Mehdi Eftekhari

In order to provide ductility of structures and rehabilitation of vulnerable structures against
earthquake, extensive research have been done in recent years among which metallic dampers,
ADAS (Added Damping and Stiffness), was one of the most prevalent methods. In ADAS dampers,
yielding of metallic plates results in earthquake input energy dissipation. In the following paper, a
comprehensive parametric study of ADAS damper is presented for a more proper design. Therefore,
numerical finite element analysis by ANSYS software is employed. Nonlinear static analysis of the
frames indicates that by increasing ADAS stiffness, frame ductility does not change significantly,
but frame force level increase. In addition, Stiffness Ratio (SR) and U are related together by a
  polynomial of second order relationship. The relationship SR and variables is linear and value
rises up while SR increases

Production of High strength steel (HSS) with well ductility and weldability has been developed in recent 50 years. Usage of HSS did not expanded, because civil engineers were not familiar with them. Until their characteristic and their performance in cyclic loading are not being studied, their final usage will not be expanded. So in this research, rigid connections have been modeled by finite element software Ansys and have been applied to quasi-static cyclic loading. The results show that, using HSS reduced ductility only 30 percent and this reduction can be counteracted by increasing allowable stress in High strength steel.

Vacancy defects can reduce mechanical properties of carbon nanotubes. In this paper, a molecular structural mechanics approach is adopted in order to evaluate the buckling analysis of defective carbon nanotubes. The modified Morse potential is used to simulate carbon bond. The results indicate that defects can substantially reduce the bearing capacity of tubes under compression.

The local buckling behavior of perfect/defective and single/multi-walled carbon nanotubes (CNTs) under axial compressive forces has been investigated by the molecular dynamics approach. Effects of different types of defects including vacancy and Stone–Wales (SW) defects and their configurations on CNTs with different chiralities at room temperature are studied. Results show that defects largely reduce the buckling stress and the ratio of immediate reduction in buckling compressive stress of the defective CNT to the perfect one, but have little influence on their compressive elastic modulus. SW defects usually reduce the mechanical properties more than vacancy defects, and zigzag CNTs are more susceptible to defects than armchairs. In addition, increasing the number of defects leads to higher deterioration in mechanical properties of CNTs. The results of simulations show that in the case of slender single-walled CNTs, the behavior is primarily governed by the Euler buckling law. On the other hand, in the local shell buckling mode, two distinct behaviors are observed, including the primary local shell buckling mode for intermediate CNTs, and the secondary local shell buckling mode for short CNTs. In the local buckling response, CNTs with smaller diameters sustain higher buckling stresses than CNTs with larger diameters.

An XFEM multiscale approach is adopted in order to investigate the mechanical properties and fracture behavior of carbon nanotube reinforced concrete specimen. At the nanoscale, molecular dynamics simulation is used to find the mechanical properties of carbon nanotube (CNTs). Afterwards, a hydration model is adopted to find the chemical composition of cement paste. The hydrated model and CNTs are then converted into a finite element mesh for further analysis. Finally, at the meso scale the fracture behavior of the CNT reinforced concrete is simulated by the XFEM approach. The results indicate that the fracture energy of samples with similar volume fractions but reinforced by longer CNTs increase significantly , but addition of CNTs has little influence on the elastic modulus. In addition, the extent of crack propagation under a similar load level becomes considerably lower for the concrete samples reinforce by longer CNTs.

Calcium–Silicate–Hydrate (C–S–H), which is the major constituent of the cement at the nanoscale, is responsible for the strength and fracture properties of concrete. This research is dedicated to the numerical study of enhanced mechanical properties of C–S–H reinforced by embedding carbon nanotube (CNT) in its molecular structure. Series of molecular dynamics (MD) simulations indicate that the tensile strength of CNT-reinforced C–S–H is substantially enhanced along the direction of CNT as compared to the pure C–S–H. The results of tensile loading reveal that CNT can efficiently bridge the two sides of cracked C–S–H. In addition, CNTs can severely intensify the ''transversely isotropic " response of the CNT-reinforced C–S–H. Furthermore, the pull-out behavior of CNT reveals that the force-displacement response can be estimated by a bilinear model, which can later be used for simulation of cohesive crack propagation and multiscale simulation of crack bridging at macro scale specimen of CNT-reinforced cement.

In this paper, the impact behavior of the carbon nanotube reinforced concrete is investigated through the multiscale simulation. At the nano scale, properties of carbon nanotubes are determined through the molecular dynamics simulation. Afterwards, a finite element based hydration model of the cement is adopted to disperse the CNTs on the surface of the cement. At the meso scale, the concrete is simulated by considering all three phases including, cement, aggregates and interfacial transition zone (ITZ), to obtain a homogenized response. Finally, at the macro scale, the homogenized response is used to find the behavior of CNT-reinforced concrete under impact loading. The results indicate that less damaged areas are generated in the CNT-reinforced concrete model. It responses with higher resistance and energy absorption capacity, which results in considerable reduction of the penetration depth to contain a projectile.