PSI - Issue 2_A

Jürgen Bär / Procedia Structural Integrity 2 (2016) 2105–2112 Author name / Structural Integrity Procedia 00 (2016) 000–000

2109

5

0.58

0.08

a

b

0.56

0.06

0.54

0.04

0.52

E-Amplitude [°C]

D-Amplitude [°C]

0.02

0.50

0.48

0.00

0

50000

100000

150000

200000

250000

0

50000

100000

150000

200000

250000

cycle number

cycle number

1.8

1.2

44

c

d

1.6

42

1.0

1.4

40

1.2

38

0.8

1.0

36

0.8

0.6

34

0.6 Heat-Flow [W]

Potential [mV]

32

0.4

0.4

specimen temperature [°C]

30

0.2

0.0

0.2

28

0

50000

100000

150000

200000

250000

0

50000

100000

150000

200000

250000

cycle number

cycle number

Fig. 5. Results of thermographic measurements (a-c) and heat flow measurement (d) of an experiment with a constant force of F=8 kN.

3.2. Experiments with constant stress intensity The experiments were started with a constant force of 8 to 8.5 kN. When the stress intensity reached the desired value the force was reduced with increasing crack length so that the maximum stress intensity K max as well as the stress intensity factor  K was kept constant. The curve progression of the stress intensity is given in figure 6(a). The experiments were undertaken with stress intensities of 12.5, 15, 17.5, 20 and 22.5 MPa  m, respectively. The corresponding crack propagation rates are shown in figure 6(b). For stress intensities up to 17.5 MPa  m the crack propagation rate is constant within the scatter over the complete crack length. For the experiments with stress intensities of 20 and 22.5 MPa  m the crack propagation rates decreases with the crack length.

10 -6

24

a

b

K max = 12.5 MPa  m K max = 15 MPa  m K max = 17.5 MPa  m K max = 20 MPa  m K max = 22.5 MPa  m

22

20

K max [MPa  m]

10 -7

18

16

14

crack propagation rate [m/cycle]

10 -8

12

2

3

4

5

6

7

8

2

3

4

5

6

7

8

crack length a [mm]

crack length a [mm]

Fig. 6. Measured stress intensities (a) and crack propagation rates (b) for the experiments with constant stress intensity.