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This thesis focuses on the use of scaled physical experiments to better understand the development and long-term evolution of fault systems that are otherwise impossible to observe directly. The document is divided into three chapters. The first chapter documents the implementation of an inexpensive stereo vision method for acquiring high resolution three-dimensional strain data for table-top experiments. The second chapter applies the stereo vision method to a tectonic problem—the development of slip partitioning in obliquely loaded crustal systems. Slip partitioned fault systems accommodate oblique convergence with different slip rake on two or more faults and are well documented in the crust. In this chapter, we simulate oblique convergence using blocks with 30° dipping contacts under wet kaolin clay. The experiments reveal three styles of slip partitioning development—contingent upon convergence angle and the presence or absence of a pre-existing vertical fault. Across all experiments, the slip rates along slip-partitioned faults vary temporally suggesting that the faults continuously adjust to conditions produced by the other fault. The lack of steady state in the experiments suggests that slip-partitioned crustal systems may also evolve with oscillating behavior rather than developing a single efficient active fault structure to accommodate oblique convergence. The third chapter documents rheological tests of wet kaolin for applications to crustal deformation experiments. This chapter investigates thixotropy in the clay as well as the role of grain size distribution and water content on its shear strength.

The uterosacral ligaments (USLs) are important anatomical structures that support the uterus and apical vagina within the pelvis. As these structures are over-stretched, become weak, and exhibit laxity, pelvic floor disorders such as pelvic organ prolapse occur. Although several surgical procedures to treat pelvic floor disorders are directed toward the USLs, there is still a lot that is unknown about their function. These surgeries often result in poor outcomes, demonstrating the need for new surgical approaches and biomaterials. The first chapter of this dissertation presents a review of the current knowledge on the mechanical properties of the USLs. The anatomy, microstructure, and clinical significance of the USLs are first reviewed. Then, the results of published experimental studies on the {emph{in vivo}} and {emph{ex vivo}}, uniaxial and biaxial tensile tests are compiled. Based on the existing findings, research gaps are identified and future research directions are discussed. The second chapter proposes the use of planar biaxial testing, digital image correlation (DIC), and optical coherence tomography (OCT) to quantify the deformations of the USLs, both in-plane and out-of-plane. Using virgin swine as an animal model, the USLs were found to deform significantly less in their main direction (MD) of {emph{in vivo}} loading than in the direction perpendicular to it (PD) at increasing equibiaxial stresses. Under constant equibiaxial loading, the USLs deformed over time equally, at comparable rates in both the MD and PD. The thickness of the USLs decreased as the equibiaxial loading increased but, under constant equibiaxial loading, the thickness increased in some specimens and decreased in others. The third chapter presents new experimental methods for testing the {emph{ex vivo}} tensile properties of the uterosacral ligaments (USLs) in rats. USL specimens were carefully dissected to preserve their anatomical attachments, and they were loaded along their main {emph{in vivo}} loading direction (MD) using a custom-built uniaxial tensile testing device. This chapter reports the first mechanical data on the rat USLs in isolation from surrounding organs. It is also the first experimental study to provide measurements of the inhomogeneous deformations of the USLs during loading along their main textit{in vivo} loading direction, revealing that the USLs may behave as auxetic structures. The fourth and final chapter presents preliminary findings on novel imaging applications to characterize the evolving structure of the USLs before, during, and after tensile pulling along the ligaments' main textit{in vivo} axis of loading. Rat USLs were excised using the proposed novel dissection method and pulled uniaxially as was performed in the previous chapter. Before and after mechanical testing, second harmonic generation (SHG) was used to image collagen and muscle within the three anatomical regions of the USLs. During mechanical testing, OCT was used to collect out-of-plane images of the cervical/intermediate regions of the USL specimens, resulting in 3D volume scans of the regions. SHG images showed the USLs to