S690 High Strength Research Articles

Investigations into mechanical properties of 50 and 70 mm thick high strength S690 butt-welded sections

A comprehensive investigation into the mechanical properties of 50 and 70 mm thick high strength S690 steel plates and their butt-welded sections under tension was presented in this paper, and a total of 40 tensile tests were conducted. Firstly, a total of 18 proportional coupons with circular cross-sections were extracted at three different layers within the plate thicknesses. Tensile tests on all of these coupons were carried out to obtain their mechanical properties, and their variations across the plate thicknesses were also examined. Secondly, submerged arc welding was adopted to prepare butt-welded sections between these thick steel plates with various heat input energy. Micrographic examinations on typical heat-affected zones of the welded sections were also performed. Standard tensile tests on a total of 22 proportional coupons with rectangular cross-sections were carried out to obtain their mechanical properties, and the full range deformation characteristics of these coupons were assessed and compared, in particular, their tensile strengths and elongations at fracture. This paper presents important experimental evidence on the mechanical properties of these high strength steel plates of practical thicknesses in construction, and also of their butt-welded sections. It is demonstrated that there is little or even no reduction in their mechanical properties of these butt-welded sections, provided that the welding processes are properly controlled according to established welding practice. Hence, full strength butt-welded sections between these thick high strength S690 steel are readily achieved in practice, similar to those of the commonly adopted S355 steel. These important findings are contrary to general understanding as many researchers and engineers consider that the mechanical properties of these high strength S690 steel are reduced significantly after welding, irrespective of their plate thicknesses.

Experimental investigations into stocky composite columns of concrete-filled circular S690 steel tubes under compression

Composite columns of concrete-filled steel tubes (CFSTs) are very efficient in providing large structural resistances and rigidities. However, high strength S690 steel tubes are not covered in current design codes. Therefore, a series of experimental investigations into stocky composite columns with circular CFSTs under compression were conducted. A total of 21 stocky composite columns with different tube diameters and thicknesses made of Grades S355, S460 and S690 steel and Grade C60/75 concrete were tested under different loading conditions. Moreover, extensive measurements on the surface strains of the steel tubes were made during tests. Data analyses on the test results reveal that all composite columns exhibit enhanced section resistances and ductility at large deformations, indicating that the use of S690 steel and C60/75 concrete in these composite columns are highly efficient. A simple method for determining confining pressures provided by the steel tubes to the concrete cores in these composite columns is proposed, which involves a systematic analysis on the measured data and the use of von Mises yielding criterion without any complicated plasticity formulation. Based on the proposed method, the resistance contributions of both the steel tubes and the concrete cores are successfully quantified. Comparison between the measured and the predicted resistances using EN 1994–1-1 of these composite columns reveals that while the section resistances of these composite columns are safely predicted, the resistances of the steel tubes are underpredicted while those of the concrete cores are overpredicted correspondingly. The findings highlight the need to improve the predictability of the current design rules, and to give increased resistances of the steel tubes when high strength S690 steel tubes are used.

Improvement of CSM for high-strength S690 and S960 unlipped channel columns, beams and beam-columns

Existing design methods that are mainly based on cross-section classification usually predict conservative resistances for high strength steel (HSS) singly symmetric unlipped channel section members. This paper conducted numerical studies on the resistance of cold-formed S690 and S960 HSS channel section members under different loading types. Numerical simulations were firstly carried out to establish suitable finite element (FE) models, and then applied to parametric studies. The obtained numerical results were adopted to assess and modify the continuous strength method (CSM) for the design of cold-formed HSS unlipped channel section members under axial compression or bending loads. An improved beam-column design method of unlipped channel sections was proposed through providing a new linear design curve incorporating the modified CSM formulae, then to evaluate the newly method and the M - N interaction curves specified in existing standards, finally the reliabilities of proposed method were confirmed.

