PSI - Issue 23

M.A. Artamonov et al. / Procedia Structural Integrity 23 (2019) 257–262 Author name / Structural Integrity Procedia 00 (2019) 000 – 000

260

4

the entire duration of loading. Therefore, the obtainment of m and C values for the specimens, tested at the temperature of 20 °C by this method , is not correct. The distribution of SIF over the crack front was determined by solving the finite element model with the use of J integral. For the plane strain state of tensile crack the SIF values are determined by the equation = √ /(1 − 2 ) , where E – Young ’ s modulus, ν – Poisson ’ s ratio [11]. The SIF value at the front was taken at the sites where the fatigue striations were analyzed. Then the dependence of the SIF range on depth was plotted. For each specimen the dependences of the FCG rate, obtained by the methods of fractography, on the SIF range were plotted in the logarithmic axes. By approximating of these dependencies with the Paris equation, the C and m coefficients were obtained for each cylindrical specimen (Fig. 2b).

Fig. 2. (a) Finite element model of the specimen with crack; (b) Dependence of da/dN on SIF for the specimen tested at 650 °C, R=0.

Results

The obtained values of lifetime for the specimens tested on LCF at different temperatures and cycle asymmetry are shown in Fig. 3a. One can note the relative high spread of values and the expected decrease of lifetime as the temperature rises. There is a difference in the results on lifetime in the temperature range from 350 °C to 550° C for the dependencies with different cycle asymmetry (Fig. 3a). There was carried out the fractographic analysis of fractures obtained during the LCF tests. On all the specimens, one can observe the multi-origin initiation of fatigue cracks, at that the origin areas on all the specimens are located on the side surfaces of specimens (Fig. 1). In the origin of fatigue initiation one can repeatedly observe the facet limited by the dimensions of austenite grain and turned against the load axis (Fig. 1b). The facets come out on the side surface of specimens. Generally, the focus of origin is located on the surface, but sometimes it is located at a certain distance from the surface. The further propagation of crack takes place with the formation of fatigue striations (Fig. 1c). The measurement of the fatigue striation pitch at the local sites of fracture made it possible to obtain the kinetics of FCG, which in its turn made it possible to estimate the duration of fracture propagation and, consequently, to calculate the durability. The obtained results on the specimen durability and period of the crack initiation as a function of temperature are shown in Fig. 3c and 3d. One can observe different dependences of the period of crack initiation (Fig. 3d) and the period of crack propagation (Fig. 3c) on temperature during the tests of specimens with different cycle asymmetry ratios. The increase of the asymmetry ratio brings about a sharp decrease in the period of crack initiation, especially at low temperatures of the specimen testing. The situation with the period of crack propagation is different. The tests at temperatures of up to 550 °C, carried out with the asymmetry ratio equal to 0.5, demonstrate higher results in comparison with tests carried out with the asymmetry ratio equal to 0. Further, at a higher temperature one can observe the identical dependence of durability on temperature. There were determined the values of SIF magnitudes which were compared with the crack growth rate obtained by measuring the pitch of fatigue striations (Fig. 2b). This made it possible to determine the Paris factors. The same analysis was carried out for all the specimens under investigation and there were obtained the values of m and C Paris factors depending on the temperature and cycle asymmetry ratio (Fig. 3b).