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Volume 22, Number 6, December 2018
 
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Abstract
One of the methods for investigation of mechanical behavior of materials is numerical simulation. For simulation, its need to model behavior is close to real condition. PFC is one of the rock mechanics software that needs calibration for models simulation. The calibration was performed based on simulation of unconfined compression test and Brazilian test. Indeed the micro parameter of models change so that the UCS and Brazilian test results in numerical simulation be close to experimental one. In this paper, the effect of four micro parameters has been investigated on the uniaxial compression test and Brazilian test. These micro parameters are friction angle, Accumulation factor, expansion coefficient and disc distance. The results show that these micro parameters affect the failure pattern in UCS and Brazilian test. Also compressive strength and tensile strength are controlled by failure pattern.

Key Words
PFC2D; friction angle; accumulation factor; expansion coefficient and disc distance

Address
M.R. Ajamzadeh and Vahab Sarfarazi: Department of Mining Engineering, Hamedan University of Technology, Hamedan, Iran
Hadi Haeri: Young Researchers and Elite Club, Bafgh Branch, Islamic Azad University, Bafgh, Iran
H. Dehghani: Academic Member, Hamedan University of Technology, Hamedan, Iran

Abstract
In this study, the effect of particle size on the cracks propagation and coalescence or cracking pattern of the edge notched disc specimens are investigated. Firstly, calibration of PFC3D was performed using Brazilian experimental test output. Then micro parameters were used to build edge notched disc specimen. The horizontal wall of the assembly is let to move downward with a standard low speed of 0.016 m/s. The numerical results show that the tensile cracks are dominant failure pattern for the modeled discs. These tensile cracks initiate from the pre-existing notch tip and propagate parallel to the loading direction then interact with the upper boundary of the modeled specimen. As the size of the balls (ball diameter) decrease the number of tensile cracks increase. The tensile fracture toughness of the samples also decreases as the particle size increases. Understanding the crack propagation and crack coalescence phenomena in brittle materials such as concretes and rocks is of paramount importance in the stability analyses for engineering structures such as rock slopes, underground structures and tunneling.

Key Words
END test; pre-existing edge crack; PFC3D

Address
Hadi Haeri and Zheming Zhu: MOE Key Laboratory of Deep Underground Scienceand Engineering,
School of Architectureand Environment, Sichuan University,Chengdu 610065, China
Vahab Sarfarazi: Department of Mining Engineering, Hamedan University of Technology, Hamedan, Iran
Mohammad Fatehi Marji: Department of Mining Engineering, Yazd University, Yazd, Iran



Abstract
In the previous studies on the porous rock strength the effect of pore number and its diameter is not explicitly defined. In this paper crack initiation, propagation and coalescence in Brazilian model disc containing a single cylindrical hole and or multiple holes have been studied numerically using PFC3D. In model with internal hole, the ratio of hole diameter to model diameter was varied between 0.03, 0.17, 0.25, 0.33, and 0.42. In model with multiple hole number of holes was different in various model, i.e., one hole, two holes, three holes, four holes, five holes, six holes, seven holes, eight holes and nine holes. Diameter of these holes was 5 mm, 10 mm and 12 mm. The pre-holed Brazilian discs are numerically tested under Brazilian test. The breakage load in the ring type disc specimens containing an internal hole with varying diameters is measured. The mechanism of cracks propagation in the wall of the ring type specimens is also studied. In the case of multi-hole Brazilian disc, the cracks propagation and b cracks coalescence are also investigated. The results shows that breaking of the pre-holed disc specimens is due to the propagation of radially induced tensile cracks initiated from the surface of the central hole and propagating toward the direction of diametrical loading. In the case of disc specimens with multiple holes, the cracks propagation and cracks coalescence may occur simultaneously in the breaking process of model under diametrical compressive loading. Finally the results shows that the failure stress and crack initiation stress decreases by increasing the hole diameter. Also, the failure stress decreases by increasing the number of hole which mobilized in failure. The results of these simulations were comprised with other experimental and numerical test results. It has been shown that the numerical and experimental results are in good agreement with each other.

Key Words
concrete rings; pre-holed Brazilian disc; crack analyses; coalescence; PFC3D

Address
Vahab Sarfarazi: Department of Mining Engineering, Hamedan University of Technology, Hamedan, Iran
Hadi Haeri: Young Researchers and Elite Club, Bafgh Branch, Islamic Azad University, Bafgh, Iran
Alireza Bagher Shemirani: Department of Civil Engineering, Sadra Institute of Higher Education, Tehran, Iran

Abstract
This paper presents a model of the elastic curve for rectangular beams with straight haunches under uniformly distributed load and moments in the ends considering the bending and shear deformations (Timoshenko Theory) to obtain the deflections and rotations on the beam, which is the main part of this research. The traditional model of the elastic curve for rectangular beams under uniformly distributed load considers only the bending deformations (Euler-Bernoulli Theory). Also, a comparison is made between the proposed and traditional model of simply supported beams with respect to the rotations in two supports and the maximum deflection of the beam. Also, another comparison is made for beams fixed at both ends with respect to the moments and reactions in the support A, and the maximum deflection of the beam. Results show that the proposed model is greater for simply supported beams in the maximum deflection and the traditional model is greater for beams fixed at both ends in the maximum deflection. Then, the proposed model is more appropriate and safe with respect the traditional model for structural analysis, because the shear forces and bending moments are present in any type of structure and the bending and shear deformations appear.

