Spatially Continuous Strain Monitoring using Distributed Fiber Optic Sensors Embedded in Carbon Fiber Composites

A distributed fiber optic strain sensor based on Rayleigh backscattering, embedded in a fiber-reinforced polymer composite, has been demonstrated. The optical frequency domain reflectometry technique is used to analyze the backscattered signal. The shift in the Rayleigh backscattered spectrum is observed to be linearly related to the change in strain of the composite material. The sensor (standard single-mode fiber) is embedded between the layers of the composite laminate. A series of tensile loads is applied to the laminate using an Instron testing machine, and the corresponding strain distribution of the laminate is measured. The results show a linear response indicating a seamless integration of the optical fiber in the composite material and a good correlation with the electrical-resistance strain gauge results. The sensor is also used to evaluate the strain response of a composite-laminate-based cantilever beam. Distributed strain measurements in a composite laminate are successfully obtained using an embedded fiber optic sensor

Soy-based polyurethane foam for insulation and structural applications

Polyurethane (PU) foams are widely used as insulation materials due to their high insulation properties and low cost compared to conventional materials such as styrene and mineral wool. PU foams are traditionally fabricated with petroleum-based precursors. However, high crude price and higher carbon footprint has lead interest of researchers to synthesis PU foams using plant-based raw materials, that are inexpensive and renewable. In this dissertation, PU foams were fabricated using soy-based polyol and its thermal and mechanical properties were investigated. In the first part, of PU foam samples with different formulations were fabricated using soy-based polyol HB230, and varying amounts of blowing agent, catalyst and surfactant. The prepared samples were tested for density, mechanical properties, thermal insulation, and thermal stability. It was observed that soy-based polyol had comparable thermal and mechanical properties to petroleum-based foam. In the second part, PU foam samples were fabricated by blending HB230 polyol with soy-based crosslinker HB530, and petroleum-based polyol in varying ratios. Blended foam samples exhibited better thermal resistivity, compressive properties, and tensile strength of 8% higher, 512% higher, and 287% higher, respectively as compared to pure petroleum-based and pure HB230. In the third part, soy-based foams were optimized for maximum thermal insulation and mechanical strength by investigating the effects of different surfactants, used in varying amounts. The fabricated samples were investigated for effect of morphology on mechanical and thermal properties. It was observed that the cell size of the foam samples can be controlled by varying the surface tension of the reactants of the foam, resulting in variation of the properties --Abstract, page iv

Investigation of Grain, Grain Boundary, and Interface Contributions on the Impedance of Ca₂FeO₅

Conductivity properties such as the impedance contributions of grain, grain boundary, and electrode–material interface of brownmillerite-type Ca2Fe2O5 are studied using alternate current (AC) impedance at different temperatures over a wide range of frequencies. The compound was synthesized at 1000 °C by a solid-state reaction. Powder X-ray diffraction confirmed the pure and single-phase formation. The correlation of the electrical properties with the microstructure of the compound was studied by an AC impedance spectroscopic technique at different temperatures (25–300 °C), which demonstrated the contribution of both the grain (bulk) and grain boundary to the impedance. The frequency-dependent electrical conductivity was used to study the conductivity mechanism. The electric impedance and the frequency at different temperatures supported the suggested conduction mechanism

Effect of Reverse Micelles Size on the Electron Transfer Reaction within the Ion Pair of Co (III)/Fe (II) Complexes

The electron transfer process between pentammineaquacobalt (III) and hexacyanoferrate (II), [Co(NH3)5H2O]3+/Fe(CN)6]4− ion pair was investigated in water/dioctyl sodium sulfosuccinate (AOT)/Isooctane reverse micelles. The study observed that the electron transfer rate depends on the size of the reverse micelles. The concentrations of Fe (II) ions were varied in different-sized (Wo) reverse micelles of Wo = [H2O]/[AOT] = 10 to 30, but the concentration of Co (III) ions was kept constant. The rate of electron transfer in the ion pair [Co(NH3)5H2O]3+/[Fe(CN)6]4− increased with decreasing size (Wo) of reverse micelles. The smallest reverse micelles Wo = 10 demonstrated the fastest electron transfer rate, and the biggest Wo = 30 reverse micelles showed the slowest electron transfer rate. The change of reaction environment and the location of the reactants in the reverse micelles due to confinement are considered the factors responsible for the results