Experimental Study on Seismic Fatigue Capacity of High- and Mild-Strength Structural Steels with and without Corrosion

The test results on hourglass specimens of steel under repetitive sine loads provide graphs that indicate the stress range in terms of the number of cycles to failure and are known as S-N curves. Using this curve, it is determined that if the applied stress is less than a certain level, failure will not occur as the number of load cycles increases. The S-N curve can be affected by several factors such as yield stress, temperature, surface properties, and corrosion. In this research, the S-N curve has been investigated for two types of high-strength steels, S690 and S460, as well as two types of mild-strength steels, S235 and S355, at 25°C, and S355 with corrosion. The numbers of samples used for S235 and S460 steels were 45 each while S355 and S690 steels were 36 each and for S355 with corrosion was 15 with the high cycle fatigue curve obtained for them. To investigate the effect of plate thickness on the high cycle fatigue of the samples, four sets of 24-piece S235 steel samples, being 96 samples in total, were made of plates with different thicknesses of 8, 12, 15, and 20 mm and tested. Finally, a four-story three-span steel moment frame was designed, and under the Northridge earthquake record, the high cycle fatigue was investigated. It was observed that the high cycle fatigue was not effective for the mentioned structure under the Northridge earthquake record, but in the corroded structure, damage from high cycle fatigue occurs under this record.

Advanced numerical simulations on S690 cold-formed square hollow sections under compression

Based on a number of numerical investigations into welding simulations, highly non-linear welding-induced residual stresses within high strength S690 welded H-sections have been evaluated successfully, and surface residual stresses along their flange / web junctions have been carefully calibrated against measured data. This modelling technique has been extended to cover residual stresses of cold-formed circular hollow sections recently, and also of cold-formed square hollow sections as described in this paper. Hence, a full presentation on the proposed approach of integrated and coordinated numerical simulations on various mechanical, heat transfer and thermomechanical, and stresses analyses on these cold-formed square hollow sections is described. Subsequent structural analyses of these models on both stocky and slender columns of these cold-formed square hollow sections are then carried out. Structural adequacy of the proposed approach is thoroughly demonstrated through a systematic calibration against various measured data – both temperatures and residual stresses of these sections induced during fabrication, and load-deformation curves of these sections under compression. It should be noted that the proposed approach provides a scientific mechanism to determine directly residual stresses due to transverse bending and longitudinal welding as initial materials imperfections in these high strength S690 cold-formed square hollow sections. With an adoption of the lowest eigenmodes of these columns as their initial geometric imperfections after adjusted with measured or design values of the member out-of-straightness, both the effects of initial materials and of initial geometric imperfections onto the structural performance of these columns are readily quantified separately.

Integrated and coordinated numerical simulations on fabrication processes of high strength S690 cold-formed circular hollow sections

In order to simulate transverse bending and longitudinal welding in fabricating high strength S690 cold-formed circular hollow sections, and more importantly, effects due to these fabrication processes on structural behaviour of these sections under axial compression, an integrated and coordinated numerical modelling approach using coupled analyses is proposed. Details of these mechanical, thermal, thermomechanical and structural analyses of these sections under various boundary and loading conditions imposed during fabrication are presented. Two-dimensional models with large deformation plane strain elements are employed to perform transverse bending while two compatible three-dimensional models with heat transfer elements and stress elements are employed to simulate sequentially coupled heat transfer and thermomechanical analyses to simulate longitudinal welding. These fabrication-induced residual stresses and strains are then incorporated as initial material imperfections into the third compatible three-dimensional model with stress elements for structural analyses under axial compression. These models have been carefully calibrated against data obtained from various stages of fabrication and also from compression tests. The proposed modelling approach is one of the few attempts available in the literature that develops a series of integrated and coordinated models to examine the structural behaviour of high strength S690 cold-formed circular hollow sections under axial compression with a direct simulation on various fabrication processes without the use of simplified residual stress patterns.

Compression tests on stocky and slender columns of high strength S690 cold-formed square tubes

An experimental investigation into the structural behaviour of both stocky and slender columns of high strength S690 cold-formed square tubes (CFST) under compression is presented in this paper. A total of 8 stocky columns with two section types of four different cross-sectional dimensions were tested. Moreover, a total of 24 slender columns with two section types of six different cross-sectional dimensions were tested. Typical residual stresses of these two section types were also obtained. As expected, all of these columns of S690 CFST failed under compression similar to those stocky and slender columns of S355 CFST, namely, i) local plate buckling in stocky columns, and ii) overall axial buckling in slender columns. Hence, these tests may be regarded to be confirmatory tests to provide experimental evidence and test data on cross-section resistances of the stocky columns, and member resistances of the slender columns of these S690 CFST. In addition, the measured resistances of these columns were directly compared with those design resistances determined according to Structural Eurocode EN 1993-1-1 based on measured geometrical dimensions and mechanical properties. It is demonstrated that the measured resistances of these columns are significantly larger than those design values, and hence, these test data provide a scientific basis to enhance structural efficiency of the design rules.

