27-1high-temperaturepropertiesofsteelforfireresistanceok-外文文献(编辑修改稿)

27-1high-temperaturepropertiesofsteelforfireresistanceok-外文文献(编辑修改稿)内容摘要：

operties. Generally, for a typical beam with 2 h firerated protection, the heating rate of steel can vary between 3 and7 176。 C / min. However, for unprotected steel sections, the heatingrate can vary between 25 and 40176。 C / min. In the literature Outinen 2007 , the reported transient mechanical property tests wereconducted at heating rates ranging between 10 and 50176。 C / min.This heating rate can be suitable for unprotected steel members,but not for protected members with slow heating rates.On the other hand, the steadystate tests are generally fasterand easier to conduct than the transientstate tests. In the steadystate tests, the test specimen is heated to a specific temperatureFig. 3. Yield strength of steel as predicted by different models and asmeasured in different testsand after that a tensile test is carried out. Stress and strain valuesare recorded continuously under constant temperature. The testcan be either loadcontrolled loading rate is constant or strainFig. 4. Elastic modulus of steel as predicted by different models andas measured in different testsperatures for longer time duration, therefore, other factors, suchas hightemperature creep timedependent plastic strain undercontrolled strain rate is constantOutinen 2007。 Anderbergconstant stress and temperature , can influence the resulting1988 . Despite the fact that strain rate has a significant effect onthe test results, a large amount of test data on conventional steel ispublished without the information on strain rates. Therefore, teststandards are still concerned with defining limits for strain rates intests Outinen 2007。 Anderberg 1988。 Cooke 1988 .These variations in test methods resulted in variations in thereported mechanical properties, which in turn resulted in variations in the constitutive models specified in codes and standards.The following sections present parative study of these variations.Yield Strength and Elastic ModulusAs mentioned earlier, different test regimes were used to obtainyield strength and elastic modulus of steel at elevated temperatures. The variations in test parameters resulted in different testmeasurements, thereby leading to differences in constitutive relationships presented in different codes and standards. Generally,tensile strength tests are conducted to obtain elastic modulus andyield strength of steel. There is a lack of experiments on themodulus of steel under pression. This is because in tensilestrength tests, plications that may arise due to geometric instabilities and confinement of specimen is eliminated. However, itis generally assumed that the modulus of elasticity for steel, derived based on tensile strength tests, is the same for pressionstate.Fig. 3 and 4 show the yield strength and modulus of elasticityof steel as a function of temperature, respectively. The test dataplotted in the figures are piled from various hightemperatureproperty tests as shown on the figures. Both the yield strength andelastic modulus decrease as temperature increases. This decreasecan be attributed to the nucleus of the iron atoms in steel movingfarther apart due to rising temperature in steel, leading to decreased bond strength, which in turn reduces the yield strengthand elastic modulus.It can be seen in the figures that there is significant variation intest data on yield strength and modulus of elasticity at temperatures above 300176。 C. This variation can be attributed to many factors, primarily variable heating and strain/load rates during thetest. For example, when the heating rate of the stressed specimenis small, the specimen will be subjected to stress at elevated temtemperaturestressstrain curves of the tested specimen.The yield strength and elastic modulus constitutive relationships from the ASCE manual, Eurocode, and those proposed byPoh 2001 are also shown in Figs. 3 and 4, respectively. Thehightemperature reduction factors for the yield strength and elastic modulus of steel are also presented in the “Mechanical Properties” section of the Appendix for ASCE, EC3, and Poh.As is the case with test data, there is also a considerable variation in the constitutive models for yield strength and modulus ofelasticity. These variations in constitutive models are due to thelarge variation in the test data used to pile the respectiveconstitutive models. A review of the models shows that the Eurocode model predicts less reduction in yield strength of steel withtemperature as pared to the ASCE or Poh models. However,the Eurocode model provides a higher reduction in elastic modulus of steel with temperature as pared to the ASCE and Pohmodels as shown in Fig. 4. Also, the Eurocode model assumes noreduction in steel yield strength up to 400176。 C, while ASCE andPoh models assume a loss of 30 and 40%, respectively, at 400176。 C,as shown in Fig. 3.Fig. 3 also sho。