A complement to Hoek-Brown failure criterion for strength prediction in anisotropic rock

Bagheripour, Mohammad Hossein,Rahgozar, Reza,Pashnesaz, Hassan,Malekinejad, Mohsen Techno-Press 2011 Geomechanics & engineering Vol.3 No.1

In this paper, a complement to the Hoek-Brown criterion is proposed in order to derive the strength of anisotropic rock from strength of the corresponding truly intact rock. The complement is a decay function, which unlike other modifications or suggestions made in the past, is multiplied to the function of the original Hoek-Brown failure criterion for intact rock. This results in a combined and extended form of the criterion which describes the strength of anisotropic rock as a varying fraction of the corresponding truly intact rock strength. Statistical procedures and in particular regression analyses were conducted into data obtained in experiments conducted in the current research program and those collected from the literature in order to define the Hoek-Brown's criterion complement. The complement function was best described by a simple polynomial including only three constants to be empirically evaluated. Further investigations also showed that these constants can be related to the other readily available parameters of rock material which further facilitate determining the constants. A great and prime advantage of the proposed complement is that it is mathematically simple including the least possible number of empirical constants which are easily estimated with minimum experimental effort. Moreover, proposed concept does not suggests any change to the original Hoek-Brown criterion itself or its constants and serves whenever anisotropy does exist in the rock. This further implies on the possibility of using any other failure criterion for intact rock in conjunction with the compliment to reach the strength of anisotropic rock.

Efficient analysis of SSI problems using infinite elements and wavelet theory

Bagheripour, Mohamad Hossein,Rahgozar, Reza,Malekinejad, Mohsen Techno-Press 2010 Geomechanics & engineering Vol.2 No.4

In this paper, Soil-Structure Interaction (SSI) effect is investigated using a new and integrated approach. Faster solution of time dependant differential equation of motion is achieved using numerical representation of wavelet theory while dynamic Infinite Elements (IFE) concept is utilized to effectively model the unbounded soil domain. Combination of the wavelet theory with IFE concept lead to a robust, efficient and integrated technique for the solution of complex problems. A direct method for soil-structure interaction analysis in a two dimensional medium is also presented in time domain using the frequency dependent transformation matrix. This matrix which represents the far field region is constructed by assembling stiffness matrices of the frequency dependant infinite elements. It maps the problem into the time domain where the equations of motion are to be solved. Accuracy of results obtained in this study is compared to those obtained by other SSI analysis techniques. It is shown that the solution procedure discussed in this paper is reliable, efficient and less time consuming as compared to other existing concepts and procedures.

Novel composite graphene oxide/chitosan nanoplates incorporated into PES based nanofiltration membrane: Chromium removal and antifouling enhancement

E. Bagheripour,A.R. Moghadassi,S.M. Hosseini,B. Van der Bruggen,F. Parvizian 한국공업화학회 2018 Journal of Industrial and Engineering Chemistry Vol.62 No.-

In this work, novel nanofiltration membranes with outstanding performance and antifouling properties were fabricated by incorporating composite graphene oxide/chitosan (GOC) nanoplates into the membrane structure. GOC composite nanoplates were prepared by surface modification of GO with chitosan. The GOC nanoplates were then introduced as additives in a polymeric phase in different concentrations (up to 1%) in view of membrane synthesis. these membranes were thoroughly characterized and assessed for their Na2SO4 and CrSO4 rejection and water flux. Furthermore, the antifouling performance of GO and GOC filled membranes in high concentration (1 wt%) was investigated. Modified membranes with addition of GOCs showed a higher hydrophilicity, pure water flux and rejection, and a smoother surface compared to a bare PES and a GO incorporated membrane. SEM surface images indicated a more uniformed distribution of GOCs at a high loading rate (1 wt%) compared to GO. Due to the uniform dispersion of GOCs, a better antifouling performance was observed than for GO filled membranes. The results indicate that surface modification of GO with chitosan can enhance the membrane performance and properties, due to availability of sites with higher activity.

Evaluation of thymolphthalein-grafted graphene oxide as an antioxidant for polypropylene

Mona Bagheripour-Asl,Reza Jahanmardi,Hasan Tahermansouri,Erfan Forghani 한국탄소학회 2018 Carbon Letters Vol.25 No.-

In the present work, capability of thymolphthalein-grafted graphene oxide, which was successfully synthesized in this study, in stabilization of polypropylene against thermal oxidation were investigated and compared with that of SONGNOX 1010, a commercially used phenolic antioxidant for the polymer. The modified graphene oxide were incorporated into polypropylene via melt mixing. State of distribution of the nanoplatelets in the polymer matrix was examined using scanning electron microscopy and was shown to be homogeneous. Measurements of oxidation onset temperature and oxidative induction time revealed that thymolphthalein-grafted graphene oxide modifies thermo-oxidative stability of the polymer in the melt state remarkably. However, the efficiency of the nanoplatelets in stabilization of polypropylene against thermal oxidation in melt state was shown to be inferior to that of SONGNOX 1010. Furthermore, oven ageing experiments followed by Fourier transform infrared spectroscopy showed that the modified graphene oxide improves thermo-oxidative stability of the polymer strongly in the solid state, so that its stabilization efficiency is comparable to that of SONGNOX 1010.

