In this paper, we propose a method to prevent wrinkling of the facings of a sandwich panel structure, whose outer and inner facings are at different temperatures. So, by using this method the wrinkling of the outer facing of a sandwich panel structure such as a sandwich panel refrigerator box can be prevented. In order to prevent wrinkling of the facings of the structure, it is necessary to determine the wrinkling stress by using a compression test, and to reduce the thermal stress under the wrinkling stress for a sandwich panel structure. And it is observed that a sandwich panel structure with a slit in the facing reduces the thermal stress. In this study, we use sandwich panel refrigerator containers to examine whether the prevent method can prevent wrinkling.

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Stress distributions in stepped-lap adhesive joints of dissimilar adherends subjected to tensile loadings are analyzed using a three-dimensional finite-element method (FEM). For establishing an optimum design method of the joints, the effects of some factors are examined. As the results, it is found that the maximum value of the maximum principal stress σ_1 occurs at the butted edge of the adherend's interfaces with higher Young's modulus. The maximum value of σ_1 decreases as the adherends Young's modulus ratio between two adherends and the adhesive thickness decreases. In addition, the joint strength is estimated using the interface stress distributions. For verification of the FEM calculations, experiments were carried out to measure the strains at the interfaces and the joint strengths. Fairly good agreements were found between the numerical and the experimental results. The joint strength of dissimilar adherends was found to be smaller than that of similar adherends.

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A numerical simulation was presented to discuss the microscopic damage and its influence on the strength and energy-absorbing capability of short-fiber reinforced plastic composites. The dominant damage includes matrix cracking and/or interfacial debonding, when the fibers are shorter than the critical length for fiber breakage. The simulation addressed the matrix cracking with a continuum damage mechanics model and the interfacial debonding with an embedded process zone (EPZ) model. The fictitious free-edge effects on the fracture modes were successfully eliminated with the periodiccell simulation. The advantage of our simulation was pointed out by demonstrating that the simulation with edge effects significantly overestimates the dissipative energy of the composites. We then investigated the effect of the material microstructure on the fracture modes in the composites. The simulated results clarified that the inter-fiber distance affects the breaking strain of the composites and the fiber orientation angle affects the positions of the damage initiation. These factors influence the strength and energy-absorbing capability of short fiber-reinforced composites.

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Fracture mechanics analyses using B-spline wavelet Galerkin method are carried out. B-spline wavelet Galerkin basis functions have the so-called muliresolution properties. These properties enhance the solution resolution of high stress concentration region around the crack tip. However, there are difficulties to represent the discontinuous displacements in the crack problem. In this study, we propose a new technique based on the concept of eXtended finite element method (X-FEM). Enrich functions are introduced to represent the displacement discontinuity and the near crack tip asymptotic solutions. In this paper, mathematical formulation and numerical implementation for the crack analyses are discussed. Stress intensity factors to the mixed mode crack problems are evaluated by using M-integral method for the domain integral form. Then, some numerical examples to the elastostatic crack problems are shown.

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This study proposes the cohesive zone model (CZM) for fatigue damage growth in the notched CFRP cross-ply laminates. In this model, the damage growth in the fracture process of cohesive elements due to cyclic loading is represented by the conventional damage mechanics model. We preliminary investigated how this model can appropriately express the fatigue damage growth for the circular crack embedded in the isotropic solid material. This investigation demonstrated that this model could reproduce the results with the well-established fracture mechanics model plus the Paris law by tuning adjustable parameters. We then numerically investigated the damage progress in notched CFRP cross-ply laminates under tensile cyclic loading and compared the predicted damage patterns with experiments reported by Spearing et al. (Compos. Sci. Technol. 1992). The predicted damage patterns agreed with the experiment results that exhibited the extension of the multiple types of damage (i.e., splits, transverse cracks and delaminations) near the notches.

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This study presents a numerical simulation to predicting damage progress in notched composite laminates under cyclic loading by using cohesive zone model. A damage-mechanics concept was introduced directly into the fracture process in cohesive elements in order to express the crack growth by cyclic loading. This approach then conformed to the established damage mechanics and also facilitated understanding of the procedure and the reduction of computation costs. We numerically investigated the damage progress in holed CFRP cross-ply laminates under tensile cyclic loading and compared the predicted damage patterns with experiments. The predicted damage patterns agreed with the experiment results that exhibited the extension of the multiple types of damage (i.e., splits, transverse cracks and delamination) near the hole. A numerical study indicated that the change in the distribution of in-plane shear stress due to delamination induced the extension of splits and transverse cracks near the hole.

