An Investigation On The Performance Of AFRP Strengthened Circular Steel Tubes Under Axial Compression And Impact

The use of fiber-reinforced polymers(FRP)such as carbon fiber-reinforced polymer(CFRP)and glass fiber-reinforced polymer(GFRP)as external confinement has recently become a very important technique in many industries.In the civil engineering and construction industry,the FRP has been mainly used for retrofitting/strengthening of concrete and steel structural elements.Also,it has been used for rehabilitation,upgrading,and repair of deteriorated structures.The FRP has become increasingly attractive to researchers and structural designers due to their high mechanical properties,easy installation,short construction time,no maintenance required after installation.Therefore,it is an excellent technique for improving structural performance while maintaining the aesthetic appearance.External strengthening using FRP composite significantly improves the performance of the structural elements by increasing their load-carrying capacity,buckling behavior,stiffness,and energy absorption performance.The common FRP used for strengthening was CFRP and GFRP,where aramid fiber-reinforced polymer(AFRP)was rarely taken into account.Based on the above background,the main objective of this research work is to carry out further studies to understand the behavior of AFRP retrofitting thin-walled circular hollow section(CHS)steel tubes.The aramid fiber used for this research is available under the commercial name of Kevlar 49.Many experiments have been conducted to find out the best and optimum wrapping schemes that can be applied for axial members and the influence of various parameters for improving the structural behavior.Also,to determine the feasibility of retrofitting using filament winding techniques rather than the hand lay-up technique.The filament winding being an automated process suitable for high-volume manufacturing resulting in simple,fast,and better cost production techniques.Experiments and simulations have been carried out on both static axial compression and axial impact load,to understand the enhancement in strength,ductility,and collapse characteristics of AFRP composites.The expected result is to complement existing knowledge in this area.The first part of this work consists of investigating the behavior of AFRP strengthened thin-walled steel tubes subjected to a static axial compressive load.The specimens were tested to study the structural performance enhancement due to the AFRP and also to investigate the effect of different parameters such as the AFRP layers thickness,the steel tube thickness,and the slenderness ratio.The load vs.displacement,load vs.strain relationships were measured and compared.Moreover,the load-carrying capacities,stiffness,and failure modes are discussed.It has been observed that the efficiency of AFRP strengthening is more distinct for specimens with a small slenderness ratio where the outer buckling is mostly to occur.On the other hand,the AFRP confinement wasn’t effective for specimens with a large slenderness ratio where overall buckling is the typical failure mode(Euler’s mode).Another part of the work focused on the determination of the energy absorption capacity of AFRP strengthened steel tubes under axial impact load.The specimens used share the same geometrical properties as those used in static axial compression except having a height of 100 mm.The specimens were subjected to an axial impact force using a mass of400 kg dropped from a height of 0.77 m and 1.53 m,resulting in an impact velocity of 3.88m/s and 5.47 m/s,respectively.The force vs.displacement and energy absorption capacity will be measured and compared to characterize the crushing capacity.Through research,it is found that AFRP strengthening contributed largely to improving the peak force,mean force,and energy dissipation capacity by providing the external confinement pressure in the hope.The enhancement effect mainly depends on the AFRP thickness.In addition,this research focuses on developing a 3D finite element(FE)constitutive model of AFRP strengthened CHS to investigate its structural behavior and failure modes.The model considers the geometric and material nonlinearity also includes AFRP damage and interlaminar failures of the bonded interface to provide an accurate simulation.ANSYS/Structural is used for static analysis and ANSYS/LS-DYNA for impact analysis.ACP(ANSYS Composite Prep/Post)tool was used to model the AFRP composite.Comparing the results of the numerical simulation to the experimental tests,it is verified that using proper parameters and material constituents,the FE model can predict with tolerable accuracy the load capacity and energy absorption,as well as the possibility to accurately simulate AFRP damage and contact interface failures.Finally,the development of an artificial neural network(ANN)offers a better alternative with a strong prediction capability.An ANN model is used to predict the load capacity,maximum stress,and strain of AFRP strengthened CHS steel tubes.Following,a parametric study and a sensitivity analysis were carried out to investigate the effect of different parameters on the load capacity.The successfully trained ANN is further used to predict new cases.The predicted results of the ANN models show a good correlation with the experimental and FEM results.The ANN is effective for predicting the load capacity.Such a method can be used to reduce computation time and labor.