Proposed residual stress model for roller bent steel wide flange sections

The manufacturing process of structural wide flange steel sections introduces residual stresses in the material. These stresses due to hot-rolling or welding influence the inelastic buckling response of structural steel members and need to be taken into account in the design. Based on experimental data standardized residual stress models have been proposed for inclusion in inelastic buckling analyses. By incorporating these residual stress models their effect on the resistance of beams and columns can be obtained. Residual stress models for roller bent steel sections are currently not available. Roller bent wide flange sections are manufactured by curving straight members at ambient temperature. This manufacturing technique, which is also known as roller bending, stresses the material beyond its yield stress, thereby overriding the initial residual stresses prior to bending and generating an entirely new pattern. This paper proposes a residual stress model for roller bent wide flange sections, based on earlier conducted numerical investigations which were validated by experimental research performed at Eindhoven University of Technology. The proposed residual stress model can serve as an initial state of a roller bent steel section in fully non-linear finite element analyses to accurately predict its influence on the inelastic buckling response.     

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.     

受栓钉强度控制的钢板-混凝土界面黏结滑移性能试验与本构模型

以预应力型钢-混凝土界面为研究对象,旨在建立钢板-混凝土栓钉连接界面黏结滑移的本构关系。研究以混凝土强度、栓钉数量及法向压应力为变量,采用正交试验法对18个钢板-混凝土界面试件进行剪切试验。试验界面取为原型界面足够小的单元,使其能代表该单元内一点处的性能,从而探讨一点处的黏结滑移性能,并得到一点处的黏结应力-滑移关系曲线。结果表明：试验后期均发生栓钉剪断造成黏结应力骤降的破坏。混凝土强度和法向压应力的增加对界面的屈服、峰值和残余黏结应力均有提高作用;栓钉含量对界面的屈服及残余黏结应力影响不大,但可提高界面的峰值黏结应力,还可增大峰值黏结应力点的滑移及残余段初始滑移,提高延性。基于试验结果分别建立了无栓钉、含单栓钉和含双栓钉的钢板-混凝土界面黏结滑移本构模型,统计回归了模型中特征点的应力及滑移等参数的算式。所提模型与试验结果吻合良好,达到了在黏结滑移本构关系中考虑栓钉影响的目的。     

不锈钢构件截面的残余应力分析

对碳钢结构截面的残余应力的大小及分布的研究已经比较成熟,但是对不锈钢截面的残余应力研究却还很少见。不锈钢与碳钢有着不同的材料应力-应变特性和热性能,它们都影响着残余应力的形成。这意味着已确定的碳钢残余应力模型可能并不适合不锈钢。随着不锈钢的应用日益增多,对其残余应力的研究显得尤为重要。     

Residual stresses in cold-rolled stainless steel hollow sections

Stainless steel exhibits a pronounced response to cold-work and heat input. As a result, the behaviour of structural stainless steel sections, as influenced by strength, ductility and residual stress presence, is sensitive to the precise means by which the sections are produced. This paper explores the presence and influence of residual stresses in cold-rolled stainless steel box sections using experimental and numerical techniques. In previous studies, residual stress magnitudes have been inferred from surface strain measurements and an assumed through-thickness stress distribution. In the present study, through-thickness residual stresses in cold-rolled stainless steel box sections have been measured directly by means of X-ray diffraction and their effect on structural behaviour has been carefully assessed through detailed non-linear numerical modelling. Geometric imperfections, flat and corner material properties and the average compressive response of stainless steel box sections were also examined experimentally and the results have been fully reported. From the X-ray diffraction measurements, it was concluded that the influence of through-thickness (bending) residual stresses in cold-rolled stainless steel box sections could be effectively represented by a rectangular stress block distribution. The developed ABAQUS numerical models included features such as non-linear material stress-strain characteristics, initial geometric imperfections, residual stresses (membrane and bending) and enhanced strength corner properties. The residual stresses, together with the corresponding plastic strains, were included in the FE models by means of the SIGINI and HARDINI Fortran subroutines. Of the two residual stress components, the bending residual stresses were found to be larger in magnitude and of greater (often positive) influence on the structural behaviour of thin-walled cold-formed stainless steel sections.     

