Characterization of fractures in an arkosic sandstone from the western damage zone of the San Andreas Fault (SAF) at San Andreas Fault Observatory at Depth (SAFOD) was used to better understand the origin of damage and to determine the scale dependence of fracture fabric and fracture density. Samples for this study were acquired from core taken at approximately 2.6 km depth during Phase 1 drilling at SAFOD. Petrographic sections of samples were studied using an optical petrographic microscope equipped with a universal stage and digital imaging system, and a scanning electron microscope with cathodoluminescence (SEM-CL) imaging capability. Use of combined optical imaging and SEM-CL imaging was found to more successfully acquire true fracture density at the grain scale. Linear fracture density and fracture orientation were determined for transgranular fractures at the whole thin section scale, and intragranular fractures at the grain scale. The microscopic scale measurements were compared to measurements of mesoscopic scale fractures in the same core, as well as to published data from an ancient, exhumed trace of the SAF in southern California. Fracturing in the damage zone of the SAF fault follows simple scaling laws from the grain scale to the km scale. Fracture density distributions in the core from SAFOD are similar to distributions in damaged arkosic sandstone of the SAF along other traces. Transgranular fractures, which are dominantly shear fractures, indicate preferred orientation approximately parallel to the dominant sets of the mesoscale faults. Although additional work is necessary to confirm general applicability, the results of this work demonstrate that fracture density and orientation distribution over a broad range of scales can be determined from measurements at the mesoscopic scale using empirical scaling relations.

This research investigates the mechanism of transgranular stress corrosion cracking (TGSCC) on fuel transmission pipelines. This research proposes that in near-neutral pH environments, hydrogen can be generated by the dissociation of carbonic acid and the reaction of metal ions with bicarbonate solutions, significantly increasing the available hydrogen for diffusion into the pipeline steel. This research has shown that TGSCC of pipeline steels is possible in simple groundwater solutions containing bicarbonate ions and carbon dioxide. Microstructural characterization coupled with hydrogen permeation indicates that the level of strain in the microstructure has the most influence on hydrogen diffusivity. Hydrogen accumulation occurs preferentially in at high energy discontinuous interfaces such as inclusion interfaces. It was determined that a stress concentration is required to facilitate sufficient hydrogen accumulation in the pipeline steel in order to initiate TGSCC. It was discovered that these stress concentrations develop from inclusions falling out of the pipeline surface. Slow strain rate tests found that TGSCC occurred in a wide range of compositions and temperatures as long as near-neutral conditions were maintained. Microcracks ahead of the crack tip provide evidence of hydrogen in these cracking processes. Morphology of these microcracks indicates that cracks propagate by the coalescence of microcracks with the main crack tip. Further research findings, scientific impact, and potential future work are also discussed.

No description available.

La corrosion sous contrainte par l’iode (CSC-I) est l’un des mécanismes de rupture potentiels des crayons combustibles en alliage de zirconium, pouvant intervenir au cours des transitoires de puissance des réacteurs nucléaires. La fissuration par CSC-I comporte trois étapes : amorçage de la fissure, développement intergranulaire puis propagation transgranulaire. Le but du travail est d’identifier des paramètres critiques gouvernant les transitions entre ces différentes étapes. En premier lieu, des essais sur des éprouvettes en Zircaloy présentant des finitions de surface et des états métallurgiques variés permettent de discriminer l’influence de différents paramètres sur l’amorçage des fissures. Nous mettons en évidence le rôle critique du niveau des contraintes résiduelles, de leur répartition en surface ainsi que de leur profil au sein du matériau. La sensibilité des alliages à l’amorçage des fissures n’est pas directement corrélée à la rugosité de la surface. Cependant, la dispersion des paramètres de rugosité traduit l’irrégularité du profil, l’hétérogénéité du niveau des contraintes résiduelles, et donc l’existence de zones où les contraintes résiduelles sont localement moins protectrices. Dans un second temps, des éprouvettes de Zircaloy-4 possédant différents états d’écrouissage sont sollicitées sous charge constante, en présence de méthanol iodé. Les modifications microstructurales induites par l’écrouissage favorisent l’apparition de la propagation transgranulaire des fissures de CSC-I. Des observations des faciès de rupture en MET révèlent que la transition inter/transgranulaire intervient dans des zones où les grains sont fortement désorientés les uns par rapport aux autres, suite à l’augmentation des contraintes locales résultant des incompatibilités de déformation grain à grain. / Iodine-induced stress corrosion cracking (I-SCC) is one of the potential failure modes of zirconium alloy fuel claddings during power transients in nuclear reactors. I-SCC failures are usually described in three steps: initiation of cracks, intergranular development and transgranular propagation. The objective of this work is to identify critical parameters controlling transitions between crack propagation modes. First of all, experiments conducted on Zircaloy samples with various surface conditions and metallurgical states lead to discriminate the influence of several parameters responsible for cracks initiation. The critical role of residual stresses level, their distribution at the subsurface and their evolution in the bulk of the material is evidenced. Sensitivity to I-SSC is not directly correlated to surface roughness. However, dispersion in roughness parameters indicates the presence of surface irregularities, heterogeneities of residual stresses and the existence of surface areas where residual stresses are less protective. In a second step, Zircaloy-4 samples with various strain-hardening pre-treatments are submitted to constant load tests in an iodine methanol solution. Microstructural modifications induced by a strain-hardening pre-treatment enhance transgranular propagation of I-SCC cracks. TEM observations of fracture surfaces show that the intergranular to transgranular crack transition takes place preferentially where the relative crystallographic orientation is large between two adjacent grains, because of local stress concentrations resulting from strain incompatibilities between neighbouring grains.

No description available.