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J. Bär et alii, Frattura ed Integrità Strutturale, 34 (2015) 456-465; DOI: 10.3221/IGF-ESIS.34.51 458 Figure 2 : Specimen chamber with Infrared Camera. The evaluation of the lock-in thermography is illustrated in Fig. 3. The specimen is loaded with an alternating force and the infrared camera, working with a frequency at least twice of loading frequency, records a noisy temperature signal. This temperature signal is filtered by a pixel wise fourier transformation using the frequency of the loading signal. The filtered signal shows the reaction of the specimen due to the mechanical loading. As shown on the right hand side of Fig. 3 two different values can be determined. At first the amplitude value of the filtered temperature signal and second the phase lag between the temperature curve and the mechanical loading. lock in filtered temperature Force magnitude time Temperature Phase Image magnitude time Amplitude Image Figure 3 : Lock In evaluation. The software Altair LI delivers two different modes: the E-mode and the D-mode. The E-mode is based on the thermo elastic effect and can therefore be used for analysis of the elastic stresses as shown by Harwood et al. [9]. The D-mode provides information about the dissipated energy as shown by Brémond [10]. The software calculates the resulting amplitude of temperature variations (amplitude image) as well as the distribution of phase lags between the thermographic signal and the mechanical loading (phase image) for the E-mode and D-mode, respectively. As shown by Sakagami et al. [11] the D-Mode images can be achieved when the evaluation is performed with the double loading frequency. During the crack propagation experiments amplitude and phase images in the E- and D-Mode were received from lock-in evaluations, which were performed automatically in an defined interval of 60 s. To enhance the emissivity of the specimens the surfaces were coated with a thin graphite layer. In Fig. 4 the resulting images of the lock in evaluation are shown. In the E-Amplitude image, the stress-field in front of the crack tip can be seen. In case of plastic deformation, the reversible adiabatic conditions are injured and therefore a phase-lag between mechanical loading and temperature signal occurs. In the E-Phase image the plastic zone can be easily identified. In the amplitude image of the D-mode a higher amplitude due to the plastic deformation is visible. In this