The article considers the existing stages of the process of fatigue destruction of ship structural elements on the example of the structural material of the propeller shaft of the vessel. The technique of the experiment to study the process of origin and further growth of fatigue cracks in samples made of carbon steel 35 is briefly characterized. The analysis of the results of fatigue tests was carried out, according to the results of which it was noted that crack-like defects appear on the surface of the test samples after a short loading. The orientation of the nucleated cracks relative to the axis of loading application is chaotic, and the origin of cracks mainly occurs in the grain bodies of the micro structure of the material. Tests of the material at stresses exceeding the endurance limit showed that the microcrack develops within the grain boundaries, and then stops its growth. It is noted that the boundaries of grains, phases or inclusions play the role of force barriers, to overcome which the microcrack must accumulate the necessary deformation energy. The main result of the work is to determine the boundary of the transition of a small crack in the stage of development of a macro-crack. The author proposes to assume that such a boundary corresponds to the crack length equal to the size of ten average grain diameters of the micro structure.
ship's shaft line, ships propeller shaft, fatigue failure, small crack
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