Development of a Stress Corrosion Cracking test approach for multi-mode measurements

Experimentation to study stress corrosion cracking (SCC) often requires simulation of aggressive environments that include both tensile loading with simultaneous chemical exposure. This demands complex testing approaches that will benefit from multi-mode measurements to capture in-situ degradation. Multi-mode measurements aid in characterizing the nature of the reaction, quantifying the formation of corrosive products and in calculating the breakdown potential of the sample. This paper details a testing approach for the chemical aspect of SCC on aluminum samples where sample and experimental design facilitates in-situ imaging. In initial tests, 3.5 wt. % NaCl solution is applied to AA7075 for specific time frames which initiate the pitting process. The testing approach enables to produce optical microscope images of real-time reactions. Raman and microscopy measurements help capturing failure initiation mechanisms which eventually lead to crack propagation. Identification of specific Raman peaks reveals the nature of the products formed on the surface using the pre-exposure values as a baseline for the characterization. These results from multi-mode measurements can be compared to understand SCC processes with both qualitative and quantitative information. Knowledge gained can be used to design materials and processes to better withstand corrosive environments

High-resolution stress mapping of polycrystalline alumina compression using synchrotron X-ray diffraction

The ability to achieve uniform stress in uniaxial compression tests of polycrystalline alumina is of significance for the calibration of piezospectroscopic coefficients as well as strength studies in ceramics. In this study high-energy X-rays were used to capture powder diffraction profiles over a half-section of a polycrystalline alumina parallelepiped sample under an increasing uniaxial compressive load. The data were converted to strain and results were used for stress mapping of the sample. Stress maps from the study quantify the higher stresses at the sample-platen contact interface and reveal the evolution of the stress distribution in these specimens with load. For the geometry of the samples used, at the center section of the specimen the overall magnitudes of the compressive stresses were found to be 20% higher compared with the average expected theoretical stress based on the applied load and cross-sectional area. The observed compressive stresses at the corners of the parallelepiped specimen were 62% higher and shear stresses were observed at the specimen interface to the load mechanism. The effects, seen at the interface, can lead to premature failure at these locations and can affect the accuracy of calibration of spectral peaks with stress as well as compression strength measurements. The results provide important information that can be used to establish guidelines on material and geometry considerations in developing compression tests on high-strength ceramics

Comparison of 3D confocal Raman and high energy X-ray diffraction for the measurement of molten sand infiltration in turbine blade coatings

Thermal barrier coatings (TBCs) are used to protect metallic blades subjected to the extreme temperatures found within the turbine sections following the combustion chamber of jet engines. Yttria-stabilized zirconia (YSZ) is the standard high temperature ceramic material used for TBCs. One of the primary deposition methods of these TBCs on the metallic blades is electron-beam physical vapor deposition (EB-PVD), which gives YSZ a characteristic columnar, porous structure. This allows for a higher strain tolerance than that of other deposition methods; however, it also increases the susceptibility of the coating to molten sand and volcanic ash infiltration, referred to as calcium-magnesium-alumino-silicates (CMAS), which greatly reduces the lifetime of TBCs. A variety of techniques are used to analyze and quantify the damage to the coating; two of which will be discussed and compared in this paper. 3D confocal Raman spectroscopy and high-energy x-ray diffraction (XRD) measurements were acquired non-destructively from CMAS-infiltrated samples. The results were compared to show how the similarities and differences in the data collected can reveal a complete picture of the chemical degradation within the TBC due to CMAS. This includes the comparison of phase volume fractions found by each method. Both methods showed that higher temperatures and longer annealing times lead to a greater volume of the monoclinic phase, which is linked to the eventual failure of the coating. 3D confocal Raman provides local phase volume fractions, spatially resolved for the assessing the effects of infiltration on single columns. Meanwhile, XRD provides a more global quantification of phase volume fractions throughout the probed volume. This work highlights the complimentary nature of 3D confocal Raman and XRD for high accuracy determination of degradation mechanisms for improved lifetime predictions of TBCs

