Nominal Yield Stress Research Articles

Experimental testing and plastic strain analysis of high strength steel longitudinal plate to tubular X joints

Applying high strength steels to tubular joints can increase the propensity to fracture failure due to their lower ductility. However, to simulate the fracture behavior in the finite element (FE) analysis, sophisticated material damage model with rigorous fracture criteria should be incorporated. Recently, the latest draft of ISO 14346 has advocated the use of 5% strain limit in FE analysis of tubular joints. The strain limit concept has been further developed by the authors’ previous work, for its use as a practical alternative to the complicated material damage model. Specifically, a lowered limit of 2.5% was recommended for longitudinal plate to circular hollow section (CHS) joints subjected to in-plane bending. This study extends the strain limit investigation to longitudinal plate to tubular X joints subjected to tensile loads. An experimental program is first presented which included both CHS and rectangular hollow section (RHS) chord members fabricated from two high strength steels with nominal yield stresses of 460 MPa and 700 MPa. Based on test-validated supplemental numerical analyses, it is suggested that the joint load corresponding to 2.5% strain limit can be taken as the load-bearing capacity to suppress occurrence of a fracture failure for the tubular joint configurations considered in this study.

Tensile Coupon Testing and Residual Stress Measurements of High-Strength Steel Built-Up I-Shaped Sections

High strength structural steels (with yield stresses greater than 65 ksi) may have notably different material characteristics when compared to structural steels conventionally used in building construction [i.e., ASTM A992/A992M (2022) or A572/A572M Gr. 50 (2021)]. This paper presents findings from an experimental program that investigated the material characterization of ASTM A656/A656M Gr. 80 (2024) plate steel. The results obtained were compared to conventional ASTM A572/A572M Gr. 50 steel. Two types of testing were performed for this work: tensile coupon testing and residual stress testing. The tensile coupon testing was carried out for both the A656/A656M Gr. 80 and A572/A572M Gr. 50 plate material. The A656/A656M Gr. 80 plate material showed more variation between the two different plate thicknesses in both mechanical behavior and microstructure due to differences in steel production. The 0.375 in. thick plate exhibited a clear yield plateau with an ultimate/yield stress ratio similar to the Gr. 50 material. In contrast, the 0.5 in. plate did not have a yield plateau and reached lower ultimate strain. The residual stress testing was performed using a sectioning technique for one A572/A572M Gr. 50 and five A656/A656M Gr. 80 built-up sections that were fabricated from 0.5 in. and 0.375 in. plate material. Residual stresses obtained from measurements were compared to previously published predictive models. The ECCS model and BSK99 models were found to be reasonable predictors of residual stresses for all specimens except the one section fabricated from 0.5 in. thick Gr. 80 plate. When comparing the Gr. 50 and Gr. 80 specimens of the same cross-sectional geometry, the residual stresses were similar, implying that cross-sectional geometry is more prevalent than the nominal yield stress in determining residual stresses in built-up I-sections.

Subassemblage test and width-thickness design limit for steel built-up box columns subjected to axial load and cyclic lateral drift

This study investigates the behavior of first-story columns under reversed cyclic lateral loading and constant axial loading by using a full-scale, one-and-a-half story subassemblage testing scheme. Four subassemblages were designed with steel beams at the second floor and a moderately ductile built-up box column with a width-thickness ratio (b/t) of either 16.2 or 20.5, which are between the highly ductile (=12.9) and moderately ductile (=23.5) requirements in AISC 341 (2016). All built-up box columns were made by SM570MB steel with a nominal yield stress, Fy= 420 MPa. A high axial load (=4200–8000 kN, corresponding to 0.2–0.4 times the axial yield strength Py) was applied to the one-and-a-half story subassemblage column before cyclic lateral drifts. The second-floor beams and column provide rotational flexibility at the top end of first-story columns in the subassemblages, significantly changing column’s stiffness, moment distribution, and inflection point location observed from typical column testing. The subassemblage column and the isolated column had a similar maximum moment at the first cycle of 0.04 rad drift, but the later showed larger strength degradation in the following cycles due to larger fixity at the column top end. An experimental solution that relates the column width-thickness ratio to the moment strength deterioration was developed to obtain design b/t limits for highly and moderately ductile built-up box columns under combined high axial load (0.3–0.4Py) and cyclic lateral drift.

