Masoud Rais-Rohani

Mississippi State University, Mississippi, United States

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Publications (58)27.82 Total impact

  • M. Islam, A. Buijk, M. Rais-Rohani, K. Motoyama
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    ABSTRACT: A welding process design tool is proposed for arc welding parametric optimization.•It is based on integrated Finite Element Method, Response Surface Method and Genetic Algorithms.•Simulation based process parameter optimization is possible without expensive experiments.•The method effectively determines optimum parameters for minimum distortion.
    Advances in Engineering Software. 01/2015; 79.
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    ABSTRACT: A vehicle–dummy multidisciplinary design optimization problem is treated as a multilevel system composed of structural and occupant restraint system design elements. The vehicle-based responses and the dummy-based responses are obtained from nonlinear transient dynamic finite element simulations of full frontal impacts and side impacts. The wall thicknesses of a set of energy-absorbing components together with the occupant restraint system control parameters associated with the seat belt and the airbag are treated as design variables and used to optimize the multilevel system to minimize both the structural mass and the selected injury criteria. Each element optimization problem is modeled using the augmented Lagrangian with exponential penalty function formulation. A single-loop coordination strategy is used to solve the multilevel optimization problem. To maximize the computational efficiency, surrogate models are used to approximate the vehicle-based responses and the dummy-based responses. The results of the vehicle–dummy design problem are used to examine the computational efficiency and the accuracy of the decomposed multilevel optimization methodology.
    Proceedings of the Institution of Mechanical Engineers Part D Journal of Automobile Engineering 08/2014; · 0.58 Impact Factor
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    M. Islam, A. Buijk, M. Rais-Rohani, K. Motoyama
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    ABSTRACT: This paper presents an effective numerical approach for welding process parameter optimization to minimize weld-induced distortion in structures. A numerical optimization framework based on coupled Genetic Algorithm (GA) and Finite Element Analysis (FEA) is developed and implemented for a low and a high fidelity model. Classical weakly coupled thermo-mechanical analysis with thermo-elasto-plastic assumptions is carried out for distortion prediction of numerical models. The search for optimum process parameters is executed by direct integration of numerical models and GA-based optimization technique. The developed framework automatically inserts the process parameters into the simulation models, executes the FE-based welding simulations and evaluates the required simulation output data for iterative evolutionary optimization. The optimization results show that the proposed approach can contribute substantially to enhance final welded product quality while facilitating and accelerating the product design and development.
    Finite Elements in Analysis and Design 07/2014; 84:54–64. · 1.39 Impact Factor
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    ABSTRACT: Car body design in view of structural performance and lightweighting is a challenging task due to all the performance targets that must be satisfied such as vehicle safety and ride quality. In this paper, material replacement along with multidisciplinary design optimization strategy is proposed to develop a lightweight car body structure that satisfies the crash and vibration criteria while minimizing weight. Through finite element simulations, full frontal, offset frontal, and side crashes of a full car model are evaluated for peak acceleration, intrusion distance, and the internal energy absorbed by the structural parts. In addition, the first three fundamental natural frequencies are combined with the crash metrics to form the design constraints. The wall thicknesses of twenty-two parts are considered as the design variables. Latin Hypercube Sampling is used to sample the design space, while Radial Basis Function methodology is used to develop surrogate models for the selected crash responses at multiple sites as well as the first three fundamental natural frequencies. A nonlinear surrogate-based optimization problem is formulated for mass minimization under crash and vibration constraints. Using Sequential Quadratic Programming, the design optimization problem is solved with the results verified by finite element simulations. The performance of the optimum design with magnesium parts shows significant weight reduction and better performance compared to the baseline design.
    Journal of Magnesium and Alloys. 06/2014;
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    ABSTRACT: An approach is presented to evaluate the structural performance of a vehicle model in terms of the joint stiffness. Seven major joints on the left and right sides of the vehicle body are identified, and each joint is decomposed in the finite element model and assigned a separate set of material properties. By adjusting the elastic modulus of each structural member, the effects of the joint stiffness on the full and offset frontal impacts as well as the vibration characteristics are examined. Latin hypercube sampling is used in the design of experiments to approximate the acceleration, the intrusion distance, and the fundamental vibration frequencies using full quadratic polynomial response surface models. Through direct differentiation, the sensitivities of the crash responses and the vibration responses to the joint stiffness are calculated. A constrained multi-objective optimization problem is formulated and solved to improve the structural responses by adjusting the stiffness at each joint. Evaluation of the car body structure based on the optimum joint stiffness showed a superior performance relative to the baseline model without a weight penalty. The results of both the sensitivity analysis and the design optimization are presented and discussed.
    Proceedings of the Institution of Mechanical Engineers Part D Journal of Automobile Engineering 01/2014; 228(6):689-700. · 0.58 Impact Factor
  • Ali Najafi, Erdem Acar, Masoud Rais-Rohani
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    ABSTRACT: The stochastic uncertainties associated with the material, process and product are represented and propagated to process and performance responses. A finite element-based sequential coupled process–performance framework is used to simulate the forming and energy absorption responses of a thin-walled tube in a manner that both material properties and component geometry can evolve from one stage to the next for better prediction of the structural performance measures. Metamodelling techniques are used to develop surrogate models for manufacturing and performance responses. One set of metamodels relates the responses to the random variables whereas the other relates the mean and standard deviation of the responses to the selected design variables. A multi-objective robust design optimization problem is formulated and solved to illustrate the methodology and the influence of uncertainties on manufacturability and energy absorption of a metallic double-hat tube. The results are compared with those of deterministic and augmented robust optimization problems.
