PSI - Issue 17

Jan Klusák et al. / Procedia Structural Integrity 17 (2019) 576–581 Author name / Structural Integrity Procedia 00 (2019) 000 – 000

579

4

320 330 340 350 360 370 380 390 400 410 420 1.0E+04 1.0E+05 1.0E+06 1.0E+07 1.0E+08 1.0E+09 1.0E+10 1.0E+11  a [MPa] log N S355 J0A/J0B/J2A/J2B J0A J0B J2A J2B

Fig. 3: Results of fatigue life tests of S355 J0A and J0B and S355 J2A and J2B

The S-N diagram shows that the direction of orientation of the samples in relation to the direction of rolling has not significant influence on the fatigue life. On the other hand it can be said that the grade S355 J2 exhibit higher fatigue life resistance for the same loading levels in comparison to S355 J0 grade. Conventional fatigue limit is determined as the level of stress amplitude under which all and at least two samples of the material withstand 1  10 10 cycles without failure, see Table 2.

Table 2. Fatigue limit of the studied S355 steel grades Steel grade S355 J0A

S355 J0B

S355 J2A

S355 J2B

Fatigue limit [MPa]

340.21

337.42

346.18

346.18

4. Fracture surface analysis

Selected samples were analysed after failure in a scanning electron microscope (SEM) in order to determine points of crack origin. Cracks have initiated both from surface and subsurface. Usually subsurface (interior) crack initiation is expected in cases of very high cycle fatigue after 1  10 7 cycles. Here we observed mainly surface crack formation independently on number of cycles (between 7.4  10 4 and 1.06  10 10 cycles), see e.g. Fig. 4. Interior crack initiation was observed for several cases for failure between 2.8  10 7 and 1.09  10 10 cycles with the one exception of subsurface crack initiation in the inclusion in S355 J0A for lower number of cycles (specimen No. 4 –  a = 362.5 MPa, N f = 3.2590  10 6 ), see Fig. 5. The reason for that early failure originating in interior can be stress concentration and non-ideal adhesion at the interface between the inclusion and the matrix. The dimension of the ball-shape inclusion in this case was about 120  m. Similar inclusions were observed at fracture surface also in other samples. In some cases the inclusions led to the crack initiation, in some cases they were near to the crack origin place, see e.g. Fig. 6.