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  1. 1. International Journal of Civil Engineering and Technology (IJCIET), ISSN 0976 – 6308 (Print), ISSN 0976 – 6316(Online) Volume 5, Issue 3, March (2014), pp. 301-307 © IAEME 301 A STATE OF THE ART REPORT ON FATIGUE BEHAVIOUR OF STEEL STRUCTURES STRENGTHENED WITH FIBRE-REINFORCED POLYMER COMPOSITES N. Umamaheswari*, Dhanya Mary Alexander** *Associate Professor, **M.Tech (Str.) Student, Dept. of Civil Engineering, SRM University, Kattankulathur, Kancheepuram Dist., Tamilnadu, INDIA ABSTRACT Fatigue damage is a major concern for many infrastructures such as steel bridges and offshore structures, with fatigue cracks being developed in areas of high stress concentrations. Although these structures are still in service, many of them need to be strengthened or repaired. Repairing and retro- fitting fatigue cracks are more economical and environmental friendly than replacing the aged steel structures. The cracks are generally repaired by welding in accordance with international welding standards. However, cracking recurs in a short period after repair due to stress concentrations near welds, causing significant loss in repair costs. Over the past two decades, Fibre-reinforced Polymer composites (FRP) have gradually gained wide acceptance in civil engineering applications due to some of their unique advantages. More recently, the use of FRP composites to strengthen existing steel structures has received much attention. This paper presents a state of the art review of fatigue performance of steel structures strengthened with fibre-reinforced polymer composites, which includes a comprehensive review of literature on experimental and analytical studies conducted, the factors influencing their behaviour and the type of failure modes. Keywords: Steel Structures, Fatigue Behaviour, Fibre-Reinforced Polymer Composites, Retrofitting, Stress Concentration. INTRODUCTION The majority of trucks in mining operations are experiencing structural problems, with fatigue cracks appeared at various locations on truck chassis due to rough road conditions on mine sites. Repairing and retro-fitting fatigue cracks are attracting much research attention worldwide. INTERNATIONAL JOURNAL OF CIVIL ENGINEERING AND TECHNOLOGY (IJCIET) ISSN 0976 – 6308 (Print) ISSN 0976 – 6316(Online) Volume 5, Issue 3, March (2014), pp. 301-307 © IAEME: www.iaeme.com/ijciet.asp Journal Impact Factor (2014): 7.9290 (Calculated by GISI) www.jifactor.com IJCIET ©IAEME
  2. 2. International Journal of Civil Engineering and Technology (IJCIET), ISSN 0976 – 6308 (Print), ISSN 0976 – 6316(Online) Volume 5, Issue 3, March (2014), pp. 301-307 © IAEME 302 FRP composites have gradually gained wide acceptance in civil engineering applications due to their unique advantages including their high strength-to-weight ratio and excellent corrosion resistance properties. In particular, many possibilities of using FRP in the strengthening and construction of concrete structures have been explored. More recently, the use of FRP to strengthen existing steel structures has received much attention. Since steel is also a material of high elastic modulus and strength, the use of FRP in strengthening steel structures calls for innovative exploitations of the advantages of FRP. The main advantage of using FRP over steel in strengthening of steel structures is its high strength-to-weight ratio, leading to ease and speed of transportation and installation, thus reducing disturbance to services and traffic. Another significant advantage of FRP, which applies only to FRP laminates formed via the wet lay-up process, is the ability of such FRP laminates to follow curved and irregular surfaces of a structure. This is difficult to achieve while using steel plates for strengthening. A third advantage of FRP is that its material properties in different directions can be tailored for a particular application. As a result of the second and third advantages, FRP jackets with fibres oriented only or predominantly in the circumferential direction can be used to confine steel tubes/shells or concrete-filled steel tubes to delay/eliminate local buckling problems in steel tubes/shells, thereby enhancing the strength and/or seismic resistance of such structures. USE OF COMPOSITE MATERIALS Bonding composite materials to strengthen structural elements has attracted a great deal of attention in recent years and a large number of studies have been devoted to this area. The outstanding properties of composite materials such as high strength, high elastic modulus, light weight and good durability have made them a suitable alternative to steel plates in strengthening work. In addition, the adhesive bonding technique offers several advantages such as ease of application and improved fatigue behaviour. This has made bonding a good alternative to conventional jointing techniques such as welding and bolting. Composite laminates used for strengthening purposes usually consist of unidirectional fibres embedded in an adhesive matrix or so-called unidirectional laminates. One inherent feature of this structure is the clear anisotropy in the properties of the laminate in terms of both stiffness and strength. In the longitudinal direction, the stiffness and the strength are very high, whereas the transverse (i.e. perpendicular to the fibre direction) stiffness and shear strengths are much lesser. The through-thickness modulus of unidirectional laminates is only two or three times that of the matrix and strength is of the same order as that of the matrix or, in some cases, even less. On the other hand, the strain capacity of structural adhesives is very limited in tension, as compared to shear. Compared with concrete structures, in which failure mostly takes place in the concrete cover due to poor behaviour of concrete in tension, the state and modes of failure are more complex in strengthened steel members. The strength of steel is usually superior to that of conventional adhesives used in strengthening applications. This, in addition to the low strength of unidirectional composite laminates, result in a variety of possible failure modes in steel members strengthened with bonded CFRP-laminates. Failure of the adhesive, through-thickness failure of composite laminate, i.e. delamination, or failure along the interfaces, i.e. debonding are the possible modes needed to be considered in the design of CFRP-strengthened steel members. Owing to this variety of possible failure modes and the lack of knowledge regarding the behaviour of composite materials in adhesively bonded joints, more research is needed before a wide implementation of the strengthening techniques is feasible in practice.
