Showing papers on "Micromechanics published in 1989"

The Mechanical Behavior of Ceramic Matrix Composites

Abstract: This article summarizes the current understanding of relationships between microstructure and mechanical properties in ceramics reinforced with aligned fibers. Emphasis is placed on definition of the micromechanical properties of the interface that govern the composite toughness. Issues such as the debond and sliding resistance of the interface are discussed based on micromechanics calculations and experiments conducted on both model composites and actual composites.

The mechanical performance of fiber-reinforced ceramic matrix composites

Abstract: This article evaluates the current understanding of relationships between microstructure and mechanical properties in ceramics reinforced with aligned fiber. Emphasis is placed on definition of the micromechanical properties of the interface that govern the composite toughness. Issues such as the debond and sliding resistance of the interface are discussed based on micromechanics calculations and experiments conducted on both model composites and actual composites.

Microdamage analysis of fibrous composite monolayers under tensile stress

Abstract: The quasi-static deformation and fracture modes of several types of fibrous composite materials are studied from a fundamental viewpoint using a new experimental approach. Microcomposite monolayers, consisting of single fibres accurately positioned into a thin poly-meric matrix, were manufactured using a specially developed technique, and tested for strength by means of a custom-made miniature tensile testing machine. The materials used were E-glass, and Kevlar 29, Kevlar 49 and Kevlar 149 para-aramid fibres, and a room-temperature curing epoxy resin. The tensile testing machine was fitted to the stage of a polarized light stereozoom microscope and the fracture process was recorded both via a standard 35 mm camera and a colour video camera. The fibre content of the first generation of micro-composite monolayers used in this work was low (<0.025) but definite effects on the modulus and strength were obtained as the experimental data followed the rule-of-mixtures quite accurately in most cases. The failure process was different in each type of composite and current statistical models for strength are unable to account for the modes of failure observed in some of the systems studied. The experimental approach proposed is potentially useful in the study of the effects of interface chemistry modifications, fibre-fibre interactions, matrix toughness modification, misalignment effects, and more, on the deformation and failure micromechanics of composites.

Cracking and damage: strain localization and size effect

Abstract: Observation and measurement of damage and localization. Micromechanics and micro-macro relationships. Stability, bifurcation and localisation. Nonlocal models, localization limiters and size effect. Summary of discussions in workshop.

Micromechanical stresses in SiC-reinforced Al2O3 composites

Abstract: Applying an Eshelby (1957) approach, the internal micromechanical stresses within an SiC-inclusion-reinforced (platelet to whisker geometries) polycrystalline alumina matrix composite were calculated. The results are compared to the experimental residual stress measurements of a SiC-whisker-reinforced Al2O3 by Predecki, et al. (in press) and found to be in excellent agreement. The calculations are then extended to SiC-reinforced composites with polycrystalline mullite, silicon nitride, and cordierite matrices. It is concluded that the internal stresses are significantly influenced by the inclusion geometry as well as the thermoelastic differences between the inclusion and the matrix and also the volume fraction.

Micromechanics of compression failures in open hole composite laminates

Abstract: The local softening in the damage zone that precedes compression failures in laminates with holes was modeled using a Dugdale-type approach, and the calculated load to damage-size relationships were compared to the actual data obtained in two composite systems, AS4/PEEK and T2C145/F155, loaded to compression The comparison indicates that the Dugdale model, which works well for metals, did not accurately predict the load-damage zone size relationship in these specimens Possible reasons for the poor agreement are considered to be the fact that the constitutive relationship used in the Dugdale model is not an accurate description of material response in the crush zone, and the fact that the crush zone size is measured on the surface of the specimen, while this size is not an accurate indication of the actual crush zone size through the laminate

Micromechanics Prediction of Fatigue Failure of Composite Materials

Abstract: A micromechanics theory is proposed for the prediction of the fatigue failure of unidi rectional composites and laminates under any loading system. The method is based on micro-failure criteria which are applied separately on the fiber and matrix regions. The resulting failure stresses of the composite depend explicitly on the elastic constants of the phases and the reinforcement volume ratio. Comparisons between the theoretical fatigue failure diagrams and experimental results are given for unidirectional composites and angle-ply laminates.

