Abstract
The presented paper brings the results of metallographic and fractographic analysis of fracture surfaces, fatigue crack propagation and fatigue life measured at the high-frequency cyclic loading of Mg-alloy AZ91D. Experimentally measured values of the fatigue resistance of the alloy AZ 91D have been influenced, first of all, by the presence of cast defects. The occurrence of these defects, their character, size, orientation, amount, and location on the surface and in the subsurface layers influenced, in a negative way, the fatigue resistance of the studied alloy. The fatigue behaviour and character of the fatigue fracture of the alloy AZ 91D are highly dependent on structural factors and on the level of technical perfection of their production. The character of the fracture surface morphology depends on the size of the stress amplitude σ_a. Decreasing of expenses as well as increasing of efficiency of whole process of mechanical design and providing of operability during the overall lifetime of parts and machineries allow us to make progress in the field of the utilization of the computational technologies and the application of numerical methods for the solution of huge amount of mechanical engineering praxis problems. Nowadays we have several commercial programs at our disposal, which allow us to solve the crack propagation. Many authors have dealt with influence of the crack growth on the functionality of the particular parts from global point of view.

Key words
magnesium alloy, high-frequency cyclic loading, fatigue tests, numerical methods

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