Q690 High Strength Steel Research Articles

Behavior of randomly pitted Q690 high-strength steel H-section members under eccentric compression

Six Q690 high-strength steel H-section specimens with pits were tested under eccentric compression loads. The results showed that the pits changed the failure modes of the members under eccentric compression by inducing local buckling, which resulted in a significant reduction in the ultimate resistance, reaching 31.7 % when the volume loss ratio was 15 %. A numerical study was then conducted, which revealed that the ultimate resistance loss ratio varied with the volume loss ratio and the eccentricity ratio, but not with the slenderness ratio. Finally, the existing rules for determining the ultimate resistance were modified by using the equivalent residual thickness, and the applicability of the modification was assessed.

Local buckling behaviour of Q690 high strength steel press-braked elliptical hollow section stub columns: Testing, numerical modelling, and design

This paper presents comprehensive investigations on the local bucking behaviour of Q690 high strength steel (HSS) press-braked elliptical hollow section (EHS) stub columns subject to concentric compression experimentally. In comparison with conventional hot-finished EHS or cold-rolled EHS, the press-braked EHS provides high precision, great construction productivity and the fabricated geometries are not limited to production assembly instrumentation. HSS press-braked EHS fabricated through press-braking process was expected to exhibit distinct behaviour compared with their counterparts constructed from the conventional fabrication routes. Well-understanding of their performance at the levels of material, and cross-section level is essential. Material properties were obtained through tensile coupon tests. Residual stresses in both membrane and bending types were measured and recorded and their impact on structural performance was also assessed. Initial local geometric imperfection measurements were conducted for all the specimens. In conjunction with experimental studies, the finite element (FE) models of Q690 HSS press-braked EHS stub columns were developed. The accuracy of the FE model for HSS press-braked EHS stub column was asserted by comparing the numerical predictions with the tested results. An extensive parametric study was conducted to generate structural performance data with a wider range of cross-section slenderness to complement the test data bank. The cross-section classification for conventional CHS codified in existing standards and the proposed slenderness limits by various researchers were evaluated. The existing slenderness limits cannot be extended to HSS press-braked EHS investigated in this study, a new slenderness limit is proposed. Moreover, the advanced design approaches of Direct Strength Method (DSM) and Continuous Strength Method (CSM) were assessed for their applicability to the cross-section resistance predictions of the examined EHS stub columns.

Membrane Residual Stress of Q690 High-Strength Steel Welded H-Section

Membrane Residual Stress of Q690 High-Strength Steel Welded H-Section

Local buckling behaviour of Q690 high strength steel cold-formed round-ended oval hollow section stub columns under axial compression: Experimental investigation, numerical modelling and design

This study presents a comprehensive investigation of Q690 high strength steel (HSS) cold-formed round-ended oval hollow section (ROHS) stub columns subjected to concentric compression, utilising a combination of experimental and numerical analyses. Material properties were determined through tensile coupon tests. Residual stresses in both membrane and bending types were measured and recorded and their impact on structural behavior was evaluated. Initial local geometric imperfection measurements were conducted for all specimens. In addition to experimental studies, the numerical investigation was conducted to develop a finite element (FE) model of Q690 HSS cold-formed ROHS stub column. The accuracy of the FE model for HSS cold-formed ROHS stub column was validated through a comparison of the numerical predictions with the experimental test results. An extensive parametric study was conducted to produce structural performance data for HSS cold-formed ROHS with an expanded range of cross-sectional slenderness, aiming to enhance the existing data repository. The cross-sectional resistance of stub columns obtained from experimental and numerical investigations were compared with the design strengths predicted by the advanced design methods of Direct Strength Method (DSM) and Continuous Strength Method (CSM). The comparison indicate that the existing design methods provide over-conservative and unreliable cross-sectional resistance predictions. Modifications on DSM and CSM are proposed, based upon which notable improvements in cross-sectional resistance in predictions are obtained in a reliable manner.

