The Range of Use of the Critical Distance Concept to Predict the Endurance Limit in the Presence of Stress Raisers
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THE RANGE OF USE OF THE CRITICAL DISTANCE CONCEPT TO PREDICT THE ENDURANCE LIMIT IN THE PRESENCE OF STRESS RAISERS O. M. Herasymchuk1 and O. V. Kononuchenko
UDC 539.4
The paper presents results of fatigue tests in the tension–compression and tension–tension modes of specimens of 45 steel: smooth specimens and specimens with a blind hole of different diameters of 0.6 and 1.6 mm. The fatigue resistance of specimens with a 1.6 mm hole is higher than that of specimens with a 0.6 mm hole. This is accounted for by the different mechanisms of reaching the endurance limit in both cases, which is a consequence of different gradients of local stresses in the vicinity of the hole. For the specimens with a smaller hole, the endurance limit is determined by the nominal stress for the initiation of a crack of the size not exceeding the grain size, as this is the case in smooth specimens too. For the specimens with a larger hole, the endurance limit is determined by the threshold nominal stress required for the propagation of a crack whose size is larger than the grain size. To theoretically estimate the endurance limit in the presence of a stress raiser in the form of a hole, the critical distance concept as a single parameter for a particular material can be successfully used, but if the radius of the hole does not exceed a certain critical value. A formula for the estimation of this value as a function of the material characteristic and theoretical stress concentration factor is proposed. The same formula can be used to determine the critical value of the theoretical stress concentration factor, above which the endurance limit in the presence of a stress raiser of the same depth and different sharpness remains unchanged. Keywords: endurance limit, stress raisers, critical distance. Introduction. The endurance limit of specimens and structural elements in the presence of a stress raiser is analyzed using different approaches depending on the stress raiser geometry [1, 2]. To estimate the endurance limit of specimens with “blunt” and “shallow” stress raisers (the theoretical stress concentration factor K t £ 4), the critical distance concept is used [1]. The endurance limit in the presence of a blunt stress raiser is determined by the nominal stress at which the local stress at the critical distance from its tip is equal to the endurance limit of smooth specimens. The critical distance characterizes the depth of the surface layer with mechanical properties differing from those of the bulk material, in which during cyclic loading, local plastic strains occur and then cracks nucleate [3]. The thickness of this layer can reach the size of one grain for coarse-grained materials to several tens of grains for fine-grained materials. Its absolute value can be an order of magnitude smaller for fine-grained materials [2]. When using the critical distance concept, it is assumed that at the endurance limit of specimens with a blunt stress raiser, no crack occurs, or if it is formed from the stress raiser tip and does not propagate furthe
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