DEBONDING STRENGTH OF STEEL JOINTS STRENGTHENED USING STRAND CFRP SHEETS UNDER AXIAL TENSION

Transcription

1 Fourth Asia-Pacific Conference on FRP in Structures (APFIS 2013) December 2013, Melbourne, Australia 2013 International Institute for FRP in Construction DEBONDING STRENGTH OF STEEL JOINTS STRENGTHENED USING STRAND CFRP SHEETS UNDER AXIAL TENSION C. Taylor 1, H Jiao 1, X. L. Zhao 2 and A. Kobayashi 3 1 School of Engineering, University of Tasmania, Australia 2 Department of Civil Engineering, Monash University, Vic 3168, Australia 3 Nippon Steel & Sumikin Materials Co., Ltd, Japan ABSTRACT Strand CFRP sheets have the unique features of containing factory impregnated CFRP strands and having the flexibility that make strand sheets suitable for strengthening circular hollow sections along the longitudinal direction. Sufficient gaps are designed between the strands that can be easily filled with epoxy resin during a wet layup process. These features make strand CFRP sheets a promising alternative to conventional CFRP materials in strengthening metallic structures. In these applications, proper bonding between steel and CFRP is vital. Therefore the debonding mechanism and debonding strength are of interests to many researchers. In this study, strand CFRP sheets, with a nominal Young s modulus of 430GPa, were used for the connection of double trap steel joints. Tensile tests were conducted to investigate the bonding behaviour and bond strength. Different failure modes were observed for specimens with and without a primer layer. The bond strength and measured stresses in the bond region were compared with those reported by previous researchers. KEYWORDS Strand CFRP sheets, strengthening, bond strength INTRODUCTION Cabon fibre reinforced polymer (CFRP) is an advanced composite material that is growing in popularity for application in strengthening steel structures. It possesses desirable characteristics such as high strength, lightweight and resistance to corrosion. Among different types of CFRP materials, strand CFRP sheet has the features that it consists of factory impregnated strands that are tied together with sufficient gaps between the strands. The gaps are filled with epoxy resin during a wet layup process to form a high quality composite material. These features help to avoid the difficulties in applying epoxy resin evenly in between CFRP fibres when conventional CFRP sheets are used, where air bubbles are often embedded in CFRP sheets, causing fibre breakage failure under static tensile loads. As a result, the quality of bonding involving strand sheets may be comparable to the bonding quality of factory impregnated CFRP plates. One advantage of strand CFRP sheets over CFRP plates is that strand CFRP sheets can be applied to curved surface, such as CHS, in the longitudinal direction. Due to these features, strand CFRP is becoming an attractive alternative reinforcement polymer that can provide noticeably different to traditional carbon fibre sheets or strips. While recent studies showed that applying strand sheets to steel members can effectively increase the stiffness of the steel members (Hidekuma et al. 2011; Hidekuma et al. 2012), the debonding strength of CFRP strand sheets on steel elements has not been fully investigated. The debonding mechanism and debonding strength of CFRP strengthened steel structures have been of interests to many researchers (Fawzia et al. 2005; Teng et al. 2012; Wu et al. 2012). In this study strand CFRP sheets, with a nominal Young s modulus of 430GPa, were used for the connection of steel plates. Tensile tests were conducted to examine the bonding behaviour and the bond strength of the strand CFRP connected steel joints. The effect of bond length on the bond strength and the respective failure modes were investigated. The effect of the manufacturer specified primer resin on the bond strength was also investigated to reveal any connection between the primer and the debonding strength. The results were compared with the data reported by previous researchers. The aim of this study was to investigate the failure modes and the debonding strength of steel joints strengthened using the strand CFRP sheets.

