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Residual stresses and plastic strains in steel tubes are induced by their manufacturing processes and play an important role in determining their mechanical and structural behaviour. The manufacturing process of cold-formed steel hollow sections can be divided into four main stages: (1) the coiling and uncoiling of a steel strip, (2) the transverse bending of the uncoiled strip into a circular shape, (3) the welding of the circular tube along the longitudinal strip edges, and (4) the shaping of the welded circular steel tube into a specific cross-sectional shape. In this paper, a finite element-based method is presented for predicting residual stresses and equivalent plastic strains in cold-formed steel hollow sections (including elliptical, square, and rectangular shapes of both normal grade steel and high strength steel). In this method, residual stresses and equivalent plastic strains due to the coiling, uncoiling and transverse bending operations are predicted analytically, and applied as the initial state for the subsequent finite element simulations of both welding and shaping operations. Predictions from this method show satisfying agreement with experimental measurements, which demonstrate the validity of the proposed method. This method can predict the distributions of residual stresses and equivalent plastic strains across the thickness and over the cross section, which are too complex to be measured completely in laboratory. The method provides a tool for investigating the effects of different material and forming parameters on the resulting cold work and structural behaviour.

In this study, a novel ultra-high-performance concrete (UHPC) composite plate that is reinforced with a fibre-reinforced polymer (FRP) grid (herein FRP-UHPC composite plate ) is developed and reported. The durability and mechanical performance of FRP-UHPC composite plates in harsh environments are expected to be superior to conventional materials when used in optimal configurations; because i) both UHPC and FRP composites in isolation have excellent mechanical properties in specific directions, and ii) they are also durable materials. The flexural and tensile behaviour of FRP-UHPC composite plates, with and without the inclusion of steel fibres in the UHPC mix, were investigated via experimentation. The test results, which demonstrate the excellent interaction between FRP and UHPC, confirm the viability of the system: i) the inclusion of an FRP grid enhances the ultimate flexural capacity by over 150% and the ultimate tensile capacity by over 200%; ii) the FRP-UHPC composite plates exhibit tensile elastic-strain hardening behaviour and the average ultimate tensile stress of the composite plates with an FRP reinforcement ratio of about 0.69% is over 25 MPa; and iii) the interaction between the FRP grid and UHPC is sufficient to transfer stresses in the longitudinal FRP strips to the UHPC through the transverse strips of the FRP grid, while the steel fibres in the UHPC passing through the openings of the FRP grid reduce the likelihood of FRP debonding. The proposed strain-hardening FRP-UHPC plates are expected to be promising for structural elements with various purposes.

Fibre-reinforced polymer (FRP) bars have become increasingly popular as internal reinforcement in reinforced concrete (RC) structures due to their excellent corrosion resistance. However, the compressive strength of FRP bars is generally much inferior to their tensile strength due to fibre micro-buckling under compression, and their transverse shear performance is much inferior to that of steel bars with the same diameter. To this end, a novel form of steel-free hybrid bars, which consist of an outer FRP confining tube, a central FRP bar and a layer of ultra-high performance concrete (UHPC) (without steel fibres) in the annular space between them (referred to as FRP-UHPC hybrid bars), have been proposed. In this study, compressive and transverse shear behaviour of FRP-UHPC hybrid bars have been investigated via experimentation. The key test variables include fibre winding angles of the FRP tube, fibre types of the FRP tube, the FRP tube thickness and the diameter of the central FRP bar. The test results confirm the validation of the novel hybrid bars: i) the compressive stress-strain curves of hybrid bars exhibit a ductile behaviour with a strain hardening segment, and the compressive behaviour of the central FRP bar in hybrid bars is superior to that of FRP bars in isolation; ii) the stress-strain response of hybrid bars can be designed to meet an elastic-plastic response with a post-yielding strain-hardening response; and iii) the transverse shear performance of hybrid bars is much better than that of FRP bars in isolation due to the contribution of FRP-confined UHPC section. Refereed/Peer-reviewed

