Mines Repository
Communities in Mines Repository
Select a community to browse its collections.
Recent Submissions
Publication Effect of microstructure on edge ductility and fracture toughness of hot-rolled single- and multi-phase AHSS grades, The(Colorado School of Mines. Arthur Lakes Library, 2024)Hot-rolled AHSS grades are utilized in automotive parts where high formability is required, but the mechanical shearing process produces a sheared edge that reduces local ductility. Understanding the edge ductility and fracture toughness of these hot-rolled AHSS grades is critical to predict their performance in automotive applications. This study investigates the edge ductility and fracture toughness of four hot-rolled HSLA grades and four hot-rolled AHSS grades with strengths of 600 and 800 MPa achieved with single- or multi-phase matrix microstructures. The experimental and structural single-phase grades include microstructures comprised of ferritic matrices with mixed microalloyed precipitates and secondary microconstituents. The multi-phase grades include dual-phase (DP), ferritic bainitic (FB), and complex-phase (CP) steels, whose microstructures include a combination of ferrite, bainite, martensite, and mixed microalloy precipitates. Scanning electron microscopy, light optical microscopy, and electron backscatter diffraction were used to characterize the microstructure of each investigated grade. Hole expansion testing (HET) and deep draw cup (DDC) testing were performed to correlate microstructure and sheared face characteristics to edge ductility. Crack tip opening angle (CTOA) testing was performed to correlate microstructure and mechanical properties to fracture toughness. The results of this work confirmed that the strain incompatibility present between a matrix and secondary phases or microconstituents is detrimental to the hole expansion ratio (HER). Single-phase ferritic grades that exhibited high HER values exhibited low crack susceptibility in the DDC test, suggesting that edge ductility behavior can vary based on the stress state acting on the sheared edge, i.e. microstructural features that are beneficial to edge ductility in states of tension can be detrimental to edge ductility in multiaxial stress states. The measured CTOA of all but one investigated grades was similar, suggesting that differing microstructures and material properties had little impact on fracture toughness. A steel with an anisotropic grain structure and with both coarse and fine ferritic grains distributed throughout the matrix had the lowest measured edge ductility and fracture toughness, suggesting that these features can be detrimental to both HER and CTOA. It is interpreted that edge ductility and fracture toughness did not correlate to one another because they measure different microstructural conditions in the investigated grades. Specifically, the HER measures the behavior of a damaged microstructure via the sheared edge, and the CTOA measures toughness of the base material.Publication Scale-model investigation of brittle tunnel failure using a true-triaxial device(Colorado School of Mines. Arthur Lakes Library, 2024)Brittle instabilities in tunnel excavation have led to severe consequences such as spalling, violent rock ejection or rock burst, and tunnel collapse. Detached rocks from the tunnel boundary not only compromise the structural integrity of tunnels but also pose significant safety risks to personnel and construction equipment, disrupting the operations. Over the past decades, in-situ studies and laboratory tests have contributed to understanding the brittle failure mechanisms. However, the factors contributing to the progression and extent of brittle failure damage remain not fully understood, limiting the predictive and preventive capabilities. This dissertation aims to improve the understanding of tunnel brittle instabilities through laboratory experiments. The research focused on (1) developing an analog brittle rock model, (2) performing a series of tunnel model tests using a true-triaxial cell, a mock miniature tunnel boring machine (TBM), and acoustic emission (AE) monitoring, and (3) proposing more robust analytical methods and predictive models for brittle failures. The analog rock was designed to mimic the brittle behavior of sedimentary rock while maintaining a low uniaxial compressive strength (UCS). This combination of weak and brittle response was vital, as it allowed for using a lower-capacity true-triaxial loading apparatus and enabled large specimens, making it accessible to observe and document the progressive failure inside the excavated tunnel. The true-triaxial setup and mock miniature TBM allowed for realistic tunneling simulations, capturing unloading stress paths and face support behavior. Damage progression observation and post-mortem investigation provide detailed identification of damage mechanisms around the tunnel boundary, such as surficial spalling and damage zone development, as a V-shaped notch formed from progressive shear fracturing and resembled brittle-to-ductile transition. Real-time data on microcracking progression was captured using acoustic emission (AE) sensors during the loading stage, revealing potential precursors on brittle failure onset. Additionally, triaxial extension (TE) tests were introduced to improve the accuracy of spalling predictions. These tests provided a more representative biaxial stress state and unloading conditions in spalling that proved more robust than conventional triaxial compression (TC) tests, providing factors such as the entry angles and damage depth. In this study, the thin spalling with a steep angle measured from the minor principal stress direction in the tunnel model provided evidence of shear-driven fractures rather than extensional fractures. While steep angles may resemble extensional fractures, appearing perpendicular to minor principal stress, this likely results from shear-induced dilation along the fracture surface, triggering the mobilization of friction angle. Another novel contribution of this work is the development of quadratic Bézier curves in capturing the successive progression of fracturing based on damage factors and breakout