Cyclic behavior of high-strength steel beam-to-column welded flange-bolted web connections

Four Q690 high-strength steel beam-to-column moment connections, which use welded joints between beam and column flanges as well as a bolted beam web to the column flange, were tested under cyclic loading. The effects of beam-to-column welding details and panel zone strength were studied. Two pairs of welded flange details were incorporated, including the Chinese code-specified complete-joint-penetration (CJP) welded connections with backing plates where the bottom backing plate is reinforced by a fillet weld, and enhanced CJP welded connections with backing plates removed, weld roots backgouged, and further reinforced by fillet welds. Three panel zone thicknesses were designed to characterize strong, intermediate and weak panel zones, respectively. The test results show that backing plates should be removed in these moment connections to prevent brittle weld fracture, but still rendering only limited plastic hinge rotations in the order of 0.01–0.02 rad in the beam end before ductile fracture of the beam flange. The panel zone, however, survived even after a plastic shear rotation of 0.03 rad, demonstrating much higher plastic deformation capacity than the beam plastic hinge. This suggests that ductility can be exploited in 690 MPa high-strength steel panel zones, but not reliably in 690 MPa steel beams.

Experimental and Numerical Study of Membrane Residual Stress in Q690 High-Strength Steel Welded Box Section Compressed Member.

High-strength steel (HSS) members with welded sections exhibit a notably lower residual compressive stress ratio compared with common mild steel (CMS) members. Despite this difference, current codes often generalize the findings from CMS members to HSS members, and the previous unified residual stress models are generally conservative. This study focuses on the membrane residual stress distribution in Q690 steel welded box sections. By leveraging experimental results, the influence of section sizes and welding parameters on membrane residual stress was delved into. A larger plate size correlates with a decrease in the residual compressive stress across the section, with a more pronounced reduction observed in adjacent plates. Additionally, augmenting the number of welding passes tends to diminish residual stresses across the section. Results showed that membrane residual stress adhered to the section's self-equilibrium, while the self-equilibrium in the plates was not a uniform pattern. A reliable residual stress simulation method for Q690 steel welded box sections was established using a three-dimensional thermal-elastic-plastic finite element model (3DTEFEM) grounded in experimental data. This method served as the cornerstone for parameter analysis in this study and set the stage for subsequent research. As a result, an accurate unified residual stress model for Q690 steel welded box sections was derived.

Residual behavior of UHPC-filled Q690 high-strength steel tubular members after axial impact

Structures may suffer from accidental impact loads during the service life. While most impacts will not destroy structures completely, quantifying their residual behavior after impact is crucial for evaluating the damage and then providing recommendations for repair or removal. This may greatly reduce the time and economic losses caused by impacts. Hence, this study experimentally and numerically investigated the residual behavior of UHPC-filled high-strength steel tube (UHPC-FHST) members subjected to axial impact. Parameters including impact energy, steel ratio, specimen length, and steel grade (Q690 and Q355) were carefully considered. After tests, different failure modes and axial load-displacement curves were obtained. The residual ratio (β), a ratio of the bearing capacity of a damaged specimen to that of a specimen without impact, was employed to assess the damage level of UHPC-FHST members after axial impact. Tests showed that UHPC-FHST members have excellent axial impact resistance, high residual strength, and high residual ratio, especially under low-energy impact conditions. β decreases rapidly and linearly with increasing nominal strain and then stabilizes after a certain value. A finite element (FE) model was thereafter established and benchmarked by the available test results, where two analysis steps were established to continuously simulate the impact and residual compression processes. The FE results demonstrated a great agreement with the tests, which could provide a reference for further investigations in related studies.

Numerical Study on High Strength Q690 Steel Flush End-Plate Connections at Elevated Temperatures

Numerical Study on High Strength Q690 Steel Flush End-Plate Connections at Elevated Temperatures

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.

Continuous damage model for structural steels and weld metals under ultra-low-cyclic loading

For the fracture problem of structural steel under strong earthquakes, a continuous damage model which comprehensively considers the influence of stress triaxiality and Lode parameter was proposed in this paper. According to the variation law of Mises stress under cyclic loading, this model decomposes the effective cumulative plastic strain into isotropic hardening and kinematic hardening components, assuming a linear proportional relationship between the fatigue damage induced by these two components. The basic mechanical properties and constitutive relationships of five common Chinese structural steels and four corresponding weld metals were investigated. The fracture failure tests for both notched round bar specimens and pure shear specimens subjected to monotonic tensile and ultra-low-cyclic loading were thoroughly examined on these materials. The continuous damage model parameters were calibrated by means of test results and complementary finite element simulations. Experimental and numerical simulation results indicate that Q690 high strength steel exhibits cyclic softening behavior, whereas the other steels show cyclic hardening characteristics. For pure shear specimens, plane strain specimens and shear energy dissipation cover plates, the continuous damage model was adopted to simulate and analyze the damage development and fracture failure process of the specimens. The predictions for crack initiation, crack propagation and fatigue life are consistent with experimental results, lending further credibility to the model. The calibrated continuous damage model of various steels provides an effective means for analyzing the ULCF fracture failure of structural steel under intense earthquakes.

