Azam Tafreshi

Buckling and post-buckling analysis of composite cylindrical shells with cutouts subjected to internal pressure and axial compression loads

A numerical study using the finite element method has been carried out to investigate the response of composite shells with cutouts subjected to internal pressure and axial compression. The CAE package ABAQUS, has been employed for the analysis. The buckling and post-buckling responses in a series of shells with different size cutouts are presented. The numerical results show that the buckling load decreases as the size of the cutout is increased and the buckling load increases as the internal pressure is increased. Results also show that for the equivalent cutout areas the cutout with higher width (measurement in the circumferential direction) creates a lower buckling load. The results indicate that the response of a compression loaded cylinder with a cutout is influenced by the internal pressure, cutout area and orientation.

Global buckling behaviour and local damage propagation in composite plates with embedded delaminations

Finite element models were developed to study global, local and mixed mode buckling behaviour of composite plates with embedded delaminations under compression. The global modelling results were compared with corresponding experimental results. It is shown that the numerical results for embedded delaminations agree very well with the experimental results, whereas the difference between the results was high for delaminations located at the edge of the plates. It is also shown that at lower loading levels the interaction of global and local buckling is negligible. At higher loading levels the strain energy release rate distribution and the delamination growth potential at the delamination front strongly depend on the shape of the debonded region and the local buckling mode. It was observed that the local buckling mode was highly influenced by the laminate stacking sequence. In the course of global buckling, a parametric study was carried out to investigate the influence of the delamination size, shape and alignment of a series of composite plates.

Shape design sensitivity analysis of 2D anisotropic structures using the boundary element method

A directly differentiated form of boundary integral equation with respect to geometric design variables is used to calculate shape design sensitivities for anisotropic materials. An optimum shape design algorithm in two dimensions is developed by the coupling of an optimising technique and a boundary element stress analyser for stress minimisation of anisotropic structures. Applications of this general-purpose program to the optimum shape design of bars and holes in plates with anisotropic material properties are presented.

Stress analysis of three-dimensional contact problems using the boundary element method

Abstract This paper presents a technique based on the boundary element method [1] to analyse three-dimensional contact problems. The formulation is implemented for the frictionless and infinite friction conditions. Following a review of the basic nature of contact problems, the analytical basis of the direct formulation of the boundary element method is described. The numerical implementation employs linear triangular elements for the representation of the boundary and variables of the bodies in contact. Opposite nodal points in similar element pairs are defined on the two surfaces in the area which are expected to come into contact under the increasing load. The use of appropriate contact conditions enables the integral equations for the two bodies to be coupled together. Following an iteration procedure, the size of the contact zone is determined by finding a boundary solution compatible with the contact conditions. Different examples have been analysed in order to verify the applicability of the proposed method to various contact situations. The results have been compared with those obtained using the finite element method in conjunction with the ABAQUS [2] and IDEAS [3] packages which are shown to be in good agreement.

Numerical analysis of thin torispherical end closures

A numerical analysis study using the finite element method has been carried out to investigate the design sensitivity of thin torispherical end pressure vessels. A detailed dimensional survey of a number of thin full-size production-quality torispherical ends has shown that actual dimensions may differ significantly from nominal dimensions. The sensitivity of the end stresses to particular forms of shape imperfections were analysed numerically and compared with those in the corresponding ‘perfect’ end which are presented here in more detail. The effect of these shape imperfections on buckling pressure is also investigated. The buckling pressure and stresses in a series of torispherical ends are evaluated to show the effect of variation of shape parameters in these ends. It is also shown that, for stainless steel ends, small displacement analysis agrees much better with the experimental results than do those given by the large displacement analysis.

Optimum Shape Design of Composite Structures Using Boundary-Element Method.

The boundary-element method, combined with a numerical optimization algorithm, has been employed for the shape optimization of two-dimensional anisotropic structures. To find the optimum shape of a structure with the highest stiffness, the elastic compliance of the structure has been minimized subject to constraints upon stresses, weight, and geometry. The optimum shapes of a series of anisotropic structures are obtained for maximum stiffness and minimum weight and stress, for specified loading conditions. The results are compared with the optimum shapes, that were already created by the minimization of the structural weight while satisfying certain constraints upon stresses and geometry. A directly differentiated form of boundary integral equation with respect to geometric design variables is used to calculate shape design sensitivities of anisotropic materials. Because of the nonlinear nature of the mean compliance, weight, and stresses, the numerical optimization algorithm used is the feasible direction method, together with the golden section method for the one-dimensional search. Hermitian cubic spline functions are used to represent boundary shapes that offer considerable advantages in fitting a wide range of curves and in the automatic remeshing process. Five example problems with anisotropic material properties are presented to demonstrate the applications of this general-purpose program.