Experimental investigation into high strength S690 cold-formed circular hollow sections under compression

This paper presents an experimental investigation into the structural behaviour of both stocky and slender columns of high strength S690 cold-formed circular hollow sections (CFCHS) under compression. CFCHS are readily fabricated with cold-bending and welding on S690 steel plates, and these CFCHS are considered to behave similarly to those of hot-finished circular hollow sections. A total of 16 columns of S690 CFCHS were tested under compression, and these included eight stocky columns and eight slender columns with four different cross-section dimensions were tested. Tensile tests on typical curved coupons were also conducted to determine mechanical properties of these CFCHS while the sectioning method was employed to measure their residual stresses. All the stocky columns were found to have failed in material yielding in the sections while all the slender columns were found to have failed in overall buckling together with apparent plastic local buckling in the sections.The measured resistances of these columns of S690 CFCHS are then compared with those predicted resistances according to the current design rules given in EN1993–1. It is demonstrated that these design rules are effective to predict cross-section resistances of those stocky columns, and rather conservative to predict member resistances of those slender columns. The test results presented in the experimental investigation provide scientific data on their resistances for subsequent numerical investigations and design development for effective design rules.

Prediction of residual stresses in high-strength S690 cold-formed square hollow sections using integrated numerical simulations

In general, residual stresses are induced in structural members during various fabrication processes, such as welding, bending, press-braking, folding, flame cutting and punching. The presence of those residual stresses in cold-formed square hollow sections (CFSHS) primarily caused by i) transverse bending (or cold-forming), and ii) longitudinal welding, is widely considered to have modified both initial stress and strain conditions of these steel members significantly. Hence, these residual stresses are widely considered to have significant adverse effects onto the structural performance of these CFSHS under various actions.In order to examine and quantify both magnitudes and distributions of these residual stresses in S690 CFSHS, an investigation is undertaken to measure residual stresses due to transverse bending and longitudinal welding. Moreover, an approach of integrated numerical simulations is adopted in which a total of three co-ordinated finite element models are established together with various element types and block data transfers to generate compatible meshes for the following three analyses: i) two-dimensional plane-strain bending analyses with large plastic deformations and springback, ii) three-dimensional heat transfer analyses for transient temperature distributions under a heat source, and iii) three-dimensional thermomechanical analyses for welding-induced residual stresses.The predicted results of these three analyses have been carefully calibrated against various experimental data, such as surface temperatures during welding, and residual stresses and strains after welding. Good comparisons between the measured and the predicted data of these S690 CFSHS are demonstrated. The complete residual stress distributions within typical S690 CFSHS due to transverse bending and longitudinal welding are illustrated in three-dimensional plots while simplified residual stress patterns of these sections with specific values and parameters are also provided for subsequent advanced structural analyses and design.The proposed modelling technique for transverse bending and longitudinal welding with compatible meshes of two-dimensional and three-dimensional models with various element types and block data transfers is demonstrated to be highly effective. The technique is readily applicable to simulate residual stresses of all fabricated sections manufactured with transverse bending and longitudinal welding, and, thus, the simulated residual stresses can be employed in subsequent structural analyses of all fabricated members.

Structural testing and numerical modelling of T-joints between cold-formed S690 circular hollow sections under brace in-plane bending

This paper presents a comprehensive investigation into the structural behaviour of T-joints between high strength S690 cold-formed circular hollow sections (CFCHS) under brace in-plane bending, and deformation characteristics of these T-joints under both monotonic and cyclic actions are examined experimentally and simulated numerically. These T-joints between S690 CFCHS were found to deform with a high level of resistances and ductility under both monotonic and cyclic actions.Through an integrated numerical modelling approach developed by the authors, advanced three-dimensional finite element models of CFCHS with solid elements were established. With a proper definition of weld collars at the brace/chord junctions of the T-joints between S690 CFCHS with different diameters and thicknesses, these models were readily adopted to perform a heat transfer analysis, a thermomechanical analysis and a structural analysis sequentially with compatible element types and meshes. Hence, the effects of the welding-induced residual stresses at the welded brace/chord junctions onto the structural behaviour of these T-joints were readily assessed. Consequently, these advanced models are demonstrated to be able to predict rationally deformation characteristics of these T-joints between S690 CFCHS under monotonic and cyclic actions of brace in-plane bending with a high degree of structural accuracy.

