A.R. Alian

Atomistic Modelling of Nanoindentation of Multilayered Graphene-Reinforced Nanocomposites

The force-displacement curves, obtained from a nanoindentation experiment, are generally analysed using continuum contact mechanics models. However, the applicability of these models at the nanoscale is questionable due to several inherited nanoscale phenomena, e.g., discreteness, quantum manifestations, and scale effects. Atomistic simulations such as molecular dynamics could provide better insight into the contact mechanics of nanoscale systems. In this chapter, we present a comprehensive molecular dynamics simulations of the contact behaviour of multilayered graphene-reinforced composite systems. Three aspects of the work were considered. The first was concerned with the force-displacement curves resulting from nanoindentation of a polyethylene matrix reinforced by multilayered graphene sheets. The second is concerned with the associated deformation patterns as well as the atomic adhesion associated with the retraction stage of the indenter. The third is concerned with the reinforcement mechanism and fracture behaviour associated with the increase in the number of graphene sheets and their spatial locations within the composite. The results of our work reveal: (a) strong interlayer interaction of graphene results in higher indentation resistance, (b) indentation resistance of a single-layer graphene-coated polyethylene is about 13-fold of the indentation resistance of pure polyethylene, (c) strong atomic adhesion between the indenter and the graphene prevails at the nanoscale, and (d) the proper choice of interlayer separation is critical in achieving the best performance of multilayered graphene-reinforced nanocomposites.

Multiscale Modeling of Nanoreinforced Composites

In this chapter, we present different multiscale modeling techniques to determine the elastic and interfacial properties of carbon nanotube (CNT)-reinforced polymer composites. The elastic properties of CNT-reinforced composite (hereinafter the “nanocomposite”) are obtained in a two-step approach. First, at the nanoscale level, molecular dynamics (MD) and atomistic-based continuum (ABC) techniques are used to determine the effective elastic properties of a representative volume element (RVE) that is comprised of a nanofiller and its immediate surrounding. Second, at the microscale level, several micromechanics models and hybrid Monte Carlo finite-element (FE) simulations are used to determine the bulk properties of nanocomposite. The interfacial properties are determined through pullout test using MD and ABC techniques. The effect of length, diameter, agglomeration, waviness, defects, and orientation of CNTs on the elastic and interfacial properties of nanocomposites is also investigated. The development of multiscale modeling and the proper selection of simulation parameters are discussed in detail. The results of several studies are presented and compared to show the inherited limitations in each technique.

Finite Element simulation of In-Service Sleeve Repair Welding of Gas Pipelines

The purpose of this study is to investigate the influence of welding sequence on the risk of burn-through, cold cracking and residual stresses during in-service sleeve repair welding of gas pipelines. Based on ABAQUS software, axisymmetric finite e1ement models were conducted to calculate transient temperature distributions and resulting residual stress field after multi-pass sleeve fillet welding of in-service API 5L-X65 36” Schedule pipes. Influence of welding sequence was investigated by comparing residual stresses and transient deformations for sequential welding; in which welding of the two circumferential pipe/sleeve fillet welds was made in sequence, to the case of performing the two welds at the same time. Sequential welding was found to reduce weld zone distortion and residual stresses due to the sleeve freedom to expand axially while conducting the first fillet weld. The upper limit of heat input was found to generate higher pipe wall peak temperature, but not to the extent of initiating pipe wall melt through.