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<p dir="ltr">Since before the Apollo era, the rocket propulsion sector has been a key player in developing standards of cleanliness and compatibility when designing, building, and operating systems with toxic propellants. The advent of hypergols and the widespread use of propellants like N₂O₄, Mixed Oxides of Nitrogen (MON), and hydrazine have forced new standards to be developed to meet the ever-growing need for safety when working with dangerous substances. These systems have only continued to grow more complex and many propellant combinations remain toxic and corrosive to most substances as the industry seeks the optimal methods for deriving the most efficient, highest performing, and generally more capable. ASTM International and other standards organizations carry on documenting standards for cleaning and passivation to ensure safe use today to meet the needs of the ever-expanding propulsion industry.</p><p dir="ltr">This thesis aims to determine the feasibility of using Raman spectroscopy as a method of characterizing interactions between metals and propellants. First, a background of knowledge regarding the spectroscopic method, propellants, and industry practices was researched and current areas of possible application were identified. The passivation and propellant storage phases of system lifecycles were determined to be the scope and target for experimentation. A multilevel passivation study consisting of exposing three metal types to different concentrations of nitric acid for various durations was conducted to begin developing a greater understanding of the applicability of and the techniques required to make Raman spectroscopy work as a complement to the ASTM passivation verification tests. Lessons learned from this and a short-duration compatibility study with MON and similar metal samples were documented and will be used for a larger scale and longer duration compatibility study in conjunction with NASA White Sands Test Facility (WSTF). The buildup of safe and adequate facilities for such a study was undertaken, completed, and documented in this work.</p><p dir="ltr">The results of testing in this thesis suggest the promising and desirable non-destructive and minimally invasive features of Raman spectroscopy have the potential to be used extensively in the propulsion sector. Suggestions for developing key techniques and methods for this application are developed and outlined as they were learned throughout the study's conduction.</p>

<p dir="ltr">Carbon-Carbon (C/C) composites have been shown to be a preferred material for high temperature applications as they retain their properties and performance at temperatures in excess of 2000°C. This study shows that High Temperature Treatment (HTT) at 2400°C for 4 hrs followed by two subsequent Polymer Infiltration and Pyrolysis (PIP) cycles using SC1008 phenolic resin changes the failure mechanism of 2D C/C which has been subject to directional ablation prior to flexural testing. The study observes that prior directional ablation of the non-HTT C/C condition decreases flexural strength by 50.2%, whereas negligible change for the HTT C/C condition was observed (6.6%). This is attributed to the significant degradation of the tensile surface of the non-HTT C/C during ablation corresponding to an average linear thickness loss of 0.321mm (Std Dev = 0.223mm) and average mass loss of 0.364g (Std Dev = 0.196g) while the HTT recorded 0.033mm (Std Dev = 0.005mm) and 0.032g (Std Dev = 0.008g) respectively. The difference in degradation is attributed to the microstructure which was characterised through X-Ray Diffraction and Scanning Electron Microscopy. It is shown that HTT transformed the carbon matrix from a glassy/amorphous matrix to a layered matrix with an indicative increased degree of graphitisation (from 0.52 to 0.69). This not only increased the average density from 1.511 g/cm³ (Std Dev = 0.002 g/cm³) to 1.652 g/cm³ (Std Dev = 0.003 g/cm³) but also increased the average thermal conductivity from 9.1 W/mK (Std Dev = 1.06 W/mK) to 13.3 W/mK (Std Dev = 1.32 W/mK). This ultimately contributed to a reduction in available sites for the oxidation reaction to occur, while also allowing for thermal energy to be conducted away from the ablation surface reducing the amount of heat related damage. For conditions without and with prior ablation damage, the non-HTT C/C is found to fail in matrix dominated tension with the fibres and matrix breaking in a single plane originating at the tensile surface and propagating towards the neutral axis whereas the HTT C/C is found to fail in shear at the neutral axis with the fibres-matrix debonding being the primary failure mechanism. The non-HTT C/C is found to have an average flexural strength of 88.8 MPa (Std Dev = 13.7 MPa) and flexural modulus 81.0 GPa (Std Dev = 10.5 GPa), where the HTT C/C has 196.7 MPa (Std Dev = 31.4 MPa) and 115.2 GPa (Std Dev = 3.3 GPa) respectively. Lastly, this study found that a square notch in the non-HTT C/C condition resulted in a 23.9% and 26.4% reduction in flexural strength for conditions without and with prior ablation damage, respectively. No change in the failure mechanism was observed for notched specimens compared to un-notched specimens, and the debit in strength was attributed to broken fibers created by the notch.</p>

