Showing papers on "Micromechanics published in 1987"

Composite Materials: Science and Engineering

Abstract: 1. Introduction.- 2. Reinforcements.- 3. Matrix Materials.- 4. Interfaces.- 5. Polymer Matrix Composites.- 6. Metal Matrix Composites.- 7. Ceramic Matrix Composites.- 8. Carbon Fiber/Carbon Matrix Composites.- 9. Multifilamentary Superconducting Composites.- 10. Micromechanics of Composites.- 11. Macromechanics of Composites.- 12. Monotonic Strength and Fracture.- 13. Fatigue and Creep.- 14. Designing with Composites.- 15. Non-Conventional Composites.

Crack propagation in ceramics under cyclic loads

Abstract: Stable crack growth is observed in notched plates of polycrystalline alumina subject to fully compressive far-field cyclic loads at room temperature in a moist air environment andin vacuo. The fatigue cracks propagate at a progressively decreasing velocity along the plane of the notch and in a direction macroscopically normal to the compression axis. The principal failure events leading to this effect are analysed in terms of notch-tip damage under the far-field compressive stress, microcracking, frictional sliding and opening of microcracks, and crack closure. An important contribution to such Mode I crack growth arises from the residualtensile stresses induced locally at the notch-tip when the deformation within the notch-tip process zone leaves permanent strains upon unloading from the maximum nominal compressive stress. It is shown that the phenomenon of crack growth under cyclic compressive stresses exhibits a macroscopically similar behaviour in a wide range of materials spanning the very ductile metals to extremely brittle solids, although the micromechanics of this effect are very different among the various classes of materials. The mechanisms of fatigue in ceramics are compared and contrasted with the more familiar examples of crack propagation under far-field cyclic compression in metallic systems and the implications for fracture in ceramic-metal composites and transformation toughened ceramic composites are highlighted. Strategies for some important applications of this phenomenon are recommended for the study of fracture mechanisms and for the measurement of fracture toughness in brittle solids.

Simplified composite micromechanics for predicting microstresses

Abstract: A unified set of composite micromechanics equations is summarized and described. This unified set is for predicting the ply microstresses when the ply stresses are known. The set consists of equations of simple form for predicting three-dimensional stresses (six each) in the matrix, fiber, and interface. Several numerical examples are included to illustrate use and computational effectiveness of the equations in this unified set. Numerical results from these examples are discussed with respect to their significance on microcrack formation and, therefore, damage initiation in fiber composites.

A micromechanics analysis of the influence of the interface on the performance of polymer-matrix composites

Abstract: A specially developed two-dimensional finite element micromechanics analysis was used to predict the transverse tensile response of three different carbon fiber-reinforced, polymer matrix unidirectional composites. Experimental data were available for four different fiber sizings. The composites were tested both dry and moisture-conditioned, at room and elevated temperatures. Analytical/experimental correlations are presented and discussed.

Fiber orientation and its effect upon thermoelastic properties of short carbon fiber reinforced poly(etheretherketone) (PEEK)

Abstract: Experimental and analytical techniques are employed in the present study to investigate the influence of microstructure on thermoelastic properties of short carbon fiber reinforced poly(etheretherketone). The test specimen geometry is an edge gated, injection molded dogbone tensile bar. Typical of injection molded structures, three distinct layers of fiber orientation were discernable through the sample thickness. The thermoelastic properties of the surface layer (machined from the specimen) are measured for direct correlation with a micromechanics model. In addition to measuring the volume fractions and constituent properties, microstructural features such as fiber aspect ratio and the process-induced fiber orientation distribution are quantified. Correlation of experimental data with micromechanics model predictions is found to be quite good.

Mechanical response of unidirectional boron/aluminum under combined loading

Abstract: Three test methods were employed to characterize the response of unidirectional Boron/Aluminum metal matrix composite material under monotonic and cyclic loading conditions, namely, losipescu shear, off-axis tension and compression The characterization of the elastic and plastic response includes the elastic material properties, yielding and subsequent hardening of the unidirectional composite under different stress ratios in the material principal coordinate system Yield loci generated for different stress ratios are compared for the three different test methods, taking into account residual stresses and specimen geometry Subsequently, the yield locus for in-plane shear is compared with the prediction of an analytical, micromechanical model The influence of the scatter in the experimental data on the predicted yield surface is also analyzed Lastly, the experimental material strengths in tension and compression are correlated with the maximum stress and the Tsai-Wu failure criterion

Micromechanics of time-dependent deformation in a dispersion-hardened polycrystal