Lateral torsional buckling of partially restrained beams of high strength S690 welded I-sections

This paper reports a systematic experimental investigation into the structural behaviour of a total of eighteen simply supported beams of high strength S690 welded I-sections with various restraint conditions, focussing on lateral torsional buckling. Deformation characteristics of these beams, critical failure modes and moment resistances, are fully presented and discussed. In general, these beams of S690 welded I-sections were found to behave in an essentially similar manner to beams of S355 I-sections. Lateral torsional buckling in these beams was thoroughly assessed and compared with the predictions of EN1993-1-1 using their non-dimensionalized slenderness, determined using the procedures of EN 1993-1-1, and measured mechanical properties, geometrical dimensions and restraint conditions. It was demonstrated that the codified design rules were very conservative when being extended to cover these beams of S690 welded I-sections.This comprehensive experimental investigation provides scientific evidence and test data that highlights a need to improve structural efficiency of the design rules in EN 1993-1-1 so that structural engineers can fully exploit the structural benefits offered by these high strength S690 steels in construction.

Structural response of high strength S690 welded sections under cyclic loading conditions

High strength S690 steel offers an attractive solution for use in buildings and bridges, owing to the inherent high strength to self-weight ratio of the material which can lead to significant savings in terms of cost and time. However, take up in practice has been hampered by concerns related to the deterioration in mechanical properties of the S690 steel plates after welding as a result of changes in microstructure. Whilst recent studies have illustrated the high levels of ductility that can be provided by the S690 steel, it is essential to assess and quantify the inelastic cyclic response of welded sections made from the same material under earthquake attacks. This paper therefore presents an experimental investigation into the structural response of high strength S690 welded sections under various cyclic loading conditions. A detailed description of 32 cyclic tests, carried out under two different cyclic protocols with various combinations of target strains and loading frequencies, is provided. Particular focus is given to examining the strength degradation in the material, the number of effective cycles completed before fracture, and the energy dissipation performance. A direct comparsion is also provided between the cumulative cyclic response of the S690 welded sections and their unwelded S690 plate counterparts, based on which the deterioration in behaviour is readily quantified.It is shown that a significant deterioration in strength similarly occurs in both the unwelded and welded S690 sections with the increase in number of cycles, particularly under large target strains. However, in terms of ductility, the number of effective cycles to fracture of the S690 welded sections is found to be considerably lower than for the S690 steel plates, typically by a factor of 2 to 3. Moreover, the energy dissipation densities of the S690 welded sections are shown to be only 41% to 47% of those for the counterparts. Overall, in addition to highlighting the importance of conducting realistic cyclic tests for assessing ductility, the findings provide detailed insights into the structural response of the S690 welded sections in steel structures under earthquake attacks, including the influence of target strains and loading frequencies. The results also enable detailed quantification of the cyclic response for the purpose of numerical modelling as well as for determining reliable ductility criteria.

Structural behaviour of T-joints between high strength S690 steel cold-formed circular hollow sections

This paper reports an experimental investigation into structural behaviour of T-joints between S690 cold-formed circular hollow sections (CFCHS). A total of twelve T-joints of both S355 and S690 CFCHS under i) brace axial compression, and ii) brace in-plane moments are tested to failure in order to examine their deformation characteristics, in particular, their joint resistances and failure modes. These tests are conducted with extensive instrumentation on both overall deformations of the T-joints and local deformations of their welded brace/chord junctions. Welding of all these T-joints between S690 CFCHS is carefully performed so that the heat input energy during welding varies narrowly between 1.2 and 1.5 kJ/mm. Standard tensile tests are also conducted on both flat and curved coupons extracted from the sections to provide mechanical properties for subsequent strength analysis.All the tests have been successfully conducted. For those T-joints between S690 CFCHS under brace axial compression, chord plastification is always found to be a critical failure mode while for those T-joints between S690 CFCHS under brace in-plane moments, both brace fracture and chord punching shear failure are found to be critical at the end of the tests.All the measured joint resistances of the T-joints between S690 CFCHS are found to be larger than those design resistances obtained with EN 1993-1-8 and Design Guide 1 of CIDECT. Consequently, this investigation provides structural insights and scientific data for possible improvements onto prediction capabilities of the design methods.

