Thesis (M.S.)--West Virginia University, 2005. / Title from document title page. Document formatted into pages; contains x, 65 p. : ill. (some col.). Includes abstract. Includes bibliographical references (p. 63-65).

Thesis (M.S.)--West Virginia University, 2006. / Title from document title page. Document formatted into pages; contains xi, 72 p. : ill. (some col.). Includes abstract. Includes bibliographical references (p. 69-72).

This work begins by establishing the necessary and sufficient conditions for hygrothermal stability of composite laminates. An investigation is performed into the range of coupling achievable from within all hygrothermally stable families. The minimum number of plies required to create an asymmetric hygrothermally stable stacking sequence is found to be five. Next, a rigorous and general approach for determining designs corresponding to optimal levels of coupling is established through the use of a constrained optimization procedure. Couplings investigated include extension-twist, bend-twist, extension-bend, shear-twist, and anticlastic. For extension-twist and bend-twist coupling, specimens from five- through ten-ply laminates are manufactured and tested to demonstrate hygrothermal stability and achievable levels of coupling. Nonlinear models and finite element analysis are developed, and predictions are verified through comparison with test results. Sensitivity analyses are performed to demonstrate the robustness of the hygrothermal stability and couplings to deviations in ply angle, typical of manufacturing tolerances. Comparisons are made with current state-of-the-art suboptimal layups, and significant increases in coupling over previously known levels are demonstrated.

In light-frame residential construction wood-based composite panels used externally or internally are exposed to relative humidity and/or temperature changes. The subsequent moisture content change of the panels will result in two types of deformations as follows: 1. elastic deformation of the panel due to the constraint, 2. warpage of the panel due to the unbalanced expansion of the layers. Such deformations can cause unacceptable serviceability problems in light-frame wood construction. A model was developed to predict quantitatively the global deflection of wood-based composite panels exposed to relative humidity changes. The model was based on the constitutive relationship of the Classical Lamination Theory and the thermal stress analogy in composites. As an alternative solution, the applicability of the eccentrically loaded column formula was evaluated. The developed models were experimentally validated for OSB and plywood sheathing. Test variables included the panel type, exposure (symmetric and non-symmetric MC gradient) and specimen configuration (single span, multiple span). The comparison of measured and predicted deflections are presented. The important elastic and hygroscopic material properties were acquired through testing. Statistical analyses of test results are discussed. The uncertainty analysis was used to make statistical inference comparing the means of measured deflection to the uncertainty interval of predictions. Good agreement between predicted and measured deflections was found for single span test structures. Also, for double span structures the models predicted the experimental response fairly veil. Uncertainties in Me measurements made the prediction less reliable when symmetric moisture content gradient developed during the exposure. Due to its lower variability in material properties, the response of OSB sheathing to moisture content changes is more predictable. / Ph. D.

In this study, a Jk-integral based computational method will be developed to conduct fracture analysis of fibrous composite laminates that possess variable fiber spacing. This study will be carried out for the fibrous composites exposed to not only thermal but also hygroscopic boundary condition, which results hygrothermal load. Formulation of the Jk-integral will be carried out by using the constitutive relations of plane orthotropic hygrothermoelasticity. One of the most important challenges of this study is to change Jk-integral formulation into domain independent form, because dealing with infinitely small domains in solving the integral would be frustrating. Developed form of Jk-integral will be merged to ANSYS, a finite element analysis software. Numerical results will be generated so as to assess the influence of variable fiber spacing on the modes I and II stress intensity factors, energy release rate, and the T-stress. For validation and comparison, some of the results are also obtained using Displacement Correlation Technique (DCT).