behrooz hassani

The isogeometric method was introduced to bridge the gap between computer-aided design (CAD) and analysis. This method offers advantages such as precise geometric modeling, suitable refinement methods, easy access to higher-order functions, and higher computational accuracy. The aim of this research is to provide an efficient method for applying the distributed loads on curved surfaces in isogeometric analysis. One of the main challenges in this method is how to apply boundary conditions on complex geometries. In curved models, some control points may not lie on the geometry, leading to ambiguity in the distribution of loads on these points. This study uses NURBS functions, which are standard non-interpolatory functions in CAD systems, to approximate the solution space and describe the geometry. Additionally, to leverage the capabilities of CAD tools, the process of importing geometries created in Rhino into isogeometric analysis using Bézier extraction is explained. The results confirm the accuracy and efficiency of the proposed method.

Flexure joints are one of the most widely used and crucial elements in the design of precision mechanisms. Owing to their monolithic and elastic structure, these joints facilitate highly precise movements. In this study, we present a kinetoelastic model for designing various types of flexure joints with single and multiple degrees of freedom. To reduce computational costs, two beneficial approaches for defining the objective function and constraints are presented, based solely on the strain energy criterion and predetermined displacements. The resulting self-adjoint optimization problem exhibits computational efficiency and improved convergence. The topology optimization problem utilizes the Finite Element Method (FEM) and the Solid Isotropic Material with Penalization (SIMP) model, employing the Method of Moving Asymptotes (MMA) to solve and identify the optimal topology. A comprehensive mathematical framework, including the relevant two-dimensional (2D) boundary conditions and sensitivity analysis, is meticulously developed and extensively examined. For this purpose, MATLAB code is developed for designing 2D flexure joints with single and multiple degrees of freedom. Finally, the results obtained from the comparison of two optimization approaches presented in this study are discussed. In these joints, the stiffness ratio of the structure has increased significantly, up to 208 times, indicating the practicality and effectiveness of this method in the topology optimization of flexure joints.

 Cardiac complications are the leading cause of death in thalassemia patients. It is assumed that progressive iron accumulation results in myocyte damage. Myocardial T2* measurement by cardiac MRI quantifies iron overload. We aimed to study the association between left and right ventricular (LV and RV) function and iron deposition estimation by cardiac MRI T2* in a sample of Iranian patients.

 Cardiac MRI exams of 118 transfusion-dependent thalassemia major patients were evaluated retrospectively. Biventricular function and volume and myocardial and liver T2* values were measured. The demographic and lab data were registered. Poisson and chi-square regression analyses investigated the correlation between the T2* value and ventricular dysfunction.

 The study participants’ mean (SD) age was 32.7y (9.02), and 47.46% were female. Forty-nine cases (41.52%) revealed at least uni-ventricular dysfunction. LV dysfunction was noted in 20 cases, whereas 47 patients revealed RV dysfunction. The risk of LV dysfunction was 5.3-fold higher in patients with cardiac T2* value less than 10msec (RR=5.3, 95% CI=1.6, 17.1, P<0.05). No association was found between age, liver T2* value, serum ferritin level, and chelation therapy with the risk of LV and RV dysfunction.

 Cardiac MRI T2* measure is a good indicator of LV dysfunction. Moreover, MRI parameters, especially RV functional measures, may have a substantial role in patient management. Therefore, cardiac MRI should be included in beta-thalassemia patients’ management strategies.

Although the nonlocal integral (NI) model circumvents the inconsistencies associated with the differential model, it is shown in the present study that the way its nonlocal kernel function is normalized noticeably affects the dynamic response of nanobeams. To this aim, a two-phase nonlocal integral nanobeam model with different boundary conditions and kernel functions is considered and its natural frequencies are obtained using the Rayleigh-Ritz method. Also, the kernel function is normalized via two procedures to see the influence of each one on the vibration characteristics of nanobeam. From the results it is found that kernel normalization has a significant effect on vibration response of nanobeam and therefore must be taken into account. Further, it is found that the results from each normalized model are noticeably different from the other. Furthermore, by comparing the results of continuum NI models with those from atomistic models, it is revealed that for certain normalization schemes a calibrated nonlocal parameter cannot be found due to twofold hardening-softening behavior. Moreover, the effect of kernel type, boundary conditions and mode number is thoroughly studied. The results from current study can shed light on the way of choosing or developing more reliable equivalent continuum NI models for nanostructures.

