Dans cette thèse, j’ai conçu un nouveau test de rupture adapté à l’étude des matériaux fragiles sur une grande gamme de vitesse de fissure. Il a été mis en œuvre sur le PMMA, permettant de caractériser la transition entre un régime de rupture à grande vitesse v>vc=15 mms-1 avec des faciès optiquement plats et un autre régime pour v<vc avec une rugosité caractérisée par des motifs triangulaires. L’étude de la déformation du front dans le plan moyen de fissuration montre que le matériau est plus tenace à l’intérieur de ces triangles qu’à l’extérieur. Qui plus est, ces triangles sont décorés par de petits motifs en forme de toit d’usine, caractéristiques de l’instabilité de fragmentation sous mode I+III. Pour comprendre l’émergence de ces formes, nous revisitons tout d’abord les modèles de fragmentation en supposant que l’énergie de rupture dépend du cisaillement comme Gc(KIII/KI)=GcI[1+ (KIII/KI)2]. Le seuil de fragmentation (KIII/KI)thc ainsi prédit décroit significativement, réconciliant théorie et expérience. Le motifs triangulaires permettant de mesurer le paramètre exp à partir de la déformée du front et le degré de cisaillement (KIII/KI)exp par l’inclinaison des facettes qui est compatible avec la valeur de seuil prédite (KIII/KI)thc. A partir des valeurs exp et (KIII/KI)thc ainsi déterminées, nous prédisons que les facettes dérivent le long du front avec un angle compatible avec celui formé par les triangles. Cette compréhension fine de ces motifs nous permet donc de mettre en relation la transition rugueuse observée dans le PMMA avec l’instabilité de fragmentation et ouvre de nouvelles perspectives pour la compréhension de motifs similaires dans d’autres matériaux. / During this PhD thesis, a new fracture test geometry is designed for the accurate measurement of the fracture properties of brittle solids, subsequently applied to study failure in PMMA. At high crack speeds, their fracture surfaces are optically smooth. But below vc = 15 mms-1, a transition to rough surfaces occurs through the formation of puzzling triangular patterns. These patterns lead to significant toughening of the material that reflects through the pinned shape of the crack front as it crosses triangles. In addition, these triangles are found to be decorated by faceted features reminiscent of the crack front fragmentation instability in mode I+III. Assuming a shear-dependent fracture energy Gc(KIII/KI) = GcI[1+ (KIII/KI)2] we theoretically predict a fragmentation threshold (KIII/KI)thc that can be as low as a few percent while earlier models (that assumes = 0) predict a much larger value, inconsistent with various experimental observations. Applied to our experiments, this model allows us to measure exp from the deformation amplitude of the pinned front and the amount of applied shear (KIII/KI)exp from the facet inclination which is found to be compatible with the theoretically predicted threshold (KIII/KI)thc . Using the values (KIII/KI)exp and exp thus determined, one finally predict a drift of the facets from the propagation direction accounting for the triangle angle observed experimentally. To conclude, our study shows that the roughening transition in PMMA is a signature of front fragmentation under mode I+III. As a result, deciphering the triangular patterns at the transition led to significant improvements in the understanding of this instability.

