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Composite structure of steel and shape memory alloy for fire resistance design
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posted on 21.02.2017by Fang, Han
Steel is one of the most widely used construction materials in buildings which may be subject to fire. In fire, the strength of steel drops significantly with increasing temperature and structural failures may occur. In this research study, an innovative approach to strengthen the steel beams in fire using shape memory alloys (SMAs) was investigated. The prestrained SMAs have the unique ability to generate large recovery stress with increasing temperatures which can be taken advantage of to reduce the internal bending action of a steel beam by installing SMAs to form a steel-SMA composite beam. Thus, the steel beam can sustain loads even though the load bearing capacity of steel decreases in fire, resulting in higher fire resistance capacity in the steel beam. In order to obtain a robust design of steel-SMA composite beam and determine the enhancement of fire resistance capacity of steel beams using SMAs, three stages of research study were performed.
Stage one is focused on the characterisation of the thermal properties of steel which are needed for the determination of the behaviour of both bare steel and steel-SMA composite beam at high temperatures. Both specific heat capacity and emissivity of steel were investigated. The effect of heating rate on these properties was examined since both the bare steel and steel-SMA composite beams are subject to heating at varying rates in fire. Through the kinetic analysis using existing data, the specific heat capacity of steel during phase transformation stage at high temperatures was found to be dependent on the heating rate. A kinetic model was developed for estimating the specific heat capacity at varying heating rates. A simplified method with validated accuracy was also proposed to simplify the calculation of the specific heat capacity for fire resistance design. The emissivity of steel was measured experimentally and found to be both temperature and heating rate dependent. The emissivity increased significantly at high temperatures when thermal oxidation occurred at the surface of steel. During the stage of thermal oxidation, the emissivity of steel increased at a higher rate under a lower heating rate. A model based on the kinetics of thermal oxidation was also proposed in order to accurately estimate the emissivity of steel for determining the behaviour of both bare steel and steel-SMA beams at high temperatures.
Stage two consists of the characterisation of the thermophysical and thermomechanical properties of NiTi shape memory alloys which can generate large recovery stress being used in the current research. Since the recovery stress generated in SMAs is due to the phase transformation from martensite phase to austenite phase upon heating, the start and finish temperatures of the phase transformation needed for predicting the behaviour of steel-SMA composite beams subject to heating were measured under both constant stress and stress-free condition at varying heating rates. From the results, the phase transformation temperatures were found to be significantly sensitive to the heating/cooling rate. The phase transformation temperatures were lower during heating under a lower heating rate but higher during cooling under a lower cooling rate. The effect of heating rate on the recovery stress was also found. The recovery stress in the NiTi alloy heated under a lower rate was much greater at any given temperature than that under a higher rate. In addition, the specific heat capacity needed for determining the temperature of the NiTi alloys subject to heating was investigated experimentally at varying rates. The specific heat capacity was found to be nearly constant when the alloys were in complete martensite or austenite phase but dependent on heating/cooling rates during phase transformation. A kinetic model for estimating the specific heat capacity taking into account the heating/cooling rate effect was also developed for the design and analysis of steel-SMA composite beams at high temperatures.
The final stage investigated the fire resistance capacity of full-size bare steel and steel-SMA composite beams in both experiments and finite element modelling which are compared in order to determine the enhancement of fire resistance capacity of steel beams using NiTi shape memory alloys. In the experimental investigation, the bare steel beams lost their structural stability at high temperatures. Compared with the bare steel beam, the steel-SMA composite beam had smaller deflections than the bare steel beam when loaded at the same level, indicating that the effective reduction of the bending moment in the steel beam by the generation of recovery stress. The steel-SMA composite beams also had much better structural stability and longer fire endurance period than those of the bare steel beams. Therefore, the fire resistance capacity of steel beams is significantly improved by using shape memory alloys. Structural models for both bare steel and steel-SMA composite beams were also developed in the finite element modelling using ABAQUS. The methodology and material properties for estimating the thermomechanical behaviours of the structures in finite element analysis were determined. The results of the calculated structural behaviours including the deflection, failure mode and fire endurance period agreed well with those observed in experiments for both bare steel and steel-SMA composite beams loaded at different levels. Therefore, the models and methodology can be applied to predict the performance of any design of steel-SMA composite beam at high temperatures.
The findings in this research give new insights into the material properties of both steel and NiTi shape memory alloys. The heating/cooling rate effect must be taken into consideration to obtain accurate design for steel structures with or without shape memory alloys used under heating at varying rates. The innovative approach to enhance the fire resistance capacity of steel structures using shape memory alloys has also been shown to be effective. This improvement is significant so that building structures will be able to not only sustain for longer time but also maintain their stability in the early stage of fire in order for the occupants to escape the buildings.