V.B.C. Tan

Large-scale synthesis and field emission properties of vertically oriented CuO nanowire films

Using a simple method of direct heating of bulk copper plates in air, oriented CuO nanowire films were synthesized on a large scale. The length and density of nanowires could be controlled by growth temperature and growth time. Field emission (FE) measurements of CuO nanowire films show that they have a low turn-on field of 3.5?4.5?V??m?1 and a large current density of 0.45?mA?cm?2 under an applied field of about 7?V??m?1. By comparing the FE properties of two types of samples with different average lengths and densities (30??m, 108?cm?2 and 4??m, 4 ? 107?cm?2, respectively), we found that the large length?radius ratio of CuO nanowires effectively improved the local field, which was beneficial to field emission. Verified with finite element calculation, the work function of oriented CuO nanowire films was estimated to be 2.5?2.8?eV.

Perforation of high-strength double-ply fabric system by varying shaped projectiles

Work done previously on single-ply ballistic resistant fabric systems is now extended to double-ply systems. Experiments were carried out to investigate the impact phenomenon of double-ply systems that consists of Twaron® CT 716 fabric and projectiles of the following nose shapes: hemispherical, flat, ogival (CRH 2.5) and conical (half-angle of 30°). Results obtained revealed that the increase in energy absorption does not necessarily double when the ply number is increased to two. In fact, the ratio of energy absorbed in the double-ply system to that of the single-ply system varies with impact velocity and projectile geometry. At points corresponding to maximum energy absorption, the ratio for all projectile types scatters about an average of 2.1. The amount of deviation from this value depends on the mechanisms that lead to perforation. Failure mechanisms of a double-ply system are similar to those of a single-ply system, but the degree of damage of the impact and distal plies differs.

Effect of chirality on buckling behavior of single-walled carbon nanotubes

In this paper, molecular dynamics simulations (MDS) are performed on single-walled carbon nanotubes (SWCNTs) in order to study the effects of chirality on their buckling behavior under axial compression. In the MDS, the Tersoff-Brenner potential is used to describe the interaction of carbon atoms in the SWCNTs. The sensitivity of the buckling strains and buckling modes with respect to the chirality of SWCNT is investigated by modeling SWCNTs with different chiral angles, varying from 0° to 30°, but keeping the length-to-diameter ratio constant. The carbon nanotubes are also analyzed using a continuum cylindrical shell model based on the theory of nonlocal elasticity so as to assess its validity in predicting the buckling strains when compared with the results that are obtained by MDS. The differences between the buckling strains at the continuum scale and that at the nanoscale are also studied. The present analysis and results are helpful in understanding the buckling behaviors of axially compressed carbo...

Crystallinity and surface effects on Young’s modulus of CuO nanowires

The authors investigate the crystallinity and surface effects on Young’s modulus of cupric oxide (CuO) nanowires by performing three-point bend test using atomic force microscopy. Young’s modulus of the nanowires obtained ranges from 70to300GPa and is dependent on two factors. Firstly, it depends on whether the nanowire is mono- or polycrystalline, as indicated by the absence or presence of an amorphous surface layer. Second, the modulus increases with decreasing diameter for both types of nanowires. Combined with transmission electron microscopy and computational simulation studies, the nanostructure-mechanical property relationship of CuO nanowires is elucidated.

First-principles calculations of structural and mechanical properties of Cu6Sn5

The elastic constants of polycrystalline Cu6Sn5—an intermetallic in lead-free alternatives of several material systems—are presented. The results are obtained by applying: (i) Reported crystallographic structure of monoclinic single crystal Cu6Sn5, (ii) structure optimization and determination of single crystal elastic constants from first principle calculations, and (iii) limit analysis of polycrystal stiffness based on single crystal properties. The agreement between the calculated Young’s modulus (120 GPa) and those from nanoindentation experiments (112–125 GPa), and the tight bounds on the predicted polycrystal values give a measure of confidence in other calculated properties for which experimental data are unavailable.

Structure-Mechanical Property of Individual Cobalt Oxide Nanowires

We present a comprehensive approach to address the correlation between mechanical properties of nanowires (NWs) with their characteristic size, microstructure, and chemical composition. Using this technique, the Youngs modulus of Co3O4 NWs with different sizes was evaluated. Thermal annealing in inert atmosphere was found to induce chemical reduction of as-grown Co3O4 NWs into CoO NWs without modifying their geometrical shape. Both Co3O4 and CoO NWs exhibited a size-dependent variation in Youngs modulus.