Ultra-high performance concrete (UHPC) usually exhibits a weak ductility under axial compression. However, the ductility can be enhanced by implementing confining devices such as fiber-reinforced polymer (FRP) wraps. In order to gain in-depth understandings on the compressive behavior of FRP-confined UHPC, axial compression tests on 52 FRP-confined and 12 unconfined UHPC cylinders were carried out in this study. The failure modes, stress-strain behaviors, confinement efficiency, and the influences of crucial variables (steel fiber contents, specimen sizes, FRP fiber types, and FRP thickness) on the compressive behavior of FRP-confined UHPC were investigated and discussed. Results show that the increase in steel fiber content could increase the first peak stress and its corresponding strain, hoop strain efficiency, avoid the abrupt stress reduction after the first peak stress, as well as alleviating the effect of specimen sizes. Addition of steel fibers also alters the actual confinement ratio threshold for sufficient confinement. Comparisons suggest that FRP confinement fiber types have limited influence on the normalized stress-strain behavior of specimens with similar level of actual confinement ratio. Additionally, performances of four existing axial stress-strain models for FRP-confined concrete are verified and the results reveal that Teng et al.’s model performs the best although this model is incapable in capturing the strain softening behavior after the first peak stress.

Purpose– Although the actual residual stress distribution in any structural steel member can be only obtained by experimental measurements, it is known to be a difficult, tedious and inefficient piece of work with limited accuracy. Thus, besides aiming at clarifying structural designers and researchers about the possible ways of modelling residual stresses when performing finite element analysis (FEA), the purpose of this paper is to provide an effective literature review of the longitudinal membrane residual stress analytical expressions for carbon steel non-heavy sections, covering a vast range of structural shapes (plates, I, H, L, T, cruciform, SHS, RHS and LSB) and fabrication processes (hot-rolling, welding and cold-forming).Design/methodology/approach– This is a literature review.Findings– Those residual stresses are those often required as input of numerical analyses, since the other types are approximately accounted for through thes-ecurves of coupons cut from member walls.Practical implications– One of the most challenging aspects in FEA aimed to simulate the real behaviour of steel members, is the modelling of residual stresses.Originality/value– Besides aiming at clarifying structural designers and researchers about the possible ways of modelling residual stresses when performing FEA, this paper also provides an effective literature review of the longitudinal membrane residual stress analytical expressions for carbon steel non-heavy sections, covering a vast range of structural shapes (plates, I, H, L, T, cruciform, SHS, RHS and LSB) and fabrication processes (hot-rolling, welding and cold-forming).

Numerical modelling and design of cold-formed stainless steel members require accurate knowledge of the material stress-strain behavior over wide ranges of tensile and compressive strains. While many stress-strain models for stainless steels have been developed, when both tensile and compressive behaviors are considered, they are either only capable of accurate predictions over limited strain ranges or defined by rather complex mathematical expressions. This paper presents a new stress-strain model for stainless steel alloys. The proposed stress-strain model, while expressed using simple mathematical expressions, can accurately predict both tensile and compressive full-range stress-strain curves. The proposed stress-strain model is defined using the three basic Ramberg-Osgood parameters and based on a careful interpretation of a large experimental database covering wide ranges of strains. While the proposed stress-strain model is based on the staged approach similar to the existing stress-strain models, a novel method is used to define the second stage strain hardening exponent. Observing the fact that the strain hardening exponent tends to vary with the stress level, the second stage strain hardening exponent is defined in the proposed model as a function of stress level. For use in this model, the best existing equations for predicting the nominal ultimate stress and the corresponding strain (i.e., nominal ultimate strain) respectively are selected through an assessment of existing equations with experimental data. Comparisons between predictions from the proposed model and experimental stress-strain curves for three common structural classes of stainless steel alloys are presented. These comparisons demonstrate clearly the better accuracy of the proposed model over the existing full-range stress-strain models

The manufacturing process of cold-formed steel members induces cold work which has a significant effect on their structural performance. In order to assess this cold-working effect, residual stresses and equivalent plastic strains need to be quantified. A finite element-based method was recently proposed by the authors for predicting the coexistent residual stresses and plastic strains in cold-formed steel hollow sections. In the present study, this finite element-based method was employed to carry out a parametric study in order to generate numerical data of the residual stresses and coexistent equivalent plastic strains in cold-formed steel elliptical hollow sections (EHS) for a wide range of steel grades (including both normal grade steels and high strength steels) and section sizes. By analyzing these numerical data, simplified models for residual stresses and coexistent equivalent plastic strains in cold-formed steel EHS are proposed. The simplified models can be employed to define the initial states of cold-formed steel EHS members in a nonlinear finite element analysis for their structural performance, and thus can be used to examine the cold-working effect on the member behavior in future studies.

