The complexity of the fiber-matrix interphase in a composite is largely due to the myriad of variables (material, processing, and design) that affect its formation. The interphase, thus formed, has to be characterized at several levels (micro-structural, chemical, and mechanical) in order for one to fully understand the nature of the bond between the fiber and matrix and in order to perform a stress analysis of the fiber-interphase-matrix assemblage. A thorough thermo-mechanical characterization of the interphase is difficult, at present, due to the necessity of studying the interphase in situ, its small dimension (usually on the order of a micrometer), and its general complexity. However, a cursory glance at the literature shows that great progress has been made in all of the three levels of characterization mentioned above for various composite systems. Several recent attempts have focused on the physical characterization (evaluation of volume fraction, thickness, Young's modulus, shear modulus and coefficient of thermal expansion) of the interphase. Models of physical properties (thickness, Young's modulus, Poisson's ratio and coefficient of thermal expansion) of the interphase have been considered by several researchers in an effort to study the influence of the interphase on overall composite properties and behavior. Hypotheses on interphase formation and properties have been proposed and tested by some researchers. Both experimental characterization as well as modeling studies are necessary to achieve a more profound understanding of the interphase and its behavior. The interphase, in a composite, is usually modeled as a homogeneous region, despite the fact that it may have spatial property variations.However, it is important to the understanding of composite behavior to incorporate a realistic interphasial region into the analysis and testing of composite material systems. A new thermo-elastic model for the interphase properties in fiber-reinforced thermosets is proposed. The Young’s modulus and coefficient of thermal expansion of the interphase are assumed to be functions of distance from the fiber in this model. The Poisson’s ratio of the interphase is assumed to be the same as that of the matrix. The new model is used in a concentric cylinder assemblage analysis for the determination of the residual thermal stresses in unidirectional fiber-reinforced epoxies. The governing field equations in terms of displacements are solved in “closed form”. It is found that, although the solution is dilute, the property variations in the interphase have a distinct effect on the residual thermal stresses. This is significant, considering the fact that residual thermal stresses play an important role in controlling the structural performance of a composite. The new model is used in Mori-Tanaka analyses for the determination of non-dilute local stress fields in unidirectional fiber-reinforced epoxies under thermo-mechanical loading situations. The governing field equations in terms of displacements are solved in “closed form”. It is found that property variations in the interphase have a distinct effect on the local stresses. This is also significant, considering the fact that local stresses play an important role in controlling the structural performance of a composite. A model composite system consisting of a coated glass rod embedded in Epon 828 is considered; coatings are applied to the glass rod in succession to simulate two different interphase types. The model composite specimens are loaded in transverse compression and transverse shear, and the resulting in-plane displacements are measured by the use of the Moire interferometry technique. Differences in displacement fields between the various specimens, due to the presence of interphasial regions, are found to be minimal. More sensitive measurements are needed to measure pointwise displacements in the interphasial (coatings) region. / Ph. D.

An adaptation of an extrinsic Fabry-Perot interferometer (EFPI) strain sensor is described, which permits the state of cure of an epoxy matrix to be monitored, when the sensor is embedded in a polymeric matrix composite. By using a glass rod with a retroreflecting end for the target fiber in the EFPI sensor, the intensity of the light reflected depends on the refractive index of the host matrix, if a low coherence source is used. As the epoxy cross-links during cure, the refractive index of the epoxy will increase to a value exceeding that of the target fiber. The resulting increased loss in the fiber can be detected at the sensor output and correlated to the state of cure of the epoxy. After cure, the sensor may be operated as a conventional extrinsic Fabry-Perot interferometric strain sensor if a coherent source is used. Using the modified extrinsic Fabry-Perot sensor, we monitor the cure of Devcon® 5-Minute® Epoxy, and show that it cures in approximately 60 minutes. / Master of Science

The state of cure of epoxies is an important issue in the manufacture of graphite epoxy composites used in aerospace structures. Variations in the initial state and process used to cure the epoxy resin in a composite material lead to variations in the mechanical properties of the part manufactured from the composite._[12] Control of these variation can be accomplished by monitoring the bulk and shear moduli of the epoxy resin as it cures. The moduli properties of the resin determine the acoustic properties of the epoxy._{[12],[13],[14]} Hence measurement of the acoustic longitudinal velocity and attenuation of the epoxy during its cure cycle provides a good indicator of the state of cure. Optical fiber waveguides can be embedded within a host material and used to detect longitudinal acoustic waves._[15],[16] Herein, the mechanisms allowing the detection of ultrasound with optical fiber are presented. An analysis of optical fiber waveguides and optical fiber based interferometric detection methods is performed in detail. The interaction of radial strain fields, induced by longitudinal acoustic waves, with singlemode optical fibers is described. Experimental results obtained in epoxy cure monitoring, using an optical fiber based method for acoustic detection, are compared with results obtained using conventional piezoelectric based acoustic detection methods. / Master of Science

