Issue 48

J. Bär et alii, Frattura ed Integrità Strutturale, 48 (2019) 563-570; DOI: 10.3221/IGF-ESIS.48.54 569 and an image size of 160x120 pixel. In a second step, an elastic-plastic loading was applied on the specimen. The recorded mean temperature of all pixels within the image  T measured is shown in Fig. 7a as a green line. The corresponding thermoelastic temperature change  T elastic was calculated according to Eq. (1) (red dashed line in Fig. 7a) and subtracted from the measured temperature change  T measured (green line in Fig. 7a). As a result, the temperature change due to dissipative effects  T dissipative is obtained (magenta line in Fig. 7a). The run of this temperature change,  T dissipative , is shown in Fig. 7 b in higher resolution. Due to dissipative effects, a remarkable temperature increase is visible starting when the load maximum is reached. In compression, only a slight increase is visible. These results clearly indicates that the temperature change due to dissipative effects cannot be described by a sine wave with double loading frequency. When a complete DFT according to Eq. (3) is used for the dissection of the temperature signal, amplitude values for the higher harmonic modes (D1 and D2) appear. These higher harmonic modes cannot be assigned to physical effects in the material. The entirety of the higher harmonic modes are just a mathematical description of the deviation of the temperature signal from a pure sine wave (see magenta line in Fig. 7b). Figure 6 : E-Amplitude images of (a) short (4.5 mm) and (b) long crack (8 mm). For a qualitative determination of the dissipated energies, the DFT-approach is sufficient. For a quantitative determination of the temperature change due to dissipative effects and therefore the dissipated energy, a summation of the amplitudes of the D, D1 and D2-Modes may provide an improvement, but for a realistic quantitative description, a new evaluation method has to be developed. Therefore, a detailed analysis of the temperature change during cycling in the elastic-plastic regime is necessary. Without this knowledge the development of a method for a quantitative determination of dissipated energies with the thermoelastic stimulated Lock-In-Thermography is impossible. Figure 7 : (a) extracting the temperature change due to dissipative effects from the measured temperature change, (b) run of the temperature change due to dissipative effects. 0,0 0,1 0,2 0,3 0,4 -6000 -4000 -2000 0 2000 4000 6000 0,0 0,1 0,2 0,3 0,4 -6000 -4000 -2000 0 2000 4000 6000 Force [N] Force -1,0 -0,8 -0,6 -0,4 -0,2 0,0 0,2 0,4 0,6 0,8 1,0  T measured  T elastic  T dissipative  T [K] time [s] (a) Force [N] time [s] (b) -0,05 0,00 0,05 0,10 0,15  TDiss [K]