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Knowledge of in-situ stress is essential to well development technologies such as wellbore stability, hydraulic fracturing, well spacing design, and the monitoring of induced seismicity. As energy exploration has become more prevalent in unconventional reservoirs and as it has shifted focus to sustainable energy such as geothermal energy, the knowledge of in situ stresses plays a crucial role in the success of energy production. In this project, the in-situ stress S_Hmax in the geothermal reservoir of Utah FORGE is computed and analyzed using multiple stress analysis methodologies in both vertical and horizontal wells. In a stress analysis, the understanding of three principal stresses is needed and they are vertical stress (S_v), minimum horizontal principal stress (S_hmin), and maximum horizontal principal stress (S_Hmax). In general, S_v is calculated by integrating the density log over the depth, while S_hmin is determined by hydraulic fracturing-based tests such as DFIT, leak-off test, and micro frac test. The direction of S_hmin and S_Hmax can be interpreted from the azimuth of drilling induced fractures and/or borehole breakouts. However, the determination of the magnitude of maximum horizontal principal stress (S_Hmax) is challenging especially in an inclined wellbore. Traditionally, S_Hmax can be estimated using hydraulic fracturing data and the Kirsch solution. However, this method of using hydraulic fracturing data imposes uncertainties as it is sensitive to the models and the assumption to rock formation is too sample (Schmitt and Zoback, 1989; Zi et al., 2022). With the gaining use of image log tools such as FMI and UBI, the method of using borehole failures such as breakouts and drilling induced fractures gives rise to more access to assess in situ stresses (Zoback, et al., 2003). Thus, the need of integrating multiple stress analysis methods and the incorporation of drilling induced fractures and breakouts around the wellbore is required to constrain a more comprehensive S_Hmax magnitude estimation. In this paper, we present the wellbore in-situ stress models of the vertical and deviated wells in Utah FORGE based on the drilling-induced fractures and breakouts observed from borehole image logs. We also applied an in-situ stress inversion program based on focal mechanism and validated the program using the data from the Geyser geothermal reservoirs. This program can be used for the Utah FORGE reservoir once its focal mechanism data is available. In Utah FOREG, we integrated multiple methods such as fracture mechanics, and wellbore failure analysis with the incorporation of drilling-induced fractures, borehole breakouts, and thermal stresses to constrain the wellbore stress model and input parameters. The validated wellbore in-situ stress models and parameters were then applied to establish the in-situ stress profiles of wells. The stress results in all wells yield a range of S_Hmax of 0.80 – 1.05 psi/ft, suggesting a normal faulting regime in Utah FORGE. The orientation of S_Hmax has a range of N20E to N40E with a mean of N33E based on drilling induced fractures in vertical wells. In Geyser, an in-situ stress inversion program based on the focal mechanism has been applied and validated. The stress variation analysis confirmed that the Geyser reservoir has a mixture of normal and strike-slip fault regimes sandwiched by strike-slip fault regimes on the top and bottom of the reservoir. The Geyser injection test also was shown to alter the stress regimes of the reservoir and cause the stress regime to transit from strike slip to normal and to transtensional fault during a cycle of injection activities.

