Showing papers in "JSME international journal. Series A, mechanics and material engineering in 1995"

Application of Electric Potential Method to Smart Composite Structures for Detecting Delamination

Abstract: CF/epoxy composites have been applied to many kinds of aircraft structures. The composite laminated plate compression properties, however, are easily degraded by delaminations. Thus an approach for detecting the delaminations is widely desired. In this study, an electric potential method was discussed for realtime nondestructive evaluation of delaminations because this method has several advantages in comparison with the embedded optical fiber. In order to obtain the basic properties of the CF/epoxy composites, mode I and II delamination tests were carried out and the relations between the electric resistance change and the delamination crack length were measured. As a result, the electric resistance bridge circuit approach was proven to be excellent for detecting delaminations from the inside surfaces of aircraft structures.

Mechanisms of fretting fatigue

Abstract: Fretting fatigue is one of the most important phenomena for inducing a dramatic reduction of fatigue strength and consequently, of unexpected failure accidents. There are numerous works on fretting fatigue. The objective of the present review is to clarify fretting fatigue mechanisms based on the vast amount of research work. A fretting fatigue crack is initiated at the highest point of tangential force on a contact surface in the very early stage of fatigue life. The tangential force is the dominant factor in enhancing crack initiation and propagation and hence, in reducing fatigue strength. Further studies on short crack and mixed-mode crack growth behavior in fretting fatigue are needed.

On the Distribution of Rainflow Range for Gaussian Random Processes with Bimodal PSD

Abstract: The distribution of rainflow range is investigated for the Gaussian random process using bimodal PSD. The new method evaluates the distribution of range by taking the sum of two Rayleigh distributions which correspond to two dominant frequencies in bimodal PSD. Representative distribution data have been obtained from rainflow analysis for simulated time histories. The applicability of the proposed method is examined for the range distribution generated by the simulation. It is shown that the method estimates the distribution of range with high accuracy even, in the case of a random wave with a large irregularity factor. Since range data for unimodal PSD can be evaluated by Rayleigh approximation, the new method for bimodal PSD greatly extends the region of analytical treatment.

Inverse analysis of the magnitude and direction of impact force

Abstract: This paper treats an inverse problem to estimate the magnitude and direction of impact force acting on a body of arbitrary shape. Three directional components of the impact force vector can be estimated from responses measured at more than three points of the body by means of deconvolution. It is pointed out that the impact force cannot always be estimated satisfactorily since the inverse problem is ill-posed or illconditioned in some situations. To improve the estimation accuracy, the least squares method based on the singular value decomposition is adopted. The validity of this method is demonstrated through an experiment in which the impact force acting on a simply supported beam is estimated from strain responses measured by strain gages.

Effect of out-of-plane stress on the forming limit strain of sheet metals

Abstract: In press forming of sheet metals, the material sheet is usually subjected to very large plastic strain under in-plane stressing Moreover, the sheet also very often is subjected to out-of-plane compressive force between tools such as the upper and lower dies, the blank holder and the die, and so forth However, the forming limit strain of sheet metals has been investigated so far only for the plane stress state without the effect of such out-of-plane force both theoretically and experimentally In this paper, it is clearly demonstrated theoretically that out-of-plane stress (even as small as one tenth of the yield stress) may notably raise the forming limit strain and thus it can be effectively utilized to avoid earlier fracture of the sheet in press forming The classical Swift's or Hill's criterion of instability, Storen & Rice's condition together with the J2G plasticity constitutive equation (previously proposed by one of the authors, Gotoh) for localized necking, and the numerical simulation of plane-stress and plane-strain tension of a sheet by the finite-element method (FEM) are applied here to discuss this problem, all leading us to the same conclusion

Application of Genetic Algorithms to Stiffness Optimization of Laminated Composite Plates with Stress-Concentrated Open Holes

Abstract: Recently, laminated composite plates have been applied to many aircraft structures because their mechanical properties are superior to those of conventional materials. Since the laminates have anisotropic elastic properties, an optimum design is needed to make advantageous use of composite laminates. The authors have proposed a successful object-oriented expert system to design laminated composites with actual constraints. A solution to optimize the fiber orientation of stiffeners around open holes, however, has not yet been developed because it is very difficult and requires stress redistribution analysis. In this study, therefore, genetic algorithms (GA) were applied to solve the optimization problem by the object-oriented finite-element stress analysis method. Results obtained were as follows. (1) The GA method is applicable to an open hole model. (2) The random search method is easily applicable to any model.