H13 is a typical high-performance tool steel, and has a wide application in the mold making industry. To optimize mechanical properties of additively manufactured H13, a detailed investigation on tempering was conducted at 600 oC and 700 oC. Tensile testing, transmission electron microscopy (TEM) and atom probe tomography (APT) were utilized in the present study. It was found that the selective laser melted H13 showed unsatisfactory tensile properties in the as-printed condition, especially in terms of the elongation at fracture (~2.42%). After tempering at 600 oC, the tensile properties, including the ultra-high yield strength (1483 ± 48 MPa), ultimate strength (1938 ± 62 MPa) and ductility (~5.8%), were significantly improved, which was attributed to the strengthening effect mainly from the presence of a large number of fine V8C7 precipitates and the high density of dislocations. After tempering at 700 oC, the material showed a high ductility (~10.95%) and moderate tensile strength (1076 ± 21 MPa). The fracture mode was a mixed ductile-brittle fracture for the specimen after tempering at 600 oC, and became a ductile fracture for the specimen after tempering at 700 oC. It is clarified that a transition in the microstructure explains the difference in the mechanical behavior between the specimens tempered at 600 oC and 700 oC. The present study provides a basis for manipulating the SLM-fabricated H13 for applications requiring either high strength or high ductility.

The cold work from the manufacturing process of cold-formed steel members can enhance the strength but reduce the ductility of materials. Due to a high cost of stainless steels, it is desirable to utilize this enhanced strength and avoid the early fracture in cold-formed stainless steel members. The paper is concerned with the prediction of the enhanced stress–strain behaviour and reduced ductility of corner materials in cold-formed stainless steel sections. The enhanced strength of corner materials has been traditionally determined using empirical models. However, most of these empirical models are only able to predict the enhanced 0.2% proof strength, but are neither capable of predicting the enhanced ultimate strength nor able to determine the reduced ductility. This paper first presents a modified weighted-average method for predicting the post-ultimate stress–strain behaviour and the fracture strain for stainless steels. An advanced numerical approach is next presented for predicting the full-range stress–strain behaviour of corner materials in cold-formed stainless steel sections, in which the modified weighted-average method is incorporated. The accuracy of this approach is demonstrated by comparing its predictions with test results. The proposed approach is generally applicable to cold-worked materials for predicting their enhanced strength, reduced ductility and full-range stress–strain behaviour. The proposed method and numerical results can explain why and how the ultimate strength of cold-formed steels can be increased and how the post-ultimate stress–strain behaviour can be utilized through cold working.

An accurate stress-strain relationship is needed for use in the advanced numerical modelling of cold-formed light-gauge steel members covering a wide range of strains. Existing stress-strain models for carbon steels are either capable of accurate predictions over a limited strain range or defined using many material parameters for a full range of strains. The values of some material parameters are not always available in existing design codes. This paper presents two stress-strain models for light-gauge carbon steels, which are capable of accurate predictions over the full range of tensile strains up to the strain at the ultimate stress. Both stress-strain models are solely defined using either three basic Ramberg-Osgood parameters ( E0, σ0.2, and n) or even two basic parameters ( E0, and σ0.2), and are based on a careful interpretation of existing experimental data of virgin materials and materials from flat portions of cold-formed steel sections. The accuracy of these two proposed models is demonstrated by comparing their predictions with experimental stress-strain curves.

The paper presents a numerical study on the stub-column behavior of cold-formed steel elliptical hollow sections (EHS). In the paper, a finite element model for the stub columns of cold-formed tubular sections is presented, in which residual stresses, strength enhancement, geometric imperfection resulting from cold forming are predicted by using a rigorous finite element simulation of the roll-forming process. The accuracy of the finite element model for stub columns was verified by comparing the numerical predictions with test results. By using the verified finite element model, a parametric study on the stub-column behavior of cold-formed steel EHS was carried out for a wide range of section sizes and steel grades (covering both normal grade steels and high strength steels). Based on the generated finite element data, predictive models for the strength enhancement and local geometric imperfection of cold-formed steel EHS are proposed. Furthermore, the cross-section classification for cold-formed steel EHS were examined according to the slenderness limits of circular hollow sections (CHS) in current design codes. It can be found that the equivalent diameter method together with the current CHS slenderness limits in existing design codes can be safely adopted for cold-formed steel EHS. In addition, a new yield slenderness limit for cold-formed steel EHS is proposed.

