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Investigation of Mechanical Properties of Particulate Metal Matrix Nanocomposites Using Surface Elasticity Theory

Farajzadeh Moshtaghin, Alireza | 2011

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  1. Type of Document: M.Sc. Thesis
  2. Language: Farsi
  3. Document No: 42361 (08)
  4. University: Sharif University of Technology
  5. Department: Mechanical Engineering
  6. Advisor(s): Naghdabadi, Reza; Asghari, Mohsen
  7. Abstract:
  8. Excellent mechanical properties and high aspect ratio of carbon nanotubes have been the reason for extensive research on nanocomposites reinforced by this nanostructure, and recently metal matrix type of these nanocomposites, due to their advantages, have received great attention. Moreover, nanoporous materials are another type of nanostructured materials which due to their very high porosity have special applications in hydrogen storage, separating molecules, purification and energy absorption. Because of completely neglecting the details of nanoscale structure and not paying attention to the length scale, classical continuum mechanics methods are not precise enough for evaluating different material properties at nano scale. Therefore non-classical methods such as surface elasticity theory have been developed which have the ability to evaluate properties of nanomaterials with enough precision and lower calculation cost with respect to atomic methods. The nanoscale structure of heterogeneous materials results in an increase in surface to volume ratio which, in turn, makes the behavior of surfaces/interfaces a major factor in controlling mechanical properties of these materials. Consequently, the material properties become size-dependent as has been reported in experimental observations. This work modifies the classical equations from this point of view. The main objective of this project is to understand the effects of surface/interface stress on the overall yield strength of metals containing aligned nanoscale cylindrical inhomogeneities. Within a micromechanical framework, modified Hill's condition with size-dependent moduli is used, and for incorporating the effect of surface/interface residual stress, the related deformations are resolved into two parts: one caused by the external loading, and another caused by the surface/interface residual stress. Next, applying the field fluctuation method, the general form of the yield function will be derived. Then, it will be simplified for different practical loading conditions such as longitudinal, transverse, and axisymmetric. Finally, the effects of both parts of the surface/interface stress and size-dependency of the yield surface is illustrated by some numerical examples. The results show that the surface/interface stress makes the yield strength of nanoporous materials and nanocomposites size-dependent if the radii of cylindrical inhomogeneities are less than 50nm and 10nm, respectively. In addition, the effect of surface/interface residual stress is much more important than surface/interface elasticity
  9. Keywords:
  10. Nanocomposite ; Yield Surface ; Surface Elasticity ; Cylindrical Inhomogeneity ; Metal Matrix Composite (MMC)

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