Showing papers in "Jsme International Journal Series A-solid Mechanics and Material Engineering in 1999"
TL;DR: In this article, a new structural health monitoring technique capable of in-service, on-line incipient damage detection has been proposed by the Center for Intelligent Material Systems and Structures, physical changes in a structure cause changes in the mechanical impedance.
Abstract: A new structural health monitoring technique capable of in-service, on-line incipient damage detection has been proposed by the Center for Intelligent Material Systems and Structures, Physical changes in a structure cause changes in the mechanical impedance. Due to the electromechanical coupling in piezoelectric materials, this change causes a change in the electrical impedance of the piezoelectric sensor. Hence, by monitoring the electrical impedance and comparing this to a baseline measurement, we can determine when structural damage has either occurred or is imminent. However, in almost all practical health monitoring applications, the structure being monitored is constantly undergoing change due to the effect of external boundary conditions. One of the important factors that leads to this change is the temperature variations. In this paper, temperature effects on the electrical impedance of piezoelectric materials and the structures have been investigated. A computer algorithm was developed which incorporates temperature compensation into our health monitoring applications. Three experimental investigations were performed successfully under the temperature varying condition, in the range of 25 to 75°C, including a bolted pipe structure, composite reinforced aluminum and precision part such as gears. It was found that, by this compensation procedure, the impedance based health monitoring tehcnique is able to detect damage in the incipient stage, even with the presence of significant temperature variation.
265 citations
TL;DR: In this paper, it was shown that inelastic deformation of silicon monocrystals is solely caused by amorphous phase transformation, and the onset of deformation can be predicted by a criterion considering either the octahedral or maximum shear stress.
Abstract: This paper aims to investigate some fundamental problems of mechanics and physics in the nano-indentation of silicon monocrystals with the aid of molecular dynamics analysis. The study showed that inelastic deformation of silicon monocrystals is solely caused by amorphous phase transformation. Dislocations do not appear and purely elastic deformation exists only in an extremely narrow regime. The onset of inelastic deformation can be predicted by a criterion considering either the octahedral or maximum shear stress. Due to phase transformation, the conatct area between indenter and specimen varies in a very complex manner. The study offers a new theory for the nano-indentation of monocrystalline silicon.
126 citations
TL;DR: In this article, a review of impact perforation of fiber composite laminates by foreign objects traveling at high speed has been conducted, where the target materials include glass, carbon, and aramid fiber reinforced plastic composites, while the foreign objects consist of steel ball projectiles and cylindrical projectiles with a conical, hemispherical or flat tip.
Abstract: In this paper, recent works on impact perforation of fiber composite laminates struck by foreign objects traveling at high speed have been reviewd. The review focuses on two major subjects in impact perforation research, i.e., an assessment of perforation characteristics and an understanding of the perforation process and its mechanisms. The target materials include glass, carbon, and aramid fiber-reinforced plastic composites, while the foreign objects consist of steel ball projectiles and cylindrical projectiles with a conical, hemispherical or flat tip. The ballistic limit velocity, residual velocity, and perforation energy obtained from the impact tests and predicted from the analytical and computer models are reviewed first. Then the perforation process and mechanisms are discussed based on the experimental observations and computer simulations. The present article provides state-of-the-art information in the rapidly developing field of impact perforation of fiber composite laminates, by referring mainly to literature published over the past decade.
34 citations
TL;DR: In this article, the authors considered the electroelastic problem for a piezoelectric layer with a crack bonded to two elastic half planes under antiplane shear loading and used Fourier transforms to reduce the problem to the solution of a pair of dual integral equations.
Abstract: Following the theory of linear piezoelectricity, we consider the electroelastic problem for a piezoelectric layer with a crack bonded to two elastic half planes under antiplane shear loading. The crack parallel to the interfaces is in the mid-plane of the piezoelectric layer. Fourier transforms are used to reduce the problem to the solution of a pair of dual integral equations. The solution of the dual integral equations is then expressed in terms of a Fredholm integral equation of the second kind. Numerical values of the stress intensity factor for some piezoelectric laminates are obtained, and the results are plotted to display the electroelastic interactions.
