Yield Plateau Research Articles

Monotonic and Cyclic Modelling of Structural Steel for Finite Element Analysis

Nowadays, as a response to the promoted concept of performance-based seismic design, the finite element method (FEM) has become an omnipresent practice that supports the simulation of structural elements. This approach requires constitutive numerical models, which could reproduce both elastic and plastic behaviour of steel. Looking forward to a reliable FEM simulation, the modelling and calibration of steel undergoing monotonic and cyclic loading tests is investigated in this paper. With respect to monotonic tensile test, a sensitivity analysis regarding mesh refinement and finite element type is developed. Another inspected issue, such as high-strength steel modelling, taking into consideration low ductility and absence of yield plateau, is also presented. And finally, a model for structural steel, using the combined isotropic-kinematic hardening model, is introduced to simulate metal non-linear behaviour under ultra-low-cycle fatigue regime. All these numerical simulations are processed via Abaqus software package, and are validated by a set of experimental tests.

Experimental studies on the effect of nano silica modified novel concrete CFST columns

Under axial compression, the present article analyses and compares the behaviour of Plain Concrete filled and Nano Silica modified concrete filled with light gauge steel rectangular columns. In 18 work samples, plain and nano silica-modified concrete was tested on light gauge steel sections. Investigations were conducted into several essential variables, including the geometry of the cross-section, the tubular thickness of light gauge steel, the depth-to-thickness ratio, and the type of concrete infill used. From the experimental results, effects of flat width-to-thickness ratio (w/t), axial load-end shortening, axial load-deflection, axial load-strain characteristics, and failure modes have been studied under the linear stage. To evaluate their confinement effects, this study compared plain and silica-modified nano concrete tube columns with light gauge steel hollow hollow hollow columns. The experimental results are compared using BS5400, EC4, and ACI codes to determine the ultimate sectional capacity. Test outcomes showed that tested nano-silica modified concrete filled steel tubular (CFST) columns exhibited higher strength and outstanding ductility than the plain CFST and hollow light gauge steel columns. The nano silica-modified concrete in-filled columns showed a good yield plateau under the non-linear stage compared with the other columns.

Patch Loading Design Recommendations for Titanium–Clad Bimetallic Steel Plate Girder

For steel structures service in corrosive environments, corrosion reduces the mechanical properties and effective thickness of the structural steel, which has a significant effect on the durability of steel structures. Titanium–clad bimetallic steel (TCBS), a bimetallic material composed of titanium alloy and carbon steel with excellent corrosion resistance has been suggested to solve the corrosion issue. The TCBS consisting of the TA1 titanium cladding and Q235 carbon steel substrate was introduced in the plate girder as the web to improve durability in this study. The stress–strain properties and failure performance of TCBS and Q235 carbon steel were discussed. Notably, the separation between the cladding and the substrate in the TCBS was observed during the necking stage in the tensile coupon test. Welding method between TCBS web and Q235 carbon steel flange was suggested for processing the plate girder, which was validated to be appropriate according to the experimental results. No crack in welding and TCBS web of plate girder was observed during the patch loading test. A numerical parametric study on the ultimate resistance of TCBS plate girder subjected to patch loading was conducted using the finite element software ABAQUS, in which three different key parameters were included. A comparison between the numerical and predicted results indicated that the design equations suggested in EN 1993–1–5 were too conservative to be directly used for predicting the ultimate resistance of the TCBS plate girder under patch loading, which was caused by the strain–hardening effect and yield plateau of the structural steel. The current design equations were modified to effectively predict the resistance properties of TCBS plate girders.