Additive manufacturing, commonly known as 3D printing, is an evolutional technology in the manufacturing industry. This technology has already been embraced by different industries, such as aerospace and biomedical engineering, and the recent advance in this technology has generated a vast of societal excitement for its future. Despite the exciting prospect of its application in civil engineering, additive manufacturing technology is still at a perceived stage for construction industry. The benefits and potential in construction industry are rarely known in the field at this stage. This paper aims to investigate the mechanical properties and structural performance of additively manufactured high strength steel tubular sections at the cross-sectional level through experimental program. The specimens were additively manufactured from H13 steel powder by selective laser melting (SLM) with three different scanning patterns, where the typical yield strength (0.2% proof stress) of the conventionally manufactured H13 steel is around 1650 MPa. The test program comprised tensile coupon tests, compression coupon tests, hardness tests, microstructural characterization as well as geometric imperfection measurements and stub column tests. The anisotropy on mechanical properties was examined through tensile coupon tests in both longitudinal and transverse directions. Compression coupon tests were also conducted to investigate the differential strength of material in tension and compression. The influences of scanning patterns on mechanical properties and structural behavior of as-printed high strength steel tubular sections stub columns were also investigated. The test results were used to assess the applicability of existing design provisions in the American Specification, Australian and New Zealand Standard as well as European Code that originally developed for conventionally produced hot-rolled and cold-formed steel tubular sections to the additively manufactured high strength steel tubular non-slender sections in this study.

The determination of microstructural details for powder materials is vital for facilitating their selective laser melting (SLM) process. Four widely used steels (316L, H13, P20 and 18Ni300) have been investigated to detail their powders&rsquo, microstructures as well as laser absorptivity to understand their SLM processing from raw material perspective. Phase components of these four steel powders were characterized by X-ray diffraction (XRD), synchrotron radiation X-ray diffraction (SR-XRD) and scanning electron microscopy (SEM). X-ray photoelectron spectroscopy (XPS) and transmission electron microscopy (TEM) were utilized to reveal the surface structure of these four steel powders. It is found that phase components of H13, P20 and 18Ni300 are mainly composed of martensite and a small amount of austenite due to the high cooling rate during gas atomization processing, while 316L is characterized by austenite. XPS results show that the four steel powders all possess a layered surface structure, consisting of a thin iron oxide layer at the outmost surface and metal matrix at the inner surface. It is found that the presence of such oxide layer can improve the absorptivity of steel powders and is beneficial for their SLM process.

Advanced numerical modeling for cold-formed light gauge steel structures, from manufacturing to the structural response under the applied loading, requires the knowledge of the stress-strain behavior of the material over the full range of tensile strains. Existing stress-strain models for carbon steels are either only capable of accurate predictions over a limited strain range or defined by many material parameters and the values of some material parameters are not available in most of existing design codes. Therefore, a new stress-strain relationship for light gauge carbon steels up to the ultimate strength is required for the advanced numerical modeling and needs to be modeled on the basis of three basic material parameters, the so-called Ramberg-Osgood parameters (the 0.2% proof stress σ 0.2 , the initial elastic modulus 0 E and the strain-hardening exponent η). This paper presents such new stress-strain models for light gauge carbon steels, which are able to describe the stress-strain relationship over the full range of tensile strains by using only three basic Ramberg-Osgood parameters. In the present study, the stress-strain data obtained from tensile coupon tests reported in existing literatures have been collected and analyzed, and these tested coupons were cut from both virgin steel sheets and cold-formed steel sections. The new models have been developed by a careful interpretation of these existing experimental data. The accuracy of the proposed models has been demonstrated by comparing their predictions with experimental stress-strain curves.

Advanced numerical modeling of cold-formed stainless steel members, from manufacturing to full-range response under applied loading, requires knowledge of the stress-strain relationship of the material over a wide range of tensile and compressive strains. Although a number of stress-strain models have been developed for stainless steels, they are only capable of accurate predictions either over a limited strain range or for the tensile stress-strain behavior only. This paper presents a three-stage stress-strain model for stainless steels, which is capable of accurate predictions over the full ranges of both tensile and compressive strains. The new stress-strain model is defined using the three basic Ramberg-Osgood parameters and is based on a careful interpretation of existing experimental data. The accuracy of the proposed model is demonstrated by comparing its predictions with experimental stress-strain curves. These comparisons also clearly demonstrate the advantage of the proposed model over the only existing full-range stress-strain model.