notch root radius ρ on the values of K_Ic and K_Id is also studied in the ρ range from 0 to 1.0 mm under R. T. condition. Moreover, the visualized AE-wave which is emitted from the notch tip at the initiation of fracture is shown.

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An elastic-plastic fracture toughness test was conducted at 4 K and 290 K to investigate the factors influencing fracture behavior of IN 718. First, the effect of grain size on the fracture toughness and tearing modulus was clarified. It was found that at 4 K the pop-in behavior was dominant in both coarse- and fine-grained materials. However, the fracture surface examination showed that ductile dimple formation prevailed in the coarse-grained meterial while alternating narrow dimple bands and cleavage facets were observed in the fine-grained material. Results showed that fracture toughness increases with increasing grain size. Second, IN 718 with few inclusions was used to clarify the effect of cleanliness. It was revealed that at 4 K the fracture toughness was comparatively increased due to the presence of few inclusions while the tearing modulus was decreased since the presence of many fine brittle carbides contributed to sudden crack growth.

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Development of internal damage in CFRP [±45°]₄ laminate tubes free from the edge effect under cyclic tension-compression with stress ratio R=-0.25 was evaluated quantitatively. The damaged specimens subjected to specific life fractions were sliced perpendicularly to the specimen axis. Then the development and distribution of matrix cracks and delamination were observed on the sliced surfaces with a microscope. The matrix cracks were classified into three stages of degradation with respect to the crack length, and the number and the location of these three kinds of cracks were measured. The length and the location of the delamination were also measured. It was observed that once a matrix crack initiates, it grows rapidly and traversely through the layer. The matrix cracks developed uniformly in the radius and circumferential direction. The delamination appeared in the last stage of fatigue as the result of the extension of the matrix cracks to the interlayer, and progressed mainly in the fiber direction. The influence of internal damage on Young's modulus was also discussed.

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Classification of the ultrasonic wave signals emitted from defects such as delamination and inclusions in layered media has been attempted using the neural network technique. In providing both reflected waveform from defects to the input and information of defect type, as well as location to the output, the relationships between the input and the output were learned beforehand by the neural network. In the system, the network learned in terms of detected waveform, however, could not infer correct information of defects for a waveform of a different phase. Therefore, appropriate parameters sampled from the detected waveform were added to the network. Locally connected networks restricting the connection between input layer and output one were proven in a highly precise estimation. It was revealed that the locally connected network is an effective technique to estimate the location of each defect, types of defect, and the amount of defects, within the range of the application of the model.

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The objective of this investigation is to predict the natural frequency of a laminated carbon fiber reinforced thermoplastics (CF/PEEK) plate with a hole by using the finite element method. In the finite element formulation, a parabolic isoparametric plate bending element is used and the first-order shear deformation theory based on the Mindlin type is considered. The good agreement between the numerical and the experimental results of CF/PEEK unidirectional and cross-ply laminated plates with a hole shows that the natural frequencies and modes can be predicted by the present method. Futhermore, the changes of frequencies and modes of the antisymmetrically angle-ply and the cross-ply laminated plates with the size of the hole, the stacking sequence of lamina and the plate length-to-width ratio are examined by F. E. M. under simply supported and clamped boundary conditions.

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An AFM/STM hybrid system is a potential tool because it enables one to obtain the current or force images in addition to ordinary AFM or STM topographic images. The current image in the AFM mode reflects the electric properties on the specimen surface. In fact, 200-nm-diameter ZrO₂particles dispersed in a Mo matrix were clearly detected in the current image. Furthermore, a new method for thickness measurement of surface films was proposed, combining current imaging and nano-machining techniques. The thickness of the oxide film on Mo single crystal was 16-24 nm, 170-350 nm, and 370-490 nm for the specimens oxidized at 300°C for 72 hours, at 400°C for 24 hours, and at 400°C for 72 hours, respecitively.

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Recently, the homogenization method is becoming a matter of interest as a new analysis method of composite materials. This method aims at stricter modeling and more precise simulation for any kind of reinforcement. Discretization of complicated microstructures of such reinforcement, however, becomes the bottleneck in the analysis procedure. Therefore, a new simulation technique based on a fixed grid is proposed in this paper. Fundamental investigation of the accuracy of the homogenization method based on a fixed grid for unidirectional FRP is presented. The obtained elastic constants show good agreement with the solution of the Halpin-Tsai equation. Microscopic stress distribution has also been evaluated well compared with conventional smooth finite-element discretization.

