| Nowadays,aluminum alloy has been used in the construction because of its environmental protection,corrosion resistance,light and other characteristics,and it is often used to construct large-span space structures such as landmark buildings.However,the aluminum alloy melting point is low,its post-fire mechanical properties cannot be ignored.Usually,the building structure is designed with redundancy.Even if the internal force of the structure is redistributed and will not completely collapse when a fire occurs,it is necessary to study the post-fire mechanical properties of building structure materials.At the same time,terrorist attacks such as the 911 aircraft crashing and the Urumqi Railway Station explosion in 2014 also threaten the overall stability of large public infrastructure buildings,especially for public buildings that have experienced fire but have not collapsed and are still in service.For basic buildings,it is of great engineering significance to study the dynamic post-fire mechanical properties of building materials.This thesis studies the dynamic post-fire mechanical properties and impact resistance of 6060-T6 aluminum alloy.First,6060-T6 aluminum alloy is under five temperatures and two cooling methods（Air cooling and spray cooling）,followed by a quasi-static tensile test to compare the mechanical properties of materials at different temperatures,then the lowest yield strength and tensile strength of the material at 450 ℃ are obtained,which is set as the post-fire group.There are lots of studies about the mechanical properties of the unfire 6060-T6 aluminum alloy specimens and the post-fire6060-T6 aluminum alloy specimens,which in different temperatures,strain rates and stress states.Through the analysis of pure shear and plane strain state,the ASCE fracture criterion is calibrated.Finally,a combination of experiment and numerical analysis was used to carry out the flat-nosed projectile impact target test,and the influence of the Lode angle and analyze the ballistic limit of the target’s fracture.This article mainly carried out the following work:1.The effect of different fire temperatures on the mechanical properties of6060-T6 aluminum alloy.A smooth round rod tensile test was carried out.Using the engineering stress and strain data obtained from the experiment,the residual coefficient of elastic modulus,yield strength,tensile strength and ductility were obtained.In order to improve the prediction of the mechanical properties of post-fire 6060-T6 aluminum alloy,the calculation formulas of the above-mentioned four residual coefficients with the temperatures are respectively fitted.The tensile fracture with a scanning electron microscope,found that the morphology change trend at the fracture is consistent with the calculation formulas.2.Dynamic constitutive model and fracture criterion of 6060-T6 aluminum alloy subjected to post-fire and unfire.Tensile tests at room temperature and high temperature and Hopkinson pressure bar tests were carried out on two groups of specimens of 6060-T6 aluminum alloy post-fire and unfire.Calibrated the MJC constitutive model and fracture criterion,ASCE fracture criterion parameters.The test found that when the temperature is higher than 250°C,the yield strength of the fired group specimens decreases with the temperature more than that of the unfire group,and there is obvious temperature softening.3.Research on the influence of Lode angle on fracture strainUsing the method of combining numerical simulation and experiment to carry out the research,the two sets of 6060-T6 aluminum alloy target plates of 2mm post-fire and unfire were carried out.Comparing the numerical simulation with the test results,the validity of the parameter calibration of the constitutive model and the fracture criterion is verified.Through the comparison results,it is found that the Lode angle has no obvious influence on the fracture criterion of the unfire group,but has a significant influence on the fracture criterion of the post-fire group. |
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