Proceedings of the International Symposium on Bond Behaviour of FRP in Structures (BBFS 2005) Chen and Teng (eds) 2005 International Institute for FRP in Construction THE EFFECT OF FATIGUE LOADING ON BOND STRENGTH OF CFRP BONDED STEEL PLATE JOINTS H.B. Liu, X.L. Zhao * and R. Al-Mahaidi Department of Civil Engineering, Monash University, Clayton, Victoria 3168, Australia. *corresponding author: Email: ZXL@eng.monash.edu.au ABSTRACT This paper describes a series of tests on CFRP (Carbon Fibre Reinforced Polymer) bonded steel plate joints subjected to fatigue loading. The aim is to study the effect of fatigue loading on the bond strength, bond slip and failure modes. Both normal modulus (240 GPa) and high modulus (640 GPa) CFRPs were used. The test results were compared with those subjected to static tension alone. It was found that when the maximum applied load is less than 40% of the ultimate static strength there is no fatigue failure in the specimens. When the maximum applied load is less than about 35% of the ultimate static strength, the influence on the bond strength is not significant (less than 10%). A reduction in bond slip stiffness was observed due to accumulated damage caused by the fatigue loading. The failure modes were not affected much by the fatigue loading except for those bonded with high modulus CFRP, where fibre fracture extended over more than one cross-section. KEYWORDS CFRP sheets, steel plates, fatigue, bond strength, debonding. INTRODUCTION In recent years, with the utilization of advanced polymer composite materials growing rapidly, adhesively bonded joints have become more importance because the thin sheet materials could be joined by adhesive efficiently. The continuing increase in the use and widespread appreciation of the benefits of adhesive technology must be based on a thorough understanding of the bond characteristics of the joints. In practice, adhesively bonded joints are often subjected to large number of live load oscillation. This repetitive cyclic action introduces fatigue damage that could initiate either partial or complete detachment of composite materials. Therefore the fatigue effect on the bond needs to be considered when evaluating the service life and effectiveness of this technique. During the last decade, some experimental studies on fatigue behaviour of CFRP (Carbon Fiber Reinforced Polymer) repaired metal specimens have been carried out (Domazet, 1996, Bassetti et al., 1998, Okura et al., 2000, Suzuki, 2002, Suzuki and Okamoto, 2003, Tavakkolizadeh and Saadatmanesh, 2003, Jones and Civjan, 2003, Matta, 2003). It was found that the fatigue life of these structures was affected by CFRP system, stress range of fatigue cycles, number of CFRP layers, CFRP bond length, crack arrest hole and prestressing technique. However, the effect of fatigue cycles on the bond strength has not been identified. The aim of this study is to explore the effect of cyclic loading on bond strength and the fatigue behaviour of CFRP bonded steel plates. MATERIALS, SPECIMENS AND TEST PROCEDURES A total of twelve CFRP/steel joint specimens were designed and tested. The configuration is illustrated in Figure 1. For each of the joints, two pieces of steel plates, 210mm long, 50mm wide and 5mm thick, were used. Its mechanical properties were determined through tensile coupon tests. The mean elastic modulus, yield stress and tensile strength were 195GPa, 359MPa and 484MPa respectively. Carbon fibre sheets, MBrace CF 130 and MBrace CF 530, were used as a patching system in this test program. They have uni-directional fibres and are placed in the longitudinal direction. Based on the technical data provided by the manufacturer, MBrace CF130 has an elastic modulus of 240GPa, an ultimate tensile strength of 3800 MPa and an ultimate tensile elongation of 1.55%. MBrace CF530 has an elastic modulus of 640GPa, an ultimate tensile strength of 2650 MPa and an ultimate tensile elongation of 0.4%. In this paper MBrace CF130 and CF530 are called normal modulus CFRP 451
