Showing papers on "Micromechanics published in 1992"
TL;DR: In this article, the pseudostrain-hardening phenomenon of brittle matrix composites reinforced with discontinuous flexible and randomly distributed fibers is analyzed based on a cohesive crack-mechanics approach, and the first crack strength and strain are derived in terms of fiber, matrix, and interface micromechanical properties.
Abstract: This paper analyzes the pseudostrain‐hardening phenomenon of brittle matrix composites reinforced with discontinuous flexible and randomly distributed fibers, based on a cohesive crack‐mechanics approach. The first crack strength and strain are derived in terms of fiber, matrix, and interface micromechanical properties. Conditions for steady‐state cracking and multiple cracking are found to depend on two nondimensionalized parameters that embody all relevant material micromechanical parameters. The results are therefore quite general and applicable to a variety of composite‐material systems. Phrased in terms of a failure‐mechanism map, various uniaxial load‐deformation behaviors for discontinuous fiber composites can be predicted. The influence of a snubbing effect due to local fiber/matrix interaction for randomly oriented crack‐bridging fibers on the composite properties is also studied.
975 citations
TL;DR: In this article, the authors compare homogenization and standard mechanics RVE-based analyses for periodic porous composites with finite ν, and conclude that homogenisation is preferable over standard mechanics of materials approaches for periodic composites even when the material is only locally periodic and ν is finite.
Abstract: Composite material elastic behavior has been studied using many approaches, all of which are based on the concept of a Representative Volume Element (RVE). Most methods accurately estimate effective elastic properties when the ratio of the RVE size to the global structural dimensions, denoted here as ν, goes to zero. However, many composites are locally periodic with finite ν. The purpose of this paper was to compare homogenization and standard mechanics RVE based analyses for periodic porous composites with finite ν. Both methods were implemented using a displacement based finite element formulation. For one-dimensional analyses of composite bars the two methods were equivalent. Howver, for two- and three-dimensional analyses the methods were quite different due to the fact that the local RVE stress and strain state was not determined uniquely by the applied boundary conditions. For two-dimensional analyses of porous periodic composites the effective material properties predicted by standard mechanics approaches using multiple cell RVEs converged to the homogenization predictions using one cell. In addition, homogenization estimates of local strain energy density were within 30% of direct analyses while standard mechanics approaches generally differed from direct analyses by more than 70%. These results suggest that homogenization theory is preferable over standard mechanics of materials approaches for periodic composites even when the material is only locally periodic and ν is finite.
446 citations
01 Mar 1992
TL;DR: In this paper, a brief outline of research findings and a list of publications and technical reports related to micromechanics of defects, nondestructive testing and cracking in fiber reinforced composites are presented.
Abstract: : This report contains a brief outline of research findings and a list of publications and technical reports related to micromechanics of defects, nondestructive testing and cracking in fiber reinforced composites.
284 citations
TL;DR: In this paper, a finite element method has been employed to study the micromechanics and micro-echanisms in rubber-toughened cross-linked-epoxy polymers.
Abstract: The finite element method has been employed to study the micromechanics and micromechanisms in rubber-toughened cross-linked-epoxy polymers. A two-dimensional plane-strain model has been proposed and has successfully been used to identify the stress fields associated with the dispersed rubbery phase and to simulate the initiation and growth of localized plastic shear-bands running between the rubbery particles. The effects of the microstructure and mechanical properties of the multiphase polymer on the nature and magnitude of the stress fields have also been examined.
240 citations
TL;DR: A finite-element micromechanics model for Haversian cortical bone tissue has been developed and studied and it is demonstrated that the predicted variation in the elastic modulus with porosity is not as sensitive as that observed experimentally.
94 citations
TL;DR: In this article, the micromechanics of reinforcement of a model composite system consisting of a continuous high-modulus carbon fiber embedded in an epoxy resin were investigated.