Employing the Isogeometric Analysis method for solution of nonlinear compressible elastic materials, generally known as hyperelasticity, is the subject of this article. For this purpose, the matrix of coefficients is derived and by the linearization of governing equations the discretized equilibrium equations are obtained and a solution algorithm is presented. To study the performance and accuracy of the method in compressible hyperelastic problems, the obtained results are compared with those of finite elements. The presented approach, besides providing a good flexibility in geometrical modeling, results in a smaller system of equations and consequently reducing the computational cost. Furthermore, despite having large deformations, the need for remeshings is alleviated. Also, the effects of the number of load increments, as well as, the number of Gauss integration points on the convergence of the solution are studied.

This paper presents an improved approach for handling stress constraints in minimum weight topological design. The Finite Element Method (FEM) and the material model of Solid Isotropic Material with Penalization (SIMP) is used to formulate the topology optimization problem. To evaluate the stress values in elements, the von Mises stresses are calculated at the so called super-convergent Gauss quadrature points. To reduce the time and computational cost, a clustering approach is here adopted and the P-norm integrated stress constraints are used. Doing this, a large number of local constraints are replaced with a few global ones and consequently the stress constraint sensitivities are calculated by using the adjoint method. The employed formulation as well as a complete explanation of the sensitivity analysis is provided. Due to the complexity of the topology optimization problem in the presence of stress constraints, the Method of Moving Asymptotes (MMA) is here employed. To demonstrate the performance and capability of the procedure, a couple of plane stress elasticity problems are taken into consideration. The resulted layouts indicate the superiority of the approach in generating acceptable and practical topological designs.

This paper presents an isogeometric analysis approach for static and free vibration analysis of laminated composite plates covered with piezoelectric layers using the Reissner-Mindlin theory. Isogeometric analysis (IGA) aims at simplifying the Computer Aided Design (CAD) and Computer Aided Engineering (CAE) by using the functions describe the geometry (CAD) and the unknown fields (Analysis). The isogeometric approach has here been employed that utilizes the Non-Uniform Rational B-Splines (NURBS) of quadratic, cubic and quartic orders to approximate the variables defining geometry as well as the unknown functions. Using the Reissner-Mindlin first order shear deformation theory requires the C0-continuity of generalized displacements and the NURBS basis functions are well suited for this purpose. The electric potential is assumed to vary linearly through the thickness for each piezoelectric sublayer. To alleviate the shear locking problem, a stabilization technique is employed in the stiffness formulation. Since study of the performance and accuracy of IGA in solving laminated composite plates is one of the main objectives of this article. Several numerical examples are presented and compared with those available in the literature. The obtained results indicate desirable accuracy and efficiency of the proposed approach.

Self-centering rocking walls are known as viable alternatives to typical shear walls, as they provide a number of solutions for eliminating seismic flaws of conventional designs. These rocking walls have a generally positive impact on the seismic behavior of structural systems, but their design makes them susceptible to concrete crushing around their base, which can lead to significantly adverse effects on their seismic performance. This paper first models the dynamic behavior of these walls under cyclic loading and then uses a new approach to estimate the extent and quality of damage incurred by the wall at element level. The damage index used for this purpose acts as a quantitative scale measuring the quality of damage incurred by the concrete and therefore gauging the status of the wall. This paper uses the PERFORM 3D software for the procedure of modeling and damage estimation. To assess the accuracy of the modeling technique, results of numerical analyses are compared with the results of a full-scale load test. The quantitated damage incurred by the wall is then plotted for its surface and these damages are then compared with the actual results obtained from the test. The results indicate that the technique used by this paper to model the dynamic behavior of these walls can accurately simulate their behavior. Also, the damage index used in this paper provides an adequately accurate estimate of the damages incurred by this type of walls.

In this research, for the first time, the net of control points in the isogeometric analysis has been improved by employing an error estimator based on a stress recovery method. First, an error estimation algorithm based on stress recovery is used to obtain the energy norm for each element. Then, artificial rods are defined between control points and the estimated values of errors in the control points located at the vicinity of a typical point is assigned to each rod as a thermal gradient. Now, by analyzing this hypothetical truss problem under temperature changes a new arrangement of control points and consequently the knot vectors can be obtained. Repeating this process in isogeometric analysis will lead to a better distribution of errors in the domain of the problem and results in an optimal net of control points to calculate the integrals. To evaluate the efficiency of this method, the results of modeling and analysis of two elasticity problem is presented. The obtained results show that this innovative approach has a good performance and can be employed for reducing the analysis errors.

