The Effect of Elevated Temperature on the Bond between High Modulus CFRP Sheet and Steel

Transcription

1 The Effect of Elevated Temperature on the Bond between High Modulus CFRP Sheet and Steel H.B. Liu 1, X.L. Zhao 1*, Y. Bai 1, R.K. Singh Raman 2, S. Rizkalla 3 and S. Bandyopadhyay 4 1 Department of Civil Engineering, Monash University, Melbourne, Australia 2 Department of Mechanical & Aerospace Engineering, Monash University, Melbourne, Australia 3 Constructed Facilities Laboratory, North Carolina State University, Raleigh, USA 4 School of Materials Science and Engineering, University of New South Wales, Sydney, Australia *corresponding author: ZXL@monash.edu ABSTRACT: The technique of strengthening steel structures with Carbon Fibre Reinforced Polymer (CFRP) has attracted growing attention in research field and in practice and its environmental durability is of high importance. This paper describes an investigation on the bond characteristics between high modulus CFRP sheet and steel plates under elevated temperature exposures. Tensile tests were carried out on CFRP/steel plate double strap joints at several environmental temperatures of 20 C, 40 C and 50 C, which represent the usually encountered conditions for civil infrastructure. High modulus (640 GPa) unidirectional carbon fibre sheets were applied in wet lay-up fabrication method. Some joints were formed by one layer of CFRP sheet and others by three layers of CFRP sheets. They were all tested to failure by tensile static load. The fracture surfaces of the failed specimens were studied using the scanning electron microscope (SEM) and the failure mechanism were discussed. The objective of the current experimental work is to determine the strain distribution along the bond length, ultimate strength, failure patterns and effective bond length under those thermal exposures. It was found that the ultimate load decreased significantly when the test temperature was above the glass transition temperature of the adhesive. Larger effective bond lengths were found for the joints tested at glass transition temperature. It is obvious that the short-term thermal exposure has little effect on their failure patterns. All the specimens failed by fibre breakage at the joint and cohesive failure accompanied some joints with very short bond length. Keywords: CFRP (Carbon Fibre Reinforced Polymer), Steel Plate, Double Strap Joints, Elevated Temperature, Ultimate Strength.

2 1 INTRODUCTION Applications of adhesively-bonded FRP composite materials in repairing and strengthening infrastructures have gained significant attention (Hollaway and Teng 2008), due to their excellent material properties such as lightweight, high strength to weight ratio, corrosion and fatigue resistance, high specific stiffness, easy to form strong bond, excellent formability to curved surface, flexibility in adapting to field configurations (Chester, 2002). However, the polymeric matrix/adhesive in an FRP strengthening application is the weak link within the system at elevated temperatures, even though fibres alone can retain their strength (Sauder et al, 2004). Although extensive research has been conducted on the bond behavior at ambient temperatures (Miller et al, 2001; Moy, 2001; Hollaway & Cadei, 2002; Cadei et al, 2004; Shaat et al, 2004; Liu et al, 2005; Xia & Teng, 2005; Fam et al, 2006; Zhao et al, 2006; Zhao & Zhang, 2007; Fam et al, 2008;Teng et al, 2012), yet information on the bond behavior of CFRP/steel systems subjected to elevated temperatures remains limited (Al-Shawaf et al 2009). The effect of elevated temperature on bond between normal modulus (230 GPa) CFRP sheet and steel was studied by Ngyuen et al (2011). It was found that the joint ultimate load and stiffness decreased and the effective bond length increased with temperature increasing. Its failure mode was changed with temperature as well. High modulus (640 GPa) CFRP sheet is often used in fatigue strengthening. There is a need to understand the influence of elevated temperatures on its bond behaviour. This paper presents results concluded from a series of shear tensile tests performed at Monash University in order to characterize the effects of short-term exposure to different elevated environmental temperatures, on the bond characteristics of wet lay-upped CFRP/steel plates. These specimens were loaded statically in axial tension under 20 C, 40 C and 50 C. There are two major types of configurations included: some joints were formed by one layer of CFRP sheet and others by three layers of CFRP sheets. The effective bond length at different temperatures was identified by testing