Influence of cold bending on the resistance of wide flange members

Cold rolling is used for bending straight members with hot rolled wide-flange sections to create arches. Extensive studies have shown that the influence of residual stresses due to hot rolling on the resistance of wide-flange steel sections is nonnegligible. On the contrary, the residual stress pattern due to roller bending has been only recently identified. Its effects on the elasto-plastic behavior of curved members have not been studied sufficiently. In this paper, an in-depth study of the influence of such residual stresses is performed. Considering the residual stress pattern due to cold bending, interaction diagrams and buckling curves for cold bent steel arches are developed. The results are quantified and compared with those for hot-rolled and stress free members. This allows designers to appreciate the available margins of safety when using standard interaction equations and buckling curves for cold bent members. Furthermore, the results suggest the necessity for the development of buckling curves for cold bent members including initial imperfections.     

The assessment of residual stresses in welded high strength steel box sections

Much work on the investigation of the magnitude and distribution of residual stresses in mild carbon steel sections have been made previously. However, limited efforts have been put on residual stress measurements of high strength steel sections. The differences of stress–strain curves and high-temperature material properties between the high strength steel and mild carbon steel demands a necessary study of the residual stresses in high strength steel welded sections. In the present study, three box columns fabricated from Q460 steel plates of 11 mm in thickness with different details were used for the examination. Both sectioning and hole-drilling methods are adopted for the measurement. The measured residual stress distributions of three different box sections are presented, and the corresponding simplified residual stress pattern is proposed. By comparing with the residual stress patterns for mild carbon steel, it is found that the box section fabricated from HSS plates has the lower compressive residual stress ratio. The differences in the measurement by using sectioning and hole-drilling methods are also compared.     

对焊接高强度钢材箱型构件中残余应力的评估

关于残余应力在低碳型钢中的大小和分布之前已经进行了很多研究,然而,对于高强度型钢中残余应力的测定研究甚少。高强钢和低碳钢在应力-应变曲线及高温材料特性上的差异是研究高强度焊接型钢残余应力的必要研究方面。在当前研究中,采用以厚11mm的Q460钢板焊接而成、不同的3个箱型柱做试验。切条法和钻孔法都可用于测量。最终将可看到测定的残余应力在3个箱型柱中的分布状况及其相应的简易残余应力分布图。与低碳钢的残余应力图相比较,可以发现,由高强度钢板焊接而成的箱型柱有较低的残余应力比。同时也对采用不同测定方法(切条法、钻孔法)所造成的差异进行了比较。     

Ductile extension of 3-D external circumferential cracks in pipe structures

This study investigates the ductile fracture resistance of 3-D external circumferential cracks in the wall of a steel pipe under remote tension, using a damagemechanism model originally proposed by Gurson and Tvergaard. The ductile crack extension utilizes an element extinction technique implemented in the computational cell framework. The key parameter for the computational cell method, i.e., the initial porosity ratio f ₀, is calibrated using both the fracture resistance and the load-deformation responses obtained from fracture tests of multiple single-edge bend [SE(B)] specimens made of high-strength steel, HY80, which has a yield strength of 630 MPa. The fracture resistance along the 3-D semi-elliptical crack front is computed from the calibrated cell model. Based on the similarity concept in the near-tip stress-strain fields, this study demonstrates that an equivalent 2-D axi-symmetric model provides conservative estimations of the fracture resistance for 3-D circumferential cracks in pipes.     

Residual stress of 460 MPa high strength steel welded i section: Experimental investigation and modeling

A reliable estimation of residual stress within steel sections is important in steel structural design and construction, especially for high strength steel which has been increasingly used in recent years. An experimental investigation was conducted in this paper to quantify the residual stresses in 460 MPa steel welded I sections using sectioning method. The magnitude and distribution of both compressive and tensile residual stresses were obtained based on 1972 sets of original data measured from eight different sections. The effects of width-thickness ratios of the flange and web, steel plate thickness, weld type and interaction of the flange and web were investigated. It was found that the compressive residual stress magnitude was largely related to the sectional dimension, while no direct correlation was found with the weld type and size for tensile ones. No residual stress interaction between the flange and web was identified because of the stress equilibrium within each individual part. In addition, a distribution model was proposed in this paper and well described the experimental results, which can be used to investigate and design the buckling behavior of 460 MPa high strength steel members.     

Combining fiber-reinforced concrete with traditional reinforcement in tunnel linings

New procedures to design cast-in-situ steel fiber reinforced concrete (SFRC) tunnel linings are briefly presented in this paper. The ductile failure of such cement-based structures is ensured by adding a suitable amount of steel fibers to ordinary steel bars. The capability of SFRC to carry tensile stresses, also in the presence of cracks, allows designers to reduce the minimum area of ordinary steel reinforcement, generally computed in compliance with American or European code requirements. In the serviceability stage, to evaluate crack widths more accurately, a suitable block model is introduced. This model is able to take into account the bridging effect of fibers, as well as the bond slip between steel bars and concrete in tension. The proposed approaches have been successfully applied to the design of tunnel linings in Italy.     