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The effect of local plastic deformation on the fatigue life in a holed specimen was investigated. The local plastic deformation was applied around the hole by inserting a pin into the hole, and the pin was removed before the testing. The material used was aluminum alloy 2024-T3. After removing the pin, there was a circular hole or elliptical hole in the center of the flat section of the specimen. Due to the application of local deformation, the fatigue life of holed specimens became longer. Especially, in specimens where the hole shape was made elliptical, the fatigue life was clearly improved. Hardness distribution around the hole was measured to discuss the effect of the local plastic deformation on the fatigue life expansion. Consequently, it is concluded that the local plastic hardening and compression residual stress in the vicinity of the hole are the cause of the strengthening of the holed specimen.

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Rotating bending fatigue tests were performed using heat-treated 0.45%C steel specimens under the constant amplitude and the two-step loading and then the cumulative fatigue was investigated. Firstly, both the original S-N curve for the virgin specimen and the S-N curves for the specimens damaged by prior fatigue were plotted on the double-logarithmic scale. All curves could be linear approximately. The values of the cumulative fatigue damage defined by modified Miner's rule under two-step loading were less than unity. The fatigue life and fatigue limit for the damaged specimen tended lower, as the degree of prior damage increased. Next, the life prediction method for the damaged specimens was proposed. The estimated results were in good agreement with the experimental ones. Then the lower limit of cumulative fatigue damage was examined based on the proposed method. The minimum value of the estimated lower limit was about 0.6, but it was 0.4 when the scatter of the experimental results was considered.

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This study was conducted to investigate effect of DLC (diamond-like carbon) multi-layer coating on fatigue properties of stainless steel SUS304. The DLC multi-layer of 2.2μm thickness possessed a laminated structure composed of very thin DLC layers in which hardness was different. This structure was selected to prevent loss of the functionalities of the DLC layer through fracture or peeling which could occur under impact force. The DLC multi-layer was generated by UBMS (unbalanced magnetron sputtering) method. The result showed that microstructure of the substrate was unchanged by the DLC multi-layer coating conducted at relatively low temperature, so that the mechanical properties was maintained to the same level of the untreated material. The fatigue strength was improved by the DLC multi-layer coating since the layer having a high strength sufficiently adhered to the substrate and prevented initiation of fatigue cracks from the surface. However, its fatigue strength was slightly lower than that of the material coated by DLC single-layer. Such difference in the fatigue strength between the materials coated by the DLC multi-layer and single-layer would result from the difference in the fracture strength of the layers.

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In the rules on fitness-for-service for nuclear power plants of the Japan society of mechanical engineers (JSME), the two-parameter approach is prescribed for failure assessment of defects. In JSME rule, the failure assessment curve (FAC) for the two-parameter approach has to be determined using J-integral value when existing FAC is not available. The reference stress method can estimate J-integral value using stress-strain relation of the material. However, the accuracy of estimation depends on the limit load used for an evaluation of the reference stress. In this study, applicability of several limit load solutions was investigated by comparing with results of elastic-plastic finite element analyses. Pipes containing axial or circumferential flaw were analyzed under internal pressure or bending load. Appropriate limit load solutions were suggested for evaluation of FAC, based on analyses of 2880 cases for different flaw and pipe geometries and materials used in nuclear power plants.

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In nuclear power plants, the crack networks caused by thermal fatigue loading are shallow and do not penetrate the wall thickness. Mechanical interaction between the cracks and stress gradient in the depth direction are thought to suppress crack growth. In this study, in order to investigate the influence of the shape of the crack network on cracking behavior, finite element analyses were conducted using a three-dimensional model of the crack network under thermal fatigue loading. A Monte Carlo simulation of the initiation and growth of cracks was carried out to simulate the evolution of the crack network. It was revealed that the crack network can develop under a bi-axial stress field together with significant stress gradient and that the mechnical interaction between cracks reduces the crack growth rate in the depth direction. It was concluded that a well-developed crack network in an operating plant is observed only when the crack growth in the depth direction is interrupted and the structural integrity of cracked components is assured.

