Browsing by Author "Dr. Eric C. Klang, Committee Member"

Filter results by typing the first few letters
Now showing 1 - 2 of 2
  • No Thumbnail Available
    Atomistic Simulations of Fracture of 2d Graphene Systems and the Elastic Properties of Carbon Nanotubes
    (2004-11-15) Jin, Yun; Dr. Kara Peters, Committee Member; Dr. Eric C. Klang, Committee Member; Dr. Jeffrey W. Eischen, Committee Member; Dr. Fuh-Gwo Yuan, Committee Chair
    The essential feature of all the macroscopic mechanical properties of a material are governed by constitutive atoms and the basic laws of physics. The traditional descriptions of fracture phenomena in solids have been developed almost exclusively from continuum concepts. These continuum approaches have successfully described many fracture mechanism in solid materials and continue to be of great use. Nevertheless, they still have some shortcomings, such as the spurious singularity of the stress field at crack tip. Hence, atomistic predictions may open new avenues in the studies of microscopic origins of material failure behavior. Also the highly active researches on the lightweight nano-structured materials in the past decade revealed the emergence of interests in predicting the properties of materials in the atomic level before they are synthesized. In this dissertation the fracture mechanism of a nanostructure material has been investigated by atomistic simulations. Macroscopic fracture parameters have been examined from both atomistic simulation and continuum models. There is a very good agreement between atomistic simulation and theoretical results from LEFM for the energy release rate in a semi-infinite graphene sheet containing cracks. The atomic description of the stress field in this case is also obtained and matches very well with the linear elastic solutions. Then another case in which a center-cracked graphene sheet with finite width is proposed. The energy release rates are obtained in both global energy approach and local force approach. These simulations show that, in macroscopic fracture mechanics under small deformation, linear elastic fracture mechanics is sufficient for the description of cracking behavior for this covalently bonded material. The results merge the discrete (atomistic) and continuum (macroscopic) description of facture. Meanwhile, the method to calculate J integral in the atomic system is successfully developed. The numerical results of J integral agree very well with the energy release rate in the same systems. Then after a necessary modification on the Tersoff-Brenner potential, the critical value of J integral, denoted by , is eventually reached as the measure of the fracture toughness of graphene sheet. The mechanical properties of single-walled carbon nanotube (SWNT) have also been evaluated in this dissertation. Several elastic moduli of SWNTs using the MD simulations were obtained at the atomic scale. The values of the in-plane Young¡¯s modulus, rotational shear modulus, and in-plane Poisson's ratio are in the range of existing theoretical and experimental results. It has been shown from simulations that overall the elastic constants of SWNTs are insensitive to the morphology pattern such as nanotube radius and thus the effect of curvature on the elastic constants can be neglected. Assumption of the transversely isotropic properties on the cylinder surface of the single-walled nanotube was confirmed by numerical calculations. Besides the conventional energy approach, a new method, which denoted as force approach in the dissertation, has been developed to analysis the elastic properties of carbon nanotubes. The results from two approaches matched very well. The advantage of the force approach is that it can provide more accurate prediction than the energy approach. Furthermore, the force approach can predict the nonlinear behavior without assumption of assumed total potential energy in quadratic form described for small-strain deformation in the energy approach.
  • No Thumbnail Available
    Behavior of FRP-Concrete Beam-Columns under Cyclic Loading
    (2003-07-02) Shao, Yutian; Dr. Amir Mirmiran, Committee Chair; Dr. Sami H. Rizkalla, Committee Member; Dr. Mervyn J. Kowalsky, Committee Member; Dr. Eric C. Klang, Committee Member
    Use of concrete-filled fiber-reinforced polymers (FRP) tubes (CFFT) for columns and piles has been studied extensively over the last decade. The focus, however, has been exclusively on the monotonic behavior of CFFT. An issue that has received little attention is the implications of using CFFT in seismic regions. Survey of damaged structures in recent earthquakes indicates that catastrophic failure of an entire structure may result from failure of few columns in a chain action. Since it may not be economical to design columns to respond to earthquake loads in their elastic range, dissipation of energy by post-elastic deformation is desired. Although, FRP materials are known for their linear elastic behavior, some FRP systems may exhibit non-linearity due to their laminate architecture and inter-laminar shear. Also, confinement of concrete core in CFFT improves its ductility. This study was carried out to evaluate the cyclic behavior of CFFT beam-columns, and determine whether non-linearity of FRP or confinement of concrete can provide seismic performance comparable to reinforced concrete (RC) columns or concrete-filled steel tubes (CFST). The experimental work consisted of cyclic loading and unloading of FRP-wrapped concrete cylinders and FRP coupons, and reverse cyclic loading of CFFT beam-columns under constant axial load. Some measures of hysteretic performance, including cumulative energy dissipation, ductility and pinching effect were used to evaluate the cyclic response of tested CFFT beam-columns. The study resulted in a cyclic model for FRP-confined concrete in compression, and cyclic models for linear and non-linear FRP materials in tension and compression. A fiber element model was employed to predict the cyclic behavior of CFFT beam-columns. A parametric study was carried out on the cyclic behavior of CFFT beam-columns, and to compare the hysteretic response of CFFT beam-columns with those of RC and CFST members. The two types of CFFT beam-columns tested under this study represented two different failure modes; a brittle compression failure for the over-reinforced white tube specimens with thick FRP tube and with majority of the fibers in the longitudinal direction, and a ductile tension failure for the under-reinforced yellow tube specimens with thin FRP tubes and off-axis fibers. The study showed feasibility of designing ductile CFFT members for seismic applications comparable to RC members. Significant ductility can result from the inter-laminar shear in the FRP tube. Moderate amounts of internal steel reinforcement can further improve the performance of CFFT members. Adding internal steel can be ineffective and may lead to premature failure. Slender CFFT members have less capacity than their short stocky counterparts. However, they are less susceptible to pinching effect and premature shear failure.