Quenched‐and‐tempered high‐strength steels at elevated temperatures: Mechanical properties and effects of plate thickness

AbstractHigh‐strength steels can be used in civil structures that demand high strength/weight ratios, but their advantages may be weakened by fire. For this reason, this study has been concerned with the mechanical properties of a type of quenched‐and‐tempered high‐strength steel at elevated temperatures up to 800°C, the steel grade is S690. A standard axial tensile test was conducted on plate specimens with a reduced section. The test results demonstrated that the stress–strain curves of S690 steel at elevated temperatures had no distinctive yield plateau but post‐yield hardening, the effective yield strengths and elastic modulus deteriorated dramatically with the increasing temperature due to microstructure changes. Besides, this study also investigated the effects of steel plate thickness on said temperature‐dependent mechanical properties. The plate thicknesses are 8 and 12 mm nominally. The comparisons showed that, different from the trend at room temperature where the thinner steel plates usually have higher strength than their thicker counterparts, the thinner plates exhibited lower strengths at elevated temperatures, this implies that with the same cross‐sectional area, a steel member formed by thinner plates would have a lower cross‐sectional load‐carrying capacity under fire.

Enhancement effect of polyurea coating on the impact resistance of aluminum foam

AbstractA polyurea coating was prepared to enhance the impact protection performance of aluminum foam, and its mechanical properties were analyzed. Polyurea‐coated aluminum foam composite specimens were prepared by coating the polyurea on the surface of aluminum foam. The mechanical properties of composite specimens under quasi‐static and dynamic loading were tested, and the stress–strain relationship at different strain rates was analyzed. The deformation process and failure mode of typical specimens under different loading conditions were discussed. The influence of polyurea coating on the energy absorption characteristics of aluminum foam was explained. The results show that when the polyurea is coated on the end surface of aluminum foam specimen, the sawtooth oscillation in the yield plateau stage of the stress–strain curve is weakened. The existence of polyurea coating makes the stress transfer of cell wall of aluminum foam uniform, and can effectively prevent the damage caused by the splash of aluminum foam fragments under high‐speed impact load. Coating polyurea on the opposite to load‐receiving surface of aluminum foam specimen can effectively improve the yield stress of composite specimen under dynamic load, and make the specimen still maintain good integrity after failure. When the polyurea is coated on the end surface of aluminum foam specimen, the final energy absorption per unit volume and the ideal energy absorption efficiency peak of the specimen are improved.

Phenomics based prediction of plant biomass and leaf area in wheat using machine learning approaches.

Phenomics has emerged as important tool to bridge the genotype-phenotype gap. To dissect complex traits such as highly dynamic plant growth, and quantification of its component traits over a different growth phase of plant will immensely help dissect genetic basis of biomass production. Based on RGB images, models have been developed to predict biomass recently. However, it is very challenging to find a model performing stable across experiments. In this study, we recorded RGB and NIR images of wheat germplasm and Recombinant Inbred Lines (RILs) of Raj3765xHD2329, and examined the use of multimodal images from RGB, NIR sensors and machine learning models to predict biomass and leaf area non-invasively. The image-based traits (i-Traits) containing geometric features, RGB based indices, RGB colour classes and NIR features were categorized into architectural traits and physiological traits. Total 77 i-Traits were selected for prediction of biomass and leaf area consisting of 35 architectural and 42 physiological traits. We have shown that different biomass related traits such as fresh weight, dry weight and shoot area can be predicted accurately from RGB and NIR images using 16 machine learning models. We applied the models on two consecutive years of experiments and found that measurement accuracies were similar suggesting the generalized nature of models. Results showed that all biomass-related traits could be estimated with about 90% accuracy but the performance of model BLASSO was relatively stable and high in all the traits and experiments. The R2 of BLASSO for fresh weight prediction was 0.96 (both year experiments), for dry weight prediction was 0.90 (Experiment 1) and 0.93 (Experiment 2) and for shoot area prediction 0.96 (Experiment 1) and 0.93 (Experiment 2). Also, the RMSRE of BLASSO for fresh weight prediction was 0.53 (Experiment 1) and 0.24 (Experiment 2), for dry weight prediction was 0.85 (Experiment 1) and 0.25 (Experiment 2) and for shoot area prediction 0.59 (Experiment 1) and 0.53 (Experiment 2). Based on the quantification power analysis of i-Traits, the determinants of biomass accumulation were found which contains both architectural and physiological traits. The best predictor i-Trait for fresh weight and dry weight prediction was Area_SV and for shoot area prediction was projected shoot area. These results will be helpful for identification and genetic basis dissection of major determinants of biomass accumulation and also non-invasive high throughput estimation of plant growth during different phenological stages can identify hitherto uncovered genes for biomass production and its deployment in crop improvement for breaking the yield plateau.