Determination of true stress strain characteristics of structural steels using Instantaneous Area Method

This paper presents recent research findings on true stress strain characteristics of normal strength S275, S355 and high strength S690 steels to EN 10025:2005. It aims to provide generalized constitutive models for analysis and design of steel structures. Comprehensive experimental and numerical investigations into mechanical properties of the structural steels have been carried out based on test results of nine monotonic tensile tests. Non-uniform stress and strain distributions within the necked region are found when the test coupons are under very large deformation. After corrections to the stress and strain non-uniformities by Instantaneous Area Method with successive approximations, true stress strain characteristics of the tested steels were successfully quantified as generalized constitutive models. Moreover, the microstructures of the typical structural steels haven been carefully examined, and their microstructural constituents are successfully quantified by Digital Image Analytics. The research findings are very important for subsequent numerical investigations into the structural performance of steel structures under very large deformations up to fracture. The authors are grateful for the financial supports from the Research Grants Council of the Government of Hong Kong SAR, and the CNERC for Steel Construction (HKB).

Determination of a full range constitutive model for high strength S690 steels

In order to enable accurate analysis and prediction on structural behaviour of high strength S690 steel member at large deformations, an integrated experimental, theoretical and numerical investigation to formulate a full range constitutive model is reported in this paper. It is well known that standard tensile tests are effective means to obtain key mechanical properties of coupons for deformations up to on-set of necking. However, owing to non-uniform cross-sectional distributions in both stresses and strains within the necking regions of the coupons, it is important to allow for these effects when analysing measured forces and extensions to give true stresses and strains of the coupons.The proposed instantaneous area method has been developed and calibrated against test results of a total of 30 standard tensile tests on S690 cylindrical coupons, and it is formulated with key mechanical properties of engineering stress-strain curves readily obtained from these tests. High resolution digital images of the deformed coupons are captured and analysed to obtain instantaneous dimensions of the necking areas. Through theoretical formulation, true stress-strain curves of the coupons after on-set of necking are derived, and modified with numerical correction factors through successive approximations. After normalization on all the corrected true stress-strain curves, a set of formulae to describe the full-range constitutive model for the high strength S690 steels is proposed. Consequently, the proposed model is readily applicable to both analytical and numerical analyses of the high strength S690 steels and their structural members undergoing both small and large deformations up to fracture.

Cyclic deformation characteristics of S355 and S690 steels under different loading protocols

Despite of excellent high strength to self-weight ratios of the S690 steels, when compared with the S355 steels, there is a widespread concern regarding the ductility of the S690 steels. It is generally considered that the ductility of the S690 steels is significantly lower than that of the S355 steels – this is the general understandings the authors attempt to investigate.This paper presents an experimental investigation into cyclic deformation characteristics of both S355 and S690 steels through low-cycle high-strain cyclic tests with two different loading protocols. A detailed account of the results of 32 cyclic tests on both the S355 and the S690 funnel-shaped coupons is presented. Effects of four different target strains and two different loading frequencies are also examined in details. For the ranges of loading protocols, strain amplitudes, and frequencies considered, the hysteretic responses of these coupons of the two steels are compared directly in terms of engineering stress–strain curves based on their nominal diameters. Microstructures of the fractured coupons of the two steels are also identified for comparison.Contrary to the general understandings, it is demonstrated that the high strength S690 steels do have a good ductility under both monotonic and cyclic actions. Moreover, depending on specific loading protocols and target strains, the cyclic deformation characteristics of the S690 steels are demonstrated to be superior to those of the S355 steels in terms of the number of cycles completed prior to failure and their corresponding energy dissipation characteristic under various target strains up to ±10.0%.The findings of this experimental investigation highlight the importance of establishing ductility requirements and cyclic deformation characteristics for the high strength S690 steels in accordance with specifically designed cyclic tests rather than relying solely on conventional monotonic tensile tests.

Mechanical properties of high strength S690 steel welded sections through tensile tests on heat-treated coupons

High strength S690 steels achieve their greater strength through heat treatment, the benefits of which may be to some extent reversed if the steels are subsequently welded. In order to investigate this, a systematic experimental investigation into the mechanical properties of small coupons of the S690 steels representing 3 regions within the heat affected zone (HAZ) of the welded joints produced using temperature-time histories derived from different practical welding procedures was carried out. By employing specially designed funnel-shaped coupons, it was possible to generate highly consistent microstructures for the different phases of the recrystallized S690 steels that are representative of those produced by welding. These microstructures were identified using scanning electron microscopes to demonstrate an evolution of microstructure within the HAZ of the welded sections. Using the stress-strain curves from monotonic tensile tests on these coupons, it has been possible to correlate key features of various deformation characteristics in the different regions with their corresponding microstructures. The findings show that the potentially deleterious effects of welding on the mechanical properties of S690 steel may be substantially reduced through careful control of the welding process.