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The microstructure of an yttrium-oxide-dispersed tungsten alloy, newly developed, was investigated by examining the effect of sintering temperature on the particle size of yttrium oxide and the crystal size of tungsten. Also, it was confirmed that the bending strength of yttrium-oxide-dispersed tungsten alloy was affected by the sintering temperature in comparison with a sintered tungsten sample. The residual stress, induced by the coefficient of thermal expansion mismatch, is analyzed for the tungsten matrix composite with a particle of yttrium oxide using the finite element method. Because of the high residual stress, particles of yttrium oxide become crack initiation sites under the fabrication process. Finally, it is also shown that the bending strength of yttrium-oxide-dispersed tungsten alloy can be estimated simply by the fracture mechanics approach, based on the assumption of a flaw introduction effect by the yttrium oxide dispersed.

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Tensile and torsional tests were carried out on the plain specimens of isotropic carbon steels. One of the materials is an annealed cast carbon steel and the other is a rolled round carbon steel bar (S45C) which was diffusion-annealed. Fine grid lines were drawn on the surface of specimens with a diamond-point needle. The lines were fine enough not to influence the structure of the slip bands. The local strains were measured from the elongated or distorted grids. The local deformations were not uniform in either tension or torsion, and most of the local strains were not the same as the given macroscopic strain. That is, there existed various local strains which were from near zero to two or three times the given macroscopic strain. It showed the minor differences of the variation of local strains between tensile test and torsional test.

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The one-dimensional transient heat conduction problem of a nonhomogeneous plate with arbitrarily distributed and continuously varied material properties, such as FGM (functionally gradient materials), is treated theoretically in this paper. For such a nonhomogeneous plate, the heat conduction equation becomes nonlinear, therefore the theoretical treatment is very difficult and the exact solution is almost impossible to obtain. Introducing the analytical procedure of the laminated plate model, and thereafter, taking into account the constraint that the number of lamine becomes sufficiently large, the analytical temperature solution for such a completely nonhomogeneous plate is derived. Furthermore, the associated thermal stress components for an infinitely long non-homogeneous plate are formulated under the mechanical condition of being traction free. As a numerical example, the plate composed of alumina and aluminum alloy is considered. The numerical results for temperature change and the associated thermal stress distributions are shown in the figures, and the effects of nonhomogeneity (change of the volume fraction of two different materials)on thermoelastic behaviors are briefly examined.

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A thermal stress analysis system has been developed, which enables convenient analysis of multilayered materials such as coatings. The analysis method of thermal stresses is based on the strain suppression technique and beam theory. In this study, the thermal stress properties of graded thermal barrier coatings were analyzed by focussing on the reductionability of thermal stress. Graded thermal barrier coatings with a surface layer of yttria-stabilized zirconia were chosen for the thermal stress analyses in the case of uniform heating and one-side heating with temperature difference. It was verified that the analysis technique based on equivalent thermal conductivity (=constant) was useful for optimizing the thermal barrier coatings. As a result, it was found that the effects of graded distribution on thermal stress mitigating properties were strongly dependent upon the graded thickness and heating condition.

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In this paper, the structural change in the heat-affected zone during welding of an SUS-304 plate is investigated in relation to the reverse transformation from work-induced martensite to austenite. The transformation occurs in HAZ and affects the mechanical properties of the welded component. The kinetic equations for the transformation are evaluated by comparing the calculated structural change with experimental results. The stress/strain analysis of the welding process is conducted based on the viscoplastic constitutive equations associated with the structural change. Emphasis is placed on the structural change in HAZ and its analysis in the simulation of the welding process, and then the properties of the welded component are also discussed using the calculated results of distributions of stresses and structures.

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The hybrid finite element-boundary element method was applied to solve the two dimensional problem of composite materials. The method combines the differential and integral approaches and gives specific advantages to analyze the system which includes different materials. In this paper, the base matrix of the composite material was modeled using the boundary element method, while the fibers imbedded in the matrix were represented using the finite-element method. Two-dimensional elastic problems were solved to demonstrate the effectiveness of this method in the analysis of composite materials. It was shown that the solutions with high accuracy can be obtained by the hybrid FE-BE method using far smaller degrees of freedoms compared with the finite-element method.

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Cylindrical shells subjected to lateral loads buckle in shear or bending modes. The bending buckling mode results in a diamond-shaped pattern or elephant-foot bulge at the edge. In this paper, by developing a special-purpose finite-element analysis program using the 8-node isoparametric shell element, the postbuckling behaviors and residual deformations under cyclic lateral loading are investigated in both shear buckling and bending buckling cases. The shear buckling patterns on the shell change according to the direction of loading in the shear buckling case. In the bending buckling case, shear mode also appears in addition to elephant-foot bulge.