Experimental study on bond behavior of steel bar embedded in thin UHPC

Due to the high compressive strength and excellent tensile properties of UHPC, reinforced UHPC with a thickness of 45 mm–60mm and a concrete cover of 15 mm–19mm has been used in the steel-UHPC lightweight composite deck. However, the researches about the bond behaviors of steel bar in UHPC with the concrete cover less than 20 mm were still limited. This paper aimed to study the bond behaviors of steel bars in UHPC with a thinner concrete cover (≤20 mm) and develop the calculation method for the lower bound of bond strength. A program of pullout test was conducted using the tensile test setup mimicking the tensile stress condition of structural members. The effects of concrete cover (10 mm, 15 mm and 20 mm), steel fiber volume fraction (0.5%, 1% and 2%), steel bar diameter (10 mm, 12 mm, 14 mm and 16 mm), embedment length of steel bar (4db, 6db, 8db, 10db) on the bond behaviors of steel bar in UHPC were investigated through 11 groups of pullout test specimens. The test results showed that most specimens failed in concrete cover splitting and the bond strength was influenced to different degrees by the test variates. The bond strength for steel bar in UHPC with a steel fiber volume fraction of 2% varied from 12.29 MPa to 15.88 MPa, and an embedment length of 8db with a concrete cover of 15 mm could ensure the steel bar with a diameter less than 16 mm reached its nominal yield stress of 400 MPa before the bond failure. On the basis of pullout test results in this study and the published test data, an empirical equation was proposed to predicted the lower bound of bond strength for steel bar in UHPC.

In-situ observation of ‘chemical’ strengthening induced by compositional fluctuations in Nb-Mo-Ta-W

The mechanical response of refractory high-entropy alloys (RHEA) can be greatly influenced by local chemical fluctuations. Here, we perform in-situ transmission electron microscopy (TEM) nanocompression tests at room temperature of body-centered cubic Nb-Mo-Ta-W RHEA single-crystal nanopillars to understand the reasons behind this at atomic scale. We find that (i) chemical composition is not uniform in space, following certain oscillatory patterns and (ii) that specimen near-surface regions with local ‘soft’ spots favor dislocation nucleation over ‘hard’ ones. Under stress these dislocations glide inwardly, inevitably running into hard local regions adding hardening stresses of up to 400 MPa to the nominal yield stress. These hard local regions act as ‘chemical’ obstacles as revealed by the occurrence of double cross-slip and the formation of prismatic loops. Using electron dispersive spectroscopy along with atomic resolution TEM, we demonstrate that the obstacles correspond to atomic environments presumably rich in W surrounded by misfit edge dislocations.

Lateral‐torsional buckling experiments of built‐up section CFS beams

AbstractThis paper investigates the lateral‐torsional buckling of built‐up cold‐formed steel beams featuring two or three component C‐ or sigma‐sections arranged to have a closed loop. Several thicknesses of each cross‐section with nominal yield stress values of 450 MPa and 550 MPa were tested to obtain lateral buckling strength data for a range of cross‐section slenderness values. Complementary reference tests were conducted on the same built‐up sections braced laterally to obtain the in‐plane section capacities, precluding the effect of lateral‐torsional buckling. Coupon tests were also conducted to determine the mechanical properties of the steel material. The test rig featured a dual‐actuator set‐up, with one actuator (applying vertical load) mounted on a trolley that was moved horizontally by the second actuator in response to the signal from a transducer measuring the lateral buckling displacement at the mid‐length of the beam, thus ensuring vertical loading throughout the experiments. The rig also featured specialized end bearings allowing free flexural rotations and warping, while restraining lateral displacements and twist rotations. The paper describes the test rig and the results of the in‐plane and lateral‐torsional buckling experiments. Particular attention is paid to the use of closed parts to enhance the torsional rigidity of the built‐up sections and thereby their lateral‐torsional buckling capacity.

EVALUATING MESOSCALE STRAIN LIMIT FOR TRANSVERSE PLATE-TO-RHS X JOINTS

Recently, mesoscale strain limit has been introduced for finite element (FE) analysis and FE-based design of welded steel tubular joints. The strain limit criterion appears to be useful particularly for the joints made of high strength steels, considering their lower ductility and higher propensity to fracture failure when subjected to tensile strains. While the latest revised draft of ISO 14346 recommended the use of 5% principal strain as the strain limit for all kinds of tubular joints, a previous study by the authors has demonstrated that a limit lower than 5% principal strain may be required for welded plate connections. In this study, the load-deformation relationship and fracture failure of transverse plate-to-RHS X-joints are investigated to evaluate appropriate strain limit for such type of joints. Based on test and test-validated FE analysis with a high strength steel of grade 700 (i.e., nominal yield stress of 700 MPa), it is suggested that more conservative strain limit should be applied to transverse plate-to-RHS X-joints.