钢丝裂纹扩展估算模型及其在预腐蚀疲劳寿命计算中的应用

为给桥梁缆索高强度钢丝损伤容限分析提供实用的裂纹扩展计算参数,根据钢材相关试验的结果统计了珠光体钢丝门槛值ΔKth和裂纹扩展计算数据,建立适用于不同屈服强度和应力比的钢丝裂纹扩展经验模型。在此基础上,提出基于一维裂纹扩展假定的预腐蚀钢丝疲劳寿命预测方法,并对锈蚀钢丝恒幅和变幅疲劳寿命进行实例分析。结果表明,珠光体钢材的屈服强度和门槛值之间的相关性随着屈服强度和应力比的提高而增强|提出的钢丝裂纹扩展模型和初始裂纹深度假定可较好地模拟腐蚀钢丝恒幅疲劳试验的离散性和疲劳极限性质|腐蚀钢丝变幅疲劳评估结果对门槛值变化较敏感|按保守门槛值的预测寿命显示,轻微均匀锈蚀钢丝若及时采取养护措施,可在桥梁常规检修期内继续使用。     

Low-Cycle Fatigue Crack Propagation Behavior of Cracked Steel Plates Considering Accumulative Plastic Strain

AH-32 steel has been widely used in ship and marine structures but not been extensively studied for low-cycle fatigue (LCF) crack growth characteristics. The objective here is to explore the evolution mechanism behind LCF crack propagation behavior under different load conditions, for which the investigation of experimental and numerical simulation is considered in a detailed manner. An analytical model is presented to consider the effect accumulative plastic strain for ship cracked plate subjected to high stress cyclic loading. Different numerical parameters are proposed to quantify stress–strain field near crack-tip, and an integral model is developed to calculate the magnitude of the compressive stress. The effect of the maximum and minimum load on crack closure level is analyzed. The results show that there is an excellent correlation between LCF crack closure parameter and the magnitude of the compressive stress in the plastic wake, which indicates the presented integral model provides a new way for LCF crack propagation analysis.     

高温作用后高强Q690钢焊接截面残余应力试验研究

焊接中的不均匀受热和冷却使钢材内部产生残余应力,残余应力会降低钢构件的刚度、稳定承载力和疲劳性能。钢构件在高温下的强度退化、塑性和蠕变变形会引起截面残余应力改变,从而影响其火灾下及火灾后承载力。为此,采用高强Q690钢材制作了6个焊接构件,包括H形、箱形两种截面形式。利用切条法测量其在常温（20℃）下、600℃和800℃高温作用后的截面残余应力,得到不同温度作用后残余应力的大小与分布。结果表明：高温作用后,残余应力有不同程度的释放;经历600℃作用后,箱形与H形截面残余应力幅值均下降至常温下的约35%;经历800℃作用后,箱形与H形截面残余应力幅值均下降至常温下的15%以下。基于试验结果,提出Q690焊接截面残余应力分布模型及残余应力幅值随受火温度变化的折减系数。     

A new residual stress distribution for hot-rolled I-shaped sections

During the procedure of manufacturing structural steel profiles the considerable temperature changes cause residual stresses in the material. These stresses can have a significant effect on the stability resistance of structures consisting of such profiles. Therefore engineers have paid special attention to the consideration and modelling of residual stresses in the comprehensive research into column buckling. They developed several stress distribution forms based on experimental and/or theoretical results, which worked well in column buckling problems. However, when the member has twisting displacements-e.g. in the case of lateral-torsional buckling-these stress distributions can lead to malfunction, because they do not satisfy certain equilibrium equations connected with torsion and warping of the profile. In order to avoid this problem a new stress pattern is proposed which satisfies all the equilibrium equations, thus being applicable in stability problems including torsional and warping effects.     

460MPa高强度钢焊接箱形截面残余应力的试验研究和模型分析

使用分段法对460MPa高强度钢焊接箱形截面的残余应力进行试验研究和模型分析。对由不同宽厚比和厚度的钢板焊接而成的6个箱形截面进行测试,得到超过2000个原始数据,以此量化压缩和拉伸残余应力的大小和分布。阐明了宽厚比和板厚对残余应力的影响、人为误差以及各构件的相互作用。研究结果表明,压缩残余应力的大小与截面尺寸相一致,而焊缝附近区域的拉伸应力与截面尺寸关系不大。由2个人对相同截面进行测量,对测量结果进行比较,结果表明人为测量误差较小。由于每个板内的残余应力平衡,各构件之间没有残余应力。最后,建立分布模型及其简化形式,模型分析结果与试验结果相符。     

