Issue34

J. Bär et alii, Frattura ed Integrità Strutturale, 34 (2015) 456-465; DOI: 10.3221/IGF-ESIS.34.51 464 amount of decrease of the heat flow with rising crack length is also increasing with the applied stress intensity. To confirm the results some more experiments have to be undertaken. D ISCUSSION he experiments have shown that both methods, the lock in thermography as well as the peltier element based heat flow measurement, provide interesting information about the crack propagation behavior. The lock in thermography delivers detailed space resolved information about the temperature changes due to cyclic loading. With this information, elastic stresses (E-mode) as well as dissipated energies (D-mode) can be determined qualitatively. To enhance the surface emissivity metallic specimen have to be coated with black paint. The kind of the coating as well as the thickness are influencing the results [12]. In case of thin coatings the emissivity is a combination between the specimen surface and the coating, in case of thick coating the measured response is that of the coating. Therefore, the coating of the specimen has to be done with great accurateness and the thickness has to be the same to get comparable results. In this study, a graphite spray was used as coating and was sprayed in the same manner on all specimens, but the thickness was not measured and may vary from specimen to specimen. Therefore, the results obtained on different specimen are not comparable. With the peltier sensor it is easy to gather quantitative results during the fatigue experiments. The sensor is pressed on the specimen surface with a constant force. The thermo-paste between the sensor and the specimen enhances the heat flow and minimizes sliding effects. Consequently, it is possible to measure quantitatively but the integral measurement is limited to the dimensions of the peltier element. Small differences between the specimens can be explained by different temperatures in the chamber during the experiment. In further experiments the temperature at the backside of the measuring peltier element will be kept constant. The experiments with constant stress intensity showed considerable differences in the run of the thermography and the heat flow measurements (Fig. 10 and 11). Except the experiment with a stress intensity of 15 MPa  m the thermography measurements show a nearly linear decrease of the summarized amplitude with the crack length. The decrease in the heat flow measurement is clearly nonlinear. To visualize the correlation between thermography and heat flow measurement in Fig. 13 the heat flow is plotted against the summarized E- and D-Amplitude for the experiments with a constant stress intensity of K max =17.5; 20 and 22.5 MPa  m. 50,000 60,000 70,000 80,000 90,000 100,000 110,000 0.00 0.05 0.10 0.15 0.20 0.25 0.30 K max = 17,5 MPa  m K max = 20 MPa  m K max = 22,5 MPa  m Heat Flow [W] Summarized E- and D-Amplitude [°C] Figure 13 : Heat flow experiments at different stress intensity levels. T