Bond behaviour and debonding failures in CFRP-strengthened steel members

Strengthening of steel structures with adhesively bonded carbon fibre reinforced polymer (CFRP) plates (or laminates) has attracted much recent research attention. 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.;A systematic experimental study is first presented 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 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 investigated through a series of tests using this test setup. Based on the experimental observations and results, bond-slip models were developed for CFRP-to-steel interfaces with a linear adhesive and those with a nonlinear adhesive respectively. Analytical solutions were also developed for predicting the full-range bond behaviour and the bond strength of CFRP-to-steel bonded joints.;By employing the bond-slip model for linear adhesives developed in the present study and a mixed-mode cohesive law which considers the interaction between mode I loading and mode II loading, a finite element (FE) model was developed to simulate debonding failures in steel beams flexurally-strengthened with CFRP. Predictions from the FE model are shown to compare well with existing test results. The capability of FE analysis was also extended to the prediction of debonding failures in the more complex problem of CFRP-strengthened rectangular steel tubes subjected to an end bearing load. A FE model was developed and the predictions were compared with results from a series of tests conducted in the present study. The FE model is shown to closely predict the experimental results.