Showing papers by "George A. Kardomateas published in 2007"

Thermal Buckling of Multi-Walled Carbon Nanotubes by Nonlocal Elasticity

Abstract: The small internal length scales of nanomaterials/nano-devices may call the direct application of classical continuum models into question. In this research, a nonlocal elastic shell model, which takes the small scale effects into account, is developed to study the thermal buckling behavior of multi-walled carbon nanotubes. The multi-walled carbon nanotubes are considered as concentric thin shells coupled with the van der Waals forces between adjacent nanotubes. Closed form solutions are formulated for two types of thermal buckling of a double-walled carbon nanotube: Radial thermal buckling (as in a shell under external pressure) and axial thermal buckling. The effects of small scale effects are demonstrated, and a significant influence of internal characteristic parameters such as the length of the C-C bond has been found on the thermal buckling critical temperature. The study interestingly shows that the axial buckling is not likely to happen, while the "radial" buckling may often take place when the carbon nano-tubes are subjected to thermal loading. Furthermore, a convenient method to determine the material constant, "e o " and the internal characteristic parameter, "a," is suggested.

Vibration Characteristics of Multiwalled Carbon Nanotubes Embedded in Elastic Media by a Nonlocal Elastic Shell Model

Abstract: In this paper, the vibrational behavior of the multiwalled carbon nanotubes (MWCNTs) embedded in elastic media is investigated by a nonlocal shell model. The nonlocal shell model is formulated by considering the small length scales effects, the interaction of van der Waals forces between two adjacent tubes and the reaction from the surrounding media, and a set of governing equations of motion for the MWCNTs are accordingly derived. In contrast to the beam models in the literature, which would only predict the resonant frequencies of bending vibrational modes by taking the MWCNT as a whole beam, the current shell model can find the resonant frequencies of three modes being classified as radial, axial, and circumferential for each nanotube of a MWCNT. Big influences from the small length scales and the van der Waals' forces are observed. Among these, noteworthy is the reduction in the radial frequencies due to the van der Waals' force interaction between two adjacent nanotubes. The numerical results also show that when the spring constant k 0 of the surrounding elastic medium reaches a certain value, the lowest resonant frequency of the double walled carbon nanotube drops dramatically.

The Mixed Mode I and II Interface Crack in Piezoelectromagneto–Elastic Anisotropic Bimaterials

Abstract: Taking the electric-magnetic field inside the interface crack into account, the interface crack problem of dissimilar piezoelectromagneto (PEMO)-elastic anisotropic bimaterials under in-plane deformation is investigated. The conditions to decouple the in-plane and anti-plane deformation is presented for PEMO-elastic biaterials with a symmetry plane. Using the extended Stroh's dislocation theory of two-dimensional space and the analytic continuition principle of complex analysis, the interface crack problem is turned into a nonhomogeneous Hilbert equation in matrix notation. Four possible eigenvalues as well as four eigenvectors for the fundamental solution to the corresponding homogeneous Hilbert equation are found, so are four modes of singularities for the fields around the interface crack tip. These singularities are shown to have forms of r -(1/2)±i∈ 1 and r- (1/2)±i∈ 2, in which the bimaterial constants ∈ 1 and ∈ 2 are proven to be real numbers for practical dissimilar PEMO-elastic bimaterials. Compared with the solution for the interface crack of dissimilar elastic bimaterials without electro-magnetic properties, two new additional singularities are discovered for the interface crack in the PEMO-elastic bimaterial media. The electric-magnetic field inside the crack is solved by employing the "energy method, " which is based on finding the stationary point of the saddle surface of the energy release rate with respect to the electro-magnetic field inside the crack. Closed form expressions for the extended crack tip stress fields and crack open displacements are formulated, so are some other fracture characteristic parameters, such as the extended stress intensity factors and energy release rate (G) for dissimilar PEMO-elastic bimaterial solids. Finally, fundamental results and some conclusions are presented, which could have applications in the failure of piezoelectro /magneto-elastic devices.

An Investigation of the Growth of Multi-site Fatigue Cracks

Abstract: Test data obtained within the small crack growth regime of multi-site fatigue cracks are examined. Swain’s partitioning criterion (1992) is used to distinguish between cracks that arrest and cracks that continue to grow until fracture. A statistical application of this criterion indicates that the total collection of multi-site cracks on multiple specimens has a bi-modal distribution.

Thermal Buckling of Multi-Walled Carbon Nanotubes by Nonlocal

Abstract: The small internal length scales of nanomaterials/nano-devices may call the direct application of classical continuum models into question. In this research, a nonlocal elastic shell model, which takes the small scale effects into account, is developed to study the thermal buckling behavior of multi-walled carbon nanotubes. The multi-walled carbon nanotubes are considered as concentric thin shells coupled with the van der Waals forces between adjacent nanotubes. Closed form solutions are formulated for two types of thermal buckling of a double-walled carbon nanotube: Radial thermal buckling (as in a shell under external pressure) and axial thermal buckling. The effects of small scale effects are demonstrated, and a significant influence of internal characteristic parameters such as the length of the C-C bond has been found on the thermal buckling critical temperature. The study interestingly shows that the axial buckling is not likely to happen, while the "radial" buckling may often take place when the carbon nano-tubes are subjected to thermal loading. Furthermore, a convenient method to determine the material constant, "e o " and the internal characteristic parameter, "a," is suggested.

The Growth of Micro -Structurally Small Multi -Site Cracks

Abstract: [] The growth of multi-site fatigue-cracks on smooth surfaces in multiple specimens is considered. It is suggested that the total crack population for all of the specimens is bi-modal. That is, it consists of primary cracks that grow until failure by fracture in each specimen, and secondary cracks that ultimately arrest. An experimental procedure for separating the two distributions is used to obtain confidence intervals for crack length versus loading cycles from crack initiation to failure by fracture. This procedure utilizes a micro-structurally based transition point from small to long crack behavior which is defined using observations of the evolution of scatter found in the measured small crack data.