This dissertation is composed of three papers and related unpublished work providing a foundation that helps understand near threshold fatigue crack growth in both vacuum and environment conditions in the atomic scale. All studies are made possible by utilization of a recent implementation of the multiscale coupled atomistic and discrete dislocation (LF-CADD) methodology. First, near threshold fatigue crack growth of a ductile material in vacuum is investigated via atomistic modeling of dislocation motions and slip interactions. The simulation results indicate that sustained fatigue crack growth in vacuum requires emitted dislocations to change slip planes prior to their reabsorption into the crack on the opposite side of the loading cycle. This finding assesses the validity of long-hypothesized material separation mechanisms using state-of-the-art computational resources, conforms to reports of crack growth below experimentally assisted fatigue crack growth thresholds, and opens the door for improved prognosis and the design of novel fatigue resistance alloys. Second, the role of material dissolution is studied for intrinsically ductile and semi-brittle materials isolated from other environmentally assisted fatigue crack growth mechanisms. The multiscale simulations and subsequent analysis suggest that dissolution can be dual natured depending on the material. For a ductile material, dissolution at crack tip acts to blunt the crack with increasing crack tip opening displacement (CTOD). On the other hand, dissolution will work as a driving force for a slow growing in a brittle and semi-brittle material where the crack remains atomically sharp. The latter will eventually trigger brittle, unstable crack propagation as the applied load exceeds a critical threshold. Third, a crack with surface film is studied under mode I fatigue loading. Using our concurrent multiscale LF-CADD model, we conduct a series of simulations with varying loading conditions and find that the crack grows via film rupture mechanism, which requires film thickness and brittleness. Surface film swelling is needed to produce experimentally observed R-effect on $\Delta K$ threshold. In order to achieve sustained fatigue crack growth, a critical K ($K_{max}$) is necessary to overcome the compressive stress ahead of the crack tip at the bottom of the loading cycle due to film swelling.

The first paper, “Dye Penetrant Induced Microcracking in High-Performance Thermoplastic Polyimide Composites”, studied the possibility of spurious microcracking in three high-performance thermoplastic polyimide composite materials due to zinc iodine dye penetrant. The material systems were IM7/LaRC™-IAX, IM7/LaRC™-IAX2, and IM7/LaRC™-8515. Specimens from each material system were subjected to one of three immersion tests. The first immersion test involved soaking composite specimens previously prepared with different polishing techniques in dye penetrant. In the second test, specimens were immersed in the individual components of the dye penetrant. The final test involved exposure of specimens to one of six solvents followed by exposure to dye penetrant. Results showed that the composite materials have sufficiently high thermal residual stresses to drive microcracking in the presence of dye penetrant without external mechanical loading. There was no evidence that the different polishing techniques had an effect on dye penetrant-induced stress cracking. The dye penetrant components did not produce microcracks in the composites. Some combination of the components must be present to induce microcracking. Observations also revealed that polishing had an effect on the microcracking process of the composites that were initially exposed to solvents then dye penetrant. The second paper, “The Effect of Environmental Stress Cracking on High-Performance Polymeric Composites”, studied solvent stress cracking and solvent-induced strength degradation on four polyimide matrix materials developed at NASA-Langley Research Center. These materials are LaRC™-IAX, LaRC™-IAX2, LaRC™-8515, and LaRC™-PETI-5. Cross-ply specimens were used to characterize solvent stress cracking in composites. Matrix cracking due to solvent exposure was observed in all of the materials. The solvent exposure time of the materials ranged from 1 minute to 96 hours. The results show that residual thermal stresses due to processing in the cross-ply composite specimens are sufficient to drive solvent stress cracking in the matrix. Solvent application lowers the microcracking toughness, Gmc ,values such that the available strain energy, Gm, within the transverse ply groups is sufficient to initiate microcracking. In the absence of a solvent, the same Gm value would not induce microcracking. Transverse flexure tests were performed on unidirectional specimens to determine the effects of the solvents on the material strengths. The presence of certain solvents severely degraded the materials. The manner in which the solvents were applied to the materials determined the degree of material degradation. The results revealed a synergistic effect between stress and solvent. The tests showed that diglyme, MEK, and acetone produced the most severe damage to the materials. The most solvent resistant material was LaRC™-PETI-5. This is followed by LaRC™-8515, LaRC™-IAX2, and LaRC™-IAX respectively. LaRC™- PETI-5 is a thermoset whereas the remaining materials are thermoplastics.

