An explicit stress formulation for stainless steel applicable in tension and compression

In this paper an explicit stress formulation for stainless steel is presented. The formulation is applied to both tension and compression and it is an approximation to the closed form inversion of an existing two-stage stress-strain relation which is based on a modified Ramberg-Osgood equation. The validity of the explicit expression is tested over a wide range of material parameters. The tests demonstrate that the explicit formula is in excellent agreement with the fully iterated numerical solution of the full-range stress-strain relation. By incorporating an existing expression for deformation capacity, the proposed explicit formula is also used to obtain an explicit formulation for the computation of local buckling stress and compression resistance in terms of cross sectional slenderness.     

Explicit full-range stress-strain relations for stainless steel at high temperatures

This paper presents a new explicit stress-strain relation for stainless steel at high temperatures. The relation is an approximation of the closed form inversion of an existing two-stage stress-strain relation based on a modified Ramberg-Osgood equation with corresponding temperature-dependent parameters. This new expression for the stress σ, as an explicit function of the total strain ?, is obtained by making a modified power law assumption on the fractional deviation of the actual stress-strain curve from an idealized linear elastic behaviour. The validity of the inverted expression is tested over a wide range of material parameters and a wide range of temperatures. The tests show that the new expression results in stress-strain curves which are both qualitatively and quantitatively consistent with the fully iterated numerical solution of the full-range stress-strain relation for normal as well as high temperatures.     

拉压下不锈钢三阶段应力-应变关系的逆表达式

提出了不锈钢合金应力-应变关系的新表达式,应力为应变的显式函数。基于改进的Ram-berg-Osgood方程,本公式是现有三阶段应力-应变关系的一种近似逆表达。三阶段关系公式比两阶段关系公式更准确,并对拉应力和压应力均适用。基于理想线弹性性能和实际应力-应变曲线误差的合理假定,提出本表达式,其在应力的全范围阶段都是有效的。选取不同的材料参数,对本表达式进行试验验证。结果表明:新公式结果与全范围应力-应变关系的迭代数值求解结果很吻合,最大误差低于4%。     

Elevated temperature material properties of stainless steel alloys

Appropriate assessment of the fire resistance of structures depends largely on the ability to accurately predict the material response at elevated temperature. The material characteristics of stainless steel differ from those of carbon steel due to the high alloy content. These differences have been explored in some detail at room temperature, whilst those at elevated temperature have been less closely scrutinised. This paper presents an overview and reappraisal of previous pertinent research, together with an evaluation of existing elevated temperature stainless steel stress-strain test data and previously proposed material models. On the basis of examination of all available test data, much of which have been recently generated, revised strength and stiffness reduction factors at elevated temperatures for a range of grades of stainless steel have been proposed, including four grades not previously covered by existing structural fire design guidance. A total of eight sets of strength reduction factors are currently provided for different grades of stainless steel in EN 1993-1-2 and the Euro Inox/SCI Design Manual for Structural Stainless Steel, compared to a single set for carbon steel. A number of sets of reduction factors is appropriate for stainless steel since the elevated temperature properties can vary markedly between different grades, but this has to be justified with sufficient test data and balanced against ease of design — it has been proposed herein that the eight sets of reduction factors be rationalised on the basis of grouping grades that exhibit similar elevated temperature properties. In addition to more accurate prediction of discrete features of the elevated temperature material stress-strain response of stainless steel (i.e. strength and stiffness reduction factors), a material model for the continuous prediction of the stress-strain response by means of a modified compound Ramberg-Osgood formulation has also been proposed. The proposed model is less complex than the current provisions of EN 1993-1-2, more accurate when compared to test results, and the model parameters have a clear physical significance.     

Interaction of bending and axial compression of stainless steel members

The paper deals with the buckling behaviour of stainless steel members with the main focus on developing design formulae for use in the latest version of the European Standard EN 1993-1-4: Eurocode 3-Design of steel structures-Part 1-4: General rules — Supplementary rules for stainless steel. Brussels; 2005.It is based on numerical simulations of single span members of various section type, which are subjected to axial compression and bending. Both flexural buckling and lateral-torsional buckling are dealt with so that the buckling behaviour of both I-sections and hollow sections can be covered.On the basis of these numerical results interaction factors have been derived in context with the design model for member design in Eurocode 3-1-1. For statistical evaluation the test results available from other authors have been used.The outcome of this investigation has been incorporated in the present EN 1993-1-4 as a recommendation in restricted form.     