Experimental evidence on structural adequacy of high strength S690 steel welded joints with different heat input energy

Quenching and tempering is a highly developed heat treatment method to produce high strength steels with certain quantities of expensive alloy elements. This process has been highly industrialized and widely adopted in modern steel mills to produce high strength S690 steels. However, a heating/cooling cycle induced during welding may initiate phase transformation, re-crystallization and grain growth in microstructures of these steels. This will cause a significant reduction in their mechanical properties if both the maximum temperatures during welding and the cooling rates after welding are not properly controlled. Over the past twenty years, conflicting research findings have been reported on mechanical properties of these S690 welded sections due to different welding procedures and parameters adopted during welding.In order to quantify adverse effects on mechanical properties of the S690 steel welded joints under static loads, a series of pilot tests on a total of 18 coupons of S690 steel plates, welded joints and weld metals with different heat input energy during welding have been conducted to examine their deformation characteristics under tension. Moreover, 12 reference and 12 spliced S690 welded H-sections with different heat input energy adopted in the welding processes have been conducted to examine their deformation characteristics under compression, in particular, their section resistances under compression. It is demonstrated that by a proper control on the heat input energy during welding, it is possible to control or even eliminate any reduction in the mechanical properties of these S690 welded joints under either tension or compression. Consequently, experimental evidence on structural adequacy of these high strength S690 steel welded joints with different heat input energy adopted in the welding processes is provided scientifically to confirm applications of these high strength S690 steels in construction.

Steels with Yield Strength Higher than 700 MPa for Metallic Structures and the Strength of Their Welded Joints

Abstract—Thick rolled products (plates) made of high-strength S690 steels (σy ≥ 690 MPa), which have not been used in the Russian building industry to date, are shown to be used in bearing welded metallic structures. Technological approaches, which can impart the high service properties that meet the requirements of norms and records to thick-sheet steels (50 mm thick or larger), are described. The advantages of hardening of rolled products up to 50 mm thick by thermomechanical rolling followed by rapid cooling and by thermal toughening of rolled products more than 50 mm thick are revealed. Criteria are proposed to determine the properties of the welded joints of S690 plate steels and to ensure service ability of these joints. The results of testing real welded joints of rolled products made of the S690 steels fabricated according to the proposed technologies are presented.

Modelling tensile tests on high strength S690 steel materials undergoing large deformations

Standard tensile tests are commonly used to determine mechanical properties of metallic materials, such as yield strength and tensile strength as well as ductility. In general, these standard tensile tests are able to provide basic mechanical properties of steel materials, commonly referred as engineering stress-strain curves. These curves are considered to be effective for linear elastic and post yielding deformations up to mobilization of tensile strengths, and they are widely adopted in structural design and analysis of steel structures. However, for detailed investigations into structural behaviour of steel structures in large deformations beyond onset of necking, cross-sectional changes in the steel materials often become very large. Hence, an improved stress-strain curve, commonly known as a true stress-strain curve, with proper adjustment according to large longitudinal deformations should be adopted in advanced finite element models.In order to develop full range true stress-strain curves of various steel materials for large deformations, a research project is conducted to perform an integrated experimental and numerical study. Standard tensile tests on two S275 steel coupons and two S690 steel coupons are carried out, and advanced optical measurements using a digital imaging correlation technique are adopted to measure deformation fields of these coupons with a high precision during the entire deformation ranges. After data analyses using three different transformation rules, namely, i) Power Law Method, ii) Linear Law Method, and iii) Instantaneous Area Method, three different true stress-strain curves for each of S275 and S690 steel materials are derived. These curves are then incorporated into advanced finite element models to simulate large deformations of these steel coupons observed in the tensile tests. Improvements to the true stress-strain curves derived from Instantaneous Area Method are made through successive corrections according to measured and predicted deformation characteristics of the steel coupons. Consequently, the proposed true stress-strain curves determined with Instantaneous Area Method are shown to be highly acceptable for numerical analyses of steel structures undergoing large plastic deformations up to fracture. Expressions of the proposed full range true stress-strain curves for S275 and S690 steel materials are also provided.