Innovative ultrasonic tests for the Very-High-Cycle Fatigue characterization of a carbon fiber woven fabric

  • Boursier Niutta, Carlo (Politecnico di Torino)
  • Tridello, Andrea (Politecnico di Torino)
  • Paolino, Davide Salvatore (Politecnico di Torino)

In session: - Fatigue

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In this work, the Very-High-Cycle Fatigue (VHCF) response of carbon fiber woven fabric laminate has been investigated through ultrasonic tests. To the authors’ best knowledge, this is the first work addressing the VHCF response of woven laminates through ultrasonic test and with symmetric tensile-compressive stress amplitudes. The work aims at i) proving the feasibility of ultrasonic tests for the VHCF characterization of composites; ii) investigating the VHCF response at zero mean stress of woven composites. The testing setup has involved an ultrasonic wave generator at 20 kHz, a piezoelectric transducer, and a horn for displacement amplification. The specimen has been attached to the horn and the displacement at the free end has been monitored, thus realizing a closed loop control. An infrared sensor has controlled the temperature on the specimen surface and a cooling air system has been adopted for fast refrigeration. As high loading frequency induces consistent self-heating in composites, which can be in turn detrimental for the mechanical properties, the internal temperature has been also investigated through an optic fiber embedded in the material. A hourglass-shaped specimen has been considered for the tests, designed to be in resonance at 20 kHz and to maximize the stress-to-displacement ratio. The dynamic modulus of the woven composite has been assessed through the Impulse Excitation Technique on bar specimens. Calibration of the input voltage - output strain relationship has been performed with strain gauges. Furthermore, the absence of bending modes has been verified through two strain gauges mounted on the two specimen surfaces. With failures even above 10^8 cycles, results have shown a significant interaction of the normal and shear stresses on the fatigue response of the material, with compression governing the final failure. Furthermore, investigation with the optic fiber has shown that the internal temperature is only slightly higher than that measured on the specimen surface thus guaranteeing the reliability of temperature control on specimen surface.