Issue34

J. Bär et alii, Frattura ed Integrità Strutturale, 34 (2015) 456-465; DOI: 10.3221/IGF-ESIS.34.51 462 In contrast to the E-amplitude the last recorded D-amplitude image shows a symmetric plastic zone in front of the crack tip (Fig. 9). An explanation for this difference may be found in the phase images. In case of the E-Mode an extreme inhomogeneous phase shift can be observed whereas in case of the D-Mode the phase shift don’t show such great deviations. Figure 9 : Second to last (left) and last D-Amplitude image received at a constant force of 8.8kN referring to Fig. 7. Constant stress intensity The constant stress intensity experiments were performed on loading levels of 15; 17.5; 20 and 22.5 MPa  m. The experiments consist of two phases. At the beginning, the specimen was loaded with a constant force of 8.4 to 8.8 kN until a crack was initiated and reached a length according to the desired stress intensity. Starting from this point the stress intensity K max and  K was kept constant by reducing the force with increasing crack length. In Fig. 10 and 11 the results of the thermography and the heat flow measurements as a function of the crack length are shown. Due to the rising crack length in phase 1 the summarized E-Amplitude as well as the heat flow is rising, too. The maximum of these curves marks the transition from constant force (phase 1) to constant stress intensity (phase 2). This peak appears in both, the thermography and the heat flow measurement, but it’s more pronounced in the heat flow curve. After the peak an steep decrease can be observed in the heat flow curve. The slope decreases with increasing crack length. In the Fig. 11 especially at K max = 22.5 MPa  m steps are visible. These steps are caused by small phases with constant stress due to some small irregularities caused by the control software. 2 4 6 8 0 20,000 40,000 60,000 80,000 100,000 120,000 140,000 160,000 K max = 15 MPa  m E-Amplitude D-Amplitude E+D Summarized Amplitude [°C] crack length [mm] 1 3 5 7 0.00 0.05 0.10 0.15 0.20 Heat Flow [W] 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 = 17,5 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] Figure 10 : Experiments with constant stress intensity of 15 and 17.5 MPa  m.