A practical method of cycle jumps for cyclically loaded structures |
We consider failure mechanisms in multilayered systems subjected to cyclic loading, using an accelerated computational approach in the context of the finite element method. In particular, this research is focused on simulating the cyclic degradation in films and coatings with evolving material properties. An accelerated finite element computation scheme is developed and implemented in the commercial software ABAQUS. The scheme takes advantage of the long term evolution of the structure's cyclic response by replacing portions of finite element computations by cycle jumps. The accuracy of the solution is maintained through a control function which calculates the appropriate jump length based on the non-linearity of the predicted long term evolution and a set of user supplied control parameters. Computational efficiency is assessed based on applications to finite element models of coating systems. Accuracy of obtained solutions is evaluated by comparisons with fully conducted finite element simulations. It is shown that a reduction of 60-70% of the computational time can be obtained on account of a small accuracy loss.
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| Figure: Solution given by the proposed cycle-jump scheme vs. the complete cycle-by-cycle FEM solution. |
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Details in: Cojocaru, D., Karlsson, A.M., A simple numerical method of cycle jumps for cyclically loaded structures. International Journal of Fatigue, 2006. 28 (12): p. 1677-1689.
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An object-oriented approach for modeling and simulation of crack growth in cyclically loaded structures
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This work relates to developing an object-oriented modeling frame for simulating 2D crack propagation due to cyclic loadings. Central to the approach is that the crack propagates when a user defined propagation criterion is fulfilled, i.e. the crack propagation rate is not prescribed but predicted. The approach utilizes the commercial FE software ABAQUS and its associated Python based scripting interface. The crack propagation is simulated by a generalized node release technique. If the propagation criteria are satisfied in the end of a cycle, the crack is allowed to propagate. The incremental crack growth is inferred from an iterative investigation of the propagation criteria. The propagation criteria are user-defined, and can be based on any set of quantities that can be obtained from the simulations. The modeling frame has the advantage of generating fully parametric FE models and can be used for problems ranging from classical fracture mechanics specimen to multilayered structures and structures requiring geometric modeling of the micro-structural components.
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Application of the above modeling frame for simulating: |
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Multi-layered systems
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Coated particles with debonded coating layer
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Systems with grain-like micro-structure
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Structures containing random voids (figures showing crack nucleation and propagation between voids)
 
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Thermal barrier coating systems (figure showing crack developing in the bond-coat layer of TBC)
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Details in: Cojocaru, D., Karlsson, A.M., An object-oriented approach for modeling and simulation of crack growth in cyclically loaded structures Advances in Engineering Software (in press, 2008) . |
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| An ABAQUS plug-in for computing the effective elastic properties |
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Using this ABAQUS plug-in, the effective shear and bulk modulus can be computed for macroscopically isotropic media based on various analytical methods including: Voigt-Reuss bounds, Hashin-Shtrikman bounds, Mori-Tanaka method, self-consistent and differential method. XY Data are created for each of the selected method and can be further plotted in the Visualization module in ABAQUS/CAE. Download: for ABAQUS/CAE v.6.6 and ABAQUS/CAE v6.7
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Generation of Representative Volume Elements (RVE) for materials with micro-structure |
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Predicting the effective properties of media with random microstructure plays an important role in improving the material design. Numerical computation of effective properties can be performed using finite element based approaches. However, before the finite element model is generated a description of the 3D RVE microstructural architecture is needed. Here, we focused on developing algorithms for generating 3D RVE of media with random configurations of various microstructural features ( A- disks, B-fibers, C-spheres).
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Finite element modeling of media with microstructure |
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Representative volume elements of media with random configurations of microstructural features can be generated in a parametric manner. Various probabilistic distribution can be used. Three-dimensional (3D) FEM models of the random configurations are generated automatically. Using numerical averaging the effective properties can be computed. The figure below shows the Mises stress provided by FEM when the RVE is subjected to constant macro-strain for random configurations of micro-structural features: A-disks, B-fibers, C-spheres, D-a cut through the RVE with the particles removed, E-a cut through an RVE containing penny-shaped cracks and F-a random configuration of penny-shaped cracks
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| Application of finite element weight functions to cracked disks |
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Stress intensity factors are computed for a cracked disk using a practical approach based on the weight function method. This approach employs the finite element method in order to obtain an approximation of the displacements solution for a reference case. Further, the stress intensity factor for the reference case and the weight function equation are established. Using the weight function equation, the stress intensity factor can be computed for various other loading conditions. The results given by the proposed approach were found to agree with those in the literature.
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Details in: Cojocaru, D., S.D. Pastram, and P.M.S.T. de Castro, Finite element weight function application for a cracked disk. International Journal of Fracture, 2002. 116 (1): p. L9-L14.
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