Showing papers on "Micromechanics published in 1983"

Micromechanics of Crystals and Polycrystals

Abstract: Publisher Summary This chapter focuses on micromechanics of crystals and polycrystals. In Section II of the chapter, a brief outline of only some of the important features of the micromechanics of crystalline plasticity is given. The discussion is confined to plastic flow caused by dislocation slip, and face-centered-cubic crystals are used in the examples of dislocation mechanisms. Particular attention is paid to kinematics and to the phenomenology of strain hardening, because these are shown to play dominant roles in macroscopic response. In Section III, constitutive laws for elastic-plastic crystals are developed. The framework draws heavily on Hill's analysis of the mechanics of elasticplastic crystals, but the theory is extended by incorporating the possibility of deviations from the Schmid rule of a critical resolved shear stress for slip. Deviations from the Schmid rule are motivated by micromechanical models for dislocation motion and are shown to lead to deviations from the “normality flow rule” of continuum plasticity. The implications of these “non-Schmid effects” regarding the stability of plastic flow are brought out via some examples of models for kinks bands and shear bands in Section IV. In Section IV some examples of analyses of elastic-plastic deformation in crystals are discussed. The chapter concludes with some suggestions for fruitful research. These involve extensions of the theory to finite-strain rate-dependent polycrystalline models.

Simplified composite micromechanics equations for hygral, thermal and mechanical properties

Abstract: A unified set of composite micromechanics equations of simple form is summarized and described. This unified set can be used to predict unidirectional composite (ply) geometric, mechanical, thermal and hygral properties using constituent material (fiber/matrix) properties. This unified set also includes approximate equations for predicting (1) moisture absorption; (2) glass transition temperature of wet resins; and (3) hygrothermal degradation effects. Several numerical examples are worked-out to illustrate ease of use and versatility of these equations. These numerical examples also demonstrate the interrelationship of the various factors (geometric to environmental) and help provide insight into composite behavior at the micromechanistic level.

Size effects and micromechanics of a porous solid

Abstract: The rigidity of rods of a polymeric foam in bending and torsion is measured as a function of diameter. The dependence of rigidity upon specimen size is found to be inconsistent with a classically viscoelastic continuum model. The Cosserat continuum, which admits additional degrees of freedom associated with rotation of the microstructure, describes the foam more accurately than the classical continuum. Evidence is presented that additional degrees of freedom associated with the deformation of the microstructure, must be incorporated in a complete continuum model of foamed materials.

A micromechanical theory of grain-size dependence in metal plasticity

Abstract: T he effect of grain-size on the elastoplastic behavior of metals is investigated from the micromechanics standpoint. First, based on the observations that dislocation pile-ups, formation of cell structures, and other inelastic activities influenced by the presence of grain boundary actually take place transcrystallinely, a grain-size dependent constitutive equation is proposed for the slip deformation of slip systems. By means of a modified Hill's self-consistent relation the local stress of a grain is calculated, and used in conjunction with this constitutive equation to evaluate the plastic strain of each constituent grain. The grain-size effect on the plastic flow of polycrystals then can be determined by an averaging process. To check the validity of the proposed theory it was finally applied to predict the stress-strain curves and flow stresses of a copper at various grain-sizes. The obtained results were found to be in good agreement with experimental data.

Fracture Mechanics of Sublaminate Cracks in Composite Laminates.

Abstract: : This paper presents an overview of a fracture mechanics approach to some of the most frequently encountered matrix-dominated, sub-laminate cracks in epoxy-based composite laminates. By 'sub-laminate', it is meant that the cracks are internal to the laminate, generally invisible macroscopically; but are much larger in size than those microcracks considered in the realm of micromechanics. The origin of sub-laminate cracks is assumed to stem form the coalescence of natural flaws (also microcracks) which occur under a certain favorable laminate stress condition. Thus, the modelling of the mechanisms of sub-laminate crack initiation and propagation is essentially mechanistic and probabilistic in nature. Some specific results form several analytical/experimental investigations using graphite-epoxy laminates are presented and discussed in this paper. (Author)

Finite Element Analysis of the Composite Fiber, Alpha Keratin

Abstract: The individual mechanical properties of each of the phases, microfibrils, and matrix in α-keratin fibers were analyzed in terms of a two-phase composite of microfibrils and matrix. The authors applied finite element techniques appropriate for composite micromechanics to the data available on the mechanical properties of various α- keratins with varying moisture contents. Despite some simplifying assumptions the result of the analysis was in general agreement with our present understanding of keratin structure. The results indicated the following: Absorption of water by the keratin structure has a pronounced weakening effect confined mainly to the matrix. Differences existing between different keratins were in the main due to variation of matrix properties produced by the presence of high glycine-tyrosine and high sulphur proteins, both of which increase the mechanical stiffness of the matrix. Data obtained for porcupine quill show that whereas the microfibrils can be considered as isotropic at all moistur...

Experimental mechanics of composite materials.

Abstract: Experimental methods are applied to the study of structural integrity, deformation, and fracture of composite materials Specific applications include characterization of constituent materials; verification of micromechanics analyses; qualification and characterization of the basic unidirectional lamina; strain (stress) analyses, especially around discontinuities; fracture characterization; assessment of structural integrity and reliability; and life prediction Experimental methods employ modeling and prototype studies and give point-per-point information or full-field representations The most commonly used experimental methods are photoelastic, strain gage, moire, interferometric, ultrasonic, acoustic, radiographic, and thermographic methods In the area of material characterization biaxial testing is still one of the most challenging problems Considerable effort has been devoted in recent years to developing procedures and systems for testing tubular composite specimens New methods have been developed for material characterization at the high strain rates encountered under projectile impact Progress has been made in material characterization including temperature, moisture, and nonlinear viscoelastic effects Experimental methods have been applied extensively to the study of deformation and fracture in composite laminates with stress concentrations The effects of material, stacking sequence, notch geometry, and stress biaxiality have been studied