3 specimens with varying bond lengths. Comparisons are presented on the failure modes and ultimate strength of the joints. 2 EXPERIMENTAL PROGRAM 2.1 Materials The specimens were made of flat steel plates with a width of 50mm and a thickness of 5 mm. Uniaxial tension testing was performed on some steel coupons using the Baldwin Universal testing machine at a load range of 100 kn. The tensile coupon tests were carried out according to AS1391 (Standards Australia, 1991). The longitudinal strain at the mid section was measured by two strain gauges fixed on opposite sides. The mean elastic modulus, yield stress and tensile strength were 200 GPa, 362 MPa and 478 MPa, respectively. A Poisson s ratio of 0.3 was assumed. Reinforcements were made of carbon fibre sheets, MBrace CF 530 (BASF, 2012), with a thickness equal to 0.19 mm. They have uni-directional fibres and are usually placed in the longitudinal direction. As reported in the technical datasheet provided by the manufacturer, MBrace CF530 has an elastic modulus of 640 GPa, an ultimate tensile strength of 2650 MPa and an ultimate tensile elongation of 0.4%. It was also called as high modulus CFRP in this paper. The structural adhesive, Araldite 420, was used to join the CFRP and steel plate together. This room temperature curing structural epoxy is characterized with high shear and peel strength. Based on the experimental tests carried out by Fawzia (2007) and Fernando et al (2009), the average values of its elastic modulus, ultimate strength and ultimate strain are 1865MPa, 25 MPa and 2.65%, respectively. Its glass transition temperature (T g ) was found to be 42 C by Al-Shawaf (2010) at Monash University. The Poisson s ratio was assumed to be 0.35 (Jones, 2002; Matta, 2003), which is a typical value for this type of adhesive.

4 2.2 Preparation of Specimens Double sided reinforcements consisted of two steel plates adhesively bonded by CFRP sheets on both sides. The surfaces of the steel plates were sandblasted to remove the rust and create a rough surface. The substrates were then cleaned with acetone to remove grease and dust to expose a fresh chemically active surface to ensure better mechanical interlocking. The adhesive was then applied uniformly using a brush and one layer of CFRP sheet was laid onto the adhesively-coated steel plates. A roller was used to apply uniform pressure over the fibre sheet until the sheet was immersed in the resin and extra epoxy and air pockets were forced to bleed out. For the specimens with more than one layer of carbon fibres, other layers of CFRP sheets were applied in the same way. The specimens were cured for more than ten days at room temperatures as per the manufacturer s instructions (Huntsman, 2012). Typical geometry and configuration of the steel plates, adhesive layers and composite patches are shown in Figure 1. In each joint unequal bond lengths were applied on two sides, i.e. L 1 was always 30 mm shorter than L 2. It was designed to make the specimen failed in the region with a shorter bond length. L 1 was therefore defined as the bond length of the joint. In the current experimental program two types of fibre layouts, namely one layer (C1) and three layers (C3) of CFRP sheets were chosen for investigation. Three different temperatures, 20 C (T20), 40 C (T40) and 50 C (T50), were chosen to represent the normal room temperature, the glass transition temperature (T g ) and above T g, respectively. Meanwhile, these temperatures could be usually encountered in steel structures. The bond lengths were varied from 10 mm to 120 mm to test their effect bond length. The variables tested in this experimental program were listed in Table 1. There are a total number of 46 specimens, including 22 joints with one layer of carbon sheet and 24 joints with three layers of carbon sheets. To study the thermal effect on the strain distribution along the bond length, ten strain gauges were instrumented on the top of the CFRP composites of the joints with 100mm bond length. Their positions were illustrated in Figure 2.