Investigation of Sandwich Composite Failure under Three-Point Bending: Simulation and Experimental Validation

A sandwich structure consists of a two thin and strong facesheets, bonded to a thick lightweight core material. The mechanical response of a sandwich structure depends on the properties of its constituents. A numerical model and experimental validation of the three-point bending test of sandwich composites are presented in this study. The core material is aluminum honeycomb. The facesheets are made of IM7/Cycom5320-1, which is a carbon fiber/epoxy prepreg system. A comprehensive model of the failure under flexural loading was developed. Facesheet failure was modeled using Hashin’s failure criteria. A detailed meso-scale model of the honeycomb core was included in the model. The experiments indicated that failure initiation was due to local buckling in the honeycomb core. Failure propagation was in the form of core failure, facesheet compressive failure, and interlaminar failure. The developed meso-scale model was able to accurately simulate failure initiation and propagation in the composite sandwich structure. The effect of elevated temperature on the three-point bending behavior was studied numerically as well as experimentally. An increase in test temperature to 100°C resulted in a drop of 9.2% in flexural strength, which was also predicted by the numerical model

Evaluation of Low-Velocity Impact Properties for Stitched Foam-Core Polyurethane Sandwich Structural Panel

Composite sandwich structures with a variety of core materials are increasingly utilized for wide range of structural applications. This paper presents experimental and numerical investigation on low-velocity impact response of sandwich composite panels composed of stitched foam core and E-glass fiber/Polyurethane (PU) facesheets. The samples were fabricated using low-cost vacuum assisted resin transfer molding (VARTM) process. Low velocity impact response of the sandwich panels was investigated under four different impact energy levels (10J, 15J, 20J, and 30J) using a Dynatup drop tower Instron impact machine. Based on the load and energy histories, parameters including maximum load, penetration depth, and total energy absorbed have been investigated under the four different impact energy levels listed above. A three-dimensional dynamic finite element model was developed for the stitched sandwich structures under low velocity impact

Flexural Behavior of Cross-Ply Thermally Aged Bismaleimide Composites

Thermal aging in oxidative environments is a critical degradation mechanism for polymer composites in aerospace applications. After degradation, the composite strength is reduced and the working life is shortened. In the present work, thermal-oxidation effects on the bending behavior were studied experimentally and numerically. A cross-ply laminate was manufactured using, IM7G/AR4550, a unidirectional bismaleimide prepreg system. Thermal aging experiments were conducted in air at 176.7 °C (350 °F) for 1,700 hours. Weight loss of the samples was monitored during aging and three-point bending test was performed to characterize thermal oxidation impact on the bending behavior. 3D transient coupled diffusion-reaction simulations were conducted based on representative volume elements (RVE) using COMSOL Multiphysics. Three-point bending test was simulated using ABAQUS taking the effect of the transient oxidation growth and degradation into consideration. Thermal aging for 1,700 hours resulted in a weight loss of 0.5 % and a reduction of 19 % in the flexural modulus of the samples. Weight loss and flexural modulus simulation results showed a good match with the experimental findings

Investigation of Laminate Debonding in Horizontal Axis Water Turbine Composite Blades