Nonlinear Mechanical Properties of a Thermoset Matrix Composite at Elevated Temperatures

Abstract: Models are presented for describing the stress-strain relationships and the strengths of a thermoset matrix composite exposed to elevated temperature. The models take into account the nonlinear elastic characteristics of the material. For Fiberite 976 resin and for unidirectional Fiberite T300/976 composite the constants required in the models were determined by measuring the tensile, compressive, and shear properties, and Poisson ratios at 75, 250, 300, and 350°F. The strengths, initial (stress → 0) moduli, and initial Poisson's ratios of the unidirectional composite were calculated from the known resin and fiber properties using micromechanics equations. A procedure is also described which, together with the stress-strain-strength models developed in this study, can be used to calculate the deformations and strengths of loaded multidirectional laminates at high temperatures. To evaluate the accuracies of the models and the calculation procedure deformations and strengths of multidirectional Fiberite T30...

A Review of Plasticity Theory of Fibrous Composite Materials

Abstract: Continuum modeling of the elastic-plastic behavior of fibrous composites is concerned with predictions of the macroscopic behavior of a composite aggregate, which consists of an elastic-plastic metal matrix reinforced by unidirectionally aligned elastic fibers. Mechanical properties and volume fractions of the phases are assumed to be known; the objective is to predict the overall response of the composite under incrementally applied uniform macroscopic stresses or strains in terms of the phase properties, and of the geometry of the microstructure. This paper reviews some recent solutions of this problem. Those include micromechanical models that derive the overall response from uniform local fields, and also models that approximate the actual nonuniform local fields and arrive at upper and lower bound solutions. Recent experimental results pertaining to plastic behavior of fibrous composite materials are also discussed.

Nonlinear mesomechanics of composites with periodic microstructure

Abstract: This work is concerned with modeling the mechanical deformation or constitutive behavior of composites comprised of a periodic microstructure under small displacement conditions at elevated temperature. A mesomechanics approach is adopted which relates the microimechanical behavior of the heterogeneous composite with its in-service macroscopic behavior. Two different methods, one based on a Fourier series approach and the other on a Green's function approach, are used in modeling the micromechanical behavior of the composite material. Although the constitutive formulations are based on a micromechanical approach, it should be stressed that the resulting equations are volume averaged to produce overall effective constitutive relations which relate the bulk, volume averaged, stress increment to the bulk, volume averaged, strain increment. As such, they are macromodels which can be used directly in nonlinear finite element programs such as MARC, ANSYS and ABAQUS or in boundary element programs such as BEST3D. In developing the volume averaged or efective macromodels from the micromechanical models, both approaches will require the evaluation of volume integrals containing the spatially varying strain distributions throughout the composite material. By assuming that the strain distributions are spatially constant within each constituent phase-or within a given subvolume within each constituent phase-of the composite material, the volume integrals can be obtained in closed form. This simplified micromodel can then be volume averaged to obtain an effective macromodel suitable for use in the MARC, ANSYS and ABAQUS nonlinear finite element programs via user constitutive subroutines such as HYPELA and CMUSER. This effective macromodel can be used in a nonlinear finite element structural analysis to obtain the strain-temperature history at those points in the structure where thermomechanical cracking and damage are expected to occur, the so called damage critical points of the structure.

