Thesis (Ph. D.)--University of Sydney, 2005. / Title from title screen (viewed 27 May 2008). Submitted in fulfilment of the requirements for the degree of Doctor of Philosophy to the School of Aerospace, Mechanical and Mechatronic Engineering, Graduate School of Engineering. Includes bibliographical references. Also available in print form.

Mode of access: Internet via World Wide Web. Available http://hdl.handle.net/1947/9738. / "November 2008" Includes bibliographical references.

Thesis (M.S.)--University of North Texas, Dec., 2009. / Title from title page display. Includes bibliographical references.

Thesis--Auburn University, 2009. / Abstract. Vita. Includes bibliographic references (p.118-121).

Aluminum alloys are used on aerospace vehicles due to their high strength-to-weight ratio, formability and machinability. However, they become vulnerable to stress corrosion cracking (SCC) during their service life. SCC is primarily caused by the material's stress condition, a suitable corrosive environment and material susceptibility. It is also influenced by a mixture of electrochemical, mechanical, and chemical factors. Due to the complexity of SCC, tools with better resolution and sensitivity are needed to better understand the impact and interaction of the contributing factors. A vast amount of research has been done to study SCC behavior, but the scale of characterization must be reduced to elucidate the key initiation mechanisms. In this work, it is shown that SCC initiation was detected early via micro-digital image correlation (micro-DIC) prior to the crack being discernible in microscopy images. The initial effort to monitor stress corrosion cracking in AA7075-T6 involved using a pixel resolution of 3.825 microns/pixel, frame rate of 10-15 min/image and an airbrush nozzle diameter of 0.3 mm for the speckle pattern, which led to the detection of crack initiation at 98% failure load. By using a pixel resolution that is 6 times smaller, a frame rate of up to 60 times less time per image, and an airbrush nozzle that is 2 times smaller, the first observation of strain concentration marking the eventual failure region of the AA7075-T6 sample was detected as early as 58% failure load. When the micro-DIC technique was applied to study SCC behavior in additively manufactured AlSi10Mg, the first observation of localized strain marking the eventual failure region of the sample was detected at 78% failure load. X-ray synchrotron tomography was used to qualitatively assess the hydrogen bubble and precipitate formation and to quantitatively assess the post initiation crack growth in AA7075-T651. With improved micro-DIC parameters and correlation with experimental outcomes from x-ray synchrotron tomography, multiple factors contributing to SCC can be assessed to better understand the mechanisms of SCC initiation. Correlations of material exposure time and load with SCC initiation can provide data for developing corrosion control strategies and new and improved alloys or heat treatment, as well as understanding SCC behavior in alloys made through unconventional means, such as additive manufacturing. The impact of this work lies in the life extension of alloys and greater reusability and fatigue life extension of aerospace vehicles.

Wear-resistant ceramic and ceramic composite coatings are significant to provide durability and support long-duration missions to the moon's surface for rovers, landers, robotic systems, habitats, and many other components. On the Lunar surface, structural components are continuously exposed to lunar dust projectiles that can cause protective coating delamination around the affected area, of protective coatings and this may not be physically visible. Ceramic coatings, composed of alumina, present excellent resistance to different types of wear due to their high strength and hardness as well as the ability to protect structural components from regolith impacts, wear, and abrasive damage. Air Plasma Spray (APS) already plays a vital role in the aerospace industry by protecting engine structures against wear, friction, corrosion, high temperatures, and harsh environments. In this study, an Inconel 738 substrate was grit blasted, following a 100-μm layer bond coat, and a 200-μm layer of alumina was deposited using APS. Prior to performing destructive experiments, the microstructure and characteristics of the APS alumina coating were studied and analyzed. Scanning electron images were collected to observe the anisotropic properties of the APS alumina coating. The X-Ray diffraction measurements demonstrated that α-phase and γ-phase are the dominant phases present in the APS alumina coating. The roughness of the APS alumina coating was measured with a profilometer, resulting in an average of 4.063 μm. The surface energy plays a role in enhancing the adhesion of the regolith particles to the surface of the components and systems used for lunar exploration. In this study, the surface energy had an average advancing contact angle of 61.13°, relating to low surface energy. Artificial damage was introduced by indenting the coating using Rockwell and Vickers indenters. The hardness of the APS alumina coating was measured around the different indentation locations. The measured Vickers hardness values at 1000 gf and 2000 gf were found to be 0.2913 GPa and 0.5677 GPa, respectively. An initial Rockwell hardness value of 38.9 was found and was reduced to 22.5 after two surrounding indentations were applied to the coating. Results showed that the Rockwell hardness value decreases as the number of indentations around the initial indent increases. The fracture toughness of the APS alumina coating was calculated using the cracks formed during the last two Rockwell indentations and was found to be 2.48 and 2.95 MPa√ m. Considering the optical properties of the alumina, piezospectroscopic (PS) measurements were taken to detect the underlying coating delamination and determine the mechanical properties of the APS alumina coating. The peak shifts from the characteristic alumina peaks revealed the underlying damage, quantifying the effect of projectiles on the overall coating integrity. The multifunctional properties of alumina, utilized in the studies performed here, have offered a unique means for understanding the durability of a material with high spatial and stress resolution.