The Impact of Innovation in the New Era of Space Exploration

Every once in a while, a confluence of discoveries, events and initiatives results in a breakthrough so significant that it propels the entire world to a higher level, redefining what is possible in so many different fields. This breakthrough is taking centerstage now, as the new era of space exploration — catalyzed by increasing launch access — dawns upon us. The surge of innovation that comes with this will create new opportunities and inspire the next generation of doers. When this happens, boundaries between scientific and social impact are blurred. Innovation leading to scientific discovery can benefit society in the same way that social innovation can diversify and support scientific innovators, who can contribute to global progress. To ride this wave of progress, we must all participate and innovate in the new era of space exploration

Shattered Glass Ceilings that Pave the Way to Inspiration

Academic institutions need to come to the realization that to grow and retain a diverse student population, there is a need to demonstrate commitment to representation

When Flying is Truly Out of This World

My fascination with flight — like that of many others — is simple: There is something about the way an aircraft dances gracefully with the wind to lift itself off the ground, seemingly defying the impossible, that has always intrigued me

Non-destructively capturing CMAS degradation of EB-PVD thermal barrier coatings through 3D confocal Raman renderings

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3D Printed Stress Sensors for NonDestructive Evaluation of Space Structures

Self-sufficient and non-contact sensors play multiple roles in lunar, planetary exploration, and Earth structures. These sensors allow engineers to accurately examine structural integrity and defects on mechanical components for optimal operations. Structural integrity allows the industry to ensure the safety and capacity of key structures. Materials like α-alumina can be employed as sensors due to the photoluminescent properties that they possess. Piezospectroscopy is a non-destructive evaluation (NDE) method capable of capturing in-situ stress using α-alumina due to the chromium ion impurities that it contains. The chromium ion impurities carry spectral characteristics, that when excited with an Nd: YAG laser (532 nm), demonstrate capabilities for structural integrity monitoring. In this work, a 3D printing method is developed to autonomously create sensors that are compatible with use in space environments. The 3D printing method intends to provide the industry flexible and adaptive solutions for structural integrity monitoring. This method includes a modified Fused Deposition Method printer by exchanging its original nozzle with a syringe base nozzle. The printing parameters such as printing speed, printing bed temperature, coating thickness, and syringe volume are determined during the testing process. Challenges include achieving uniform integration and nanoparticle dispersion as well as adhesion between the matrix and the substrates. The parameters to encounter these challenges will depend on the materials used. Experiments with three different volume fractions (VF) of α-alumina within an epoxy were performed to address the printing challenges. The sensors were applied to nine specimens, three of each VF but with varying deposition rates after the mixture process. These experiments considered the mixing and deposition method while testing the dispersion within the α-alumina and the epoxy matrix. The substrates, on which the epoxy matrix was deposited, underwent a surface treatment to ensure adhesion between the substrate and the sensor matrix. During this experiment, the epoxy matrix was deposited with a syringe onto a substrate and cured at room temperature. The specimens were tested with a tensile load using an electromechanical MTS. While the samples are tensile loaded, the sensors were characterized via photoluminescent piezo spectroscopy to determine which VF demonstrates the best stress sensing capabilities, along with the adhesion between the matrix and the substrate. The data collected allows the optimal VF to be established for future applications

Stress and structural damage sensing piezospectroscopic coatings validated with digital image correlation

The piezospectroscopic effect, relating a material\u27s stress state and spectral signature, has recently demonstrated tailorable sensitivity when the photo-luminescent alpha alumina is distributed in nanoparticulate form within a matrix. Here, the stress-sensing behavior of an alumina-epoxy nanoparticle coating, applied to a composite substrate in an open hole tension configuration, is validated with the biaxial strain field concurrently determined through digital image correlation. The coating achieved early detection of composite failure initiation at 77% failure load, and subsequently tracked stress distribution in the immediate vicinity of the crack as it progressed, demonstrating non-invasive stress and damage detection with multi-scale spatial resolution