Experimental study on the seismic behaviour of a corroded T-shaped RC shear wall

The available research shows that the corrosion of reinforcement is the main reason for the decrease in durability of reinforced concrete (RC) structures. In this study, wet-dry cycles under constant current conditions were used to perform accelerated corrosion and quasi-static tests of five T-shaped RC shear wall specimens. The effects of corrosion and longitudinal reinforcement ratio of boundary elements on the seismic performance of the T-shaped shear walls were studied. The results showed that the bearing, deformation, and energy dissipation capacities of the specimens gradually decreased with increasing degree of corrosion, and the maximum decreases were 16.73%, 15.95%, and 21.06%, respectively. For the same degree of corrosion, the yield plateau of the T-shaped RC shear wall specimens lengthened with decreasing diameter of the longitudinal reinforcements. The bearing capacity gradually decreased, the longitudinal reinforcements buckled more quickly, and the specimen failed sooner. In addition, increasing the longitudinal reinforcement ratio of the boundary element increased the bending capacity of the specimen, resulting in greater average shear deformation and ratio at different stages and less bending deformation. The maximum increases in the bearing, deformation, and energy dissipation capacities of the specimens were 17.18%, 10.63%, and 61.22%, respectively. Finally, based on the experimental results, the current theoretical equation was modified, and a model was established for the calculation of the skeleton curve characteristic point parameters of the T-shaped RC shear wall. This model can account for damage caused by corrosion and the ratio of boundary element longitudinal reinforcement.

Post-fire constitutive model on explosively welded stainless-clad bimetallic steel after cold-forming process

This study conducted an experiment on corner stainless-clad bimetallic steel (SBS) specimens to clarify the coupling effects of the cold-forming process and fire exposure on the monotonic mechanical properties of SBS, where different exposure temperatures and cooling methods were considered. The SBS produced through the explosive welding method was used to obtain cold-formed specimens. After the cold-forming process and fire exposure, no obvious cracking was observed in the stainless-steel cladding and metallurgical bonding layer. Based on the tensile coupon test results, the yield plateau in the stress–strain curve of the virgin SBS specimen disappeared because of the cold-forming process. The effects of the exposure temperature and cooling method on the stress–strain curves of the corner SBS specimen were clarified. The variation trends of the key mechanical parameters were discussed. Predictive equations were proposed for the key mechanical parameters of corner SBS specimen after fire exposure. Two constitutive models were suggested for the corner SBS stress–strain curves with and without the yield plateau, which could be used in the numerical and theoretical investigations on the post-fire-resistant performance of cold-formed SBS components. Comparing the experimental and predicted results indicated that the residual monotonic stress–strain curves of the corner SBS could be accurately described using the proposed constitutive models.

Elastoplastic hysteretic behavior and constitutive models of in-service structural steel considering fatigue-induced pre-damages

Fatigue damages are commonly observed in the in-service steel components and progressive deterioration in their mechanical behavior is triggered with increasing service period. Hence the present work aims to investigate the degradation of the mechanical properties of the fatigue-damaged structural steel Q345qD and establish the practical constitutive models. Various fatigue tests before the elastoplastic cyclic loading were conducted to simulate the fatigue-induced pre-damages. The residual mechanical properties of the fatigue-damaged material were investigated and refined versions of the hysteretic constitutive models were proposed. Moreover, the effects of the deterioration in the material properties on the mechanical behavior of the steel components were studied. Experimental results indicate that a prior history of fatigue loading significantly affects the subsequent elastoplastic hysteretic behavior of the in-service material. As the accumulation of the prior fatigue damage, the yield and ultimate strengths of the steel material deteriorate. In particular, the yield plateau disappears when the pre-damage level exceeds 0.5. Both of the strength and ductility of the in-service steel components deteriorate remarkably. This implies that the fatigue damages accumulated in the service history should be included in the seismic design of structures.

