Showing papers in "Journal of Engineering Materials and Technology-transactions of The Asme in 1994"

Approximate Models for Ductile Metals Containing Nonspherical Voids—Case of Axisymmetric Oblate Ellipsoidal Cavities

Abstract: The aim of this paper is to extend the classical Gurson (1977) analysis of a hollow rigid ideal-plastic sphere loaded axisymmetrically to an ellipsoidal volume containing a confocal oblate ellipsoidal cavity. An “expansion” velocity field satisfying conditions of homogeneous boundary strain rate is used to derive a two-field estimate of the overall yield criterion. The latter is shown to be reducible, with a few approximations, to a Gurson-like criterion depending on the “shape parameter” of the cavity. The accuracy of this estimate is assessed through comparison with some results derived from a numerical minimization procedure. An approximate evolution equation for the shape parameter is also presented; comparison with some finite element simulations suggests a slight modification of the theoretical formula leading to considerably enhanced agreement.

Double-Inclusion Model and Overall Moduli of Multi-Phase Composites

Abstract: The double inclusion model consists of an ellipsoidal inclusion of arbitrary elasticity, containing another ellipsoidal heterogeneity of arbitrary elasticity, size, and orientation, which are embedded in an infinitely extended homogenous domain of yet another arbitrary elasticity. Average field quantities for the double inclusion are obtained analytically, and used to estimate the overall moduli of two phase composites. The technique includes the self consistent and other related methods as special cases. Furthermore, exact bounds for the overall moduli are obtained on the basis of the double inclusion model

A Micromechanics Model for Thermoelastic Properties of Plain Weave Fabric Composites

Abstract: A micromechanics model is presented to predict thermoelastic properties of composites reinforced with plain weave fabrics. A representative volume element is chosen for analysis and the fiber architecture is described by a few simple functions. Equations are developed to calculate various phase fractions from geometric parameters that can be measured on a cross section. Effective elastic moduli and effective thermal expansion coefficients are determined under the assumption of uniform strain inside the representative volume element. The resulting model is similar to the classical laminated theory, and hence is easier to use than other models available in the literature. An experimental correlation is provided for a number of Nicalon SiC/CVI SiC and Graphite/CVI SiC composite laminates.

Deformation and Failure Behavior of Woven Composite Laminates

Abstract: A micromechanistic deformation model, that could realistically be incorporated into structural finite element codes, is proposed where loading direction and weave parameters are allowed to vary. Comparisons are made to previous models and experimental results for woven materials, indicating that the proposed model provides improved estimates for the linear elastic stiffness. The model further provides predictions for internal stresses in the longitudinal, transverse, and interlace regions of the woven laminate which qualitatively correspond to the experimentally observed failure mechanisms

A Progressive Damage Mechanics in Particle-Reinforced Metal-Matrix Composites Under High Triaxial Tension

Abstract: The energy approach recently proposed by Qiu and Weng (1992) is introduced to estimate the equivalent stress of the ductile matrix in Tohgo and Chou’s (1991) incremental damage theory for particulate-reinforced composites containing hard particles In such a composite debonding of the particle-matrix interface is a significant damage process, as the damaged particles have a weakening effect while the intact particles have a reinforcing effect In Tohgo-Chou’s theory, which describes the elastic-plastic behavior and the damage behavior of particulate-reinforced composites, it was assumed that the debonding damage is controlled by the stress of the particle and the statistical behavior of the particle-matrix interfacial strength, and that the debonded (damaged) particles are regarded as voids, resulting in an increased void concentration with deformation On the other hand, Qiu-Weng’s energy approach provides a reasonable equivalent stress of the matrix in the porous material and particulate-reinforced composite even under a high triaxiality The incremental damage theory developed here enables one to calculate the overall stress-strain response and damage evolution of the composite under high triaxial tension The stress-strain relations for porous material obtained by the present incremental theory are completely consistent with that obtained by Qiu and Weng The influence of the debonding damage on the stress-strain response is demonstrated for particulate-reinforced composites