Abstract: A micromechanical theory is developed to study the time-dependent creep behavior of a two-phase alloy, consisting of a polycrystalline matrix and uniformly dispersed spherical particles. It is shown that there are two major sources of dispersion hardening-metallurgical, involving the bypassing of particles by dislocations, and mechanical, concerning the stress redistribution among the constituent phases and the calculation of creep strains. The metallurgical effect is implemented in a dispersion-dependent micro constitutive equation of slip systems. The problem of stress redistribution due to both the elastic and the plastic inhomogeneity is also analyzed. The results indicate that, while the stress in the creeping matrix of a constituent grain tends to decrease, the stress in the embedded inclusions is dependent upon the grain orientation in which they are embedded, and may increase or decrease. The overall creep behavior is calculated from the averaging process over the orientation of its constituent grains. This theory is finally applied to a cobalt system reinforced with rutile particles. Some merits and limitations of the theory are also discussed.

Effective thermoelastic properties of short-fiber composites

Abstract: The effective elastic properties of short fiber composites are calculated in terms of tensors which are measures of the strain concentration and excess thermal strain in the fibers relative to to the average matrix strain. These tensors are estimated on the basis of the so-called equivalent inclusion method which utilizes the Eshelby's solution. A comparison of the results with the available bounds and limited experimental data suggests that the proposed estimates of effective properties may be quite reliable.

Morphology, Microstructure and Micromechanics of Ice Fields

Abstract: A model of constitutive behavior of ice fields treated as ensembles of ice floes with random physical and geometrical properties is presented. A graph-based micromechanical model permits the introduction of an elastic/viscoplastic law at the microscale, and the derivation of constitutve coefficients of the statistical continuum approximation. The elastic-plastic transition is characterized by a fractal dimension of plastic ridges, a feature typically observed in many ice field morphologies.

The micromechanics of fatigue-induced delamination of a graphite/epoxy composite

Abstract: Fatigue crack growth in the resin layer between 0 and 90 plies of an AS/3501-5A graphite fibre/epoxy composite is discontinuous. Regularly spaced extensions of the crack front occur after periods of arrest. Crack compliance and tip strain fields have been measured to determine how the local minimum (K min l) and maximum (K max l) crack tip stress intensities affect growth. Contact of the fracture surfaces and swelling of the 90° ply modify these local stress intensities by an amount sensitive to load ratio (R), and the resulting propagation rate depends strongly onR. A model capable of describing thisR effect relates the distance of each individual crack advance to Kmax l and the duration of each arrest toK mnax l -K min l, i.e., to ΔK eff. We discuss the genesis of this model, and its explanation of the large Paris law coefficient which results if growth rates are instead expressed against the applied cyclic stress intensity.

Strain energy release rates of composite interlaminar end-notch and mixed-mode fracture: A sublaminate/ply level analysis and a computer code

Abstract: A computer code is presented for the sublaminate/ply level analysis of composite structures. This code is useful for obtaining stresses in regions affected by delaminations, transverse cracks, and discontinuities related to inherent fabrication anomalies, geometric configurations, and loading conditions. Particular attention is focussed on those layers or groups of layers (sublaminates) which are immediately affected by the inherent flaws. These layers are analyzed as homogeneous bodies in equilibrium and in isolation from the rest of the laminate. The theoretical model used to analyze the individual layers allows the relevant stresses and displacements near discontinuities to be represented in the form of pure exponential-decay-type functions which are selected to eliminate the exponential-precision-related difficulties in sublaminate/ply level analysis. Thus, sublaminate analysis can be conducted without any restriction on the maximum number of layers, delaminations, transverse cracks, or other types of discontinuities. In conjunction with the strain energy release rate (SERR) concept and composite micromechanics, this computational procedure is used to model select cases of end-notch and mixed-mode fracture specimens. The computed stresses are in good agreement with those from a three-dimensional finite element analysis. Also, SERRs compare well with limited available experimental data.

Fracture Testing of Brittle Polymers

Abstract: Three optical methods will be presented which, together with adequate measuring devices of short time physics, are powerful tools in the investigation of relevant fracture parameters during crack propagation. By reflection optics, crack speeds can be measured in a very large range. By microscopic optical interferometry, the size and shape of the craze zone at the crack tip and the crack opening can be determined thus leading to an insight into the micromechanics in the crack tip region. By the shadow optical method of caustics, information on the stress field around the crack tip is used in the determination of stress-intensity factors. The methods are applied to fracture induced by different types of loading such as quasistatic, fatigue and impact. For PMMA, life times measured in creep rupture will be compared with the fracture mechanics approach. Critical craze dimensions of PMMA measured during crack propagation will be given and results of the micromechanics of fatigue crack propagation in PMMA and PVC will be presented. For dynamic loading some recent results will be reviewed.