  3. 3. International Journal of Civil Engineering and Technology (IJCIET), ISSN 0976 – 6308 (Print), ISSN 0976 – 6316(Online) Volume 5, Issue 3, March (2014), pp. 301-307 © IAEME 303 FRP STRENGTHENING OF STEEL STRUCTURES A large number of steel structures, such as bridges, offshore platforms, large mining equipment and buildings, need retrofitting. The conventional method of repairing or strengthening of steel structures is to cut out and replace plating, or to attach external steel plates. These plates are usually bulky, heavy, difficult to fix and are prone to corrosion and fatigue. There is a need to look for alternatives. The use of FRP appears to be an excellent solution. FRP has high strength to weight ratios, and excellent resistance to corrosion and environmental degradation. It is very flexible and forms all kinds of shapes, and is easy to handle during construction. FRP has been widely used in strengthening concrete structures, and extensive research has already been conducted. Xiao-Ling Zhaoa and Lei Zhang(2007) published a state of the art review on FRP strengthened steel structures combining the data from CIRIA Design Guide (2004) and works of Hollaway and Cadei(2002), Shaat A and Fam A(2006). It was concluded that the possible failure modes in a CFRP bonded steel system subjected to tensile force include: (a) Steel and adhesive interface failure (b) Cohesive failure (adhesive layer failure) (c) CFRP and adhesive interface failure (d) CFRP delamination (separation of carbon fibres from the resin matrix) (e) CFRP rupture (f) Steel yielding The failure modes depend upon the modulus of elasticity of the CFRP sheets, and types of adhesive and the thickness of adhesive. It was found that cohesive failure (mode (b)) tends to occur for thin adhesives, whereas the failure mode changes to delamination of CFRP plates (mode (d)) for thick adhesives, for the CFRP plate bonded system. Mode (d) was found to be more brittle than mode (b). Failure modes (a) and (c) were not observed, indicating the strong bond capacity of the adhesives to the roughened steel and the FRP plate surfaces. Steel yielding (failure mode (f)) is often avoided in bond testing by using sufficient plate thickness. In compression members, local buckling may occur in steel hollow sections in compression if the width-to-thickness ratio is larger than a certain value. Shaat and Fam (2006) identified three typical failure modes for short steel hollow section columns strengthened by CFRP. They are delamination between the steel and the longitudinal CFRP at the end of the specimen, rupture of the transverse CFRP near the corners and delamination between the steel and the transverse CFRP at an inward buckling location. For long columns without CFRP, failure was mainly due to excessive overall buckling, followed by a secondary local buckling near the mid-length of the specimen. The local buckling was in the form of inward buckling of the compression face and outward buckling of the two side faces. For CFRP strengthened specimens, the delamination and crushing of CFRP was observed on the compression side where inward buckling occurred. CFRP rupture was found at the outward buckling location. Shaat and Fam (2006) also found that for short columns, transverse CFRP layers are effective in confining the outward local buckling. The maximum load carrying capacity increased upto 18% for short columns and upto 13–23% for long columns. The highest capacity gain was associated with three layers of CFRP applied on four sides of specimens. Studies on fatigue crack propagation in steel members strengthened by CFRP were carried out by Tavakkolizadeh and Saadatmanesh(2003) and Jones SC and Civjan SA(2003). It was found that in specimens of I-section, crack always initiated from the tip of the cut and started to move towards the web. After reaching the fillet section of the web, debonding at the near edge (the side where the crack was initiated) of the CFRP plate started. While the crack front moved to the far edge
