A viable aspect of fiber-reinforced composites is their elastic tailoring ability. Anticlastic curvature is one example of elastic tailoring that occurs with an unsymmetric cross-ply layup. Residual stresses resulting from differences in coefficients of thermal expansion and elastic properties in each lamina can cause large out-of-plane deformations. In the case of thin unsymmetric cross-ply laminates under thermal curing load, a cylindrical shape is observed because of this inherent geometrical nonlinearity as opposed to the saddle shape that classical lamination theory predicts. In this article, a finite element approach, using ABAQUS 2 , is implemented in order to predict the unsymmetric cross-ply laminate shapes under thermal curing stresses and understand the underlying limit point instability. Numerical results for curvatures of the predicted shapes are in agreement with published experimental and analytical data. The stability of the cylindrical laminates is also investigated. Depending on the aspect ratio of the rectangular laminate, a cylindrical shape may snap-through from its current stable configuration to another stable cylindrical shape with a different curvature. In particular, both the critical aspect ratio where snap-through will cease to occur and the buckling load are reported.
This work could not have been completed without the help of many people to whom I would like to give my thanks. I would like to express my deepest gratitude to Dr. Erian Armanios for his patient guidance throughout my life at Georgia Tech. This work would have never been possible without his immeasurable professional and emotional support, encouragement and above all his trust. I am also grateful to Dr. D. Stefan Dancila for his support in this work. His valuable critiques and guidance helped me improve my work. I also would like to acknowledge Dr. Olivier A. Bauchau. I am thankful to him for providing the DYMORE and VABS programs. I would like to thank Dr. Dewey H. Hodges and Dr. Abdul-Hamid Zureick for their willingness to serve on my thesis committee. This work has been completed under a grant from the National Rotorcraft Technology Center. I am sincerely thankful for their support. Additionally I would like to thank my officemates, true friends, Samer Tawfik, Xinyuan Tan and Dr. Roxana Vasilescu for their technical and emotional support. I sincerely thank my parents Nedime and Engin Ozbay. Without their love, support and sacrifices, I would not be who I am now. Thank you for your belief in what I could achieve eventually. Finally I would like to express my great gratitude to my lovely wife Tuba, for her unconditional love, support and confidence. It is a huge understatement to say that I could achieve this without her. I love her. The rest of my days are special because of her. v TABLE OF CONTENTS
Optimum stacking sequences for extension-twist coupled composites under thermal stresses are determined. An optimization technique is utilized to determine the optimum stacking sequences. An alternative method is developed based on the identification of a parameter controlling the optimum stacking sequence. Comparison of associated extension-twist coupling values demonstrates their accuracy. It is observed that as the number of plies increases, the optimum stacking sequence follows an asymptotic behavior and that an angle ply system does not represent an optimum stacking sequence. The results are validated through manufacturing of candidate laminates.
This paper discusses a method aiming at providing tailored education that would increase students understanding of the material and enhance their success. The method is being applied to COE 3001 (Mechanics of Deformable Bodies), the second structures course in Aerospace Engineering undergraduate program at the Georgia Institute of Technology. COE 2001 (Statics) is the first class in structures and introduces students to the elements of statics in two and three dimensions, free-body diagrams, distributed loads, centroids and friction. In COE 3001 the students are further exposed to stress and strain analysis applied to beams, vessels, pipes, and combined loading, stress and strain transformations, beam deflection as well as column buckling. The method consists of weekly problem sessions, a test evaluation concept and a tailored workplan. The weekly problem sessions are conducted by teaching assistants. The students are given a chance to discuss with the TA a set of suggested textbook problems assigned by the instructor as well as any other recommended problems pertaining to the topics covered in the lectures. These include practical problems that are recommended by the instructor as bonus problems. The students are encouraged to submit their work on the suggested problems a few days after the problem sessions for evaluation by the TA. During this period the TA maintain an open-door policy where students can have one-on-one interaction.
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