32 citations
TL;DR: In this article, a genetic algorithm is applied to obtain the optimal stacking sequences for maximization of the buckling load of composite cylinders, where three constraint stacking rules are implemented. And the design reliability of the genetic algorithm with the new repair system is examined in detail by comparing the reliability with a conventional penalty method.
Abstract: A genetic algorithm is applied to obtain the optimal stacking sequences for maximization of the buckling load of composite cylinders. In the problems, three constraint stacking rules are implemented. The first is that the angle plies of laminates have to be balanced. The second is that more than four contiguous plies of the same fiber orientation are not allowed. The third is that the difference in fiber orientation between adjacent plies must not be more than 45 degrees. The difficulty of handling the combinatorial constraints in genetic optimizations is overcome by adoption of a new repair system. When a chromosome violating the stacking constraints is decoded to a stacking sequence, the new repair system operates. The new repair system does not alter genes of the chromosome but changes only the decoding rules to introduce the constraints. This is similar to recessive genes in biology. By using this new repair system, a chromosome of {2 2 211 1 0 0 0} is decoded to [90/90/90/45/90/- 45/- 45/ 0/45] S. The design reliability of the genetic algorithm with the new repair system is examined in detail by comparing the reliability with a conventional penalty method. As a result, the genetic algorithm with the new repair system is shown to provide higher design reliability.
30 citations
TL;DR: In this paper, a unified constitutive model considering dynamic strain aging effect was developed in order to describe inelastic deformation behavior of modified 9Cr-1Mo steel precisely, and numerical simulations were conducted to evaluate the validity of the model.
Abstract: A unified constitutive model considering dynamic strain aging effect was developed in order to describe inelastic deformation behavior of modified 9Cr-1Mo steel precisely. The inelastic behavior of the material was summarized as follows. A rate-dependent deformation was observed above 500°C, and there was no rate-dependency under 400°C. However, stress relaxation behavior under constant strain was observed even at rate independent temperature region. Further, the stress after relaxation depended on prior loading strain-rate, and it showed a higher value as the strain-rate was decreased. A feature of the proposed constitutive model was that an applied stress consisted of three components:a back stress, an overstress and an aging stress which corresponded to dynamic strain aging effect and showed a negative strain-rate-dependency. The aging stress was measured by stepwise strain-rate change tests, and it showed a larger value at smaller strain-rate and lower temperature. The back stress and the overstress were measured by strain dip tests. The back stress was approximately rate-independent below 400°C, however it showed rate-dependency above 500°C. The overstress showed a larger value as the strain-rate or the temperature was increased. Material constants involved in the constitutive model were determined systematically based on the measured values of these stress components. In order to evaluate the validity of the constitutive model, numerical simulations were conducted for various inelastic deformation behavior of modified 9Cr-1Mo steel. The simulations agreed with experimental results very well in all cases.
25 citations
TL;DR: In this paper, a precise method for estimating axial stress for short and highly stressed bolts used for automobile is proposed, which takes into account of nonlinear extension due to non-uniform stress distribution.
Abstract: A precise method for estimating the axial stress is proposed for the short and highly stressed bolts used for automobile These bolts show non-linear extension, because of local plastic deformation as well as non-uniform stress distribution in bolt cross section The proposed method consists of(a)precise measurement of the time-of-flight of longitudinal and transverse waves using a digital pulse-echo technique, (b)to establish the calibration curves which take into account of nonlinear extension due to non-uniform stress distribution based on simultaneous measurement of the axial elongation, time-of-flight of longitudinal and transverse waves and(c)evaluation of the axial stress with the change in the time-of-flight and the assumed length of equivalent stressed length The axial stress is estimated within 1% error for one type bolts and within 2% error for another type bolts
22 citations
TL;DR: In this article, the effect of water environment on delamination fatigue crack propagation behavior was investigated using unidirectional CFRP laminates made from Toray prepregs T800H/3900-2.
