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Theoretical Analysis of Mechanical Properties for Metal Matrix Nanocomposites Reinforced by Carbon Nanotubes Using Nonlocal Elasticity

Amini Niaki, Sina | 2011

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  1. Type of Document: M.Sc. Thesis
  2. Language: Farsi
  3. Document No: 42238 (08)
  4. University: Sharif University of Technology
  5. Department: Mechanical Engineering
  6. Advisor(s): Naghdabadi, Reza; Sohrabpour, Saeed
  7. Abstract:
  8. Nanocomposites are fabricated by incorporating nano-scale particles in polymer, metal, or ceramic matrices. Metal matrix nanocomposites are a new kind of materials which have shown superior mechanical features, such as high strength, stiffness, and hardness. However, they are not sensitive to high temperature, in contrast to the polymer matrix nanocomposites. Nanoporous materials, also, have wide range of applications in different fields, such as geology, biology, biomechanics, and electronic. Carbon Nanotubes (CNTs) have shown wonderful properties like high Young’s modulus, high resonance frequency and unique electrical behavior. Wonderful mechanical and electrical properties make CNTs superior candidates as reinforcing elements for composites. Continuum theories have been used to study mechanical properties of nanocomposites. As the classical continuum does not representeffects of length scale, they are not appropriate theories for studying the nano-scale structures. On the other hand, nonclassical continuum theories, such as the nonlocal elasticity, lead to attainment accurate results in small scales by usingmaterial length scale in the constitutive equations. Interaction of CNTs is a phenomenon that occurs in nanocomposites especially in the case of high volume fraction of the CNTs. In this thesis, effects of two nearby CNTsinteraction, as well as two nearby nanovoids, on the stress distribution in matrix is studied comparatively using the nonlocal integral elasticity. For this purpose, stress distribution around two CNTs/nanovoids, modeled as two similar cylindrical inclusions/holes in an infinite medium, is investigated. Classical stresses are obtained employing the complex stress potential method. Then, the nonlocal stresses are calculated applying the nonlocal integral equation to the classical stresses. Moreover, the influence function of the nonlocal integral equation is modified considering different nonlocal effects of the CNTs/nanovoids on the matrix points. As a result of this modification, a good agreement is observed for the maximum stress concentrations values and locations of the modified nonlocal analysis as compared with those published before in the literature using the surface elasticity analysis. Strengthening effects of the CNTs/nanovoids on the matrix, effects of the CNTs/nanovoids size and the distance between their centers as well as effects of far-field loading ratio on the classical, nonlocal and modified nonlocal stresses are studied. For the modified nonlocal analysis, the maximum stress concentration factor at the nanovoids and matrix interface decreases slightly for the nondimensional distance parameters less than a critical value. This critical distance is smaller than the predicted distance by the classical analysis. For the CNTs, similar to the nanovoids, the critical distance parameter of the modified nonlocal analysis is smaller than that corresponding to the classical one. However, the maximum stress concentration factor increases when the CNTs approach to each other. Moreover, the modified nonlocal analysis showed that increase of the maximum stress concentration factor due to CNTs interaction is less than the classical analysis prediction. In conclusion, using the modified nonlocal analysis indicated that nanocomposites with higher volume fraction of CNTs can be synthesized without concerning about devastating effects of CNTs/nanovoids interactions.

  9. Keywords:
  10. Nanoporous Materials ; Carbon Nanotubes ; Nonlocal Elasticity Theory ; Metal Matrix Nanocomposite ; Circular Inclusion

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