Issue34

J. Bär et alii, Frattura ed Integrità Strutturale, 34 (2015) 456-465; DOI: 10.3221/IGF-ESIS.34.51 463 1 2 3 4 5 6 7 8 0 10,000 20,000 30,000 40,000 50,000 60,000 70,000 80,000 90,000 100,000 110,000 K max = 20 MPa  m E-Amplitude D-Amplitude E+D Summarized Amplitude [°C] crack length [mm] 0.00 0.05 0.10 0.15 0.20 0.25 0.30 Heat Flow [W] 1 2 3 4 5 6 7 8 0 20,000 40,000 60,000 80,000 100,000 120,000 K max = 22,5 MPa  m E-Amplitude D-Amplitude E+D Summarized Amplitude [°C] crack length [mm] 0.0 0.1 0.2 0.3 0.4 0.5 Heat Flow [W] 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. 2 3 4 5 6 7 8 0,00 0,05 0,10 0,15 0,20 0,25 0,30 0,35 0,40 0,45 Heat Flow [W] crack length [mm] K max = 15 MPa  m K max = 17,5 MPa  m K max = 20 MPa  m K max = 22,5 MPa  m 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