Laminated carbon/epoxy composite structures are increasingly used in the aerospace industries. Low weight, elastic tailoring, and high durability make the composite materials well suited for replacement of conventional metallic structures. However the difficulty to capture structural failure phenomena is a significant barrier to more extensive use of laminated composites. Predictions are challenging because matrix (resin) dominated failure mechanisms such as delaminations and matrix cracking contribute to the structural failure in addition to fiber-dominated failures. A key to rigorous failure predictions for composites is availability of measurements to quantify structural model parameters including matrix-dominated stress-strain relations and failure criteria. Novel techniques for measurement of nonlinear interlaminar constitutive properties in tape composites have been recently developed at Georgia Institute of Technology. Development of methods for accurate predictions of failure in carbon/epoxy tape laminate configurations with complex lay-ups is the main focus of this work. Failures through delamination and matrix cracking are considered. The first objective of this effort is to implement nonlinear interlaminar shear stress-strain relations for IM7/8552 carbon/epoxy tape in ABAQUS finite element models and validate structural delamination failure predictions with tests. Test data for composite configurations with wavy fibers confirm that nonlinear interlaminar shear stress-strain response enables accurate failure prediction. The problem of the presence of porosity and its influence on failure was noted. The second objective is to assess the ability to simulate initiation and propagation of matrix-ply cracking. Failure models for IM7/8552 carbon/epoxy tape open-hole tensile coupons are built and validated.

No description available.

This investigation is focused on the understanding of mechanical and chemical reaction behaviors of stoichiometric mixtures of nano- and micro-scale aluminum and hematite (Fe2O3) powders dispersed in epoxy. Epoxy-cast Al+Fe2O3 thermite composites are an example of a structural energetic material that can simultaneously release energy while providing structural strength. The structural and energetic response of this material system is investigated by characterizing the mechanical behavior under high-strain rate and shock loading conditions. The mechanical response and reaction behavior are closely interlinked through deformation characteristics. It is, therefore, desirable to understand the deformation behavior up to and beyond failure and establish the necessary stress and strain states required for initiating chemical reactions. The composite s behavior has been altered by changing two main processing parameters; the reactants particle size and the relative volume fraction of the epoxy matrix. This study also establishes processing techniques necessary for incorporating nanometric-scale reactants into energetic material systems. The mechanochemical behavior of epoxy-cast Al+Fe2O3 composites and the influence of epoxy volume fraction have been evaluated for a variety of loading conditions over a broad range of strain rates, which include low-strain rate or quasistatic loading experiments (10-4 to 10-2 1/s), medium-strain rate Charpy and Taylor impacts (103 to 104 1/s), and high-strain rate parallel-plate impacts (105 to 106 1/s). In general, structural strength and toughness have been observed to improve as the volume fraction of epoxy decreases, regardless of the loading strain rate regime explored. Hugoniot experiments show damage occurring at approximately the same critical impact stress for compositions prepared with significantly different volume fractions of the epoxy binder phase. Additionally, Taylor impact experiments have indicated evidence for strain-induced chemical reactions, which subject the composite to large shear accompanied by temperature increase and associated softening, preceding these reactions. Overall, the work aims to establish an understanding of the microstructural influence on mechanical behavior and chemical reactivity exhibited by epoxy-cast Al+Fe2O3 materials when exposed to high stress and high-strain loading conditions. The understanding of fundamental aspects and the results of impact experiment measurements provide information needed for the design of structural energetic materials.

No / Acrylamide is a common dietary exposure that crosses the human placenta. It is classified as a probable human carcinogen, and developmental toxicity has been observed in rodents. OBJECTIVES: We examined the associations between prenatal exposure to acrylamide and birth outcomes in a prospective European mother-child study. METHODS: Hemoglobin (Hb) adducts of acrylamide and its metabolite glycidamide were measured in cord blood (reflecting cumulated exposure in the last months of pregnancy) from 1,101 singleton pregnant women recruited in Denmark, England, Greece, Norway, and Spain during 2006-2010. Maternal diet was estimated through food-frequency questionnaires. RESULTS: Both acrylamide and glycidamide Hb adducts were associated with a statistically significant reduction in birth weight and head circumference. The estimated difference in birth weight for infants in the highest versus lowest quartile of acrylamide Hb adduct levels after adjusting for gestational age and country was -132 g (95% CI: -207, -56); the corresponding difference for head circumference was -0.33 cm (95% CI: -0.61, -0.06). Findings were similar in infants of nonsmokers, were consistent across countries, and remained after adjustment for factors associated with reduced birth weight. Maternal consumption of foods rich in acrylamide, such as fried potatoes, was associated with cord blood acrylamide adduct levels and with reduced birth weight. CONCLUSIONS: Dietary exposure to acrylamide was associated with reduced birth weight and head circumference. Consumption of specific foods during pregnancy was associated with higher acrylamide exposure in utero. If confirmed, these findings suggest that dietary intake of acrylamide should be reduced among pregnant women.