In this study, experimental study of profile geometry effect on Polycrystal Diamond Compact (PDC) drill bits performance and durability was conducted. In an extensive field study, three samples of bits was choosed among NIOC bits collection. PDC bit profile consists of apex, cone, nose, shoulder and gage, which all are effective on stability, penetration rate, aggressiveness and durability. Verification of the effect of PDC bit profile geometry needs to first determine the exact geometry. Complicated geometry of these PDC bits was obtained by 3D-scan and cloud of points. Then cutters arrangement of the profile was produced. In experimental study, field test of these bits was conducted in same condition (WOB, ROP, fluid velocity and drilling mud weight) in Ahwaz oil rig in Asmari formation. Drilling metrage and penetration rate was measured and the bits dull grading based on IADC standards was determined. Results of the bits test showed the effect of profile geometry on PDC bits performance and durability. optimization of profile geometry of PDC bits causes an increase in penetration rate, stability and durability.

This article is devoted to the derivation of formulation and isogeometric solution of nonlinear nearly incompressible elastic problems, known as nearly incompressible hyperelasticity. After problem definition, the governing equations are linearized for employing the Newton-Raphson iteration method. Then, the problem is discretized by using concepts of isogeometric analysis method and its solution algorithm is devised. To demonstrate the performance of the proposed approach, the obtained results are compared with finite elements. Due to large deformations in this kind of problems, the finite element method requires a relatively large number of elements, as well as the need for remeshings in some problems, that results in a large system of equations with a high computational cost. In the isogeometric analysis method, using B-Spline and NURBS (Non-Uniform Rational B-Spline) basis functions provides us with a good flexibility in modeling of geometry without any need for further remeshings. The examples studied in this article indicate that by using the isogeometric approach good quality results are obtained with a smaller system of equations and less computational cost. Also, influence of different volumetric functions for the nearly incompressible materials are investigated.

This research is devoted to the adaptive solution and control point net improvement of axisymmetric problems in isogeometric analysis using the error estimation based methods for stress recovery. For this purpose، after the calculation of the energy norm، the estimated value of error in the vicinity of each control point is assigned to the neighboring members of a hypothetical truss-like structure as an artificial thermal gradient. By analysis of this network of rods under the temperature variations a new arrangement of control points is obtained. Repeating this process of thermal isogeometric analysis will eventually lead to a better distribution of errors in the domain of the problem and results in an optimal net of control points for the calculation of the integrals. To demonstrate the performance and efficiency of the proposed method، two axisymmetric elasticity problems with available analytical solutions are considered. The obtained results indicate that this innovative approach is effective in reducing errors of axisymmetric problems and can be employed for improving the accuracy in the context of the isogeometric analysis method. Innovated method of this research focuses on adaptive analysis and Network improving of axisymmetric problems in isogeometric analysis using error estimation methods based on stress recovery. For this purpose after calculation the energy norm، estimated value of error in the vicinity of control points is assigned to each rod as the thermal gradient. Thus after analyzing the hypothetical rods network under the temperature changes a new arrangement of control points and knot vectors can be obtained. The use of multi-cycle of this process in isogeometric analysis will lead to a better distribution of errors in the domain and thus achieve optimal network to calculate the integrals. To measure the efficiency of this method and demonstrate the increased carefully in axisymmetric problems، which has the analytical solution، two elasticity problem is evaluated. The results show that innovative network improving method has good efficiency to reduce the error rate and can be used to increase the accuracy of isogeometric analysis results. Innovated method of this research focuses on adaptive analysis and Network improving of axisymmetric problems in isogeometric analysis using error estimation methods based on stress recovery. For this purpose after calculation the energy norm، estimated value of error in the vicinity of control points is assigned to each rod as the thermal gradient. Thus after analyzing the hypothetical rods network under the temperature changes a new arrangement of control points and knot vectors can be obtained. The use of multi-cycle of this process in isogeometric analysis will lead to a better distribution of errors in the domain and thus achieve optimal network to calculate the integrals. To measure the efficiency of this method and demonstrate the increased carefully in axisymmetric problems، which has the analytical solution، two elasticity problem is evaluated. The results show that innovative network improving method has good efficiency to reduce the error rate and can be used to increase the accuracy of isogeometric analysis results.

Study of the critical buckling load as well as post-buckling behavior of single-walled nanotubes (SWNT) under various thermal conditions is the subject of this article. For finding critical buckling loads the clamped-free and simply supported conditions are considered and for post-buckling analysis the boundary conditions are assumed to be clamped. A space-frame model is here employed for the zigzag and armchair nanotubes with different chiralities and aspect ratios. In this approach، the linkage between carbon atoms is modeled as three dimensional elastic beam. By establishing a linkage between structural mechanics and molecular mechanics، the sectional property parameters of these beam members are obtained. The obtained results indicate that، as it is expected، by increasing the aspect ratio of the nanotubes as well as chirality، the critical buckling load decreases. Also، it is noticed that the post-buckling behavior of both armchair and zigzag nanotubes are quite similar. In addition، the effect of temperature on the critical buckling load is investigated. Due to the influence of temperature over the linkage length and force field constants it is shown that when the temperature increases a decline in the critical buckling loads is observed.

