Non-linear kinetics of damage accumulation and anomalies in the elastic properties of metals at very high cycle loading

O. Naimark1, M. Bannikov1
1ICMM UB RAS, Russia, Perm
Paper describes current understanding of the mechanism of generation of cyclic irreversible shifts as structurally induced plastic shear strains that, are generally a part of the accumulated plastic strain, which lead to the formation of fatigue cracks. Statistical approach is proposed, which allowed one to determine the types of collective modes in ensembles of mesoscopic defects and establish their role in the mechanisms of fatigue fracture, scaling mechanisms during fracture. A system of constitutive equations for low- and highcycle fatigue is presented. An attempt is made to describe the relation between the microscopic mechanisms of fatigue and model representation and consideration of damage stages on the basis of non-linear kinetics of defects accumulation during cyclic loading. A technique for «in situ» determination of fatigue damage accumulation is presented based on an analysis of the nonlinearity of feedback signal in the closed system of ultrasonic fatigue testing set. In the ultrahigh cycle fatigue regime anomalies of elastic properties of the material are manifesting that leads to an effect of non-linearity of the oscillation amplitude. This effect increases with the initiation and growth of fatigue cracks. The method has been applied to samples with different pore sizes and at different levels of average stress in order to determine the moment of initiation and growth of fatigue cracks in the regimes of high- and gigacycle fatigue. The technique can be used for an early detection of fatigue cracks both on the surface and within the material during cyclic loading in ultrasonic regimes.
Received: 02 October 2015   Revised: 13 November 2015   Accepted: 19 November 2015
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