Interplay between nano zinc oxide-coated urea and summer green manuring in basmati rice under basmati rice-wheat cropping system: implications on yield response, nutrient acquisition and grain fortification

Identifying appropriate nutrient management options is crucial for reversing the yield plateau and enhancing the nutritional status of basmati rice under the basmati rice-wheat cropping system of the Indo Gangetic Plain (IGP). Alternative to the conventional chemical fertilizer, ZnO nanoparticles as carrier material for the micronutrient Zn has shown promise in reducing the bulkiness of fertilizer use in the soil–plant environment. But whether its integration and interaction with an organic source such as green manuring could enrich basmati rice grain with micronutrients and promote protein nutrition is not well investigated. Therefore, we conducted a field experiment during the summer and rainy seasons (April–October) of 2020 and 2021 at the research farm of the ICAR-Indian Agricultural Research Institute, New Delhi in a split-plot design with two summer green manuring (SGM) options (Sesbania and cowpea, along with fallow) as main plots and six fertilization strategies as subplots: 5 kg Zn ha−1 as bulk ZnO, N at 120 kg N ha−1 as prilled urea (PU), N at 120 kg N ha−1 as PU + 5 kg Zn ha−1 as bulk ZnO, 1% bulk ZnO-coated PU, 0.1% nano ZnO coated PU and 0.2% nano ZnO coated PU replicated thrice. On average, SGM increased basmati rice grain yield by 13.2 and 12.3% during 2020 and 2021, respectively compared to fallow. Integrated application of zinc with urea significantly (p < 0.05) increased the grain yield of basmati rice by 9.56% and by 10.5% relative to urea without zinc and by 33.7 and 33.8% than the sole application of 5 kg Zn ha−1 through ZnO, respectively during 2020 and 2021. On average, SGM boosted Zn, Cu, Mn, and Fe content in milled rice by 25, 22.38, 20.0, and 18.85% during 2020 and 23.75, 21.4, 19.6, and 13.3% during 2021, respectively compared to fallow. Relative to sole urea application, zinc, and urea together improved the Zn and Fe content in milled rice by 2.99 mg kg−1 and 2.62 mg kg–1, respectively during the first year and by 2.83 mg kg−1 and 2.6 mg kg−1, respectively during the second year of study. The highest protein content in basmati rice grain was observed when it was grown after Sesbania aculeata residue incorporated plot during both the years and it decreased in the order: Sesbania aculeata > Vigna unguiculata > summer fallow. Our findings revealed that with the application of 1% bulk ZnO coated PU with Sesbania; the yield response, micronutrient acquisition, and protein accumulation in milled rice was higher than in other plant fertilization methods. However, in plots treated with Sesbania, along with 0.2% nano ZnO-coated PU exhibited statistically equivalent yield and micronutrient loading in edible tissues. Hence, this study unveils the critical role of nano ZnO-coated urea and summer green manuring in elevating micronutrient and protein bioavailability in basmati rice and concurrently reducing Zn dose by 20%, making it a profitable option for farmers.

Combined effects of freeze–thaw cycles and chemical corrosion on triaxial mechanical properties of sandstone

To study the combined effects of freeze–thaw cycles and chemical corrosion on deterioration mechanism of triaxial mechanical properties of sandstone in cold regions, triaxial compression tests were carried out on homologous sandstone specimens after corrosion action of acidic, alkaline or neutral solution and cyclic freeze–thaw action, by taking the pH value of the hydrochemical solution and the number of freeze–thaw cycles as control parameters. The failure mode of triaxial compression is shear-slip failure, and the deviatoric stress–strain curves can be divided into four stages of compaction, linear elastic, yield, and failure. The stress–strain curves and deterioration law of triaxial mechanical properties were analyzed. With the increase of freeze–thaw cycles, the peak stress, peak strain and modulus all decrease, the deterioration degree of the rock specimens increases, and the yield plateau and plastic characteristics in the yield stage become weaker. The strong acid solution shows the largest degree of damage, and the peak stress, peak strain and modulus decreased significantly. The precipitates produced by the reaction between the strong alkaline solution and the specimens adhere to the specimen surface, pores, and fissures, thereby inhibiting further damage and deterioration. Confining pressure is an essential factor affecting the rock mechanical properties. As confining pressure increases, the strength and deformation-resistant ability of specimens increase, and the yield stage on the stress–strain curve is more obvious. The sensitivity of acidic solution to the deterioration of mechanical properties of sandstone is obviously greater than that of alkaline solution. These results provide a theoretical basis for the construction of geotechnical engineering structures in cold regions.

