Author: Zhu, Jia-ming
Title: Behaviour of CFRP-timber bonded joints
Advisors: Lin, Guan (CEE)
Teng, Jin-guang (CEE)
Degree: M.Sc.
Year: 2019
Subject: Fiber reinforced plastics
Polymeric composites
Composite materials -- Bonding
Adhesive joints
Hong Kong Polytechnic University -- Dissertations
Department: Department of Civil and Environmental Engineering
Pages: xii, 119 pages : color illustrations
Language: English
Abstract: A review on damages and messes of trees caused by typhoon and other natural disturbances indicates that innovative methods are required to strengthen the trees in urban area. One such approach involves the use of fibre reinforced polymer (FRP) composites. This method strengthens trees through inserting FRP plates into the trunk and finally increases the ultimate strength as well as the stiffness of trees. This research aims to quantify the strengthening effect of the implanted FRP plate on trees by investigating the behaviour of FRP-to-timber bonded interface. One of the most important issues about the FRP-to-timber bond is the FRP plate debonding which provides insight into the mechanism of bonded interface. The debonding between timber and FRP may happen at different locations of the joint as follows: (1) timber-adhesive interface in the wood substrate (2) timber-adhesive interface (3) adhesive-FRP interface (4) within adhesive. There are different factors affecting the debonding failure mode including surface preparation, material properties (timber, FRP and adhesive) and curing conditions such as temperature, humidity etc. As the properties of FRP composites are usually available from the manufacturer or can be easily obtained by conducting the standard test e.g. ASTM 3039, emphasis have been put on wood mechanical properties which varied significantly under different environmental conditions and were affected greatly by natural imperfections including knots and splits. Timber was observed to fail brittlely when subjected to tension, whereas it exhibited some ductility under compression. Timber specimens cut from different layers of the section inclined to have different strengths with Heartwood (close to the core) being weaker and the Sapwood (close to the bark) being stiffer. Influences of moisture content also differentiated among layers, and Heartwood was more likely to be affected by changes of moisture content than Sapwood. With the present of natural defects, wood mechanical properties decrease remarkably, but the influences of the imperfections were yet quantified. Four tests of single-lap bonded joints were conducted to explore the bond relationship between FRP and wood substrate. Two types of epoxy adhesives, namely 3M Scotch-Weld 2216 and Six 10, were tested for comparison on the suitability of bonding timber. The result shows that the strain of the FRP plate deceases as the distance from loaded end increases and then stays at a relatively low level till the end of bond length. This implies that the strain distribution of FRP plate bonded with timber is similar to that of FRP-to-concrete bond. Bonded joints fabricated by using Six 10 were tested to have higher bond strength but fail suddenly with little debonding, while load plateau occurred in joints made by using 3M Scotch-Weld 2216 indicating the propagation of debonding. At the trial stage, load eccentricity and lack of support at the far end of the joint induced some degree of bending to the FRP plate, which affected the overall strain profile. It was suggested by the single shear tests that accurate test results depend on both precise fixture fabrication and careful experimental execution. Research focus has been diverted to near-surface mounted (hereafter NSM) bonded joints test, for it directly simulates the bond condition between FRP and wood for the proposed strengthening method. Three series of tests with eight NSM bonded joints have been conducted to investigate the interfacial behaviour and corresponding influential parameters including bond length, timber types and humidity. There exists an important concept that the bond strength of FRP to timber cannot always increase with a longer bond length. The specific limit of bond length is known as effective length beyond which the bond strength does not increase. Different moisture contents were achieved by curing the bonded joints in the moisture chamber for different time, namely one week or two weeks. This aimed to emulate the actual living conditions where humidity level varies considerably and its effect on the bond performance should be accounted for. The test results showed that NSM joints behaved similarly to the single-shear bonded joints. However, NSM bonded joint did not always have a desired failure mode, which can be attributed to the insufficient surface preparation and lack of pressure in the bond area during the curing process. The humidification tended to weaken the bond of the NSM joints, probably because the adhesive strength cannot fully develop under high moisture condition.
Rights: All rights reserved
Access: restricted access

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