Corrosion behavior and microstructure analysis of butt welds of Q690 high strength steel in simulated marine environment

Steel components that have served in the marine environment for extended periods often face challenges in preventing surface damage caused by corrosion protection measures. These challenges can ultimately diminish the expected service life of structures. To investigate the corrosion behavior and time-varying effects in butt welds of Q690 high strength steel in the ocean splash zone, a macroscopic morphology analysis was conducted using various corrosion times involving indoor saltwater immersion and hot-humid cycling accelerated corrosion. Quantitative analysis of corrosion pits' size and distribution range was performed in both the weld zone (WZ) and heat-affected zone (HAZ) to establish a relationship between roughness parameters and corrosion time. Experimental results were compared with corrosion parameters of regular steel. The findings reveal a gradual reduction in metallic luster with increasing corrosion time on specimen surfaces. Moreover, a greater accumulation of corrosion products is observed in the butt weld region, leading to progressive corrosion damage. High strength steel (HSS) exhibits superior corrosion resistance compared to ordinary steel. However, the corrosion damage of HSS weld joints gradually approaches that of ordinary steel in the later stage of corrosion. Microscopic scanning indicates that the accumulated corrosion layer effectively impedes the penetration of corrosive media into the matrix's interior, while the surface microstructure transitions from pitting to corrosion pits. These research findings significantly contribute to assessing the performance and analyzing the reliability of high strength steel in marine environments.

Numerical Investigation and Design Suggestion on Patch-Loading Strength of Q690 High-Strength Steel Plate Girders at Elevated Temperature

Numerical Investigation and Design Suggestion on Patch-Loading Strength of Q690 High-Strength Steel Plate Girders at Elevated Temperature

Axial compressive bearing capacity of high-strength concrete-filled Q690 square steel tubular stub column

Concrete-filled high-strength steel tubular (CFHSST) column is a kind of composite structure that uses high-strength steel instead of ordinary steel to strengthen the transverse restraint of concrete. In this work, in order to investigated the axial bearing capacity of CFHSST square column, a series of 22 CFHSST square stub columns were prepared by using two kinds of Q690 high-strength steel plates with different thickness of 3 mm and 6 mm with the strength of 741–749 MPa and 819–871 MPa, respectively. The static axial compression tests were carried out to understand the confinement effect of high-strength steel tube on concrete of different strength under static axial pressure and the influence of local buckling on the specimen. The axial compressive performances of CFHSST square stub columns are analyzed, including the failure mode, the relationship between load and deformation, and the stress-strain relationship. The test results show that the core concrete strength and area are major factors which have affected on bearing capacity of CFHSST square stub columns. Furthermore, width-to-thickness ratio exhibits significantly improving the failure mode and ductility of CFHSST square stub columns, while width-to-thickness ratio, the ratio of steel or the coefficient of hoop determine confinement effect. Q690 steel tube has good confinement effects, but the confinement effects on low strength concrete is small, especially for the specimen with relatively small width-to-thickness ratio. Finally, the calculation method of the ultimate bearing capacity of CFHSST square stub columns under axial compression is proposed, which can effectively evaluate the ultimate bearing capacity of CFHSST square stub columns.

Fire-resistance design of ultimate and serviceability limit states for Q690 high-strength steel plate girders under patch loading