Abstract: Attention is given to the micromechanics of reinforcement of a model composite system consisting of a continuous high-modulus carbon fiber embedded in an epoxy resin. The composite was subjected to incremental tensile loading up to full fiber fragmentation, while the strain in the fiber was monitored at each level of load using a laser Raman spectroscopic technique. The average strain in the fiber increased linearly with applied matrix strain up to a value of 0.8 percent, when the first fiber fracture occurred. After fracture, the strain in the fiber was found to build from the tips of the fiber breaks, reaching a maximum value in the middle of each fragment. The shape of the load transfer profiles at the locality of the fiber tips indicated that the stress transfer efficiency had been affected by the fracture process. The length of interfacial debonding at the point of fiber fracture was found to be driven by the strain energy of the fractured fragments. The interfacial shear stress distributions at various levels of the applied load along individual fragments are derived from the load transfer profiles. 51 refs.
92 citations
TL;DR: In this paper, a review of the use of a strain energy-based finite element approach to the analysis of various aspects of damping of composite materials and structures is presented, focusing on the design and analy sis flexibility offered by finite element implementation of the strain energy method.
Abstract: This paper presents a review of the authors' recent work on the use of a strain energy-based finite element approach to the analysis of various aspects of damping of composite materials and structures. Particular interest was paid to the design and analy sis flexibility offered by finite element implementation of the strain energy method, and both micromechanical and macromechanical models are presented for the analysis of damping in composites. Previous work using micromechanical models to study the effects of fiber interaction, fiber aspect ratio, and fiber/matrix interphase size on the composite damping are described. Various macromechanical models for study of the effects of in terlaminar stresses, fiber orientation, vibration coupling, and constrained layer damping treatments in composites are discussed. Some experimental results from previous work are compared with analytical results from the strain energy-based finite element models. Finally, recommendations on ways of improving and optimizing damp...
82 citations
TL;DR: In this paper, a method is presented to estimate the influence of the matrix and the interfacial bond strength itself on the composite shear strength for a given fiber/matrix composite.
Abstract: Presently there is great interest in understanding and improving the bond ing between the fibers and matrix in high performance composite materials. Indeed, the fiber-matrix adhesion in many recently-developed systems is poor. To improve bonding, various fiber surface treatments have been developed. These treatments are often evaluated by measuring their effect on a composite property sensitive to the interfacial bond strength (e.g., the composite shear strength). Such methods are, however, inferential, and do not provide a direct measure of the strength of the interfacial bond.A method is presented to estimate the influence of the matrix and the interfacial bond strength itself on the composite shear strength for a given fiber/matrix composite. Analyti cal prediction of these effects has been achieved using a finite element micromechanics model. Numerical results generated using this model have been shown to compare well with experimental data, which suggests that the micromechanics approach to predictin...
74 citations
TL;DR: In this paper, a micromechanical model is constructed for the compressive strength of fiber-reinforced cementitious composites (FRCs), based on the classical models of compressive failure of brittle soilds in which sliding microcracks induce wing-crack growth under compression loads.
Abstract: A micromechanical model is constructed for the compressive strength of fiber-reinforced cementitious composites (FRCs). This model is based on the classical models of compressive failure of brittle soilds in which sliding microcracks induce wing-crack growth under compression loads. The concept of increased microcrack sliding resistance and wing-crack growth retardation associated with fiber bridging is exploited to produce a strengthening effect of fibers in composite strength. The concept of defect introduction associated with fiber volume fraction is included to produce a composite strength degradation. The combined effect results in composite compressive strength which increases initially and subsequently drops with increasing fiber content, as has been observed in FRCs reinforced with a variety of fibers.
73 citations
TL;DR: In this paper, the stiffness tensor of the composite is calculated by globally averaging the approximate stresses in the representative fibers and matrix embedded in the composite, which admits the possibility of load transfer between the fiber ends and matrix.
71 citations
TL;DR: In this article, a whole-field in-plane displacement measurement method was developed for micromechanics studies and the method increased the mean square error of the displacement measurement.
Abstract: A whole-field in-plane displacement measurement method was developed for micromechanics studies. The method increased the sen-
TL;DR: In this article, the authors measured fiber displacements, measured through small port holes in the matrix made by electropolishing, to determine fiber stresses, which ranged from 1.1 to 4 GPa.