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A finite-element numerical model is presented to simulate electromagnetic acoustic transduction into a half-space of a ferromagnetic metal. The model deals with a two-dimensional electromagnetic phenomenon induced by an rf burst signal to a flat coil situated close to the surface ; a pair of permanent magnets, having opposite magnetization directions, applies a static field into the metal. The simulation requires four steps. First, the initial magnetic field is calculated, including the nonlinear initial magnetization curve. Second, after adding the oscillating magnetic field induced by the driving current to the initial field, the differential magnetic susceptibility is determined for each element in the metal with reference to its own initial magnetization. The minor hysteresis loop, not the main loop, relating the magnetic field and the magnetization is approximated to a linear relation-ship, since the burst signal is long enough. Third, the magnetic field and eddy current distributions are separately calculated using these susceptibilities. Finally, the Lorentz force distribution is calculated, which is the product of the interaction between the magnetic field and the eddy current. The longitudinal wave excited is found to be less effective, due to the susceptibility decreased by the oscillating magnetic field, than the shear wave excited, as observed in experiments.

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This paper presents a new analysis scheme for the two-dimensional elastostatic contact Problems with and without friction. The principal objective is to promote high computational efficiency for the above-mentioned problem, specifically for the unknown boundary identifications corresponding to a variety of contact phases including separation (no cantact). Two elastic bodies are treated individually by the new formulation similar to the flexibility method, and are coupled to satisfy the conditions specified on the contact region, which is oriented for the best fit in parallel computation. The scheme can reduce recomputation of system matrices for each iteration and their modification is sufficient of only small part for the assumed contact region. Some numerical examples are shown for the cases with and without friction.

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This paper describes a new adaptive remeshing technique for the three-dimensional finite element analysis based on the fuzzy knowledge processing and the computational geometry. First, a three dimensional geometry model is defined through some solid modelling operations, and analysis conditions are attached on each part of the model. A tetrahedral finite element model is generated following an initial density distribution of nodes specified by a user. After its analysis is finished, a posteriori error estimation is performed. An optimal density distribution of nodes is estimated, and a new finite element model is created. These procedures are iterated fully automatically until solution accuracy satisfies the criterion specified by a user. The error estimation method proposed by Zienkiewicz and Zhu (Z-Z estimator) is applied to the system, and its practical performances are demonstrated through its application to some static heat conduction problems of three-dimensional complex geometry.

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Mixed finite element method has been successfully applied to the analysis of incompressible solid and fluid, shell and beam structures and electromagnetic fields, etc. However it is well known that mixed finite element method may work well in the solution of certain problems but perform very poorly on other problems. In this paper, first, a brief review of the stability conditions for mixed variational principle, which is known as the LBB condition, is given. Then a numerical prediction procedure for the discrete LBB condition which has recently been developed by Malkus (Int. J. Eng. Sci., 19 (1981), 1299) and Chapelle et al. (Compt. & Struct., 47 (1993), 537) is described. Finally the performances of two-and three-dimensional incompressible mixed elements, such as quadrilateral, triangular, hexahedral and tetrahedral elements are systematically investigated based on the discrete LBB condition.

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The concept of specific gravity was employed in this paper as an indicator for measuring the progress achieved in reducing the weight of transportation equipments. According to this indicator, the weight of domestic passenger cars has been reduced by a value of 0.93 over the past 19 years. However, only slight weight reduction has been achieved in the past six years despite rationalizing vehicular structure throught the extensive use of CAE. This indicates that the weight of vehicles constructed primarily of steel materials is approaching a natural limit. What this suggests is that it will be necessary to switch from steel to lightweight materials to substantially reduce the weight of vehicles in the years ahead.

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The standardized transformation methods are discussed from the viewpoint of the theory of structural reliability and safety. These methods involve transforming non-normal dependent vectors into normal independent vectors. In the structural reliability field, it will be possible to estimate safety measures such as safety index or failure probability if these transformation methods are formulated in order to transform random vectors with correlation into normalized random vectors without correlation. Since there are correlations among limit state functions and design variables in the practical design, these transformation methods enable one to evaluate the effectiveness of correlation among probabilistic characters of statistical uncertainty factors on structural design. For these reasons, some procedures for the transformation have been proposed. Despite these proposals, however, each transformation method has not been studied comparatively. In this study, some typical transformation procedures proposed to date are evaluated comparatively in terms of their features and differences. From these evaluations, a new transformation procedure is also presented which has the improved algorithm of decomposition of correlation matrix. It is verified from numerical analyses that the design point is invariant regardless of the standardized transformation method applied and there are differences in the transformed paces.

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This study presents a reliable method for allowable stress predictions for silica optical fibers based on a probabilistic failure theory. The key to accurate predictions is the use of a crack growth law in which the crack velocity increases exponentially with the applied stress intensity factor. Such a growth law, i. e., an exponential law, can explain crack growth behavior of silica optical fibers better than the commonly used power law. Using the exponential law, we derive useful equations providing relationships between failure probabilities and allowable stresses for optical fibers in long-term service situations after proof testing. By comparing allowable stresses predicted based on the equations with those predicted using the power law, we also discuss risks of the use of conventional prediction methods based on the power law.

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