Masoud Asgari

Energy absorption characteristics and a meta-model of miniature frusta under axial impact

ABSTRACT In this paper, crushing characteristics of small-sized conical tubes called miniature frusta under axial loading have been studied. Finite-element model is developed using non-linear explicit code LS_DYNA to investigate the non-symmetrical fold patterns as well as material and geometrical non-linearity of frusta. Numerical simulation is first validated by confirming the results using experimental test data. Effects of shell thickness, semi-epical angle of cone and frustas length on energy absorption characteristics are then studied by carrying out a parametric study. Based on the crushing parameters of numerous examples, a simplified analytical model as a meta-model for the mean crushing force of miniature frusta is presented using a Genetic Algorithm optimization for finding the meta-model coefficients. Miniature frusta show promising behaviour in lightweight design and crash analysis as their response results in low peak force and efficient-specific energy absorption. Obtained results from the developed meta-model showed good concurrence with finite elements method (FEM) model.

Thermo-Mechanical Analysis of 2D-FGM Thick Hollow Cylinder Using Graded Finite Elements

In this paper a thick hollow circular cylinder with finite length made of two-dimensional functionally graded material (2D-FGM) and subjected to steady state thermal and mechanical loadings is considered. The volume fraction distribution of materials, geometry and thermo-mechanical load are assumed to be axisymmetric but not uniform along the axial direction. The finite element method with graded material properties within each element (graded finite elements) is used to model the structure. The effects of variation of materials distribution in two radial and axial directions on the temperature, displacements and stress distributions are studied. Also the effectiveness of the graded finite elements on accuracy of the stresses is investigated. The achieved results show that using 2D-FGM leads to a more flexible design so that behavior of structure, maximum amplitude of stresses and uniformity of temperature and stress distributions can be modified to a required manner by selecting suitable material distribution profiles in two directions. Also for identical meshes, the graded element formulations do better than conventional homogeneous elements.

A unit-cell-based three-dimensional molecular mechanics analysis for buckling load, effective elasticity and Poisson's ratio determination of the nanosheets

By using a three-dimensional (3D) space-frame-like model, a molecular mechanics (MM) approach is proposed for determination of the buckling loads, effective Youngs modulus and Poissons ratio of the nanosheets, using a proper unit cell. The governing equations are derived based on the 3D kinematics of deformations and the principle of minimum total potential energy. The unit-cell-based results are employed for the space-frame-like finite element model of the nanosheet. The nonlinear MM equations are solved by representing bonds of the boron nitride nanosheet (BNNS) by beam elements to extract the local characteristics. These properties are employed in modelling of the nanosheet, as a space-frame-like finite element structure. The force field constants are chosen according to the Morse, AMBER, UFF and DREIDING models to determine the buckling strength, and effective Poissons ratio and in-plane rigidity of the whole graphene and BNNSs. Silicon Carbide nanosheets are analysed based on different force constants. These results are concordant with the results available in the literature. The comparisons reveal that the DREIDING force field usually gives the most accurate predictions.

Material optimization of functionally graded heterogeneous cylinder for wave propagation

In this paper, optimization of volume fraction distribution in a thick hollow heterogeneous cylinder subjected to impulsive internal pressure is considered. Dynamic behavior and wave propagation are considered in radial and axial directions. Volume fractions of constituent materials on a finite number of design points are taken as design variables and the volume fractions at any arbitrary point in the cylinder are obtained via cubic spline interpolation functions. The objectives are to minimize the amplitude of stress waves propagating through the structure during a specified time interval, while the total mass of the structure is also minimized. Minimizing the displacement amplitude of the outer surface of the cylinder will also be considered as another objective function. Multi-objective Genetic Algorithm jointed with interior penalty-function method for implementing constraints is effectively employed to find the global solution of the optimization problem. Obtained results indicates that by using the mentioned objective functions, considerably more efficient usage of materials can be achieved compared with the common power law volume fraction distribution. Based on our results, the proposed methodology provides a framework for designing functionally graded structures with optimum material tailoring.

Optimal material tailoring of 2D heterogeneous cylinder for a prescribed temperature field in transient heat conduction

In this paper, optimization of volume fraction distribution in a finite length thick hollow cylinder made of heterogeneous material is considered. The cylinder has two-directional functionally graded constituent and subjected to transient thermal loading. Two-dimensional heat conduction in the cylinder is considered and variation of temperature with time as well as temperature distribution through the cylinder is investigated. A graded finite element method combined with direct time integration method is used to model the structure and solve time-dependent equations. Volume fractions of constituent materials on a finite number of design points are taken as design variables and the volume fractions at any arbitrary point in the cylinder are obtained via cubic spline interpolation functions. The objective is to minimize the difference between the actual distribution of the temperature through the structure and a desired target distribution after a prescribed time while the total mass of the structure is also minimized. Multi-objective Genetic Algorithm jointed with interior penalty-function method for implementing constraints is effectively employed to find the global solution of the optimization problem. Obtained results indicate that by using the uniform distribution of temperature and minimum mass as objective functions, considerably more effective usage of materials can be achieved compared with the common power law volume fraction distribution. The proposed methodology provides a framework for designing functionally graded structures with optimum material tailoring for a prescribed field variable problem.

