In this paper, the mechanism of stress concentration due to defects in honeycomb sandwich structure is studied by using finite element method. It is found that the stress concentrantion can be expressed as a function of E_ct_c=E_pt_p, where t_c and t_p are the thickness of core and plate, respectively, and E_c and E_p are the Young's modulus of core and plate, respectively. Also, it is found that the stress concentrantion can be evaluated by an approximation of an elliptical inclusion. The validly of this technique is verified through comparison of the evaluation values with the numerical results obtained by FEM.

抄録全体を表示

Change in electric capacitance with an advance of ceramic top coat delamination in a thermal barrier coating (TBC) specimen was measured by an AC impedance method, as a trial to develop a new method for a life assessment of TBCs. It was found by the experiments that the capacitance showed significantly lower value at the delaminated area, compared with at the non-delaminated one. It was also found that the capacitance continuously changed with the advance of delamination crack during the delamination test. These results strongly suggest that the delamination can be detected and contentiously monitored by the AC Impedance method. The engineering background in the impedance change was semi-quantitatively discussed based on an electrical condenser model.

抄録全体を表示

The authors have reported a significant reduction in fretting fatigue strength of austenitic stainless steels due to hydrogen. One of the causes of the reduced fretting fatigue strength in hydrogen is adhesion between contacting surfaces and following formation of small cracks which emanate from the adhered spots. The objective of this study is to understand the effect of hydrogen on the initiation of the small cracks under fretting fatigue conditions. Since the adhesion between contacting surfaces during fretting in hydrogen is very localized, a small contact length was used in this test in order to facilitate understanding by avoiding such localization. The fretting fatigue test of an austenitic stainless steel SUS304 was performed in air and 0.13MPa hydrogen. In the fretting fatigue test, hydrogen participates in the initiation of the fretting fatigue crack. It can be presumed that high strain at the contact edge activates hydrogen assisted fracture in terms of dislocation mobility. Adhesion mimic test, in which a small contact area was welded, was also performed. As the result, the crack initiation limit evaluated by the maximum range of shear stress was significantly lower in hydrogen than in air. Hydrogen assists small crack initiation under fretting fatigue conditions. This is one of the possible causes of the significant reduction of fretting fatigue strength in hydrogen.

抄録全体を表示

In this research, a technique has been developed for quantitatively evaluating the amount and distribution of tensile and compressive biaxial residual stresses by using combined strain measurements under the spherical indentation loading together with the finite element method (FEM). When the spherical indentation is applied to the top surface of a steel plate, the elastic strain at an optimized position near the indentation is measured by strain gauges, where the biaxial residual stresses and applied indentation stresses are largely superimposed. In order to analyze the biaxial residual stress, FEM analysis has been conducted to establish the relationship between the elastic strain adjacent to the indentation and the indentation load for steel plates subjected to various uniform tensile and compressive biaxial bending stresses. A strain energy term (U^*) has been newly introduced by integrating along the trajectory between the indentation load and elastic strain in a range from 0(zero) to the maximum load. The technique explained herein has been applied to quantify the biaxial residual stress from the maximum and minimum principal stresses at two directionally perpendicular points U^*_ax and U^*_by, which is expected contribute for enhancing reliability verification of steel parts.

抄録全体を表示

In this study, a new welding method which utilizes conductivity of CFRTP (Carbon Fiber Reinforced Thermo-Plastic) is proposed. Electrical current is applied through the CFRTP single-lap joints in thickness direction. Since resistance of joining surface is the highest in the current path, the joining surface is preferentially heated by Joule heating. In this study, spot welding and surface welding were conducted. By applying constant voltage of 5.0V or 10.3V, the joint-surfaces were heated above melting temperature T_m of the material and were welded. In order to evaluate lap-shear strength (LSS) of welded joints, tensile lap-shear strength tests were conducted. As a result, the tensile lap-shear true stress of spot welding is 31.0MPa and LSS of surface welding is 22.0MPa. These strengths were comparable to those that were welded by traditional resistance welding method. It is also found that LSS is changed with welding pressure of joint parts.

抄録全体を表示

In this study, we fabricated piezoelectric strain sensors from multi-walled carbon nanotubes (MWNTs) and PVDF nanocomposites, i.e., PVDF/MWNT for measuring dynamic strains. The influence of MWNT loading on the sensor performance was evaluated by changing the MWNT loading as 0.0wt%, 0.05wt%, 0.2wt%, and 0.3wt%. To increase MWNT dispersion in PDVF matrix, a mixing process by using a planetary stirring machine and sonication processing by using an ultrasonic mixer were firstly employed together to produce nanocomposite films. Then, these films were stretched under uniaxial loading and poled under 60MV/m to fabricate the strain sensors. Moreover, crystallinity of the PVDF/MWNT nanocomposites was analyzed by using X-ray diffraction (XRD) analysis, and the fractured surfaces of pre-stretched PVDF/MWNT nanocomposites were observed by using a polarized optical microscope (POM). The piezoelectricity and signal tracking capability of the PVDF/MWNT nanocomposites sensors in vibration were investigated. From experimental results, the piezoelectricity, i.e., the sensor output voltage of the PVDF/MWNT nanocomposites reaches to a maximum peak value at 0.05wt% MWNT loading, and then decreases with further more addition of MWNTs. The result of XRD intensity was consistent with the piezoelectric sensor output results of PVDF/MWNT nanocomposites. From POM observations, compared to that of pure PVDF, the spherulite's size in the PVDF/MWNT nanocomposites becomes smaller and its number increases. Moreover, there should exist an optimum content of MWNTs, which can result in high piezoelectric properties of the nanocomposite strain sensors.

