R.J. Cano, J.H. Kang, B.W. Grimsley, J.G. Ratcliffe, E.J. Siochi
NASA Langley Research Center,
Keywords: hybrid composites, carbon nanotubes, multi-functionality
Summary:Carbon nanotubes (CNTs) have been studied extensively since their discovery and demonstrated at the nanoscale to have superior mechanical, electrical and thermal properties in comparison to micro and macro scale properties of conventional engineering materials. This combination of properties [1-6] suggests CNTs potential to enhance multi-functionality of composites in regions of primary structures on aerospace vehicles where lightweight materials with improved thermal and electrical conductivity are desirable. For example, the low electrical conductivity of carbon fiber reinforced polymer (CFRP) compared to aluminum alloy requires the application of a conductive layer of copper mesh over the CFRP structure to disperse up to 200 kA of current that can be delivered during a lightning-strike. The current state-of-the-art for lightning strike mitigation can benefit from the development of a multifunctional CFRP laminate with improved electrical and thermal conductivity to dissipate the energy during a lightning strike with a reduced weight penalty. In this study, hybrid multifunctional polymer matrix composites were fabricated by interleaving layers of CNT sheets from Nanocomp Technologies, Inc. into Hexcel® IM7/8552 prepreg, a well-characterized toughened epoxy CFRP composite. The resin content of these interleaved CNT sheets, ply-stacking location, pretreatment of the CNT, CNT alignment, CNT growth mechanism and CNT density were varied to determine the effects on the electrical, thermal, and mechanical performance of the composites. The direct-current electrical conductivity of the hybrid CNT composites was characterized by in-line and Montgomery four-probe methods. For initial 20 laminates containing a single layer of CNT sheet between each ply of IM7/8552, in-plane electrical conductivity and in-plane thermal conductivity of the hybrid laminate increased significantly in comparison to the control IM7/8552 laminates. The thermal conductivity of the control and hybrid CNT laminates was measured according to ASTM E1461 in both the in-plane and through-the-thickness directions. Photo-microscopy and short beam shear (SBS) strength tests following ASTM D2344 were used to characterize the consolidation quality of the fabricated laminates. Hybrid panels fabricated without any pretreatment of the CNT sheets resulted in a short beam shear (SBS) strength reduction of 70%. Aligning the tubes and pre-infusing the CNT sheets with resin significantly improved the SBS strength of the hybrid composite. To determine the effect of the CNT layers on performance, and assess the effects of various processing parameters, Mode I and Mode II fracture toughness of the CNT sheet to CFRP interface were characterized by double cantilever beam (DCB), ASTM D5528, and end notch flexure (ENF) testing, ASTM D7905, respectively. Initial DCB testing of hybrid laminates resulted in intralaminar failure in the CNT layer, with Gic values being half of that found for the CFRP control laminate, likely due to poor CNT adhesion to the polymer matrix. Initial ENF testing of hybrid laminates resulted in a doubling of the Giic value in comparison to the CFRP control laminate. Results of this effort are compared to the control IM7/8552 laminate as well as to the previous findings.