Bond behaviour and debonding failures in CFRP-strengthened steel members

Abstract

Strengthening of steel structures with adhesively bonded carbon fibre reinforced polymer (CFRP) plates (or laminates) has attracted much recent research attention. In the strengthening of steel structures, CFRP is preferred to glass FRP (GFRP) due to the much higher elastic modulus of the former. Existing studies have revealed that debonding of the CFRP plate from the steel substrate is one of the main failure modes in CFRP-strengthened steel structures. This thesis presents a series of experimental and theoretical studies aimed at the development of a good understanding of the mechanisms of and reliable theoretical models for debonding failures in CFRP-strengthened steel structures. Debonding failures between steel and CFRP may occur in the following modes: (a) within the adhesive (cohesion failure); (b) at the bi-material interfaces between the adhesive and the adherends (adhesion failure); (c) a combination of adhesion failure and cohesion failure. Among these failure modes, cohesion failure in the adhesive is the preferred mode of debonding failure at CFRP-to-steel interfaces as for such debonding failure, the design theory can be established based on the properties of the adhesive. A systematic experimental study is first presented in this thesis to examine the effects of steel surface preparation and adhesive properties on the adhesion strength between steel and adhesive. The test results show that the adhesive bonding capability of a steel surface can be characterised using three key surface parameters and that adhesion failure can be avoided if the steel surface is grit-blasted prior to bonding. Following an experimental study with sophisticated instrumentation which confirmed the suitability of the single-shear pull-off test method for studying the behaviour of CFRP-to-steel interfaces subjected to pure shear loading, the full-range behaviour of CFRP-to-steel interfaces was then investigated through a series of tests using this test setup. The focus of these tests was on the interfacial behaviour when governed by cohesion failure. The parameters examined include the material properties and the thickness of the adhesive layer and the axial rigidity of the CFRP plate. The test results demonstrated that the bond strength of such bonded joints depends significantly on the interfacial fracture energy among other factors. Non-linear adhesives with a lower elastic modulus but a larger strain capacity are shown to lead to a much higher interfacial fracture energy than linear adhesives with a similar or even a higher tensile strength. The bond-slip curve is shown to have an approximately triangular shape for a linear adhesive but to have a trapezoidal shape for a non-linear adhesive. Based on the experimental observations and results from single-shear pull-off tests, bond-slip models were developed for CFRP-to-steel interfaces with a linear adhesive and those with a non-linear adhesive respectively. The bond-slip models include an explicit formula which predicts the interfacial fracture energy from the properties of the adhesive. Analytical solutions were also developed for predicting the full-range bond behaviour and the bond strength of CFRP-to-steel bonded joints, with the definition of the effective bond length addressed as an important issue. By making use of the bond-slip model for linear adhesives developed in the present work, finite element modelling was conducted to simulate debonding failures in steel beams flexurally-strengthened with CFRP. In the FE model, the bond-slip model was employed with a mixed-mode cohesive law which considers the effect of interaction between mode I loading and mode II loading on damage propagation within the adhesive. Predictions from the FE model are shown to compare well with existing test results. The proposed FE model represents a significant advancement in the modelling of debonding failures in CFRP-strengthened steel structures. The last part of the thesis extends the capability of FE analysis to the prediction of debonding failures in the more complex problem of rectangular steel tubes with CFRP plates bonded on the webs subjected to an end bearing load. A series of tests is first presented, in which the effects of adhesive types and web slenderness on the effectiveness of CFRP strengthening were examined. The failure of such members is normally controlled by the debonding of the CFRP plates, so the properties of the adhesive used are shown to be very important. The test results show that an adhesive with a larger strain energy (e.g. a softer nonlinear adhesive) leads to a larger load-carrying capacity for a strengthened rectangular hollow section (RHS) steel tubes. A FE model is next presented, in which the effect of interaction between mode I loading and mode II loading on damage propagation within the adhesive is duly considered. This FE model is shown to closely predict the experimental behaviour of these CFRP-strengthened tubes.