The crack-tip displacement concept has been applied to austenitic steels at 25°C and in the temperature range 400-800°C. The measurement of a maximum load crack-tip displacement in a notch bend test, provided a good indication of fracture-toughness since it included a portion of slot: crack-growth. The ability of these alloys to tolerate slow crack-growth, and even to arrest cracks without becoming unstable, is of considerable engineering importance since many service failures originate at stress concentrations produced by welding of poor design. Slow crack-growth was intermittent and highly dependent upon microstructural irregularities in the immediate vicinity of the crack-tip. The stress concentration at the crack-tip could be relieved (and the fracture-toughness improved) by localized plastic deformation. The degree of stress relief depended upon the locality and proximity of the various microstructural features as determined by the alloy manufacturing process. The temperature sensitivity of the bond strengths of the various interfaces particularly in cast alloys, had a marked-effect on fracture-toughness. This may have been the result of segregation of trace elements e.g. lead to the interface. Above the equicohesive temperature, the greatest contribution to fracture-toughness was associated with the presence of large amounts of second phase particles in the boundaries. In many cases, and particularly cast alloys, more than one crack formed at the notch root. Only one of these cracks propagated to failure. Heterogeneous cracking of this kind (itself an indication of fracture-toughness), lead to difficulties in correlating an initiation C. O. D. with the macroscopic plastic properties of the material. Similar correlations were however possible with the maximum load crack opening displacement.

A new preconditioner for the quick solution of a crack growth problem in 2D adaptive finite element analysis is proposed. Numerical experiments demonstrate the power of the method.

A large-area electronics (LAE) strain sensor, termed soft elastomeric capacitor (SEC), has shown great promise in fatigue crack monitoring. The SEC is capable to monitor strain changes over a large structural surface and undergo large deformations under cracking. Previous tests verified that the SEC can detect and localize fatigue cracks under low-cycle fatigue loading. In this paper, we further investigate the SEC's capability for monitoring high-cycle fatigue cracks, which are commonly seen in steel bridges. The peak-to-peak amplitude (pk-pk amplitude) of the SEC measurement is proposed as an indicator of crack growth. This technique is is robust and insensitive to long-term capacitance drift. To overcome the difficulty of identifying the pk-pk amplitude in time series due to high signal-to-noise ratio, a signal processing method is established. This method converts the measured SEC capacitance and applied load to power spectral densities (PSD) in the frequency domain, such that the pk-pk amplitudes of the measurements can be accurately extracted. Finally, the performance of this method is validated using a fatigue test of a compact steel specimen equipped with a SEC. Results show that the crack growth under high-cycle fatigue loading can be successfully monitored using the proposed signal processing method.

No description available.

No description available.

No description available.

La modélisation numérique de la multi-fissuration et son influence sur les ouvrages du Génie Civil reste un sujet ouvert et nécessite le développement de nouveaux outils numériques de plus en plus performants. L’approche retenue dans cette thèse est basée sur l’utilisation des concepts des équations intégrales de Galerkin accélérées par la méthode multipôle rapide. Les méthodes intégrales sont bien connues pour leur souplesse à définir des géométries complexes en 3D. Elles restent également très performantes en mécanique de la rupture, lors de la détermination des champs singuliers au voisinage des fissures. La Méthode Multipôle Rapide, quant à elle, permet via une judicieuse reformulation des fonctions fondamentales propres aux formulations intégrales, de réduire considérablement le coût des calculs. La mise en œuvre de la FM-SGBEM a permis de pallier les difficultés rencontrées lors de la phase de résolution et ce lorsqu’on traite de domaines de grandes tailles par équations intégrales de Galerkin pures. Les présents travaux, viennent en partie optimiser et renforcer cette phase dans les environnements numériques existants. D’autre part, des adaptations et des développements théoriques des formulations FM-SGBEM pour prendre en compte le caractère hétérogène des domaines en Génie Civil qui en découlent, ont fait l’objet d’une large partie des travaux développés dans cette thèse. La modélisation du phénomène de propagation de fissures par fatigue a également été étudiée avec succès. Enfin, une application sur une structure de chaussée souple a permis de valider les modèles ainsi développés en propagation de fissures par fatigue dans des structures hétérogènes. De réelles perspectives d’optimisations et de développements de cet outil numérique restent envisagées. / The modeling of cracks and its influence on the understanding of the behaviors of the civil engineering structures is an open topic since many decades. To take into consideration complex configurations, it is necessary to construct more robust and more efficient algorithms. In this work, the approach Galerkin of the boundary integral equations (Symmetric Galerkin Boundary Element Method) coupled with the Fast Multipole Method (FMM) has been adopted. The boundary analysis are well-known for the flexibility to treat sophisticated geometries (unbounded/semi-unbounded) whilst reducing the problem dimension or for the good accuracy when dealing with the singularities. By coupling with the FMM, all the bottle-necks of the traditional BEM due to the fully-populated matrices or the slow evaluations of the integrals have been reduced, thus making the FM-SGBEM an attractive alternative for problems in fracture mechanics. In this work, the existing single-region formulations have been extended to multi-region configurations along with several types of solicitations. Many efforts have also been spent to improve the efficiency of the numerical algorithms. Fatigue crack propagations have been implemented and some practical simulations have been considered. The obtained results have validated the numerical program and have also opened many perspectives of further developments for the code.

