PSI - Issue 14

A.N. Savkin et al. / Procedia Structural Integrity 14 (2019) 429–434 Author name / Structural Integrity Procedia 00 (2018) 000–000

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The simulation of the test under MiniFalstaff-spectrum loading is shown in Fig. 4

Fig. 4. The local near-tip stress σ * (a), threshold SIF К th (b) and fatigue crack length (c) under variable amplitude loading with MiniFalstaff spectrum

A Sunder model feature of the use is the definition of the asymmetry ratio for each semi-cycle of random cyclic loading and crack closing factor U and the local near-tip stresses and the values of the threshold SIF Kth used in Eq. (3). However, the random nature of the loads applied to the specimen leads with the field of spread of the points of change of the threshold SIF as a function of the local near-tip stress, which makes it difficult to study the function of the change of these parameters as the fatigue crack grew. The mean values were calculated for studying the kinetics of these parameters during the loading process. Their variation during the loading process is shown on Fig. 4 b and 4 c. It is noted that, the local near-tip stress σ* during 100-200 load blocks remains constant, and then increases up to failure, and irrespective P max . These stresses are negative at P max = 2500 N, with P max increasing it move to the positive area. SIF K th remains practically constant in the initial period of loading. As the fatigue crack grows, the SIF at the crack tip increases, and its threshold value decreases, contributing to an increase in the rate of crack growth. Comparison of the calculated and experimental data on the fatigue cracks growth life with different nature of variable loading with allowance for local near-tip stress and the change in the value of the threshold stress intensity factor are shown in Fig. 5.

Fig. 5. Comparison of the calculated and experimental fatigue crack growth lives for tests under different types of loading