width. This method offers a more robust representation of tunnel damage under anisotropic stress conditions and improves the theoretical logarithmic spiral plastic slip lines that rely solely on friction angle. In conclusion, this research improves the understanding of brittle failure mechanisms in tunneling, contributing to safer and more reliable tunnel designs in brittle rock environments. The developed experimental methods and predictive models offer valuable tools for mitigating the risks of spalling and rockburst during underground excavations.Publication Manipulation of colloidal particles under electric and magnetic fields(Colorado School of Mines. Arthur Lakes Library, 2024)Colloidal particles have garnered significant attention over the past few decades. Beyond their everyday applications in food science, lubricants, and cosmetics, they also serve as fundamental building blocks for advanced materials. An interesting subset of these particles, known as anisotropic particles, with their asymmetric geometric, interfacial, or compositional properties, have demonstrated remarkable potential across a spectrum of applications, ranging from self- and directed assembly to the development of microrobots. Extensive research efforts have been dedicated to synthesizing, assembling, and actuating anisotropic particles using external fields. However, a knowledge gap exists concerning the combined effects of electric and magnetic fields on both the assembly and active motion of colloidal particles. The convergence of these fields promises to address several unresolved issues in particle assembly and motion. This thesis is dedicated to exploring new propulsion and assembly behaviors of colloidal particles under the influence of electric and magnetic fields. Previously, we found that colloidal dimers with asymmetric shapes can propel along the substrate when subjected to a perpendicular electric field. However, their moving directions are random. Inspired by the separate engine and steering wheel systems in automobiles, we use orthogonally applied alternate-current electric field and direct-current magnetic field to control the dimer’s speed and direction independently. To this end, we first synthesize magnetic dimers by coating dopamine-functionalized nanoparticles on geometrically asymmetric polystyrene dimers. We then characterize their static and dynamic susceptibilities by measuring the hysteresis diagram and rotation speed experimentally and comparing them with theoretical predictions. The dimers can align their long axes quickly with a planar direct-current magnetic field, allowing us to control the particles’ orientation accurately. The propulsion speed of dimers is tuned by an alternating-current electric field applied perpendicularly to the substrate. As a result, we can direct the particle’s motion with pre-designed trajectories of complex shapes. Our bulk-synthesis approach has the potential of making other types of magnetically anisotropic particles. The synthesized magnetic dimers can be further assembled into chiral clusters under AC electric fields. However, similar to their molecular counterparts, these assemblies often result in racemic mixtures. We invent an approach to obtain single-handed clusters from colloidal dimers using orthogonal electric and magnetic fields. By superimposing a planar rotating magnetic field, we break the image symmetry so that one chirality is favored over the other. By adjusting the magnetic field’s direction and strength, as well as the electric field frequency, we can not only control the handedness of a cluster precisely but also induce uniform chirality in initially achiral clusters when exposed solely to the electric field. This work demonstrates the potential of integrating external fields and provides a viable way to create reconfigurable chiral colloidal structures. Alternating current electric fields can assemble microspheres into various colloidal clusters, but the mechanisms driving this process remain unclear. We investigate how particle concentration, salt concentration, and electric field frequency influence the formation and transformation of these clusters. By experimentally measuring the strengths of dipolar and electrohydrodynamic interactions and analyzing the balance of these forces under different conditions, we explain the observed morphological changes in the clusters. Moreover, we find that as the frequency increases at high particle concentrations, colloidal tetramers transform into square-shaped pentamers, which can further assemble into square or sigma-phase arrays–complex structures that are difficult to achieve with isotropic particles. Overall, this thesis explores the multifaceted realm of colloidal particles and their dynamic response to external fields, offering fundamental insights toward practical applications across various domains.Publication Better together: the case for cooperative regionalism in U.S. energy policy(Colorado School of Mines. Arthur Lakes Library, 2025-04-03)As U.S. Environmental Protection Agency Administrator Lee Zeldin recently announced plans for the most significant rollback of federal regulations in U.S. history, a critical question emerges: Will delegating power to states foster innovation and economic growth or could it jeopardize essential protections for public health, safety, and the environment?Publication Eyes on the Arctic: satellite monitoring of the Arctic LNG 2 terminal(Colorado School of Mines. Arthur Lakes Library, 2025-04-10)The Arctic LNG 2 project operated by Novatek, Russia's largest independent natural gas producer, represents a significant undertaking in the global energy sector [1]. Situated on the Gydan Peninsula in the Arctic region, this ambitious project aims to tap into vast natural gas reserves and establish Russia as a leading exporter of LNG. The terminal is designed to eventually consist of three liquefaction trains, with a total planned annual production capacity of 19.8 million metric tons of LNG. This capacity is crucial for Russia's strategic goal of significantly increasing its share in the global LNG market, targeting a substantial portion of the expanding demand, particularly in Asia.