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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UT experimental data are elaborated with MATLAB codes for damage analysis and results are implemented to check results with strain and compliance data acquired during tests. Authors make use of open source platform Ncorr-2D DIC to obtain displacement and strains on specimen surface for static study, proposed by Blaber and Adair (2015); data elaboration and post-processing of all experimental results are finally discussed. Experimental results indicated fatigue life of CFRP elements obviously decreased with increase of stress levels, but the same cannot be said about damage evolution levels and structural specimen state before final rupture, as well as the amount of strength and stiffness after fatigue testing on specimens apparently undamaged.

Nomenclature F Amp_adim normalized amplitude fatigue load F max_adim normalized maximum fatigue load 2. Materials and methods 2.1. CFRP elements and experimental procedures

K adim N adim

normalized stiffness

normalized fatigue number of cycles

Both static/ fatigue tensile and bending tests are executed on INSTRON 8850 servo-hydraulic testing machine, with maximum load capacity of 250 kN in the Experimental Mechanical Laboratory at the University of Salento (Lecce, Italy). The standard configuration Open Hole is the selected geometry for the study of wrinkles presence on CFRP fatigue behavior, which consists on a flat rectangular specimen [304 × 39 × 6.78 mm in dimension] with a hole in middle section, in this case of nominal diameter of 6.35 mm. The artificial wrinkle was introduced in the reduced section pressing several layers of calibrated thicknesses during cure process (Dattoma et al. (2018)); the measured parameters defining the artificial wrinkles and final configuration are reported in Fig. 2a. However, mechanical tests are done only on specimens defined series 50-33-24-WD20 (lay-up [45, 90, 0, 0, -45, 0, 0, -45, 0, 0, 90, 45] s ) in previous works (Dattoma et al. (2018)). a b

Fig. 2. (a) Example of wrinkle samples with central hole for NDT controls; (b) σ / ε diagram of wrinkle specimen static tests.

Each specimen’s surface is opportunely covered with a black paint for IRT control and the static test ones with a speckle pattern for DIC analysis, as shown in Fig. 2a. Wrinkle samples are tested following experimental experience and indication of ASTM D 5766 and ASTM D 3479, respectively standard test method for open hole tensile strength and fatigue tests are performed in HCF regime at low frequency (Dattoma et al. (2018)). As in Fig. 2b, four preliminary static tests provide a suitable average value of maximum traction breaking load (F static ) and relative displacement (u static ) for wrinkle samples, used as reference value for fatigue tests to evaluate initial stiffness and compare residual fatigue stresses. Four fatigue tests denoted with W type are performed with load frequency 8 Hz and stress ratio 0.05.