不锈钢材料的应力-应变模型

不锈钢材料与普通碳素钢的力学性能有较大不同,其应力-应变关系呈显著非线性,且各向异性,因此精确描述不锈钢应力-应变关系是不锈钢构件受力及稳定性分析等相关研究工作的基础.本文回顾了国内外关于不锈钢应力-应变本构模型的研究成果,通过介绍、分析和比较研究指出,Quach三段式应力-应变模型采用Ramberg-Osgood三参数表示,可直接应用现行规范进行结构设计,其准确性已得到相关试验和有限元分析证实.因此,Quach模型是目前可供选用的最佳本构模型.     

Residual stress analysis of structural stainless steel sections

The magnitude and distribution of residual stresses in structural carbon steel sections have been thoroughly investigated. However, few residual stress measurements have been made on structural stainless steel sections. Stainless steel has differing material stress-strain characteristics and thermal properties to carbon steel, both of which influence the formation of residual stresses. This suggests that established carbon steel residual stress models may not be appropriate for stainless steel. With increased use of stainless steel in load bearing applications, it is important to establish the residual stresses that exist within structural members. An experimental program to quantify the residual stresses in stainless steel sections from three different production routes has therefore been carried out. Comprehensive residual stress distributions have been obtained for three hot rolled angles, eight press braked angles and seven cold rolled box sections, with a total of over 800 readings taken. This paper presents the experimental techniques implemented and the residual stress distributions obtained as well as discussing the assumptions commonly made regarding through thickness residual stress variations. In the hot rolled and press braked sections, residual stresses were typically found to be below 20% of the material 0.2% proof stress, though for the cold rolled box sections, whilst membrane residual stresses were relatively low, bending residual stresses were found to be between 40% and 70% of the material 0.2% proof stress.     

The direct strength method for stainless steel compression members

The paper presents a complete set of direct strength equations for stainless steel members and sections in compression. The direct strength equations are based on recent research on the local, distortional and member bucking of stainless steel compression members, including the interaction of local and member buckling. The paper summarises the underlying research and presents the direct strength equations in a consistent format using a notation similar to that used in the North American Specification and the Australian Standard for (carbon) steel structures. Direct strength equations are proposed for local, distortional and combined local and member buckling, which fit within the framework of the Australian, North-American and European standards for stainless steel.     

Tests of cold-formed high strength stainless steel compression members

The paper describes a test program on cold-formed high strength stainless steel compression members. The duplex stainless steel having the yield stress and tensile strength up to 750 and 850 MPa, respectively, was investigated. The material properties of the test specimens were obtained from tensile coupon and stub column tests. The test specimens were cold-rolled into square and rectangular hollow sections. The specimens were compressed between fixed ends at different column lengths. The initial overall geometric imperfections of the column specimens were measured. The strength and behaviour of cold-formed high strength stainless steel columns were investigated. The test strengths were compared with the design strengths predicted using the American, Australian/New Zealand and European specifications for cold-formed stainless steel structures. Generally, it is shown that the design strengths predicted by the three specifications are conservative for the cold-formed high strength stainless steel columns. In addition, reliability analysis was performed to evaluate the current design rules.     

A design model for stainless steel box columns in fire

A study of stainless steel cold-rolled box columns at elevated temperatures is presented, which is a part of an on-going RFCS project “Stainless Steel in Fire”, 2004-2007. Experimental results of six, class 4, stub columns at elevated temperature, tested by Ala-Outinen [Members with Class 4 cross-sections in fire: Work package 3, ECSC project stainless steel in fire. Contract no. RFS-CR-04048, Espoo, Finland; 2005], were used to evaluate the finite element (FE) model. The FE analysis obtained using the commercially available software, ABAQUS, shows that the critical temperature was closely predicted. Further, a parametric study was performed using the same numerical model. This was a basis to check the quality of prediction of a newly proposed improvement for design rules of class 4 cross-sections in fire according to EN 1993-1-4 [Eurocode 3 - Design of steel structures — Part 1-4: General rules — Supplementary rules for stainless steels, CEN; 2006] and EN 1993-1-2 [Eurocode 3: Design of steel structures — Part 1.2: General rules — Structural fire design, CEN; 2005].     