Unified micromechanics of damping for unidirectional fiber reinforced composites

Abstract: An integrated micromechanics methodology for the prediction of damping capacity in fiber-reinforced polymer matrix unidirectional composites has been developed. Explicit micromechanics equations based on hysteretic damping are presented relating the on-axis damping capacities to the fiber and matrix properties and volume fraction. The damping capacities of unidirectional composites subjected to off-axis loading are synthesized from thermal effect on the damping performance of unidirectional composites due to temperature and moisture variations is also modeled. The damping contributions from interfacial friction between broken fibers and matrix are incorporated. Finally, the temperature rise in continuously vibrating composite plies is estimated. Application examples illustrate the significance of various parameters on the damping performance of unidirectional and off-axis fiber reinforced composites.

Micromechanics and fatigue crack growth in an alumina-fiber-reinforced magnesium alloy composite

Abstract: A study has been made of fatigue crack growth through the magnesium alloy ZE41A and a composite of this alloy reinforced with alumina fibers. Crack growth rates were measured and failure mechanisms characterized for specimens with fibers parallel to the loading axis and for two off-axis orientations. Crack opening displacements and matrix and fiber strains in the vicinity of the crack tip were measured using the stereomaging technique. Crack growth rates through the composite were retarded by the fibers. For the composite with fibers at 22.5 deg to the loading axis, fibers were found to fracture in the composite at the same stress as measured for the fibers alone. Fiber fracture was the dominant growth-controlling mechanism for fibers oriented on and 22.5 deg to the loading axis, and little fiber pullout was observed. However, for crack growth through material with fibers oriented at 45 deg to the loading axis, crack growth was found to exist principally through the interface. Driving forces for cracks in interfaces were determined to be smaller than the applied δK. It was found that approximate fatigue crack growth rates through the composites could be predicted from those through the matrix by adjusting the tensile modulus. The upper and lower bounds of fatigue crack growth rate were also computed for the composite using a micromechanics-based model that incorporated observed failure mechanisms.

Metal matrix composite micromechanics: In-situ behavior influence on composite properties

Abstract: Recent efforts in computational mechanics methods for simulating the nonlinear behavior of metal matrix composites have culminated in the implementation of the Metal Matrix Composite Analyzer (METCAN) computer code. In METCAN material nonlinearity is treated at the constituent (fiber, matrix, and interphase) level where the current material model describes a time-temperature-stress dependency of the constituent properties in a material behavior space. The composite properties are synthesized from the constituent instantaneous properties by virtue of composite micromechanics and macromechanics models. The behavior of metal matrix composites depends on fabrication process variables, in situ fiber and matrix properties, bonding between the fiber and matrix, and/or the properties of an interphase between the fiber and matrix. Specifically, the influence of in situ matrix strength and the interphase degradation on the unidirectional composite stress-strain behavior is examined. These types of studies provide insight into micromechanical behavior that may be helpful in resolving discrepancies between experimentally observed composite behavior and predicted response.

A comparison of various micromechanics models for metal matrix composites

Abstract: Four micromechanics models currently used in the analysis of metal matrix composites are reviewed. The four models are the vanishing fiber diameter model, the Aboudi model, the multicell model, and discrete fiber-matrix model. Results predicted by computer programs based on each of the models are compared. Comparisons are made for the laminate properties and laminate stress-strain behavior for boron/aluminum and silicon-carbide/titanium composites. The predictions of constituent stresses are also compared. The predictions are compared to experimental data. Each of the models did a reasonably good job predicting laminate properties and stress-strain behavior. Thus, the discriminator between the models is a question of what type of results are required, the capabilities of the model and program, or the ease of operation. The choice of a model and program depends on several factors and the same program may not be the best choice for all analysis needs. The descriptions and comparisons made in this paper should aid in the choice of a model and program.