have complex microstructures with significant wavy collagen bundles interwoven with muscle bundles. Preliminary observation of the microstructure during testing revealed interwoven sections of tissue with collagenous fibers that reoriented in all directions illustrating how the USLs may expand laterally during uniaxial loading, causing the auxetic properties documented in the previous chapter. Though more quantitative work remains to be done, the findings presented in this dissertation improve our understanding of how the USLs deform with increasing load, such as what occurs during pregnancy. Together, these studies serve as a springboard for future investigations on the supportive function of the USLs in animal models by offering guidelines on testing methods that capture their complex mechanical behavior. / Doctor of Philosophy / The uterosacral ligaments (USLs) are important anatomical structures that support the uterus and vagina and are often used to restore the support of pelvic organs during surgeries for pelvic organ prolapse. These surgeries often result in poor outcomes, demonstrating the need for new surgical approaches and graft materials. Due to their supportive role, the mechanical properties of the USLs are important for their physiological function, and they must be investigated to improve current treatment strategies for pelvic organ prolapse. To this end, we designed new equipment, dissection, and testing methods to characterize the mechanical behavior of the USLs using swine and rats as animal models. We provided the first three-dimensional characterization of time-dependent deformations of swine USLs as they were pulled along their two physiological loading directions using advanced imaging methods, including digital image correlation and optical coherence tomography. We isolated the USLs from rats with their anatomical attachments and mechanically tested them along their main physiological loading direction, reporting the first mechanical data on the rat USLs in isolation from surrounding organs. Finally, we used the advanced imaging techniques optical second harmonic generation microscopy and optical coherence tomography to determine how the microstructure (e.g., collagen and muscle) of the rat USLs evolves before, during, and after mechanical testing. These findings advance our understanding of the three-dimensional, nonlinear, heterogeneous, elastic, and viscoelastic deformations of the USLs. Our work may serve as a springboard for future investigations on the supportive function of the USLs by offering guidelines on testing methods that capture their complex mechanical behavior.

Lightweighting demands have been achieved by third generation (3G) Advanced High Strength Steels (AHSSs) by a means of increased strength. The challenge faced in doing so, however, is in ensuring that ductility and crashworthiness is efficiently retained. Key methods in which automotive research has been invested to achieve this strength-ductility balance is by microalloying to promote grain refinement, the introduction of precipitates, and the effective use of plasticity enhancing mechanisms. Specifically, the ability to tailor the stability of retained austenite during deformation has been crucial in manipulating the strength-to-ductility ratio of 3G AHSSs using the Transformation Induced Plasticity (TRIP) effect. On the other hand, dual phase (DP) (i.e: non-TRIP-assisted steels) continue to be most significantly manufactured due to their robust thermomechanical processing but are also compromised by their poor damage tolerance. Hence, considerable reports are available regarding the damage tolerance of DP steels, but the ability for the volume expansion associated with the austenite-to-martensite transformation to suppress damage evolution and enhance a steel’s local formability has not yet been thoroughly investigated. Nonetheless, the damage processes that lead to fracture in 3G AHSSs are complex. A full understanding of the underlying phenomena requires a careful assessment of the strain partitioning amongst phases, how the microstructure evolves with strain and how damage, in the form of voids and micro-cracks, nucleates and grows. This can only be accomplished by applying a range of methodologies, including microscopic Digital Image Correlation (µDIC), X-ray Computed Microtomography (µXCT), Electron Backscattered Diffraction (EBSD) and X-ray Diffraction (XRD), all of which can be tracked as deformation proceeds. This PhD thesis uses a novel post µDIC data processing technique to prove that a reduction in strain gradient, linked to the evolution Geometrically Necessary Dislocations (GNDs), at dissimilar phase interfaces is attainable with vanadium-microalloying and with use of the TRIP effect. A local strain gradient post µDIC data processing technique was