Nonlinear analysis of concrete-filled square stainless steel stub columns under axial compression

Concrete-filled stainless steel tubes (CFSST) can be considered as a new and innovative kind of composite construction technique, and have the potential to be used extensively in civil engineering. This paper employs a nonlinear analysis of square CFSST stub columns under axial compression. A three-dimensional nonlinear finite element (FE) model is developed using ABAQUS, where nonlinear material behaviour, enhanced strength corner properties of steel, and initial geometric imperfections are included. Close agreement is achieved between the test and FE results in terms of load-deformation response and ultimate strength. In light of the numerical results, the behaviour of stainless steel composite columns is compared with that of carbon steel composite columns. A simple model is proposed to calculate the ultimate strength of square CFSST stub columns.     

Assessment of stainless steel reinforcement for concrete structures rehabilitation

The aim of this study was to revise the factors influencing the service life of galvanic coupling between carbon steel and stainless steel reinforcements in simulated concrete pore solution, simulating the condition of a damaged structure repaired with stainless steel reinforcing bars. Numerous investigations have reported that austenitic stainless steel rebar, compared to carbon steel, when embedded in concrete, offer superior corrosion resistance in aggressive environments, especially chloride contaminated concrete. In concrete, contact with other metals should be avoided because of the risk of galvanic corrosion. When passive, both carbon steel and stainless steel have comparable corrosion potentials and the coupling of the two materials is of little effect on the corrosion behavior of either material. Galvanic current values measured between carbon and stainless steel are negligible.     

Discussion on the use of stainless steel in constructions in view of sustainability

Recent years have seen an increase in the use of stainless steel in buildings, mainly owing to its corrosion properties and therefore long service life. Among stainless steels, ferritic and lean duplex grades are characterized by low nickel content resulting in a more cost-stable and economic material compared to austenitic stainless steels. These grades have comparable (or even higher) strength than carbon steel and good corrosion resistance at lower cost. That is why, lately, they have been more often used in structural components. In this paper, attention is first paid to the advantages associated with the use of stainless steel in recent construction projects in view of sustainability. Second, life cycle analysis and the background of the new European standard EN 15804 are introduced, including module D, which allows credits to be taken now for the eventual reuse or recycling of material in the future, at the end-of-life stage. Life cycle inventories of stainless steel products (cold-rolled coils and quarto plate) are presented. Depending on the fraction of material recovered at the end of the lifespan, two potential impacts (Primary Energy Demand and Global Warming Potential) are presented for four grades: 1.4301 (AISI 304) and 1.4401 (AISI 316) austenitic grades, 1.4016 (AISI 430) ferritic grade and 1.4462 (AISI 2205) duplex grade. The influence of module D is underlined.     

Heat transfer in concrete-filled carbon and stainless steel tubes exposed to fire

Nowadays, performance-based design methods are increasingly used for fire resistance assessment of structures. To implement these methods, it is paramount to determine the temperature development within a structural member exposed to fire as accurately and efficiently as possible. Numerical models are developed in this paper to simulate the temperature development in concrete-filled carbon and stainless steel tubes. It was found that the influence of the moisture content in concrete and the thermal contact conductance at the steel–concrete interface is significant. New models for thermal conductivity of concrete and thermal contact conductance at the interface are proposed in this paper. Comparisons of temperature development are made between numerical simulations and extensive experimental results. Improved agreement with test results is achieved when the proposed models are used in the heat transfer analysis.     