A Yield Surface Approach to the Estimation of Notch Strains for Proportional and Nonproportional Cyclic Loading

Abstract: An analytical method is developed to estimate notch root strains in a notched bar of elastic-plastic, isotropic material subjected to proportional and nonproportional multiaxial nominal loading. The method uses anisotropic plasticity theory to define a structural yield surface in nominal stress space that incorporates both the isotropic material properties and the anisotropic geometry factors of the notch. Notch root plastic strain increments and anisotropic work-hardening effects are then related to this yield surface using standard methods of plasticity. Comparisons of the proposed method with previously published strain estimates using the finite element method for a notched shaft under proportional nominal bending and torsion, and with strain gage measurements of a circumferentially notched solid steel shaft subjected to a series of box-shaped nonproportional loading paths in tension-torsion nominal stress space are presented. The strain calculations agree well both qualitatively and quantitatively using an appropriate nominal load-notch plastic strain relationship, and are suitable for strain-life fatigue calculations.

Micromechanical approach of the coated inclusion problem and applications to composite materials

Abstract: A micromechanical model using simultaneously Green's function techniques and interfacial operators is proposed in order to solve the elastic inhomogneous coated inclusion problem. For a composite material made of a non dilute concentration of coated inclusions and a homogenous matrix, the interactions between the reinforcements are solved by a self consistent scheme. The theoretical results for a composite of hollow sphere of glass in a polyester matrix are presented

Damage Mechanisms Modeling for Ceramic Composites

Abstract: For ceramic composites, continuum damage mechanics models are built, which include information coming from both the micro and macro scales. These models are constitutive relations which, when included in a structural analysis code, are able to predict the damage state of the studied structure at any time and at any point until final fracture.

Micromechanics of Active Composites With SMA Fibers

Abstract: The study of the effective thermomechanical response of active fibrous composites with shape memory alloy (SMA) fibers is the subject of this work. A 3-D constitutive response for the SMA fibers is formulated first. To model thermomechanical loading path dependence, an incremental approach is used assuming that within each stress and temperature increment the volume fraction of the martensitic phase remains constant in the SMA fibers. The Mori-Tanaka averaging scheme is then used to give an estimate of the instantaneous effective thermomechanical properties in terms of the thermomechanical properties of the two phases and martensitic volume fraction. A unit cell model for a periodic active composite with cubic and hexagonal arrangement of fibers is also developed to study the effective properties using finite element analysis. It is found that since the fibers and not the matrix undergo the martensitic phase transformation that induces eigenstrains, the Mori-Tanaka averaging scheme accurately models the thermomechanical response of the composite, relative to the finite element analysis, for different loading paths. Specific results are reported for the composite pseudoelastic and shape memory effect for an elastomeric matrix continuous SMA fiber composite.

Strain-Rate Sensitivity, Relaxation Behavior, and Complex Moduli of a Class of Isotropic Viscoelastic Composites

Abstract: A micromechanical principle is developed to determine the strain rate sensitivity, relaxation behavior, and complex moluli of a linear viscoelastic composite comprised of randomly oriented spheoidal inclusions. First, by taking both the matrix and incluions as Maxwell or Voigt solids, it is found possible to construct a Maxwell or a Voigt composite when the Poisson ratios of both phases remain constant and the ratios of their shear modulus to shear viscosity (or their bulk counterparts) are equal; such a specialized composite can never be attained if either phase is purely elastic. In order to shed some light for the obtained theoretical structure, explicit results are derived next with the Maxwell matrix reinforced with spherical particles and randomly oriented disks. General calculations are performed for the glass/ED-6 system, the matrix being represented by a four parameter model

Micromechanics and effective elastoplastic behavior of two-phase metal matrix composites