A new preconditioner for the quick solution of a crack growth problem in 2D adaptive finite element analysis is proposed. Numerical experiments demonstrate the power of the method.

<div>An important challenge in predicting fatigue and creep crack growth is describing crack growth rates under transient conditions. Transient conditions occur when similitude is violated at the crack tip due to the applied loads or material behavior. Crack growth models like the Paris law, valid for homogeneous materials under constant-amplitude cyclic loading or sustained loading, no longer apply. Transient crack growth rates are strongly influenced by changes in plastic deformation at the crack tip. Activation of time-dependent damage and viscoplastic deformation at high temperatures further complicates the problem.</div><div><br></div><div>This thesis advances knowledge and predictive capabilities for transient creep and fatigue crack growth in metals, with specific applications to two technologically-relevant nickel-base superalloys. Finite element computations of crack growth following overloads and in multilayered materials are conducted. Crack extension is an outcome of the boundary value problem through an irreversible cohesive zone model and its interaction with plasticity and viscoplasticity in the bulk material.</div><div><br></div><div>First, fatigue crack growth in rate-independent materials is analyzed. The plasticity formulation considers both plastic strain and gradients of plastic strain, which produce hardening beyond that predicted by classical plasticity models. The computations demonstrate that hardening due to plastic strain gradients plays a significant role in transient fatigue crack growth following overloads. Fatigue crack growth transients associated with material inhomogeneity are studied through the case of a crack growing toward interfaces between plastically dissimilar materials. Interactions between the interface strength and the yield strength mismatch are found to govern crack growth rates near the interface. Hardening due to plastic strain gradients is important for finding the critical conditions associated with crack bifurcation at an interface and penetration through an interlayer.</div><div><br></div><div>Subsequently, crack growth in rate-dependent materials is analyzed. For materials characterized by power-law viscoplasticity, fatigue crack growth rates following overloads are found to depend strongly on the material rate sensitivity. The computations predict a transition from acceleration- to retardation-dominated post-overload crack growth as the rate sensitivity decreases. The predicted post-overload crack growth rates show good agreement with high-temperature experimentally-measured trends for Alloy 617, a solid solution strengthened nickel-base superalloy proposed for use in next-generation nuclear power plants. The results demonstrate why Alloy 617 behaves in a relatively brittle manner following overloads despite being characterized as a creep-ductile material. Crack growth is also studied in materials where rate dependence is captured through time-dependent damage and dislocation storage and dynamic recovery processes. This approach is relevant for high-strength creep-brittle materials, in which the viscoplastic zone grows with the advancing crack. The computations predict crack growth retardation for several loading waveforms containing overloads. The amount of retardation depends strongly on the overload ratio and subsequent unloading ahead of the crack tip. The predicted post-overload crack extension shows good agreement with high-temperature experimentally-measured trends for Alloy 718, a precipitation-hardened nickel-base superalloy used in turbine engines and power generation applications. The results demonstrate why Alloy 718 behaves in a ductile manner following overloads, despite being characterized as a creep-brittle material.</div>

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.