5 3 TEST SETUP AND THERMAL CONDITIONING 3.1 Test setup Figure 3(a) shows the experimental setup, including a temperature chamber and a MTS laser extensometer. Figure 3(b) shows the ready-to-test specimen with the reflective laser tapes. In the current experimental program all the double strap joints were thermally-aged inside the environmental chamber to their target temperatures for a period of 45 minutes, and then tested individually in tension to failure at a constant displacement rate of 2 mm/min using Shimadzu Universal Hydraulic Testing machine. The temperature chamber was manufactured by the Instron with a temperature range of -150 C up to 600 C. The internal testing space is accessed through a front door equipped with a window constructed of two glass panels specially designed to prevent any heat gradient with the external environment. Its temperature controller and all other controlling switches for the internal fan and light are mounted in a convenient handset unit. The MTS laser extensometer was used to measure the relative slip between the CFRP and steel plates. The laser extensometer is capable of performing strain and elongation measurements by aiming a visible laser light at the specimen with reflective laser tapes set at the gage length desired. It uses a unique scanning laser beam technique to measure elongation. It can be used for testing at elevated or sub zero temperatures. 3.2 Thermal conditioning Two dummy specimens were prepared with the same materials and method, one dummy specimen with one layer of carbon fibre sheet and the other with three layers. Each dummy specimen was equipped with one thermocouple, embedded in the joint specimen close to the interface, as shown in Figure 4. The thermal soaking time was decided by measuring its corresponding dummy specimen s temperature. The soaking time of 45 minutes was applied to each specimen ensuring that the satura-

6 tion of the entire specimen was maintained at the specified target temperature. During this period the specimen was maintained in a free mode to exclude stresses from thermal elongation. 4 EXPERIMENTAL RESULTS AND DISCUSSION 4.1 Strain distribution along the bond length The strain distribution along the instrumented bond length was captured by the readings of strain gauges 1 to 7 (refer to Figure 2) bonded on the surface of the CFRP composites. These readings were plotted with distance from the joint under varied thermal exposures in Figure 5(a)-(c) for the joint with one layer of CFRP sheet and in Figure 6(a)-(c) for the joint with three layers of CFRP sheets. The general pattern of the strain distributions was observed that ultimate strain values at the joint x=0) followed by gradual reductions towards the other end of the bond length x=96) at all load levels (i.e. from 10%P ult to P ult ). The trends displayed in these figures were comparable for all exposure temperatures (i.e. 20 C, 40 C and 50 C). Comparing to the curves in Figure 6, the decreasing trends are not as steeper as those in Figure 5, and the discrepancy of the strains at the joint and the other end is less pronounced. To make the results more comparable, the relationship between the axial strain at the joint (i.e. x=0) and load level was plotted in Figure 7 for each type of joint. It was observed that at each temperature the strain in the joint with one layer of CFRP sheet is much higher than that with three layers of CFRP sheets even though the ultimate strengths of the joints with three layers of CFRP sheets are always higher, as introduced in Section 4.5. One reason is that the force distributed on thicker composites leads to smaller strains. The thickness (2.37 mm) of the composites with three layers of CFRP sheets is thicker than that (0.89 mm) with one layer of CFRP sheet. Another reason is that the strain distribution through the layers is different in the composites formed by three layers of CFRP sheets (Liu et al., 2010). The strain in the top layer is smaller than that in the bottom layer which is closest to the