Abstract: A micromechanical framework is presented to predict effective (overall) elasto-(visco-)plastic behavior of two-phase particle-reinforced metal matrix composites (PRMMC). In particular, the inclusion phase (particle) is assumed to be elastic and the matrix material is elasto-(visco-)plastic. Emanating from Ju and Chen's (1994a,b) work on effective elastic properties of composites containing many randomly dispersed inhomogeneities, effective elastoplastic deformations and responses of PRMMC are estimated by means of the effective yield criterion'' derived micromechanically by considering effects due to elastic particles embedded in the elastoplastic matrix. The matrix material is elastic or plastic, depending on local stress and deformation, and obeys general plastic flow rule and hardening law. Arbitrary (general) loadings and unloadings are permitted in the framework through the elastic predictor-plastic corrector two-step operator splitting methodology. The proposed combined micromechanical and computational approach allows one to estimate overall elastoplastic responses of PRMMCs by accounting for the microstructural information (such as the spatial distribution and micro-geometry of particles), elastic properties of constituent phases, and the plastic behavior of the matrix-only materials.

Measurement of Near Surface Residual Stresses Using Electric Discharge Wire Machining

Abstract: It has been shown that near surface residual stresses may be deduced form surface strains produced by making a cut of progressively increasing depth. The process of electric discharge wire machining (EDWM) greatly improves the ability of the method to resolve a stress gradient near the surface. However, the EDWM process may also introduce residual stresses

Small Punch Testing for Determining the Material Toughness of Low Alloy Steel Components in Service

Abstract: The toughness of the low alloy ferritic steel material of structural components operating at elevated temperatures can degrade during service due to embrittling phenomena such as carbide coarsening and temper embrittlement. The extent of degradation and the current level of toughness are critical inputs to component structural integrity assessments and to operation and maintenance planning. Conventional test methods for measuring toughness require the removal of large material samples from the in-service component, which is generally impractical. However, the recent development of relatively nondestructive, miniature sample removal systems and the small punch test technique (which utilizes nonstandard, miniature specimens) now provides a convenient, practical means of evaluating the material of an in-service component for toughness and related mechanical properties. This paper describes the small punch test technique with selected examples of its application to various grades of low alloy ferritic steel.

Effective Thermomechanical Behavior of Plain-Weave Fabric-Reinforced Composites Using Homogenization Theory

Abstract: A two scale asymptotic homogenization theory is used. The model is based on the properties of the constituents and an accurate, three dimensional simulation of the weave microarchitecture, and is used for predicting the thermomechanical behavior of glass epoxy (FR-4) woven fabric laminates typically used by the electronics industry in multilayered printed wiring boards

High Temperature Multiaxial Low Cycle Fatigue of Cruciform Specimen

Abstract: This paper describes high temperature multiaxial low cycle fatigue lives of type SUS304 stainless steel and 1Cr-1Mo-1/4V steel cruciform specimens at 923 K and 823 K in air. Strain controlled multiaxial low cycle fatigue tests were carried out using cruciform specimens at the principal strain ratios between [minus]1 and 1. The principal strain ratio had a significant effect on low cycle fatigue lives. Fatigue lives drastically decreased as the principal strain ratio increased. Multiaxial low cycle fatigue strain parameters were applied to the experimental data and the applicability of the parameter was discussed. The equivalent strain based on crack opening displacement (COD strain) developed in the paper and [Gamma][sup *] -- plane parameter successfully predicted multiaxial low cycle fatigue lives. The crack morphology was also extensively discussed from not only the surface crack direction but also the crack inclination into the specimen.

Local Buckling and Failure of Pultruded Fiber-Reinforced Plastic Beams

Abstract: An experimental investigation of the local compression flange buckling and failure of commercially produced pultruded fiber-reinforced plastic (FRP) I-shaped beams is described in this paper. Results of tests on pultruded E-glass/polyester and E-glass/vinylester composite material beams are described. The test configuration was designed to cause local buckling and ultimate failure of the compression flange of the beams and to prevent global lateral-torsional buckling. The beams were stiffened to prevent crippling and warping at the supports, and local tensile failure at the load points. All beams were monitored with strain gages and LVDT's. Buckling loads, failure loads, buckling stresses, deflections, and failure modes are reported. Effective mechanical properties of the beams, obtained from overall flexural and shear strain data, are presented. A discussion of the different failure characteristics of the polyester and the vinylester beams is provided.