Abstract: The effect of water environment on delamination fatigue crack propagation behavior was investigated using unidirectional CFRP laminates made from Toray prepregs T800H/3900-2. T800H/3900-2 has toughened interlayer which is composed of epoxy resin and thermoplastic polyamide particles. Tests were carried out under mode II loading using end notched flexure specimens which were moisture-conditioned up to the saturated level. In the static tests, the value of the fracture toughness for the moisture-saturated specimen was slightly smaller than that for the dry specimen in air. In the fatigue tests, the crack propagation rate, da/dN, was expressed as a power-low function of the maximum energy release rate, GII max in the region where da/dN>5×10-10m/cycle. Below this region, there exists the threshold value of GII max. The threshold value for the moisture-saturated specimen in water was smaller than that for the dry specimen in air. Glass transition temperatures were measured both in moisture-saturated state and in dry state using differential scanning calorimetry. The decrease in the glass transition temperature indicated the ductility increase of epoxy. Microscopic observation revealed that the ductility increase of the epoxy affected the morphology of the fracture surfaces under fatigue loading.
21 citations
TL;DR: In this article, three types of rubber toughened poly(methyl methacrylate)(RT-PMMA) were investigated under mode I loading condition by optical and electron microscopies in conjunction with quantitative evaluation of mode I fracture toughness.
Abstract: Toughening mechanisms of three types of rubber toughened poly(methyl methacrylate)(RT-PMMA) were investigated under mode I loading condition by optical and electron microscopies in conjunction with the quantitative evaluation of mode I fracture toughness. Polarizing optical microscopy clearly exhibited damage zone development ahead of a crack-tip in the RT-PMMA's. The three RT-PMMA's revealed different shapes of the damage zone. Transmission electron microscopy exhibited microcrazes generated in the equator of rubber particles within the damage zones. Extensive deformation of rubber particles corresponding to localized shear yielding of the PMMA matrix was also found in a region close to a propagating crack-tip. In addition, cavitation of rubber particles was observed in the vicinity of the crack. It is therefore understood that the toughening of the RT-PMMA's is due to energy dissipation caused by the microdamage formations such as microcrazing, matrix shear deformation and rubber particle cavitation ahead of the crack-tip.
21 citations
TL;DR: In this paper, the effect of the material used for a tool on atomic scale indentation and cutting mechanisms of metal workpieces is described by means of molecular dynamics simulations, where the interatomic force between the tool and workpiece is assumed to be a two-body interatomic potential using parameters based on the ab-initio molecular orbital calculation for a(Cr, Ni)-(C, Si)6H9 atom cluster.
Abstract: This paper describes the effect of the material used for a tool on atomic scale indentation and cutting mechanisms of metal workpieces, by means of molecular dynamics simulations. The interatomic force between the tool and workpiece is assumed to be a two-body interatomic potential using parameters based on the ab-initio molecular orbital calculation for a(Cr, Ni)-(C, Si)6H9 atom cluster. Molecular dynamics simulated the atomic scale indentation and cutting process of the chromium and nickel workpieces using the diamond, silicon and diamond-like carbon(DLC) tools. The diamond and DLC tools formed the indentation mark. Young's modulus of the chromium and nickel in indentation simulations was larger than that in experiments. This was qualitatively explained by the effect of the surface energy for the workpiece on the elastic modulus. The machinability of the chromium and nickel with the diamond tool was better than that of the silicon tool in atomic scale cutting simulations. The depth of the cut for the workpieces in nano scale cutting experiments with AFM, was similar to that in atomic scale cutting by molecular dynamics simulations.