L’Inconel 718 est un superalliage base nickel largement utilisé par les motoristes tels Safran Aircraft Engines pour l’élaboration des disques de turbine. Après forgeage des disques, un traitement de vieillissement appelé « Direct Aged » est appliqué. En service, le régime de croisière représente un temps de maintien sous chargement constant pour les disques. Bien que pas complètement compris, il est largement admis qu’un temps de maintien dans un cycle de fatigue a un effet néfaste sur le comportement en fissuration de l’Inconel 718 ainsi que d’autres superalliages. Cette étude porte donc sur la fissuration en fatigue-fluage dans l’Inconel 718 DA à 550°C et 650°C. Des essais sont menés pour des temps de maintien allant jusqu’à 1h. Des développements de la méthode de suivi de fissure par mesure de potentiel (DCPD) ont permis d’identifier la décharge-recharge (contribution de fatigue) d’un cycle de fatigue-fluage comme la partie la plus néfaste du cycle. L’application d’un temps de maintien amplifie cette contribution. Le temps de maintien induit également des fronts de fissure extrêmement courbes et tortueux, contrairement à de la fatigue pure. Une stratégie numérique a été développée, couplant la simulation 3D de la propagation et la méthode dite DCPD, permettant de réaliser des « essais numériques ». La propagation de fronts courbes et tortueux est simulée. Il a été démontré que le comportement en propagation est directement lié à la forme du front de fissure et son évolution. Des essais complexes ont été menés, sous vide, ou impliquant des surcharges. Lorsque l’effet du temps de maintien est annihilé, les morphologies complexes des fronts disparaissent. Elles sont alors associées à une inhibition locale de l’effet endommageant de l’environnement due à la plasticité et aux vitesses de déformation locales. Tous les essais présentés sont analysés en considérant l’effet de la vitesse de déformation locale qui influe largement le comportement en fissuration de l’Inconel 718. / Inconel 718 is a nickel-based superalloy widely used by aeroengines manufacturers like Safran Aircraft Engine to manufacture turbine disks. After forging, disks are given an ageing treatment called “Direct Aged”. In service, during cruise, these critical components handle hold-time periods at constant loading. It is well known, although not fully understood, that hold-time increases crack growth rates (CGR) in Inconel 718 as well as others superalloys. Therefore, this study focuses on crack propagation under hold-time conditions in DA Inconel 718, at 550°C and 650°C. Experiments were carried out for different hold-times, up to 1h. Developments on the crack monitoring technique (DCPD) led to the conclusion that the most damaging part of the cycle is load-reversal (fatigue contribution). This contribution is enhanced by the hold-time period. Holdtime leads to dramatically curved and tortuous crack front, contrary to pure fatigue cycles. A numerical framework was developed, combining crack growth and DCPD simulations, so that “numerical tests” can be carried out. Using this method, crack growth simulations were performed from curved and tortuous, experimentally reproduced, crack front. It was concluded that increased crack CGR under hold-time conditions are closely related to the crack front morphology and its evolution during propagation. More complex tests, with overloads or under vacuum, were carried out. When the hold-time effect is inhibited, complex morphologies vanish. Such morphologies were associated to local inhibition of the environmental damaging effect due to local high plastic strain and strain rates. The large variety of experiments, presented in this study, was then successfully analyzed considering the effect of local strain rates which greatly influence the crack growth behavior of Inconel 718.

This thesis was written under the supervision of Assoc. Prof. Luboš Náhlík, Ph.D. and Assoc. Prof. Pavel Hutař, Ph.D. The topic of this thesis is the study of a free surface effect and gradient change of material properties on a crack behavior. The common theme of the work is a fracture mechanics description of a crack behavior near a material nonhomogeneity. Here, the material nonhomogeneity can be understood either as a boundary of a body (interface between body and surrounding) or as a continuous change of material properties. The thesis is introduced by a review part where the state-of-the-art of the concerned topic is described. This part presents several stress state descriptions in the vicinity of some general singular stress concentrators, particularly a crack and a V-notch. The influence of the free surface on a fatigue crack front shape is discussed here. The review part follows with the fracture mechanics description of the graded materials. The problem formulation and the main aims of the thesis are stated in the following chapter. The core of this work is the fracture mechanical description of a crack propagating near the material nonhomogeneity. The thesis focuses on a stress field description near the free surface of the body where a change in a type of the singular stress field occurs. Methods used in generalized fracture mechanics are applied here to describe the stress field near the free surface. The difference between crack behavior in thin-walled and thick-walled structures is shown and supplied by relevant examples. Methods and procedures used are later utilized for estimation of a crack behavior in graded structures. The thesis is concluded by the discussion of obtained results in appropriate context.