This study numerically investigated the serviceability and ultimate limit states of Q690 high-strength steel (HSS) plate girders exposed to fire under patch loading. A shell element numerical model of the Q690 HSS plate girder was created using Abaqus software, and its accuracy was verified through experimental results. A comprehensive parametric analysis involving 512 numerical models was conducted to examine the effects of elevated temperatures, web aspect ratio, loading length, and web depth-to-thickness ratio. Because of the absence of specific design guidance in EN 1993–1-2 for Q690 HSS plate girders exposed to fire, the accuracy of the resistance model specified in EN 1993–1-5 that was applied to steel plate girders at room temperature was verified by employing the material properties of Q690 HSS at elevated temperature. The verification results revealed that the ultimate resistance of the Q690 HSS plate girder obtained from the resistance model in EN 1993–1-5 was considerably lower than that obtained from the numerical analysis. A modified resistance reduction function that applies to the Q690 HSS plate girder at room temperature was proposed based on the resistance model in EN 1993–1-5. Comparisons indicated that the accuracy of the resistance model in EN 1993–1-5 was improved when applied with the proposed resistance reduction function. Additionally, predictive approaches were introduced to determine the ultimate resistance, effective resistance, and effective stiffness of Q690 HSS plate girders in fire. The comparison with numerical results highlighted that the proposed approaches exhibited high accuracy and provided a sound basis for the engineering design of Q690 HSS plate girders.

Hysteretic performance of thin-walled Q690 high-strength steel H-section beam-columns bent about the weak-axis

High-strength steel has broad application prospects in steel structures as its weight to strength ratio is low, but the reduced ductility limits its application in seismic design. In order to evaluate the application potential of high-strength steel in light steel structures, four Q690 high-strength steel beam–columns with large width to thickness ratios were tested under cyclic loading about the weak axis. The failure mode, ultimate strength, hysteretic curve, stiffness degradation, ductility and energy dissipation capacity of the tested specimens are discussed. Subsequently, a finite element analysis model is established, and an extensive parametric study is carried out using the verified finite element model. The test results show that the failure mode is the local buckling of column base. The hysteretic behavior of the specimens is mainly affected by the axial load ratio and the flange width to thickness ratio. The results of the parametric study show that the extent of development of plasticity in the cross-section is related to the axial load ratio. The calculated ultimate strength of the beam–column shows that Eurocode 3 is conservative for the classification of the beam–column as having a Class 4 cross-section, members with Class 4 cross-sections should in fact be classified as having Class 3 cross-sections. Accordingly, a section classification for a high-strength steel beam–column subjected to weak axis bending is discussed.

Hysteretic energy dissipation capacity of dual-steel welded-plate preloaded bolted T-stubs

With the high- and ultrahigh-strength steels applied in the construction industry, the seismic safety of their structural components is very concerned due to the inferior material ductility of those steels. The current studies on bolted high-strength steel T-stubs mainly focus on their monotonic loading behavior. There is limited research work conducted to study their cyclic loading behavior. In this study, cyclic tests of bolted dual-steel T-stubs were conducted. Their T-elements were produced by Q690 high-strength steel flanges and Q355 normal-strength steel webs, and class 10.9 high-strength bolts with pretension were used to connect the groove-welded T-elements. Failure modes, force-deformation hysteretic and backbone curves, cumulative plastic deformation and energy dissipation capacities were tested and analyzed. The test results indicated that an optimal β ratio between the plastic resistances of Mode 1 (neglecting the influence of bolt action on a finite contact area) and Mode 3 in Eurocode 3 Part 1–8, could be 0.3 for those bolted T-stubs to maximize hysteretic energy dissipation. Accordingly, predictive formulas with good precision were proposed for the low-cycle fatigue failure criterion and hysteretic energy dissipation capacity of those bolted T-stubs, respectively.

Experimental and numerical studies on UHPC-filled Q690 high-strength steel tubular members subjected to axial impact

With the advances in high-performance materials and structures, the impact behavior of ultra-high-performance concrete (UHPC) filled Q690 high-strength steel tubular columns was systematically investigated in this study. A total of 19 axial impact tests on UHPC-filled steel tubular columns were conducted by a drop hammer test apparatus, experimentally obtaining the failure model, impact force and displacement time histories, and energy absorption. The main parameters, including steel strength, steel ratio, impact energy, and specimen length, were considered. The results showed that UHPC-filled Q690 high-strength steel tubular members exhibited excellent impact resistance and could withstand higher energy impacts. As the impact energy increased, the impact force also increased owing to the strain-rate effect, and the deformation was significantly enlarged. Increasing the steel ratio significantly enhanced the impact resistance and energy absorption efficiency of components; the use of high-strength steel and UHPC also improved the impact resistance and energy absorption; the effect of specimen length on the axial impact behavior was limited. Additionally, a finite element model considering the strain-rate effects of high-strength steel and UHPC was established and benchmarked with the test results from the present study and literature. The simulated results were in good agreement with the experimental results, which contributed to the relevant research on UHPC-filled steel tubular components subjected to axial impact.

