PSI - Issue 28

Sahand P. Shamchi et al. / Procedia Structural Integrity 28 (2020) 1664–1672 Sahand Shamchi et al. / Structural Integrity Procedia 00 (2020) 000 – 000

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1. Introduction The progressive substitution of metallic materials with lightweight composite materials in the primary structures of modern aircrafts, due to their high specific strength and stiffness features, creates new challenges. One issue with polymeric composite materials is their weak electrical conductivity, which makes them vulnerable to damage inflicted by a lightning strike. The current lightning strike protection (LSP) solution of modern aircrafts consists of insertion of conductive metallic meshes or perforated foils on the outer surface of the composite panels. However, this solution comes with the expense of adding extra weight to the composite structure and it is often challenging to conform to a complex structural geometry [1], [2]. A possible future candidate for an airframe material is the use of advanced nano based composites, by incorporating conductive nanofillers such as carbon nanotubes (CNTs) and graphene [2]. The objective is to alter the resistivity of the polymer matrix by forming a conductive network by means of nanofillers, thereby safely dissipating the electrical currents. In spite of the numerous studies regarding the polymer-matrix nanocomposites with enhanced electrical conductivity, the technology has not yet matured enough to offer an accountable protection mechanism against a lightning strike [3] – [6]. In addition to the electrical properties, any successful technology also requires sufficient mechanical performance, such as high impact damage tolerance in order to be integrated into the primary load bearing structures of an aircraft. To tackle these issues, a multifunctional composite material has been developed by AVIC Composite Center (ACC) using a method called Functionalized Interlayer Technology (FIT) [7]. The method employs an interleaf material surface-loaded with nano-sized electrically conductive fillers. By interleaving this conductive film into the carbon/epoxy laminate, it simultaneously improves the electrical conductivity and the interlaminar fracture toughness of the composite material [7], [8]. The present work focuses on the strain rate effect of the electrically modified UD carbon/epoxy composite laminate, produced via FIT, under the longitudinal compressive loading using a split Hopkinson pressure bar. The study also covers the non-modified UD carbon/epoxy laminate with the intention of examining the influence of this modification on the compressive behavior of the material. The influence of interleaving conductive film on the interlaminar fracture toughness of the laminated composite will be addressed in a future work. 2. Material and specimen The base material under the investigation , referred to as “reference material”, was a 16-ply unidirectional (UD) laminate of carbon/epoxy prepreg system with T800 continuous reinforcing carbon fibers, purchased from Toray, and 180ºC cure epoxy resin system. The ply thickness was around 0.095 mm. The second material configuration, labelled as “modified material”, was an electrically modified UD carbon/epoxy system using a procedure called as Functionalized Interlayer Technology (FIT). The concept includes a porous non-woven nylon carrier and nano-sized electrically conductive silver nanowires (AgNWs), which are deposited into the interleaf carrier material. The plain nylon veil has a surface density of around 14 g/m 2 and a thickness of 53 μm. S ilver nanowires (AgNWs) with average diameter of 70 nm and the length of 20 – 80 μ m were dispersed in isopropanol with a concentration of 5 mg/mL. The nylon veil was then immersed into this prepared solution to deposit the conductive fillers on the carrier. The procedure was repeated several times to stablish a well-dispersed conductive network of AgNWs, and subsequently, air dried at room temperature. The surface-loaded conductive veil was then interleaved into the carbon/epoxy prepreg layers, forming a laminate with the same lay-up configuration as the reference material. The prepreg was cured in an autoclave. Further information regarding the fabrication of the conductive veil and FIT can be found in [7]. The electrical conductivity of the material was increased from 6.8×10 3 S/m to 2.1×10 4 S/m in the in-plane perpendicular to the fiber orientation, and from 3.1 S/m to 72.3 S/m in the through-thickness direction. Test samples for compression tests were cut into rectangular strips parallel to the fiber orientation with nominal dimension given in table 1.