Design based on damage tolerance concepts has become mandatory in high technology structures. These concepts are also essential for evaluating life extension of aged structures which are in service beyond originally stipulated life. Fracture analysis of such structures in the presence of single or multiple three-dimensional flaws is essential for this approach. Surface cracks are the most commonly occurring flaws and development of accurate methods of analysis for such cracks is essential for structural integrity evaluation of newly designed or aged structures. The crack fronts of these surface flaws are usually approximated mathematically to be of either part-elliptical or part-circular in geometry. In this thesis, some of the issues related to fatigue crack growth of single and multiple surface cracks are studied in detail. Here emphasis is given to the development of simple and accurate post-processing techniques to estimate stress intensity factors for surface cracks, development and/or implementation of simple numerical methods to simulate three-dimensional single and multiple cracks in fatigue and their experimental verification. Modified virtual crack closure integral (MVCCI) technique for estimation of strain energy release rates has been improved (chapter II) to deal with curved crack front and unequal elements across the crack front. The accuracy of this method is evaluated and presented in this chapter for certain benchmark surface flaw problems. The improved MVCCI is used in the investigation of interaction between multiple surface cracks in three-dimensional solids. The interaction effects are studied for both interacting and coalescing phases as observed to occur in the growth of multiple surface cracks. Extensive numerical work is performed to study the effects of various parameters such as aspect ratio, thickness ratio, interspacing on the interaction factors. These solutions are used in formulating empirical equations to estimate interaction factors. This facilitated the development of a simple semi-analytical method to study fatigue crack growth of multiple cracks. The growth of surface cracks under fatigue loading in the finite width specimens of an aero-engine superalloy has been studied experimentally (presented in chapter III). Four configurations for single semi-elliptical cracks are considered. Fatigue crack growth is simulated by two models viz. two degrees of freedom and "multi degrees of freedom with ellipse fit'. These models are sometimes referred to as semi-analytical models as the crack growth is predicted by numerical integration combining Paris equation with an empirical form of stress intensity factor solution. In order to use two degrees of freedom model for fatigue crack growth prediction of semi-elliptical cracks, empirical solution for the Ml range of geometric parameters for stress intensity factor is required for the considered configurations. The available Newman-Raju solution is useful for this purpose within a limited range of surface crack length to width (c/W) of the specimen. Based on the present finite element results, the empirical equations are developed for extended values of c/W. It is well understood that the fatigue prediction for two-dimensional crack can be improved by inclusion of crack closure effects. Usually, in semi-analytical models for growth of surface cracks under fatigue loading, the crack closure is included as a ratio of crack closure factor at surface and depth locations of semi-elliptical crack. In the present work, this ratio for the considered material of specimens is obtained by an experimental study. The difference in characteristics of preferred propagation path between semi-elliptical crack in a finite width plate and a wide plate is clearly brought out. Current crack growth predictions for most of the structures are based on the presence of only a single crack. However, in structures several cracks may initiate simultaneously within a stress critical zone and may interact depending upon their geometry, spatial location, structure geometry and mode of loading. In this work various configurations of twin semi-elliptical cracks have been studied by experiments. The beachmarks created on the specimens during experiments are used in the investigation of crack shape progression during fatigue. A three degrees of freedom crack growth model for interacting and coalescing cracks has been proposed. The experimentally determined crack shape and lives have been compared with the corresponding values from numerical simulation. The correlation of experimental results with numerical predictions was carried out through improved MVCCI for eight-noded brick elements. This has worked well in the configurations analysed. However, it is known in literature that there are benefits of using 20-noded singular elements. There could be special situations where the regular elements could fail, and singular elements could be essential. For this purpose, further development of MVCCI were carried out using 20-noded quarter-point elements (presented in chapter IV). Also a novel technique of decomposed crack closure integral (DCCI) was developed (presented in chapter V) for both regular and singular elements to represent the variation of MVCCI more accurately along the crack front. It is well known that quarter-point elements at crack front produce the required singularity at the crack tip and give accurate stress distribution with fewer degrees of freedom than conventional elements. Thus to develop more efficient post-processing tools, the MVCCI expressions are formulated for 20-noded singular quarter-point element for various assumptions regarding stress and displacement distributions in the elements across the crack front. A comprehensive study is presented (chapter IV) on MVCCI for 20-noded singular brick element including various simplified expressions for three-dimensional part-through cracks in pure and mixed-mode state of deformation of fracture. The developed MVCCI expressions are also valid for 15-noded quarter-point Penta elements. The reduction in model size can further be obtained if 12-noded three-dimensional singular element is employed at the crack front and eight-noded elements are used away from the crack front. The MVCCI expressions are also developed for 12-noded singular element and their accuracy is evaluated by numerical solutions. Presently, MVCCI, estimates the average stress intensity factor at the center of each element along the crack front. In this thesis, a Decomposed Crack Closure Integral (DCCI) is formulated to represent an assumed variation of stress intensity factor along the crack front in each element. The DCCI is formulated for 8-noded brick, 20-noded conventional brick and 20-noded singular brick elements. The numerical examples presented here deal with three-dimensional problems of patch repair technology and part-through cracks. The technique showed a major advantage for the patch repair problems where SIF variations along the crack front are of significance and large mesh sizes are computationally expensive. This along with MVCCI for 12-noded and 20-noded singular elements formed a part of the work on development of accurate and effective post-processing tools. It is expected that the present work will be helpful in damage tolerance design and assessment of aerospace structures and the experimental work performed as a part of this thesis will enhance confidence in the damage tolerance analysis. The thesis is concluded in chapter VI presenting the contributions of this thesis and projecting future lines of work possible in this area.