21 citations
TL;DR: In this article, an incremental constitutive relation of the composites with progressive cracking damage of the reinforcements has been developed based on Eshelby's equivalent inclusion method and Mori and Tanaka's mean field concept.
Abstract: In particle or short-fiber reinforced composites, cracking or debonding of the reinforcements is a significant damage mode because the damaged reinforcements lose load carrying capacity. This paper deals with a theory of the reinforcement damage in discontinuously-reinforced composites and its application. The composite with progressive cracking damage contains intact and cracked reinforcements in a matrix. To describe the load carrying capacity of the cracked reinforcement, the average stress of a broken ellipsoidal inhomogeneity in an infinite body which was proposed in the previous paper is introduced. An incremental constitutive relation of the composites with progressive cracking damage of the reinforcements has been developed based on Eshelby's equivalent inclusion method and Mori and Tanaka's mean field concept. This damage theory can describe not only cracking damage but also debonding damage of the reinforcements by modifying teh load carrying capacity of damaged reinforcements. Influence of the reinforcement damage on the stress-strain response and elastic stiffness of the composites is discussed. It is noted that tha full-debonding damage gives the lower limit of the stress-strain relation of the composite with progressive reinforcement damage.
TL;DR: In this paper, a new X-ray method is proposed to measure the residual stress in cubic polycrystalline films having the fiber texture with the axis of and perpendicular to the film surface.
Abstract: A new X-ray method is proposed to measure the residual stress in cubic polycrystalline films having the fiber texture with the axis of and perpendicular to the film surface. The elastic constants of textured thin films were calculated on the bases of Reuss and Voigt models. According to the analysis based on Reuss model, the relation between the strain measured by X-ray, e^-L and sin2 ψ for the equi-biaxial stress is linear for the cases of and fiber textures. For the other cases, the relation is non-linear. A method to determine the stress from non-linear relations is proposed. The analysis based on Voigt model gives the linear relation between e^-L and sin2 ψ for any case of fiber texture. In thin films made of materials with low anisotroy, the both models give nearly identical relation.
TL;DR: In this paper, a solution procedure based on displacement and electric potentials is reviewed, and the determination of unknown thermal loadings inferred from prescribed electric potential differences is illustrated for a number of cases.
Abstract: Reference is made to investigations by various authors dealing with piezothermoelastic response of beams, plates, shells and adaptive structural systems. The emphasis of this paper is, however, directed at solutions to inverse problems considered by the present authors during the past few years. A solution procedure based on displacement and electric potentials is reviewed, and the determination of unknown thermal loadings inferred from prescribed electric potential differences is illustrated for a number of cases.
TL;DR: In this article, the aging in quenched and strained polymethyl methacrylate (PMMA) has been studied using a differential scanning calorimetry (DSC) and a thermally stimulated deformation recovery measurement as well as a mechanical compression test.
Abstract: Aging in quenched and strained polymethyl methacrylate (PMMA) has been studied using a differential scanning calorimetry (DSC) and a thermally stimulated deformation recovery measurement as well as a mechanical compression test. The aging in quenched and strained PMMA was compared with the physical aging in quenched PMMA with no strains. The PMMA samples strained in compression before the aging showed an evolution of yield stress with aging time much faster than that in quenched samples with no strains. The I)SC analysis, however, showed that the endothermic peak for the quenched and strain-aged PMMA was less developed than that for the quenched PMMA with no strains. This paradox was analyzed using the deformation recovery data for quenched and strain-aged PMMA. The analysis led us to the conclusion that the evolution of yield stress in quenched and strain aged PMMA is probably due to the structural change locking the strain energy tightly in the polymeric structure.
TL;DR: In this article, the residual stress distributions in a 4-inch-diameter carbon-steel pipe butt-welded joint were evaluated using five methods, including inherent strain analysis, thermal elastic-plastic analysis, X-ray diffraction and strain-gauge measurement for surface residual stress and as well as neutron diffraction for internal stress.
