Steel pipelines are widely used in offshore oil/gas facilities. To achieve economically feasible designs, regulatory codes permit utilization of the pipelines well past their elastic response limit. This requires thorough integrity check of the pipeline subject to large scale yielding (LSY). Engineering criticality assessments (ECA) are used to justify the integrity of a cracked pipeline against fracture failure. The currently used ECA crack driving force equation was developed for load-controlled components subject to very limited crack-tip plasticity. Moreover, fracture toughness data are extracted from deeply-cracked laboratory specimens that produce the lowest margin of toughness values. Therefore, the current framework can be overly conservative (or include non-uniform inaccuracies) for ECA of modern pipelines that undergo LSY and ductile crack growth prior to failure. The two main goals of this thesis are: (i) Development of an alternative crack driving force estimation scheme, (ii) Justification of the use of use of shallow-cracked single edge notch tensile (SENT) specimens for the ECA. Strain concentration in concrete coated pipelines, and effect of Lüders plateau on the fracture response are also investigated. A new reference strain J-estimation scheme is proposed and calibrated to 300 nonlinear parametric FE models, which takes advantage of the linear evolution of the J with LSY bending strains. The scheme is hence strain-based and needless of limit load solutions, providing additional accuracy and robustness. The near-tip stress and strain fields of cracked pipelines were also investigated and compared to those obtained from a K-T type formulation. It is shown that the J-Q constraint theory can satisfactorily characterize these fields up to extreme plastic bending levels. Similar J-Q trajectories were also observed in the SENT and pipeline models. Subsequently, FE models utilizing a voided plasticity material were used to parametrically investigate ductile crack growth and subsequent failure of pipelines subject to a biaxial stress state. Plastic strain and stress triaxiality fields ahead of the propagating crack, along with R-curves, were compared among SENT and pipeline models. It is concluded that the SENT specimen could be a viable option for ECA of such pipes based on the observed crack tip constraint similarity.

La thèse porte sur la propagation de fissures de fatigue sous conditions de plasticité cyclique généralisée en Mode I dans l’acier 316L. Un modèle incrémental de fissuration est proposé. Deux types d’essais de fatigue en plasticité généralisée ont été réalisés pour identifier et valider le modèle, un essai sous chargement mécanique cyclique et un essai de fatigue thermique. Pour les deux catégories d'essais, les examens des surface de rupture ont permis de vérifier que le mécanisme de propagation des fissures était analogue à celui observé en conditions de plasticité confinée.Une étude numérique par éléments finis a permis de montrer que :- Les champs de vitesses au voisinage du front de la fissure peuvent être représentés de manière approchée par une somme trois produits d'un facteur d'intensité, utilisé comme degré de liberté du problème, et d'un champ spatial de référence défini a priori. cette thèse a permis d'ajouter aux deux composantes élastiques et de plasticité confinée, un troisième champ permettant d'étendre la représentation à la plasticité généralisée.- L’unique moteur de la propagation de la fissure est l’émoussement plastique de la fissure que l’on peut associer au facteur d’intensité du champ de plasticité confiné.- Il est possible d’exprimer l’évolution de l’émoussement plastique à partir du chargement appliqué et de quelques variables internes, en plasticité confinée, comme sous conditions de plasticité généralisée.Un modèle incrémental de fissuration par fatigue en conditions de plasticité généralisée a été proposé puis identifié à partir des essais de propagation sous chargement mécanique.Ce modèle a ensuite été utilisé pour interpréter un essai de propagation en condition de plasticité généralisée sous chargement cyclique thermique. / This thesis focuses on the propagation of fatigue cracks in large scale yielding conditions in Mode I in the 316L stainless steel. An incremental model is proposed for fatigue crack growth. Two types of fatigue tests in large scale yielding conditions were conducted to identify and validate the model, a cyclic mechanical loading test, develped specifically for this project, and a thermal fatigue test (PACIFIC). For both tests, the observations of the fracture surface have shown that the crack propagation mechanism is similar to that observed in small scale yielding conditions.A finite element analysis has shown that:- The velocity field in the crack front region may be approximated by a sum of three products of an intensity factor, used as a degree of freedom for the problem, and a spatial reference field, defined once for all. This thesis has added a third field, dedicated to capture large scale yielding contribution, to the two existing terms for elastic and small scale yield contributions.- The sole mechnism for fatigue crack propagation is plastic blunting at crack tip which can be associated to the intensity factor of the small scale yielding field.- It is possible to express the evolution of plastic blunting from the applied loading scheme and a set of internal variables, in small scale yielding conditions as well as under large scale yielding conditions.An incremental model was proposed to predict fatigue crack growth in large scale yielding conditions and identified using fatigue tests under mechanical loadings.This model was then used to interpret a crack propagation test in thermal fatigue conditions on the PACIFC set up.