Experimental study on the constitutive relation of austenitic stainless steel S31608 under monotonic and cyclic loading

In order to study the constitutive relation of Chinese-made austenitic stainless steel S31608 (EN 1.4401, AISI 316) under monotonic and cyclic loading, different types of coupons were tested. Based on the Ramberg–Osgood model, modified by Gardner and Nethercot, parameters that described stress–strain relationship under monotonic loading were obtained. Comparison between data obtained using different types of coupons was made and the influences of welding and rolling directions were reviewed. Parameters of the hardening model of cyclic plasticity were calibrated from cyclic test data and the tests were simulated using ABAQUS finite element analysis software. The results show that stainless steel exhibits remarkable nonlinearity, and the stress–strain relationship may vary from different rolling directions. Under cyclic loading, with the increase of cyclic loops and change in strain amplitudes, stainless steel exhibits cyclic hardening behavior, and the simulated curves agree fairly well with the test curves. It is therefore recommended that the influences of welding and rolling directions on the stress–strain relationship should be taken into consideration and the constitutive relation under cyclic loading should be used if the component is subjected to cyclic loads.     

Lateral-torsional buckling of stainless steel I-beams in case of fire

This work presents a numerical study of the behaviour of stainless steel I-beams subjected to lateral-torsional buckling in case of fire and compares the obtained results with the beam design curves of Eurocode 3.New formulae for lateral-torsional buckling, that approximate better the real behaviour of stainless steel structural elements in case of fire are proposed. These new formulae were based on numerical simulations using the program SAFIR, which was modified to take into account the material properties of the stainless steel.     

Experimental investigation of cold-formed lean duplex stainless steel beam-columns

This paper describes a test program on cold-formed lean duplex stainless steel members in combined compression and minor axis bending. The test specimens were cold-rolled from flat strips of lean duplex stainless steel grade EN 1.4162. In this study, square and rectangular hollow sections were compressed at different eccentricities, in order to obtain a beam-column interaction curve for each series of tests. Initial overall geometric imperfections of the members were measured prior to testing. The ultimate loads and the failure modes of each specimen were obtained. The observed failure modes include local buckling, flexural buckling and interaction of local and flexural buckling. The test strengths obtained from this study and other available data were compared with the design strengths predicted by the American Specification, Australian/New Zealand Standard and European Code for stainless steel structures. It should be noted that these specifications do not cover the material of lean duplex stainless steel. Therefore, the suitability of the beam-column design rules in these specifications for lean duplex stainless steel is assessed in this study. Generally, these specifications are capable of predicting the beam-column strengths of the lean duplex stainless steel test specimens, and the design rules in the specifications are considered to be reliable. It is observed that the European Code generally provides quite conservative predictions for the beam-column specimens compared to the American Specification and Australian/New Zealand Standard predictions.     

Strength and stability criteria for thin-walled stainless steel circular hollow section members under bending

A series of tests consisting of various cross-section geometries were performed on structural stainless steel circular hollow sections (CHS) subjected to bending. The test program comprised a total of eight tests on CHS in two grades of stainless steel, namely 304 and Duplex 2205. For each grade four sections, each with a different slenderness, were tested, in order to cover a range of structural responses. Measurements of overall geometric imperfections and material properties were conducted. The test strengths are compared with the strengths predicted using the American, Australian and European specifications for cold-formed stainless steel structures. In the light of the test results and code recommendations, strength and cross-section classification criteria for stainless steel circular hollow section members in bending are examined.     

Structural uses of stainless steel — buildings and civil engineering

Stainless steels have not traditionally been widely used as structural materials in building and civil engineering. Where the steels have been used for this purpose there has been some other imperative driving the design, usually corrosion resistance or architectural requirements rather than the inherent structural properties of the steel. The primary reason for this low use in structural applications is usually the perceived and actual cost of stainless steel as a material. Developments over the last 10 years, both in available materials and attitudes to durability, are now offering a new opportunity for stainless steels to be considered as primary structural materials.     

Experimental and numerical investigation of high strength stainless steel structures

The paper summarises research on high strength stainless steel tubular structures conducted at the University of Hong Kong, and the Hong Kong University of Science and Technology. Square and rectangular hollow sections were investigated. The test specimens were cold-rolled from high strength austenitic and duplex stainless steel sheets. The material properties of the test specimens were determined by tensile coupon tests at normal room and elevated temperatures. The initial geometric imperfection and residual stress of the specimens were measured. The experimental and numerical investigation focused on the design and behaviour of cold-formed high strength stainless steel structural members. The results were compared with design strengths calculated using the American, Australian/New Zealand and European specifications for cold-formed stainless steel structures.