Abstract: Residual stress distributions in a 4-inch-diameter carbon-steel pipe butt-welded joint were evaluated using five methods. The analytical evaluation methods used were inherent strain analysis and thermal elastic-plastic analysis. The experimental methods were X-ray diffraction and strain-gauge measurement for the surface residual stress and as well as neutron diffraction for the internal stress. The residual stress distributions determined using these methods agreed well with each other, both for surface stress and internal stress. The characteristics of the evaluation methods were summarized, and it was found that the most suitable method for any particular situation can be selected depending on the purpose by considering the evaluated location and the operating conditions of the object to be evaluated.
TL;DR: In this article, a moving finite element method for dynamic interfacial fracture analysis was developed to make it possible to analyze transonic and supersonic interfacial crack propagation, and it was found that compliant material supplies larger fracture energy than the stiff material does.
Abstract: First, the concepts of separated dynamic J integral and separated energy release rate were presented for dynamic interfacial fracture mechanics. The separated dynamic J integral has the physical meaning of the energy flow rate to a propagating interfacial crack tip from each material component. Next, a moving finite element method for dynamic interfacial fracture analysis was developed to make it possible to analyze transonic and supersonic interfacial crack propagation. The moving finite element simulations revealed the shock waves (Mach waves) emanated from transonically and supersonically propagating crack tips. The dynamic J integral and the separated dynamic J integrals showed excellent path independence for all crack velocity regimes even for supersonic interfacial crack propagation. It was found that, in the subsonic crack-velocity regimes, the compliant material supplies larger fracture energy than the stiff material does. From the energy flow rate to the propagating interfacial crack tip, the theoretical limit of interfacial crack velocity without macro contact zone was found to be the shear wave velocity of the compliant material.
TL;DR: In this article, the authors analyzed the composition profile of semi-infinite functionally graded materials (FGMs) with a view to improving the brittle fracture characteristics of these materials, and derived the stress intensity factor of an edge crack under a far-field uniform applied load.
Abstract: In this study, we analyze the composition profile of semi-infinite functionally graded materials(FGMs) with a view to improving the brittle fracture characteristics of these materials. First, a method is developed to calculate the stress intensity factor of an edge crack in semi-infinite homogeneous media with distributed eigenstrain under a far-field uniform applied load. The crack is represented by the distribution of edge dislocations, and a singular integral equation with a Cauchy-type kernel is obtained using the complex potential functions of the edge dislocations. Second, a useful approximation method is established to simulate the nonhomogeneity of semi-infinite FGMs by an equivalent eigenstrain. The stress intensity factor is calculated for this equivalent eigenstrain. Finally, the stress intensity factor of the original semi-infinite FGMs is obtained by principle of superposition. By equating the stress intensity factor to the intrinsic fracture toughness of the FGMs, the apparent fracture toughness is calculated for prescribed composition profiles of the semi-infinite FGMs. Conversely, the composition profiles of prescribed apparent fracture toughness are also determined.
TL;DR: In this article, the authors considered the stress separation problem to determine individual stress components from the sum of the principal stresses and found that the combination of Tikhonov regularization with supplementary data and Hansen's L-curve method is effective for attaining an accurate result of stress separation.
Abstract: Thermoelastic stress analysis(TSA) is a technique for measuring stresses through temperature changes induced by the thermoelastic effect of elastic bodies. Unfortunately, TSA can only provide the sum of the principal stresses. In this paper, the stress separation problem to determine individual stress components from the sum of the principal stresses is considered. The problem is divided into two parts:(1)an inverse problem to estimate unknown boundary values from the sum of the principal stresses inside the body, and(2)a direct problem to determine the stress components inside the body based on the estimated boundary values. This two-step approach is formulated and solved by BEM. It is found that the combination of Tikhonov regularization with supplementary data and Hansen's L-curve method is effective for attaining an accurate result of stress separation.