SIF evaluation and stress analysis of drillstring threaded joints

Abstract In a study by Tafreshi and Dover [1–3] stress analysis of drillstring threaded joints under axial, bending and torsion loadings was carried out using the finite element method. Stress concentrations were determined on a variety of threaded joints in order to predict the fatigue life of these components. Here some of those results are presented in more detail. In this study it is shown that in the case of bending, axisymmetric solid elements with non-linear, asymmetric deformation with fourier interpolation can be employed which reduces the computational time and modelling in comparison with the full three-dimensional analysis. Quasi semi-elliptic circumferential internal and external cracks in tubes with the same dimensions of the pins or boxes of a series of joints are modelled and analysed using the J -integral method. The result of this fully three-dimensional crack analysis shows the same trend as experimental [3] and published results [4] .

Fracture mechanics analysis of composite structures using the boundary element shape sensitivities

Based on the shape sensitivity analysis ofmultiregion domains using the boundary element method, stress intensity factors of the cracks of arbitrary geometric shapes in anisotropic elastic solids are calculated. The results obtained agree very well with the existing analytical or numerical solutions. In contrast to the J-integral method, which would require the computation of stresses and strains at a series of internal points around the crack for evaluation of the path-independent integrals, the fracture mechanics parameters are evaluated here by direct differentiation of the structural response for a multiregion domain. Therefore, the present method is computationally more accurate and efficient. The length of the crack of arbitrary geometric shape is treated as the shape design variable. Then the shape variable is associated with the coordinates of a series of boundary nodes located on the crack surface. Thus, the relevant velocity terms are applied together in the sensitivity analysis with respect to that variable to determine the energy release rate, which is the total derivative of the strain energy with respect to crack length. Five example problems with anisotropic material properties are presented to validate the applications of this formulation. The results show that although the stress intensity factor is of fundamental importance in the prediction of brittle failure using linear elastic fracture mechanics, the direct evaluation of the strain energy release rate would easily characterize the crack instability of a loaded laminated composite for different fiber orientations. The results show that the strain energy release rate is highly influenced by the fiber orientation of the composite lamina. Therefore, a laminate can be tailored to crack-growth resistance.

Delamination buckling of composite cylindrical shells

Publisher Summary Laminated composites are gaining importance in aircraft structural applications as a result of their very high strength-to-weight and high stiffness-to-weight ratios. However, owing to the lack of through-the-thickness reinforcement, structures made from these materials are highly liable to failures caused by delamination. Therefore, within a design process, a structures resistance to delamination should be addressed to maximize its durability and damage tolerance. Delaminations in composite cylinders might be due to manufacturing defects, transportation impacts and environmental effects during their service life. The presence of delaminations leads to a reduction in the overall buckling strength of the structure. Thin-walled circular cylindrical shells are very often loaded in such a way that the three buckling membrane forces: axial compression, circumferential compression and shear, are not applied individually but in combination. Therefore, a designer not only has to consider the buckling characteristics of a cylindrical shell under fundamental loading conditions, but also the buckling interactions.

Delamination Buckling of Composite Cylindrical Panels Under Axial Compressive Load

Composite cylindrical shells and panels are widely used in aerospace structures. Delaminations within the composite structure reduce the compressive strength of laminates, and often result because of damage incurred during manufacturing and in-service use. This paper investigates the buckling behaviour of laminated cylindrical panels loaded in axial compression using the finite element method. The use of three-dimensional finite elements for predicting the delamination buckling of these structures is computationally expensive. Here the analysis has been carried out using a layerwise shell finite element based on the first-order, shear deformation theory. Contact elements were placed between the delaminated regions to avoid physical interpenetration of the elements. It is shown that through-the-thickness delamination can be modelled and analysed effectively without requiring a great deal of computing time and memory. Delamination shapes considered in this study were square and rectangular — extended longitudinally over the entire length or extended along the entire circumference of the panel. Some of the results were compared with the corresponding analytical results which were in good agreement. The most influential parameters for a given laminated panel were the size of the delamination and its through-the-thickness position. The effect of the curvature on the global buckling strength of a delaminated panel was also studied. Depending on the size and through the thickness position of delaminations, three different modes of buckling behaviour occur. The local mode occurs when the delamination is near the free surface of the laminate and the area of the delamination is large. The global mode occurs when the delamination is deeper within the laminate and has a small area. The mixed mode is a combination of global and local modes.© 2004 ASME