Carbon fiber composites are continually seeing increased use in aerospace applications. It is necessary to understand their failure modes in order to properly design and perform analysis on structures constructed primarily from them. This thesis studies woven carbon fiber composites with and without double-edge notches in a series of static and fatigue tests performed on an Instron 8801 servo-hydraulic testing system. Specimens were constructed of Advanced Composites Group product # LTM45EL woven carbon fiber pre-preg/epoxy and were cut to approximately 9-inch in length and 1-inch in width. Notches were cut into some of the specimens using a slitting saw blade of 0.006-in. thickness. Ultimate strength, Young’s modulus and Poisson’s ratio for specimens were determined to be 119,418 psi, 7,149,000 psi and 0.05, respeictively. Fracture stress for specimens with notch depths of 0.10, 0.15, 0.20, 0.25, 0.30 and 0.35 was determined to be 93,481 psi, 88,193 psi, 86,968 psi, 81,112 psi, 84,197 psi and 81,955 psi, respectively. The results from these tests showed that the specimens followed Griffith’s model for brittle failure. Average number of cycles to failure was determined to be 6,600, 37,200, 94,300 and 293,400 for fatigue tests with maximum stresses of 72.5%, 75%, 77.5% and 80% of the ultimate strength. Fatigue tests performed on notched specimens produced data that was too scattered from which to draw a statistically significant result. Numerical modeling in Abacus showed comparable results to experimental tests for stress and strain.

Impact strength is one of the most important structural properties for a designer to consider, but it is often the most difficult to quantify or measure. A major concern for composite structures in the field is the effect of foreign objects striking composites because the damage is often undetectable by visual inspection. The objective for this study was to determine the effectiveness of using dynamic testing to identify the existence of damage in a small scale composite wing design. Four different impact locations were tested with three specimens per location for a total of 12 wings manufactured. The different impact locations were over the skin, directly over the rib/spar intersection at the mid-span of the wing, directly over the middle rib, and directly over the leading edge spar. The results will be compared to a control group of wings that sustain no damage. The wing design was based on an existing model located in the Cal Poly Aerospace Composites/Structures lab. The airfoil selected was a NACA 2412 airfoil profile with a chord length of 3 inches and a wingspan of just over 8 inches. All parts cured for 7 hours at 148°F and 70 psi. The wings were each tested on a shaker-table in a cantilever position undergoing 1g (ft/s2) acceleration sinusoidal frequency sweep from 10-2000 Hz. The 1st bending mode was excited at 190 Hz and the 2nd bending mode was excited at 900 Hz. After the pre-impact vibrational testing each wing was impacted, excluding the control group. To verify the experimental results, a finite element model of the wing was created in ABAQUS. The frequency and impact numerical results and the experimental results were in good agreement with a percent error for both the 1st and 2nd mode at around 10%.

This study presents the effect of the stacking sequence and fiber orientation on a composite sandwich panel subjected to static in-plane bolt loading. Six plates were constructed with laminates of unidirectional carbon fiber and cross ply weaves of fiberglass. The orientations that were examined included 0, +/- 45, and 90 degrees. Half of the plates had fiberglass lamina on the outside of the laminate while the other three plates had the carbon fiber on the outside. Experimental and analytical tests were performed to determine the best orientations and stacking sequence. For the numerical analysis, plates with fibers oriented at +/- 45 degrees showed the highest strength. The experimental data also showed high strengths for the +/- 45 degree plates. However the experimental data also showed high strengths for the 90 degree laminate but with very high displacements. These high displacements would not allow the joint to maintain its relative position to the adjacent part. The discrepancy between the strength of the FEA models and the experimental data is attributed to inaccurate strength properties. The effect of in situ strength and compression strength was found to have a significant effect on the accuracy of the FEA solution. Good correlation was found between the FEA and experimental data in predicting the trend of the stiffness of the plates.