抄録全体を表示

Recently, high strength steel material is used increasingly for plates which constitute the frame structures of vehicles. Since these plates become thinner, the buckling on the plates has been recognized as an important issue for automotive industries. In this paper, the expression is derived to obtain simply the shear stress on plates at torsional buckling for the box beams which represent the frame structure. Assuming the deflection shape of the buckled plate simply supported on all sides and the right angle at four corners, the precise expression is derived based on the energy method. However, this expression is complicated and hard to use easily. Therefore, the approximate expression is proposed by the knowledge from the aforementioned derivation. The accuracy of that expression is investigated as compared with the results acquired in the buckling eigenvalue analysis by FEM. As the results, the difference of shear stress at torsional buckling is less than about 5% as for the aspect ratio of the cross section between 0.4 - 1.0.

抄録全体を表示

The index U^* was introduced in the previous paper by the authors to show the load paths in a structure. The complementary index U^** to U^* has also been studied to ascertain the conditions of distributed loading. The main problem encountered in calculating U^* and U^** is the high computation time required—especially for the actual application of large scale FEM models. In this study, we propose two methods for reducing the time taken by FEM calculations to obtain the U^** distributions. In the first method, the essential term is extracted from the global form of the flexibility matrix for the calculation of an internal flexibility matrix in each node. Since the first method cannot be utilized for large-scale commercial FEM codes, a second method is introduced. In the second method, three independent inspection loadings are applied to each node of a structure. In this operation, the multi-displacement condition is converted into the multi-loading condition. After calculating the cab structure for a heavy-duty truck cab using the second method, we obtained the result that the reduction in time required for calculations was better than 90 percent.

抄録全体を表示

A circular-shell deep-drawing test and an FEM simulation utilizing LS-DYNA with Barlat-Lian anisotropic yield locus (1989) were conducted in order to investigate the occurrence of earring for low-carbon aluminum-killed steel sheet and pure titanium sheet. The simulation method was developed that can accurately estimate the earring pattern of whole 360° circular wall height, where R-value is supposed to be measured within the range of uniform elongation, in particular for the case of pure titanium. The well known opinion that the earring value is proportional to ΔR/R_ave does not always hold for the whole sheet materials. Despite the fact that the value of ΔR/R_ave of pure titanium is nearly zero, it exhibits large earring. This is considered to stem from the characteristic planer anisotropy of R-value of pure titanium, namely large ΔTL_ave ((R₉₀ - R₀₀))/R_ave) together with very small ΔR. Furthermore, it is interesting to note that the earring in 45° direction was observed experimentally and was reproduced by simulation, despite the fact that pure titanium shows the straight planer anisotropy with the maximum in 90° direction. The optimum m-values in yield locus for pure titanium and low-carbon aluminum-killed steel, which led to the fairly good reproduction of experimental results, is 8 and 2, respectively. This difference in optimum m-value between the two metals is presumably related to the difference in the number of slip system; namely the number of active slip system is very limited for pure titanium.

抄録全体を表示

This study describes an efficient directional importance sampling method for the simulation-based estimation of the structural failure probability. Samples are generated in the orthogonal coordinates from importance sampling p.d.f.s centered at the design points on the limit state surfaces of the respective failure modes and directional vector samples are determined by the directions from the origin to the respective sampled points. The probability content of the importance sampling p.d.f.s contained in a hyperconical domain subtended by an infinitesimal increment at the respective sampled direction is evaluated numerically and adopted as a directional importance sampling probability density at the sampled direction. When estimating the structural failure probability, the effect of the importance sampling in each simulation cycle is compensated by the ratio of the uniform density distributed over the unit hypersphere and the respective directional importance sampling probability density thus determined. Numerical examples to estimate the failure probability of structures with multiple failure modes show that the proposed method gives accurate estimations of structural failure probabilities efficiently.

抄録全体を表示

In this paper, the bending properties of micro-lattice beam (second moment of area and natural frequency) are presented. In particular, the effects of overall lengths of the beam and geometries of the unit-cell on these properties are investigated by using FE analysis. The second moment of area for the micro-lattice beam depends strongly on the geometries of the unit-cell (strand's length, diameter and angles among the adjacent strands), and its trend is not the same as the continuous solid beam. On the other hand, the observed vibration modes for the micro-lattice beam are similar to those for a solid beam, and their natural frequencies can be estimated precisely using the initial stiffness, the second moment of area obtained by structural analysis and the mass per unit length for the lattice beam, if the wavelength for each mode is ten times as long as the width of the unit-cell.