Cette étude s'intéresse à la fissuration des structures multi-perforées, soumises à des sollicitations pouvant aller de la plasticité confinée à la plasticité généralisée. Le cas d'étude considéré est celui des chambres de combustion des turbomachines aéronautiques. Le matériau utilisé est le Haynes 188, un superalliage à base de Cobalt, spécialement conçu pour ce type d'applications. La fissuration des structures multi-percées a été souvent étudiée dans des conditions de plasticité confinée. Ces études doivent être étendues au domaine de la plasticité généralisée.Afin d'étudier les interactions entre fissures et perçages, une éprouvette originale a été conçue. Dans le but de reproduire un motif de base simplifié correspondant aux trous de refroidissement des chambres de combustion, l'éprouvette est percée en son centre par trois trous. Des essais de fissuration isotherme à 900°C sous des chargements de fatigue ont été réalisés avec des niveaux de chargement allant de la plasticité confinée à la plasticité généralisée. Ces essais ont permis d'étudier la durée de vie du motif de base. A partir des résultats expérimentaux de contrainte et de déformation, les essais ont été modélisés à une échelle dite macroscopique avec un modèle de fissuration en énergie. Dans le but d'améliorer la description des essais, la modélisation a été ensuite réalisée à une échelle intermédiaire, dite mésoscopique, à partir de calculs par éléments finis. / The purpose of this study is to investigate crack growth of multi-perforated structures when loading can vary from small scale yielding to large scale yielding conditions. In this study we focus on combustion chambers of aerospace engines. The material used in crack growth tests is the Haynes 188, a cobalt based superalloy, specially developed for this type of applications. Studies on crack growth of multi-perforated structures are often made in small scale yielding conditions. These studies have to be extended to large scale yielding conditions.In order to study interactions between cracks and perforations, an original specimen has been developed. The specimen is perforated in the center by three holes inspired by cooling holes of combustion chambers. Fatigue crack growth tests at 900°C have been conducted with loads from small scale to large scale yielding conditions. These tests were used to study life of a base pattern. Using experimental stresses and strains, tests were modeled at a macroscopic scale with an energy based crack growth model. In order to improve experimental results description, tests were modeled at an intermediate mesoscopic scale using finite element calculations.

This thesis is on fatigue crack growth experiments and assessments of fatigue crack growth rates. Both constant and variable amplitude loads in two different materials are considered; a nickel based super-alloy Inconel 718 and a stainless steel 316L. The considered load levels extend from small scale yielding (SSY) to large scale yielding (LSY) for both materials. The effect of different load schemes on the fatigue crack growth rates is investigated on Inconel 718 and compact tension specimens in Paper A. It is concluded that load decreasing schemes give a to high Paris law exponent compared to constant or increasing load amplitude schemes. Inconel 718 is further analyzed in Paper B where growth rates at variable amplitude loading in notched tensile specimens are assessed. The predictions are based on the fatigue crack growth parameters obtained in Paper A. The crack closure levels are taken into consideration and it is concluded that linear elastic fracture mechanics is incapable of predicting the growth rates in notches that experience large plastic cyclic strains. Even if crack closure free fatigue parameters are used and residual stresses due to plasticity are included. It is also concluded that crack closure free and nominal fatigue crack growth data predict the growth rates equally well. However, if the crack closure free parameters are used, then it is possible to make a statement in advance on the prediction in relation to the experimental outcome. This is not possible with nominal fatigue crack growth parameters. The last three papers consider fatigue crack growth in stainless steel 316L. Here the load is defined as the crack tip opening displacement parameter. Paper C constitutes an investigation on the effect of plastic deformation on the potential drop and consequently the measured crack length. It is concluded that the nominal calibration equation obtained in the undeformed geometry can be used at large plastic deformations. However, two conditions must be met: the reference potential must be taken in the deformed geometry and the reference potential needs to be adjusted at every major change of plastic deformation. The potential drop technique is further used in Paper D and Paper E for crack length measurements at monotonic LSY. Constant amplitude loads are considered in Paper D and two different variable amplitude block loads are investigated in Paper E. The crack tip opening displacement is concluded in Paper D to be an objective parameter able to characterize the load state in two different geometries and at the present load levels. Furthermore, if the crack tip opening displacement is controlled in an experiment and the local load ratio set to zero, then only monotonic LSY will appear due to extensive isotropic hardening, i.e. elastic shake-down. This is also the reason why the linear elastic stress-intensity factor successfully could merge all growth rates, extending from SSY to monotonic LSY along a single line in a Paris law type of diagram, even though the generally accepted criteria for SSY is never fulfilled. For the variable amplitude loads investigated in Paper E, the effect of plastic deformation on measured potential drop is more pronounced. However, also here both the crack tip opening displacement parameter and the linear elastic stress-intensity factor successfully characterized the load state. / <p>QC 20130108</p>