PSI - Issue 18

R. De Finis et al. / Procedia Structural Integrity 18 (2019) 781–791 Author name / Structural Integrity Procedia 00 (2019) 000–000

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represents the advantage of producing isotropic structures capable to carry load in any direction. Each tape is pressed to the mould by a roller for proper compaction which is installed on robotic arm (Fig. 1). The tested material was CFRP panel (fifteen plies of an epoxy-type resin reinforced by carbon fibers with a stacking sequence of [0/-45/45/90/90/45/-45/0]2), from which specimens were extracted. The global panel was 560 mm (weight) and 695 mm (length), samples extracted was designed according to the Standard ASTM D3039. The dimension of specimens were: 25 mm width, 250 mm length and thickness of 3.5 mm. All the specimens were tested on a INSTROM 8850 (250 kN capacity) servo-hydraulic machine. Nine samples were tested with conventional constant amplitude procedure, in load control, in order to obtain the S-N curve conducted at stress ratio of 0.1 and a loading frequency of 7 Hz, runout 2*10^6 cycles. The imposed loading level of SN curve were compared to ultimate tensile strength (UTS) of the material which is 827 MPa, previously evaluated. The imposed stress levels were six: 50 UTS% (maximum stress: 413.5 MPa), 60% UTS (maximum stress: 496 MPa), 65%UTS (maximum stress: 537.5 MPa), 70%UTS (maximum stress: 579 MPa), 75% UTS (maximum stress: 620 MPa), 80%UTS (maximum stress: 661 MPa).

(a)

(b) Fig. 2. Obtained SN curve (a), Layout of loading frame and infrared camera (b).

Fig. 2 reports the experimental SN curve for material (a) and the set-up (b). The equipment involves also an extensometer with the purpose of acquiring strain and stress cycles per cycles. This leads to calculating the global secant Young’s modulus for specimens. The evaluation of Young modulus was to provide a metric with which to compare the thermoelastic signal data by fixing the residual stiffness of the specimens after cycles. The SN curve reported in Fig. 2a shows as the endurance limit is present in correspondence of 50%UTS at 412.5 MPa in terms of maximum stress. All the data lies in the 95% confidence bounds. In Fig. 2a, is represented the loading