Abstract: Micromechanics parameters for fatigue cracks growing perpendicular to fibers were measured for the center-notched specimen geometry. Fiber displacements, measured through small port holes in the matrix made by electropolishing, were used to determine fiber stresses, which ranged from 1.1 to 4 GPa. Crack opening displacements at maximum load and residual crack opening displacements at minimum load were measured. Matrix was removed along the crack flanks after completion of the tests to reveal the extent and nature of the fiber damage. Analyses were made of these parameters, and it was found possible to link the extent of fiber debonding to residual COD and the shear stress for fiber sliding to COD. Measured experimental parameters were used to compute crack growth rates using a well-known fracture mechanics model for fiber bridging tailored to these experiments.
TL;DR: In this article, a multiobjective optimal design methodology is developed for lightweight, low-cost composite structures of improved dynamic performance, which may include minimization of damped resonance amplitudes (or maximization of modal damping), weight, and material cost.
Abstract: A multiobjective optimal design methodology is developed for lightweight, low-cost composite structures of improved dynamic performance. The design objectives may include minimization of damped resonance amplitudes (or maximization of modal damping), weight, and material cost. The design vector includes micromechanics, laminate, and structural shape parameters. Constraints are imposed on static displacements, static and dynamic ply stresses, dynamic amplitudes, and natural frequencies. The effects of composite damping tailoring on the dynamics of the composite structure are incorporated. Applications on a cantilever composite beam and plate illustrate that only the proposed multiobjective formulation, as opposed to single objective functions, may simultaneously improve the objectives. The significance of composite damping in the design of advanced composite structures is also demonstrated, and the results indicate that the minimum-weight design or design methods based on undamped dynamics may fail to improve the dynamic performance near resonances. Nomenclature A = area [C],[c] = global and modal damping matrices, respectively E = normal modulus F(z) = objective functions / = frequency fd = damped frequency G = shear modulus G(z) = inequality constraints h = thickness [/£],[/:] = global and modal stiffness matrices, respectively k = volume ratio [M],[m] = global and modal mass matrices, respectively p^p = global and modal excitation force, respectively q — modal vector 5 = strength t = time U = dynamic amplitude u = displacement vector
TL;DR: In this article, the authors measured the Raman spectrum associated with the E2-TOquasi and LOquasi modes of 6H-SiC particles as a function of hydrostatic pressure using a diamond anvil cell.
Abstract: We measured the Raman spectrum associated with the E2-TOquasi and LOquasi modes of 6H-SiC particles as a function of hydrostatic pressure using a diamond anvil cell. The results of this calibration experiment were used to analyze the residual stress in 6H-SiC particles within Al2O3/SiC composites with 12%, 20%, and 30% SiC by volume. The Raman spectra show that residual stress in the SiC near the surface of the composites is −2040 ± 120, −1841 ± 110, and −1615 ± 100 MPa for the 12%, 20%, and 30% SiC composites, respectively. The measured decrease in stress with increasing packing fraction is consistent with theoretical predictions based on micromechanics.
TL;DR: In this article, a large collection of data on Young's modulus and density of unfilled polyethylenes at ambient conditions has been compared with various competing theoretical mixing rules developed for composite micromechanics.
Abstract: A large collection of data on Young's modulus and density of unfilled polyethylenes at ambient conditions has been compared with various competing theoretical mixing rules developed for composite micromechanics. The objective was to see if such theories usefully predict the dependence of stiffness on crystalline content in an archetypal isotropic semicrystalline thermoplastic polymer above its glass trnsition temperature. It was found that the self-consistent scheme derived by Hill and Budiansky from continuum micromechanics appears to have valid application to this system. The scheme naturally and coherently incorporates information on bulk and shear moduli and Poisson's ratios, while giving a good account of the main trend in the Young's modulus data. Conversely, other theoretical models frequently invoked in the polymer literature were explicitly found to be unsuitable for representing principal features of modulus-density relationships dectated by the data.
Journal Article•
TL;DR: It is now possible for stress patterns within the interfacial zone to be quantified and the local micromechanical behavior around individual surface structures for whole implants accounted for.