2 EXPERIMENT SETUP Material Properties The CFRP material used in this project was FORCA FSS-MM600 strand sheets that have a nominal Young s modulus of 430GPa and a nominal ultimate tensile strength of 4500MPa. The thickness of the strand sheet, which was controlled by the diameter of the strands, was around 1.2mm that is comparable to CFRP laminates or plates, such as MBrace Laminate 460 that has a nominal thickness of 1.45mm. However, the design thickness of FSS-MM600 which is calculated from net five cross section without resin is 0.33mm. The strand sheets were thicker than conventional CFRP sheets, such as SikaWrap Hex-230C that has a nominal thickness of 0.13mm. Table 1 shows the properties of the strand CFRP sheets used in this study. For comparison purpose, other two types of CFRP materials used by previous researchers were also listed in Table 1. Table 1. Properties of CFRP materials Types of CFRP Young s modulus Tensile Strength Ply thickness (GPa) (MPa) (mm) FSS-MM600 strand sheet (this study) MBrace Laminate 460 (Wu et al. 2012) CFRP Plate (Yu et al. 2012) A blue coloured two components epoxy resin, FB-E9S(WT), was used as the adhesive in this study. It was generally believed that the bonding behaviour of CFRP to steel structures was related to the properties of the bonding epoxy resins. It was reported by Xia & Teng (2005) that the failure modes were related not only to the epoxy strength but also to the thickness of the adhesive layer. In addition to FB-E9S(WT), another epoxy resin, the primer FP-WE7W, was recommended by the manufacturer of the CFRP strand sheets. According to the Work Procedures provided by the manufacturer, the primer resin should be applied on the steel surface firstly. The bonding of CFRP strand sheets could proceed after the primer was cured for at least 12 hours. This process has not been used for the bonding of conventional CFRP sheets. In order to study the effect of the primer layer on the bond strength, three specimens were prepared without the primer, i.e., with the FB-E9S(WT) and strand sheets being directly applied to the steel surface. Table 2 shows the properties of FB-E9S(WT) together with Araldite 420 that was commonly used by other researchers (Wu et al. 2012; Yu et al. 2012). Table 2. Properties of adhesive materials Types of CFRP Young s modulus Tensile Strength Shear Strength (GPa) (MPa) (MPa) FB-E9S(WT) (this study) Araldite 420 (Wu et al. 2012; Yu et al. 2012) The steel plates used for the steel joints were Grade 300 mild steel that had a nominal yield stress of 300MPa, which was the same as the steel used by Wu et al. (2012). The steel plates had a width of 75mm and a thickness of 6mm. Tensile coupon tests were conducted on the steel in accordance with the Australian Standard AS1391 (SAA 1991). An average yield stress of 322MPa and an ultimate tensile strength of 495MPa were obtained. Specimen Preparation and Test Setup Strand sheets were bonded to both sides of the steel plates to form a double strap joint. Double strap joints were uses by other researchers in determining the bond characteristics of CFRP strengthened steel elements (Fawzia et al. 2005; Wu et al. 2012). A schematic view of a specimen is shown in Figure 1. In order to study the effect of the bond length on the bond strength, specimens with the bond lengths (L 1 ) of 30mm, 50mm and 75mm were prepared. The bond lengths on each side of the joint (L 1 and L 2 ) for each specimen were made unequal, with L 2 being longer than L 1. This was to ensure that failure happened on the side with the designed bond length, i.e., L 1. A total of six specimens were prepared with varying parameters as shown in Table 3. The specimen were labelled based on the bonding method followed by the bond length, with the letters SP representing strand sheets with the primer and the letters SN referring to strand sheets without a primer layer. The specimens were prepared by firstly cutting steel plates to the size of 75x200x6mm (width x length x thickness). Then a grinder was used to remove the rough surface of the steel at the bonding region. The steel was cleaned with acetone to remove any fine particles. This process helped to remove any impurities from the steel surface that could affect the bonding of the strand sheets. Three specimens were applied with the primer resin by following the manufacturer s instructions and cured for 24 hours. Stand sheets were then applied to all test

