تجزیه و تحلیل شکست و رفتار سازه تیرهای I فولادی تقویت شده CFRP

This paper reports the experimental and numerical investigations on the Carbon Fibre Reinforced Polymer (CFRP) failure analysis and structural behaviour of the CFRP flexural strengthened steel I-beams. Understanding the CFRP failure modes is useful to find solutions for preventing or retarding the failures. One non-strengthened control beam and twelve strengthened beams using different types and dimensions of CFRP strips in both experimental test and simulation modelling studies were investigated. In the experimental test, four-point bending method with static gradual loading was applied. To simulate the specimens, the ANSYS software in full three dimensional (3D) modelling case and non-linear analysis method was utilized. The results show the CFRP failure modes used in flexural strengthening of steel I-beams include below point load splitting (BS), below point load debonding (BD), end delamination (EDL), and end debonding (ED). The occurrences and sequences of CFRP failure modes depended on the strengthening schedule. The structural performance of the CFRP strengthened steel beams also varied according to the strengthening specifications investigated in this research.

The use of Fibre Reinforced Polymer (FRP) for strengthening of steel structures has gained significant interests recently. There are various methods used for the strengthening of steel structures. Amongst them are the application of additional steel parts, external pre-stressing of parts, and reducing or bridging the gap between the supports. These methods require considerable time and cost. In contrast, FRP is high in strength, light in weight, strong resistance to corrosion, and suitable for upgrading of steel structures. Normally, the FRP for flexural strengthening are installed to the bottom (tensile) flange. Edberg et al. [1] presented an experimental study in which five different configurations of glass (GFRP) and carbon (CFRP) fibre reinforced polymers were attached to the tensile flange of small scale steel wide flange beams using adhesive bonding. Also, a similar study was carried out by Ammar [2]. In addition, Tavakkolizadeh and Saadatmanesh [3] tested small scale steel beams in four-point bending. All these aforementioned researches showed that it is feasible to flexural strengthen steel beams using CFRP plates. Identification of CFRP failure modes in flexural strengthening of steel I-beams is useful in order to overcome or retard these failures. Deng et al. [4] had highlighted an important feature of the reinforced steel beam which is the significant stress intensity on the adhesive at the tip of the CFRP plate due to discontinuity by the abrupt termination of the CFRP plate. Buyukozturk et al. [5] reviewed their achievements in the strengthening of both reinforced concrete and steel members. They concluded that failures of FRP flexural strengthened reinforced concrete (RC) and steel members occur due to different mechanisms, and it is dependent on the parameters of strengthening. They found that shear failure takes place when the shear capacity of the beam is not able to accommodate the increment of the flexural capacity due to flexural strengthening. They indicated that the following are the failure modes of an FRP strengthened steel member: (a) buckling of top flange in compression, (b) buckling of web in shear, (c) FRP rupture, and (d) FRP debonding. Schnerch et al. [6] and [7] investigated the flexural strengthening of steel structures and bridges by using FRP materials. They indicated that the bonding behaviour of FRP to steel structures completely different from concrete structures in terms of failure modes. The test results also indicated that for steel structures and bridges, very high bonding stresses had occurred. Al-Emrani and Kliger [8] examined different types of fracture mode by testing composite elements with different combinations of CFRP-laminates and adhesives. The effect of various material parameters on the behaviour and strength of bonded steel–CFRP elements was examined. The delamination failure of steel beams flexurally strengthened by externally bonded FRP was presented by Colombi [9]. He used the simplified fracture mechanics based approach to investigate the edge delamination of the reinforcement strips. Benachour et al. [10] developed a closed-form rigorous solution for interfacial stress in simply supported beams strengthened with bonded prestressed FRP plates. The results show that high concentration of both shear and normal stresses occurred at the ends of the laminate which can result in premature failure of the strengthened specimens at these locations. Czaderski and Rabinovitch [11] investigated the displacements between the steel beam and the FRP plate. These displacements resulted from the interaction between the steel surfaces and FRP plates. One of the findings was that the slip values calculated with the shear stress–slip approach are notably different from the ones measured experimentally and determined by the FE model. The objective of this research is to investigate the CFRP failure modes used in strengthening of steel I-beams. Also, the effects of the strengthening schedule on some structural behaviour of steel beams such as load bearing capacity, deformations, and strain in different regions will be investigated. Different types and dimensions of CFRP strips are chosen, and both numerical simulation and experimental test studies are employed.

This study shows the CFRP failure modes in strengthening unrestrained steel I-beams include: (a) below point load-splitting (BS), (b) below point load-debonding (BD), (c) end-delamination (EDL), and (d) end-debonding (ED). Lateral deformation caused that the CFRP strips could not carry the load in full-capacity due to splitting. Application of shorter CFRP strips caused premature end-debonding, and applying longer CFRP plates increased the resistance against end-debonding (ED). By increasing the thickness of CFRP plate, below point load-splitting (BS) was overcame. Also, applying too thick CFRP plate caused premature debonding. Load bearing capacity of the strengthened steel beams increased by increasing the thickness and length of plate. Using CFRP length longer than the CFRP effective length is not effective as similar increment in load bearing capacity was obtained. Using higher Module of Elasticity further improved the load bearing capacity. Applying CFRP on the steel beam decreased the vertical deflection significantly. High strain occurred on CFRP below the point loads and the CFRP tips while this behaviour was not the same before and after yielding. For higher loads, the thickness of CFRP plate has more influence on the reduction of strain on the CFRP plate, and for lower loads, the Modulus of Elasticity plays an important role. By increasing the length of the CFRP plate, the strain on adhesive decreased while strain intensity at the CFRP plate tip increased. In addition, using much longer plates indicated more reduction of strain at the CFRP tip compared to the shorter CFRP plate lengths.