Adapting the performance and physico-chemical properties of PES nanofiltration membrane by using of magnesium oxide nanoparticles

Sayed Mohsen Hosseini,Ehsan Bagheripour,Mohsen Ansari 한국화학공학회 2017 Korean Journal of Chemical Engineering Vol.34 No.6

A new mixed matrix Polyethersulfone (PES)-co-Magnesium oxide nanoparticles (MGO) nanocomposite nanofiltration membrane was prepared through phase inversion method by using polyvinylpyrrolidone (PVP) as pore former and N, N dimethylacetamide (DMAc) as solvent. The influence of MGO nanoparticles concentration in the membrane matrix on the separation performance and physico-chemical characteristics of prepared membrane was studied by scanning electron microscopy, surface analysis, porosity measurement, water contact angle, permeability flux, salt rejection, antifouling property, and tensile strength. SEM images exhibited situating of MGO nanoparticles on the top surface of mixed matrix prepared membranes. SEM analysis also showed formation of a dense nanoparticle layer on the surface of prepared membrane at high additive concentration. Surface analysis results that revealed membrane surface roughness was increased initially by addition of MGO and then was decreased. Measured porosity showed reduction behavior for all prepared membranes filled with MGO nanoparticles. The membrane surface hydrophilicity was enhanced 35% by incorporating MGO nanoparticles into the membrane matrix. Results showed that membrane permeation flux was improved 32% by utilizing of MgO nanoparticles into the membrane matrix. Salt rejection was also improved 49% by using MGO nanoparticles in the membrane matrix relatively. The modified membranes filled with different concentrations of MGO nanoparticles showed higher antifouling properties and tensile strength compared to the neat PES membrane.

Free vibration analysis of non-prismatic beams under variable axial forces

Saffari, H.,Mohammadnejad, M.,Bagheripour, M.H. Techno-Press 2012 Structural Engineering and Mechanics, An Int'l Jou Vol.43 No.5

Despite popularity of FEM in analysis of static and dynamic structural problems and the routine applicability of FE softwares, analytical methods based on simple mathematical relations is still largely sought by many researchers and practicing engineers around the world. Development of such analytical methods for analysis of free vibration of non-prismatic beams is also of primary concern. In this paper a new and simple method is proposed for determination of vibration frequencies of non-prismatic beams under variable axial forces. The governing differential equation is first obtained and, according to a harmonic vibration, is converted into a single variable equation in terms of location. Through repetitive integrations, integral equation for the weak form of governing equation is derived. The integration constants are determined using the boundary conditions applied to the problem. The mode shape functions are approximated by a power series. Substitution of the power series into the integral equation transforms it into a system of linear algebraic equations. Natural frequencies are determined using a non-trivial solution for system of equations. Presented method is formulated for beams having various end conditions and is extended for determination of the buckling load of non-prismatic beams. The efficiency and convergence rate of the current approach are investigated through comparison of the numerical results obtained to those obtained using available finite element software.

Optimization of ground response analysis using wavelet-based transfer function technique

Moghaddam, Amir Bazrafshan,Bagheripour, Mohammad H. Techno-Press 2014 Geomechanics & engineering Vol.7 No.2

One of the most advanced classes of techniques for ground response analysis is based on the use of Transfer Functions. They represent the ratio of Fourier spectrum of amplitude motion at the free surface to the corresponding spectrum of the bedrock motion and they are applied in frequency domain usually by FFT method. However, Fourier spectrum only shows the dominant frequency in each time step and is unable to represent all frequency contents in every time step and this drawback leads to inaccurate results. In this research, this process is optimized by decomposing the input motion into different frequency sub-bands using Wavelet Multi-level Decomposition. Each component is then processed with transfer Function relating to the corresponding component frequency. Taking inverse FFT from all components, the ground motion can be recovered by summing up the results. The nonlinear behavior is approximated using an iterative procedure with nonlinear soil properties. The results of this procedure show better accuracy with respect to field observations than does the Conventional method. The proposed method can also be applied to other engineering disciplines with similar procedure.

