Showing papers on "Micromechanics published in 1982"

Micromechanics of defects in solids

Abstract: Preface Numerical simulation of intergranular and transgranular crack propagation in ferroelectric polycrystals Microstructure and stray electric fields at surface cracks in ferroelectrics Double kink mechanisms for discrete dislocations in BCC crystals The expanding spherical inhomogeneity with transformation strain A new model of damage: a moving thick layer approach On configurational forces at boundaries in fracture mechanics HotQC simulation of nanovoid growth under tension in copper Coupled phase transformations and plasticity as a field theory of deformation incompatibility Continuum strain-gradient elasticity from discrete valence force field theory for diamond-like crystals

Rate Sensitive Tensile Impact Properties of Fully and Partially Loaded Unidirectional Composites

Abstract: An instrumented tensile impact test system with partial loading capabilities has been developed and tested. Hardware development included a low mass gripping system for tensile impact and a vibration-damped base and load cell. A standard pendulum type impact testing machine was modified for use. The rate sensitive material properties of unidirectional glass/epoxy and graphite/epoxy composite materials were investigated. The micromechanics of fracture within the composite caused by impact were studied, using partially loaded impact test specimens and scanning electron microscopy.

Micromechanics approach to the indentation hardness of glass matrix particulate composites

Abstract: A theoretical analysis is made of the indentation hardness of glass matrix, particulate composites. It is hypothesized that glass is an elastic-plastic solid on a microscopic scale. Based upon the Marsh theory of indentation, expressions are formulated for indentation hardness of two-phase composites containing spherical particles. When hard particles are dispersed in a soft glass matrix, the overall hardness depends upon the matrix hardness, the volume-fraction of dispersed phase, the elastic properties of the two phases and also the matrix flow stress. On the other hand, when soft particles are dispersed in a hard glass matrix, the hardness and the elastic moduli vary in parallel with the volume-fraction of dispersed phase. Furthermore, the present analysis indicates that the hardness of a composite is independent of the particle size and interparticle spacing if the volume-fraction of the particles is kept constant. Experimental results of the Vickers hardness of phase-separated glasses as well as published hardness data for a glass-ceramic are used for the verification of the theory. The proposed theory explains well the hardness behaviour of such glass matrix composites in terms of the properties and amounts of the individual phases and the microstructural effects.

A Fatigue Reliability Distribution Based on Probabilistic Micromechanics

Abstract: The first portion of this paper reviews the probabilistic micromechanics techniques of converting the effects of various microdamage mechanisms, attributable to the eventual fatigue failure of industrially important metals, into an analytic form that is numerically tractable. Following from this micro-structural approach, both theoretical and experimental justifications of a new fatigue reliability distribution are presented. Finally, the interrelations between this micromechanics fatigue life distribution and several of the common empirical expressions describing the observed scatter in fatigue data are discussed.

Finite‐element and boundary‐element analysis of craze micromechanics

Abstract: The two numerical methods are used to estimate craze surface displacements and stresses for both isolated crazes and crazes at crack tips. The results are compared with the predictions of craze micromechanics models. The investigation includes the computation of the craze surface stress profile required to maintain a given craze opening displacement profile. The boundary element program requires less computer time than the finite element one, and similar results are obtained. The analysis also considers the craze surface displacement profile corresponding to an assumed craze surface stress distribution. The element methods produce results which are approximately the same as those obtained using the model of Verheulpen-Heymans and Bauwens.

Viscoelastic Characterization of a Random Fiber Composite Material Employing Micromechanics

Abstract: The mechanical creep and recovery behavior for uniaxial loading of a short glass-fiber reinforced polyester composite, SMC-R50, with random fiber orientation was investigated experimentally and theoretically. The strain at different stress levels and temperatures was measured during isothermal tests involving several loading-unloading cycles; each cycle consisted of a period of constant load and zero load. Repeatable creep and recovery response was observed only after several cycles; an accelerated mechanical conditioning method was used, in which the duration of each cycle was progressively increased. It is shown that the thermally complex, linear viscoelastic response of SMC-R50 in the mechanically conditioned state can be described by the time-dependent and temperature-dependent behavior of the matrix system and micromechanics calculations of the effective properties of the composite. The data analysis methods for parameters of the model are discussed and illustrated. Master curves for the matrix and for the in situ matrix, the matrix in the composite, were obtained and found to be virtually the same. In order to achieve this agreement it was necessary to use a micromechanics analysis in which the reinforcement is properly modeled as viscoelastic, orthotropic ribbons rather than isotropic, elastic fibers.

Analytical/Experimental Correlations of Stiffness Properties of Unidirectional Composites

Abstract: A finite element micromechanics analysis is used to predict the longitudinal and transverse moduli of both graphite/epoxy and glass/epoxy unidirectional composites, as a function of temperature, for various moisture preconditionings. These predictions are then correlated with corresponding experimental data. The micromechanics analysis includes temperature- and moisture-dependent matrix material properties, inelastic matrix stress-strain response, and anisotropic fibers. Thermal residual stresses caused by cooldown from the cure temperature and moisture-induced swelling stresses are included in the analysis. Good correlation is obtained between theory and experiment.