3 specimens at varying bond lengths using the FB-E9S(WT) epoxy resin. Two CFRP strand sheets, with the same width as the steel plate (75mm), were bonded on both sides of a specimen using the same batch of epoxy resin (one layer on each side). The steel plates of each specimen were carefully aligned in position. The specimens were cured for 2 weeks before testing. Strain gauges were applied to the specimens at the locations shown in Figure 1. Each specimen was tested under axial tension using a universal testing machine until failure. Table 3. Specimen details Specimen Label Bond Length L 1 (mm) With Primer resin SP Yes SP Yes SP Yes SN No SN No SN No TEST RESULTS Failure Modes Figure 1. Schematic view of a specimen All specimens showed FRP adhesive interface failure or adhesive steel interface failure as defined by Zhao & Zhang (2007). No fibre breakage of the CFRP strand sheets was observed. By examining the failed specimens, it was found that specimens with a primer failed in the interface between the primer layer and the strand CFRP sheet as shown in Figure 2(a). This was different to the failure mode of specimens bonded with the same primer and CFRP strand sheets subject to fatigue pure bending (Jiao et al. 2013), in which failure happened in the interface between the steel and the primer resin as shown in Figure 2(b). Further tests are expected to study the effect of loading on the bonding of strand sheets when the primer resin was used. (a) (b) Figure 2. Typical failure modes of specimens with a primer (a) Under static tension (Specimen SP-50, this study) (b) Under fatigue pure bending (Jiao et al. 2013)

4 Specimens without the primer layer showed adhesive failure with the epoxy resin being left on both of the steel plate and the strand sheet as shown in Figure 3(a). The same failure mode was observed for beams strengthened with the same strand sheets without a primer layer under fatigue pure bending (Jiao et al. 2013) as shown in Figure 3(b). When failure happened, two plates separated at the joint with hardened epoxy resin particles being spread over the testing region accompanied by a loud noise. (a) (b) Figure 3. Failure modes of specimens without a primer (a) Under static tension (Specimen SN-30-1, this study) (b) Under fatigue pure bending (Jiao et al. 2013) Specimen Bond Strength and Effect of Bond Length The ultimate loads of all specimens are listed in Table 4 together with the bond strength and the ratio of the ultimate load of the specimens without the primer to that of specimens with a primer layer and with the same bond lengths. The bond shear strength was calculated using Eq. (1), i.e., the load over a unit bond area. The ultimate loads are plotted in Figure 4 versus the bond length. It can be seen from Figure 4 that the load carrying capacity for specimens without a primer layer was higher than that of specimens with a primer layer when both specimens were bonded with the same length of strand CFRP sheets. The ultimate load was increased between 15% and 43% with the bond length ranging from 30mm to 75mm when no primer resin was applied. Table 4. Ultimate loads and bond strength Specimen Label Ultimate load P ult Ratio of P ult.no_primer Bond shear strength (V) (kn) /P ult with Primer (MPa) SP SP SP SN SN SN

5 Figure 4. The ultimate loads versus the bond length V = Pult /( 2 width L1 ) (1) It can be seen from Figure 4 that the ultimate load increases with the increase of the bond length. The same trend was reported by previous research (Xia & Teng 2005; Wu et al. 2012). It appeared that more tests are needed for bond lengths beyond 75mm so that an effective length could be determined for the strand CFRP sheets bonded with the FB-E9S(WT) epoxy resin. Figure 5 shows the bond shear strength versus the bond length together with the data reported by Jiao & Zhao (2004) and Wu et al. (2012). As all specimens failed by adhesive failure, the shear strength of the epoxy resin seems more relevant to the bond strength. It can be seen from Figure 5 that the bond shear strength of the specimens in this study is slighly lower than those in (Jiao & Zhao 2004; Wu et al. 2012). This may be due to the nominar shear strength of FB-E9S(WT) resin (16MPa) is lower than that of Araldite 420 (37MPa). More tests are expected to verify the effect of the shear strength of a epoxy resin on the bond strength. Strain Distribution in the Bond Region Figure 5. Bond shear strength versus the bond length Figure 6(a) shows the typical stress and strain curves of the test specimens (e.g. SN-75). The stress was calculated by the load over the cross-sectional area of the plate. The labels of SG-1 to SG-4 represent the four strain gauges illustrated in Figure 1, with SG-1 being the closest to the joint and SG-4 the furthest from the joint. It can be seen from Figure 6(a) that the specimen remained in the linear range since the stress corresponding to the ultimate load was less than the yield stress of the steel (322MPa). The measured strains along the bond length were plotted in Figure 6(b). It can be seen that the strain in the steel at a location closer to the joint is smaller than that at the end of the bond, indicating that an effective composite was formed with the CFRP strand sheets carried more load in the centre of the joint. This measurement agreed with the observations by Wu et al. (2012) and Yu et al. (2012). SG-1 SG-2 SG-3 SG-4 (a) (b) Figure 6. (a) Typical stress-strain relationship in the bond region (Specimen SN-75) (b) Typical measured strain distribution along the bond (Specimen SN-75)

