PSI - Issue 43

Jan Klusák et al. / Procedia Structural Integrity 43 (2023) 142–147 Author name / Structural Integrity Procedia 00 (2022) 000 – 000

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3. Results of fatigue tests 3.1. Fatigue life curves

The fatigue lifetime of the studied steels is presented in S-N curves, see Fig. 4. The results of fatigue life of both steels are comparable, where the steel 1.4306 exhibits slightly higher numbers of cycles to fracture than 1.4307. The main difference between both steels is the presence of two step S-N curve in the case of 1.4306 steel. It is apparent that the life-time of the specimens loaded by 270 and 280 MPa failed in two regions: below 1  10 6 cycles (high cycle fatigue region - HCF) and between 1  10 7 and 1  10 9 cycles (very high cycle fatigue region - VHCF). In the case of 1.4306 steel, no failure was observed in VHCF region. The fatigue limit was determined from the samples capable to withstand 1  10 10 loading cycles without failure. The slope of the curve in high cycle fatigue region (below 1  10 7 ) can be described by the Basquin’s law in the form:

,

(1)

where the coefficients A and B of the relation (1) are in the Table 2.

Table 2. Coefficients of the Basquin’s law relation and the fatigue limit. Material A B

σ f [MPa]

304L/4306 304L/4307

750 553

-0.078 -0.055

255 245

3.2. Frequency dependence During fatigue tests, frequency was monitored and evaluated. We have observed frequency changes related to changes of stiffness of the materials tested. During the initial phases of the tests an increase of frequency was observed with the following very slow frequency decrease. The initial growth lasted about 10 million cycles, while the decrease in frequency lasted billions of cycles. The frequency dependence on the number of cycles in logarithmic scale is shown in Fig. 5. There are records of tests of the 1.4307 specimens No. 09 and 10, run outs at 240 and 245 MPa, respectively.

Fig. 4. S-N curves of fatigue data measured on steels 304L/4306 and 304L/4307.