PSI - Issue 2_B

K.-H. Lang et al. / Procedia Structural Integrity 2 (2016) 1133–1142 K.-H.-Lang et al. / Structural Integrity Procedia 00 (2016) 000–000

1137

5

1,5

650

1,3 1,4 1,5

900

90 7

570 16

570 16

90 7

T N = 4,44

T N = 3,57

339

600

236

800

10

1,25

k = 5 / k * = 25

k O = 9,4 (6) /8,7(5)

k = 7

k O = 10 /7

550

600  /2 (MPa) 700

1  /2R w/6 (MPa)

90

90

1,1  /2R w/6 (MPa) 1,2

500  /2 (MPa)

-

10

641

236

T  = 1,14

216

11

-

14

456 

- -

15

-

561

-

- -

450

-

17

Regression line

1/5 3/7 2/2 0/5 1/4

-

1

500

-

25 26

18 24

408 

50 Hz mit

-

- -

63

27

T  = 1,02

400

20

29

-

-0,11

 /2R w/6 = 4N B

32

22 28 30

19

63

0,75

21

Regression line 50 Hz mit  /2R w/6 = 3,8  N B

1 kHz OA /50 Hz OA /1 kHz

Failure 50 Hz Failure 1 kHz Run Out

-

31

0,9

400

2/3

350

-

Failure 50 Hz Failure 1kHz Run Out

-0,1

1 kHz OA /50 Hz OA /1 kHz

mit R 50%

23

236 V 0 =  r 2

0  l 0 in (mm³)

300

w/6/50Hz

300

0,8

mit R 50%

339 V 0 =  r 2

0  l 0 in (mm³)

641 R 50% 561 R 50%

w/6/50Hz in (MPa) w/6/1kHz in (MPa)

w/6/1kHz DoK /N 1 kHz VoA 1 kHz VoA/ODA 1 kHz

456 R 50% 408 R 50%

w/6/50Hz in (MPa) w/6/1kHz in (MPa)

mit R 50%

1 250

1 200

w/6/50Hz

mit R 50%

0/5 Run Out /Test

0,7

0,5

w/6/1kHz

10 -1 10 3 0,25

10 5

10 7

10 9

10 11

10 -1 0,25

10 1

10 3

10 5 N f

10 7

10 9

10 11

10 -1 10 3 0,25

10 5

10 7

10 9

10 11

10 0

10 3

10 6

10 9

10 11

0,25

N f

N f

N f

Fig. 2. S-N diagrams for the heat-treatment condition 570 (left) and 90 (right) with fracture probability lines P f = 10 %, 50 % and 90 and normalized S-N diagrams for the respective heat-treatment condition. In case of high tempered condition (570) no fatigue resistance drop can be observed between 3∙10 6 and 10 9 cycles, but a reduced fatigue limit for a testing frequency of 1 kHz. On the other hand shows the low tempered condition (90) a fatigue resistance drop of 36 %/decade in the range of 10 6 and 10 9 cycles. Based on the examination of fracture surface of every failed specimen by scanning electron microscope (SEM) the fatigue failure is classified into surface (open symbols) and interior inclusion-induced fracture with fish-eye formation (solid and half solid symbols). The crack initiation side is divided in the following groups: surface, surface defect (in contact with the surface or in a depth t I < 100µm), Volume with and without ODA formation (t > 100µm). As it is presented in the normalized S-N diagrams the ODA-formation determine the lifetime in the VHCF-regime (N > 10 7 ). The normalized S-N diagrams for both heat treatment conditions are fitted to the fatigue data for surface-induced failure by the standard linear regression method is shown by the black (50 Hz) and red (1 kHz) line. Based on the results of the 10 % and 90 % fracture probability lines, the scatter ranges T σ = σ 90% /σ 10% and T N = N 90% /N 10% are also labeled. It can be seen in Fig. 3, that the scatter range of fatigue limit for surface induced failure and for number of cycles to failure increases with decreasing tempering temperature in the HCF-regime typically. It seems that the heat treatment condition 90 has a fatigue limit for inner crack initiation at 10 9 cycles, but a further drop in fatigue strength at higher cycles could not excluded. Generally the high tempering conditions (570 , 450) show no difference between the fatigue strength at 10 6 and 10 9 cycles and a linear relationship involving tensile strength (or hardness) and the respective fatigue limit, as shown in Fig. 3. As expected the fatigue limit for surface induced failure increases with decreasing tempering temperature (Fig. 3). The relation between tensile strength and fatigue strength for surface initiated failure (R w/O ) could be estimated by a modified quadratic equation (solid blue (1 kHz) thin and black (50 Hz) thin line, Fig. 3), according to (Pan 2014): R w/O = F O ∙ F m ∙ R w0 with R w0 = (0.67 - 31.5 ∙ R m /E) ∙ R m and E = 210318 MPa, R m is the respective tensile strength in MPa. Based on (fkm 2003), the influence of surface roughness and residual stresses on the fatigue strength can be taken into account by the two parameters F O (R Z ) and F m (σ RS ). However, it must be said that the parameter F m overestimates the influence of surface residual stresses caused by the heat treatment. In our case it is sufficient to take the surface roughness for an accuracy of less than 5 % into account. A reason could be the low depth of the acting residual stresses. Also can be seen in Fig. 3 (left), the testing frequency has a non-negligible influence on the fatigue strength of surface induced fracture. Compared to the 50 Hz – tests, the fatigue strength for 1 kHz is less. It is believed that the reduced fatigue strength is caused by a local self-heating process at micro-notches (roughness or brush-marks) on the specimen surface. It seems, that a macroscopic self-heating of less than 10 K in combination with a roughness of R z = 3.3 µm is critical for surface induced failure. This thermal influence reduces the critical shear-stress for dislocation movements and thus the formation of persistent slip bands. The low tempering conditions (300, 250, 180 and 90) show a marked difference in their surface and volume resistance of up to 32 %. This comparison indicates a different VHCF sensitivity with a critical tensile strength of 1400 MPa of the chosen tempering conditions. It seems that the difference between R w/1E6 and R w/1E9 increases with decreasing tempering temperatures and could be described for the investigated heat treatment condition by a linear relationship R w/1E9 = (1-Z * ) · R w/O . The respective fatigue