Issue 47

Yu. G. Matvienko et alii, Frattura ed Integrità Strutturale, 47 (2019) 303-320; DOI: 10.3221/IGF-ESIS.47.23 315 2 100 1 F F N k N  = 2.51. It is necessary to note that a ratio between 60% lifetime indicators for specimens of different groups has very close value, namely, 2 60 1 H H N k N  = 2.25. a b c Figure 9: Experimental SIF values 1  I K (a) , 2  I K (b) and 3  I K (c) for cracks of 1  a , 2  a and 3  a length as a function of lifetime percentage for specimens of T5-H1 and T5-H2 group. CMOD values for the first crack length are connected with circumferential compressive stresses caused by cold hole expansion. Comparing CMOD distributions in Fig. 8a reveals residual stress relaxation for specimens of T5-H1 group. But there is no residual stress redistribution for N > 1000 cycles for T5-H2 specimens. Fig. 10 concentrates on the effect of low-cycle fatigue on T-stress redistribution. The value of T is the stress acting parallel to the crack plane in the direction of crack extension with a magnitude proportional to the applied nominal stress. The non- singular term T represents a tension or compression stress. Positive T-stress strengthens the level of crack tip stress triaxility