Five angle-ply laminates ([0₄₈], [(±10)₁₂]_s, [(±20)₁₂]_s, [(±30)₁₂]_s, and [(±45)₁₂]_s with central circular holes were tested under uniaxial compressive loading. The results from these tests show that the [(±45)₁₂]_s laminate exhibited plastic deformation, with ultimate applied strains exceeding -1%. All other laminates failed in a brittle manner with ultimate strains of less than -0.5%. A three-dimensional finite element stress analysis was performed for the same laminates. A failure analysis based on the three-dimensional stress tensor polynomial predicted that failure will initiate at the intersection of the ply interface with hole edge for all laminates, and be due to a combination of the out-of-plane and in-plane shear stresses. Use of the state of stress directly on the hole edge in the prediction of laminate failure resulted in predictions of laminate ultimate strengths which were less than experimentally observed values by as much as a factor of ten. In addition, symmetry considerations for three-dimensional finite element modelling of composite laminates are discussed, and a two-dimensional finite element model based on shear-deformable plate theory is predicted. / M.S.

Vlasov’s one-dimensional structural theory for thin-walled open section bars was originally developed and used for metallic elements. The theory was recently extended to laminated bars fabricated from advanced composite materials. The purpose of this research is to provide a study and assessment of the extended theory. The focus is on flexural and torsional-flexural buckling of thin-walled, open section, laminated composite columns. Buckling loads are computed from the theory using a linear bifurcation analysis, and are compared to available experimental data. Also, a geometrically nonlinear beam column analysis by the finite element method is developed from the theory. Results from the nonlinear compression response analysis are compared to limited available test data. The merits of the theory and its implementation are discussed. / Master of Science / incomplete_metadata

A method for reinforcing the free edges of a symmetric 11-ply graphite-epoxy laminate by adding a one-layer Kevlar-epoxy edge cap is studied. Generalized plane strain finite element analysis is used to predict that interlaminar stresses are reduced when an edge cap is added to the laminate. Different edge cap designs are evaluated. A three-dimensional composite failure criterion and finite element analysis are used in a progressive laminate failure analysis to predict the failure load of the reinforced luminate. The results of an experimental program are presented. Cappad laminates are shown to be on average 130% to 140%. stronger than uncapped laminates when subjected to static tensile or tension-tension fatigue loading. In addition, the coefficient of variation of the static tensile failure load decreases from 24% to 8% with the addition of edge caps. The predicted failure load which is calculated with the finite element results is 10%. lower than the actual failure load. For both the capped and the uncapped laminates, actual failure loads are much lower than those predicted using classical lamination theory stresses and a 2-D failure criterion. Possible applications of the free edge reinforcement concept are given. Suggestions for future research are made. / M.S.

A change in laminate thickness due to terminating internal plies acts as a stress riser for both intralamina and interlaminar stresses. This laminate configuration is referred to as a ply drop. The linear elastic, three-dimensional stress distributions in the vicinity of a ply drop are determined for a graphite-epoxy laminate subject to axial tension and compression by a finite element analysis. It is shown that the interlaminar stresses have a maximum magnitude at the ply drop-off, and decrease proceeding away from the drop-off. Two modes of failure initiation are analyzed. In the pure resin regions surrounding the dropped plies, the maximum stress criterion is assumed to govern failure. The Tsai-Wu criterion is used for intralamina failure prediction. The influence of two laminate lay-ups and a variety of ply drop geometries on the response and failure are presented. / M.S.

The characteristics of thermally-induced transverse cracks in T300/5208 graphite-epoxy cross-ply and quasi-isotropic laminates were investigated both experimentally and analytically. The formation of transverse cracks and the subsequent crack spacing present during cooldown to -250°F (116K) and thermal cycling between 250 and -250°F (116 and 394K) was investigated. The state of stress in the vicinity of a transverse crack and the influence of transverse cracking on the laminate coefficient of thermal expansion (CTE) was predicted using a generalized plane-strain finite element analysis and a modified shear-lag analysis. It is shown that a majority of the cross-ply laminates experienced transverse cracking during the initial cool-down to -250°F whereas the quasi-isotropic laminates remained uncracked. All cross-ply laminates and the [0/±45/90]_s quasi-isotropic laminate exhibited transverse cracking following 20 thermal cycles. The uniformity of crack spacing increased with an increasing number of thermal cycles. The cross-ply laminates exhibited a rather sharp drop in CTE at crack densities less than 50 cracks/in. (19.7 cracks/cm) whereas the quasi-isotropic laminates exhibited a smaller decrease in CTE. The in situ transverse strength of the 90° layers was more than 1.9 times greater than the transverse strength of the unidirectional 90° material for all laminates investigated. / M.S.