Inhibition mechanism of heterogeneous grain structures on plastic instability behavior in 3Mn steel

Isothermal Bainite Transformation (IBT) generates heterogeneous grain structures, leading to increased strain-hardening ability and inhibiting yield plateau and stress serrations. IA-steel and IBT-steel have ultimate tensile strengths of 1306 MPa, 1354 MPa, and elongations of 20.0%, 19.2%, respectively. IBT effectively suppresses plastic instability while maintaining mechanical properties.

Highly compressible concrete: The effect of reinforcement design on concrete’s compressive behavior at high strains

In squeezing ground tunnel construction, yielding elements must absorb the large tunnel deformation without damaging the tunnel lining. Various designs for these highly compressible structures exist. Still, they all share one commonality: they are complicated to manufacture, and it is difficult to alter their design to match desired compressive properties. A new yielding element design is presented here, consisting of corrugated metal plates embedded within fiber-reinforced concrete. As this yielding element is compressed, the corrugated plates are gradually flattened, increasing the plates’ stiffness. This mechanism enables the engineering of structures with monotonically increasing compressive stress–strain curves and matches the target compressive properties. When compared against alternate reinforcement schemes, including fiber reinforcement, flat plate reinforcement, and polymeric lattice reinforcement, compression results indicate that only the corrugated metal plate reinforcement produced monotonically increasing stress–strain curves while keeping stress levels below a prescribed limit. Additionally, the corrugated metal plate-reinforced specimens began densification at a strain 280% larger than the strain at which the fiber-reinforced samples began densification, indicating that the corrugated metal plates extended the yield plateau. Fiber-reinforced concrete in conjunction with corrugated metal plates shows promise for use as a yielding element.

Association of photosynthesis of flag leaves with grain yield in pearl millet (Pennisetumglaucum (L.) R. Br.)

The grain yield of pearl millet has increased continuously in past decades through development of hybrids and improved production practices. Elite inbred lines are crucial for innovating new germplasm/traits to break the yield plateau. Hence, 64 inbred lines of pearl millet were evaluated to assess the extent of genetic variability and association among agronomic, physiological and grain yield contributing traits. Analysis of variance showed significant differences among the lines for all the traits indicating sufficient variability for genetic exploitation. The preferred arid adaptation traits, early flowering and high tillering were associated with each other making them amenable for simultaneous selection and these two traits were not associated with grain yield. Hence, for deriving ideotypes suitable for arid adaptation, intensive selection for grain yield in early flowering background was suggested. In productive regions, the plants having greater plant height, larger size of leaves including flag leaf, larger panicle size to accommodate the photosynthates are desirable and these traits were found to be associated among themselves and with grain yield in the current study. Though the maximal photosynthetic efficiency, Fv/Fm was significantly associated with leaf length, flag leaf length, flag leaf width and actual PSII efficiency (Y(II)), these physiological parameters did not significantly associate with grain yield directly and their effects are confounded within the leaf morphological variation.

