PSI - Issue 42

Tereza Juhászová et al. / Procedia Structural Integrity 42 (2022) 1090–1097 Juhaszova/ Structural Integrity Procedia 00 (2019) 000–000

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ratio R applied during the fatigue testing, was set to 0.1 and the maximal force P max used for testing was 25 kN. To obtain the values of CMOD, clip-on extensometer was placed on the lower edge of the beam, from where the crack propagated. To secure the measuring device, thin plastic plates were glued to the specimen, as is visible in Fig. 4(a). The results obtained with this experimental method were values of the maximal and minimal CMOD per each 100 cycles, from which values of mean and amplitude of CMOD were calculated.

(a)

(b)

Fig. 4 Experimental setup with specimen initially notch 0.5 mm after test - (a) and (b) - results obtained by fatigue measurements for P max =25 kN and R =0.1.

The measured values of ΔCMOD depending on number of cycles are plotted in Fig. 4(b). The results of the measurement show the biggest resistance of the A02 specimen, when the distinctive propagation of the crack started after the biggest amount of cycles. The other three specimen did not show as large dispersion of the cycles needed for crack propagation. 6. Results and discussion The experimental data obtained from the fatigue tests were evaluated using the dependence of SIF and relative crack length on CMOD. These results were used to graphically display relation between SIF range � and FCGR d a /d N in double logarithmic scale. The complete results are shown in Fig.5(a). Due to the similar composition of AISI 304 (Tab.1) and main alloying elements, and the same processing of both the steel grades, very similar mechanical properties of the material can be assumed. Linear part of the measured graph was approximated using power function, determined in the interval of (30, 45) MPam 1/2 . The function prescription establishes the Paris’ equation for each specimen, with the material characteristic values of constants m and C. The results of fitting are plotted graphically in Fig. 5(b) and the values of Paris law constants are displayed in Tab. 4 together with coefficient of determination of fitting R 2 . Linear fitting of double logarithmic plot shows good approximation according to R 2 greater than 0.9 for each specimen. The values of Paris’ law constants for specimens of the same production show minimal dispersion. The constant m shows decrease for A specimens and higher values for E specimens, which means higher slope of fitted curve and therefore faster crack propagation rate. The constant C for these specimens shows lower values, which predicts lower crack growth rate in the beginning of crack propagation. All of the specimens show results corresponded well with literature, in which the ratio for m for stainless steel lays in interval <2.2 ÷ 5.4> and for C <10 −12 ÷ 10 −8 >, as can be observed in studies by Kim (2006) , Azouggagh (2017), Jambor (2021) or Seitl (2022 ) .