PSI - Issue 2_B

Manuela Sander et al. / Procedia Structural Integrity 2 (2016) 034–041 M. Sander et al./ Structural Integrity Procedia 00 (2016) 000–000

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large scatter band (Fig. 3a). Fig. 3b shows the results of the modified S-N curves using the approach by Murakami (2002) with

α

1/ 6 ( 120) 1,56 (HV   area )

2      1

R

  

(1)





th

for internal defects and

α

1/ 6 ( 120) 1,43 (HV   area )

2      1

R

  

(2)





th

for surface defects (SI) in order to account for the different defect sizes in terms of √ area . The exponent α has been calculated from the fatigue strength experiments with 0.5472. It becomes obvious that the scatter is reduced and the S-N curves of all R -ratios fall within one scatter band.

R = -1

R = -0.8

R = 0

R = 0.5

200 300 400 500 600 700 1.0E+05 1.0E+06 1.0E+07 1.0E+08 1.0E+09 1.0E+10 σ a [MPa] N f (log) R = -1 R = -0.8 R = 0 R = 0.5 R = -1 (SI) R = -0.8 (SI) R = 0 (SI) R = 0.5 (SI) a)

R = -1 (SI)

R = -0.8 (SI) R = 0 (SI)

R = 0.5 (SI)

1.0 1.2 1.4 1.6 1.8 2.0 2.2 1.0E+05 1.0E+06 1.0E+07 1.0E+08 1.0E+09 1.0E+10 σ a / σ th N f (log) b)

Fig. 3. (a) S-N curve and (b) modified S-N curve for different R -ratios (Müller (2016)).

In order to investigate the influence of variable amplitude loadings and especially the loads beneath the fatigue strength on the lifetime, systematic experiments with repeated two-step loadings have been performed. Therefore, the load levels of the high block ( R HL = ͞ σ ai / σ D,VHCF ) as well as the cycle number n 2 of the low block have been varied, whereby the load level of the low block was 90% of the fatigue strength determined for a limit of 10 9 cycles (Fig. 4a). The cycle number n 1 of the high block is constant 10,000 cycles. The sequence of these two blocks has been repeated until the specimen fails or the limit of 10 9 cycles has been reached.

0.0 0.2 0.4 0.6 0.8 1.0 1.2 0.0E+00 2.0E+05 4.0E+05

0.0 0.2 0.4 0.6 0.8 1.0 1.2

a)

b)

c)

a1 σ a2 σ a0 σ a σ

1.1· σ D 1.2· σ D

σ a /  σ a

σ a /  σ a

0.9· σ D

H

n 1 = 10.000

n 2

0.0E+00

2.0E+06

H

H

Fig. 4. Investigated variable amplitude sequences (a) 2-step block loading; (b) Felix ( R = -1) and (c) WISPER ( R = 0).