<p dir="ltr">In composites manufacturing, predicting the shape change in parts is vital for making sure part dimensions are properly compensated. Different factors in the manufacturing process, such as the temperature change throughout a thermoset cure cycle, can influence shape change. The compensation process becomes more difficult for geometries with double curvature, as interactions between the two radii of curvature can reduce the effectiveness of applying methodologies for single curvature geometries. Additionally, using finite element analysis (FEA) to predict shape change can be costly and time-consuming depending on part geometry.</p><p dir="ltr">This thesis studies an approach for predicting the shape change of a symmetric thermoset laminate with a double-curved L-angle section in its geometry. Specifically, the conservation of Gaussian curvature is applied to predict shape change. The geometry studied in this thesis can be broken down and analyzed as a segment of a torus, which is attached on one end by a cylinder and on the other end by a curved flange. Varying the length of the cylinder and flange sections, the effectiveness of Gauss’s theorem is determined for the different part geometries, with developed formulas compared against finite element simulations and experimental measurements.</p><p dir="ltr">By approximating torus segments with certain geometric criteria as cylinders, linear elasticity equations for a cylinder undergoing free thermal strain can be solved and the change in the larger arc length in the double-curved geometry is predicted after deformation. The integral form of Gauss’ theorem is then applied to determine the deformed angle of the larger arc, from which geometric relations can be applied to extract the deformed radius. Abaqus is used first to study the torus segment on its own, and then to see the effects of the cylinder and flange segments on the overall geometry. Experimental measurements are also used as a comparison.</p><p dir="ltr">Generally, the formula derived using Gauss’ theorem predicts shape change very well for the torus segment on its own. When cylinder and flange segments are included in the geometry, an empirical correction factor can be introduced to account for geometrically induced stiffening effects. Future developments and next steps in this research are discussed.</p>

<div> <p>Fiber reinforced polymer composites are widely used in manufacturing advanced light weight structures for the aerospace, automotive, and energy sectors owing to their superior stiffness and strength. With the increasing use of composites, there is an increasing need to monitor the health of these structures during their lifetime. Currently, health monitoring in filament wound composites is facilitated by embedding piezoelectrics and optical fibers in the composite during the manufacturing process. However, the incorporation of these sensing elements introduces sites of stress concentration which could lead to progressive damage accumulation. In addition to introducing weak spots in the structure, they also make the manufacturing procedure difficult. </p> <p> </p> <p>Alternatively, nanofiller modification of the matrix imparts conductivity which can be leveraged for real time health monitoring with fewer changes to the manufacturing method. Well dispersed nanofillers act as an integrated sensing network. Damage or strain severs the well-connected nanofiller network thereby causing a local change in conductivity. The self-sensing capabilities of these modified composites can be combined with low cost, minimally invasive imaging modalities such as electrical impedance tomography (EIT) for damage detection. To date, however, EIT has exclusively been used for damage detection in planar coupons. These simple plate-like structures are not representative of real-world complex geometries. This thesis advances the state of the art in conductivity-based structural health monitoring (SHM) and nondestructive evaluation (NDE) by addressing this limitation of EIT. The current study will look into damage detection of a non-planar multiply connected domain – a filament-wound glass fiber/epoxy tube modified by carbon black (CB) filler. The results show that EIT is able to detect through holes as small as 7.94 mm in a tube with length-to-diameter ratio of 132.4 mm-to-66.2 mm (aspect ratio of 2:1). Further, the sensitivity of EIT to damage improved with decreasing tube aspect ratio. EIT was also successful in detecting sub-surface damage induced by low velocity impacts. These results indicate that EIT has much greater potential for composite SHM and NDE than prevailing work limited to planar geometries suggest.</p> </div> <br>