Composite blade structural analyzer (COBSTRAN) user's manual

Abstract: The installation and use of a computer code, COBSTRAN (COmposite Blade STRuctrual ANalyzer), developed for the design and analysis of composite turbofan and turboprop blades and also for composite wind turbine blades was described. This code combines composite mechanics and laminate theory with an internal data base of fiber and matrix properties. Inputs to the code are constituent fiber and matrix material properties, factors reflecting the fabrication process, composite geometry and blade geometry. COBSTRAN performs the micromechanics, macromechanics and laminate analyses of these fiber composites. COBSTRAN generates a NASTRAN model with equivalent anisotropic homogeneous material properties. Stress output from NASTRAN is used to calculate individual ply stresses, strains, interply stresses, thru-the-thickness stresses and failure margins. Curved panel structures may be modeled providing the curvature of a cross-section is defined by a single value function. COBSTRAN is written in FORTRAN 77.

The isothermal fatigue behavior of a unidirectional SiC/Ti composite and the Ti alloy matrix

Abstract: The high temperature fatigue behavior of a metal matrix composite (MMC) consisting of Ti-15V-3Cr-3Al-3Sn (Ti-15-3) matrix reinforced by 33 vol percent of continuous unidirectional SiC fibers was experimentally and analytically evaluated. Isothermal MMC fatigue tests with constant amplitude loading parallel to the fiber direction were performed at 300 and 550 C. Comparative fatigue tests of the Ti-15-3 matrix alloy were also conducted. Composite fatigue behavior and the in-situ stress state of the fiber and matrix were analyzed with a micromechanical model, the Concentric Cylinder Model (CCM). The cyclic stress-strain response of the composite was stable at 300 C. However, an increase in cyclic mean strain foreshortened MMC fatigue life at high strain ranges at 550 C. Fatigue tests of the matrix alloy and CCM analyses indicated this response was associated with stress relaxation of the matrix in the composite.

Asymptotic Fields in Adhesive Fracture

Abstract: This paper presents the asymptotic singular fields associated with the fracture analysis of adhesive joints and the micromechanics of adhesive failure. The fracture parameters used in adhesives are examined and their validity and use in applications is evaluated. Contrary to conventional fracture mechanics of homogeneous media the asymptotic field in most adhesive fracture is a function of the following: the adhesive and adherend properties, the dimensionality of the crack geometry, and their relationship to the interface. The Finite Element Iterative Method (FEIM) is used in analyzing the asymptotic fields. The results of the singularities for interfacial cracks of various geometries and material properties are presented and discussed in relation to adhesive failures.

Micro-to-Macro Analysis of Viscoelastic Laminated Plates

Abstract: This paper deals with the analysis of viscoelastic laminated plates subjected to static, dynamic or random loads The methodology is based on a micromechanics approach by which the time-dependent properties of the composite are established, followed by a macromechanics analysis of the laminated plate The following problems are analyzed: quasi-static deflection, quasi-static buckling, dynamic response and random vibration

Prediction of Process Induced Thermal Residual Stresses in Injection Molded Composites

Abstract: Several models exist for predicting residual thermal stresses in inorganic glasses. A few of these have been applied with some success, to predicting residual stresses in amorphous polymers. One such theory, that of Indenbom, has been used in this study to conduct a sensitivity analysis on the effect of material properties and processing parameters on the residual stress distribution in neat polymers. This theory has then been coupled to a micromechanics theory to predict residual stresses in isotropic short fiber composites (composites which have a 3-D random fiber orientation distribution). Finally a thermoelastic formulation based on incremental stress analysis is proposed for predict ing residual thermal stresses in orthotropic, short-fiber composites with depth varying properties due to the skin-core effect.

An Integrated Methodology for Optimizing Structural Composite Damping

Abstract: A method is presented for tailoring plate and shell composite structures for optimal forced damped dynamic response. The damping of specific vibration modes is optimized with respect to dynamic performance criteria including placement of natural frequencies and minimization of resonance amplitudes. The structural composite damping is synthesized from the properties of the constituent materials, laminate parameters, and structural geometry based on a specialty finite element. Application studies include the optimization of laminated composite beams and composite shells with fiber volume ratios and ply angles as design variables. The results illustrate the significance of damping tailoring to the dynamic performance of composite structures, and the effectiveness of the method in optimizing the structural dynamic response.