PSI - Issue 14

Ilyin A.V. et al. / Procedia Structural Integrity 14 (2019) 964–977 Ilyin A.V., Filin V.Yu. / Structural Integrity Procedia 00 (2018) 000 – 000

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the stage of a micro crack nucleation, micro crack extension through the structural barriers, a short crack and partly a macro crack propagation, because the registered value N i as a rule corresponds to a visually detectable crack of the depth not less than 0.5 or 1.0 mm. For type (b) tests it is necessary also to get the data concerning the effect of cyclic asymmetry and corrosion medium onto da / dN . Figure 4 gives the most important results of type (a) tests, i.e. the cyclic life expectations. In relation to the data for polished specimens loaded by a symmetric cycle in air, the effect of all three mentioned factors is a sharp decrease of the number of cycles to failure in the high-cycle area and absence of the fatigue limit (or, as minimum, a shift of the breakpoint of the fatigue curve to by an order higher numbers of cycles). At the same time all these factors show no dependence on the strength of tested metal, and their effect is not additive. There is no tendency to monotonic increase of damage rate with lowering of the loading frequency. It has been found that a test duration more than 3 to 5 days is representative enough to account for the corrosion effect. Results of type (b) tests are generalized as follows:   q ef da dN C K    / . (10)

Figure 4 – Effect of cyclic asymmetry, surface quality and corrosion medium on the cyclic life of unnotched specimens made of high-strength steel. 1. Polished specimens, r = -1, in air,

2. Polished specimens, r = -1, in a corrosion medium, 3. Polished specimens, maximum r (  max =  Y ), in air, 4. Specimens with rough surface, r = 0, in air, 5. Specimens with rough surface, r = 0, in a corrosion medium, 6. Specimens with rough surface, maximum r (  max =  Y ), in a corrosion medium

An experimental research by Sadkin et al (2016) has shown that the effect of cyclic asymmetry depends on the load interval between the start of a crack opening ( K op ) and the maximum SIF in a cycle ( K Imax ). When r = 0, K op almost coincides with a threshold SIF value K th . For r > 0 K op reduces. For alternating loading ( r < 0) the compressive part of a cycle brings a contribution into a crack kinetics because K op is below zero. However the maximum of Δ K ef does not exceed 1.3 K Imax even when the compressive part of a cycle is infinitely large. The