Wen Hui Duan

Reinforcing Effects of Graphene Oxide on Portland Cement Paste

In this experimental study, the reinforcing effects of graphene oxide (GO) on portland cement paste are investigated. It is discovered that the introduction of 0.03% by weight GO sheets into the cement paste can increase the compressive strength and tensile strength of the cement composite by more than 40% due to the reduction of the pore structure of the cement paste. Moreover, the inclusion of the GO sheets enhances the degree of hydration of the cement paste. However, the workability of the GO-cement composite becomes somewhat reduced. The overall results indicate that GO could be a promising nanofillers for reinforcing the engineering properties of portland cement paste.

Assessment of continuum mechanics models in predicting buckling strains of single-walled carbon nanotubes

This paper presents an assessment of continuum mechanics (beam and cylindrical shell) models in the prediction of critical buckling strains of axially loaded single-walled carbon nanotubes (SWCNTs). Molecular dynamics (MD) simulation results for SWCNTs with various aspect (length-to-diameter) ratios and diameters will be used as the reference solutions for this assessment exercise. From MD simulations, two distinct buckling modes are observed, i.e. the shell-type buckling mode, when the aspect ratios are small, and the beam-type mode, when the aspect ratios are large. For moderate aspect ratios, the SWCNTs buckle in a mixed beam-shell mode. Therefore one chooses either the beam or the shell model depending on the aspect ratio of the carbon nanotubes (CNTs). It will be shown herein that for SWCNTs with long aspect ratios, the local Euler beam results are comparable to MD simulation results carried out at room temperature. However, when the SWCNTs have moderate aspect ratios, it is necessary to use the more refined nonlocal beam theory or the Timoshenko beam model for a better prediction of the critical strain. For short SWCNTs with large diameters, the nonlocal shell model with the appropriate small length scale parameter can provide critical strains that are in good agreement with MD results. However, for short SWCNTs with small diameters, more work has to be done to refine the nonlocal cylindrical shell model for better prediction of critical strains.

Nonlinear bending and stretching of a circular graphene sheet under a central point load

Understanding of the bending and stretching properties of graphene is crucial in guiding its growth and applications. In this paper, we investigate the deformation of a single layer, circular, graphene sheet under a central point load by carrying out molecular mechanics (MM) simulations. The bending and stretching of the graphene sheet are characterized by using the von Kármán plate theory. Stress concentrations near the loaded region and the boundary due to bending rigidity of the graphene sheet are highlighted. It is shown herein that, with properly selected parameters, the von Kármán plate theory can provide a remarkably accurate prediction of the graphene sheet behavior under linear and nonlinear bending and stretching.

Carbon nanotube-cement composites: A retrospect

Although ordinary Portland cement (OPC) is widely used in the construction industry, its weak tensile strength, to some extent, limits its application. A carbon nanotube (CNT), on the other hand, has outstanding mechanical properties with a tensile strength of 63 GPa and Youngs modulus of 1 TPa, making it a candidate as nano-scale reinforcements in OPC. Past research studies have reported improved mechanical and electrical properties of carbonnanotube–ordinary Portland cement (CNT–OPC) composites, which show future promise in practical civilengineering applications. In this study, recent research studies in developing CNT–OPC composites are comprehensively reviewed. Highlighted herein are the considerable efforts been made in the study of fabrication, hydration, porosity and transport properties of the CNT–OPC composites. There are, however, future investigations needed to provide a better understanding in the areas of uniform dispersion of CNTs within the OPC paste, durability, impact, fatigue properties and the theoretical modelling of CNT–OPC interaction.

Calibration of Eringen's small length scale coefficient for initially stressed vibrating nonlocal Euler beams based on microstructured beam model

In this paper, we calibrate Eringens small length scale coefficient e0 for an initially stressed vibrating nonlocal Euler beam via a microstructured beam modelled by some repetitive cells comprising finite rigid segments and elastic rotational springs. By adopting the pseudo-differential operator and Pades approximation, an analytical solution for the vibration frequency in terms of initial stress may be developed for the microstructured beam model. When comparing this analytical solution with the established exact vibration solution from the nonlocal beam theory, one finds that the calibrated Eringens small length scale coefficient e0 is given by where σ0 is the initial stress and is the mth mode buckling stress of the corresponding local Euler beam. It is shown that e0 varies with respect to the initial axial stress, from at the buckling compressive stress to when the axial stress is zero and it monotonically increases with increasing initial tensile stress. The small length scale coefficient e0, however, does not depend on the vibration/buckling mode considered.

Fatigue Tests of Cracked Steel Plates Strengthened with UHM CFRP Plates

Carbon fibre reinforced polymer (CFRP) has shown promise for improving the fatigue performance of steel structures. Previous studies have indicated that increasing the Youngs modulus of CFRP can be beneficial for decreasing the stress intensity factor at the fatigue crack tip. In this project, ultra high modulus (UHM) CFRP plates with Youngs modulus of 460 GPa were adopted to study their fatigue repair effectiveness. A series of fatigue tension tests was carried out on steel plates with an initial crack in the centre. Five strengthening configurations were used and a constant amplitude fatigue loading was applied to all the specimens. The beach marking technique was utilized to record the fatigue crack propagation. The effects of CFRP bond length, bond width and bond locations on the fatigue performance of cracked steel plates were also studied. The experimental results show that UHM CFRP plates can greatly increase the fatigue life of cracked steel plates by a factor ranging from 3.26 to 7.47. When CFRP plates cover the whole crack surface, the fatigue crack of the steel plate is arrested. The strengthening effectiveness of UHM CFRP plates is also compared with those using high Youngs modulus CFRP sheeting and normal Youngs modulus CFRP plates with or without prestressing.