DYNAMIC ANALYSIS OF HEALTHY AND EDGE-TO-EDGE REPAIRED MITRAL VALVE BEHAVIOR SUBJECTED TO HIGH G ACCELERATIONS

As the mitral leaflets have the greatest area among heart valves and bear the highest pressure load during systole, high G accelerations may result in mitral valve dysfunctions and it might affect the cardiovascular system drastically. In this study, dynamic behavior of healthy and repaired human mitral valves have been numerically simulated during the Early Systolic Phase and the Rapid Filling Phase in a cardiac cycle in high G accelerated environments. The aim of this study is to investigate the effects of accelerations on the stress and strain patterns and the configuration of human mitral valve. The geometrical model of the mitral valve has been developed based on in vivo and ex vivo anatomical measurements and the anisotropic nonlinear behavior of mitral leaflets has been modeled by a discrete constitutive approach. Mitral valve behavior has been simulated using an explicit dynamic finite element method to take into account inertial effects and dynamic responses. Analysis results reveal beside different stress–strain patterns generated on mitral leaflets, abnormal deformed configurations result from accelerations which can affect the circulation and the cardiovascular system. It is observed that situations similar to mitral diseases could rise from high G accelerated environments even though the valve maintains its normal physiological structure.

Dynamical Stress Distribution Analysis of a Non-Uniform Cross-Section Beam Under Moving Mass

One of the engineers concern in designing bridges and structures under moving load is the uniformity of stress distribution. In this paper the analysis of a variable cross-section beam subjected to a moving concentrated force and mass is investigated. Finite element method with cubic Hermitian interpolation functions is used to model the structure based on Euler-Bernoulli beam and Wilson-Θ direct integration method is implemented to solve time dependent equations. Effects of cross-section area variation, boundary conditions, and moving mass inertia on the deflection, natural frequencies and longitudinal stresses of beam are investigated. Results indicates using a beam of parabolically varying thickness with constant mass can decrease maximum deflection and stresses along the beam while increasing natural frequencies of the beam. The effect of moving mass inertia of moving load is found to be significant at high velocity.Copyright

Efficient crushable corrugated conical tubes for energy absorption considering axial and oblique loading

Thin-walled structures are of much interest as energy absorption devices for their great crashworthiness and also low weight. Conical tubes are favorable structures because unlike most other geometries, they are also useful in oblique impacts. This paper investigated the effect of corrugations on energy absorption characteristics of conical tubes under quasi-static axial and oblique loadings. To do so, conical tubes with different corrugation geometries were analyzed using the finite element explicit code and the effects of corrugations on initial peak crushing force and specific energy absorption were studied. The finite element model was validated by experimental quasi-static compression tests on simple and corrugated aluminum cylinders. An efficient analytical solution for EA during axial loading was also derived and compared with the FEM solution. The crushing stableness was analyzed using the undulation of the load-carrying capacity parameter and it was shown that corrugations made collapsing mode, more predictable and controllable. The findings have shown that corrugated conical tubes have much better energy absorption characteristics compared with their non-corrugated counterparts. It was also discovered that during oblique loadings, introducing corrugations can significantly increase the specific energy absorption compared with simple cones.

Vibration Interaction Analysis of Non-uniform Cross-Section Beam Structure under a Moving Vehicle

One of an engineer’s concern when designing bridges and structures under a moving load is the uniformity of stress distribution. The dynamic behavior of a vehicle on a flexible support is also of great importance. In this paper, an analysis of a variable cross-section beam subjected to a moving load (such as a concentrated mass), a simple quarter car (SQC) planar model, and a two-axle dynamic system with four degrees of freedom (4DOF) is carried out. The finite element method with cubic interpolation functions is used to model the structure based on the Euler-Bernoulli beam and a direct integration method is implemented to solve time dependent equations implicitly. The effects of variation of a cross-section and moving load parameters on the deflection, natural frequencies, and longitudinal stresses of the beam are investigated. The interaction between vehicle body vibration and the support structure is also considered. The obtained results indicate that using a beam of parabolically varying thickness with a constant weight can decrease the maximum deflection and stresses along the beam while increasing the natural frequencies of the beam. The effect of moving mass inertia at a high velocity of a moving vehicle is also investigated and the findings indicate that the effect of inertia is significant at high velocities.

Structural simulation of human mitral valve behaviour cosidering effects of material nonlinearities

Simulation of human heart mitral valves is a challenging biomechanical problem due to its complex anatomical structure, material properties and time dependent loading conditions. This study presents a modeling and simulation of human mitral valve behavior considering the effects of material nonlinearity and Chordae tendineae rupture via a numerical analysis. Three-dimensional sized geometrical model obtained from anatomically measurement used as structural model The transient finite element method including inertia effects and time dependencies implemented for numerical solution. Two different material models have been considered to illustrate the effect of material nonlinearity on the stress and strain imposed by leaflets. On the other hand Chordae tendineae rupture caused by bacterial endocarditis, rheumatic valvular disease or trauma can be a deadly defect leads to malfunction of human heart. Chordae tendineae rupture has been also simulated to investigate the effects on leaflet stresses and strains. Based on the results, although the linear elastic model exhibits an acceptable correlation in the location of high stress regions with the hyperelastic model but Stress magnitudes differ between the elastic and hyper elastic model Depending on the strain energy function used to describe the nonlinear material, different stress magnitudes release from the analyses. Chordae rupture causes an unintended increase in the magnitude of leaflet stresses and the closed valve configuration. The increment value depends on the location and number of ruptured chordae.