L’assemblage par collage est une technique de plus en plus utilisée dans les industries aéronautique, aérospatiale et médicale. L’étude des assemblages est d’une grande importance et nécessite le développement de techniques de contrôle des performances, autant au niveau mécanique, que physico-chimique. Dans cette étude nous abordons plusieurs problématiques liées au collage. La première approche est purement mécanique, avec la mise au point d’un essai à simple levier ou « Single Cantilever Beam adhesion test » (SCB), variante du « Wedge test » dans lequel une charge constante est appliquée a une extremité libre d’un adhérent causant un moment de flexion nécessaire à la rupture du joint. A travers cet essai, différents phénomènes (pré-fissuration, plasticité, …) on été observés et traités afin d’analyser les énergies de ruptures dans le joint de colle.L’une des préoccupations majeures, et seconde approche de ce projet sur l’étude des assemblages collés, est le vieillissement humide. Les effets du vieillissement sont analysés par SCB. La compréhension du vieillissement doit passer par une meilleure connaissance de la cinétique de diffusion, une technique d’impédancemétrie, dérivée de l’électrochimie, est donc proposée pour analyser la prise d’eau dans le joint collé. L’atout de cette dernière technique est de donner accès à la cinétique de diffusion in situ et dans des géométries proches des échantillons utilisés lors des essais mécaniques. / Structural adhesives are increasingly used in the aeronautics, aerospace and medical industrie, leading to the necessity to develop new tools for the assessment of adhesives, both mechanically and physico-chemically.This study deals with two types of technique to quantify the durability of adhesive joints. The first approach is purely mechanical. We adopt a Single Cantilever Beam adhesion test (SCB) in which a constant load is applied at the free end of a flexible beam, leading to a couple inducing fracture of the adhesive joint. From this test, different phenomena have been observed (pre-crack behaviour, plasticity, ...), analysed and quantified, to give a better appraisal of adhesive fracture.The second approach deals with adhesive ageing in water. The characterisation of water diffusion is of prime importance in many practical situations. A technique of impedancemetry, based on electrochemical techniques, is applied to the characterisation of diffusion and absorption properties of adhesives. A major asset of this technique is to be able to follow diffusion measurement in situ.

The presented master’s thesis deals with fatigue crack front shape estimation. The aim of this thesis is to create an iterative process leading to the real fatigue crack front shape. Thesis is solved using finite element method. The work is divided into two logical parts. The first part of the thesis describes the basic concepts of linear elastic fracture mechanic (LEFM), methods used for estimation of stress intensity factor and stress singularity exponent. The first part further describes some phenomenon’s accompanying the mechanism of fatigue crack growth as for example crack tip curving and crack closure. In the second part of the thesis there is studied an affect of the free surface on the fracture parameters, especially the affected distance from the free surface is determined. Based on the assumption of a constant stress intensity factor and stress singularity exponent along the crack front, an iterative process leading to fatigue crack front shape is presented. The accuracy of the result is discussed by comparing of obtained crack front shapes with experimental data at the end of the thesis.