Post-fire Mechanical Behavior Research Articles

Study on the deterioration of mechanical properties of stainless steel S35657 exposed to overall stage of fire

Fires often have destructive effects on steel structures, putting the safety of building structures at serious risk. As a new austenite stainless steel, grade S35657, its in-fire and post-fire mechanical behavior has not been reported, which limits its application. In this paper, a total 68 stainless steel S35657 coupons were designed and tested, with 34 coupons used for in-fire tests and 34 for post-fire tests. The effects of heating temperatures, loading rates, and heating soaking times on the mechanical properties were investigated. The failure modes, as well as the stress-strain curves and mechanical properties, were acquired. Electron microscopy scanning analysis were also conducted on the coupons to study the fracture mechanisms at elevated temperatures. The test results showed a rapid decline in in-fire mechanical properties when the temperature exceeded 600 °C. The yield strength of the coupon at 1000 °C is only 9.1 % of that at room temperature. As the tensile rate increased, the strength of the coupon increased gradually. For post-fire coupons, there is no significant change in the mechanical properties of coupons at temperatures ranging from 20 °C to 900 °C. However, after reaching a temperature of 1000 °C, the coupon experienced a reduction in yield strength (up to 27.8 %) and an expansion in elongation (up to18.0 %), compared to the coupon at ambient temperature. This related to the change in grain size of the coupons after exposure to high temperatures. The heating soaking time has an insignificant effect on the post-fire mechanical properties of stainless steel S35657. Existing constitutive models in the literature cannot be well applied to stainless steel S35657, new constitutive models to predict the in-fire and post-fire mechanical properties have been established. Moreover, reduction factors for the key mechanical properties were compared with existing values, and new unified equations for stainless steel S35657 were proposed. Finally, a reliability analysis was performed to evaluate the proposed design rules, and it was deemed that the proposed design rules were reliable. The findings of this study will provide a reference for engineers in evaluating the in-fire and post-fire residual strength of stainless steel S35657, and offer guidance for fire resistance design and high-temperature repair reinforcement.

Behavior of self-sensing masonry structures exposed to high temperatures and rehydration

High temperatures can compromise the structural integrity of structural masonry. Therefore, assessing the post-fire mechanical behavior is crucial for selecting suitable rehabilitation methods. Recent research has proposed the application of self-sensing cementitious composites for monitoring strains, stresses, and damage of structural masonry in room temperature. No previous research has been found on the self-sensing performance of fire-damaged masonry prisms exposed to elevated temperatures, with or without subsequent rehydration. Masonry prisms with integrated self-sensing joints and blocks were produced and exposed to different maximum temperature levels, followed or not by rehydration. Then, experimental tests for determination of compressive strength, modulus of elasticity, gauge factor (GF) and damage-detection performance were carried out. The exposure to 300 ºC caused a 7.4 % decrease in compressive strength and a 44.5 % reduction in elastic modulus. Rehydration methods had negligible effects in this case. After 600 ºC, substantial reductions of 42.2 % in compressive strength and 81.5 % in elastic modulus occurred. In this case, rehydration led to a 15.9 % increase in compressive strength and a 117.4 % increase in elastic modulus. Regardless of the exposure temperature, the predominant rupture mode initiated by vertical cracks originating at the block-mortar interfaces. After 300 ºC, slight GF increases occurred due to the thermal decomposition of hydrates of the cementitious matrix, while exposure to 600 ºC resulted in high GF increases due to partial oxidation of nanomaterials and substantial microcracking propagation. Post-fire curing improved the GF by sealing microcracks with nonconductive rehydration products and enhancing tunneling conduction mechanisms. The self-sensing prisms also demonstrated the ability for real-time damage detection and quantification, providing valuable predictive insights into the propagation of damage.

