Improvement of Damping in Carbon/Epoxy Beams with Externally Bonded, Aligned Carbon Nanotubes

Abstract

Adding multiwalled carbon nanotubes (CNTs) to polymer composites such carbon fiber reinforced epoxies (c/ep) has the potential to add beneficial damping in vibration prone structures, such as rotorcraft blades, because of slip at the CNT/epoxy interface and between the multiple walls of the CNTs themselves. Recent research has demonstrated remarkable damping improvement by incorporating CNT-rich interlayers into c/ep laminates, and there is current interest in flight testing this damping approach in rotorcraft blades. In order to avoid the time and cost of developing and flight-certifying an entirely new blade material with CNT interlayers, it has been proposed to bond a CNT-rich patch onto the surface of an existing blade. Thus, this investigation compares and analyzes damping in three types of cantilevered [±45/0]s c/ep beams: without any CNTs; with CNT interlayers; and with CNT layers co-bonded to the surface. The CNT layers were used in 64 g/m2 pairs, covering 20% of the length of the beam near the clamped end, and comprising about 1 vol% of the cantilevered portion of the beam. Damping ratio, , was measured during free vibration in a vacuum chamber. The damping ratio increased from 0.65% in a plain c/ep beam to 0.93 (+43%) and 1.24% (+91%) by adding CNT interlayers and externally bonded CNTs, respectively. Dynamic finite element (FE) analysis of the beams confirmed these results and was further used to parametrically investigate the damping attainable with CNT surface films covering higher lengths of the beam or stacked to higher thicknesses at the root of the beam. The parametric investigation showed that increasing the length of the CNT surface film is the most efficient use of CNTs and that a damping ratio 277% greater than that of the beam without CNTs could be realized. In conclusion, externally bonded CNT layers offer a promising avenue for the rapid, cost-effective flight testing of CNT-augmented damping in composite rotorcraft blades.