    Engineering Optimization 01/2014; 46(2). · 0.96 Impact Factor
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    ABSTRACT: The traditional crashworthiness optimisation problem is augmented by inclusion of additional design criterion associated with vehicle vibration characteristics. Through finite element (FE) simulations, full frontal, offset frontal and side crashes of a full vehicle model are analysed for peak acceleration, intrusion distance and internal energy. Moreover, the FE crash model of the vehicle is modified to develop a vibration-analysis model for evaluation of natural frequencies and mode shapes. Design of computer experiments through Latin hypercube sampling is used to sample the design space defined by the wall thicknesses of 22 parts. Radial basis functions are used to generate separate surrogate models for the selected crash responses measured at multiple sites as well as the fundamental natural frequencies in bending and torsion. A nonlinear surrogate-based mass minimisation problem is formulated and solved under crash and vibration constraints with the results verified through FE simulations. The optimum vehicle design under multiple design criteria is presented and the vehicle's characteristics are compared with those of the baseline design as well as those associated with the optimum design based on crash responses alone.
    International Journal of Crashworthiness 10/2013; 18(5). · 0.88 Impact Factor
  • M. Rouhi, M. Rais-Rohani, A. Najafi
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    ABSTRACT: An enhanced thermoset polymer matrix with randomly distributed carbon nanofibers (CNFs) is combined with conventional long fibers to form a hybrid composite material for application to impact energy absorbing components. The multi-inclusion method in combination with functionally graded interphase is used for stiffness characterization and shear-lag theory combined with quasi-isotropic lamination approximation is used for strength prediction. Axial crush simulations are performed using MD Nastran with a micromechanics-based progressive failure analysis constitutive model. The stochastic uncertainties in the geometric and material properties of CNF as well as the three-dimensional, non-homogeneous CNF–matrix interphase are represented using probability theory. Through Monte Carlo simulations, these uncertainties are propagated to the homogenized macroscopic properties of the nano-enhanced matrix and subsequently to the stiffness and strength properties of the composite laminate as well as the energy absorbing characteristics of the crush tube. A probabilistic design optimization problem is formulated for minimizing the failure probability associated with the specific energy absorption of the composite tube. A dual surrogate modeling approach is used for approximating the failure probability and solving the optimization problem using sequential quadratic programming. The modeling approach, uncertainty analysis, and probabilistic optimization results are presented and discussed.
    Composite Structures 06/2013; 100:144–153. · 3.12 Impact Factor
  • Mohammad Rouhi, Masoud Rais-Rohani
    54th AIAA/ASME/ASCE/AHS/ASC Structures, Structural Dynamics, and Materials Conference; 04/2013
  • S. DorMohammadi, M. Rais-Rohani
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    ABSTRACT: An exponential penalty function (EPF) formulation based on method of multipliers is presented for solving multilevel optimization problems within the framework of analytical target cascading. The original all-at-once constrained optimization problem is decomposed into a hierarchical system with consistency constraints enforcing the target-response coupling in the connected elements. The objective function is combined with the consistency constraints in each element to formulate an augmented Lagrangian with EPF. The EPF formulation is implemented using double-loop (EPF I) and single-loop (EPF II) coordination strategies and two penalty-parameter-updating schemes. Four benchmark problems representing nonlinear convex and non-convex optimization problems with different number of design variables and design constraints are used to evaluate the computational characteristics of the proposed approaches. The same problems are also solved using four other approaches suggested in the literature, and the overall computational efficiency characteristics are compared and discussed.
    Structural and Multidisciplinary Optimization 04/2013; 47(4). · 1.73 Impact Factor
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    Amin Kargarian, Yong Fu, Saber DorMohammadi, Masoud Rais-Rohani
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    ABSTRACT: Active distribution grid is composed of autonomous systems which should collaborate with each other in order to operate the entire distribution grid in a secure and economic manner. This paper presents a system of systems (SoS) framework for optimally operating active distribution grids. The proposed SoS framework defines both distribution company (DISCO) and microgrids (MGs) as independent systems, and identifies the process of information exchange among them. As the DISCO and MGs are physically connected together, the operating condition of one might impact the operating point of other systems. The proposed mathematical model uses a decentralized optimization problem aimed at maximizing the benefit of each independent system. A hierarchical optimization algorithm is presented to coordinate the independent systems and to find the optimal operating point of the SoS-based active distribution grid. The numerical results show the effectiveness of the proposed SoS framework and solution methodology.
