Post-fire mechanical properties of Q620 high-strength steel with different cooling methods

Tension and compression tests, transverse to and in the direction of plastic prestretching, indicate that cold stretching has pronounced effects on the mechanical properties of structural sheet steels. The effects are strongly directional and different for various types of structural steels. Increases in yield strength and ultimate strength and decreases in ductility are approximately proportional to the amount of prior cold stretching. The Bauschinger effect is present in the direction of prestretching whereas an inverse Bauschinger effect exists in the direction transverse to that of prestretching. Strain aging increases the yield strength and ultimate strength, decreases the ductility, and converts the stress-strain curve from gradual to sharp yielding. Current concepts of the crystalline nature of metals and dislocation theories are used to examine experimental observation regarding strain hardening, the Bauschinger effect, and strain aging.

Closure of "Effects of Cold-Straining on Structural Sheet Steels"

Post-fire mechanical properties of titanium–clad bimetallic steel in different cooling approaches

Bimetallic steel bar (BSB) consisting of S30408 stainless steel (cladding layer) and HRB400 carbon steel bar (substrate) has outstanding durability, indicating its suitability for RC structures in corrosive environments. The effects of fire on the mechanical properties of BSB in relation to its metallographic structure (MS) were investigated experimentally by exposing BSB specimens to different elevated temperatures and cooling methods. When the exposure temperature (ET) was lower than the austenite transformation temperature, the changes in the MS were relatively small. When the ET was higher than 700°C, lamellar pearlite and lath martensite were formed in the substrate of BSB specimens with cooling in air (CIA) and cooling in water (CIW), respectively, which led to changes in the mechanical properties. The hardness of the BSB specimens also was affected by the ET and cooling method. There always was a yield plateau in the stress–strain curve of the BSB specimens with CIA. When the ET ranged from 700°C to 900°C, the formation of granular pearlite improved the homogeneity of cementite in ferrite, which enhanced the ductility. For BSB specimens with CIW, when the ETs were 800°C and 900°C, the yield plateau in the stress–strain curve disappeared because of the lath martensite in the substrate. Ductile dimples were observed in the scanning electron microscope images of the BSB specimens with CIA. However, for BSB specimens exposed to 900°C with CIW, the ductile dimples were almost negligible. 

Relation between the Metallographic Structure and Mechanical Properties of a Bimetallic Steel Bar after Fire

Fire is a frequent disaster, which has lasting effects on mechanical properties of high-strength steel (HSS). For HSS structures after fire, it is important to assess if the structure can continue to service, should be repaired or demolished. The experimental and numerical investigations are performed to investigate the effects of exposure temperature on residual resistance of Q690 HSS plate girder considering the post-buckling capacity. In this study, the post-fire mechanical properties of Q690 HSS are investigated experimentally. The appearance characteristics of Q690 HSS with different exposure temperatures are obtained. The effects of exposure temperature on mechanical properties of Q690 HSS were discussed. To investigate the residual resistance of Q690 HSS plate girder after fire, the post-fire mechanical properties of Q690 HSS are introduced in the plate girder model, which has been strictly validated. Based on numerical results, the effect of exposure temperature on the initial stiffness of HSS plate girder can be ignored. The nonlinear properties of the Q690 HSS have the certain effect on the nonlinear characteristics of resistant curve of Q690 HSS plate girder considering the post-buckling capacity, which are influenced by the exposure temperature. The effect of exposure temperature on the buckling resistance of Q690 HSS plate girder is relatively small. With the raise of exposure temperature, the postbuckling capacity of Q690 HSS plate girder decreases. Effects of exposure temperature on the resistant properties of Q690 HSS plate girder were investigated, which were quantified by a suggested equation. The increase of exposure temperature has an adverse effect on the proportion of safety reserve of the Q690 HSS plate girder with post-buckling capacity. 

Post-fire performance of high-strength steel plate girders developing post-buckling capacity

Cold-formed steels are commonly used in construction applications. However, there is a lack of understanding on its post-fire mechanical properties. This paper presents an experimental investigation on post-fire mechanical properties of cold-formed steels. The test specimens were cut from flat portion and corners of cold-formed channel sections, which were exposed to temperatures ranging from ambient temperature to 800 °C, and then cooled with water and air. The specimens are of grade Q235, with section thicknesses of 1 mm and 2 mm. The stress-strain curves and mechanical properties of the specimens were obtained from tensile coupon tests. Test results of post-fire mechanical properties are presented. Moreover, evolution of microstructure and fracture morphology of specimens with different cooling methods were examined. Finally, predictive equations are proposed for evaluating the post-fire mechanical properties of Q235 cold-formed steel channel sections.

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Chong Ren

Papers

Experimental investigation of post-fire mechanical properties of Q235 cold-formed steel

Post-fire mechanical properties of corroded grade D36 marine steel

Buckling analyses of cold-formed steel Sigma sections in purlin-sheeting systems subjected to uniformly distributed uplift loading