Compression-Softening Bond Model for Non-Water Reactive Foaming Polyurethane Grouting Material

In this study, the uniaxial compression and cyclic loading and unloading experiments were conducted on the non-water reactive foaming polyurethane (NRFP) grouting material with a density of 0.29 g/cm3, and the microstructure was characterized using scanning electron microscope (SEM) method. Based on the uniaxial compression and SEM characterization results and the elastic-brittle-plastic assumption, a compression softening bond (CSB) model describing the mechanical behavior of micro-foam walls under compression was proposed, and it was assigned to the particle units in a particle flow code (PFC) model simulating the NRFP sample. Results show that the NRFP grouting materials are porous mediums consisting of numerous micro-foams, and with the increasing density, the diameter of the micro-foams increases and the micro-foam walls become thicker. Under compression, the micro-foam walls crack, and the cracks are mainly perpendicular to the loading direction. The compressive stress-strain curve of the NRFP sample contains the linear increasing stage, yielding stage, yield plateau stage, and strain hardening stage, and the compressive strength and elastic modulus are 5.72 MPa and 83.2 MPa, respectively. Under the cyclic loading and unloading, when the number of cycles increases, the residual strain increases, and there is little difference between the modulus during the loading and unloading processes. The stress-strain curves of the PFC model under uniaxial compression and cyclic loading and unloading are consistent with the experimental ones, well indicating the feasibility of using the CSB model and PFC simulation method to study the mechanical properties of NRFP grouting materials. The failure of the contact elements in the simulation model causes the yielding of the sample. The yield deformation propagates almost perpendicular to the loading direction and is distributed in the material layer by layer, which ultimately results in the bulging deformation of the sample. This paper provides a new insight into the application of the discrete element numerical method in NRFP grouting materials.

High-Cycle Fatigue Properties of Titanium-Clad Bimetallic Steel with Different Interfacial Conditions

Titanium-clad (TC) bimetallic steel is an advanced composite steel consisting of metallurgically bonded titanium alloy and structural steel. This paper compared the high-cycle fatigue properties of three types of TC bimetallic steels, including two hot-rolled bonding types with different bonding strengths and an explosion-bonded type. The three types of TC bimetallic steels were all manufactured from TA2 titanium alloy as the cladding metal and Q355B structural steel as the substrate metal, of which the thicknesses are 2 mm and 8 mm, respectively. Based on the comparison results, the qualitative relationship between the bonding interface strength and the manufacturing methods with the basic mechanical and high-cycle fatigue properties was obtained. It was found that the different manufacturing methods and the bonding degree of the two component metals resulted in the different nonlinear yield plateau in the TC bimetallic steel. The high bonding strength seems to affect the failure mode of the tensile coupons. The bonding interface shear strength only slightly affects the tensile performance, which exhibits visible effects only when the test strain reaches the fractured state. In addition, three failure modes in total were found in the high-cycle fatigue tests for the three types of TC bimetallic steel. The manufacturing methods and the bonding interface strength significantly affect the fatigue phenomena of the TC bimetallic steel. The hot-rolled bonding TC bimetallic steel with high bonding strength has a 10% improvement in fatigue performance than the one with low bonding strength. Despite this, the manufacturing methods significantly affect the fatigue ratio, while the influence of the bonding strength on the high cycle fatigue performance is limited. The research outcomes can provide reference for the selection of different manufacturing methods and interfacial conditions for the use of TC bimetallic steel in structural engineering.

Distinguishing between yield plateaus and yield ceilings: A case study of rice in Uruguay

Distinguishing between yield plateaus and yield ceilings: A case study of rice in Uruguay

Development of technology for production of new materials on traditional wide-strip hot rolling mills using modern methods of numerical and physical modeling. Part 2

The first part of the article presents a modern approach to the technology development for the manufacture of hot-rolled steel with characteristics that exceed the passport values of the equipment of traditional hot strip mills using based on the widespread use of mathematical and simulation methods. The stages of the laboratory experiment and the carried-out investigations are described. In the second part, the results of industrial implementation and their comparison with the results of a laboratory experiment are considered. According to the developed modes of hot rolling on the hot strip mill 2000, industrial batches of rolled products were produced using the technology of direct quenching from rolling heating. The resulting microstructure is a matrix of lower bainite with a share of upper bainite, retained austenite, and Fe3Mo3C carbide phase in total no more than 10 %. The formed structure provides in rolled products a strength of 1075 MPa and a relative elongation of at least 10 %. Due to the formation of a dispersed bainitic microstructure, the tension diagram is characterized by a degenerate yield plateau. The developed deformation rolling mode in the roughing and finishing groups of stands makes it possible to form elongated “fritter-like” grains with a grain length to thickness ratio of 2.5–3.0 before accelerated cooling, due to which the KCV–40 impact strength values significantly exceed the consumer requirements (170>27 J/cm2). Using modern numerical simulation methods, the parameters of accelerated cooling on the run out table and the downcoiler settings of the hot strip mill were determined, which made it possible to achieve the desired temperature state of the strip and defect-free coiling from the first piece. The convergence of the results of laboratory and industrial experiments is shown.The research was carried out within the framework of the program of strategic academic leadership of the Russian Federation "Priority-2030", aimed at supporting the development programs of educational institutions of higher education, the scientific project PRIOR/SN/NU/22/SP5/26 "Creation of innovative digital tools for the use of applied artificial intelligence and advanced statistical analysis of big data in technological processes for the production of metallurgical products.