Influence of stress interaction on the behavior of off-axis unidirectional composites

Abstract: The influence of combined states of stress on the shear response along material principal directions in off-axis, unidirectional, composite coupons is examined for systems whose matrix material obeys the von Mises yield condition. Such analysis is motivated by trends in experimental data observed for at least two composite systems that indicate deviations from pure shear behavior along material directions for various off-axis configurations.

Fatigue Crack Initiation with Creep

Abstract: Experimental observations of the micromechanics theory of fatigue crack initiation of Lin and Ito are reviewed. This theory is shown to be applicable also for time-dependent fatigue. An aluminum polycrystal subject to cyclic tension and compression is considered. An initial plastic strain field is assumed to exist in a thin slice parallel to the sliding direction a in the most favorably oriented crystal at a free surface. The initial stress field calculated from this strain field is shown to have a positive resolved shear stress on one side of the slice and a negative one on the other. This gives slip on one side during tensile loading and a reversed slip on the other side during compressive loading. Based on the stress-strain-time relationship of aluminum single crystals, the creep strain distributions in this favorably oriented crystal under cyclic loadings have been calculated. The buildup of creep strain near the free surface was found to vary linearly with the time of loading in high frequencies and to become less dependent on the loading time and more on the number of cycles as the frequency decreases.

Laminate Analyses, Micromechanical Creep Response, and Fatigue Behavior of Polymer Matrix Composite Materials.

Abstract: : Three major topics were pursued. The two-dimensional fintie element micromechanics analysis was extended to include nonlinear viscoelastic material response. Analytical predictions of the time-dependent behavior or both glass/epoxy and graphite/epoxy unidirectinal composites subjected to transverse compression were then correlated with experimental data also generated as part of this study. The viscoelastic properties of the epoxy matrix were also determined, as required input to the analysis. A new three-dimensional finite element analysis was developed, incorporating inelastic orthotropic material response, temperature- and moisture-dependent material properties, and improved numerical solution techniques. This analysis is permitting the study of both micromechanical and laminate problems.

Sensitivity analysis results of the effects of various parameters on composite design

Abstract: Sensitivity analysis results are presented to assess the effects of a multitude of important parameters on fiber composite design and structural response. These results were obtained by using optimum design procedures in conjunction with sensitivity analyses. Sensitivity analyses were performed to assess the effects on composite optimum design and structural response of parameters such fiber transverse and shear properties, in situ matrix elastic and strength properties, correlation coefficients used in composite micromechanics and in combined strength predictions, processing variables, and perturbations of loading conditions. The results show that matrix properties, fiber volume ratio and small perturbations of the loading conditions have significant effects on certain composite structural responses. The remaining parameters have negligible effect.

Analysis of the effect of matrix degradation on fatigue behavior of a graphite/epoxy laminate

Abstract: : An analytical study of the effect of fatigue on a + OR - 45/90 SUB 2sub s graphite/epoxy laminate is conducted considering two levels of damage. The first one is microscopic damage, which is viewed as a matrix softening mechanism that is accounted for by the use of micromechanics. The second level of damage is delamination. An evaluation of lamination theory versus a finite element elasticity solution which accounts for free edge stresses is made first. Lamination theory is found to be inadequate due to its inability to account for large amounts of shear deformation as the matrix degrades. Experimental data are used to evaluate the matrix degradation as a function of fatigue cycles, and then a parametric study of the strain energy release rate as a function of delamination size at various degrees of matrix degradation is conducted. For a constant maximum stress fatigue test, the matrix degradation leads to an increase in strain energy which is predicted to eventually cause rapid delamination. Reasonably good agreement between theory and experimental data on the + OR - 45/9O sub 2sub s laminate is demonstrated. (MM)

X-Ray Measurements of Long-Range Strains: A Bridge Between Micromechanics and Macromechanics

Abstract: Long-range, elastic strains emanating from stress raisers, such as notches, holes and inclusions were investigated in bent silicon crystals which functioned as a model material. The strains and strain interactions were characterized by x-ray pendellosung fringe topography and were quantitatively evaluated by x-ray intensity measurements of traverse-oscillation topographs. The experimental results were compared to calculations based on continuum mechanics and satisfactory agreement was obtained. The x-ray analysis of long-range plastic strains and deformation gradients in technological polycrystalline materials by a computer-aided rocking curve analyzer (CARCA) was described. It was shown how CARCA can be used in alloys to obtain aggregate information of the defect structure of a large grain population and to isolate regions of intense deformation. Applications of the method were given to determine prefracture damage in fatigued and corrosion fatigued aluminum alloys and in stress corroded steel.

Axial shear modulus of a fiber-reinforced composite with random fiber cross-sections

Abstract: A study is made of the effective axial shear modulus of a fiber rein- forced material with random fiber cross-sections so that the micromechanics is governed by stochastic differential equations. A coarse-graining procedure is adopted to investigate the macroscopic behavior of the material. This analysis leads to the formula for the effective axial shear modulus,