TL;DR: In this article, a constitutive model for macroscopic sintering deformation analysis was developed. But the model was not applied to the finite element analysis to predict the shape change of the powder compacts.
Abstract: Microscopic sintering behavior in compacts formed by pressing ceramic granules is examined to develop a constitutive model for macroscopic sintering deformation analysis. Spray-dried alumina granules are compacted by CIPing and the compacts are fired at various temperatures. The internal structure composed of fractured and unfractured granules, and large crack-like cavities caused by their inhomogeneous shrinkage during sintering are observed. The macroscopic sintering rate becomes slow because of the formation of the large cavities. A basic constitutive equation for sintering of ceramic powder compacts is proposed with the grain boundary diffusion and grain growth taken into account. The relationship between the change in the microstructure and the shrinkage rate of the powder compacts is modelled by using the basic constitutive equation. The constitutive model is applied to the finite element analysis to predict the shape change of sintering bodies. The calculated results show good agreement with the experimental results.
TL;DR: In this paper, an analytical and experimental analysis for tensile and compressive creep properties of fine and coarse-grained aluminium alloy, A3003P, used for plate-fin heat exchanger is presented for temperatures from 400°C to 600°C and stresses between 0.255 MPa and 6.370 MPa.
Abstract: Analytical and experimental analysis for tensile and compressive creep properties of fine and coarse-grained aluminium alloy, A3003P, used for plate-fin heat exchanger are presented for temperatures from 400°C to 600°C and stresses between 0.255 MPa and 6.370 MPa. Investigation for parameters' effect on the behaviour of the time-dependent flow of material was carried out. For analytical investigation, the grain-size mechanism was applied to both tension and compression creep tests. Creep-strain rate increases with increasing both temperature and stress, while decreasing with coarse grain size. For loading mode, experimental results showed lower creep strain in compression than in tension for all stresses investigated. Nevertheless, numerical analysis showed identical behaviour for both tension and compression. Good agreement was found between the experimentally determined creep strain and analytical predictions at lower stress levels. However, at higher stresses considerable deviations from the experimental results were recorded for the analytical findings. Though the grain-size mechanism is applicable in case of tension for pure aluminium, modification becomes necessary when compression is applied.
TL;DR: In this article, an exact analysis of an external circular crack in a transversely isotropic piezoelectric solid subjected to normal stress as well as electric charge loadings that are symmetric with respect to the crack plane is presented.
Abstract: This paper presents an exact analysis of the problem of an external circular crack in a transversely isotropic piezoelectric solid subjected to normal stress as well as electric charge loadings that are symmetric with respect to the crack plane. The recently proposed general solution is used and the potential theory method is employed. To take account of the effect of the electric field, a new potential of a simple layer is introduced. The derived Green's functions for point force and point charge are completely exact and expressed in terms of elementary functions. Simple form expressions for intensity factors are also obtained. Numerical examples are finally performed.
TL;DR: In this paper, a review of some fundamental aspects of vibration damping in general, and of vibration properties of structural FRP composites in particular, is presented, which provides a reasonably broad appreciation of vibration characteristics of structural fiber reinforced plastic composites and of the work done to study these characteristics.
Abstract: Being considerably stiffer, lighter and more damped than metals, fibre reinforced plastic composites are increasingly replacing metal alloys used traditionally in weight sensitive structural applications. However, the hybrid nature of FRP composites has a far more significant bearing on vibration properties of structures made from these materials than metals. Presented here is a review of some fundamental aspects of vibration damping in general, and of vibration properties of structural FRP composites in particular. Both experimental and analytical aspects of studying these properties are considered. This, it is hoped, will provide a reasonably broad appreciation of vibration characteristics of structural FRP composites, and of the work done to study these characteristics.
TL;DR: In this paper, the effects of crystal anisotropy such as the elastic constants and slip directions were approximately taken into account by averaging the Young's modulus, the Poisson's ratio and the resolved shear stress along the azimuthal direction.
