Issue34

A. Shanyavskiy, Frattura ed Integrità Strutturale, 34 (2015) 199-207; DOI: 10.3221/IGF-ESIS.34.21 202 a) b) Figure 2: Mono S-N curves for tested smooth specimens: (a) symmetrical case for (1) not tempered and (2) tempered specimens; (b) at R>0 for (1) TP- specimens and (2) T-specimens. Figure 3 : Mono S-N curves for notched (1) TP-specimens, (2) specimens without tempering and short peening, and (3) T-specimens tested under R=-1.0. In all cases, fatigue tests were stopped in the range of durability (2-5)x10 7 cycles (see Fig.3). These not failed specimens were used for tensile test up to their failure to discover crack growth path from the fractographic analyses. Crack path for failed specimens Material behavior for hardened and not hardened specimens has significant difference in all range of stress levels. In hardened specimens, crack originates at the subsurface for durability being more than 5х10 5 load cycles, independent of the value of R-ratio. Number of origins has not a strong correlation with the maximum stress level. This fact can be explained on the basis of the maximum stress level value. This value was not far from the material Yield stress for high values of R-ratio. Nevertheless, in spite of high level of maximum stress (approximately 920 MPa) for R=0.67, the number of load cycles to failure can be reached (10 6 load cycles and more) that relayed to HCF regime. In the case of low R-ratio in the range of 0.3-0.33, there were several origins in the fracture surface for maximum stress levels 920 and 900 MPa. For example, in the TP-specimen tested under maximum stress level 920 MPa, crack has not reached the specimen surface before fast fracture (Fig. 4). The first step of crack origination was realized because of smooth facet formation (the one by the  - phase).