Crack-Tip Singularity Fields in Nonhomogeneous Body under Thermal Stress Fields

Abstract: In this investigation, singular fields of stresses and heat fluxes at the tip of a crack in a nonhomogeneous body, or functionally gradient material (FGM), are studied under both elastic and plastic conditions. It is found that the crack tip fields have the same forms as those in the homogeneous material, provided that the material properties of FGM are continuous. The heat fluxes have a square-root singularity at the crack tip, and the elastic crack tip stress field is characterized by the K-field of linear fracture mechanics and the plastic singular field is the HRR field for a power-law material. The relationship between stress intensity factors and the path-independent integral J proposed by Aoki, Kishimoto and Sakata (1) is discussed.

Determination of the optimum blank shape when deep drawing nonaxisymmetric cups, using a finite-element method (simplified determination of optimum blank shape and limiting drawing ratio by inverse finite-element procedure)

Abstract: The article is concerned with blank development when deep drawing nonaxisymmetric, prismatic cups with a flat bottomed punch. Two processes are considered, one of which is the calculation of optimum blank shapes when deep drawing square cups ; here optimum means that the cups have essentially a flat top after the drawing process. The second process is to design a blank such that the finished component is a partially drawn cylindrical cup possessing a square flange of some specified dimensions. An inverse finite-element technique was developed to perform the analysis for calculating the blank shapes. In addition to blank shapes, it was possible to predict the limiting drawing ratios, the maximum draw depth and the drawing load. Although the entire drawing process is not modeled, it is possible to assess the influence of certain processing parameters such as blank shape, the coefficient of friction, blank holder force, and die radius on the drawing force and limiting drawing ratio. Some of the theoretical predictions for the limiting blank size in deep drawing square cups from optimum-shaped blanks are compared with published data, and the agreement is found to be reasonable.

Reinvestigation on Optimization of Clamped-Clamped Columns and Symmetry of Corresponding Eigenfunctions

Abstract: In this paper, the optimization problem of the clamped-clamped column under buckling load, which was previously dealt with by Tadjbakhsh & Keller (Trans. ASME, J. Appl. Mech., Vol. 29, No. 1 (1962), p.159) and Olhoff & Rasmussen (Int. J. Solids Struct., Vol. 13, No.7 (1977), p.605), is reinvestigated. Both numerical and analytical methods based on the single and bimodal formulations are used in investigating the behavior of the optimum eigenvalues and the corresponding eigenfunctions with respect to the minimum constraint of cross-sectional area. The conditional optimum column under the single modal formulation is determined. The optimum column together with two mutually symmetrical eigenfunctions under bimodal formulation is obtained. Furthermore, on the basis of that we deduce the orthogonal optimum eigenfunctions, with the 1st-order eigenfunction having a symmetrical configuration and the 2nd-order eigenfunction having an anti-symmetrical one. Finally, using Keller's analytical solution, we derive several analytical solutions for the columns mentioned in previous papers.

Wavelet Analysis of Dispersive Stress Waves

Abstract: The time-frequency analysis of dispersive stress waves is reviewed. It is shown that the wavelet transform using the Gabor wavelet effectively decomposes the strain response into its time-frequency components, and the peaks of the time-frequency distribution indicate the arrival times of waves. The flexural waves induced in a beam by lateral impact are considered and it is shown that the wavelet transform enables us to identify the dispersion relation of the group velocity, and to estimate the impact location. In addition, the potential of using the wavelet transform for more detailed nondestructive evaluation of material damage is shown.

