Fiber orientation angle stands out as one of the most effective design variables in the design optimization of composite structures. During the manufacturing of the composite structures, one can change the fiber orientation according to the specific design needs and constraints to optimize a pre-determined performance index. Fiber placement machines can place different width tows in curvilinear paths resulting in continuous change of the fiber orientation angle in a layer of the composite structure. By allowing the fibers to follow curvilinear paths in the composite structure, modification of load paths within the laminate can be obtained. Thus, more favorable stress distributions and improved laminate performance can be achieved. Such structures are called as variable stiffness composites structure. This thesis presents a fundamental study on the discrete fiber angle and continuous fiber path optimization of composite structures. In discrete fiber angle optimization, application of different analysis/optimization tools is demonstrated for optimum fiber angle optimization at the element level for both orthotropic and laminated composite structures. In the continuous fiber path optimization, which can be produced with fiber placement machines, optimized fiber paths are determined for different case studies. Continuous fiber path optimization is performed by means of an interface code that is developed. It is hard to find the global optimum for complex optimization problems with hundreds of design variables. In order to find the global optimum solution for such complex optimization problems, a gradient based optimization algorithm is not appropriate because there will be a lot of local minima for the problem and gradient based optimization algorithms may be stuck at the local minimums. Therefore, an evolutionary algorithm is a better solver for such kind of complex optimization problems. In this thesis, genetic algorithm, an evolutionary algorithm, in MATLAB Optimization Toolbox is used for the optimizer and commercial finite element program Nastran is used for the structural solver. For the continuous fiber path optimizations these two programs are integrated with the interface code that is developed. Manufacturing constraints of a typical fiber placement machine is also included in the constraint definition of continuous fiber path optimization. By coupling of Nastran finite element solver and MATLAB genetic algorithm tool, with the manufacturing constraint for the fiber placement machines, the first buckling load of a continuous fiber composite plate is increased %22 with respect to a composite plate with zero degree orientations.
| Identifer | oai:union.ndltd.org:METU/oai:etd.lib.metu.edu.tr:http://etd.lib.metu.edu.tr/upload/12614127/index.pdf |
| Date | 01 February 2012 |
| Creators | Inci, Hasan |
| Contributors | Kayran, Altan |
| Publisher | METU |
| Source Sets | Middle East Technical Univ. |
| Language | English |
| Detected Language | English |
| Type | M.S. Thesis |
| Format | text/pdf |
| Rights | To liberate the content for METU campus |
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