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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Although speckle pattern generally plays a critical role in successful application of DIC, previous experience allowed to produce minimal importance on results, confirming DIC as powerful tool in performing displacement/strain measurements, especially using high res camera and optimal optics. The strain fields are evaluated for both static tests cases, showing a mechanical heterogeneity of these composite laminates since the beginning of load application. By acquired images, critical zones are identified on expected stress concentration spots and strain field are analysed in specific region of interest for bending and tensile static test, as shown respectively in Figures 6b, 7a and 7b. In bending test, strain fields highlight different stressed areas in the upper layers and border extremities of CFRP bar, in agreement with the fatigue failure modes reported in Table 1. As expected, a different the mechanical behaviour is observed for wrinkle samples whose strain fields show a high stress state near the hole’s area, as shown in Figures 7a and 7b. a b

Fig. 7. (a) ε xx [mm/mm] and (b) ε xy [mm/mm] strain plot of static traction test of wrinkle sample.

3.2. Thermographic results

Fatigue test data are recorded with a frame rate of 10 Hz and fatigue results show a significant stiffness reduction with load cycles number, as displayed by normalized stiffness vs normalized fatigue life in Fig. 8a. Before fatigue stiffness decay occurs, all W and B samples show stabilized general trend in the initial phase, followed by a rapid decrease after 70% of fatigue life, because of progressive damage accumulation before final failure. In some cases, the 80% of total fatigue life is ensured before damage start to occur. a b

Fig. 8. (a) CFRP fatigue curves’ diagram; (b) thermal coefficient T 0 map for 65% life of B11 sample.