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The objective of the current research was to contribute to the area of mechanics of composite polymeric materials. This objective was reached by establishing a quantitative assessment of the fatigue strength and evolution of mechanical property changes during fatigue loading of nanocomposite fibers and films. Both experimental testing and mathematical modeling were used to gain a fundamental understanding of the fatigue behavior and material changes that occurred during fatigue loading. In addition, the objective of the study was to gain a qualitative and fundamental understanding of the failure mechanisms that occurred between the nanoagent and matrix in nanocomposite fibers. This objective was accomplished by examining scanning electron microscopy (SEM) fractographs. The results of this research can be used to better understand the behavior of nanocomposite materials in applications where degradation due to fatigue and instability of the composite under loading conditions may be a concern. These applications are typically encountered in automotive, aerospace, and civil engineering applications where fatigue and/or fracture are primary factors that contribute to failure.

The goal of this academic project was to study the effects of different variables on the damage initiation and progression around four bolt holes of a joint in carbon fiber/graphite epoxy composite coupons. The tracked variables included the type of layup, R values, stress levels, and damage mechanisms observed in each specimen. In-situ x-ray of the individual coupons recorded the extent of damage, mostly longitudinal splitting and bearing failure, as a function of the cycle count. The following lay-ups were included: [45/90/-45/02/45/02/-45/0]s, [04/45/03/90/0]s, [±5/65/(±5)2/-65/±5]s, and [±5/65/(±5)2/-65/5/65]s, In particular, the objective was to determine the stress levels at which detectable damage starts developing by applying 50,000 cycles at incremental stress levels. Once damage was initially detected, we typically raised the stress level 2.5 ksi and cycled another 50,000 cycles until damage reached a point where the bolt holes had elongated 10% of the original diameter of 0.25 inches. This type of testing was be continued for several different R ratios and comparisons were be made between the performances of different lay-ups at varying load levels. A finite element model was created in ABAQUS to help understand the stress fields within the laminates.

Thesis (Ph.D.)--Civil and Environmental Engineering, Georgia Institute of Technology, 2008. / Committee Chair: Rami Haj-Ali; Committee Member: Arash Yavari; Committee Member: Bruce R. Ellingwood; Committee Member: Kenneth M. Will; Committee Member: Richard W. Neu.

Several modifications have been implemented to numerical simulation codes based on blade element momentum theory (BEMT), for application to the design of ocean current turbine (OCT) blades. The modifications were applied in terms of section modulus and include adjustments due to core inclusion, buoyancy, and added mass. Hydrodynamic loads and mode shapes were calculated using the modified BEMT based analysis tools. A 3D model of the blade was developed using SolidWorks. The model was integrated with ANSYS and several loading scenarios, calculated from the modified simulation tools, were applied. A complete stress and failure analysis was then performed. Additionally, the rainflow counting method was used on ocean current velocity data to determine the loading histogram for fatigue analysis. A constant life diagram and cumulative fatigue damage model were used to predict the OCT blade life. Due to a critical area of fatigue failure being found in the blade adhesive joint, a statistical analysis was performed on experimental adhesive joint data. / Includes bibliography. / Thesis (M.S.)--Florida Atlantic University, 2016. / FAU Electronic Theses and Dissertations Collection

This study introduces a new three-dimensional (3D) multi-scale constitutive framework for the nonlinear viscoelastic analysis of laminated and pultruded composites. Two previously developed nonlinear micromechanical models for unidirectional and in-plane random composite layers are modified to include time-dependent and nonlinear behavior. A new recursive-iterative numerical integration method is introduced for the Schapery nonlinear viscoelastic model and is used to model the isotropic matrix subcells in the two micromodels. In addition, a sublaminate model is used to provide for a through-thickness 3D nonlinear equivalent continuum of a layered medium. The fiber medium is considered as transversely isotropic and linear elastic. Incremental micromechanical formulations of the above three micromodels are geared towards the time integration scheme in the matrix phase. New iterative numerical algorithms with predictor-corrector type steps are derived and implemented for each micromodel to satisfy both the constitutive and homogenization equations. Experimental creep tests are performed for off-axis pultruded specimens in order to calibrate and examine the predictions of the constitutive framework for the multi-axial nonlinear viscoelastic response. Experimental creep data, available in the literature, is also used to validate the micromodel formulation for laminated composite materials. Nonlinear viscoelastic effects at the matrix level, such as aging, temperature, and moisture effects can be easily incorporated in the constitutive framework. The multi-scale constitutive framework is implemented in a displacement-based finite element (FE) code for the analysis of laminated and pultruded structures. Several examples are presented to demonstrate the coupled multi-scale material and structural analysis.

