Hessam Yazdani

Exact travelling wave solutions for the generalized nonlinear Schrödinger (GNLS) equation with a source by Extended tanh-coth, sine-cosine and Exp-Function methods

The capability of Extended tanh-coth, sine-cosine and Exp-Function methods as alternative approaches to obtain the analytic solution of different types of applied differential equations in engineering mathematics has been revealed. In this study, the generalized nonlinear Schrodinger (GNLS) equation is solved by three different methods. To obtain the single-soliton solutions for the equation, the Extended tanh-coth and sine-cosine methods are used. Furthermore, for this nonlinear evolution equation the Exp-Function method is applied to derive various travelling wave solution. Results show that while the first two procedures easily provide a concise solution, the Exp-Function method provides a powerful mathematical means for solving nonlinear evolution equations in mathematical physics.

Failure criterion for graphene in biaxial loading—a molecular dynamics study

Molecular dynamics simulations are carried out in order to develop a failure criterion for infinite/bulk graphene in biaxial tension. Stresses along the principal edge configurations of graphene (i.e. armchair and zigzag directions) are normalized to the corresponding uniaxial ultimate strength values. The combinations of normalized stresses resulting in the failure of graphene are used to define failure envelopes (limiting stress ratio surfaces). Results indicate that a bilinear failure envelope can be used to represent the tensile strength of graphene in biaxial loading at different temperatures with reasonable accuracy. A circular failure envelope is also introduced for practical applications. Both failure envelopes define temperature-independent upper limits for the feasible combinations of normalized stresses for a graphene sheet in biaxial loading. Predicted failure modes of graphene under biaxial loading are also shown and discussed.

Atomic-Scale Simulation of Sensor-Enabled Geosynthetics for Health-Monitoring of Reinforced Soil Slopes and Embankments

Safety and serviceability assessment of reinforced soil slopes and embankments includes monitoring their reinforcement strains during their service life to ensure that they are within the allowable limits. Strain gauges are used as a common method to measure and monitor strains in geosynthetic reinforcement. However, the use of strain gauges has several shortcomings which are discussed in the paper. Recently, a novel technique has been developed at the University of Oklahoma which is based on the piezoresistivity of carbon black (CB)and carbon nanotube (CNT)-filled polymers. This method is devised to eliminate the need for conventional instrumentation to measure tensile strain in modified geosynthetics. In this technique, a polymeric material (e.g. polyethylene, PE) and an electrically conductive filler (e.g. CB or CNT), are blended to fabricate Sensor-Enabled Geosynthetics (SEG) with piezoelectric characteristics. In this study, Molecular Dynamics (MD) simulations are used to examine the effect of the filler (CB in this case) on the stress-strain behavior of SEG samples. Dreiding and OPLS force fields are adopted for the simulations which are carried out in the LAMMPS environment. Results of the study indicate that adding small quantities of CB to otherwise pure PE samples may not adversely affect their tensile strength properties.

Accelerated multi-gravitational search algorithm for size optimization of truss structures

Abstract Weak local exploitation capability of the gravitational search algorithm (GSA) and its slow convergence rate in final iterations have been demonstrated in the literature. This paper presents a modified GSA denoted here as the accelerated multi-gravitational search algorithm (AMGSA) that exhibits an improved convergence rate. In AMGSA, the simplex crossover (SPX) and the operator mutation of the breeder genetic algorithm (BGA) are incorporated with the multi-gravitational search algorithm (MGSA) to achieve an algorithm with a good exploration-exploitation balance. MGSA is adopted to prevent stagnation of the search into a local optimum (i.e. to improve the exploration capability), while the SPX and the BGA mutation operator are used to bias the search toward promising areas of the search space (i.e. to promote local exploitation). The performance of AMGSA is evaluated using several benchmark truss optimization examples. Results indicate that AMGSA not only exhibits an improved balance between the exploration and exploitation schemes but also shows competitive promise in effectively and efficiently solving large-scale optimization problems as it requires a significantly lower number of structural analyses compared to other algorithms that it is checked against.

Ant Colony Optimization Method for Design of Piled-Raft Foundations (DFI 2013 Student Paper Competition Winner)

Abstract In comparison to conventional piled foundations, piled-raft foundations provide a more economical solution to support high-rise buildings constructed on compressible soils. In this type of foundation, the bearing capacity of the underlying soil is taken into account in supporting the superstructure loads, and the piles are placed to control both the total and differential movements of the superstructure. Currently, there are no universally accepted methods to design piled-raft foundations including the selection of the piles locations and dimensions. Most piled-raft foundation designs are based on empirical methods and the experience of designers. However, piled-raft foundations are massive and expensive. Therefore, developing methodologies for their optimal design could significantly help minimize their otherwise high construction costs and would make them more feasible and common practice. This paper examines the capability of the ant colony optimization (ACO) algorithm to optimize piled-raft foundations. The soil-pile interactions are taken into account by modeling the side and tip capacities of the piles using the nonlinear p-y, t-z, and Q-z springs in the OpenSees platform. The soil-raft interaction is taken into consideration using the Winkler springs beneath the raft. The objective of the optimization problem is to minimize the volume of the foundation by taking the number, configuration, and penetration depth of the piles, as well as the thickness of the raft, as design variables. The side and tip forces of the piles, the pressure applied on the underlying soil, and the total and differential movements of the foundation under the serviceability limit state are the constraints adopted for the optimization problem. Results indicate that the ACO algorithm is a suitable method for optimal design of piled-raft foundations. Findings of the study also indicate that including soil nonlinearity in the analysis (as opposed to a linear elastic soil model) can lead to a more economical design for these foundation systems.

“Laboratory tests on the engineering properties of sensor-enabled geobelts (SEGB)” by Cui et al., Geotextiles and Geomembranes 46 (2018) 66–76

The article by Cui et al. is a significant contribution, given that it is challenging to design and fabricate sensor-enabled geosynthetics (SEG) that consistently exhibit reliable tensoresistivity performances in air and soil environments. The contribution of the work is that the strains calculated from the electrical conductivity measurements of SEGB specimens were nearly the same as those interpolated from strain gauge measurements. While we commend the authors for the quality of their work, we believe that some aspects of sensor-enabled geobelts (SEGB), and of SEG in general, need further development and improvement before they can be implemented in practice. Additionally, we have identified areas in the paper that we believe further clarification and study details are warranted, and/or explanations provided may have to be reconsidered or revised, as we have discussed in detail in the following sections.