  4. 4. International Journal of Civil Engineering and Technology (IJCIET), ISSN 0976 – 6308 (Print), ISSN 0976 – 6316(Online) Volume 5, Issue 3, March (2014), pp. 301-307 © IAEME 304 (the side where the crack terminated), the debonding at the edge continued. Finally, the far edge of the CFRP plate started to debond. The stable crack growth rate was found to be 35% of that for un- retrofitted specimens on average. Colombi et al.(2003) found that for plate specimens, debonding is not a dominant failure mode. Other major conclusions include a. the use of very stiff CFRP plates does not produce a significant increase of effectiveness of the reinforcement. b. a relatively thin adhesive layer reduces the effectiveness of the reinforcement. c. pre-tensioning is strongly recommended in order to maximize the effectiveness of fatigue strengthening. Jones SC and Civjan SA (2003) found that better performance was achieved when the CFRP overlay was applied directly over the crack starters (centre hole) and in the path of the crack. This finding is valuable, since potential crack locations are not accurately defined in actual structural applications. They also found that CFRP with moderate modulus of elasticity performed best, and that an increase in performance would be realized through CFRP pre-tensioning. FATIGUE AND CRACK PROPAGATION In material science, fatigue is the progressive and localized structural damage that occurs when a material is subjected to cyclic loading. The nominal maximum stress values are less than the ultimate tensile stress limit, and may be below the yield stress limit of the material. Fatigue occurs when a material is subjected to repeated cycles of loading and unloading. If the loads are above a certain threshold, microscopic cracks will begin to form at the stress concentrators such as the surface, persistent slip bands, and grain interfaces. Eventually a crack will reach a critical size, and the structure will suddenly fracture. The shape of the structure will significantly affect the fatigue life; square holes or sharp corners will lead to elevated local stresses where fatigue cracks can initiate. Round holes and smooth transitions or fillets are therefore important to increase the fatigue strength of the structure. ASTM standards define fatigue life, Nf, as the number of stress cycles of a specified character that a specimen sustains before failure of a specified nature occurs. Jun Deng and Marcus M.K.Lee (2007) observed crack initiation and development in the adhesive layer to establish its mode of cracking during an experimental study. They also evaluated the change in stiffness of the retrofitted beams with crack development and the effect of presence of spew fillet beyond the plate ends on the fatigue behaviour of the bonded joints. They identified that the backface-strain technique that was applied to monitor crack initiation and crack growth is simple and effective way to monitor retrofitted beams. The results showed that the strain gauges could detect crack initiation immediately and track the deterioration of the adhesive layer. The measured strain showed that the spew fillet was of benefit to the fatigue performance of the bonded joints, but the improvement was not significant. The stiffness of the retrofitted beams was found to deteriorate with crack growth, but the deterioration was insignificant due to the lesser thickness of the plates used in the tests. An S–N curve was developed from the test results. Since this S–N curve shows the relationship between the maximum interfacial stress and the number of cycles, it can be used for different sizes of retrofitted metallic beams using the same adhesive. The fatigue limit, i.e. threshold, of the S– N curve is about 30% of the ultimate static failure stress. The static performance of retrofitted beam is not affected in any significant way by fatigue loading under the threshold limit. The fatigue load range will affect the fatigue life, but its significance is much less than the magnitude of the maximum load in the load range.
