Disbond arrest in fibre metal laminates

Abstract

In the development of materials used for constructing aircraft, progress has been made in the production, certification and implementation of materials such as Carbon Fibre Reinforced Polymer (CFRP) and Glass Laminate Aluminium Reinforced Epoxy (GLARE). The state-of-the-art in joining techniques however, has not changed much: fastening is still the current used method for high load transfer (HLT) joints. Unfortunately, fastening is associated with high fatigue sensitivity due to the need for holes in the joined materials, causing stress concentrations and a highly concentrated load introduction. Using bonding for HLT joints would alleviate this fatigue sensitivity by continuous load transfer over the bonded area. Currently, certification of these bonded major loadpath joints is only allowed if the design of bond-line secures a limitation of the maximum disbond size originating from manufacturing anomalies or in-service impact events. In order to meet this requirement, disbond arrest features (DAFs) need to be implemented in the bond-line. As a spin-off from the BOPACS (Boltless Assembling of Primary Aerospace Composite Structures) project, where fasteners are used for disbond arrest, the transferability of this design strategy in GLARE is investigated. The fatigue sensitivity of the metallic bonded surface in GLARE in combination with the implemented fasteners inside the bonded area prove to affect the disbond arrest performance to a large extent. A unique combination of fatigue crack growth (FCG), adherend delamination and adhesive disbonding is observed in the tested configurations, detrimental to the DAFs intended use. In the process, a qualitative in-situ disbond monitoring system is developed and verified. Additionally, the root causes of the observed failure mode and the subsequent damage progression are identified. By identification of the one-off damage progression characteristics, new DAF designs are proposed for the certification of bonded HLT joints in GLARE.