Seismic response analysis of layered soils considering effect of surcharge mass using HFTD approach. Part Ι: basic formulation and linear HFTD

Saffarian, Mohammad A.,Bagheripour, Mohammad H. Techno-Press 2014 Geomechanics & engineering Vol.6 No.6

Seismic ground response analysis is one of the most important issues in geotechnical earthquake engineering. Conventional seismic site response and free field analysis of layered soils does not consider the effect of surcharge mass which may be present on the top layer. Surcharge mass may develop extra inertial force to the soil and, hence, significantly affect on the results of seismic ground response analysis. Methods of analysis of ground response may also be categorized into time domain and frequency domain concepts. Simplicity in developing analytical relations and accuracy in considering soil dynamic properties dependency to loading frequency are benefits of frequency domain analysis. In this part of the paper, seismic ground response is analyzed using transfer function method for soil layers considering surcharge mass on the top layer. Equation of motion, wave equation, is solved using amended boundary conditions which effectively take the impact of surcharge mass into account. A computer program is developed by MATLAB software based on the solution method developed for wave equation. Layered soils subjected to earthquake loading were numerically studied and solved especially by the computer program developed in this research. Results obtained were compared with those given by DEEP SOIL computer program. Such comparison showed the accuracy of the program developed in this study. Also in this part, the effects of geometrical and mechanical properties of soil layers and especially the impact of surcharge mass on transfer function are investigated using the current approach and the program developed. The efficiency and accuracy of the method developed here is shown through some worked examples and through comparison of the results obtained here with those given by other approaches. Discussions on the results obtained are presented throughout in this part.

Seismic response analysis of layered soils considering effect of surcharge mass using HFTD approach. Part II: Nonlinear HFTD and numerical examples

Saffarian, Mohammad A.,Bagheripour, Mohammad H. Techno-Press 2014 Geomechanics & engineering Vol.6 No.6

Studies of earthquakes over the last 50 years and the examination of dynamic soil behavior reveal that soil behavior is highly nonlinear and hysteretic even at small strains. Nonlinear behavior of soils during a seismic event has a predominant role in current site response analysis approaches. Common approaches to ground response analysis include linear, equivalent linear and nonlinear methods. These methods of ground response analysis may also be categorized into time domain and frequency domain concepts. Simplicity in developing analytical relations and accuracy in considering soils' dynamic properties dependency to loading frequency are benefits of frequency domain analysis. On the other hand, nonlinear methods are complicated and time consuming mainly because of their step by step integrations in time intervals. In part Ι of this paper, governing equations for seismic response analysis of surcharged and layered soils were developed using fundamental of wave propagation theory based on transfer function and boundary conditions. In this part, nonlinear seismic ground response is analyzed using extended HFTD method. The extended HFTD method benefits Newton-Raphson procedure which applies regular iterations and follows soils' fundamental stress-strain curve until convergence is achieved. The nonlinear HFTD approach developed here are applied to some examples presented in this part of the paper. Case studies are carried in which effects of some influencing parameters on the response are investigated. Results show that the current approach is sufficiently accurate, efficient, and fast converging. Discussions on the results obtained are presented throughout this part of the paper.

Preparation of mixed matrix PES-based nanofiltration membrane filled with PANI-co-MWCNT composite nanoparticles

Abdolreza Moghadassi,Ehsan Bagheripour,Sayed Mohsen Hosseini 한국화학공학회 2016 Korean Journal of Chemical Engineering Vol.33 No.4

Mixed matrix polyethersulfone/PANI-co-MWCNTs composite nanoparticle nanofiltration membrane was prepared by casting solution technique. Polyvinylpyrrolidone was also used as membrane pore former in membrane fabrication. The effect of polyaniline-co-multi walled carbon nanotubes composite nanoparticle concentration in the casting solution on membrane structure and performance was investigated. Scanning optical microscopy and scanning electron microscopy, FTIR analysis, porosity, mean pore size, contact angle, water content, NaCl/Na2SO4 rejection, water flux, tensile strength measurements and 3D surface image were also carried out in membrane characterization. SOM images showed nanoparticle agglomeration at high additive loading ratio. SEM images showed the membrane sub-layer porosity and thickness were changed by use of nanoparticles in membrane matrix. The membrane water content, porosity and pore size were increased by increase of nanoparticle concentration, except for 1%wt. Use of PANI-co- MWCNT nanoparticles in the membrane matrix caused a decrease of membrane contact angle from 63.43 to 46.76o. Salt rejection and water flux were improved initially by increase of nanoparticle concentration up to 0.1%wt and then decreased by more additive concentration. In addition, the membranes tensile strength was reduced by increase of PANI-co-MWCNTs composite nanoparticle concentration. 3D surface images showed a smoother surface for mixed matrix membrane filled with 0.1wt% PANI-co-MWCNTs. Modified membrane containing 0.1wt% composite nanoparticles showed better performance compared to others.