and high modulus CFRP respectively. The structural adhesive Aradite 420 was used to join the two materials together. Aradite 420 is an extremely tough and resilient adhesive and it can resist moisture adequately. Its material properties could be obtained from the product data sheets. During the test session, the tension loads applied on the joints were carried by the bonding interfaces. To make sure the specimens failed on the desired side, the length of L 2 was set at 80mm which was always higher than L 1 of 40mm and 60mm. Figure 1 Schematic view of double strap joints between CFRP and steel plates (not to scale) The specimen surfaces at the bonding area were ground with Linisha and cleaned with acetone to remove grease, oil and rust to ensure a better mechanical interlocking. When the surfaces were ready, the adhesive Aradite 420 was applied uniformly using a brush. Then one layer of CFRP sheet was laid onto the adhesively coated steel plates. A roller was rolled slowly over the CFRP sheet to apply uniform pressure until the sheet was immersed in the resin and extra epoxy and air pockets were forced to bleed out. In the same way, two more layers of CFRP sheets were applied. On each side of the joint three layers of CFRP sheets were bonded. When the specimens were fully cured and ready to be tested, they underwent a pre-set number of fatigue cycles (N) ranging from 0.5 million to 6 million at different levels of constant amplitude stress range. The maximum load applied (P max ) was taken as a certain percentage of the static strength of the joint (F 1 ). The stress ratio of the constant amplitude stress range is 0.1, which gives the minimum applied load in the fatigue tests as 0.1P max. After fatigue testing, the specimens were pulled to failure by static tension loads. Displacement transducers were installed to measure the relative slip between CFRP and steel plates. TEST RESULTS The test results are summarised in Table 1 which includes N, P max /F 1, F 1, F 2, F 2 /F 1 and failure modes. In this table, F 1 denotes the ultimate strength without any fatigue loading given in Fawzia et al. (2005). F 2 denotes the ultimate strength after a pre-set number of fatigue cycles. Bond strength ratio is defined as the ratio of F 2 to F 1. When the specimens failed during the fatigue test, the value of F 2 is set equal to zero. ID stands for the failure mode of interfacial debonding and FB for fibre breakage failure. Similar experimental results reported in Matta (2003) are also listed in Table 1 in the last 3 rows. Table 1 Experimental results of CFRP/steel plate (joints) Specimen L 1 CFRP N Load F 1 F 2 Bond Failure No. (mm) E (GPa) (million) Ratio Strength (P max /F 1 ) (kn) (kn) Ratio (F 2 /F 1 ) Mode N40-1 40 240 0.8 0.50 49.90 0 0.00 ID N40-2 40 240 0.268 0.59 49.90 0 0.00 ID N60-1 60 240 1 0.26 58.80 54.9 0.93 ID N60-2 60 240 0.43 0.38 58.80 0 0.00 ID N40-3 40 240 0.5 0.34 49.90 62.5 1.25 ID N40-4 40 240 1 0.34 49.90 56.8 1.14 ID N60-3 60 240 0.5 0.26 58.80 87.7 1.49 ID N60-4 60 240 1 0.26 58.80 79.4 1.35 ID H40-1 40 640 0.523 0.50 53.10 0 0.00 FB H40-2 40 640 2 0.39 53.10 51.9 0.98 FB H60-1 60 640 1.5 0.34 52.20 61.8 1.18 FB H60-2 60 640 6 0.34 52.20 61.3 1.17 FB DSS-F-1 205 166 1 0.38 39.00 34.4 0.88 ID DSS-F-2 205 166 1 0.37 40.30 34.3 0.85 ID DSS-F-3 205 166 1 0.37 40.90 33.9 0.83 ID 452
EFFECT OF FATIGUE LOADING ON BOND STRENGTH The relations between load ratio (P max /F 1 ), bond strength ratio (F 2 /F 1 ) and pre-set number of cyclic loading (N) are shown in Figure 2. Experimental result of specimen N60-2 was not included due to the premature failure caused by strain gauges mounted between CFRP sheets. In Figure 2 there are three points shown with unfilled triangular shape. They represent the specimens which failed during fatigue tests. It is concluded from this figure that the specimen usually failed in one million cycles when the load ratio is above 0.50. The fatigue boundary is outlined by a dashed line, beyond which fatigue failure occurs. This gives a limit of about 40% of ultimate static strength which is approximately close to the limit of 60% yield strength specified in Department of Energy (1990) for fatigue application. Meanwhile the fatigue influence boundary line is drawn by a dot dashed line within which the fatigue loading does not significantly (say less than 10%) affect the bond strength. It seems that the joints can retain at least 90% of their static strengths when the load ratio (P max /F 1 ) is less than about 35%. 