Microplasticity , Microdamage, Microcracking in Ultrasonic Fatigue C Bathias 1, D Field2, S Antolovich,3 and PC Paris 4 1University of Paris Ouest ,LEME, 50 rue de Sevres, Ville d’Avray ,92410 Fr, claude@bathias.com 2Washington State University, Pullman, WA 99164-2920 , 3 GeorgiaTech, Atlanta,Ga, 30332-0245 4Washington University, Saint Louis, Mo,63130 Abstract: Depending upon the fatigue domain, different types of crack initiation occur. In low cycle fatigue, the cyclic plastic deformation is critical at the surface but exists also in the bulk of the metal. In high cycle fatigue, the plastic deformation is limited to the surface in plane stress conditions. In very high cycle fatigue, the crack initiates from the interior, starting from a defect in plane strain conditions. This means that the initiation mechanisms in gigacycle fatigue depend not only on upon the stress field but also on the stability of the microstructure. Keywords : Gigacycle fatigue, fish eye, plasticity, microstructure 1-Introduction The initiation of fatigue cracks can be considered differently from physical or mechanical points of view. At the microscopic level, Mughrabi [1] shows that the initiation of fatigue crack in the gigacycle fatigue regime can be described in terms of a microstructurally irreversible portion of the cumulative cycle strain. This means that there is no basic difference between fatigue mechanisms in low, mega, and gigacycle fatigue except for the strain localization. However, specific mechanisms can occur depending on the fatigue life. The fatigue life seems to be a key parameter to determine correctly the fatigue initiation location. In low cycle fatigue, in megacycle fatigue and in gigacycle fatigue different mechanisms can operate at different scales of plasticity. In the low cycle fatigue regime, the cyclic plastic deformation is critical at the surface but exists also in the bulk of the metal. Typically, several cracks nucleate from the surface. When the fatigue life is below 105 cycles, general plastic deformation of the specimen bulk governs the initiation. When the fatigue life is between 106 and 107 cycles the plastic deformation depends on the plane stress surface effect and the presence of flaws which explain the critical location of fatigue initiation. Typically, the initiation starts with one crack only, from the surface. However approaching 109 cycles the plastic deformation in plane stress conditions vanishes; the macroscopic behaviour of the metal is elastic except around flaws, metallurgical defects or inclusions. In very high cycle fatigue, the plane stress conditions are not enough for a surface plastic deformation according the Von Mises criteria. The initiation may be located in an internal zone. When the crack initiation site is in the interior, this leads to the formation of one fish eye on the fracture surface, typical of gigacycle fatigue. In this case, the cyclic plastic deformation is related to the stress concentration around a defect: inclusion, porosity, super grain. Since the probability of occurrence of a flaw is greater within an internal volume than at a surface, the typical initiation in Gigacycle fatigue will be most often occur in the bulk of the metal. To sum up the state of art, it is generally accepted that if the first damage in fatigue is located at the surface the typical feature is persistent slip band (PSB), intrusion or extrusion, all related to a strong planar shear deformation, in plane stress. . On the contrary, if the first
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