La détection des fissures fermées constitue un verrou important pour les méthodes conventionnelles de CND. En revanche, les méthodes basées sur le principe du contact acoustique non-linéaire (CAN) représentent un outil potentiel capable de détecter et éventuellement de caractériser les fissures fermées. Dans ce travail de thèse, le CAN et la génération du second harmonique sont au centre de l’attention dans le but d’analyser les mécanismes mis en jeu en vue de la détection et la caractérisation de fissures fermées. Notre approche repose sur une analyse numérique de l’interaction entre une interface de contact et une onde longitudinale et une analyse expérimentale du CAN sur une éprouvette contenant une fissure de fatigue.Le CAN est modélisé par une loi de contact unilatéral avec (ou sans) frottement de Coulomb, en considérant l’interaction non-linéaire entre une onde longitudinale et une interface de contact, d’abord dans un milieu unidimensionnel en utilisant la méthode des différences finies, puis dans un milieu bidimensionnel par la méthode des Eléments Finis. Cette analyse a permis d’expliquer la génération des harmoniques supérieurs et de mettre en évidence les principaux paramètres gouvernant le CAN dans un cas unidimensionnel (onde plane/interface de contact plane), et aussi dans un cas bidimensionnel plus complexe considérant une onde non-plane et des morphologies d’interfaces complexes. Afin de valider les résultats numériques et d’apprécier l’applicabilité de la méthode dans un cas réel, des mesures expérimentales du second harmonique dans le cas d’une fissure de fatigue réelle sont effectuées. Les résultats de cette étude sont en accord avec les tendances numériques obtenues.Enfin, dans le but d’exploiter au maximum la méthode de génération d’harmonique, nous proposons également une stratégie numérique/expérimentale de reconstruction des sources acoustiques, basée sur une méthode d’holographie acoustique, permettant ainsi d’accéder à la distribution des sources de génération du second harmonique au niveau de la fissure. Ces résultats sont prometteurs en vue de la caractérisation des fissures fermées. / The detection of closed cracks constitutes an important obstacle for conventional nondestructiftechnics (NDT). However, nonlinear methods based on contact acoustic nonlinearity (CAN)represent a potential tool for the detection of these defects. In this work, a detailed analysis of thecontact acoustic nonlinearity is proposed in view of the characterization of closed crack.Our approach is based on an association between a numerical analysis of the nonlinear interactionbetween a longitudinal wave and a contact interface, as well as an experimental study of the CAN inthe case of a real fatigue crack.Numerically, the CAN is modeled by using a unilateral contact law with (or without) Coulombfriction. The nonlinear interaction between a longitudinal wave and a contact interface is considered,first in a one-dimensional medium by using the Finite Differences method, and then in a twodimensionalmedium by using the Finite Elements method. This analysis explains the generation ofsuper-harmonics and defines the main physical parameters controlling the CAN in a one-dimensionalcase (plane wave / plane contact interface) and also in a more complex two-dimensional caseconsidering a non-planar wave and complex interface morphologies. In order to validate the numericalresults and to test the applicability of the method in a real case, experimental measurements of thesecond harmonic in the case of a real fatigue crack are realized. The results of this study are inagreement with the numerical tendencies obtained.Finally, in order to exploit well the harmonic generation method, a numerical / experimentalstrategy based on acoustic holography principal for the reconstruction of acoustic sources is proposed.This strategy gives access to the distribution of second harmonic sources at the crack. The results ofthis study are promising in view of the characterization of closed cracks.

Mechanical fatigue is a failure phenomenon that occurs due to repeated application of mechanical loads. Very High Cycle Fatigue (VHCF) is considered as the domain of fatigue life greater than 10 million load cycles. Increasing numbers of structural components have service life in the VHCF regime, for instance in automotive and high speed train transportation, gas turbine disks, and components of paper production machinery. Safe and reliable operation of these components depends on the knowledge of their VHCF properties. In this thesis both experimental tools and theoretical modelling were utilized to develop better understanding of the VHCF phenomena. In the experimental part, ultrasonic fatigue testing at 20 kHz of cold rolled and hot rolled stainless steel grades was conducted and fatigue strengths in the VHCF regime were obtained. The mechanisms for fatigue crack initiation and short crack growth were investigated using electron microscopes. For the cold rolled stainless steels crack initiation and early growth occurred through the formation of the Fine Granular Area (FGA) observed on the fracture surface and in TEM observations of cross-sections. The crack growth in the FGA seems to control more than 90% of the total fatigue life. For the hot rolled duplex stainless steels fatigue crack initiation occurred due to accumulation of plastic fatigue damage at the external surface, and early crack growth proceeded through a crystallographic growth mechanism. Theoretical modelling of complex cracks involving kinks and branches in an elastic half-plane under static loading was carried out by using the Distributed Dislocation Dipole Technique (DDDT). The technique was implemented for 2D crack problems. Both fully open and partially closed crack cases were analyzed. The main aim of the development of the DDDT was to compute the stress intensity factors. Accuracy of 2% in the computations was attainable compared to the solutions obtained by the Finite Element Method. / Very High Cycle Fatigue (VHCF) is considered as the domain of fatigue life greater than 10 million load cycles. Structural components that have service life in the VHCF regime include wheels and axles of high speed trains, gas turbine disks, and components of paper production machinery. Safe and reliable design, and the longevity, of these components depends on the knowledge of their VHCF properties. The overall aim of the experimental portion of this thesis was to gain in-depth knowledge of the VHCF properties of stainless steels. Fatigue test data in the VHCF regime was generated for different stainless steel grades using ultrasonic fatigue testing. The mechanisms for fatigue crack initiation and short crack growth were investigated using electron microscopes. Theoretical modelling of complex crack geometries involving kinks and branches was carried out by using the Distributed Dislocation Dipole Technique (DDDT). The main aim of this development was to compute the stress intensity factors and to analyse the stress state around the cracks. The results showed that accuracy of 2% was attainable compared to the solutions obtained by Finite Element Method (FEM). / <p>Artikel 4 publicerad i avhandlingen som manuskript</p>