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Bond characteristics and fatigue behaviour of steel structures strengthened with ultra high modulus CFRP plates
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posted on 15.02.2017, 05:00by Wu, Chao
Originated in 1940s, fibre reinforced polymer (FRP) composites are generally characterized by combining high strength and high stiffness fibres with relatively weaker matrices. The extraordinary properties of FRP composites of lightweight, high strength-toweight ratio corrosion resistance, potentially high overall durability and tailorability enable them to be used in areas where the conventional materials might be restricted. Approximately forty years ago, the concept of using bonded FRP composites as a means to maintain aging metallic aircraft was instituted in Australia. And this technique was promoted by research pioneers like Dr. Alan Baker, Dr. Francis Rose, Dr. Rhys Jones and Dr. L.J. Hart-Smith, etc. However, the retrofitting applications of FRP composite in civil infrastructures were much delayed until late 1980s, when the use of FRP composites for retrofitting concrete structures was initiated in Swinzerland by Urs Meier and his colleagues at the Swiss Federal Laboratories for Material Testing and Research (EMPA). Nowadays, the use of FRP for retrofitting of concrete structures is becoming more widely accepted in practice. Since last decade, the experiences in USA, the UK, Japan and Switzerland have showed great potential for FRP to be used in the retrofitting of steel structures. One of the most promising applications is to restore or improve the fatigue performance of steel members by externally attached FRP composites. The carbon fibre reinforced polymer (CFRP) seems to perform better in fatigue strengthening with higher elastic modulus comparing to other FRP composites, like glass fibre reinforced polymer (GFRP) or aramid fibre reinforced polymer (AFRP).
In the previous studies on the fatigue retrofitting of steel structures, the elastic moduli of adopted CFRP composites are generally lower than 300 GPa. In addition, most CFRP composites were thinner than the corresponding steel adherents, resulting in a relatively lower CFRP stiffness. Therefore, the further improvement in the fatigue performance of retrofitted steel structures relies on the increase of either the elastic modulus or the thickness of CFRP composite. Fortunately, with the development of the FRP manufacturing industry, the mechanical properties of CFRP composites have gained considerable improvement. Unidirectional CFRP sheet with a modulus of 640 GPa and pultruded CFRP plate with a modulus of 460 GPa are now commercially available. The researchers in Civil Engineering Department of Monash University in Melbourne have conducted extensive research on the retrofitting of steel members using the CFRP sheets with a modulus of 640 GPa. The high modulus CFRP sheet shows great potential in improving the fatigue performance of cracked steel members. Comparing to CFRP sheeting system, CFRP plate of 460 GPa has other benefits for fatigue retrofitting like larger thickness, more convenient installation and better quality control. However, there has been no report on its fatigue retrofitting applications in steel structures. In this thesis, the CFRP plate with a modulus of 460 GPa is named ultra high modulus (UHM) CFRP plate. The issues regarding fatigue strengthening of cracked steel members using this UHM CFRP plates are investigated, including (1) the bond behaviour between this UHM CFRP plate and steel under static loading and fatigue loading conditions; (2) fatigue behaviour of non-welded cracked steel plates with UHM CFRP plate strengthening and (3) fatigue behaviour of welded cracked steel connections with UHM CFRP plate strengthening. Experimental, numerical and analytical approaches have been resorted.
The first part of the thesis presents the static bond behaviour between this UHM CFRP plate and steel. A series of tests was conducted on the CFRP-steel double strap joints. Experimental results, including bond strength, effective bond length, failure modes, stress and strain distribution along the bond line were reported. Theoretical models were proposed for the prediction of bond strength and effective bond length of CFRP-steel double strap joint. Finite element models were built to simulate the tests. It was found that the failure mode of double strap joint was dependent on the adhesive properties. CFRP delamination and CFRP rupture were observed for specimens with Araldite 420 adhesive, whereas specimens with Sikadur 30 only experienced CFRP delamination. The effective bond lengths of Araldite and Sikadur specimens were 100~120mm and 70~100 mm respectively. The proposed theoretical and finite element models agreed well with the experimental results.
The second step of the research is to understand the effect of fatigue loading on the bond between UHM CFRP plate and steel. Similar CFRP-steel double strap joints were prepared and tested under fatigue loading. These joints were tensioned to failure after enduring a pre-set number of fatigue cycles at various load ratios and then compared with those joints subjected to static tension alone. It was found that the same failure mode of CFRP delamination was observed from visual inspection for all specimens. Unexpectedly, limited effects of fatigue loading on the bond between CFRP and steel were identified. Microscopic investigation was conducted and the failure mechanism was explained by a “fatigue damage zone” concept.
The third part of the PhD study is to investigate the effectiveness of the UHM CFRP plate on preventing fatigue crack propagation and extending the fatigue life of cracked steel plates. Steel plates with through thickness centre crack were repaired with UHM CFRP plates with various strengthening configurations. The effects of CFRP bond length, bond width, and bond locations were investigated. The experimental results showed that the fatigue crack propagation was largely delayed by the CFRP strengthening. The fatigue life was increased up to 7 times more than un-strengthened steel plates. Finally, it was recommended that it be better to attaché CFRP as close to the crack as possible, so that better strengthening effect can be achieved. Following the experimental study, fracture mechanics was introduced to analyse the stress intensity factor (SIF) of cracked steel plate considering the CFRP reinforcement. A simplified mode I SIF formula was proposed for CFRP strengthened steel plates, which provided a method for quick assessment of CFRP strengthening efficiencies.
Finally, a series of fatigue tests were conducted on welded steel joints with CFRP strengthening. The experimental results were compared with those of non-welded CFRP strengthened steel plates. The effects of welding and stress concentration due to the weld attachment on CFRP strengthening effectiveness were investigated. It was found that the strengthening efficiency of UHM CFRP plate was largely decreased due to the welding and attachment. The experimental results suggested that it be better to attaché CFRP on the welding side as close to the crack as possible to achieve longer fatigue life.
Based on the experimental, numerical and theoretical studies, recommendations were provided for the application of fatigue strengthening techniques using this UHM CFRP plate. Future research in the fatigue behaviour of composites reinforced steel structures was also identified.