Abstract Isogeomteric Analysis is a newly developed method with some features that can be considered as ‎a potential substitute to other numerical methods such as finite elements and meshless approaches. ‎The NURBS technique, that allows precise geometrical modeling, plays an important role in this ‎method. However, similar to other numerical methods, existence of errors in the approximation of ‎the unknown function is inevitable. This paper is devoted to finding points with higher accuracy in ‎stress recovery by the isogeometric analysis. It can be shown that these points are coincident with ‎the Gauss quadrature points. By making use of these superconvergent points together with the ‎NURBS technique and the least square method, a surface is constructed for each component of ‎the stress tensor that represents the improved stresses. For this purpose, three examples with ‎available analytical results has been solved. The comparison of the obtained improved stresses ‎with the exact solution is used for the sake of verification of the proposed method. It is concluded ‎that the superconvergent points location in the isogeometric analysis are the same as the minimum ‎required Gauss points for the integration of a square element.‎

Shape optimization of free form shell structures with different objective and constraint functions including the stress constraint is the subject of this article. To construct the geometry B-Splines are employed that allow generating smooth free form geometries with a small number of parameters which are considered as the design variables of the optimization problem. For analysis, the finite element method by using the Wilson’s quad shell element is employed. For shape optimization, in each step of the optimization process the mesh generation, finite element analysis, sensitivity analysis and geometry update steps are repeated until convergence. Maximization of the stiffness of structure with volume constraint and the minimization of the weight of structure with the von Misses stress constraint are the addressed problems of this article. In both kinds of problems, the applicates of the control points are considered as the design variables of the shape optimization problem and the sequential quadratic programming (SQP) is employed to solve the optimization problem. Since in this approach, the derivatives of the objective and constraint functions are needed, the sensitivity analysis is carried out in each step by the finite difference method. The quality and smoothness of the obtained results together with the convergence graphs of the presented examples are indicative of the usefulness and efficiency of the proposed approach for shape optimization of shell structures.

In this paper, a new method is introduced for calculation of stress field in the isogeometric analysis (IGA) method. ‎In this technique, by considering the forces induced on the patches of IGA, the surface denoting the variations of ‎each component of the stress tensor is approximated by the same order of NURBS’ shape functions that are used ‎for approximation of the displacements. This new surface is more accurate compared to the stress surfaces obtained ‎from isogeometric analysis, thereby, can be used error estimation of the IGA. It is shown that this method, which ‎falls in the category of stress recovery methods, is superior to the stress recovery based on the superconvergent ‎points in the IGA. To verify the performance of the method, three examples are taken into consideration and their ‎solutions with this approach as well as the method based on superconvergent points are compared with their exact ‎solutions. The obtained results of these examples indicate the effectiveness of the proposed method and therefore ‎can be considered as a simple and efficient method for error estimation and stress recovery.‎

In the paper, a new approach is presented for improving the stress field and estimating the solution error based on the isogeometrical analysis method. In this approach, by using the superconvergent points for the components of the stress field in each area, an imaginary surface is generated. The surface is described by the same NURBS’ basis functions which are employed for approximating the displacement function in the isogeometrical analysis. The performance of the method is demonstrated by comparison of the exact and approximate energy error norms for a couple of examples that the exact solution is available. It seems that the proposed method can be used as a suitable approach for error estimation in the isogeometrical analysis method.

A meshless approach, collocation discrete least square (CDLS) method, is extended in this paper, for solving elasticity problems. In the present CDLS method, the problem domain is discretized by distributed field nodes. The field nodes are used to construct the trial functions. The moving least-squares interpolant is employed to construct the trial functions. Some collocation points that are independent of the field nodes are used to form the total residuals of the problem. The least-squares technique is used to obtain the solution of the problem by minimizing the summation of the residuals for the collocation points. The final stiffness matrix is symmetric and therefore can be solved via efficient solvers. The boundary conditions are easily enforced by the penalty method. The present method does not require any mesh so it is a truly meshless method. Numerical examples are studied in detail, which show that the present method is stable and possesses good accuracy, high convergence rate and high efficiency.