Showing papers on "Micromechanics published in 1979"

Micromechanics of crack nucleation during indentations

Abstract: An analysis for the nucleation of microcracks from the inhomogeneous flow lines in soda-lime glass under Vickers indentations is considered. The minimum loads for crack nucleation are shown to depend on the hardness,H, and the critical stress intensity factor,K IC. Unlike the Lawn and Evans analysis, the present model does not require the presence of any fortuitous flaws of critical dimensions in the material, since the flaws are nucleated by the deformation in the deformed zone.

Mechanical Property Characterization of Intraply Hybrid Composites

Abstract: An investigation of the mechanical properties of intraply hybrids made from graphite fiber/epoxy matrix hybridized with secondary S-glass or Kevlar 49 fiber composites is presented. The specimen stress-strain behavior was determined, showing that mechanical properties of intraply hybrid composites can be measured with available methods such as the ten-degree off-axis test for intralaminar shear, and conventional tests for tensile, flexure, and Izod impact properties. The results also showed that combinations of high modulus graphite/S-glass/epoxy matrix composites exist which yield intraply hybrid laminates with the best 'balanced' properties, and that the translation efficiency of mechanical properties from the constituent composites to intraply hybrids may be assessed with a simple equation.

A Micromechanical Prediction of Initial Yield Surfaces of Unidirectional Composites

Abstract: For two-phase materials composed of ductile matrices reinforced with plastically nondeformable inclusions, the yield condition that has the same form as Tsai and Wu's phenomenological criterion has been deduced through a micromechanics analysis. Basic assumptions made are that the matrix material obeys the Levy-von Mises flow rule, and that matrix plastic strains are uniform. In general, the shape of the yield surface can be proved to be hyperellipsoidal.A further analysis is devoted to the case where the inclusions can be treated as identical ellipsoids with corresponding axes aligned. Based on the results, numerical computation of yield strength parameters is carried out for unidirectional boron fiber-aluminum composites subjected to uniform temperature changes prior to external loading. It is shown that yielding cannot take place under certain combined triaxial (not hydrostatic) stresses. Some discussion related to the effect of prior temperature changes is also given.

Micromechanics of intraply hybrid composites: Elastic and thermal properties

Abstract: Composite micromechanics are used to derive equations for predicting the elastic and thermal properties of unidirectional intraply hybrid composites. The results predicted using these equations are compared with those predicted using approximate equations based on the rule of mixtures, linear laminate theory, finite element analysis and limited experimental data. The comparisons for three different intraply hybrids indicate that all four methods predict approximately the same elastic properties and are in good agreement with measured data. The micromechanics equations and linear laminate theory predict about the same values for thermal expansion coefficients. The micromechanics equations predict through-the-thickness properties which are in good agreement with the finite element results.

On the micromechanics of multifilamentary superconducting composites

Abstract: The mechanical properties of multifilamentary Nb 3 Sn composites depend on several interrelated effects. The elastic modulus is influenced by crystallographic texturing as well as internal stresses. The textures as they develop in the as-drawn composite are typical of FCC and BCC metals, as is the oblate filament shape of the Nb filaments, which can be understood in terms of the orientation restriction of slip systems in such materials. Jc versus strain measurements on a 12.25% Nb composite with 1.2 micron diameter filaments show that zero strain Jc's are obtainable well beyond 1.0% strain on the first cycle. However, the internal residual stresses are relieved by applied strains well below 1% and subsequent Jc vs. strain performance is markedly impaired. The possible advantages of ultrafine filament composites are discussed.

Mathematical Modeling and Micromechanics of Fiber-Reinforced Bimodulus Composite Materials.

Abstract: : In Part I of this report are described and evaluated various mathematical constitutive equations proposed to model the mechanical behavior of fiber-reinforced composite materials having different compliances in tension and compression. Five different constitutive equations are described in detail and evaluated in the light of three criteria. In Part II of the report two entirely different micromechanistic approaches are developed to explain these observed differences in tension/compression behavior: the mean-fiber-angle approach originated by Tarnopol'skii et al. and the elastically supported tie-bar/column approach originated by Herrmann et al. (Author)

Prediction and Verification of Moisture Effects on Carbon Fiber-Epoxy Composites.

Abstract: : Moisture has an effect on the elastic as well as on the strength properties of composites. In this work we have experimentally verified the prediction of changes in the longitudinal shear modules G sub 12 as a function of moisture uptake and temperature. The properties of the bulk resin (measured independently as a function of temperature and moisture concentration) were used for predicting the composite response by using the simple Halpin-Tsai equation of micromechanics. The excellent agreement between the predicted and observed values demonstrates the feasibility of predicting changes in 'reversible properties' of composite in a natural service environment as suggested in a previous report. This predictive scheme considered simultaneously the concepts of diffusion, environmental modeling, micromechanics, and a finite difference laminate theory. (Author)

Energy absorption mechanisms during crack propagation in metal matrix composites

Abstract: The stress distributions around individual fibers in a unidirectional boron/aluminum composite material subjected to axial and transverse loadings are being studied utilizing a generalized plane strain finite element analysis. This micromechanics analysis was modified to permit the analysis of longitudinal sections, and also to incorporate crack initiation and propagation. The analysis fully models the elastoplastic response of the aluminum matrix, as well as temperature dependent material properties and thermal stress effects. The micromechanics analysis modifications are described, and numerical results are given for both longitudinal and transverse models loaded into the inelastic range, to first failure. Included are initially cracked fiber models.

Recent Results in Micromechanics

Abstract: The elastic and plastic fields (displacement, stress, plastic strain) are investigated for a uniformly moving inclusion and a dislocation loop. From the results of analysis a new concept of essential distributions of dislocations is proposed. Finally, the enerqy release rate of a moving crack is given by the J integral and dislocation density tensors.