Abstract: A series of finite element models was developed for evaluation of the micromechanics of implant/tissue interfaces Conventional finite element global models of a dental implant, assuming a continuum implant/bone interface, were developed so that general stress patterns in the implant and surrounding tissue could be obtained Stresses in bone were concentrated on the alveolar crest and apex region for all global models having a direct bone/implant contact The addition of a 100-microns-thick layer of fibrous tissue into the bone/implant interface concentrated the stresses in the middle third of the bone adjacent to the implant surface Stresses in the middle third were ten times higher than in the cases without fibrous tissue Interfaces modeled under the assumption of a volume-weighted average material stiffness of bone tissue and metal confirmed these general stress patterns, but provided no stress details of the interfacial zone Finally, the equivalent material constants of the interfacial zone with and without fibrous tissue were calculated by homogenization theory From these equivalent constants, local strains around single threads were calculated These equivalent material properties are sensitive to the microstructure Therefore, it is now possible for stress patterns within the interfacial zone to be quantified and the local micromechanical behavior around individual surface structures for whole implants accounted for
01 Jan 1992
TL;DR: In this paper, the authors describe the performance driven approach in the design of fiber reinforced cementitious composites by means of micromechanics, which is illustrated with structural durability as an example.
Abstract: This paper describes the performance driven approach in the design of fiber reinforced cementitious composites. This approach is illustrated with structural durability as an example. The identified material property, crack width, is then related quantitatively to material structures - fiber, matrix and interface properties, by means of micromechanics. It is suggested that tailoring of material structure can lead to controlled crack widths, and hence directly influences the durability of built structures. Success in the performance driven design of fiber reinforced cementitious composites will depend on future research in quantifying links between specific structural performance, material properties, and material structures. (A) For the covering abstract of the conference see IRRD 862056.
TL;DR: In this article, a model capable of predicting the process-induced macroscopic in-plane material response of semicrystalline thermoplastic matrices and their composites was developed.
Abstract: A model capable of predicting the process-induced macroscopic in-plane material response of semicrystalline thermoplastic matrices and their composites was developed. Thi sinvestigation focused on the material response of a single layer or ply of neat PEEK matrix and its carbon fiber composite (APC-2) when subjected to various processing histories. Specifically, the response of the material moduli and processing strains as a function of temperature and the degree of crystallinity were studied. The kinetic-viscoelastic response of the matrix was determined from a modified form of the Standard Linear Solid model. A constitutive relation was proposed to quantify resign shrinkage as a function of thermal history, which incorporated crystallization. For a specific process history, the effective composite mechanical properties were determined from micromechanics models. Both neat and composite processing strains were evaluated to show the effect of fibers on the matrix dominated response (90° direction). In addition, comparisons of model moduli predictions with experimental measurements were performed. This study demonstrated that an increase in the degree of crystallinity results in an asymmetric shift of the modulus in the glass transition region to higher temperatures. Also, strains due to crystallization were predicted to be much smaller in comparison to the strains resulting from thermal contraction of the PEEK matrix.
TL;DR: In this paper, the problem of nonuniform strain field under large deformation is resolved by introducing a distributive law that describes the heterogeneous strain field, and the overall stress-strain relationship is described in terms of interparticle contact behavior.
Abstract: The constitutive relationship for granular systems is extended while taking into account the effect of particle separation and sliding under large deformation. The problem of nonuniform strain field under large deformation is resolved by introducing a distributive law that describes the heterogeneous strain field. The paper reviews previous work on granular mechanics, and describes the current approach of treating the material at three levels: representative unit; microelement; and interparticle contact. On this basis the constitutive laws for each level are derived, and the overall stress-strain relationship is described in terms of interparticle contact behavior.
01 Jan 1992
TL;DR: In this article, the influence of fiber waviness and matrix nonlinearity on the compressive behavior of continuous fiber composites is studied and a micromechanics model based on the kinematics of the fibers has been developed to predict the behavior of unidirectional composites with initially wavy fibers under compressive loads.