7 steel plate. In Figure 7 the strain in the joint decreases slightly when the temperature increases from 20 C to 40 C and significant reduction occurs to the strain when the temperature increases from 40 C to 50 C. This observation is true for both one layer and three layers of CFRP sheets reinforced specimens. As introduced in Section 4.5 the reduction in bond strength is no more than 10% when the temperature increases from 20 C to 40 C but up to 40% at 50 C. It explained the change happened in Figure Joint failure modes In this experimental program the failure of high modulus CFRP sheets reinforced steel plates was characterized by fibre breakage and cohesive failure accompanied some joints with very short bond length (i.e. 10mm and 20mm). The specimens in Figure 8(a) have bond lengths of 10mm or 20mm and they mainly failed by fibre breakage but cohesive failure happened in small areas. Figure 8(b) depicts the failure mode of the joins reinforced with one layer of CFRP sheets. The three joints were the same but tested at different temperatures. They all failed by fibre rupture at the joints. Figure 8(c) illustrates the failure mode of the joints reinforced with three layers of CFRP sheets. The three identical joints were tested at varied temperatures, 20 C, 40 C and 50 C, respectively. When they failed, most fibres ruptured mainly at the joint. However a part of fibres broke within the bond region for C3T20L60, C3T40L60 and C3T50L60 in comparison to those in Figure 8(a). Such a failure mode is different from that presented in Nguyen et al. (2011), where the bond behaviour was examined on the double strap joints with normal modulus CFRP sheets at elevated temperatures. The joint configurations, the environmental temperatures, and the test procedure are very similar to the current experimental program except for the type of carbon fibres. In previous tests the carbon fibre of MBrace CF130 was adopted, with a nominal elastic modulus of 240 GPa, tensile strength of 3800 MPa, ultimate tensile elongation of 1.55% and fibre thickness of 0.17 mm (BASF, 2012). Herein the carbon fibre of MBrace CF130 is also called as normal modulus CFRP. The joints

8 with normal modulus CFRP tested at the temperature of 20 C the joints failed by CFRP delamination. When temperatures increased to 40 C and 50 C, the CFRP adherend was entirely separated from the steel plate and there were no fibres attached to the steel surface. In term of the joints with three layers of normal modulus CFRP sheets, they failed by CFRP delamination at the temperatures of 20 C. When the temperature increased to 50 C the failure mode changed to mainly cohesive failure with some fibres remaining on the steel surface. The difference of the failure modes between the joints with high and normal modulus CFRP is of interest. The reason had been studied and can be explained using the variable ultimate strains of the materials (Liu et al, 2005). When epoxy is used as the matrix material, the limiting strain is about 0.6% (Talreja, 1987). In the composites composed of normal modulus CFRP (MBrace CF130), the matrix (Araldite epoxy) has a tensile strain (0.6%) lower than that of the carbon fibres (1.2%). Therefore its strain limitation is defined by the matrix and the failure is related to matrix damage (delamination or cohesive failure). For the composites composed of high modulus CFRP (MBrace CF530), the strain limitation is decided by the strain of fibres and hence the fibres break before the matrix cracks, as the ultimate strain is 0.2% for the fibres. 4.3 SEM characterization SEM was adopted to observe the microstructure of the fracture sections of the composites of the specimens failed at 20 0 C, 40 0 C and 50 0 C, respectively. The resulting images are shown in Figures 9(a), (b) and (c). It was observed that with temperature increasing, larger holes, as highlighted by dashed line in each micrograph, were introduced in the resin matrix. This observation demonstrates that more carbons fibres break away from the section of the joint as temperature increasing.

