PSI - Issue 28

F.W. Panella et al. / Procedia Structural Integrity 28 (2020) 1709–1718 Author name / Structural Integrity Procedia 00 (2019) 000–000

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In previous work, thermal coefficient trends of selected ROIs are evaluated during fatigue life of CFRP samples. However, new thermal processing codes are employed on a selected region of interest for a quantitative comparison of damage evolution between different coefficient maps and among different tested specimen during CFRP fatigue life. Obviously, new approach shows higher processing time, therefore the need to remove external environment pixels. By Eq. (1), five coefficient maps could be established to provide different and complementary information about material damage state, if compared with respective maps at different fatigue life state. Thermal coefficient and phase maps of beam sample seem provide unsatisfactory results on fatigue behavior, as shown in example Figures 8b and 9c, respectively. From thermoelastic T the and dissipative A coefficients, delamination mechanism seems to occur at 65% of fatigue life for P11 sample, as shown in example dissipative maps of Figures 9a and 9b. Delamination initiation are also observed during fatigue life of wrinkle samples comparing TSA coefficient maps at different fatigue life state. Despite a polystyrene insulation cover introduced in the setup, the oil pipes of test machine produce a thermal gradient with higher temperature localized in the right zone of samples, as shown in Fig. 8a and 10a. Thermoelastic and dissipative maps seem to provide the position of the failure section located in the middle section, due to the hole presence; different coefficient maps are compared between two fatigue life state, as illustrated in Figures 10b and 10c for T the coefficient and in Figures 10c and 10d for A coefficient. a b c

Fig. 9. (a) Amplitude maps of 2 nd harmonic for 0% vs (b) 65% fatigue life and (c) 2 nd phase map for 65% fatigue life of B11 sample.

a

d

b

c

e

Fig. 10. (a) Thermal coefficient T 0 map for 65% fatigue life, (b) 1

st amplitude maps of 0% and (c) 68% fatigue life, (d) 2 nd amplitude maps of 0%

and (e) 65% life of wrinkle W1 sample.

3.3. Ultrasonic results Phased Array technique allows overcoming difficult limitation due to in-situ measurements of pulse-echo UT setup employed in fatigue tests. However, similar CFRP samples allows to employ the Distance Amplitude Correction (DAC) curves obtained through artificial flat bottom hole defects in previous work (Dattoma et al. (2020)) to evaluate damage size in a more objective manner the identified defects are to be detected with a ‘reference level of sensitivity’