Key Words
elastic curve; rectangular beams; straight haunches; uniformly distributed load; moments in the ends; bending and shear deformations; Timoshenko Theory; Euler-Bernoulli Theory; deflections and rotations on the beam

Address
José Daniel Barquero-Cabrero: Institute of Multidisciplinary Researches, Autonomous University of Coahuila, Blvd. Revolución No, 151 Ote, CP 27000, Torreón, Coahuila, México;
Institute for Long Life Learning IL3, University of Barcelona, Street Girona No, 24, CP 08010, Barcelona, Spain
Arnulfo Luévanos-Rojas, Sandra López-Chavarría, Manuel Medina-Elizondo, Francisco Velázquez-Santillán and Ricardo Sandoval-Rivas: Institute of Multidisciplinary Researches, Autonomous University of Coahuila,
Blvd. Revolución No, 151 Ote, CP 27000, Torreón, Coahuila, México


Abstract
With increasing traffic volumes and rising vehicle traffic, especially in cities, the number of pedestrian bridges has also increased significantly. Like all other structures, pedestrian bridges also suffer damage. In order to increase the safety of pedestrians, it is necessary to identify existing damage and to repair them to ensure the safety of the bridge structures. Owing to the shortcomings of local methods in identifying damage and in order to enhance the reliability of detection and identification of structural faults, signal methods have seen significant development in recent years. In this research, a new methodology, based on cone-shaped kernel distribution with a new damage index, has been used for damage detection in pedestrian truss bridges. To evaluate the proposed method, the numerical models of the Warren Type steel truss and the Arregar steel footbridge were used. Based on the results, the proposed method and damage index identified the damage and determined its location with a high degree of precision. Given the ease of use, the proposed method can be used to identify faults in pedestrian bridges.

Key Words
pedestrian truss bridges; damage detection; cone-shaped kernel distribution; time-frequency distribution

Address
Hamid Reza Ahmadi: Department of Civil Engineering, Faculty of Engineering, University of Maragheh, Maragheh 55181-83111, Iran
Diana Anvari: Department of Civil Engineering, Bandar Abbas Branch, Islamic Azad University, Bandar Abbas, Iran

Abstract
Most of the practical engineering structures exhibit nonlinearity due to nonlinear dynamic characteristics of structural joints, nonlinear boundary conditions and nonlinear material properties. Hence, it is highly desirable to detect and characterize the nonlinearity present in the system in order to assess the true behaviour of the structural system. Further, these identified nonlinear features can be effectively used for damage diagnosis during structural health monitoring. In this paper, we focus on the detection of the nonlinearity present in the system by confining our discussion to only a few selective time-frequency analysis and multivariate analysis based techniques. Both damage induced nonlinearity and inherent structural nonlinearity in healthy systems are considered. The strengths and weakness of various techniques for nonlinear detection are investigated through numerically simulated two different classes of nonlinear problems. These numerical results are complemented with the experimental data to demonstrate its suitability to the practical problems.

Key Words
nonlinear systems; wavelets; Hilbert-Huang transform; principal component analysis; Holder exponent; null subspace analysis

Address
J. Prawin and A. Rama Mohan Rao: Academy of Scientific and Innovative Research, CSIR-Structural Engineering Research Centre, CSIR Campus, Chennai, TN, India


Abstract
To mitigate vibrations, tuned mass dampers (TMD) are widely used for long span bridges or high-rise buildings. Due to some durability concerns, such as fluid degradation, oil leakage, etc., the alternative solutions, such as the non-contacted eddy current damping (ECD), are proposed for mechanical devices in small scales. In the present study, a new eddy current damping TMD (ECD-TMD) is proposed and developed for large scale civil infrastructure applications. Starting from parametric study on finite element analysis of the ECD-TMD, the new design is enhanced via using the permanent magnets to eliminate the power need and a combination of a copper plate and a steel plate to improve the energy dissipation efficiency. Additional special design includes installation of two permanent magnets at the same side above the copper plate to easily adjust the gap as well as the damping. In a case study, the proposed ECD-TMD is demonstrated in the application of a steel arch bridge to mitigate the wind-induced vibrations of the flexible hangers. After a brief introduction of the configuration and the installation process for the damper, the mitigation effects are measured for the ambient vibration and forced vibration scenarios. The results show that the damping ratios increase to 3% for the weak axis after the installation of the ECD-TMDs and the maximum vibration amplitudes can be reduced by 60%.

Key Words
eddy current damping tuned mass damper (ECD-TMD); arch bridges; hanger; tuned mass damper; wind-induced vibration; vibration control

Address
Huawei Niu, Zhengqing Chen and Xugang Hua: Key Laboratory for Wind and Bridge Engineering of Hunan Province, College of Civil Engineering, Hunan University, Changsha 410082,, Hunan, China
Wei Zhang: Department of Civil and Environmental Engineering, University of Connecticut, Storrs, CT 06269, USA

Abstract
In this project, buckling response of polymeric plates reinforced with carbon nanotubes (CNTs) and coated by magnetostrictive layer was studied. The equivalent nanocomposite properties are determined using Mori-Tanak model considering agglomeration effects. The structure is simulated with first order shear deformation theory (FSDT). Employing strains-displacements, stress-strain, the energy equations of the structure are obtained. Using Hamilton\'s principal, the governing equations are derived considering the coupling of mechanical displacements and magnetic field. Using Navier method, the buckling load of the sandwich structure is obtained. The influences of volume percent and agglomeration of CNTs, geometrical parameters and magnetic field on the buckling load are investigated. Results show that with increasing volume percent of CNTs, the buckling load increases. In addition, applying magnetic field, increases the frequency of the sandwich structure.

Key Words
buckling analysis; magnetostrictive; CNTs; agglomeration; magnetic field

Address
Moein Tabbakh and Mohsen Nasihatgozar: Department of Mechanical Engineering, Kashan Branch, Islamic Azad University, Kashan, Iran


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