Three-dimensional, nonlinear viscoelastic analysis of laminated composites : a finite element approach Public Deposited

http://ir.library.oregonstate.edu/concern/graduate_thesis_or_dissertations/j96023410

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  • Polymeric composites exhibit time-dependent behavior, which raises a concern about their long term durability and leads to a viscoelastic study of these materials. Linear viscoelastic analysis has been found to be inadequate because many polymers exhibit nonlinear viscoelastic behavior. Classical laminate theory is commonly used in the study of laminated composites, but due to the plane stress/strain assumption its application has been limited to solving two dimensional, simple plate problems. A three dimensional analysis is necessary for the study of interlaminar stress and for problems involving complex geometry where certain local effects are important. The objective of this research is to develop a fully three-dimensional, nonlinear viscoelastic analysis that can be used to model the time-dependent behavior of laminated composites. To achieve this goal, a three-dimensional finite element computer program has been developed. In this program, 20-node isoparametric solid elements are used to model the individual plies. The three-dimensional constitutive equations developed for numerical calculations are based on the Lou-Schapery one-dimensional nonlinear viscoelasticity model for the uniaxial stress case. The transient creep compliance in the viscoelastic model is represented as an exponential series plus a steady-flow term, which allows for a simplification of the numerical procedure for handling hereditary effects. A cumulative damage law for three dimensional analysis was developed based on the Brinson-Dillard two-dimensional model to predict failure initiation. Calculations were performed using this program in order to evaluate its performance in applications involving complex structural response. IM7/5260-H Graphite/Bismaleimide and T300/5208 Graphite/Epoxy were the materials selected for modeling the time-dependent behavior. The cases studied include: 1) Tensile loading of unnotched laminates; 2) bending of a thick laminated plate; and 3) tensile loading of notched laminates. The analysis emphasized the study of the traction-free edge-effect of laminated composites, stress distribution around a circular hole, and stress redistribution and transformation in the layers. The results indicate that the stress redistributions over time are complicated and could have a significant effect on the long-term durability of the structure.
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