Issue 47

Yu. G. Matvienko et alii, Frattura ed Integrità Strutturale, 47 (2019) 303-320; DOI: 10.3221/IGF-ESIS.47.23 316 and leads to high crack tip constraint, while negative T-stress leads to the lost of constraint. Fig. 10a evidences that for specimens of T5-H2 group ( R = –1) cracks of 1 a  length have lower crack-tip constraint compared with constraint for specimens of T5-H1 group ( R = –0.4). Comparing T-stress values related to T5-H1 and T5-H2 specimens obtained for the second crack length 2 a  demonstrates the opposite trend. Namely, negative T-stress values for T5-H1 group lie significantly below analogous data for T5-H2 specimens. These facts show that two-parameter fracture mechanics criteria are of great importance for cracks propagating under combined influence of external and residual stresses. a b Figure 10: Experimental T-stress values 1  T (a) and 2  T (b) for cracks of 1  a and 2  a length as a function of lifetime percentage for specimens of T5-H1 and T5-H2 group. Damage accumulation process Above-presented data describe an evolution of fracture mechanics parameters for specimens of two groups, which are loaded with the same stress range   = 350 MPa, but with different stress ratio R = –0.4 (T5-H1 group) and R = –1.0 (T5-H2 group). These experimental results are obtained for the same remote constant stress  = 80 MPa that corresponds to 24% of material yield stress y  = 330 MPa. Maximum cyclic stresses are equal to max  = 250 MPa and max  = 175 MPa that corresponds to 76% and 53% of yield stress for T5-H1 and T5-H2 group, respectively. Owing to this, maximum elastic-plastic circumferential strains at the hole vicinity caused by a stress concentration influence under external tensile load in specimens of T5-H1 group are much more than analogous strains in specimens of T5-H2 group. Stain concentration effect is valid, especially at initial stages of cyclic loading, despite of an influence of negative circumferential residual stresses due to cold hole expansion as it follows from Fig. 8a and 9a. This fact could mean that a rate of fatigue damage accumulation for specimens of T5-H1 group considerably exceeds analogous parameter for specimens of T5-H2 group. Thus, a correct comparison of fracture mechanics parameters, which are obtained in the case of the same stress range but different stress ratio, should be provided by a normalization of obtained experimental data. The most evident way resides in normalizing all data obtained for specimens of T5-H2 type by a coefficient that is equal to the life extension parameter 2 100 1 CF CF N k N  = 2.51 Distributions of normalized CMOD and SIF values for T5-H2 specimens over lifetime period are presented in Figs. 11 and 12. These figures also include CMOD and SIF values for T5-H1 specimens. A comparison of normalized SIF plots is of prime interest. The point is that these dependencies reflect the change in a process of fatigue damage accumulation as a result of stress ratio changing. To put it differently, normalized SIF distributions quantitatively describes how a decrease in