Leaf springs are a part of the suspension system attached between the axle and the chassis of the vehicle to support weight and provide shock absorbing capacity of the vehicle. For more than half a century the leaf springs are being made of steel which increases the weight of the vehicle and is prone to rusting and failure. The current study explores the feasibility of composite leaf spring to reduce weight by designing, manufacturing and testing the leaf spring for the required load cases. An off the shelf leaf spring of Ford F-150 is chosen for making of composite hybrid spring prototype. The composite hybrid prototype was made by replacing all the leaves with glass fiber unidirectional laminate except the first leaf. Fatigue tests are then done on steel and composite hybrid leaf spring to observe the failure locations and mechanism if any. High frequency fatigue tests were then done on composite beams with varying aspect ratio in a displacement-controlled mode to observe fatigue location and mechanism of just glass fiber composite laminate. It was observed that specimens with low aspect ratio failed from crack propagation initiated from stress concentrations at the loading tip in 3-point cyclic flexure test and shear forces played a dominant role in propagation of crack. Specimens with high aspect ratio under the same loading did not fail in cyclic loading and preserved the same stiffness as before the cyclic loading. The preliminary fatigue results for high aspect ratio composite beams predict a promising future for multi-leaf composite springs.

Fibre Reinforced Polymer Composites are extensively used in aircraft structures because of its high specific stiffness, high specific strength and tailorability. Though Fibre Reinforced Polymers offer many advantages, they are not free from problems. The damage of different nature, e.g., service mechanical damages, fatigue damage or environmental damage can be observed during operating conditions. Among all the damages, manufacturing or service induced, delamination related damage is the most important failure mechanisms of aircraft-composite structures and can be detrimental for safety. Delamination growth under fatigue loading may take place due to local buckling, growth from free edges and notches such as holes, growth from ply-drops and impact damaged composites containing considerable delamination. Delamination growth can also occur due to interlaminar stresses, which can arise in complex structures due to unanticipated loading. The complex nature of composite failure, involving different failure modes and their interactions, makes it necessary to characterize/identify the relevant parameters for fatigue damage resistance, accumulation and life prediction. An effort has been made in this thesis to understand the fatigue behavior of carbon fibre reinforced epoxy laminates under aircraft wing service loading conditions. The study was made on laminates with different lay-up sequences (quasi-isotropic and fibre dominated) and different geometries (plain specimen, specimen with a hole and ply-drop specimen). The fatigue behaviour of the composite was analyzed by following methods: . Ultrasonic C-Scan was used to characterize the delamination growth. . Dynamic Mechanical Analysis (DMA) was done to study the interfacial degradation due to fatigue loading. In this analysis, the interfacial strength indicator and interfacial damping were calculated. The DMA also provides the storage modulus degradation under fatigue loading. . Scanning electron microscope examination was carried out to understand the fatigue damage mechanisms. . A semi-empirical phenomenological model was also used to estimate the residual fatigue life. This research work reveals that the Carbon Fibre Reinforced Polymer laminates are in the safe limit under service loading conditions, except the specimen with a hole. The specimen with a hole showed delaminations around the hole due to stress concentration and higher interlaminar stresses at the hole edges and this delamination is found to be associated with fibre breakage and fibre pullout. The quasi-isotropic laminate is found to show poorer fatigue behaviour when compared to fibre dominated laminate and ply-drop also shows poor performance due to high stress concentration in the ply-drop region.

Laser cladding is a more recent approach to repair of aviation components within a damage tolerant framework, with its ability to restore not simply the geometric shape but the static and fatigue strength as well. This research analysed the fatigue performance of Ti-6Al-4V that has undergone a laser clad repair, comparing baseline specimens with laser clad repaired, and repaired and heat treated specimens. First an understanding of the microstructure was achieved by use of BSE imagery of the substrate, clad repaired region and post heat treated regions. The substrate of the material was identified with large grains which compared to a repaired clad region with a much finer grain structure that did not change with heat treatment. Next, performance of the specimens under tensile fatigue loading was conducted, with the clad specimens experiencing unexpectedly high fatigue performance when compared to baseline samples; the post heat treated specimen lasting significantly longer than all other specimens. It is theorised that the clad may have contributed to an increase in fatigue resilience due to its fine microstructure, when compared to the softer, more coarse substrate. The heat treatment is likely to have relaxed any residual stresses in the specimens leading to a reduction in any potential undesirable stresses, without impacting the microstructure. Residual stress analysis using EDD was unproductive due to the unexpected coarse microstructure and did not provide meaningful results. Fractography using the marker-band technique was explored with some success, proving a feesable method for measuring fatigue crack growth through a specimen post failure. Unfortunately fatigue crack growth throughout the entire fatigue life was not possible due to the tortuous fracture surface and potentially due to the fine micro-structure of the clad, resulting in interrupted marker-band formation. Future research shall expand on this work with a greater focus on residual stress analysis and its impact on fatigue.

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