This study develops new quantitative thermoelastic stress analysis (TSA) techniques for fracture and fatigue damage analysis of composite materials. The first part deals with the thermo-mechanical derivation of two quantitative TSA techniques applied to orthotropic composites with and without a transversely-isotropic surface coating layer. The new TSA test procedures are derived in order to relate the thermal infrared (IR) images with the sum of in-plane strains multiplied by two newly defined material constants that can be experimentally pre-calibrated. Experiments are performed to verify the TSA methods with finite element (FE) numerical results along with available anisotropic elasticity solution. The second part of this study applies the quantitative TSA techniques together with the Lekhnitskii's general anisotropic elasticity solution to calculate mixed-mode stress intensity factors (SIFs) in cracked composite materials. The cracked composite coupons are subjected to off-axis loadings with respect to four different material angles in order to generate mixed-mode SIFs. A least-squares method is used to correlate the sum of in-plane strains from the elasticity solution with the measured TSA test results. The mode-I and mode-II SIFs are determined from eccentrically loaded single-edge-notch tension (ESE(T)) composite specimens. The FE models and virtual crack closure technique (VCCT) are utilized for comparisons. In the third part, a new stochastic model is proposed to generate S-N curves accounting for the variability of the fatigue process. This cumulative damage Markov chain model (MCM) requires a limited number of fatigue tests for calibrating the probability transition matrix (PTM) in the Markov chain model and mean fatigue cycles to failure from experiments. In order to construct the MCM stochastic S-N curve, an iterative procedure is required to predict the mean cycles to failure. Fatigue tests are conducted in this study to demonstrate the MCM method. Twenty-one open-hole S2-glass laminates are fatigue-cycled at two different stress levels. The coupon overall stiffness and surface-ply TSA damage area have been used as two damage metrics. The MCM can satisfactorily describe the overall fatigue damage evolution for a limited number of coupons (less than 6) subjected to a given specific stress level. The stochastic S-N curve can be constructed using at least two sets of fatigue tests under different stress levels. Three available fatigue tests for different E-glass laminates from the literature are also investigated using the proposed MCM approach. The results show the MCM method can provide the stochastic S-N curves for different composite systems and a wide range of fatigue cycles.

The goal of this research was to investigate the tensile fatigue behavior of a carbon fiber / epoxy composite material. Specifically, the stress levels at which cracks initiated in static and fatigue loading in the 90 degree plies of a "quasi-cross ply layup" [0/905]S was investigated. For layups which contain them, cracks in composite laminates initiate and propagate from 90 degree plies (including the ubiquitous "quasi-isotropic layup" 0/±45/90). Thus, this work provides valuable insight into the fatigue behavior of the plies which originate fatigue damage. Unidirectional off-axis 90 degree and 10 degree specimens were also tested, but the bulk of testing was done on the cross-ply laminates. The project sponsors, Boeing, were in the process of extending a failure model to the case of fatigue. The body of work presented here provided empirical data for that effort. Several different inspection techniques were used to investigate for cracking in the 90 degree plies, including: x-ray images, edge replicates, dye penetrants, and optical microscopy. Plots of the stress level at which crack initiation occurred will be presented, as well as images illustrating damage development in these layups. Comparisons are made to the experimental results of other investigations of this type of layup. Explorations of the effect of R-ratio (including R = 0.1 and 0.5), loading frequency (including 3, 10, and 30 Hz), and surface roughness (hand polished specimen edges to 1500 grit smoothness) on fatigue crack initiation were also performed. For the most damaging case (10 Hz, R = 0.1, no polishing), the crack initiation strain (0.00276) was one half of the strain at which cracks initiated in static monotonic loading (0.0054), and was 16% of the cross-ply specimen's (0 degree fiber dominated) ultimate strain value of (0.018).

Made available in DSpace on 2014-12-17T14:58:19Z (GMT). No. of bitstreams: 1 EduardoCBC_DISSERT.pdf: 1858317 bytes, checksum: de7993f7a4a27b8a08342ddc43175aff (MD5) Previous issue date: 2012-12-14 / Coordena??o de Aperfei?oamento de Pessoal de N?vel Superior / The aim of this study is to create an artificial neural network (ANN) capable of modeling the transverse elasticity modulus (E2) of unidirectional composites. To that end, we used a dataset divided into two parts, one for training and the other for ANN testing. Three types of architectures from different networks were developed, one with only two inputs, one with three inputs and the third with mixed architecture combining an ANN with a model developed by Halpin-Tsai. After algorithm training, the results demonstrate that the use of ANNs is quite promising, given that when they were compared with those of the Halp?n-Tsai mathematical model, higher correlation coefficient values and lower root mean square values were observed / Este trabalho tem como principal objetivo a cria??o de uma arquitetura de rede neural artificial (RNA) capaz de modelar o m?dulo de elasticidade transversal (E2) de comp?sitos unidirecionais. Para tanto, se fez necess?rio o uso de um conjunto de dados que foi dividido em duas partes, uma parte sendo utilizada para o treinamento e a outra para teste das RNA. Para este trabalho se desenvolveu tr?s tipos de arquiteturas de rede diferentes uma delas possuindo somente duas entradas, a outra tr?s entradas e a ?ltima foi uma arquitetura mista que combina uma RNA com um modelo desenvolvido por Halpin-Tsai. Ap?s o treinamento dos algoritmos, os resultados demonstram que o uso de RNAs se mostra bastante promissor, j? que quando esses resultados foram comparados com o modelo matem?tico de Halpin-Tsai, apresentaram maiores valores de coeficiente de correla??o e menores valores de erro m?dio quadr?tico