Today’s environmental concerns have led a renewed search in industry to find new sustainable materials to replace non-renewable resources. President Barack Obama also quoted in the recent 2012 Presidential Debate “that there is a need to build the energy sources of the future and invest in solar, wind, and bio-fuels.” Bio-composites are believed to be the future and the new substitute for non-renewable resources. Bio-composites are similar to composites in that they are made up of two constituent materials; however the main difference is that bio-composites are made from natural fibers and a biopolymer matrix. This research investigates the buckling behavior of bamboo and will analyze and determine the slender ratio that will induce buckling when bamboo is used as a column. Along with the investigation of the bamboo under buckling, this study will also show the potential of bio-composites to replace non-renewable resources in industry through experimental and numerical analysis. However, in order to study the buckling behavior of the bamboo, the mechanical characteristics of the bamboo and optimal curing treatment first had to be established. This is because, in order for bamboo to acquire proper strength characteristics, the bamboo must first be treated. Due to the scarcity of bamboo material in the lab, the obtainment of the mechanical properties of the bamboo as well as the optimal curing treatment was done in collaboration with Jay Lopez. In order for bamboo to acquire proper strength characteristics, the bamboo must be treated. In the first study, a total of four different types of natural treatments were analyzed to optimize the mechanical characteristics of bamboo. To assess each curing method, tensile and compression tests were performed to obtain the mechanical properties. Due to each bamboo culm having different thicknesses and cross sections, the specific strength property is used to normalize the data and allow for easy comparison and assessing of each curing method equally. The specific strength parameter is defined as the ultimate stress divided by the density of the material. These curing treatments consisted of four thermo-treatments, three different percentages of salt treatments, one lime treatment, and one oil treatment. The thermo-treatments consisted of heating the bamboo internodes in an autoclave with no pressure at 150oF, 180°F, 200°F, and 220°F. The experimental results of the thermo-treatments determined that bamboo obtains higher mechanical properties as well as reduced weight when heated at higher temperatures. This is explained by the increasing bound water extracted from the bamboo material at higher temperatures. In addition to finding the optimal heat treatment, the internodes of bamboo were soaked in natural additives that included a 3%, 6%, and 9% Instant Ocean sea salt solution, a Bonide hydrated lime solution, and a Kirkland canola oil solution for approximately five days and then heat treated at the optimal temperature of 220°F. The experimental results showed that all of the different additives had a significant effect on the mechanical properties. After determining the mechanical properties of each curing method, the results were then analyzed through a trade study. The trade study parameters consisted of weight-drop of the material, the specific strength, and the ultimate stress for both compression and tension. Each parameter of the trade study is kept unbiased as the weighting of each parameter is set equal to each other. The results of the trade study indicated that the 3% salt solution was the optimal curing treatment, yielding a higher specific strength value for both compression and tension, along with a significantly lower weight-drop after curing. After we came up with the optimal treatment, the buckling behavior of bamboo was investigated. The buckling analysis was investigated to determine at what slenderness ratio the bamboo would buckle when used as a column. A total of seven cases were investigated using different lengths, that ranged from 1.5” to 10”. Through experimental results, it was determined that a slenderness ratio above approximately 34.7 would induce global buckling to the bamboo column. The last investigation of this study consisted of building a small prototype wall structure using bio-composites. The prototype wall structure was manufactured using a combination of bamboo and a bi-directional woven hemp fabric. The dimensions of the prototype were 15.13” long and 7.75” tall. The wall structure was tested under compression in the Aerospace Structures/Composites Lab and the Architectural Engineering Department’s high bay laboratory. The results of the experimental test on the wall showed great potential for bio-composites, as the structure withstood a force of 46,800 pounds. A numerical analysis technique was also employed through the finite element method using the Abaqus software. The purpose of the finite element method was to validate the experimental results by comparing the buckling behavior of the tests. The numerical analysis showed very good agreement with the experimental results.