Hygrothermal Durability of Adhesively Bonded FRP/steel Joints

Abstract: Fibre reinforced polymer (FRP) composites offer superior mechanical properties, such as specific strength and stiffness, corrosion resistance and light weight. Over the past four dec-ades, FRPs have been increasingly used for strengthening and repair of bridge structures, and more recently, in the manufacture of whole/hybrid FRP bridges. In such applications, adhesive bonding is usually the preferred joining technique. Although, the short-term behav-iour of FRP/steel bonded joints has been extensively studied, the subject of the durability has not been researched to the same degree. In addition, existing numerical and experi-mental approaches for the characterization of the durability of bonded FRP/steel joints have mainly been developed with reference to applications in the aerospace industry, where me-chanical and environmental loads differ considerably compared with those in civil engineering applications. Today, uncertainties regarding the durability aspects of adhesively bonded FRP/steel joints used in bridges present a major obstacle to their growing application. There is, therefore, a clear need for research work which focuses on the durability and long-term performance of bonded FRP/steel joints used in bridges. The lack of knowledge regarding the long-term performance is currently compensated by applying a multiple of large safety factors to the strength of composite materials, which dra-matically increases the material usage and reduces the design efficiency. The aim of this re-search work is to shed some light on the durability and long-term performance of adhesively bonded FRP/steel joints with emphasis on effect of hygrothermal ageing on mechanical be-haviour of such joints. To realize this aim, experimental, numerical and analytical approach-es are used. The work includes long-term experiments on double lap shear joints with differ-ent adherends subjected to various temperature ranges, humidity levels, and cyclic exposure scenarios. Complementary material characterization tests are also conducted to study the moisture diffusion kinetics, and moisture dependent mechanical and fracture properties. The results are used as input in FE simulations to predict the moisture induced stress redistribu-tion in the joints and the behaviour of the joints up to failure. The test results of hygrothermally aged joints indicate a clear change of failure mode from cohesive to interfacial or interlaminar failure. Even though all the joints exhibited increasing load-bearing capacity during the first six months, the failure load of specimens aged at high-er temperatures started to degrade afterwards. Cohesive zone modelling approach was capa-ble of accurately predicting the behaviour of the studied FRP/steel joints with different fail-ure modes. The Fickian diffusion was found to yield accurate predictions for all tested mate-rials. Moreover, the 3D moisture diffusion in orthotropic FRP composites was characterized and successfully modelled using FE mass diffusion analysis. The permeability of adherends was found to significantly affect moisture ingression into the joints. The joint level experi-ments in combination with the coupled mass diffusion-mechanical analysis indicated the ex-istence of a critical period after which the strength degradation is likely to occur.