6 CONCLUSIONS Based on the limited test results, the following conclusions were obtained: The double strap steel joints bonded with strand CFRP sheets and FB-E9S(WT) epoxy subject to axial tension showed debonding of CFRP sheets due to the adhesive failure. When no primer resin was applied failure happened in the interface between steel and CFRP, while the failure was in the interface between the primer and the FB-E9S(WT) adhesive when the joint involved a primer layer. The bond strength of specimens without a primer layer was about 15-42% higher than that of specimens bonded with a primer layer. The bond strength of the double strap steel joints was related to the shear strength of the epoxy resin. The higher the shear strength of the epoxy, the higher the bond strength of the joints. More tests are expected to verify this point. The strain in the steel was lower at the location closer to the joint than that at a location further from the joint. More research is needed to derive the bond-slip model for such CFRP-steel system based on more experimental testing and numerical simulations. ACKNOWLEDGMENTS This project was supported by a National Key Basic Research Program of China (973 Program, 2012CB026200). The authors are grateful to Nippon Steel & Sumikin Materials Co., Ltd for providing the testing materials. REFERENCES Fawzia, S., Zhao, X.-L., Al-Mahaidi, R. and Rizkalla, S. (2005). Double strap joint tests to determine the bond characteristics between CFRP and steel plates.in Z. Y. Shen, G. Q. Li and S. L. Chan. Fourth International Conference on Advances in Steel Structures. Oxford, Elsevier Science Ltd: Hidekuma, Y., Kobayashi, A., Okuyama, Y., Miyashita, T. and Nagai, M. (2012). Experimental Study on Debonding Behaviour of CFRP for Axial Tensile Reinforced Steel Plate by CFRP Strand Sheets. In The Third Asia-Pacific Conference on FRP in Structures ( APFIS2012), Hokkaido University, Japan.(pp. p. T1B04) Hidekuma, Y., Kobayashi, A., Takeshi, M. and Nagai, M. (2011). Reinforcing effect of CFRP strand sheets on steel members. Journal of Physical Science and Application, 1, Jiao, H. and Zhao, X.-L. (2004). CFRP strengthened butt-welded very high strength (VHS) circular steel tubes. Thin-Walled Structures, 42(7), Jiao, H., Zhao, X. L. and Kobayashi, A. (2013). Fatigue testing of defected steel beams repaired using medium and high modulus CFRP strand sheets. APFIS, Melbourne, December SAA (1991). Methods for tensile testing of metals. Australian Standard AS1391, Sydney. Teng, J. G., Fernando, D., Yu, T. and Zhao, X. L. (2012). Debonding Failures in CFRP-Strengthened Steel Structures. APFIS2012, 2-4 Feb Japan: Paper No. KEY02. Wu, C., Zhao, X., Duan, W. H. and Al-Mahaidi, R. (2012). Bond characteristics between ultra high modulus CFRP laminates and steel. Thin-Walled Structures, 51, Xia, S. H. and Teng, J. G. (2005). Behaviour of FRP-to-steel bonded joints. In The International Symposium on Bond Behaviour of FRP in Structures, Hong Kong, China.(pp ) Yu, T., Fernando, D., Teng, J. G. and Zhao, X. L. (2012). Experimental study on CFRP-to-steel bonded interfaces. Composites Part B: Engineering, 43(5), Zhao, X.-L. and Zhang, L. (2007). State-of-the-art review on FRP strengthened steel structures. Engineering Structures, 29(8),