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Three point bend specimen is used for ductile fracture tests with different mixed mode ratio, K_<II>/K_I, and specimen thickness. The crack growth direction in mid-plane of the specimen changes largely with the change of K_<II>/K_I value. Detailed observation of fracture surface is conducted, and it is shown that the average void diameter is affected largely by the mixed mode ratio and specimen thickness. Numerical simulation is also carried out using Gurson's yield function. By introducing stress-controlled void nucleation model, as well as plastic-strain-controlled nucleation model, it is shown that crack growth directions at mid-plane and free surface are estimated well and results agree with experimental observations qualitatively.

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To develop a non-destructive quantitative evaluation method for hydrogel, we demonstrated fundamental study for electrical impedance method by use of gelatin hydrogel. We made the gelatin hydrogel which differed in concentration of gelatin to add gelatin powder into saline solution. Also, to investigate the relation between electrical impedance and structure, cross-linked gelatin gels were prepared with formaldehyde. After that electrical impedance measurement, compression test, and water content measurement were carried out. Electrical impedance of gelatin gel was decreased with increasing water content. At the same water content, electrical impedance of cross-linked gelatin gel was higher than that of intact one. Also, we found the relation between electrical impedance and compressive stiffness for gelatin gel. These findings shows that electrical impedance for biopolymer gel was affected by water condition of electrolyte solution where exists in the network of biopolymer chain, electrical impedance method could evaluate quantitatively the change of the structure for hydrogel.

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SSRT (slow strain-rate technique) tests were carried out using smooth- or notched-tensile specimen of aluminum alloy plates 7075-T6 and 6061-T6 in gaseous hydrogen under high-pressure 70 or 85MPa, as compared with in 90% relative humidity air under atmospheric pressure. For 7075 the humid air caused a significant embrittlement, leading to intergranular cracking, while in contrast the high-pressure hydrogen gas resulted in little embrittlement, where the transgranular fracture mode attributed to hydrogen effect was however observed, showing an enhancement of void coalescence. The result indicates that high-pressre gaseous hydrogen is much less severe to provide hydrogen to aluminum materials than humid air. On the other hand, 6061 was found to be immune to hydrogen embrittlement in either environment. It was then comprehended that the aluminum alloys represented a series of change in fracture mode from transgranular to intergranular with increase in hydrogen concentration.

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Effect of post heat treatment on tensile strength distribution and its size effect of poly-p-phenylene benzobisoxazole (PBO) fiber were investigated in monofilament tests. Two-parameter Weibull distribution adapted for dependence of fiber strength for both length and diameter direction. It was found that the tensile strength of PBO fibers showed size effect regardless of post heat treatment for both length and diameter direction. The size effect of tensile strength in diameter direction was larger than that in longitudinal direction. For relatively long gauge lengths (12.5mm and over), the tensile strength distribution separated size effect in diameter direction well fitted the Weibull distribution function with 2 parameters. For relatively short gauge lengths (under 12.5mm), it did not fit because distribution of fiber strength was lower than that of strength related to end fracture. The size effect of fiber strength for both length and diameter direction was increased by post heat treatment because crystalline regions increased by heat-treatment and fiber became brittleness.

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Dia-Core is a newly devised core panel formed by gluing/welding two same shaped panel pieces which have periodical indents. It has good cost performance due to easy press forming. Therefore, it is expected to become a new core which can compete with honeycombs from the total points of view. The basic model of Dia-Core is based on Octet-Truss developed by Fuller, and it consists of tetrahedra and octahedra. Various shaped modified models are devised systematically by varying the geometrical patterns that appear on the panel surfaces. These pattern variations can be ruled by two geometric parameters, truncation [k] and separation [s]. In this paper, the single stage forming simulations of Dia-Core panel are performed to investigate the relation between formability of press working and these parameters by using explicit FEM technique. As a result, the models having the good formability are clarified.

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Recentlly inflatable structures attract attention because of their lightness and easy transport. For the practical application of inflatable structures in space, we must understand the structual property accurately. In this paper, we pay attention to inflatable beams and the analytical solution based on the bending theory developed by Main, J. et al. is examind through experiments. From our experimental result, large shift is found in the load-deflection curves under the various experimental conditions. So, we modify Main's theory assuming that the membrane material can be subjected to a compressive stress at some level. Then the calculated results of the modified theory give close agreement with experimental results.

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