Influence of Random Geometry on Effective Properties and Damage Formation In Composite Materials

Abstract: The authors study the effective moduli and damage formation in out-of-plane elasticity (i.e., two-dimensional conductivity) of matrix-inclusion composite materials with either randomly or periodically distributed inclusions (fibers). In this paper, they focus their attention on composites with isotropic phases, both of which have elastic-brittle response in damage. The elastic-brittle behavior is modeled with the help of a fine mesh finite-difference system, whereby damage evolution is simulated by sequentially removing/breaking bonds in this lattice in accordance with the state of stress/strain concentrations. The composite systems are specified by two parameters: stiffness ratio and strength ratio of both phases. In particular, the authors investigate the following aspects: basic classification of effective constitutive responses, geometric patterns of damage, varying degrees of randomness of the inclusions' arrangements, and mesh resolutions of continuum phases.

Studies of the Growth and Collapse of Voids in Viscous Solids

Abstract: The growth and collapse of isolated voids in power-law viscous matrix materials are investigated. The study is restricted to axisymmetric remote stressing and to voids which are initially spheroidal with the axis of symmetry of the voids coincident with the axis of symmetry of the remote loading. Particular attention is given to the evolution of initially spherical voids, but the effect of initial void shape on subsequent void evolution is also investigated. For linearly viscous matrix materials, the voids evolve through spheroidal shapes and the work of Budiansky et al. (1982) provides the desired information about the history of void shape and volume. For nonlinear matrix materials, the void evolution is idealized as proceeding through a sequence of spheroidal shapes, and the rate of deformation for a given instant is evaluated using a Ritz procedure developed by Lee and Mear (1992). The results of the study demonstrate that the history of void volume and void shape is influenced significantly by the material nonlinearity, the remote stress state and the initial void aspect ratio.

Data Management for the Evaluation of Residual Stresses by the Incremental Hole-Drilling Method

Abstract: The semidestructive incremental hole drilling method commonly used to evaluate residual stresses is exceedingly sensitive to experimental errors, with sensitivity increasing as hole depth increases. To determine stress variations through the engine thickness, it is necessary to use accurate drilling methods, as well as suitable mathematical models and procedures to minimize the errors associated with residual stress measurement. This work examines he effects of measurement errors on the evaluation of residual stresses with the integral method

A Finite Element Analysis of Electromagnetic Forming for Tube Expansion

Abstract: The finite element analysis of tube expansion in the electromagnetic forming was performed. The electromagnetic field analysis was also performed by the finite element method to obtain longitudinal magnetic pressure distribution which had not been considered exactly. A better agreement between the calculated and experimental results was obtained. It became possible to consider effects of some parameters on the magnetic pressure.

On the Propagation of a Shear Band in a Steel Tube

Abstract: Marchand and Duffy tested thin-walled steel tubes in a split Hopkinson torsion bar at a nominal strain-rate of approximately 1,600/s and could not determine conclusively whether a shear band initiating at a point in the tube propagated around the circumference in one direction or in both directions. They estimated the speed of propagation to be 520 m/s in the former case and 260 m/s in the latter. Here, the authors simulate their test numerically, and find that the shear band propagates in both directions around the circumference of the tube. When the tube is twisted at a nominal strain-rate of 5,000/s, the band speed varies from 180 m/s at the site of the initiation to approximately 1,000 m/s at the nearly diametrically opposite point. The band speed increases with an increase in the nominal strain-rate. The material defect is modeled by assuming that a small region near the center of the tubular surface is made of a material weaker than that of the rest of the tube.

