Characterization of GFRP interlayer as barrier layer in Al/CFRP laminate
Barbara Surowska, Monika Ostapiuk
Quarterly No. 4, 2017 pages 232-237
DOI:
keywords: CFRP, FML hybrid laminates, a GFRP interlayer, EIS, interface microstructure
abstract Carbon fibre reinforced polymers (CFRPs) are an attractive construction material with an increasingly wide scope of application, including the aircraft industry. By combining them with metal elements and producing fibre metal laminates (FMLs), it is possible to achieve higher mechanical properties than in the case of combinations with glass fibre reinforced polymer (GFRP). However, there is a problem associated with galvanic corrosion regarding combinations with aluminium and its alloys, stainless steel and with magnesium alloys because CFRP composites are electrical conductors. Adhesives with increasingly higher resistivity are applied in adhesive bonding technology. Fibre metal laminates (FMLs), particularly those dedicated for aircraft primary structures must be not only corrosion resistant, but first of all they must be characterized by a proper combination of mechanical properties, including fatigue features. Therefore, when designing the metal surface treatment and the type of interlayers, it is necessary to consider the joint adhesion, mechanical properties of the hybrid laminate and corrosion properties. This article presents the characterization of an interface microstructure: the anodic layer on the AA 2024 aluminium alloy-GFRP-CFRP interlayer of hybrid laminates with electrical properties presented in a pre¬vious publication. The observations have been carried out on cross-sections of Al/GFRP-R/CFRP, Al/GFRP-S/CFRP and Al/CFRP laminates in a 2/1 layout with fibres oriented in the 0° direction. Moreover, impedance measurement was performed for the oxide layer in contact with a 3.5% aqueous NaCl solution by means of electrochemical impedance spectroscopy (EIS). It has been found that the low contact resistivity between the laminate with the GFRP-S interlayer was caused by carbon fibre migration to the Al/GFRP-S boundary. Furthermore, the low surface resistance of the CFRP composite and the porosity of the outer part of the oxide layer on aluminium enables the diffusion of aggressive ions and migration of electrical charge towards the metal substrate, which poses a threat of corrosion initiation in moisture condensation conditions.