Normalized structure parameter of magnetorheological fluids under unidirectional monotonous squeeze

The evolution of chain structure parameters for magnetorheological (MR) fluids under unidirectional monotonous squeeze were studied. A normalized structure parameter SnT was proposed. The compressive behavior of MR fluids was experimentally researched under compressive speeds v in squeeze flow and the nominal shear yield stress was calculated according to the squeeze flow theory. So SnT can be derived. It showed that SnT rapidly increases with increasing compressive strain under a high magnetic field and low v. As an analogy for friction, the correspondence between τy/B2 and friction coefficient suggests a possible origin of friction in the mechanical behaviors of MR fluids. Moreover, the results at lower v seem to deviate from the predictions of Deborah number De and the Kelvin model. The aggregation of chain structure leading to much longer relaxation times at lower v was thought to result in the high De in squeeze flow.

A novel analytic model for sealing performance of static metallic joint considering the yield hardening effect

The static metallic joints’ sealing performance is deeply impacted by the plastic deformation and the interfacial separation of the contact surfaces with self-affine roughness. The yield hardening effect, unavoidable at the contact interface, is of vital importance to the plastic deformation and the distribution of the interfacial separation. However, most of the previous research ignores the effect of yield hardening, assuming that the contact surfaces are elastic-perfectly plastic. To address the problem, a novel analytic model for investigating the sealing performance under the effect of yield hardening has been developed in this paper. The upper boundary of contact stress in the conventional Persson model has been enlarged from a constant nominal yield stress to a maximum yield stress that varies with contact strain, making the model adaptable to actual scenarios with yield hardening effects. Utilizing the measured data of contact surfaces as input, the corresponding leakage rates are calculated. Besides, the contact stress distribution as well as the real contact area at the interface is also discussed. The sealing experiments are carried out accordingly, verifying that the proposed model owns the ability to predict the leakage rate under the effect of yield hardening.

Experimental study of DED-arc additively manufactured steel double-lap shear bolted connections

An experimental study into the structural behaviour of Directed Energy Deposition-arc or wire arc additively manufactured (DED-arc AM and WAAM, respectively) steel double-lap shear bolted connections is presented. The mechanical properties of the material, which had a nominal yield stress of 420 MPa, were first determined by means of tensile coupon tests. Sixty connection specimens of two different nominal thicknesses and two print layer orientations were then tested to failure. The geometry of the test specimens was determined by 3D laser scanning, while the deformation and strain fields were measured during testing using digital image correlation. The observed failure modes included shear-out, net section tension, bearing and end-splitting, while a new hybrid mode of shear-out and net section tension was identified for the first time. The test results were compared against the predictions of current design specifications, namely AISI S100 and AS/NZS 4600 for cold-formed steel and AISC 360 and Eurocode 3 for structural steel, to evaluate their applicability to WAAM elements. Overall, the structural behaviour of the tested specimens followed the anticipated trends, and the predicted resistances determined from the current design specifications were generally reasonable. There were, however, a number of exceptions to this, highlighting the need for new design provisions, together with appropriate safety factors, that are specific to this form of manufacture.

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.

Experimental investigation of wire arc additively manufactured steel single-lap shear bolted connections

An experimental investigation into the structural performance of wire arc additively manufactured (WAAM) steel single-lap shear bolted connections is presented in this paper. The steel wire had a nominal yield stress of 420 MPa. Sixty specimens of different thicknesses, printing strategies and geometric features including end distances and plate widths were tested and analysed. The shear-out, net section tension fracture, localised tearing and curl-bearing failure modes were observed and discussed, while end-splitting was also evident. Digital image correlation (DIC) was used for detailed monitoring and visualisation of the surface strain fields that developed during testing, providing valuable insight into the developed failure mechanisms. The experimental results, which generally followed the anticipated trends, were used to assess the applicability of current design specifications developed for conventional steel bolted connections to WAAM steel bolted connections. It was found that both the cold-formed steel specifications (AISI S100 and AS/NZS 4600) and the structural steel specifications (AISC 360 and EN 1993-1) devised for conventionally manufactured steel elements, could yield considerable overestimations and underestimations of the test capacities, depending on the geometry. The overestimations are caused by shortcomings in the existing design provisions for out-of-plane failure modes, which are particularly prevalent among WAAM steel connections due to their material ductility and surface undulations, which promote curling. The underestimations relate primarily to the conservatism of the shear-out provisions. Further research is underway to underpin the development of improved design provisions.