developed and first applied on 3G DP steels to show that the microcompatibility between ferrite and martensite directly at the interface is considerably improved with vanadium-microalloying. This in turn microscopically explains this DP steel’s increased local formability/damage tolerance with vanadium micro-additions. Moreover, when applying this novel µDIC technique on two other 3G experimental steels of interest, an ultrahigh strength Quench & Partition (Q&P) steel and a continuous galvanizing line (CGL)-compatible Medium-Mn (med-Mn) steel, an even slower evolution of microstrain gradients at dissimilar phase interfaces was observed. This indicates that, although vanadium-microalloying can improve the damage tolerance of a DP steel, its ability to achieve the ultrahigh strengths is a direct result of the severe inhibition of dislocation motion at dissimilar phase boundaries. Eventually, at high strains, these local strain gradients cannot be maintained and results in premature damage nucleation. By comparison, at such high strains, distinct evidence of damage nucleation was not apparent in the 3G TRIP-assisted steels which is the result of a slow strain gradient evolution delayed by the effective use of TRIP. This finding triggered a further investigation into isolating the impact the rate of TRIP exhaustion has on damage development. By intercritically annealing this prototype med-Mn steel (0.15C-5.8Mn-1.8Al-0.71Si) with a martensitic starting microstructure, within a narrow temperature interval (from 665 to 710°C), it was possible to make significant changes in the steel’s rate of TRIP exhaustion without making considerable changes to its physical microstructure. This steel exhibits the largest true strain at fracture (ɛf = 0.61), meets U.S. Department of Energy (DoE) mechanical targets (28,809 MPa%), and shows sustained monotonic work hardening when intercritically annealed at an intermediate IA temperature of 685°C for 120s. In addition, this IA condition showed optimal damage tolerance properties as an abundance of voids nucleated during its tensile deformation, but their growth was suppressed by prolonging TRIP over a large strain range. There is reason to believe that the heterogeneous distribution of austenite and Mn throughout this 685°C IA condition compared to the other two enabled its suppressed TRIP kinetics and in turn improved damage tolerance. The impact that changes in stress-state, from a stress triaxiality of 0.33-0.89, has on microstrain partitioning, TRIP kinetics and damage evolution was tested on this med-Mn at its 685°C IA condition. With the machining of notches on tensile specimens, it was seen that a high stress triaxiality (0.74-0.89) accelerated the rate of TRIP, whereas the introduction of shear, through a misaligned notched specimen design, delayed TRIP kinetics. The change in mean stress imposed by the notches was deemed to have played an active role in TRIP exhaustion during the material’s tensile deformation. A unique electropolishing micro-speckle patterning technique was applied to show that the amount of strain that can be accommodated by the steel’s the polygonal ferrite-tempered martensitic regions are considerably impacted by external modifications in stress-state. While damages studies using different such notched tensile geometries revealed that once a critical void size is reached in this med-Mn steel, coalescence proceeds at an increasing, exponential rate up to fracture. It continues to remain a challenge to quantify the effects microstrain partitioning, TRIP kinetics and damage evolution separately, opening new avenues for future experimental and modeling investigations. / Thesis / Candidate in Philosophy / A lot of research up to now has been invested in the automotive industry to create steels that are lightweight, strong and show improved crashworthiness. The means by which this has been achieved is with the use of innovative processing routes to manufacture and implement Advanced High Strength Steels (AHSSs) in a vehicle’s body-in-white. Nonetheless, the constant global pressure to reduce greenhouse gas emissions has eventually driven research to a third-generation class of ultrahigh strength, lightweight AHSSs. These steels retain the weight savings of their second-generation counterparts but are more cost-effective to manufacture and can be adapted to current industrial line capabilities. Considerable work has been done to enable the manufacturing of 3G steels, yet the steel characteristics which underpin fracture, thereby affecting the crashworthiness of these steels, continues to be weakly understood. As such, at a microscopic scale, this thesis uses three different promising 3G AHSSs candidates to evaluate the impact their unique steel characteristics has on the ability to resist damage evolution and fracture.

