Micromechanics

About: Micromechanics is a research topic. Over the lifetime, 6000 publications have been published within this topic receiving 162635 citations.

A micromechanics-based thermodynamic formulation of isotropic damage with unilateral and friction effects

Abstract: This paper is devoted to micromechanical modeling of isotropic damage in brittle materials . The damaged materials will be considered as heterogeneous media composed of solid matrix weakened by isotropically distributed microcracks . The original contribution of the present work is to provide a proper micromechanical thermodynamic formulation for damage-friction modeling in brittle materials with the help of Eshelby’s solution to matrix-inclusion problems. The elastic and plastic strain energy involving unilateral effects will be fully determined. The condition of microcrack opening–closure transition will be determined in both strain-based and stress-based forms. The effect of spatial distribution of microcracks will also be taken into account. Further, the damage evolution law is formulated in a sound thermodynamic framework and inherently coupled with frictional sliding . As a first phase of validation, the proposed micromechanical model is finally applied to reproduce basic mechanical responses of ordinary concrete in compression tests.

Micromechanics of ITZ–Aggregate Interaction in Concrete Part I: Stress Concentration

Abstract: At the macroscopic scale, concrete appears as a composite made of a cement paste matrix with embedded aggregates. The latter are covered by interfacial transition zones (ITZs) of reduced stiffness and strength. Cracking in the ITZs is probably the key to the nonlinear stress–strain behavior in the prepeak regime. For a deeper understanding of this effect triggered by tensile microstress peaks, we here employ and extend the framework of continuum micromechanics, as to develop analytical solutions relating the macroscopic stresses acting on a piece of concrete, to microtractions at the aggregates' surfaces and to three-dimensional stress states within the ITZs. In the latter context, a new aggregate-to-ITZ stress concentration tensor is derived based on the separation-of-scale principle, which implies that ITZs may be modeled as two-dimensional interfaces at the concrete scale, but as three-dimensional bulk phases at the scale of a few micrometers. Microtensile peaks occur both under uniaxial macroscopic tension and compression. To describe the respective microtraction and microstress fields, it is suitable to define aggregate's “poles” and “equator” by an “axis” through the aggregate center, directed in the uniaxial macroscopic loading direction. Accordingly, tensile microtraction peaks, induced by macro-tension and macro-compression, respectively, occur at the “poles” and at the “equator”, respectively. The largest tensile ITZ-microstresses occur at an offset of about π/8 from the “poles” and the “equator”, respectively. These fields of microtractions and ITZ microstresses are prerequisites for upscaling ITZ-related strength to the macroscopic concrete level, as presented in the companion paper (Part II).

Multiwalled carbon nanotube reinforced polymer composites

Abstract: Due to their high stiffness and strength, as well as their electrical conductivity, carbon nanotubes are under intense investigation as fillers in polymer matrix composites. The nature of the carbon nanotube/polymer bonding and the curvature of the carbon nanotubes within the polymer have arisen as particular factors in the efficacy of the carbon nanotubes to actually provide any enhanced stiffness or strength to the composite. Here the effects of carbon nanotube curvature and interface interaction with the matrix on the composite stiffness are investigated using micromechanical analysis. In particular, the effects of poor bonding and thus poor shear lag load transfer to the carbon nanotubes are studied. In the case of poor bonding, carbon nanotubes waviness is shown to enhance the composite stiffness.

MODELING AND STRESS ANALYSIS OF SMART CNTs/FIBER/POLYMER MULTISCALE COMPOSITE PLATES

Abstract: Modeling and nonlinear stress analysis of piezolaminated CNTs/fiber/polymer composite (CNTFPC) plates under a combined mechanical and electrical loading are investigated in this study. The governing equations of the piezoelectric CNTFPC plates are derived based on first-order shear deformation plate theory (FSDT) and von Karman geometric nonlinearity. Halpin–Tsai equations and fiber micromechanics are used in hierarchy to predict the bulk material properties of the multiscale composite. The CNTs are assumed to be uniformly distributed and randomly oriented through the epoxy resin matrix. An analytical solution is employed to determine the large deflection response and stress analysis of the nanocomposite plates. Finally, by solving some numerical examples for simply supported plates, the effects of the applied constant voltage, plate geometry, volume fraction of fibers and weight percentage of SWCNTs and MWCNTs on the deflection and stress analyses of the piezoelectric CNTs/fiber/polymer multiscale composite plate are studied. It is shown that the deflections significantly decrease with a small percentage of CNTs. Also, it is found that the SWCNTs reinforcement produces more pronounced effect on the bending and stress of the nanocomposite plates in comparison with MWCNTs.