Mechanical response of PCBs in portable electronic products during drop impact

There is a need for more detailed studies on the drop impact reliability of electronic packages, in view of the fact that solder interconnections are becoming smaller, and portable electronic products such as mobile phones can be easily dropped during usage. This paper investigates the mechanical response of printed circuit boards (PCBs) inside portable consumer electronic products, when such products are subjected to drop impact. The response of the PCB is studied because it is one possible factor causing the failure of the interconnections with IC packages. The board response is measured via accelerations and strains at specified locations on the board. The drop test experimental results are consistent with drop orientation and physical structure of the products.

Buckling of carbon nanotubes at high temperatures

Presented herein is an investigation into the buckling behavior of single-walled carbon nanotubes (SWCNT) subjected to axial compression and torsion at high temperatures. This study is carried out by performing molecular dynamics (MD) simulations at both room temperature and extremely high temperatures. It is observed that the SWCNT becomes more susceptible to buckling in a higher temperature environment, especially when the SWCNT is subject to axial compression. The high thermal energy enhances the vibration of carbon atoms in the SWCNT significantly, which leads to bond breaking and the formation of sp(3) bonds as well as Stone-Wales (SW) defects in the postbuckling stage.

Element-failure concepts for dynamic fracture and delamination in low-velocity impact of composites

An element-failure algorithm is proposed and incorporated into a finite element code for simulating dynamic crack propagation and impact damage in laminated composite materials. In this algorithm, when a crack is propagating within a finite element, the element is deemed to have partially failed, but not removed from the computations. Consequently, only a fraction of the stresses that were computed before the crack tip entered the element contribute to the nodal forces of the element. When the crack has propagated through the element, the element is completely failed and therefore can only resist volumetric compression. This treatment of crack propagation in isotropic solids allows fracture paths within individual elements and is able to accommodate crack growth in any arbitrary direction without the need for remeshing. However, this concept is especially powerful when extended to the modeling of damage and delamination in fibre-reinforced composite laminates. This is because the nature of damage in composite laminates is generally diffused, characterized by multiple matrix cracks, fibre pullout, fibre breakage and delaminations. It is usually not possible to define or identify crack tips in the tradition of fracture mechanics. Since parts of a damaged composite structure are often able to partially transmit load despite the presence of some damage, it is advantageous to model the damaged portions with partially failed elements. The damage may be efficiently modeled and tracked using element-failure concepts, with the application of appropriate failure criteria and damage evolution laws. The idea is to embody the effects of damage into the effective nodal forces of the finite element. In this paper, we report the novel use of element-failure concepts in the analysis of low-velocity impact damage of composite laminates. The initiation and propagation of delaminations arising from the impact are predicted and the results show qualitative agreement with experimental observation of the formation of multiple delaminations in impact-damaged specimens. While such delaminations do not permit transmission of tensile stress waves across the cracked surfaces, transmission of compressive stress waves are allowed in the simulation. It is further shown that, when elements are allowed to fail, the dynamic stress wave distributions are altered significantly. In the element-failure algorithm, the issue of interpenetration of delamination surfaces in the model does not arise. This is a significant advantage over the conventional method of explicitly modeling the delamination surfaces and crack front, where generally, much computational time must be spent in employing contact algorithms to ensure physically admissible solutions. Finally, we also demonstrate the simulation of crack propagation of pre-notched specimens of an isotropic material under initial conditions of mode II loading using the element-failure algorithm. The numerical results showed that the cracks propagated at an angle of about 70° with respect to the notches, in agreement with the experimental results of Kalthoff.

A Micro?Macro Approach to Modeling Progressive Damage in Composite Structures

Modeling progressive damage in composite materials and structures poses considerable challenges because damage is, in general, complex and involves multiple modes such as delamination, transverse cracking, fiber breakage, fiber pullout, etc. Clearly, damage in composites can be investigated at different length scales, ranging from the micromechanical to the macromechanical specimen and structural scales. In this article, a simple but novel finite-element-based method for modeling progressive damage in fiber-reinforced composites is presented. The element-failure method (EFM) is based on the simple idea that the nodal forces of an element of a damaged composite material can be modified to reflect the general state of damage and loading. This has an advantage over the usual material property degradation approaches, i.e., because the stiffness matrix of the element is not changed, computational convergence is theoretically guaranteed, resulting in a robust modeling tool. The EFM, when employed with suitable micromechanics-based failure criteria, may be a practical method for mapping damage initiation and propagation in composite structures. In this article, we present a micromechanical analysis for a new failure criterion called the strain invariant failure theory and the application of the EFM in the modeling of open-hole tension specimens. The micromechanical analysis yields a set of amplification factors, which are used to establish a set of micromechanically enhanced strain invariants for the failure criterion. The effects of material properties and volume fraction on the amplification factors are discussed.