抄録全体を表示

Material properties of a polycrystal piezoelectric ceramic, barium titanate BaTiO₃, were analyzed by the two-scale crystallographic homogenization method. Three-dimensional (3-D) micro-finite element (FE) model was constructed based on the electron backscatter diffraction (EBSD) measured crystal orientation distribution images. We obtained 13 EBSD images of 128 × 100pixels, which the measurement interval was 0.634μm in-plane and the average amount of polishing was 1.66μm in thickness (normal) direction of the specimen. Each voxel of EBSD was assigned into a solid FE in-plane with maintaining resolution of EBSD measurement. Since coefficient matrix of finite element equation for the electromechanical coupling problem is not positive definite and ill-condition, an iterative partitioned coupling procedure based on the block Gauss-Seidel (BGS) method with parallel conjugate gradient (CG) solver was applied to the multi-scale analysis. We investigated on micro finite element modeling with crystal orientation distribution and convergences of both the BGS method and the CG solver. The number of BGS iterations was about 15 in this analysis and it was independent on degrees of freedom of the micro-FE model based on the EBSD-measured images. The combined-method of the BGS and the CG solver was suitable for the large-scale analysis using experimental EBSD measurement-based realistic voxel FE model. The representative volume element (RVE) size was also determined based on the orientation distribution function analyses of EBSD voxel data. The least RVE size was 25000μm³, which corresponds to include 150 crystal grains.

抄録全体を表示

This paper presents a shape optimization method for designing the stiffeners on thin-walled or shell structures. The solutions to stiffness maximization problem subject to a volume constraint, and its reciprocal volume minimization problem subject to a compliance constraint are proposed. The boundary shape of the stiffener is determined under the condition where the stiffener is movable in the in-plane direction to the surface. The both optimization problems are formulated as distributed-parameter shape optimization problems, and the shape gradient functions are derived using the material derivative method and the adjoint variable method. The optimal free-boundary shapes of the stiffeners are determined by applying the derived shape gradient function to the H¹ gradient method for shell, which is a non-parametric shape optimization method proposed by one of the authors. Several optimal stiffener design examples are demonstrated to verify the validity and practical utility of the proposed method for designing stiffeners' shapes on thin-walled or shell structure.

抄録全体を表示

For ductile materials used in nuclear power plant components, allowable stress of cracked pipes under bending load is derived using limit load assessment procedure in the fitness-for-service (FFS) code. The limit load assessment is also used in the design and construction code. However, allowable stresses derived from both codes are not identical even when the size of cracks is negligibly small. In this study, an appropriate limit load assessment procedure for the FFS code was discussed. Firstly, the difference in two codes was investigated in detail. Then, four point bending tests were carried out using cracked and uncracked stainless steel pipes. It was found that, for service level A and B, the allowable stress should be determined so that significant plastic deformation does not occurred in cracked components. An alternative limit load assessment procedure, in which yield strength was used for material constant, was proposed. It was suggested that, when the finite element method is applied to derive the limit load of cracked components, a reduction factor should be introduced in order to accommodate the change in limit load due to notch effect.

抄録全体を表示

Several pieces of wire rope are used in the transport device of low noise elevator type mechanical parking system, EMPS. It is important to estimate the fatigue life of wire rope in order to keep the reliability of parking system. The formula proposed by G.Niemann in 1946, well known to be accurate in the fatigue life estimation of wire rope, has been modified and used in the design. But this formula has some problems such that it does not have the parameter of wire bending stress by sheaves, while the fatigue life depends on wire breakage by its repeated bending. In this paper, first, the damage mechanism and behavior of wire rope in this parking system were studied using the exchanged wire rope attained the criterion, 10% breakage of wire, and then the estimation method of fatigue life of wire rope is proposed, based on S-bend fatigue test simulated the actual condition, using the wire bending stress σ_b by sheaves and the fatigue strength reduction factor K_f which is related to the contact force P_UC with driving sheave as a function of wire rope tension.

抄録全体を表示

A high strength bolt fastening system for satellite structures has been developed. This fastening system enables high frictional strength by adopting titanium bolts with high strength and low torque-coefficient, as well as shims with high friction coefficient. Low torque-coefficient of the titanium bolts is achieved by PTFE (Polytetrafluoroethylene) solid-film lubricant, which is coated on the bolts. High friction coefficient of the shim is achieved by surface roughening by etching. To verify the design of the fastening system, long-term preload test, torque-coefficient test and vibration loosening test (Junker's test) were conducted and successfully completed. In addition, the fastening system was applied to a development model of satellite structure and was verified by mechanical environmental tests (static load test, system vibration test and thermal vacuum test). The observation showed no evidence of slip or alignment deviation. Thus, a conventional reamer-bolted-fastening system can be replaced with the developed high strength bolt fastening system that saves manufacturing period of satellites and furthermore enables the satellites to be competitive in terms of cost, delivery and performance.

抄録全体を表示