Post-fire mechanical behavior of iron-based shape memory alloy used for structural damping

This study sheds light on the post-fire mechanical behavior of Fe-SMA by conducting a series of tests at material level, with a particular focus on the low-cycle fatigue (LCF) performance. Three temperatures (500 °C, 750 °C, 1000 °C) and three cooling methods (air cooling, water spray quenching, water quenching) are considered to simulate different fire-cooling scenarios. After the heating-cooling cycle, 10 monotonic tensile specimens and 20 LCF specimens are tested. Essential material parameters for Manson-Coffin model and combined hardening model are also calibrated based on the experimental data. The failure mechanism and possible factors affecting the post-fire LCF performance of Fe-SMA are further explored by scanning electron microscope (SEM), metallographic observation (MO), electron backscattered diffraction (EBSD) and X-ray diffraction (XRD) analysis. Results show that Fe-SMA could exhibit even better LCF resistance after undergoing the considered fire-cooling procedure, especially those heated up to 750 °C and 1000 °C. Even under rapid cooling, no signs of brittle fracture are observed, which is superior to traditional structural steels. Different heating temperatures and cooling methods affect the grain sizes and microstructure of Fe-SMA, leading to variations in its post-fire LCF behavior.

Post-fire mechanical properties and constitutive model of Q690 high-strength structural steel

Fire hazard is one of the main threats to the integrity of steel structural buildings. Developing an accurate post-fire constitutive model for structural steel is crucial for enhancing the failure assessment of steel structures exposed to fire. In this study, the post-fire mechanical behavior of Q690 high-strength structural steel is investigated. Eighteen round bar specimens made of Q690 steel were heated with temperatures ranging from 20 °C to 900 °C. Tensile tests were conducted on these specimens to obtain the post-fire stress-strain curves and associated mechanical parameters of the material. To accurately characterize the post-fire behavior of Q690 steel, a temperature-dependent elastoplastic constitutive model combined with its numerical implementation algorithm is proposed. This model consists of four components: a temperature-dependent elasticity criterion, the von Mises yield criterion, an associated flow rule, and a temperature-dependent isotropic hardening law. The proposed constitutive model has a notable feature, i.e. the material parameters in the model can be defined as a function of temperature according to the test results. As a result, when utilizing this model for post-fire assessments of steel members, only three routine parameters, i.e. elastic modulus at room temperature, Poisson's ratio, and the temperature experienced by the material, are required in the numerical simulation to generate the material’s stress-strain relationships. To verify the proposed constitutive model, the new model was introduced into the numerical simulations of the tested round bar specimens after exposure to various temperature conditions. Both the load-displacement curves and local deformation of each round bar specimen obtained from the numerical simulations are compared with the test results to validate the accuracy of the new model. The constitutive model presented in this study provides a useful tool for assessing the post-fire behaviors of steel components.

Effects of high-temperature cooling and ultralow cyclic loading on mechanical behavior of Q450 weathering steel

The emphasis of this research is to investigate the mechanical behavior of Q450 weathering steel after high-temperature cooling and cyclic loading. A total of 174 steel specimens were tested in this study. The experiment consisted of three parts: heating and cooling tests, pre-cyclic loading tests (hysteretic tests) and tensile tests. These tensile specimens were first heated to nine target temperatures, and then cooled down to ambient temperature with water and firefighting foam. The effects of high temperatures and artificial cooling methods on the mechanical behavior of Q450 weathering steel were studied. According to the experimental results, the exposure temperature that caused a significant change in the mechanical behavior of Q450 weathering steel can be found. Meanwhile, the hysteretic behavior of Q450 weathering steel after being cooled from six target temperatures using water and firefighting foam was tested. Based on the post-fire mechanical behavior and hysteretic behavior of Q450 weathering steel, the damage degree of cyclic loading can be determined at each exposure temperature. Finally, the residual mechanical behavior of Q450 weathering steel after high-temperature cooling and cyclic loading was tested. According to the test results, hysteretic curves, skeleton curves and stress-strain curves were obtained. The mechanical properties such as elastic modulus, yield strength, ultimate strength and ductility were analyzed and discussed. To better understand the influence of high-temperature cooling and cyclic loading on the mechanical behavior, the macroscopic characteristics of Q450 weathering steel were observed. Moreover, polynomial equations and corresponding three-dimensional surfaces were proposed to predict the residual mechanical properties of Q450 weathering steel after high-temperature cooling and cyclic loading.

