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An investigation was conducted to study the constraint effects on flaw growth in composite laminates. Results were presented for the case of a transverse flaw in an interior ply perpendicular to the loading axis. Two orientations of the flawed ply were examined (0 and 90 degrees), and two distinctly different constraint situations were studied (cross-ply constraint and biaxial constraint). Throughout the study, various nondestructive testing methods were employed to evaluate the material response and to determine the damage and damage growth in the specimens. These techniques include replication, ultrasonic c-scan, ultrasonic attenuation, acoustic emission, x-radiography, thermography and stiffness measurement. The effects of constraint on the response of composite materials can be classified in two categories: (a) in-plane effects and (b) through-the-thickness effects. In-plane constraint is the principal contributor to notched strength and changes in notched strength under quasi-static loading. Through-the-thickness constraint controls the pattern and spacing of transverse cracks in the off-axis plies to form a characteristic damage state in the laminates. Out-of-plane stresses produced by constraints are influential on the growth of damage along ply interfaces, especially during cyclic loading. The mode of damage and the extent of damage in constrained notched plies are governed by the stress state in those plies, as determined by the constraining plies, and the relationship of the stress state to the strength state. Maximum constraint on the flawed ply does not produce minimum damage in the laminate; and the lesser degree of damage (in terms of axial splitting and delamination) does not necessarily result in a higher laminate strength or long fatigue lives. In the design of composite structures, a compromise has to be reached with regard to optimizing material parameters such as strength, stiffness, fatigue life, and residual strength. In maximizing one parameter, one might have to sacrifice other requirements on the other material properties in the design. / Ph. D.

Changes in the stiffness and strength of notched quasi-isotropic graphite-epoxy laminates were recorded and related to the fatigue damage. Two different laminates [0,90,+45,-45]s (type A) and [ +45, 90, -45, 0] s (type B) were considered and the effects of stacking sequence were compared. Nondestructive testing techniques such as X-radiography, moire technique, acoustic emission, deply technique, and stiffness change were performed to observe damage development. Static properties and damage initiation were related to an approximate stress analysis. Results show that the mechanical response and the fatigue damage depend strongly on the stacking sequence of laminates. In general, residual strength increased remarkably for both laminates due to stress redistributions while the continuous stiffness change curve is typical for each laminate and reflects damage characteristics. Buckling effects as well as matrix cracking and delaminations contribute to stiffness changes. / M.S.

A brief review of necessary fundamentals relative to composite materials and viscoelasticity is provided. Subsequently the accelerated characterization techniques of Time Temperature Superposition and Time Temperature Stress Superposition are treated in detail. An experimental procedure for applying the latter to composites is given along with results obtained on a particular T300/934 graphite/epoxy. The accelerated characterization predictions are found in good agreement with actual long term tests. A postcuring phenomenon is discussed that necessitates thermal conditioning of the specimen prior to testing. A closely related phenomenon of physical aging is described. The effect of each on the glass transition temperature and strength is discussed. Creep rupture results are provided for a variety of geometries and temperatures for T300/934 graphite/epoxy. The results are found to compare reasonably with a modified kinetic rate theory. / Ph. D.

This thesis is a study of various physical and statistical models which may be useful for the prediction of the composition of a ternary liquid mixture. The particular mixture considered in this study was the solvent system consisting of nitroglycerine (NG), 2-nitrodiphenylamine (2NDPA), and triacetin (TA). Several models were investigated for their adequacy and closeness of fit. An attempt has been made to relate the actual composition to a few easily measurable quantities, namely, refractive index, density, and the separate analysis of 2NDPA. Deterministic models relating the concentration of each component in the mixture with the physical determinations mentioned above have been considered first. These models are based on the known theory of physical chemistry. The deterministic model which was chosen as "best" in terms of the smallness of error of prediction, estimates the composition from the determination of density and the spectrophotometer analysis of 2NDPA. Since the latter analysis is a quick and accurate determination of the 2NDPA content and since the content of the third component could be determined by complementing to 100 percent, the models have been formulated in terms of the concentration of only one component, namely NG. The statistical models under investigation are divided into activity models and regression models. The activity model is a combination of chemical and statistical theory while the simple regression model represents an approach that a statistician might take if he disregarded the physical or chemical theory involved. Two activity models have been discussed, the first assuming the activity of the mixture constant and the second assuming the activity of the mixture to be a weighted sum of the activities of the three components. Tests of hypotheses are made to determine whether the activity models result in a significant reduction in error over that of the "best" deterministic model. The investigation that the model formulated assuming constant activity of the mixture results in the smallest error of estimation among all models under study. Thus it has been used as a basis for the preparation of control charts. The linear regression models, constructed with various functions of density, refractive index, and spectrophotometer analysis as independent variables, produced errors of estimation above those for the deterministic model. Chapter IV represents the summary of the thesis and the translation of findings into actual control charts. On the basis of this chapter, a technician can easily determine the estimate of the composition of the mixture and the attached 99 percent confidence bounds. Thus, this Chapter contains these charts combined with instructions in their use and numerical examples. / Master of Science