Hencky bar-chain model for buckling and vibration of beams with elastic end restraints

This paper presents the Hencky bar-chain model (HBM) for buckling and vibration analyses of Euler–Bernoulli beams with elastic end restraints. The Hencky bar-chain comprises rigid beam segments (of length a = L/n where L is the total length of beam and n the number of beam segments) connected by frictionless hinges with elastic rotational springs of stiffness EI/a where EI is the flexural rigidity of the beam. The elasticity and the mass of the beam are concentrated at the hinges with rotational springs. The key contribution of this paper lies in the modeling of the elastic end restraints of the Hencky bar-chain that will simulate the same buckling and vibration results as that furnished by the first-order central finite difference beam model (FDM) which was earlier shown to be analogous to the HBM. The establishment of such a physical discrete beam model allows one to obtain solutions for beam-like structure with repetitive cells (or elements) as well as to calibrate the Eringens coefficient e0 in the nonlocal beam theory that captures the small length scale effect.

Development of analytical vibration solutions for microstructured beam model to calibrate length scale coefficient in nonlocal Timoshenko beams

The present study takes an analytical approach for solving the free vibration problem of a microstructured beam model, in which transverse displacement springs are added to allow for the transverse shear deformation effect in addition to the rotational springs. The exact vibration frequencies for the discrete microstructured beam model with simply supported ends are obtained via matrix decomposition. In addition, a general solution technique involving the use of Pade approximants for the continualization procedure is proposed in order to obtain the continuous equivalent system for the discrete microstructured beam model. The analytical vibration solutions of the equivalent continuous system are obtained and their accuracy is assessed by using the exact solutions. It is found that the solutions of the equivalent continuous system have a first order accuracy when compared with the exact solutions of their discrete counterpart. The length scale coefficient in the nonlocal Timoshenko beam model is calibrated by using the analytical solutions. Two nonlocal Timoshenko beam models, i.e., the Wang model (without the length scale effect in the shear stress strain relation) and the Reddy model, are evaluated based on their ability to capture the nonlocal effect.

Tunable wrinkling pattern in annular graphene under circular shearing at inner edge

This work is concerned with the wrinkling phenomenon observed in an annular graphene sheet under circular shearing at its inner edge. By performing molecular mechanics simulations on the aforementioned loaded annular graphene sheet, it is observed that the unusual wrinkles formed are confined to within an annulus that hugs the perimeter of the inner radius. This confined wrinkling pattern is in contrast to the wrinkling patterns that spread throughout rectangular graphene sheets under tension or shear. The present wrinkling pattern is characterized by a wave number and wrinkle profile. The wave number at the bifurcation wrinkle is found to depend only on the inner radius of the annular graphene and it increases almost linearly with increasing inner radius. The orientation of these developed waves is found to be at a constant angle and independent from the radii ratio of annular graphene. The wrinkle profile in terms of wave amplitude and wavelength depends on the magnitude of the circular shearing. The predictable formation of wrinkles in annular graphene can be exploited for applications in nano-force sensors, tunable magnetic or electronic devices, as well as patterned stretchable electronics.

EXAMINATION OF CYLINDRICAL SHELL THEORIES FOR BUCKLING OF CARBON NANOTUBES

This paper examines the validity and accuracy of cylindrical shell theories in predicting the critical buckling strains of axially loaded single-walled carbon nanotubes (CNTs). The shell theories considered are the Donnell thin shell theory (DST), the Sanders thin shell theory (SST), and the first-order shear deformation (thick) shell theory (FSDST). Molecular dynamic (MD) simulation solutions for armchair and zig-zag CNTs with clamped ends were used as reference results to assess the shell models. The MD simulations were carried out at room temperature to eliminate the thermal effect on the buckling behavior. By adopting Youngs modulus of 5.5 TPa, Poissons ratio of 0.19, and tube thickness of 0.066 nm, it was found that DST is not able to capture the length dependency of the critical buckling strains and thus it should not be used for buckling analysis of CNTs. On the other hand, SST and FSDST are able to predict the critical buckling strains of armchair and zig-zag CNTs reasonably well for all aspect ratios, especially the results produced by the FSDST are found to be closer to the MD simulation results, because it allows for the effect of transverse shear deformation that becomes significant for CNTs with small aspect ratios. Thus, FSDST is recommended as a very suitable and convenient continuum mechanics model for buckling analysis of CNTs. The superior FSDST model is used to generate critical buckling strains of axially loaded single-walled CNT with different boundary conditions. These results should be useful for designers of nanodevices that make use of CNTs as axially loaded members. It is worth noting that for long and moderately long CNTs, the Timoshenko beam model may be used instead due to its simplicity.