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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tensile tests in presence of wrinkle. 2.2. NDT monitoring methods

Non-destructive monitoring strategy consists on an optimized experimental combination procedure with different methods, as thermographic and ultrasonic techniques for cyclic tests and digital image correlation for static tests. Raw thermal images are acquired with a sample rate of 117 Hz and observation time of 10 seconds, employing a cooled IR camera FLIR X6540sc of full frame 640×512 pixels (Dattoma et al. (2019)). Similar setup was employed for monitored on B-type samples and W-type samples, located at 0.25 m from lamps, as in Fig. 4b, based on previous experimental experience (Dattoma et al. (2019)). Thermoelastic Stress Analysis (TSA) is a full field and non-contact technique, that provides stress maps of loaded part because of thermoelastic effect from cyclic loading (Galietti et al. (2017)); TSA may be used also for damage detection, by means of custom Research-IR software, based on authors suitable Matlab code, employed for raw image post-processing (Dattoma et al. (2019)). Temperature signal in the frequency domain could be described by analytical model equation (Galietti et al. (2015), Galietti et al. (2017)): ( , , ) = 0 + ∗ + the ∗ sin( ∗ + + ) + A ∗ os(2 ∗ ∗ + ) (1) Where “T 0 + b*t” is long lasting temperature evolution for linear fatigue life variations, ω angular frequency of mechanical imposed load, T the and φ respectively related thermo-elastic signal and A represent the amplitude of second Fourier harmonic component, characteristic of harmonic character for each acquisition, claimed to be proportional to intrinsic energy dissipation; ψ is the phase signal of temperature component. Starting from this useful formulation, the authors developed a customized Matlab code based on the Discrete Fast Fourier transformer to determine the thermal coefficients of interest [Dattoma et al. (2019)]. All thermal acquisitions tests are performed in controlled ambient temperature between 18÷22 °C.

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Fig. 4. (a) Fatigue curves’ diagram of CFRP beam samples; (b) thermographic monitoring setup for wrinkle specimen.

For ultrasonic damage monitoring, Phased Array UT control are employed with a transportable Olympus Omni scan MX for PA/UT for damage detection with direct contact UT transmission. In fact, PA ultrasonic technology offers a suitable controlled excitation of individual elements in a multi-element probe, in terms of impulse amplitude and time delay (Dattoma et al. (2020)), to be generally preferred to single-element probes in case of composites, developed for ND control inspection of complex and large surfaces. Inspections with possibility to orient the direction of the sonic wave without moving the Probe are possible with innovative 64 elements Phased Array 2.25L64-A2 probe of thickness 25 mm with 2.25 MHz, equipped with Plexiglas longitudinal wedge, interposing a direct glycerine Coupling gel on CFRP surface, as shown in Fig. 5a and Fig. 5b. In case of bending tests, supports on experimental machine avoid contemporaneous UT inspection during fatigue loading, but the portable PA equipment offers good setup for continuous monitoring of tension-tension fatigue tests, overcoming limits found in bending tests, without