Recent progress in nonlinear FEM-based sensitivity analysis

Abstract: This paper reviews the recent progress in sensitivity analysis applied to the nonlinear finite element method (FEM). The sensitivity analysis method has been successfully developed over the years as a convenient tool for use in linear FEM, and it is reasonable to say that today the framework of the methodology is nearly completed. Nonlinear FEM-based sensitivity analysis, however, has only received considerable research attention since the mid 80's. Regarding static nonlinear problems, path-dependent problems such as elastic-plastic problems have been investigated by a number of research groups, and several competitive methods exist. Sensitivity analysis is also practically important in nonlinear buckling and postbuckling problems, for which several methods have been proposed. The performance of the proposed methods in static and buckling/postbuckling problems are compared, and some numerical examples are introduced to clarify the characteristics of the sensitivity which are peculiar to nonlinear problems.

Strip element method for characterization of flaws in sandwich plates

Abstract: The strip element method (SEM) is extended to investigate wave scattering by rectangular flaws in sandwich plates. Scattered wave fields in the frequency domain for sandwich plates with rectangular flaws are computed and investigated in comparison with results for corresponding plates without flaws. A technique for determining the length and the damage factor of a rectangular flaw in a sandwich plate is presented. In this technique, a distributed harmonic load is scanned along the surface of the plate. Responses of the plate at the central point of the load are computed by suing the SEM, and a scanning curve is obtained. The image of the flaw can be observed from the scanning curve. Numerical experiments are conducted to demonstrate this technique.

Atom probe study of the aging embrittlement of cast duplex stainless steel

Abstract: In this paper we discuss an analysis, with an atom probe, of nanometer-scale microstructural changes that occur during thermal aging of the ferritic phase of duplex stainless steel. The changes in electrochemical behavior and the magnetic hysteresis loop are examined and related to the microstructural changes. Cast duplex stainless steel containing 12.5% ferritic phase with a nominal composition of Fe-19%Cr-10%Ni-2%Mo was aged at 475°C for 443 h. By analysis, it is found that aging promotes a significant phase separation of the ferrite into Cr-rich and Fe-rich regions with spatial durations of 10 - 20 nm. An electrochemical reactivation peak observed in the polarization curve of aged steel is thought to be related to the Fe-rich region with a Cr concentration below 14 at% (13% in weight). Nanometer-scale microstructural changes are shown to cause hardening and increase the loss in magnetic hysteresis.

Stress Measurement of Transparent Materials by Polarized Laser

Abstract: Optical birefringence is measured by a high-frequency modulation method using a photoelastic modulator and a polarized laser. This measurement has the high sensitivity required to measure very small birefringence produced by stress. By this measuring method, the magnitudes of the differences in principal stresses and their directions are obtained directly and quantitatively. The photoelastic properties of glass, polymethyl methacrylate (PMMA) and two kinds of epoxy resins are measured with the equipment. The distributions of optical birefringence corresponding to stress distributions are measured for glass plates bonded with epoxy resin. The residual stress caused by the bonding process is measured. This stress measuring method is applicable to detection of the stress distribution of transparent solid materials.

Integrated FEM Formulation for Total/Updated-Lagrangian Method in Geometrically Nonlinear Problems

Abstract: This paper presents a new integrated FEM formulation for geometrically non-linear analyses. There have been two methods to solve the so-called large displacement problem, i.e., the total-Lagrangian method and the updated-Lagrangian method, and two types of FEM programming have been conventionally written. It is shown in the present paper that these two types of programming can be combined by the use of the covariant components of the incremental Green-Lagrange strain tensor and the contravariant components of the second Piola-Kirchhoff stress tensor in the convected coordinate system. The difference is only seen in the transformation of the constitutive tensorial components. The stiffness matrices for the solid and shell elements by this formulation are illustrated in detail, and advantages and disadvantages of the proposed method are also discussed.