Abstract: The influence of fiber waviness and matrix nonlinearity on the compressive behavior of continuous fiber composites is studied. A micromechanics model based on the kinematics of the fibers has been developed to predict the behavior of unidirectional composites with initially wavy fibers under compressive loads. The initial waviness has been idealized as sinusoidal. Nonlinear shear behavior of the matrix has been included. Because the shear strain in the matrix is a function of position, the deformed shape of the fibers differs from the initial shape. Therefore, the deformed fiber shape is represented by a sine series. Fiber waviness and matrix shear properties have been determined experimentally for T650-42/Radel C under several conditions. These experimental data have been used as input into the analytical model. Predicted compressive response is in reasonable agreement with experimental data.
TL;DR: In this paper, a local stress analysis model is used to represent the influence of interface and matrix yielding on stress distribution resulting from fiber fractures, and the model is combined with the concept of multiple fiber fractures in the development of a mechanistic representation of tensile strength.
Abstract: Yielding stress of matrix and interface is chosen to represent the influence of the chemical, mechanical, and thermodynamical nature of the bonding process between matrix and fiber materials. The shear stress-strain behavior of interface and matrix material is modeled as elastic-perfectly plastic. A local stress analysis model is used to represent the influence of interface and matrix yielding on stress distribution resulting from fiber fractures. The model is combined with the concept of multiple fiber fractures in the development of a mechanistic representation of tensile strength. In this strength formulation, fibers are assumed to be a statistical quantity, and as the material is loaded, fibers fracture randomly throughout the body causing localized stress concentration. The accumulation of these breaks results in overall failure. The resulting analysis is applied to numerical studies of the influence of interface/matrix yielding on stress and damage characteristics of unidirectional composites under quasistatic uniaxial loading. A discussion of optimal design parameters for composites is also presented. The predicted tensile strength clearly shows that there exists an optimum of composite strength at a certain level of interface and matrix yielding stress. A value of yielding stress larger than optimum causes a loss of composite strength as a result of the dominant increase in local stress concentration, and a value smaller than optimum causes a loss of composite strength as a result of the dominant increase of the region of influence of the local disturbance (increased bundle length). This phenomenon has also been observed by other researchers in their experimental studies of the effect of surface treatment on the tensile strength of composites. Furthermore, by comparison with experimental data, it is shown that the influence of interfacial bond on tensile strength of composites may be secondary to other effects such as change of fiber strength as a result of surface treatments or friction effects engendered by resin shrinkage or residual thermal stress.
TL;DR: In this paper, the boundary integral equation (BIE) method for two-dimensional general anisotropic elasticity, based on the quadratic isoparametric element formulation, is extended to treating some inclusion problems.
Abstract: Numerical methods for stress analysis are increasingly being employed in the micromechanics of solids. In this paper, the boundary integral equation (BIE) method for two-dimensional general anisotropic elasticity, based on the quadratic isoparametric element formulation, is extended to treating some inclusion problems. All the cases analysed involved an elliptical zirconia inclusion in an alumina matrix, noting that ZrO2–Al2O3 is an advanced ceramic increasingly used in structural applications. The BIE results are compared with those calculated using Eshelby's equivalent inclusion approach where possible, and excellent agreements between them are obtained. The present work demonstrates the suitability of using this numerical technique for analysing such problems and, in particular, the ease with which it may be used even in the case of general anisotropy.
TL;DR: In this article, a micromechanics-based constitutive model for granular material is evaluated for predicting the stress-strain strength behavior when the material is subjected to various stress paths.
Abstract: A micromechanics-based constitutive model for granular material is evaluated. The predicted stress-strain behavior for idealized material is compared with that observed from experiments for sands. Under small strain conditions, the model capability is evaluated for predicting initial moduli, secant moduli, and damping ratio when the material is subjected to low-amplitude loading. Under large strain conditions, the model capability is evaluated for predicting the stress-strain strength behavior when the material is subjected to various stress paths. In the predictions, three material parameters are used to represent the stiffness and friction of the interparticle contact. Although the predictions are made for idealized spherical particles, the predicted behaviors are found to be remarkably similar to that observed for sands in experiments. The potential capability of the proposed constitutive theory is illustrated, and the model performance is discussed on various aspects of granular material behavior, such as stress-induced anisotropy, path dependence, plastic flow, dilatancy, and noncoaxial behavior under rotation of principal stress.