Effects of post-fire curing on the residual mechanical behavior of cement-lime masonry mortars

Cement-lime mortars have been largely employed in contemporary masonry structures. Although usually analyzed under high-temperature conditions, their behavior over the post-fire curing time is often neglected. This paper addresses an experimental program that evaluated the influence of post-fire curing on the residual mechanical behavior of masonry mortars. Three distinct mortar mixes were subjected to standard fire tests and exposed to ambient air at different post-fire curing ages. After fire exposure, mechanical tests assessed the stress–strain diagrams of fire-damaged mortars. According to the results, an increase in post-fire curing time reduced mechanical properties and increased deformability, mainly for mortars with high lime content. The mortars showed a progressive deterioration and a more ductile behavior throughout post-fire curing. The paper also introduces a theoretical model with some correlations that showed good agreement in predicting the post-fire mechanical behavior of masonry mortars.

Post-fire mechanical behavior of Q345 structural steel after repeated cooling from elevated temperatures with fire-extinguishing foam

This study aims to investigate the post-fire mechanical behavior of Q345 structural steel after being repeatedly cooled from elevated temperatures using fire-extinguishing foam. The Q345 structural steel specimens were first repeatedly cooled 1–3 times from target temperatures of 200 °C, 500 °C, 800 °C and 900 °C to ambient temperatures using fire-extinguishing foam, respectively. After being repeatedly heated and cooled, the Q345 structural steel specimens were then subjected to the uniaxial tensile tests to obtain their post-fire mechanical behavior. A Scanning Electron Microscope (SEM) was used to observe the tensile fracture surfaces of the representative specimens to analyze the different tensile fracture characteristics induced by repeated heating and cooling. Meanwhile, the changes in stress-strain curves, yield strength, ultimate tensile strength and ductility were discussed by comparing with the experimental data of Q345 structural steel in the as-received state. Furthermore, the changes in the macro-mechanical properties of Q345 structural steel after repeated heating and fire-extinguishing foam cooling were explained in combination with the fracture morphology and phase transformation. It is observed that Q345 structural steel was able to remain its initial mechanical properties unchanged after being repeatedly cooled from temperatures of 200 °C and 500 °C using fire-extinguishing foam. In contrast, significant reduction can be observed in the mechanical properties when Q345 structural steel was repeated cooled from temperatures of 800 °C and 900 °C using fire-extinguishing foam. The yield strength, ultimate strength and ductility significantly reduced with the increase in the number of heating and cooling cycles. When the Q345 structural steel was repeatedly cooling from 500 °C to 900 °C, the shape and size of ferrite grains increased sharply, whereas those of the pearlites decreased accordingly with the increase in the number of heating and cooling cycles.

Post-fire mechanical behavior and recovery of structural reinforced concrete beams

The results of fire tests obtained from small-sized specimens may fail to represent the real post fire behavior of structural elements. Therefore, in this study structural concrete beams were exposed to ISO fire curve and then subjected to different curing conditions to better understand the changes in concrete during the post-fire stage. Beams were tested under four-point bending to evaluate their residual mechanical performance, and scanning electron microscopy (SEM) was performed in order to monitor the changes in the morphology of concrete due to fire exposure. The results showed that high temperature exposure caused deterioration in the morphology and reduction in residual mechanical properties of concrete, while post-fire re-curing caused an improvement in residual mechanical properties and recovery in the microstructure.

Post-fire mechanical behavior of ASTM A572 steels subjected to high stress triaxialities