0.70 0.60 (0.0) No Failure during Fatigue Testing Failure during Fatigue Testing Load Ratio Pmax/F1 0.50 0.40 0.30 0.20 (0.0) (0.0) (0.88) (0.98) Fatigue Failure Bounding Fatigue Influence Boundary based on 90% bond strength ratio (0.83) (0.85) (1.17) (1.14) (1.25) (1.18) (1.35) (1.49) (0.93) 0.10 0.00 0 1 2 3 4 5 6 7 Pre-set Number of Fatigue Cycles (Million) Figure 2 Relations between stress ratio, strength ratio and pre-set number of fatigue cycles (The value marked in parenthesis is the corresponding strength ratio.) EFFECT OF FATIGUE LOADING ON BOND SLIP The load-displacement plots of specimen SN-3 and N60-1 are shown in Figure 3. 60 SN-3 N60-1 58.8kN 54.9kN 50 40 Load (kn) 30 20 10 0 0 0.05 0.1 0.15 0.2 0.25 0.3 0.35 0.4 0.45 0.5 Slippage between CFRP and Steel Plate (mm) Figure 3 Load-displacement curves of specimen N60-1 (this paper) and SN-3 (Fawzia et al 2005) 453
These two specimens had no difference except the testing procedure. SN-3 was directly tested to failure by static tension loads (Fawzia et al., 2005). N60-1 was axially tensioned to failure after being fatigued. Both of these two specimens failed in a brittle manner. After one million fatigue cycles applied to N60-1, there was no crack or damage detected by visual inspection. The reduction in bond strength is not significant (about 7%). The slope change in the load-displacement curve suggests that plastic deformation happened to the adhesive and progressive damage occurred in the composites during fatigue tests. EFFECT OF FATIGUE LOADING ON FAILURE MODE Two different failure modes were observed in this experimental program. One was interfacial debonding, which happened to the joints with normal modulus CFRP, as shown in Figure 4. Another was fibre breakage failure, which happened to the joints with high modulus CFRP, as shown in Figure 5. These observations were similar to previous tests performed at Monash University (Jiao and Zhao, 2004, Fawzia et al., 2004, Fawzia et al., 2005). The comparison photos of typical failure modes are also included Figures 4 and 5. Generally the growth rate of the damage mechanism was determined by the applied strain. Both fibres and matrix were subjected to the same strain while stresses in the two phases differed depending on their volume fraction and the elastic moduli. Therefore strain instead of stress should be chosen as an independent variable to explain the failure mode. When epoxy was used as the matrix material, the limiting strain in fatigue was 0.006 (Talreja, 1987). Matrix cracking could happen when the strain exceeded this value. Meanwhile on the technical sheets provided by the manufacturers, the tensile elongation of MBrace CF130 was 1.55% and MBrace CF530 was 0.4%. Based on these values, two characteristic sets of stress-strain curves are plotted in Figure 6(a), in which, ε m denotes the fatigue limit of matrix, equal to 0.6%, ε c denotes the fracture strain of the fibres. ε c is equal to 1.55% for MBrace CF130 and 0.4% for MBrace530. (a) Static-to-failure test (Fawzia et al., 2005) Front View Back View (b) Static-to-failure after fatigue tensile test Figure 4 Typical failure mode of steel plates bonded with normal modulus CFRP (a) Static-to-failure test (Fawzia et al., 2005) Front View Back View Front View Back View (b) Static-to-failure after 2 million fatigue cycles (c) Static-to-failure after 6 million fatigue cycles Figure 5 Typical failure mode of steel plates bonded with high modulus CFRP When the axially tensile loads were applied gradually, the strain of the composites