Author: Guo, Dong
Title: Effect of temperature variation on the interfacial debonding mechanisms of FRP-strengthened steel beams
Advisors: Dai, Jian-guo (CEE)
Degree: Ph.D.
Year: 2022
Subject: Fibrous composites
Steel, Structural
Composite materials -- Thermomechanical properties
Hong Kong Polytechnic University -- Dissertations
Department: Department of Civil and Environmental Engineering
Pages: xxii, 232 pages : color illustrations
Language: English
Abstract: Fiber-reinforced polymer (FRP)-strengthened steel structures in service are likely to experience significant temperature variations due to seasonal and diurnal service temperature changes. The temperature variations may influence the interfacial debonding mechanisms in FRP-strengthened steel structures and reduce their load-carrying capacities. The temperature variation effects can be attributed to two aspects: (1) the interfacial thermal stress induced by the different thermal expansion coefficients of FRP and the substrate material; (2) the temperature-dependent properties of the bond line. For FRP-strengthened steel beams, the interfacial debonding may occur at the plate end (i.e., plate-end debonding) and the cracked locations at the intermediate part of beams (i.e., IC debonding). The occurrence of interfacial debonding is relevant to the interfacial bond behavior in these areas. In this thesis, the effects of temperature variation on both plate-end and IC debonding failure mechanisms of FRP-strengthened steel beams are investigated through a comprehensive research program comprising of theoretical derivations, experimental studies and finite element analyses. The work in the thesis is distributed to four major parts: the first part presents the theoretical solutions for analyzing the effect of thermal stress on the plate-end and IC debonding failure of FRP-strengthened un-notched and notched steel beams, respectively; the second part is concerned with the experimental study on the local bond behavior of FRP-to-steel bonded joints at different temperatures; the third and fourth parts are concerned with flexural tests of FRP-strengthened un-notched and notched steel beams, at various temperatures, which corresponded to the plate-end debonding and IC debonding, respectively.
In the first part of this thesis, three closed-form analytical solutions were proposed for analyzing the effect of thermal stress on the debonding failure mechanisms of FRP-strengthened steel beams. The occurrence of plate-end debonding of FRP-strengthened steel beams is associated with the high interfacial stress concentration in both tangential (mode II) and normal (mode I) directions. Two different closed-form solutions were derived based on a simplified mode II analysis and a coupled mixed-mode analysis for predicting the effect of temperature variation on the plate-end debonding failure mechanisms. Regarding the IC debonding, in view of the compressive interfacial stress in the mode I direction near the central notch of FRP-strengthened steel beams, which does not generate damage to the interface, only mode II analysis is considered in the derivation of theoretical solutions to predict IC debonding failure of FRP-strengthened notched steel beams. In all these above-mentioned theoretical analyses, bilinear bond-slip/separation relationships are used for describing the non-linear fracture process of the bond interface in both mode II and mode I directions. All the analytical solutions were validated through comparisons with finite element (FE) analysis results. Then parametric investigations were conducted to indicate how the temperature influences the debonding mechanisms, i.e., a temperature increase leads to a reduced plate-end debonding load while an enhanced IC debonding load and vice versa. The thermal stress effect is more significant when a thicker and stiffener FRP plate is applied.
In the second part of this thesis, double-lap shear tests were conducted on five FRP-to-steel bonded joints at temperatures from -20°C to 60°C , to reveal the mode II bond behavior between the FRP plate and the steel substrate. The temperature-dependent bond-slip relationships were derived based on the measured FRP strains. The interfacial shear stiffness was found to decrease as the temperature increases, while the interfacial fracture energy increases from -20°C to 45°C but drops at 60°C. While the stiffness of the bonded joints under different temperatures is influenced by the bond-slip relationships and the debonding load is solely dependent on the interfacial fracture energy.
In the third part of this thesis, three-point bending tests were conducted on eleven FRP-strengthened intact steel beams at temperatures from -20°C to 60°C , to examine the thermal effect on the structural behavior and plate-end debonding failure of FRP-strengthened steel beams. It was found that the steel beams strengthened with shorter and longer FRP plates failed due to plate-end debonding and FRP rupture, respectively. The debonding load was found to increase slightly when the temperature decreases but drops at an elevated temperature (60°C ). The plate-end debonding could be eliminated by extending the length of FRP plate to the support location at normal and decreased temperatures, but it still occurred at 60°C because of the significant deterioration of the bonding interface under thermal loading. The two-dimensional (2D) FE analysis incorporating the interfacial bond-slip relationship (i.e., obtained from the double-lap shear tests) was conducted to reproduce the test results and good agreement has been achieved between the FE and experimental results, in terms of the load-deflection curves and local interfacial bond behavior.
In the fourth part of this thesis, four-point bending tests were conducted on twelve FRP-retrofitted steel beams with precast notch at temperatures from -20°C to 80°C. All the FRP-retrofitted beams failed by IC debonding. The debonding load was found to increase continuously from -20°C to 60°C but drop significantly at 80°C. Considering the FRP-strengthened notched steel beams had different adhesive curing schemes, which may affect the bond-slip relationship between the FRP plate and the steel substrates, a new approach was deployed to define the local bond-slip relationships for FRP retrofitted notched steel beams at various temperatures. Such bond-slip relationships were then adopted in FE modeling to predict the structural responses of FRP retrofitted notched steel beams. The validation of the proposed bond-slip relationship and FE model was verified through comparisons with the experimental results.
Overall, this dissertation provides an in-depth understanding of the effects of temperature variation on the structural behavior and debonding mechanisms of FRP-to-steel bonded joints and FRP-strengthened steel beams. The scientific findings arisen from this dissertation work have provided a solid theoretical background for developing technical guidelines for predicting the debonding failure in FRP-strengthened steel beams with appropriate consideration of the service temperature effect, thus ensuring the safety of the strengthened members.
Rights: All rights reserved
Access: open access

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