The goal of this paper is to understand the post-fire mechanical behavior of ASTM A572 Gr. 50 structural steels cooled from high temperatures using both air-cooling (slow cooling rate) and water-cooling (rapid cooling rate) methods under the influence of high stress triaxiality. In this study, various levels of stress triaxialities in uniaxial tension specimens are achieved by introducing notches in the test specimens. The uniaxial notched test specimens are subjected to temperatures ranging between 500 °C and 1000 °C at 100 °C intervals to simulate fire temperatures. The furnace-heated specimens are then either left outside the furnace for air-cooling or quenched in a water bath in the case of water-cooling method. Uniaxial tension tests are performed on both air and water cooled notched specimens to extract the post-fire mechanical properties. Non-linear finite element analyses are performed to obtain the triaxiality profiles along the critical cross sections of notched specimens by employing hardening curves obtained from post-fire uniaxial tension tests. Moreover, microstructures at selected temperatures are obtained to assess the microstructural changes that caused a significant change in post-fire mechanical properties. It is observed that ASTM A572 steels do no exhibit considerable change in mechanical properties when exposed to temperatures up to 600 °C, for both cooling methods. Beyond 600 °C, air-cooling resulted in a reduction in yield strength and ultimate tensile strength and increase in ductility of ASTM A572 steels. Water-cooling from temperatures beyond 600 °C increased the ultimate tensile strength and decreased the ductility of ASTM A572 steels due to the formation of the martensite phase. High stress triaxiality leads to the reduction in ductility and an increase in yield strength and ultimate tensile strength of ASTM A572 steels in both air-cooled and water-cooled conditions. The combined influence of high stress triaxiality and formation of martensite phase resulted in significant increase of up to 158% in yield strength and up to 172% increase in ultimate tensile strength of ASTM A572 steels that are water-cooled after exposure to temperatures beyond 800 °C. On the other hand, the presence of the high stress triaxiality and water-cooling resulted in up to 89% loss of ductility of ASTM A572 steels specimens that are exposed to temperatures beyond 800 °C.

Influence of fire on temperature gradient and physical-mechanical properties of macro-synthetic fiber reinforced concrete for tunnel linings

Abstract This study aims to evaluate the effect of fire on the physical, mechanical, and microstructural properties of macro-synthetic fiber reinforced concrete. Results demonstrated that internal heating rate is constant through time and is a function of the distance from the fire. Temperature gradient values are nonlinear, reduce at greater distances from the heated surface and with the duration of the fire. Microstructural analysis shows the severe dehydration of hydrated products, which negatively influences the mechanical properties of the composite. Above 205 °C the formation of nesosilicates is verified. Residual flexural strength values reduce by over 85% after the fire exposure due to deterioration of the reinforcing mechanism. Compressive strength and elastic modulus drastically reduce with temperature increase. The present study contributes to the absent literature on the post-fire mechanical behavior of MSFRC, suggests an alternative approach to the post-fire evaluation of tunnel structures, and aids in the definition of post-fire safety parameters.

Influence of stress concentration and cooling methods on post-fire mechanical behavior of ASTM A36 steels

This study aims to investigate the influence of stress triaxiality and cooling methods on post-fire mechanical behavior of ASTM A36 steels. To this end, ASTM A36 notched steel specimens are designed to generate a range of stress triaxialities. These specimens are subjected to target temperatures of 500 °C, 600 °C, 700 °C, 800 °C, 900 °C and 1000 °C, and then cooled down to room temperature using air-cooling and water-cooling methods. These specimens are then uniaxially tested to determine their post-fire mechanical properties. Non-linear finite element analysis is conducted using post-fire mechanical properties to obtain stress triaxiality distribution in notched test specimens subjected to different target temperatures and cooling methods. Finally, a Scanning Electron Microscope (SEM) study is conducted on fractured surfaces of representative un-notched and notched test specimens to investigate the influence of high stress triaxiality and cooling methods on fracture initiation and propagation mechanisms. The post-fire mechanical properties of ASTM A36 steels are found to remain almost unaffected when cooled from 600 °C, irrespective of cooling method. ASTM A36 steels experienced up to 14% degradation in ultimate tensile strength and up to 22% increase in fracture strain when air-cooled from temperatures beyond 700 °C. Post-fire ultimate tensile strength is observed to increase by up to 146% whereas fracture strain is observed to decrease by up to 76% when ASTM A36 specimens are water-cooled from high temperatures. High stress triaxiality resulted in up to 37% increase in ultimate tensile strength and up to 74% reduction in ductility of air-cooled specimens. Presence of high stress triaxiality and water-cooling from temperatures beyond 700 °C is observed to significantly increase the ultimate tensile strength (up to 252%) and substantially reduced the ductility (up to 98%) of ASTM A36 steels.