Abstract: A computer code was developed for simulation of dislocation density in a bulk single crystal during the Czochralski(CZ) growth process. In this computer code, the effects of crystal anisotropy such as the elastic constants and slip directions were approximately taken into account by averaging the Young's modulus, the Poisson's ratio and the resolved shear stress along the azimuthal direction. Axisymmetric finite element analysis can be applied to quantitative estimation of dislocation density during single crystal growth process by using such averaging technique together with the Hassen-Sumino model as a creep constitutive equation of a single crystal at elevated temperatures. Dislocation density analyses were performed for both the[001]and [111]growth directions of an InP single crystal. As a result, although the[111]growth direction has the larger average Young's modulus than the[001]growth direction, the former gives lower dislocation density than the latter.
TL;DR: In this article, an experimental model was developed to evaluate the behavior of the chin bar on direct facial impact, and compared this model with the Snell Memorial Foundation and British Standard tests.
Abstract: The chin bar of a motorcycle helmet protects the rider from facial and head injuries. An experimental model was developed to evaluate the behavior of the chin bar on direct facial impact, and compared this model with the Snell Memorial Foundation and British Standard tests. In the model, a headform was secured against the interior of the chin bar, then dropped to impact a flat anvil. The maximum acceleration and Head Injury Criterion(HIC) were measured to assess the impact-absorbing capability of the chin bar. The results showed that this approach provides a more realistic and accurate assessment of the ability of the chin bar to protect against head injuries than the Snell or British Standard systems. A chin bar consisted only of plastic shell and comfort foam offers inadequate protection. An impact-absorbing liner is essential to increase the protective performance of chin bar. An appropriate cushioning structure at impact shell area could further improve the impact-absorbing capability. The stiffness at the connected portion between the chin bar and the helmet should be reduced to enhance the cushioning effect.
TL;DR: More than 80 fatigue tests under fluctuating internal pressure are carried out on line pipes with a gouge in a dent in the longitudinal direction to develop a systematic database of fatigue behavior of pipelines.
Abstract: More than 80 fatigue tests under fluctuating internal pressure are carried out on line pipes with a gouge in a dent in the longitudinal direction to develop a systematic database of fatigue behavior of pipelines. Unpressurized pipe specimens of different grades, diameters and wall thicknesses are first dented and then gouged prior to fatigue tests. This study clarifies that fatigue life(Nf), is dependent on dent depth(d), gouge depth(t)and hoop-stress amplitude(lσ)according to a power law Nf=c(d/D)α(t/T)β(lσ/E)γ, where c, α, β and γ are coefficients, D is diameter, T is wall thickness and E is Young's modulus. No obvious effects due to defect length and mean hoop stress are detected on fatigue life.
TL;DR: In this paper, the authors studied the time-dependent stress fields around a moving crack tip in a viscoelastic strip and crack extension resistance using photoviscoelastic technique.
Abstract: The present work studies time-dependent stress fields around a moving crack tip in a viscoelastic strip and crack extension resistance. Photoviscoelastic technique is employed for the evaluation of the time-dependent stress state around a crack tip. For the fringe pattern analysis, a new technique using an elliptically polarized white light, which can determine isochromatic and isoclinic parameters simultaneously from a colored image obtained by a single acquisition, is applied. The time-dependent stress intensity factor K * I which is extended for linearly viscoelastic materials is evaluated using a method based on least-squares. The results show that the proposed crack extension resistance K* IC may be considered as a characteristic property of the material under monotonically increasing load.
TL;DR: In this paper, the authors exploit the feasibility of piezoelectric polymer film sensors to evaluate in-plane static strain distributions of structural components, and determine the strain distributions from the measured potentials taking into account the piezelectric constitutive law of the film material.