The effects of room temperature tensile loading and five thermal loadings upon the development of transverse microcracks (TVM) in Celion 6000/PMR-15 graphite-epoxy laminates were investigated. Microcracks were observed using a replicating technique, microscopy and x-ray. The mechanical or thermal load at which microcracking initiates and the ply residual stresses were predicted using laminate analysis with stress and temperature-dependent materials. The TVM density as a function of tensile load was predicted using the multiple cracking theory proposed by Garrett, Parvizi and Bailey. Reasonably good correlation between theory and experiment was obtained for both the load at initial failure and the TVM density as a function of tensile load. It has been shown that TVM density and the load to initiate microcracking are functions of the type of loading and the laminate configuration. Generally, cross-ply laminates exhibit higher TVM density after thermal loading than do quasi-isotropic laminates, but the converse is true for tensile loading. Cross-ply laminates attain saturation TVM density prior to failure, but the TVM density of quasi-isotropic laminates continues to increase until failure. Edge effects have a significant influence on the development of TVM, and TVM present at the free edge of a laminate extend throughout the entire width of the laminate. <i>[Vita removed Feb. 15, 2011. GMc]</i> / Master of Science

This investigation concerns the frequency dependent heat generation behavior and the heat generation mechanisms for the thermal patterns of delamination in fiber reinforced composites during a vibrothermographic test, which uses real time thermography as a nondestructive evaluation of a structure or a component excited with mechanical vibration. A local resonance model was proposed in the past to describe the frequency dependent heat generation behavior during a vibrothermographic test, and this model was used as a basis for writing software for calculating the natural frequencies of a plate with the size of delamination. Vibrothermographic tests were performed on three glass-epoxy panels that each contained four different sized simulated delaminations. Comparison between the observed vibrothermal peak frequencies and the natural frequencies predicted by the local resonance model, and investigations of the thermoelastic emission field in the delamination region using SPATE, were made to determine the validity of the local resonance model. A significant conclusion of the results is that the local resonance is indeed the mechanics model for the frequency dependent heat generation behavior. A careful measurement of the degree of heating of both sides of [0₅] glass-epoxy panel with delaminations on the 2-3 ply interface, and comparison between the predicted heat patterns generated from a finite difference heat transfer program and observed heat patterns, was made to identify the heat generation mechanism. The results show that the majority of heat generation during vibrothermographic testing results from higher stresses or strains due to local resonance. The heat generation was affected by the combination of the principal strains and shear strain for the lower modes of resonant vibration, and was dominated by the shear strain for the higher modes of resonant vibration. Impact damaged graphite-epoxy panels were also inspected constituting an application of vibrothermography on real damaged components. The degree of heating of the damage were measured through a frequency range, and the damage severity was inspected by ultrasonic C-scan and edge replication. From comparison of two plots of the degree of heating versus exciting frequency, either the area under the curve or the number of vibrothermal peak frequencies, the severity of the damage can be qualitatively identified. / Ph. D.

The study addresses the coating selection for a proposed placement of a hydroturbine into the Gulf Stream. The turbine will generate energy in a similar manner to a wind turbine. The effects of biofouling and corrosion in the current project are assessed. A review of different types of traditional paint coatings is given, as well as the option for a copper-nickel alloy. Testing that should be undertaken for the coating selection is described in detail. Coating considerations are offered and discussed. Design considerations and modifications are also offered. / by Andrew Spicer Bak. / Vita. / Thesis (M.S.C.S.)--Florida Atlantic University, 2009. / Includes bibliography. / Electronic reproduction. Boca Raton, Fla., 2009. Mode of access: World Wide Web.

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