  5. 5. International Journal of Civil Engineering and Technology (IJCIET), ISSN 0976 – 6308 (Print), ISSN 0976 – 6316(Online) Volume 5, Issue 3, March (2014), pp. 301-307 © IAEME 305 Massimiliano Bocciarelli et al.(2009) aimed at investigating the fatigue performance of adhesive joints in steel members reinforced with CFRP strips. To assess the fatigue performance, double sided reinforcement specimens were used. It was concluded that fatigue tests showed a significant stiffness reduction of the reinforced concrete member due to progressive debonding of the adhesive, which usually represents the weakest link in FRP strengthened metallic structures. Stiffness dropped to 85% of the original value when CFRP debonding reached the mid-span of the specimens. The failure mechanisms found to be started with initial debonding of the CFRP plates in the zones of high stress concentration, i.e., the plate ends or the gap in case of joints. Then failure propagated along the element in the interfaces. Visual inspection of the bonds after failure revealed that the adhesive/metal interface represents the weakest point of the reinforcing system and improvements in the resin or in the surface preparation should be made in order to increase the bond strength and to avoid premature debond. The results showed that fatigue resistance of CFRP plates bonded on steel members is definitely competitive with that of welded steel cover plates. Hongo Liu et al. (2009) conducted experiments to investigate the effectiveness of CFRP sheets on arresting crack propagation. Also they conducted theoretical studies on crack growth in CFRP sheets repaired steel plates through the analysis of the strain distribution in the composite layers and the stress distribution at the cracked section. They found that for single-sided repairs, the efficiency of the composites in extending the fatigue life is reduced significantly, as crack propagates much faster on the un-patched side. The out-of-plane bending induced by unsymmetrical configuration also increases the stress intensity factors at the crack tip. Meanwhile the effect of crack closure is also reduced significantly by bending. In the analysis of single-sided repairs, the effects of crack closure and out-of-plane bending on stress intensity factor are assumed to be counteracted. It is proved to be an efficient assumption. In the analysis of double-sided repair, the in-plane loads produce no out-of-plane bending over the repaired zone. The crack closure effect is fully accounted by the application of effective stress intensity factor range. Yail J Kim and Kent A Harries (2011) made analytical predictions of the behaviour of the damaged steel beams subjected to fatigue loading conditions. Also experimental work was done in order to confirm the analytical observations. They concluded that the three dimensional nonlinear FE model is used to predict the behaviour of damaged steel beams and it showed good agreement with the experimental data. The damage propagation of the beams was abrupt and the level of damage was not influenced by the stress range up to approximately 60% of the fatigue life, whereas the stress range affected damage levels with further increases in fatigue cycles. The stress range affected the crack propagation rate of the repaired beams. The fatigue response of the CFRP–steel interface was bilinear and was dependent upon the stress range and the number of fatigue cycles. The local bond failure of the CFRP strip due to fatigue damage caused a sudden increase in the bond stress and corresponding slip of the strip. The elastic strains at the root of notch were essentially constant up to 10% of the fatigue life of the repaired beams, irrespective of the stress range. The development of the plastic strains was influenced by the stress range; however, their magnitude was not significantly affected when the stress range was below 40% of the yielding strength of the steel. Pierluigi Colombi et al. (2012) observed a significant stiffness reduction of the joints due to progressive debonding of the CFRP strips from experimental investigation. Such a stiffness reduction is an indicator of the subsequent and progressive global failure. Debonding started at the zones with high stress concentration, i.e. at the gap due to very high shear stress level. In few cases debonding started at the CFRP strip end indicating that due to the bridging stress effect the gap was not the critical zone. The gap adhesive penetration significantly reduces the relevant stress concentration in the adhesive layer. For a certain level of adhesive penetration (of about 50%) the shear stress level at the gap is significantly reduced and in this case debonding started at the CFRP strips end. After the onset of the crack, debonding propagated along the joint in the CFRP/adhesive
