Issue34

J. Bär et alii, Frattura ed Integrità Strutturale, 34 (2015) 456-465; DOI: 10.3221/IGF-ESIS.34.51

0 10,000 20,000 30,000 40,000 50,000 60,000 70,000 80,000 90,000 100,000 110,000

0.30

120,000

0.5

K

= 22,5 MPa  m E-Amplitude D-Amplitude E+D

K

= 20 MPa  m E-Amplitude D-Amplitude E+D

max

max

0.25

100,000

0.4

0.20

80,000

0.3

0.15

60,000

0.2

0.10

40,000

Heat Flow [W]

Heat Flow [W]

0.1

0.05

20,000 Summarized Amplitude [°C]

Summarized Amplitude [°C]

0.00

0

0.0

1

2

3

4

5

6

7

8

1

2

3

4

5

6

7

8

crack length [mm]

crack length [mm]

Figure 11 : Experiments with constant stress intensity of 20 and 22.5 MPa  m.

In case of the thermography measurements, also a decrease of the summarized E-amplitude is visible. The slope of the decrease is considerable smaller compared than in case of the heat flow measurement. In most cases, a nearly linear decrease can be observed. In contrast to the E-Amplitude the D-Amplitude increases with the crack length. The value as well as the increase is considerable smaller, therefore the Summation of E- and D-amplitude remains decreasing with increasing crack length. A quantitative examination of the thermography measurement shows that the specimen loaded with K max =15MPa  m shows the highest summarized E-amplitude values at the beginning of the phase with constant stress intensity. The amount of decrease is also higher in this experiment. In all other experiments, the summarized E-amplitude shows nearly the same values at the beginning of phase 2 and nearly the same decrease with increasing crack length.

0,45

K K K K

= 15 MPa  m = 17,5 MPa  m = 20 MPa  m = 22,5 MPa  m

max

0,40

max

0,35

max

0,30

max

0,25

0,20

0,15

Heat Flow [W]

0,10

0,05

0,00

2

3

4

5

6

7

8

crack length [mm]

Figure 12 : Heat flow experiments at different stress intensity levels.

In contrast to the thermography investigations, the heat flow measurements show a clear dependence of the applied stress intensity (Fig. 12). The measured heat flow at a given crack length is rising with the stress intensity. In addition, the

463