A lode-dependent plasticity model for high-strength structural steel

In this study, a new plasticity model was developed to characterize the mechanical behavior of high-strength structural steel under multiaxial stress states. This model utilizes a Lode-dependent yield criterion in conjunction with a non-associated flow rule and isotropic hardening law to determine the evolution of the plastic flow stress. Further, a detailed numerical implementation algorithm for this plasticity model is presented, including the integration algorithm of the constitutive equations using an implicit elastic predictor-return mapping method and formulation of the consistent tangent modulus. To validate the proposed plasticity model, six types of Q690 high-strength steel specimens were tested under monotonic loading. These specimens can be used to obtain the elastoplastic response of the material under uniaxial tension, compression, pure shear, tensile shear, and plane strain state, respectively. Corresponding numerical simulations were performed for each specimen using ABAQUS/Standard with the proposed model being introduced into the analysis via the UMAT user subroutine. Then, the prediction performance of the proposed plasticity model was investigated by comparing the load-displacement curves of each test specimen obtained from both experiments and numerical simulations. It is shown that the proposed plasticity model can accurately predict the mechanical response of the high-strength steel under different stress states and enables to visualize the key stress state parameters and material strength variation at the failure region of each specimen, which confirms the promising prospect of this new model in practical applications.

Study on ultra low cycle fatigue fracture behavior of Q690D high-strength steel after fire exposure

In order to investigate the fracture behavior of Q690D steel high-strength steel after exposure to elevated temperature due to the fire, standard coupon specimen, notched coupon specimen and pure shear specimen representing various stress states were tested under monotonic loading and ultra low cycle fatigue loading. For air cooling condition, the tensile test results of Q690D steel show that when exposed to the temperature above 500 °C (up to 800 °C), the strength decreases while the elongation increases. These variation trends are basically the same under different stress states, and the predictive equations are proposed to determine the residual strength and ductility after exposure to high temperature. The effect of exposed temperature on the ductility under monotonic loading is significantly greater than that on the ultra low cycle fatigue life. In addition, as compared with air cooling, water cooling can significantly reduce the deformation capacity and fatigue life of the specimens, and even lead to a failure mode close to brittle fracture. The Lode parameter enhanced cyclic void growth model (LCVGM) parameters of Q690D steel after being exposed to various temperatures and air cooling were calibrated based on the fracture failure test results, and the post-fire LCVGM of Q690D high-strength steel was established to facilitate the fracture evaluation. Finally, the LCVGM was employed to predict the post-fire ultra low cycle fatigue life of Q690D steel notched plane strain specimen, and its estimation accuracy was verified by the comparison between tests and simulations. This fracture prediction model LCVGM can be recommended to evaluate the post-fire seismic performance of Q690 high-strength steel members.

Ultra low cycle fatigue behavior of Q690 high-strength steel welded T-stub joints: Experiments and fracture prediction analysis

The paper presented an experimental and numerical investigation of the ultra low cycle fatigue behavior of high-strength steel welded T-stub joints. The normal and funnel T-stub joints in Q690 high-strength steel were tested under monotonic tensile loading and ultra low cyclic loading. The cyclic response of normal T-stub joints varying from bolt diameter, flange size and weld form were studied. It is found that fracture mode of the T-stub joints is basically the same under tensile loading and cyclic loading, i.e., cracking behavior occurs at the weld toe of the flange and tensile-shear failure is observed. The different configuration parameters of normal T-stub joints in the study show no significant influence on the fatigue life of the joints. Comparing the ultra low cycle fatigue results between normal and funnel T-stub joints indicates that the funnel T-stub joints have better energy dissipation capacity. In addition, the ultra low cycle fatigue performance of Q690 steel and ER80-G weld metal under various stress states were analyzed and discussed. In order to numerically generate the overall fracture behavior of the T-stub joins, the Lode parameter enhanced cyclic void growth model was adopted in the numerical simulation and the model parameters were calibrated according to the test results of Q690 steel and weld metal. Comparison with the experimental monotonic and cyclic tests of welded T-stub joints shows that the Lode parameter enhanced cyclic void growth model can provide accurate predictions of the ultra low cycle fatigue of the joints.