In this paper we develop a computational procedure to investigate linear fracture of two-dimensional problems in isotropic linearly elastic media. A symmetric Galerkin boundary element method (SGBEM), based on a weakly singular, weak-form traction integral equation, is adopted to model these fractures. In particular we consider multiple interacting cracks in an unbounded domain subject to internal pressure and remote stress. The growth of the cracks is driven by either linearly dependent injection pressures or volumes in each crack. A variety of crack geometries are investigated.

Graphene is a single layer two-dimensional honeycomb lattice of carbon atoms. It is one of the toughest, lightest, and most conductive materials known. Graphene was first isolated using adhesive tape in 2004, and awarded the Physics Nobel Prize in 2010. Here we focus on the mechanical properties of graphene. First we present an analytical study, together with numerical simulations, of ripples in graphene. We show that ripples observed in free-standing graphene sheets can be a consequence of adsorbed OH molecules sitting on random sites. The adsorbates cause the bonds between carbon atoms to lengthen slightly. In the second part of this work we study the fracture mechanics of graphene. Experiments on free-standing graphene can expose the graphene sheets to out-of-plane forces. Here we show that out-of-plane forces can cause free-standing graphene to fracture. This fracture mode is known as the tearing mode and is common in materials such as paper. We present a numerical study of the propagation of cracks in clamped, free-standing graphene as a function of the out-of-plane force. We also obtain an analytical expression for the minimum force required to tear a two dimensional sheet, which is our model of graphene, in terms of the initial crack length.

Water-assisted crack growth at an epoxy/glass interface was measured as a function of applied strain energy release rate, G, and temperature using a wedge test geometry. The specimens consist of two glass plates bonded with a thin layer of proprietary epoxy adhesive. The crack fronts along the epoxy/glass interfaces were measured using an optical stereomicroscope. The relationship between G and the debonding rate, v, can be measured using this method, and the threshold value of strain energy release rate, Gth, can be determined from the measured data. Two types of testing procedures were conducted in this study: ex situ, i.e., pre-conditioned wedge tests and in situ ones, in which wedges were applied before the specimens were submerged into water. A preliminary model was developed based on the thermal activation barrier concept, and allows the prediction of Gth for the temperatures beyond the testing region. Changes in interfacial strain energy release rate caused by thermal residual stresses in a triple-layered specimen were analyzed in Chapter Three. The method is based on linear elastic fracture mechanics and simple beam theory. The curvature of a bimaterial strip was chosen to characterize the residual stress in the specimen, and the strain energy release rate, caused by both tensile and compressive residual stresses in the adhesive, was derived for an asymmetric double cantilever beam (ADCB) geometry. The contribution of the thermal residual and mechanical stress to the global energy release rate was analyzed. The thermally induced energy release rate, GT, is found to be independent of crack length, but is a function of residual stress level and geometric and material parameters of the specimen. The adhesion of films and coatings to rigid substrates is often measured using blister geometries, which are loaded either by an applied pressure or a central shaft. The measurement will be affected if there are residual stresses that make a contribution to the energy release rate. This effect is investigated using analytical solutions based on the principle of virtual displacements. A geometrically nonlinear finite element analysis is conducted for comparison. Furthermore, the relationships among strain energy release rate, load, deflection, and fracture radius are discussed in detail in Chapter Four. Both analytical solutions and numerical results reveal that uniform tensile residual stresses reduce a specimen's deflection if it experiences plate behavior under small loads. However, this effect diminishes when membrane behavior is dominant. The mechanics of a single-lap joint with different boundary conditions subjected to tensile loading are investigated. Closed-form solutions are obtained for a specimen configuration considering different clamping methods. Based on the approach pioneered by Goland and Reissner, the solutions reported in this paper provide a simple but useful way to understand the effects of boundary conditions on this test geometry. The solutions in this study suggest that different grip configurations mainly affect the response of the specimens if the grip position is close to the joint edge or the loads are small. Generally, the influence caused by different gripping methods is only limited to the boundary region, and the behavior of the joint part subjected to tensile loading is almost the same as that for a simply-supported case.