Comparison of the Effects of Debonds and Voids in Adhesive Joints

Abstract: An analytical model is developed to compare the effects of voids and debonds on the interfacial shear stresses between the adherends and the adhesive in simple lap joints. Since the adhesive material above the debond may undergo some extension (either due to applied load or thermal expansion or both), a modified shear lag model, where the adhesive can take on extensional as well as shear deformation, is used in the analysis. The adherends take on only axial loads and act as membranes. Two coupled nondimensional differential equations are derived, and in general, five parameters govern the stress distribution in the overlap region. As expected, the major differences between the debond and the void occur for the stresses near the edge of the defect itself. Whether the defect is a debond or a void, is hardly discernible by the stresses at the overlap ends for central defect sizes up to the order of 70 percent of the overlap region. If the defect occurs precisely at or very close to either end of the overlap, however, differences of the order of 20 percent in the peak stresses can be obtained.

The Compliance Method for Measurement of Near Surface Residual Stresses—Analytical Background

Abstract: Introducing a thin cut from the surface of a part containing residual stresses produces a change in strain on the surface. When the strains are measured as a function of the depth of the cut, residual stresses near the surface can be estimated using the compliance method. In previous work, the unknown residual stress field was represented by a series of continuous polynomials. The present paper shows that for stress states with steep gradients, superior predictions are obtained by using ''overlapping piece wise functions'' to represent the stresses. The stability of the method under the influence of random errors and a zero shift is demonstrated by numerical simulation.

On the Modeling of Nonproportional Cyclic Plasticity of Waspaloy

Abstract: The isotropic hardening is known to play an effective role in the overhardening of materials under nonproportional cyclic loading However, the behavior of the two states of Waspaloy (namely overaged and underaged states) under these loading conditions, shows that the kinematic hardening has also a considerable role in the overhardening Experimental tests were carried out on these two states under various proportional and nonproportional cyclic loading conditions at room temperature The effect of loading paths on micro-mechanisms of deformation was studied From a microstructural point of view, it was shown that the deformation modes (quantitatively and qualitatively) depends on the loading path and the heat treatment A constitutive model is proposed to describe the effect of overhardening, under the nonproportional loading conditions, on the kinematic hardening The predicted response are in good agreement with experimental results

Creep Behavior of a Unidirectional SCS-6/Ti-15-3 Metal Matrix Composite at 450°C

Abstract: As the first step to understand the creep behavior of a unidirectional SCS-6/Ti-15-3 metal matrix composite, creep tests were performed at 450 C using specimens reinforced with fibers oriented at 0 deg (longitudinal), 45 deg (off-axis), and 90 deg (transverse) to the specimen axis. Measuring the elongation in the gage section, the authors found that creep deformation and rupture can occur even in the longitudinal creep at stress levels much lower than the tensile strength although the SCS-6 fiber itself does not creep at all, and that the 45 deg off-axis creep has some ductility, which is greater at lower stress, whereas the transverse creep behavior is relatively brittle. Results of the longitudinal creep tests were discussed on the basis of the relaxation of stress in the matrix as well as the bundle strength of fibers.

Exact results concerning the local fields and effective properties in piezoelectric composites

Abstract: This paper consists of two parts: a concise summary and discussion is given of the recent contributions of the author in the micromechanics of piezoelectric composites. The underlying them here is the derivation of exact connections for the local fields and effective moduli of heterogeneous piezoelectric solids. Composites of arbitrary phase geometry as well as fibrous systems are considered. New results are presented on the effective behavior of fibrous piezoelectric systems. Fibrous composites with transversely isotropic constituents and cylindrical microgeometry are considered

The compliance method for measurement of near surface residual stresses -- Application and validation for surface treatment by laser and shot-peening

Abstract: Residual stresses due to surface treatment are measured using the compliance method. The method makes use of the strains measured on the surface while a cut is extended progressively along a plane of interest. The experimental results for a shot peened specimen show good agreement with those obtained by the X-ray method. This experiment demonstrates that the compliance method is accurate and capable of measuring residual stresses which vary rapidly over a depth of less than 50 [mu]m. Good general agreement with results by the X-ray method is also obtained for a laser treated specimen. Some advantages and disadvantages of the present method relative to hole-drilling, layer removal and X-ray methods are discussed.