Abstract: The objective of the present study is to exploit the feasibility of piezoelectric polymer film sensors to evaluate in-plane static strain distributions of structural components. In contrast to ordinary use of piezoelectric films as strain and strain-rate sensors detecting electric charges or currents in a closed circuit, the proposed strategy consists of directly measuring the distribution of electric potentials induced in the piezoelectric film mounted on the surface of a deforming structural component. Strain distributions are determined from the measured potentials taking into account the piezoelectric constitutive law of the film material. As an illustrative example, thin films of polarized polyvinylidene fluoride (PVDF) are mounted on a holed elastic plate subjected to in-plane loading, and the induced potential distributions are measured on the film surface by an electrostatic voltmeter. It is demonstrated that the determined strain distributions are in fair conformity with those predicted by the theory of linear elasticity.
TL;DR: In this paper, a Ni-Ti-Nb SMA with a chemical composition of 51 wt% Ni, 38 Wt% Ti, and 11 wt % Nb was used as a structural element at elevated temperatures in water.
Abstract: Shape memory alloys(SMAs) are designed to be used as structural elements at elevated temperatures. The material used in this study was Ni-Ti-Nb SMA with a chemical composition of 51 wt% Ni, 38 wt% Ti, and 11 wt% Nb. The shape-recovery finishing temperature was around 350 K. When the material was heated from room temperature(293 K)to high temperature(561 K), the tensile strength and 0.2%-proof stress slightly decreased, while the elongation and reduction of area increased. Young's modulus increased with the temperature, up to 673 K. The fatigue strength at 561 K in air and in water was slightly higher than at 293 K in air. The fatigue limit in water was close to that in air at 561 K. The fatigue limits at 561K were about 290, 240, and 150 MPa for stress ratios of -1, 0, and 0.5 respectively. Fatigue cracks were found to initiate in the Ti-rich precipitates in the sub-surface, indicating that the fatigue strength is affected by the mean stress. The Ni-Ti-Nb SMA was found to have sufficient mechanical and fatigue strengths to be used as a structural element at elevated temperatures in water.
TL;DR: The boundary element method was used to determine the elastic stress distribution near the tip of a delamination crack in fiber-reinforced plastics (FRP) composites as mentioned in this paper.
Abstract: The boundary element method was used to determine the elastic stress distribution near the tip of a delamination crack in fiber-reinforced plastics(FRP). FRP composites were modeled by a two-dimensional laminated structure composed of isotropic matrix and orthotropic fiber. A straight crack parallel to the fiber direction is placed in the matrix and is subjected to mode I or II loading. The stress intensity factor and the energy release rate were calculated as a function of the crack length for the above inhomogeneous model, and compared with those for a homogeneous model. For large cracks, the energy release rate for the inhomogeneous model is equal to that obtained for the homogeneous model for both mode I and II loadings. For small cracks, the energy release rate is larger for the inhomogeneous model than for the homogeneous model. The transition condition from small to large cracks was defined for the cases of mode I and II. The characteristics of the elastic matrix stress distribution ahead of the crack tip for the inhomogeneous model were discussed in comparison with the solution for the homogeneous model.
TL;DR: In this article, two types of numerical inverse Laplace transformation (NILT) formulas, namely Krings & Waller's method and Hosono's method, are applied to 2D boundary element analysis employing regularized boundary integral equations.
Abstract: In the present paper, the boundary element method(BEM) for unsteady elastodynamic problems based on the Laplace transform is discussed. In the Laplace transformed BEM, the accuracy of teh numerical results is generally governed by the numerical treatment of the inverse Laplace transformation for the transformed solutions. Two types of numerical inverse Laplace transformation(NILT) formula, namely Krings & Waller's method and Hosono's method are applied to the 2-dimensional BEM analysis employing regularized boundary integral equations. It is shown that there is a stability condition between the element size and the time discretization. The characteristics of two types of NILT methods are found out through 2-dimensional BEM analyses for the unsteady elastodynamic problems.