  6. 6. International Journal of Civil Engineering and Technology (IJCIET), ISSN 0976 – 6308 (Print), ISSN 0976 – 6316(Online) Volume 5, Issue 3, March (2014), pp. 301-307 © IAEME 306 interface. The fatigue limit is defined as the stress range in the steel plate which produces, after 6 million of duty cycles, a stiffness reduction in the joint lower than 95%. Alex O’Neil et al. (2005) investigated the deleterious effect on the performance of externally bonded FRP retrofit systems through fundamental mechanical testing of adhesive systems suitable for bonded FRP repair systems. Commercially available adhesives having different moduli are tested. Although this study represents a limited number of results, it nonetheless clearly identifies the deleterious effects of fatigue loading on the ultimate mechanical properties of epoxy adhesive systems typically used in FRP retrofit applications. The degradation in performance is comparable to that observed in FRP retrofit concrete beams subject to fatigue conditioning suggesting that observed behaviour of such beams is a function of the adhesive bond line more so than the FRP or the substrate. This study suggests that a reduction factor for allowable FRP strain associated with fatigue is appropriate. Such a factor will be a function of the adhesive stiffness and the expected fatigue stress range. Also, the results of this study indicate that for relatively low expected fatigue stress ranges, a stiffer adhesive will exhibit minimal degradation. At higher stress ranges, however, degradation must be expected and a softer adhesive will provide greater ductility and may be expected to behave in a more predictable manner. CONCLUDING REMARKS A state of the art review of fatigue performance of steel structures strengthened with fibre- reinforced polymer composites, which includes a comprehensive review of literature on experimental and analytical studies conducted, the factors influencing their behaviour and the type of failure modes is presented. REFERENCES 1. Alex O’Neil, Kent A Harries and Patrick L Minnaugh(2005), ‘Fatigue Behaviour of Adhesive Systems used for Externally Bonded FRP Applications’ 2. Cadei JMC, Stratford TJ, Hollaway LC, Duckett(2004), ‘Strengthening metallic structures using externally bonded fibre-reinforced composites.’ London: CIRIA 3. Colombi P, Bassetti A, Nussbaumer A.(2003), ‘Analysis of cracked steel members reinforced by pre-stress composite patch’, Fatigue Fracture of Engineering Materials Structures, Vol. 26, pp 59–66. 4. Hollaway LC, Cadei J.(2002), ‘Progress in the technique of upgrading metallic structures with advanced polymer composites.’ Progress in Structural Engineering and Materials, Vol. 4(2), pp. 131–48. 5. Hongo Liu, Zhigang Xiao, Xiao Ling Zhao, Riadh Al-Mahaidi(2009), ‘Prediction of fatigue life for CFRP strengthened Steel Plates’, Thin Walled Structures, Vol. 47, pp.1069-1077. 6. Jones SC, Civjan SA.(2003), ‘Application of fibre reinforced polymer overlays to extend steel fatigue life.’, Journal of Composites for Construction, ASCE, Vol.7(4), pp. 331–8. 7. Jun Deng, Marcus M.K Lee (2007), ‘Fatigue Performance of Metallic Beam strengthened with a bonded CFRP Plate’, Composite Structures, Vol, 78, pp. 222-231. 8. Massimiliano Bocciarelli, Pierluigi Colombi, Giulia Fava, Carlo Poggi(2009), ‘Fatigue performance of tensile steel members strengthened with CFRP plates’, Composite Structures, Vol. 87, pp. 334-343 9. Pierluigi Colombi, Giulia Fava(2012), ‘Fatigue Behaviour of Tensile Steel/CFRP Joints’, Composite Structures, Vol. 94, pp. 2407-2417.
  7. 7. International Journal of Civil Engineering and Technology (IJCIET), ISSN 0976 – 6308 (Print), ISSN 0976 – 6316(Online) Volume 5, Issue 3, March (2014), pp. 301-307 © IAEME 307 10. Shaat A, Fam A.(2006) ‘Axial loading tests on CFRP-retrofitted short and long HSS steel columns’, Canadian Journal of Civil EngineeringVol.33(4), pp 458–70. 11. Tavakkolizadeh M, Saadatmanesh H.(2003) ‘Strengthening of steel–concrete composite girders using carbon fibre reinforced polymer sheets’, Journal of Structural Engineering, ASCE, Vol. 129(1), pp. 30–40. 12. Yail J Kim, Kent A Harries(2011), ‘Fatigue Behaviour of Damaged Steel Beams Repaired with CFRP strips’, Engineering Structures, Vol. 33, pp. 1491-1502. 13. Javaid Ahmad and Dr. Javed Ahmad Bhat, “Ductility of Timber Beams Strengthened using CFRP Plates”, International Journal of Civil Engineering & Technology (IJCIET), Volume 4, Issue 5, 2013, pp. 42 - 54, ISSN Print: 0976 – 6308, ISSN Online: 0976 – 6316. 14. Adnan Ibrahim Abdullah, Dr. Muyasser M. Jomaa'h and Dr. Alya'a Abbas Al-Attar, “Flexural Behavior of Fiber Reinforced Concrete I- Beams Strengthened with (CFRP)”, International Journal of Civil Engineering & Technology (IJCIET), Volume 5, Issue 1, 2014, pp. 47 - 60, ISSN Print: 0976 – 6308, ISSN Online: 0976 – 6316. 15. Javaid Ahmad, Dr. Javed Ahmad Bhat and Umer Salam, “Behavior of Timber Beams Provided with Flexural as Well as Shear Reinforcement in the Form of CFRP Strips”, International Journal of Advanced Research in Engineering & Technology (IJARET), Volume 4, Issue 6, 2013, pp. 153 - 165, ISSN Print: 0976-6480, ISSN Online: 0976-6499. 16. Dr. Salim T. Yousif, “New Model of CFRP-Confined Circular Concrete Columns: Ann Approach”, International Journal of Civil Engineering & Technology (IJCIET), Volume 4, Issue 3, 2013, pp. 98 - 110, ISSN Print: 0976 – 6308, ISSN Online: 0976 – 6316.