9 4.4 Ultimate joint strength The ultimate load carrying capacity against the bond length at varied temperatures was plotted in Figure 10 for joints with one layer of CFRP sheet (C1) and in Figure 11 for joints with three layers of CFRP sheets (C3). The figures clearly indicate that the joint capacities were generally decreased with temperature increasing. This is due to the fact that the strength and stiffness of polymer matrices decrease rapidly when the temperature exceeds the glass transition temperature (T g ), leading to a reduction of the mechanical properties of FRP composites (Bai et al, 2008). The C3 series have overall higher joint capacities compared to the C1 series, at all currently experimental exposures. These figures clearly indicate that, at a specified temperature the load carrying capacity reaches a plateau after the bond length exceeds a certain value. This length is called the effective bond length, beyond which no significant increase in load carrying capacity will occur. It was observed in Figure 10 that at 20 C, the ultimate load has no significant increase once the bond length exceeds 20 mm, and the length is 40 mm at 40 C and 70mm at 50 C. Similarly the observations in Figure 11 disclosed that for C3 series, the effective bond length is 40 mm at 20 C, 60mm at 40 C and 110 mm at 50 C. 4.5 Effective bond length The bond test carried out by Nguyen et al. (2011) on the double strap joints with normal modulus CFRP sheets at elevated temperatures also studied the effective bond lengths. For the joints reinforced with one layer of normal modulus CFRP sheet, the effective bond length is 40 mm at 20 C, 60 mm at 40 C and 100 mm at 50 C. For the joints with three layers of normal modulus CFRP sheets, the effective bond length is 50 mm at 20 C, 80 mm at 40 C and 130 mm at 50 C. The relationships between the effective bond length and varied temperatures were illustrated in Figure 12 for double strap joints with different CFRP sheets and layers. It was observed that the joints

10 with high modulus CFRP sheets generally have longer effective bond lengths at one specified temperature. With the same type of CFRP the joint reinforced by one layer of CFRP sheet always has longer effective bond length than the one with three layers of CFRP sheets. The relationship between the temperature and non-dimensional bond strength reduction factor was illustrated in Figure 13 for the joints with different CFRP sheets and layers. The bond strength is taken as the average ultimate strength of all the tested joints with bond length not less than its effective bond length. Using the bond strength of the joint tested at 20 C as reference, the nondimensional bond strength reduction factor is achieved by comparing the bond strength with it. In Figure 13 the bond strength reduction factor ranges between 0.9 and1.0 at 40 C and ranges between 0.6 and 0.7 at 50 C. Obviously the reduction in bond strength is no more than 10% at 40 C. However, the ultimate strength is reduced up to 40% when the temperature increases to 50 C. 5 CONCLUSIONS A series of tensile tests were performed to characterize the effect of elevated temperatures on the bond behaviors of CFRP/steel double strap joints. Some of the joints were reinforced with one layer of CFRP sheet and others with three layers. Meanwhile varied bond lengths were applied for each configuration. All the specimens were loaded statically in axial tension under different temperatures of 20 C, 40 C and 50 C. Based on the experimental results, the following observations and conclusions are made: (1) The general pattern of the strain distributions was that ultimate strain values at the joint followed by gradual reductions towards the other end of the bond length at all load levels. With load level increasing the strain closer to the joint rises faster. At each temperature the strain at the joint of the specimens with one layer of CFRP sheet is always higher than that with three layers of CFRP sheets.

11 (2) The failure of CFRP/steel joints with one layer or three layers of high modulus CFRP sheets was characterized by fibre breakage and cohesive failure accompanied some joints with very short bond length. (3) It was observed in the SEM graphs that with temperature increasing larger air pockets were introduced in the resin matrix by the increment of temperature, which may lead to the reduction of the strength of the composites. (4) The capacities of CFRP/steel plate joints were generally decreased with temperature increasing. At each specified temperature the joint with three layers of CFRP sheets has higher joint capacities compared to the joint with one layer of CFRP sheet when the same bond length is applied. (5) With the same type of high modulus CFRP the joint reinforced by one layer of CFRP sheet always has longer effective bond length than the one with three layers of CFRP sheets. 6. ACKNOWLEDGEMENT This project is sponsored by the Australian Research Council through the Discovery Scheme. The technical support received from Mr. Long Goh, Mr. Jeff Doddrell and Mr. Kevin Nievaart in the Civil Engineering at Monash University is greatly appreciated. The authors acknowledge use of the facilities and the assistance of Xi-Ya Fang at the Monash Centre for Electron Microscopy. 7. REFERENCES Al-Shawaf A Characterization of bonding behaviour between wet Lay-up carbon fibre reinforced polymer and steel plates in double strap joints under extreme environmental temperatures, Ph.D thesis, Monash Universty, Melbourne, Australia.

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