increased correspondingly. For the composites composed of high stiffness fibres MBrace CF530, the strain reached the limitation of 0.4% first. Therefore the fibres broke before the matrix cracked. Similarly, in the composites composed of lowstiffness fibres MBrace CF130, the limitation was defined by the limiting strain of matrix. Hence the joints bonded with normal modulus CFRP failed by interface debonding. The typical failure mode of steel plates bonded with high modulus CFRP is shown in Figure 5. For the specimens without any cyclic loading, the fibres broke at one very clear section. Whereas for the specimens undertaking a few number of cycles, the final fracture extended over more than one cross section and more fibres were broken. The situation got worse when more cycles were applied. This phenomenon could be explained using Figure 6(b), which illustrated the progressive nature of fibre breakage. 454
When applied strain exceeds a certain value, an individual fibre may break. The locations of the break are scattered randomly in the composite. Repeated application of the stress results in breakage of the fibres adjacent to the previously broken fibres, due to the stress concentration in the vicinity of the broken fibres. The number of broken fibres increases with the number of cycles applied, while the distribution of the broken fibre locations remains random (Talreja 1987). With an increasing number of broken fibres, the probability of finding a crosssection having a stress high enough to break the remaining fibres in it increases. The final fracture may extend over more than one cross-section. σ σ MBrace CF130 composite matrix MBrace CF530 composite matrix ε m ε c ε ε c ε m (a) (b) Figure 6 (a) Stress-strain characters of unidirectional polymer matrix composites (not to scale) (b) Fibre breakage in unidirectional polymer matrix composites (Talreja, 1987) ε CONCLUSIONS A series of tests were carried out on CFRP bonded steel plate joints subjected to fatigue loading to investigate the effect of fatigue loading on the bond strength, bond slip and failure modes. Both normal modulus (240 GPa) and high modulus (640 GPa) CFRPs were used. Based on the limited test results, the following observations and conclusions are made: No fatigue failure was observed when the maximum applied load is less than 40% of the ultimate static strength. The influence of fatigue loading on the bond strength is not significant (less than 10%) if the maximum applied load is less than about 35% of the ultimate static strength. A reduction in bond slip stiffness was observed due to accumulated damage caused by the fatigue loading. The failure modes were not affected much by the fatigue loading except for those bonded with high modulus CFRP, where fibre fracture extended over more than one cross-section. REFERENCES Bassetti, A.,Liechti, P. and Nussbaumer, A. (1998). "Fatigue Resistance and Repairs of Riveted Bridge Members", Fatigue Design 1998, Espoo, Finland, May 1998, 535-546. Domazet, Z. (1996). "Comparison of Fatigue Crack Retardation Methods", Engineering Failure Analysis, 3, 137-147. Department of Energy (1990), Offshore Installations: Guidance on design, construction and certification, Fourth Edition, London, HMSO, UK. Fawzia, S., Zhao, X. L. and Al-Mahaidi, R. (2004). "Investigation into the Bond between CFRP and Steel Tubes", the Second International Conference on FRP Composites in Civil Engineering, Adelaide, Australia, December, 2004, 733-739. 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", the Fourth International Conference on Advances in Steel Structures, Shanghai, China, June, 2005, 1583-1589. Jiao, H. and Zhao, X. L. (2004). "CFRP Strengthened Butt-Welded Very High Strength (VHS) Circular Steel Tubes", Thin-Walled Structures, 42, 963-978. Jones, S. C. and Civjan, S. A. (2003). "Application of Fiber Reinforced Polymer Overlays to Extend Steel Fatigue Life", Journal of Composites for Construction, 7, 331-338. 455
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