Carbon fiber reinforced polymer (CFRP) composites are becoming popular due to their superior strength to weight ratio and stiffness properties. This study highlights the interlaminar debonding growth, which is considered one of the most frequent problems with composite materials. A three-blade horizontal axis water turbine (HAWT) was manufactured using IM7/Cycom5320-1 carbon/epoxy prepreg. During the process of manufacturing, a specific number of Teflon sheets were placed between the composite layers in two locations to create a separation between the layers and to investigate the delamination growth. Three different laminate stacking sequences were selected to be tested: [0°]4, [0°/90°]S, and [45°/-45°]S. The composite blades were placed in a water tunnel and run for 3 million revolutions. A thermography analysis was carried out to evaluate the propagation and growth of the delamination. A one-way fluid-structure interaction (FSI) model was created and implemented to obtain the stress values along the blade. The results showed the influence of the composite lay-up orientation on the growth of the delamination. The unidirectional blades ([0°]4) showed the lowest amount of propagation, while the cross-ply ([0°/90°]S) showed the most delamination growth. The bottom location (near the root) showed the maximum delamination. Both sides of the blades showed significant delamination growth. However, the back side showed more interlaminar debonding growth than the front side. After three million revolutions, the percentage of debonding growth for the bottom/back side of the blades was 6.58%, 36.25%, and 27.63% for [0°]4, [0°/90°]S, and [45°/-45°]S, respectively

Multi-Scale Modeling of Thermo-Oxidation Effects on the Flexural Behavior of Cross-Ply Bismaleimide Composites

Mechanical properties of high-temperature polymer matrix composites deteriorate during their service. Oxidation plays a significant role in determining the residual elastic and strength properties of the composite. The present work investigates the oxidative aging damage of cross-ply bismaleimide composites, both experimentally and numerically. Also, this work introduces a better understanding of the significant damage mechanisms, and their respective time ranges. Micro/macro-scale thermo-oxidation behavior and flexural failure were simulated for 1,700 hours of aging. Thermo-oxidation behavior of the cross-ply laminates was simulated using a multi-fiber multi-layer representative volume element. Thermo-oxidation is a diffusion–reaction phenomenon that depends on temperature and oxidation state in time and space domains. In this work, the proportionality between oxidation state evolution and reaction rate was modeled using a new form of time-dependent parameter. Owing to aging damage, the required properties for the flexural test simulation were reduced in the meso-scale. The current study presents a damage state assuming proportionality between the average crack length and oxidized layer thickness based on a continuum damage mechanics approach. Scanning electron micrographs showed that the onset of the blunt transverse (through-thickness) micro-cracking/debonding in the superficial layers was at 500 hours under 176.7 °C (350 °F), where oxidative cracking dominates aging mechanisms. The blunt micro-cracks propagated in the transverse direction near 1000 hours rendering the crack faces in contact with the oxidative air. After about 1,700 hours of aging, the weight loss ratio was 0.5%, and the flexural modulus and strength reduced by 19% and 10%, respectively. Prior to 500 hours, where cracking was not noticeable in the crack-free region , the strengths reduction was not significant based on the numerical simulations. In the crack-free and oxidative damage dominated regions, the elastic property numerical reduction was significant

Development and Characterization of Polyurethane Foams with Substitution of Polyether Polyol with Soy-Based Polyol

Bio-based polyols can replace petroleum-based polyols for the reaction with isocyanate groups to prepare a wide range of polyurethane (PU) products. They can be readily derived from various types of abundant and renewable bio-resources, however, their utilization is still limited due to the complex molecular structure and relatively low primary hydroxyl content. This study aims to substitute the petroleum-based polyether polyols with soy-based polyols for the fabrication of rigid PU foams that have comparable or better physical properties required for potential structural and insulation applications. Commercially available soy polyols with 230 mg KOH/g hydroxyl content were blended with the petroleum-based polyether polyols in 5% increments up to 50% substitution to fabricate the rigid PU foams. Isocyanate index of 1.14 was maintained for all formulations. Other constituents such as catalyst, surfactant, and blowing agent were also kept constant. The density, compressive strength, and thermal conductivity of prepared PU foam samples were determined by the ASTM standard methods. Densities of PU samples with 25% substitution were within the 20% range of the control sample. Thermal conductivity values of the PU foams prepared with 25% polyether polyol substitution were similar when compared to the control sample. Compressive strengths of the 25% substitution samples were approximately 30% higher. It was concluded that the polyols blended with 25% soy-based polyols yield the rigid PU foams that have the overall best quality in terms of compressive strength, tensile strength, and insulation