TL;DR: In this article, a constitutive law for granular material is derived based on a micromechanics approach, taking into account the mechanisms of sliding and sliding in granular materials.
Abstract: Perceiving granular material as a collection of particles, a constitutive law for granular material is derived based on a micromechanics approach, taking into account the mechanisms of sliding and ...
TL;DR: In this paper, the authors combine bundle theory with the mechanics of local stress concentration in the development of a mechanistic representation of tensile strength, and apply the resulting analysis to numerical studies of the influence of micromechanical properties and irregular fiber spacing on the strength of polymeric composites.
Abstract: For continuous fiber reinforced polymeric composites the process domi nating tensile strength is fiber fracture. This phenomena results in stress concentrations in adjacent fibers over some distance which is directly associated with the ineffective length. This length is the controlling factor in the theory of bundle strength for polymer-based composites. The associated stress concentration factor, C, is normally associated with fracture propagation in both the matrix and surrounding fibers, and should also be in cluded as an important part of any representation of the mechanism controlling tensile fail ure in fibrous composites. In this paper, we combine bundle theory with the mechanics of local stress concentration in the development of a mechanistic representation of tensile strength. The resulting analysis is applied to numerical studies of the influence of micromechanical properties and irregular fiber spacing on the tensile strength, and a dis cussion of optimal design parameters for composites.
TL;DR: In this article, the micromechanical behavior and mechanical properties of graph ite/epoxy composite laminates were studied experimentally using high-sensitivity moire interferometry and strain gages.
Abstract: The micromechanical behavior and mechanical properties of thick graph ite/epoxy composite laminates were studied experimentally using high-sensitivity moire interferometry and strain gages. Quasi-isotropic and cross-ply laminates were in vestigated. Specimens were loaded in in-plane compression and interlaminar compres sion. Strong free-edge effects were documented, including high interlaminar shear strains and normal strains. Average or smeared values of Young's moduli and Poisson's ratio were measured.
TL;DR: In this paper, the finite element method (FEM) and acoustic emission technique (AE) were applied to the micromechanics analysis of the failure process of composites with thermoplastic matrix materials.
Abstract: The finite element method (FEM) and acoustic emission technique (AE) were applied to the micromechanics analysis of the failure process of composites with thermoplastic matrix materials. FEM calculations to local stress-strain distribution and the influence of very different intermediate layer properties are interpreted with regard to microscopic failure mechanisms in composite materials. The strongly differing AE behaviour of both chalk-filled Polyvinylchloride and high density polyethylene and short-glass-fibre reinforced polypropylene, polyamide, PBTP, SAN and ABS in tensile test experiments is demonstrated. Representative loading limits are derived from the nature and extent of the dominating failure mechanisms by comparison of theoretical and experimental results. The influence of critical strain, shear strength and fracture toughness properties of the modified matrix as well as the composite morphology and phase adhesion on significant deformation and failure stages is discussed. Finally some conclusions are drawn about a possible critical long-term strain of composites.
TL;DR: In this paper, the use of fiber optic strain sensors embedded within a macromodel composite employed to study and validate micromechanical theories is introduced, which can assist in the formulation of new concepts for interpretation and prediction of actuator/sensor response in smart materials.
Abstract: We introduce the use of fiber optic strain sensors embedded within a macromodel composite employed to study and validate micromechanical theories. Fabry-Perot (FP) fiber optic strain sensors (FOSSs) embedded within a macromodel composite are shown to be an accurate and precise means of making local internal strain measurements in and around damage events. The measurements made by the sensors compare closely to those obtained from resistance strain gauge data verified through presently accepted micromechanics. The optical strain sensors effectively measure both the signature of fiber fracture and the resulting strain concentration due to the damage event. The sensors add a new dimension to the validation and development of micromechanics. The approach assists in the formulation of new concepts for interpretation and prediction of actuator/sensor response in smart materials.