Papers by Author: Bijan Mohammadi

Abstract: In this paper multiple fatigue cracks propagation are simulated in two-dimensional plates. Since re-meshing the cracked bodies in each increment of crack extension is a time-consuming and complicated procedure, numerical simulation of mixed-mode crack propagation with FEM is a difficulty. For this purpose, a FEM software is programmed and mesh refinement in each increment of crack is performed by Delaunay Refinement Algorithm. Using different refinement methods, complex boundaries such as multiple cracks and discontinuities which are closed together are easily refined by this algorithm. Crack propagation path is predicted using domain form of J-integral. Modified tensile stress (MTS) criterion is used to predict the crack propagation path in each increment. Different numerical examples illustrate the validation and reliability of present software.

Abstract: Due to discontinuity of mechanical properties in composite laminates, failure occurs in different damage mechanisms. Delamination growth of adjacent layers is a major failure mechanism in laminates with various layup configurations. Pre existing delamination may initiate in composite laminate before use, due to impact in assembly and fabrication process. Cyclic compressive loading may cause delamination growth due to both post-bucking behavior and fatigue nature of loading. In this paper, a 3D mixed-mode interface element model has been developed to simulate the growth of multiple delaminations under compressive cyclic loading. For this purpose, the presented model should be able to handle the geometry nonlinearity of post-buckling and material nonlinearity of cohesive zone constitutive law under cyclic loading at interfaces. Because of mixed-mode condition of stress field at the delamination-front of post-buckled laminates, a mixed-mode bilinear constitutive law has been used as user material in this model. Paris Law has been used to relate the energy release rate to the fatigue crack growth in cohesive zone. A composite laminate with pre-existing delamination under buckling load, available from the literature has been reproduced with the present approach. Finally, laminates containing multiple delaminations in various interface layers have been analyzed under compressive fatigue loading. It is shown that the pre-existing delamination with more depth from the surface of laminate causes more initial static and fatigue delamination growth rate.

Abstract: In this study, finite element method is used to investigate the fracture analyses, crack growth trajectory and fatigue life of curved stiffened panels repaired with composite patches subjected to combined tension and shear cyclic loadings. For this purpose, 3-D finite element modeling are performed for consideration of real 3-D crack-front in general mixed-mode conditions. Contact elements are used between the crack surfaces on two crack sides to prevent interferences of crack surfaces and a complementary program was developed to handle the automatic fatigue crack growth modeling. The effects of various patch layups and shear-tension loading ratios on fracture parameters of the aluminum panel are investigated. It is shown that in low shear to tension ratios like 0.4, the patch layup of [90]₄ (perpendicular to the initial crack) is more efficient than the patch with layups angle along the tension loading. As the shear to tension ratio increases, effect of patch layups with orientations of almost perpendicular to the crack trajectory on fatigue crack growth life is increased comparing with the patch layups parallel to the tension orientation like [90]₄.

Abstract: This paper is concerned with the free vibration problem for micro/nano beams modelled after Eringen’s nonlocal elasticity theory and Euler beam theory. The small scale effect is taken into consideration in the former theory. The natural frequencies are obtained using the Hamilton’s principle and Chebyshev polynomial functions. The present method, which uses Rayleigh–Ritz technique in this paper, provides an efficient and extremely accurate vibration solution of micro/nano beams where the effects of small scale are significant. Numerical results for a variety of some micro/nano beams with various boundary conditions are given and compared with the available results wherever possible. Through the comparison of the results and the appropriate discussion, the knowledge of the level of capability of the developed method is promoted.

Abstract: In this paper a solid like interface element along with a fatigue constitutive law is used to study the damage behaviour of holed composite laminates under cyclic loading. For this purpose a user element routine and a material routine was developed include the interface element and to handle the formulation of progressive fatigue damages. The developed procedure is used to predict delamination initiation and growth in mixed-mode condition for a typical composite laminate. The obtained results for damage and stresses are compared with the available results in the literature.

Abstract: In this paper, the developed new micro-meso method by the authors is used for the edge-effects analyses of various angle-ply laminates such as [10/-10]2s and [30/-30]2s. It is shown that the obtained stress-strain behaviors of laminates are in well agreement with the available experimental results. The stress variations through the laminate thickness and near the free edges are also computed and compared with the available CDM results.

Abstract: In this paper, crack growth of thin-walled cylinders containing an initial crack under acoustic plane wave is analyzed. A coupled air-structure modeling is considered for the analyses. For this purpose, a methodology for computational modeling of the fatigue crack growth in cracked cylinders using finite element method is presented. Effects of various parameters such as crack length increment and boundary conditions on the crack growth rate are investigated.

Abstract: This paper presents the elastic buckling behavior of nonlocal micro- and nano- Timoshenko rods/tubes based on Eringen’s nonlocal elasticity theory. The critical buckling loads are obtained using the theorem of minimum total potential energy and Chebyshev polynomial functions. The present method, which uses Rayleigh–Ritz technique in this paper, provides an efficient and extremely accurate buckling solution. Numerical results for a variety of some micro- and nano-rods/tubes with various boundary conditions are given and compared with the available results wherever possible. Through the comparison of the results and the appropriate discussion, the knowledge of the level of capability of the developed method is promoted. The small scale effects on the buckling loads of rods/tubes are determined and discussed.

Abstract: The aim of the present study is evaluation of the element-free Galerkin method (EFGM) in progressive damage analyses of composite laminates. For this purpose, an orthotropic EFGM formulation is employed which is based on the first-order shear deformation theory (FSDT). In progressive damage analysis, the Hashin’s type failure criteria and their degradation rules are used. The obtained damage results from EFGM are compared with the experimental and FEM results.

Abstract: To predict the progressive damages including the large delamination growth in composite laminates, a new interface de-cohesive constitutive law is developed which is compatible with 3D continuum damage mechanics (CDM). To avoid the difficulties of 3D mesh generation and 3D interface modeling between the layers, the interface element is implemented in the Reddy’s full layer-wise plate theory. An angle-ply laminate is analyzed to evaluate the developed CDM+Interface procedure in edge delamination initiation and evolution at final stage of CDM damage progress.