Zr-2.5 Nb pressure tubes in CANDU reactors are susceptible to delayed hydride cracking (DHC), which is a sub-critical cracking process requiring hydrogen diffusion to a stress concentrator, precipitation, growth and fracture of hydrides. Service-induced and manufacturing flaws are present in some pressure tubes and these flaws may act as crack initiators. An engineering approach has been developed to assess the susceptibility of flaws to DHC. In this methodology, DHC is separated into the initiation and growth stages, and in terms of initiation, flaws are classified as blunt, sharp or crack-like. The experiments performed in this thesis are related to crack-like flaws, which are assessed in terms of the threshold stress intensity factor, K1H, below which DHC cannot occur. There is a large scatter in the overall KIH data base and a lower bound value is conservatively used for flaw assessment. Systematic studies on un irradiated Zr-2.5 Nb pressure tube material have shown that KIH increases with decreasing hydrogen in solution, increasing deviation from the radial-axial plane of the tube, and increasing temperature, while thermal cycling has no significant effect on K1H. Therefore, it may be justifiable to use higher KIH values for assessing flaws with known orientation, hydrogen concentration at the flaw location and operating thermal history. If crack initiation is postulated, as part of a defence-in-depth approach, crack growth is assessed under two scenarios. (1) When the hydrogen concentration is sufficient for cracking to continue under sustained hot conditions, a leak-before-break assessment is performed. DHC velocity is required to determine the time for a crack to grow to the critical crack length for unstable fracture. This thesis shows that crack velocities at different temperatures depend strongly on the thermal history, which affects the hydrogen concentration in solution. Crack velocity increases with increasing hydrogen in solution. In addition, hydrogen supersaturation is required for cracking to occur at the reactor operating temperatures of 2S0-31O°C. (2) When the hydrogen concentration is insufficient for cracking to occur at normal operating temperatures, cracking can only occur during reactor cool-down when hydrides can precipitate as a result of the lowering of temperature. The amount of postulated crack growth per cool-down cycle depends on the crack initiation temperature during cooling. This thesis shows that the crack initiation temperature decreases with increasing cooling rate, and by applying a load-reduction of 20% prior to cooling. Cracking during cooling can be suppressed altogether by allowing the crack tip stress to relax by creep, followed by a load reduction of 15-20%. Recommendations are made regarding reactor loading and thermal history which can reduce the propensity for DHC. From the observations on hydride morphologies and fracture surface features of the DHC cracks under different test conditions, evidence is presented which supports the hydride/stress interaction diffusion model. The observations also demonstrate the inadequacies of the critical length criterion for fracture of a hydrided region.

The primary focus of this research is to evaluate the time-dependent crack growth in rocks using lab tests and numerical modeling and its application to geologic hazard problems. This research utilized Coconino sandstone and Columbia granite as the study materials and produced the subcritical crack growth parameters in both mode I and II loadings using the rock materials. The mode I loading test employs three different types of fracture mechanics tests: the Double Torsion (DT), the Wedge Splitting (WS), and the Double Cantilever Beam (DCB) test. Each test measured the mode I crack velocity. The DT test indirectly measured the crack velocity using the load relaxation method. The WS and DCB tests directly measured the crack velocity by monitoring using a video recording. The different mode I subcritical crack growth parameters obtained from the three tests are discussed. For the mode II loading test, this study developed a new shear fracture toughness test called the modified Punch-Through Shear (MPTS). The MPTS test conducted at different loading rates produced the mode II subcritical crack growth parameters. These fracture mechanics tests were calibrated and simulated using the distinct element method (DEM) and the finite element method (FEM). DEM analysis employed the particle flow code (PFC) to simulate the mixed mode crack growth and to match with the failure strength envelop of the triaxial compressive tests. FEM analysis employed the Phase2 program to analyze the crack tip stress distribution and the FRANC2D program to calculate the modes I and II stress intensity factors. The fracture mechanics tests and numerical modeling showed well the dependency of the mode II subcritical crack growth parameters according to confining pressure, loading rate, and the mode II fracture toughness. Finally, the UDEC modeling based on DEM is utilized in this study to forecast the long-term stability of the Coconino rock slope, as one of geologic hazards. The fracture mechanics approach is implemented in the program using the modes I and II subcritical crack growth parameters obtained from the lab tests and numerical modeling. Considering the progressive failure of rock bridges due to subcritical crack growth, the UDEC results predicted the stable condition of the Coconino rock cliff over 10,000 years. This result was validated by comparing it with the previous planar failure case.

Nous étudions la fragilisation par l'hydrogène de l'alliage d'aluminium 7108. Une technique expérimentale spécifique a été développée : Un pré-chargement en hydrogène des échantillons, à travers un dépôt de nickel de quelques dizaines de microns, qui empêche la dissolution du substrat d'aluminium, est utilisé. Il permet la comparaison de la résistance à la fragilisation de différentes microstructures modèles. Nous étudions l'effet du traitement thermique et de la précipitation sur la sensibilité à l'hydrogène pour des vitesses de déformation macroscopiques imposées variables. Différents modes de rupture sont observés ainsi que des transitions entre eux. Au moyen de simulations numériques à l'échelle mésoscopique, l'effet de taille des précipités intergranulaires pré-fragilisés sur la ténacité des joints de grains est estimé, en utilisant un modèle de zone cohésive. Nous analysons la compétition entre la diffusion de l'hydrogène vers la pointe de la fissure et la vitesse de fissuration par un couplage mécanique - diffusion basé sur la diffusion de l'hydrogène assistée par la contrainte hydrostatique. Une vitesse critique au-delà de laquelle l'hydrogène ne peut plus suivre la fissure, est mise en évidence. L'influence de la microstructure du joint de grains sur cette vitesse est analysée. La valeur est comparée à une estimation des vitesses de propagation expérimentales obtenues pour différentes vitesses de déformation macroscopiques. Nous analysons l'effet du piégeage de l'hydrogène par les précipités intergranulaires et la désorption sur la répartition de l'hydrogène le long du joint de grains en imposant un flux au niveau de l'interface précipités - matrice. / We study the hydrogen embrittlement of the 7108 aluminum alloy. A specific experimental technique was developed : A hydrogen pre-charging, through few tens of microns of deposit of nickel, which prevents the dissolution of the aluminum substrate is used. It allows a comparison of the resistance to embrittlement of different model microstructures. We study the effect of heat treatment and intergranular precipitation on the susceptibility to hydrogen embrittlement for several macroscopic strain rates. Different failure modes and transitions between them are observed. Through numerical simulations, at the mesoscopic scale, the effect of the size of pre-weakened intergranular precipitates on the grain boundary toughness is estimated using a cohesive zone model. We further analyze the competition between the hydrogen diffusion toward the crack tip and crack velocity. For this purpose, a mechanical – diffusion coupling based on the hydrogen diffusion assisted by hydrostatic stress is elaborated. A critical crack velocity, beyond which hydrogen can no longer follow the crack, is highlighted. The influence of the grain boundary microstructure on this critical crack velocity is evaluated and its value is compared with an estimate of velocities obtained for different experimental macroscopic strain rates. We analyze the effect of hydrogen trapping by intergranular precipitates and hydrogen desorption by imposing a flux at the precipitates – matrix interfaces.