Issue 47

Yu. G. Matvienko et alii, Frattura ed Integrità Strutturale, 47 (2019) 303-320; DOI: 10.3221/IGF-ESIS.47.23 314 a b c Figure 8 : Experimental CMOD values ∆ ෤ ଴ (a) , ∆ ෤ ଵ (b) and ∆ ෤ ଶ (c) for cracks of ෤ ଵ , ෤ ଶ and ෤ ଷ length as a function of lifetime percentage for specimens of T5-H1 and T5-H2 group. Figs. 8 and 9 demonstrate the effect of stress ratio R on the redistribution of CMOD and SIF as the result of low-cycle fatigue with the same stress range. Comparative analysis of these distributions indicates a considerable difference in the CMOD and SIF values for all crack lengths. As expected, CMOD and SIF values for T5-H1 specimens are much below these values for T5-H2 specimens. The point is that increasing CMOD and SIF values for a narrow notch inserted under constant tensile loading means decreasing a rate of real damage accumulation process inherent in cyclic loading. Experimental SIF vs. the fatigue cycle number curves for T5-H1 specimens exhibit 63% lifetime indicator as a point of SIF fall beginning for 1 H N = 4000 cycles. This point is reliably detected as it is shown by vertical black line in Fig. 9a. The same situation takes place for specimens of T5-H2 group. The point of SIF fall beginning in Fig. 9b means reaching 57% of lifetime for 2 H N = 9000 cycles as it is shown by vertical red line in Fig. 9a. It is of importance for further investigations that reliable lifetime indicators are practically the same for specimens of both groups. This fact means that 60% lifetime indicator does not depend on a value of stress ratio R . The fracture occurred after 1 F N = 6300 cycles and 2 F N = 15800 cycles for specimens of T5-H1 and T5-H1 group, respectively. Thus, the life extension parameter is equal to