Issue34

A. Shanyavskiy, Frattura ed Integrità Strutturale, 34 (2015) 199-207; DOI: 10.3221/IGF-ESIS.34.21 203 a) b) Figure 4 : (a) TP-specimen fracture surface with a subsurface origin; (b) presented under different magnification. Area of origin placed inside of the circle. Smooth facet in (b) is area of the first step of crack initiation by the  - phase. Failed specimens under maximum stress level 900 MPa were cyclically loaded in the elastic-plastic regime, because this stress level is not far from material yield stress (see Tables 1, 2). Nevertheless, the area of origins in the fracture surface was in the subsurface as cleavage by the  - phase. It was the same as that found in HCF regime [9]. It was analysed the fracture surface pattern of the T - specimen subjected to maximum stress level 730 MPa with R=0.64 up to 2.3х10 7 load cycles without cracking. Then, this specimen was tested under maximum stress level 790 MPa with R=0.5 up to failure. Fractographic analyses have shown that fracture origin is near the specimen surface (Fig.5a). This result shows that, under low stress level with bigger R-ratio, material has accumulation damages at subsurface, and fatigue crack could be appeared at subsurface for longer material stressing. This conclusion is based on earlier test results with two steps on stress levels of surface hardened and not hardened specimens of steels and Al-based alloy [10], [11]. a) b) Figure 5 . Fracture subsurface origin (inside of the dashed line) (a) in the T - specimen tested, first, under maximum stress level 730 MPa with R=0.64 up to 2.3х10 7 load cycles without failure and, then, tested under 790 MPa with R=0.5 up to 4.1х10 5 cycles with failure, and (b) in NTP – specimen tested up to 1.8х10 6 load cycles with failure. A similar result was found for NTP – specimen tested under maximum stress level 730 MPa with R=0.58. This specimen experienced 1.8 х10 6 load cycles before failure. Fracture surface pattern testified crack origination subsurface (see Fig.5b), with approximately the same surface formation.