Fatigue Crack Propagation for Cast Iron Rotating Disk

Abstract: This paper studies the crack propagation rate for three kinds of cast iron rotating disks. Fatigue crack propagation tests were carried out for flake, compacted vermicular and spheroidal graphite cast iron disks under repeated spin loading. The crack propagation rate of the flake graphite disk was the largest, while that of the spheroidal graphite disk was the smallest. The crack propagation rate of the compacted vermicular graphite disk had the intermediate crack propagation rate between the two cast iron disks. The stress intensity factor range could not correlate the crack propagation rate for the three kinds of disks, while the J-integral range, which was calculated in finite-element analyses, could correlate the crack propagation rate within a factor of five scatter bands. The stress intensity factor range modified by the tensile strength of the material was also effective for correlating the crack propagation rate for three kinds of cast iron.

Creep Hardening Rule under Multiaxial Repeated Stress Changes

Abstract: Anisotropic creep behavior of polycrystalline metals under multiaxial nonproportional repeated loading conditions is modeled from phenomenological points of view. The creep model consists of basic constitutive equations (BCEs) and an auxiliary hardening rule (AUX) to enhance the predictive capability of the BCEs. The BCEs are specified on the basis of a modification of the conventional kinematic hardening model, and they are characterized by a new kinematic hardening variable which is defined as the sum of two component variables ; one represents the back stress in the conventional sense and the other a flow resistance in the opposite direction of the deviatoric stress. The AUX is governed by a memory region in which only the evolution of the back stress takes place. Two different formulations of the AUX are presented. The applicability of the creep model is discussed on the basis of simulations for multiaxial nonproportional repeated creep of type 304 stainless steel at high temperature.

Creep-Fatigue Interaction of Modified 9Cr-1Mo Steel in a Very High Vacuum Environment and Experimental Analysis of Overstress

Abstract: Creep-fatigue tests were conducted with Modified 9Cr-lMo steel at 600°C in a very high vacuum of 0.1 μPa in order to investigate a pure creep-fatigue behavior which is free from the environmental effect of the air. On the whole, the time-dependent life reduction occurs when the duration of tension is longer than that of compression. A good correlation was found between the creep-fatigue life and the fracture mode. When the fracture mode is transgranular, no time-dependent life reduction occurs. In contrast, when it is predominantly intergranular, a significant life reduction is observed. The overstress was experimentally analyzed to evaluate the creep-fatigue damage. The relationship between the overstress and the inelastic strain rate can be fitted, being independent of the strain range.

Intra-Ball and Inter-Ball Variability of Ring Crack Initiation Load of Silicon Nitride Bearing Balls

Abstract: When the ring crack initiation load P R of ceramic bearing balls is measured at one position per ball, P R shows considerable scatter as shown in a previous paper. In order to clarify which of the inter-ball variability and the intra-ball variability is responsible for this scatter, a new type of test was carried out in the present study on HIP-Si 3 N 4 bearing balls in which P R was measured at 8 positions per ball. Initiation of a ring crack was detected using acoustic emission (AE). From analysis of variance, it was shown that the scatter of P R observed when measuring P R at one position per ball was mainly due to the intra-ball variability or the spatial variation of P R within one ball.

Inverse Analysis of Galvanic Corrosion Using Fuzzy A Priori Information

Abstract: A new method for estimating the galvanic corrosion rate is developed. In the case where the electrochemical polarization curves of metal in the electrolyte are available, the galvanic corrosion rate can be estimated by solving the Laplace equation with boundary conditions based on the polarization curves. However, the polarization curves are not always available. In such a case, it is necessary to estimate the density of the current across the metal surfaces from the potential values measured at points far away from the metal surfaces. Since this inverse problem is ill-conditioned, direct application of the conventional method results in an oscillatory solution. To circumvent this problem, an inverse problem is formulated and the boundary element method is employed to estimate the polarization curves. In addition, the estimation accuracy is improved by using a priori information which is expressed as fuzzy membership functions. Sample problems are also presented to demonstrate the effectiveness of this new method.