Residual stresses in press-braked stainless steel sections,I: Coiling and uncoiling of sheets

The manufacturing process of cold-formed thin-walled steel members induces cold work which can be characterized by the co-existent residual stresses and equivalent plastic strains and has a significant effect on their structural behaviour and strength. The present paper and the companion paper are concerned with the prediction of residual stresses and co-existent equivalent plastic strains in stainless steel sections formed by the press-braking method. This manufacturing process consists of the following two distinct stages: (i) coiling and uncoiling of the sheets, and (ii) press-braking operations. This paper presents an analytical solution for the residual stresses and co-existent equivalent plastic strains that arise from the first stage. In the analytical solution, the coiling–uncoiling stage is modelled as an inelastic plane strain pure bending problem; the stainless steel sheets are assumed to obey Hill’s anisotropic yield criterion with isotropic hardening to account for the effects of material anisotropy and nonlinear stress–strain behaviour. The accuracy of the solution is demonstrated by comparing its predictions with those obtained from a finite element analysis. The present analytical solution and the corresponding analytical solution for press-braking operations presented in the companion paper form an integrated analytical model for predicting residual stresses and equivalent plastic strains in press-braked stainless steel sections.     

Modelling of hardening behaviour of cold expanded holes in medium-carbon steel

Cold expansion is a well-known approach for the enhancement of the fatigue life of fastener holes in structural components by introducing residual compressive circumferential stresses around them. In this work, the hardening of a medium-carbon structural steel is studied experimentally and numerically from the point of view of the material behaviour during the cold expansion process of holes in structural steel components. Six material hardening models obtained on the basis of symmetric strain-controlled experiments and half-cycle test data from unidirectional tension experiments have been used in a two-dimensional axisymmetric finite element model of the process. Parallel with this an x-ray diffraction analysis of the residual stresses at the entrance and exit faces of the structural component has been carried out to prove the finite element result’s authenticity. It is established that the nonlinear kinematic hardening model obtained by a strain-controlled cyclic test to achieve a stabilized cycle for this steel secures finite element results close to the experimental ones. The constitutive hardening model obtained can be applied to a corresponding finite element model for stress analysis of steel structural component with cold expanded holes, subjected to an external load.     

火灾下蠕变对轴向约束钢柱屈曲的影响

目前,大多采用考虑蠕变的Harmathy蠕变模型进行耐火性分析。Harmathy蠕变模型仅能预测恒定应力下一定精度的蠕变,不适用于应力变化的情况。轴向约束钢柱遇到火灾时,火灾过程中应力将随着时间和温度的变化而迅速变化。该文研究了火灾下蠕变对轴向受压钢柱屈曲性能的影响。使用ANSYS软件中能够预测任何时间、应力或温度下钢材蠕变应力的蠕变模型来预测蠕变应力。分别对考虑和不考虑蠕变的情况进行数值模拟,并对两种情况下的屈曲温度和轴向变形结果进行对比分析。快火和慢火工况都考虑在内。研究结果显示,慢火工况下考虑蠕变的轴向约束钢柱屈曲温度高于不考虑蠕变的屈曲温度,快火工况下考虑蠕变的轴向约束钢柱屈曲温度可能高于也可能低于不考虑蠕变的屈曲温度。     

HRB400钢筋材料疲劳裂纹扩展性能试验研究

以直径40mm的HRB400钢筋为研究对象,采用紧凑拉伸试件,裂纹扩展设计为沿钢筋径向和轴向两个方向,分别开展了应力比为0.02、0.1、0.2、0.3和0.5下的疲劳裂纹扩展速率试验以及疲劳裂纹扩展门槛值的测定试验,得到了不同应力比、不同试件取向下的疲劳裂纹扩展速率参数,测得了疲劳裂纹扩展门槛值。对试件的疲劳断口进行了微观分析,对比了两种试件取向下疲劳裂纹扩展性能的异同。试验结果表明：铁素体/珠光体沿钢筋轴向呈条带状分布;疲劳裂纹扩展速率随应力比的增大而增大,疲劳裂纹扩展门槛值随应力比的增大而减小;径向疲劳裂纹扩展速率曲线在裂纹扩展的初始阶段存在一个转折点,轴向疲劳裂纹扩展性能对应力比的变化更为敏感;钢筋在径向抵抗疲劳裂纹扩展的性能优于轴向方向;径向试件的韧性断裂特征明显,主要以穿晶断裂模式进行扩展,轴向试件脆性断裂形貌显著,主要以沿晶断裂模式进行扩展。