    IEEE Transactions on Smart Grid 01/2013;
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    ABSTRACT: An evidence-based approach is developed for optimization of structural components under material parameter uncertainty. The approach is applied to evidence-based design optimization (EBDO) of externally stiffened circular tubes under axial impact load using an isotropic–elastic–plastic plasticity model to simulate dynamic material behaviour. Uncertainty modelling considers the changes in material parameters that are caused by variability in material properties as well as incertitude and errors in experimental data and procedure to determine the material parameters. Spatial variation of material parameters across the structural component is modelled using a field joint belief structure and propagated for the calculation of evidence-based objective function and design constraints. Surrogate models are used in both uncertainty propagation and solution of the optimization problem. The methodology and the solution to the EBDO example problem are presented and discussed.
    Engineering Optimization 01/2013; 45(9). · 0.96 Impact Factor
  • M. Rouhi, M. Rais-Rohani
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    ABSTRACT: Micromechanical approaches are used in mathematical modeling of randomly distributed carbon nanofibers (CNFs) in a thermoset polymer material. Both CNF waviness and CNF-matrix interphase properties are included in the model. The interphase mechanical properties are considered to vary in a manner similar to functionally graded materials. The effects of stochastic uncertainties on the overall properties of the composite material are represented using the probability theory. The uncertainties are propagated in calculating the axial buckling load probability of a thin-walled composite cylinder, which is then optimized for minimum material volume. The probabilistic design optimization results are presented and discussed.
    Composite Structures 01/2013; 95:346–353. · 3.12 Impact Factor
  • Saber DorMohammadi, Masoud Rais-rohani
    12th AIAA Aviation Technology, Integration, and Operations (ATIO) Conference and 14th AIAA/ISSMO Multidisciplinary Analysis and Optimization Conference; 09/2012
  • Saber DorMohammadi, Mohammad Rouhi, Masoud Rais-Rohani
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    ABSTRACT: The newly developed element exchange method (EEM) for topology optimization is applied to the problem of blank shape optimization for the sheet-forming process. EEM uses a series of stochastic operations guided by the structural response of the model to switch solid and void elements in a given domain to minimize the objective function while maintaining the specified volume fraction. In application of EEM to blank optimization, a sheet forming simulation model is developed using Abaqus/Explicit. With the goal of minimizing the variability in wall thickness of the formed component, a subset of solid (i.e., high density) elements with the highest increase in thickness is exchanged with a consistent subset of void (i.e., low density) elements having the highest decrease in thickness so that the volume fraction remains constant. The EEM operations coupled with finite element simulations are repeated until the optimum blank geometry (i.e., boundary and initial thickness) is found. The developed numerical framework is applied to blank optimization of a benchmark problem. The results show that EEM is successful in generating the optimum blank geometry efficiently and accurately.
    ASME 2012 International Design Engineering Technical Conferences and Computers and Information in Engineering Conference; 08/2012
  • Ali Najafi, Erdem Acar, Masoud Rais-Rohani
    53rd AIAA/ASME/ASCE/AHS/ASC Structures, Structural Dynamics and Materials Conference 20th AIAA/ASME/AHS Adaptive Structures Conference 14th AIAA; 04/2012
  • Saber DorMohammadi, Masoud Rais-Rohani
    53rd AIAA/ASME/ASCE/AHS/ASC Structures, Structural Dynamics and Materials Conference 20th AIAA/ASME/AHS Adaptive Structures Conference 14th AIAA; 04/2012
  • Saber DorMohammadi, Masoud Rais-Rohani
    53rd AIAA/ASME/ASCE/AHS/ASC Structures, Structural Dynamics and Materials Conference 20th AIAA/ASME/AHS Adaptive Structures Conference 14th AIAA; 04/2012
  • Shahabedin Salehghaffari, Masoud Rais-Rohani
    53rd AIAA/ASME/ASCE/AHS/ASC Structures, Structural Dynamics and Materials Conference 20th AIAA/ASME/AHS Adaptive Structures Conference 14th AIAA; 04/2012
  • Andrew Parrish, Masoud Rais-Rohani, Ali Najafi
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    ABSTRACT: This paper explores the effects of replacing the baseline steel with lightweight magnesium alloy parts on crashworthiness characteristics and optimum design of a full-vehicle model. Full frontal, offset frontal and side crash simulations are performed on a validated 1996 Dodge Neon model using explicit nonlinear transient dynamic finite element analyses in LS-DYNA to obtain vehicle responses such as crash pulse, intrusion distance, peak acceleration and internal energy. Twenty-two parts of the vehicle body structure are converted into AZ31 magnesium alloy with adjustable wall thickness while the remaining parts are kept intact. The magnesium alloy material model follows a piecewise linear plasticity law considering separate tension and compression properties and maximum plastic strain failure criterion. Six different metamodelling techniques including optimised ensemble are developed and tuned for predictions of crash-induced responses within the design optimisation process. The crashworthiness optimisation problem is solved using the sequential quadratic programming method with most accurate surrogate models of structural responses considering both constrained single- and multi-objective formulations. The results show that under the combined crash scenarios with the selected material models and design constraints, the vehicle model with magnesium alloy parts can be optimised to maintain or improve its crashworthiness characteristics with up to 50% weight savings in the redesigned parts.
    International Journal of Crashworthiness - INT J CRASHWORTHINESS. 01/2012;