Ring Crack Initiation Load of Silicon Nitride Bearing Balls

Abstract: In relation to the static load rating of silicon nitride bearing balls, the ring crack initiation load P R of hot isostatically pressed Si 3 N 4 balls was investigated experimentally with emphasis on its statistical property. Initiation of a ring crack was detected using acoustic emission(AE). P R approximately followed a 2-parameter Weibull distribution with a shape parameter of about 9. An attempt was made to predict the mean and scatter of P R using bending test data based on the concept of effective area. The predicted values were compared with the experimental results.

Development of a sensitivity analysis method for nonlinear buckling load

Abstract: An analysis method for evaluating the sensitivity of buckling load in geometrically nonlinear finite element methods is developed. By searching for the pseudo critical point in the perturbed system on the surface of the hyper-cylinder where the original critical point exists, the sensitivity of buckling load is estimated. Compared with other methods, the present method can cope with both snap-through and bifurcation problems without an eigenvalue analysis. In addition, the load sensitivity at arbitrary points on the equilibrium path can be calculated under the total-displacement constraint condition. Two examples of sensitivity buckling analysis of shell structures are demonstrated to examine the validity of this method, and satisfactory results are obtained.

Damage Monitoring of Metal Materials by Laser Speckle Assisted by Image Processing Techniques : Relationship between Distribution of Laser Speckle and Surface Properties

Abstract: In this study, we investigated a noncontact method of measuring plastic strain of metals using laser speckle. This method is based on observation of the change in laser speckle pattern with surface profile change due to plastic deformation. Analysis of the laser speckle pattern is performed on a computer-based image processing system. The relationships between speckle pattern and surface roughness and frequency characteristics of the surface profile were investigated. Surface roughness and surface profile were observed for steel specimens polished with emery paper or plastically deformed. The surface profile diagram was analyzed by means of the fast Fourier transform with a computer and distribution of the spatial frequencies was obtained. The results showed that distribution of light intensity of the speckle is related to frequency distribution of the surface profile, but it is not always related to surface roughness. It was also clarified that the laser speckle pattern is closely related to the magnitude of plastic strain.

Scanning Stress Measurement Method by Laser Photoelasticity : 1st Report: Stress Measurement by Synthesized Method

Abstract: Optical birefringence is measured by a high-frequency modulation method using a photoelastic modulator and polarized laser. A modified synthesized method was developed and applied for the measurement of stress distributions of a glass plate. The distributions of the differences of principal stresses and their directions were obtained directly by this method. The stress distributions under tensile load of a pulled rectangular glass plate with a hole at its center were measured. The results of stress distributions agreed with the analytical results. It was confirmed experimentally that the spatial resolution of stress measurement was the same as the diameter of the laser light. The stress at many points could be obtained quickly by the synthesized method and scanning stress distribution measurement was realized.

Elasto/Visco-Plastic Deformation of Shells of Revolution under Thermal Loading due to Fluid

Abstract: An analytical method for the elasto/visco-plastic deformation of axisymmetrical thin shells subjected to thermal loads due to fluid is developed. First, the temperature distribution through the thickness is assumed to be a curve of the second order, and the temperature field in the shell under appropriate initial and boundary conditions is determined using the equations of heat conduction and heat transfer. Secondly, the stresses and deformations are derived from the thermal stress equations. The equations of equilibrium and the relationships between the strains and displacements are derived from the Sanders elastic shell theory. For the constitutive relations, the Perzyna elasto/visco-plastic equations which consider the temperature effect are employed. The fundamental equations derived are numerically solved using the finite difference method. As a numerical example, a simply supported internally pressurized cylindrical shell of aluminum under thermal loading due to fluid is analyzed, and the variations in displacements and internal forces with time are discussed.