PR-2930 was developed by PPG Industries, Inc. to meet the challenging performance requirements of MIL-PRF-32662 Group-I-classified adhesives. PR-2930 is a high-strength, high-toughness, epoxy-based adhesive intended for automotive and aerospace applications. As PR-2930 functions as a structural adhesive, quantification of its mechanical properties and limit-states is a necessary task for designing joints bonded with the adhesive. The combination of both strength and ductility results in material non-linearities, making experimental characterization and numerical analyses more challenging. This work explores the quantification of fracture energy for PR-2930 bonded joints. Fracture can occur in one of three different modes, or in some combination. Many practical adhesive joints fail in the mixed-mode region involving both opening (mode I) and shearing (mode II) displacements. Mode I fracture was evaluated with double cantilever beam (DCB) tests, mode II fracture was characterized by end-notched flexure (ENF) tests, and varying degrees of mixed mode I/II fracture were assessed through single leg bend (SLB), single-lap joint (SLJ), and asymmetric DCB and SLB tests. Test specimens were fabricated by bonding Al 2024-T3 adherends, ranging from 1.6 mm to 25.4 mm thick, with a 0.25 mm thick PR-2930 adhesive layer. Digital image correlation (DIC) was used to experimentally measure local displacements and surface strains on the adherends. Standard data-reduction methods often used to determine fracture energies of bonded joint specimens were used to numerically analyze test results. These methods included the Corrected Beam Theory (CBT), the Compliance-Based Beam Method (CBBM), and the Paris and Paris J-Integral approach. Linear elastic fracture mechanics (LEFM) conditions must be valid to correctly apply these methods, however plastic deformations were observed in some adherends. Drawbacks of these approaches and their validity for analyzing PR-2930 joints were discussed. To account for non-linearities, more advanced numerical analysis was performed using finite element analysis (FEA) with cohesive zone models (CZMs) to model the adhesive layer. CZM parameters such as fracture energies and traction separation law (TSL) shapes were determined from experimental data and published literature. Results from CZMs were compared to experimental load, displacement, and strain data. Recommended TSLs for mode I and mode II fracture were formed in this work as well as a mixed-mode relationship using a Benzeggagh-Kenane damage evolution law. More ideal analytical methods were suggested to simplify analysis of joints using the same or similar material compositions. / M.S. / Structural adhesives are used to safely transmit loads in our furniture, automobiles, aircraft, and buildings. PR-2930 is a newly developed epoxy that exhibits top-of-the-line strength and ductility. To safely design joints utilizing PR-2930, the bonding material and its limit states must be defined. The most pertinent mechanical limit state for adhesively bonded joints is its resistance to fracture, also known as fracture toughness. Fracture often occurs due to a combination of opening (mode I) or shearing (mode II) displacements. In this work, standard and novel advanced fracture characterization techniques are employed and subsequently compared. Adhesive joints using a 0.25 mm layer thickness are bonded to Al 2024-T3 adherends varying from 1.6 mm to 25.4 mm of thickness and tested in quasistatic conditions. Mathematical models of mode I, mode II, and combined mode I/II stress displacement responses (AKA a traction-separation laws) of PR-2930 are developed and compared with experimental data. Future experimental and numerical methods for fracture analysis of structural adhesives are discussed.

Environmental fluctuations in operating fuel cells impose significant biaxial stresses in the constrained proton exchange membranes (PEM). The PEM's ability to withstand cyclic environment-induced stresses plays an important role in membrane integrity and consequently, fuel cell durability. In this thesis, pressure loaded blister tests are used to study the mechanical durability of Gore-Select® series 57 over a range of times, temperatures, and loading histories. Ramped pressure tests are used with a linear viscoelastic analog to Hencky's classical solution for a pressurized circular membrane to estimate biaxial burst strength values. Biaxial strength master curves are constructed using traditional time-temperature superposition principle techniques and the associated temperature shift factors show good agreement when compared with shifts obtained from other modes of testing on the material. Investigating a more rigorous blister stress analysis becomes nontrivial due to the substantial deflections and thinning of the membrane. To further improve the analysis, the digital image correlation (DIC) technique is used to measure full-field displacements under ramped and constant pressure loading. The measured displacements are then used to validate the constitutive model and methods of the finite element analysis (FEA). With confidence in the FEA, stress histories of constant pressure tests are used to develop linear damage accumulation and residual strength based lifetime prediction models. Robust models, validated by successfully predicting fatigue failures, suggest the ability to predict failures under any given stress history whether mechanically or environmentally induced - a critical step in the effort to predict fuel cell failures caused by membrane mechanical failure. / Master of Science