Composite materials are being widely used in light weight structural applications due to their high specific stiffness and strength properties. However, predicting their mechanical behaviour accurately is a difficult task because of the complicated nature of these heterogeneous materials. This behaviour is not easily modeled with most of existing macro mechanics based models. Designers compensate for the model unknowns in failure predictions by generating overly conservative designs with relatively simple ply stacking sequences, thereby mitigating many of the benefits promised by composites. The research presented in this dissertation was undertaken with the primary goal of providing efficient methodologies for use in the design of composite structures considering inherent material variability and model shortcomings. A micromechanics based methodology is proposed to simulate stiffness, strength, and fatigue behaviour of composites. The computational micromechanics framework is based on the properties of the constituents of composite materials: the fiber, matrix and fiber/matrix interface. This model helps the designer to understand in-depth the failure modes in these materials and design efficient structures utilizing arbitrary layups with a reduced requirement for supporting experimental testing. The only limiting factor in using a micromechanics model is the challenge in obtaining the constituent properties. The overall novelty of this dissertation is to calibrate these constituent properties by integrating the micromechanics approach with a Bayesian statistical model. The early research explored the probabilistic aspects of the constituent properties to calculate the stiffness characteristics of a unidirectional lamina. Then these stochastic stiffness properties were considered as an input to analyze the wing box of a wind turbine blade. Results of this study gave a gateway to map constituent uncertainties to the top-level structure. Next, a stochastic first ply failure load method was developed based on micromechanics and Bayesian inference. Finally, probabilistic SN curves of composite materials were calculated after fatigue model parameter calibration using Bayesian inference. Throughout this research, extensive experimental data sets from literature have been used to calibrate and evaluate the proposed models. The micromechanics based probabilistic framework formulated here is quite general, and applied on the specific application of a wind turbine blade. The procedure may be easily generalized to deal with other structural applications such as storage tanks, pressure vessels, civil structural cladding, unmanned air vehicles, automotive bodies, etc. which can be explored in future work. / Graduate / 0548 / enginer315@gmail.com

The objective of the present work is to investigate the behavior of laminated fiber -reinforced polymer matrix composite shell structures under bending load with the help of a modified finite element computer code which was previously developed for the analysis of pseudo-layered single material shells. The laminates are assumed to be orthotropic and the formulation is adapted to first order shear deformation theory. The aim is to determine the large deformation characteristics numerically, and to predict the modes of failure by the illustration of the critical elements of the model. Therefore, several failure theories are also integrated to the code to detect first ply failure. Triangular shell elements are used and all the related data are generated from the mid-plane. Laminates under transverse loading are analyzed through several boundary conditions and ply orientations. To verify the numerical results obtained, a commercial finite element program is used to compare the outputs of the study, and the comparison is found to have shown good agreement. The onset of damage is investigated by using different failure criteria consisting of maximum stress, Tsai-Wu, and Tsai- Hill theories and close results are obtained.

The objective of this thesis is to investigate the progressive failure behavior of laminated fiber reinforced composite shell structures under different loading conditions. The laminates are assumed to be orthotropic and the first order shear deformation theory is applied. Three-node layered flat-shell elements are used in the analysis. To verify the numerical results obtained, experimental and analytical results found in literature are compared with the outputs of the study, and the comparison is found to have shown good agreement with the previous work. Rectangular graphite/epoxy composite laminates under transverse loading are analyzed through several boundary conditions and stacking sequences. Maximum stress criteria, Hashin&rsquo / s criteria and Tsai Wu criteria are employed to detect the failure and progressive failure methodology is be implemented according to instantaneous degradation approach. First ply failure, final failure loads, corresponding deformations and failure patterns are presented and compared.

V současnosti jedna z metod spojování zejména tlustých a vysoce zatížených kompozitních komponent je šroubový spoj, který je možné rozebrat pro případ opravy na rozdíl od lepeného spoje. Kompozitní konstrukce se tradičně dimenzují tak, aby během provozu nedošlo k porušení první vrstvy laminátu, nicméně důležité je taky poznat chování laminátu po porušení první vrstvy. Pro strukturální analýzu nejenom spojů, ale také dalších komponent se používá metoda konečných prvků a protože moderní nelineání řešiče jsou schopné modelovat chování laminátu po porušení první vrstvy, tato schopnost jednoho z nich byla využita v této práci při zkoumaní chování sklolaminátu spojeného s hliníkovou částí šrouby. Konečno-prvkové modely dvou spojů kovové a kompozitní části konstrukce schopné popsat progresivní porušování laminátu byly postaveny s využitím tří různých poruchových kritérií – kritéria maximálního napětí, kritéria Hill a kritéria Tsai-Wu. Problém byl řešen s využitím řešiče Nastran. Křivky síla-posuv, tuhost-posuv a hodnoty zatížení při hraničním posuvu byly porov-nány s výsledky experimentů. Jelikož faktor zbytkové tuhosti ovlivňuje výsledky ana-lýzy progresivního porušování, byly provedeny citlivostní studie zkoumajíci vliv faktoru na přesnost a stabilitu výpočtu. Shoda výpočtu s experimentem v případe prvního šroubového spoje je méně uspokojivá, nicméně shoda v případě druhého spoje, který má zesilující tenkou ocelovou destičku na spodní straně, je podstatně lepší. Vý-borná shoda je zejména při použití interaktivních kritérií Hill a Tsai-Wu.

The damage development and strength of ply-level scaled carbon/epoxy composite laminates having stacking sequence of [+Tn/-Tn/902n]s where constraint ply angle, T, was 0, 15, 30, 45, 60, and 75 degrees, and size was scaled as n=1,2,3, and 4, is reported in Part I. X-radiography was used to monitor damage developments. First-ply failure stress, and tensile strength were recorded. First-ply failure of the midplane 90 deg. plies depended on the stiffness of constraint plies, and size. All 24 cases were predicted using Zhang's shear-lag model and data generated from cross-ply tests. Laminate strength was controlled by the initiation of a triangular-shaped local delamination of the surface angle plies. This delamination was predicted using O'Brien's strain energy release rate model for delamination of surface angle plies. For each ply angle, the smallest laminate was used to predict delamination (and strength) of the other sizes. The in-situ tensile strength of the 0 deg. plies within different cross-ply, and quasi-isotropic laminates of varying size and stacking sequence is reported in Part II. No size effect was observed in the strength of 0 deg. plies for those lay-ups having failure confined to the gauge section. Laminates exhibiting a size-strength relationship, had grip region failures for the larger sizes. A statistically significant set of 3-point bend tests of unidirectional beams were used to provide parameters for a Weibull model, to re-examine relationship between ultimate strength of 0 deg. plies and specimen volume. The maximum stress in the 0 deg. plies in bending, and the tensile strength of the 0 deg. plies (from valid tests only) was the same. Weibull theory predicted loss of strength which was not observed in the experiments. An effort to model the durability and life of quasi-isotropic E-glass/913 epoxy composite laminates under steady load and in an acidic environment is reported in Part III. Stress-rupture tests of unidirectional coupons immersed in a weak hydrochloric acid solution was conducted to determine their stress-life response. Creep tests were conducted on unidirectional coupons parallel and transverse to the fibers, and on ±45°. layups to characterize the lamina stress- and time-dependent compliances. These data were used in a composite stress-rupture life model, based on the critical element modeling philosophy of Reifsnider, to predict the life of two ply-level thickness-scaled quasi-isotropic laminates. / Ph. D.