Stress-strain curves for wire arc additively manufactured steels

Interest in the use of wire arc additive manufacturing (WAAM) in construction has increased rapidly in recent years. Key to facilitating wider application is an improved understanding of the material behaviour. In particular, with structural design by finite element analysis in mind, constitutive models to describe the full range stress-strain response of WAAM steels are needed; development of such models is the focus of the present study. WAAM normal-strength steels generally exhibit a stress-strain response featuring a well-defined yield point, a yield plateau (for machined material) or slightly inclined yield plateau (for as-built material) and subsequent strain hardening, while the stress-strain response of WAAM high-strength steels is typically rounded, with no distinct yield point or plateau. This behaviour is similar to that of conventionally-produced steels, and hence can be represented analytically using existing material models, but with suitable modifications — a quad-linear or bilinear plus nonlinear hardening model and a two-stage Ramberg-Osgood model are proposed for WAAM normal- and high-strength steels, respectively. Predictive expressions or standardised values for the input parameters required in the models are developed and calibrated against a comprehensive database of WAAM steel coupon test results collected from the literature. The experimental database comprises over 600 engineering stress-strain curves and covers different feedstock wires, surface finishes (i.e. machined and as-built), material thicknesses, directions of testing and printing strategies. The proposed material models are shown to accurately predict the full stress-strain curves of WAAM steels, and are considered to be suitable for incorporation into analytical, numerical and design models for WAAM steel structures.

Discontinuous yielding in bare and embedded reinforcing bars: Implications on the determination of steel and bond shear stresses from strain measurements

Distinct jumps from the yield strain to the strain at the onset of hardening occur in bare reinforcing bar exhibiting a yield plateau once the yield limit is reached. These strain discountinuities known as Lüders bands nucleate at single locations and travel along the bar axis at a constant load. In contrast, no such strain jumps were observed in recent experiments with such reinforcing bars embedded in concrete and instrumented using distributed fibre optical strain sensing (DFOS). Instead, the strain increased gradually at locations where the yield limit was exceeded. This observation, made possible by the high sensing resolution of DFOS, cannot be explained entirely by the known effect of tension stiffening. While bond stresses cause a variation of steel stresses along the bar axis and, hence, indeed prevent Lüders bands from propagating along embedded bars, distinct strain jumps would still be expected at the locations where the yield stress is reached. The different steel stress–strain behaviour is rather attributed to the three-dimensional stress state induced by the locally concentrated bond forces at the bar ribs. These forces presumably (i) cause the direction of maximum shear stress to vary over the bar cross-section, leading to a loss of the macroscopic yield phenomena, and (ii) reduce the nominal yield stress in axial tension. As a consequence of the altered yield behaviour, a simple conversion from strains to stresses using a steel stress-strain relationship derived from uniaxial tensile tests leads to incorrect results for the strain range slightly below the yield limit up to the onset of hardening. These findings are most relevant for experimental studies investigating bond with DFOS, since researchers rely on the validity of the steel material law to derive and validate bond stress-slip relationships.This paper presents the results of the mentioned experiments, discusses the observed differences between bare and embedded reinforcing bars along with potential explanations, and outlines the implications on the determination of steel and bond shear stresses based on strain measurements in reinforcing bars embedded in concrete.