Cavity Growth in Polycrystalline Materials under Grain Boundary Diffusion Creep and Transition from Cavity to Crack

Abstract: The stress distribution along grain boundaries in polycrystalline materials under the grain boundary diffusion creep (Coble creep) condition is numerically analyzed in order to elucidate the effect of the grain boundary network on cavity growth. The stress gradient at the cavity tip, which is proportional to the changing rate of the cavity volume, points out that the triple junction of grain boundaries interferes with cavity growth, though the cavity grows faster after the tip goes beyond it. The deceleration effect due to the triple junction diminishes as the cavity becomes larger. The transition from crack-like cavity to crack is also discussed on the basis of the stress field near the tip. The crack and the cavity are distinguished by the fact that the former possesses the stress singularity in the vicinity of the tip while the latter does not. The calculation reveals that the transition under the Coble creep condition does not take place before the cavity length exceeds 15 grain boundaries. On the other hand, the transgranular deformation due to dislocation creep drastically accelerates the transition.

Cyclic Deformation of TiNi Shape Memory Alloy

Abstract: The cyclic deformation properties of TiNi shape memory alloy wires and helical springs for the martensitic transformation and the R-phase transformation were investigated experimentally. The results are summarized as follows. For cyclic deformation associated with the martensitic transformation, the transformation stress decreases and the transformation temperature rises. The amount of variation in the transformation stress and the transformation temperature is large in the early number of cycles and is small after about the 20th cycle. For cyclic deformation associated with the R-phase transformation, the transformation stress and the transformation temperature vary little. For cyclic deformation of helical springs, the recovery deflection is very large in the R-phase transformation region. The recovery deflection and the recovery force of the helical spring under cyclic deformation vary slightly even in the martensitic transformation region.

Characterization of Particle Impact Damage and Residual Strength Degradation Behaviors in Structural Ceramics

Abstract: In order to characterize particle impact damage in structural ceramics, the damage morphologies and the resulting strength degradation behaviors were investigated for silicon carbide and silicon nitride, using particles of different hardness. The types of damage induced in each specimen were closely related to the deformation behavior at the impact site. Particle impact damage could be characterized by the deformation behaviors, such as Hertzian cone cracks in the case of silicon carbide showing nearly elastic deformation and median cracks in the case of silicon nitride representing significant plastic deformation. Silicon nitride showed a somehow higher resistance for particle impact damage compared with silicon carbide. The effects of the particle hardness on the extent of damage and the residual strength degradation were significant in the case of silicon nitride but slight in the case of silicon carbide. The residual strength after impact test had a good correlation with the characterized crack size.

Neural Evaluation of Friction and Flow Stress Adaptive to Ring Geometry

Abstract: Interfacial friction and material flow stress can be evaluated through the use of calibration curves in ring compression testing. In this study the neural network approach has been extended to their evaluation adaptive to ring geometries of wider range. The ring geometries covered were in the range of 6 : 3 : 0.5 to 6 : 3 : 2 (OD : ID : T 0 ), which are the most commonly used values. Data for training the networks were acquired in the same way as in the development of the calibration curves. A serial scheme for the evaluation was found to be effective when multilayered BP (backpropagation) networks were employed. Network construction, network training including the selection of learning parameters, and implementation of the trained network are also detailed in this paper. Predictions for different ring geometries and friction factors were conducted and satisfactory results were obtained with prediction error of about 5%, at maximum, for both friction and flow stress.

Toughening mechanism for ceramics by a ductile metallic phase

Abstract: The basic objective of this study is to establish the material design concepts for the coexistence of good high-temperature mechanical properties and super heat resistance in the systems of ceramic/metal and ceramic/ceramic functionally gradient materials (FGMs). Flexural and fracture toughness tests were conducted on gas-pressure combustion-sintered Cr 3 C 2 ceramic and its composites with Ni and TiC (Cr 3 C 2 /Ni, Cr 3 C 2 /TiC) in a vacuum environment up to 1 200°C using a newly developed materials testing system. Fracture characteristics were investigated on the basis of fracture mechanics and fractography. The toughness of Cr 3 C 2 can be improved by the addition of metallic particles. It is found that the principal mechanism of toughness improvement in these composites (ductile-phase reinforced ceramic matrix composites) is attributable to crack-tip plastic blunting by a ductile metallic phase.