Testing of high strength steel welded thin-walled RHS subjected to combined compression and bending

The paper presents an experimental investigation of the local buckling of high strength steel Q460C, Q550D and Q690D welded thin-walled rectangular hollow sections (RHS) under combined compression and bending. Twenty-two short columns, of which five were fabricated from Q460C, twelve from Q550D and five from Q690D, under eccentric compression were loaded to failure. The local buckling behaviors are examined and the ultimate carrying capacities obtained from the test, as well as the numerical modeling reported in the existing literature, are compared with the results obtained from the American Specification ANSI/AISC 360-16, Eurocodes BS EN 1993−1−1 and BS EN 1993−1−5 , Chinese Standard GB 50017–2017 and Australian Standard AS 4100: 2020 to determine whether these technical standards are applicable to the high strength steels with nominal yield stress of 460 MPa, 550 MPa, 690 MPa and 960 MPa. The results indicate that the ANSI/AISC 360-16 generally overestimates the ultimate load carrying capacities and will need to be revised for the high strength steels, and the BS EN 1993−1−1 , BS EN 1993−1−5 and GB 50017–2017 are in excellent agreement with the experimental or numerical results and thus suitable for the high strength steels over the range of the parameters studied herein, and the Australian Standard AS 4100: 2020 is very conservative.

Experimental Investigation of Fracture Performances of SBHS500, SM570 and SM490 Steel Specimens with Notches

High-strength steels (HSSs) with nominal yield stress not less than 460 MPa have been increasingly employed in bridge structures. Compared with SM490 normal-strength steel (NSS), HSSs, including SBHS500 and SM570, have higher strength but lower ductility, and brittle fracture can easily occur in the HSSs members with notches. Therefore, 48 tension specimens with U-notch or V-notch made of SBHS500, SM570 and SM490 structural steels are carried out. The influences of notch depth, U-notch radius, V-notch degree and chemical composition on the mechanical and fracture performances of the steel specimens are investigated. It is concluded from experimental results that SBHS500 and SM570 HSSs with higher yield stress have a relatively higher elastic stress concentration factor, crack initiation appears earlier, and brittle fracture is more likely to occur. Compared to SM570 HSS, SBHS500 HSS has better weldability.

DSM with different strain hardening exponent for stainless steel lipped channel columns

This paper reports a comprehensive numerical investigation into the cross-section behavior and resistances of stainless steel lipped channel columns under axis compression. To obtain the influence of strain hardening index n and nominal yield stress σ0.2 on the design DSM compression resistance for a stainless steel lipped channel cross section, numerical parametric study has been firstly performed. It indicates that the strain hardening exponent n has a strong effect on the DSM strength prediction of stainless steel lipped channel columns, but nominal yield stress σ0.2 has a limited effect on the DSM curve due to it already included in the DSM. A DSM model was proposed that could take into account the strain hardening exponent n and different buckling modes, and a range of numerical parametric study was conducted. Based on the proposed DSM model, a set of DSM equations was proposed for stainless steel lipped channel column. The strength predictions formula is suitable for different types of stainless steel materials, and only the corresponding strain hardening coefficient n needs to be selected according to the stainless steel material. The comparison between the test results and design predictions indicates that the proposed equations are able to capture the nominal axial strength of the stainless steel lipped channel column

Experimental investigation of direct-formed square and rectangular hollow section beams

This paper presents an experimental study on the flexural behaviours of new-generation direct-formed square and rectangular hollow sections (collectively referred to as RHS herein). A total of 22 beam specimens covering wide ranges of cross-sectional dimensions and nominal yield stresses (350 and 690 MPa) are tested. The results are compared to those from previously tested indirect-formed and hot-finished RHS to study the effect of different production processes. The effects of post-production hot-dip galvanizing on residual stresses and flexural behaviours are also studied. The applicability of slenderness limits and flexural design formulae in the current North American steel design standards are examined using the experimental data. The experimental results demonstrate that the existing flexural design rules are generally conservative for direct-formed RHS (ungalvanized and galvanized).

Tensile stress-strain models for high strength steels

In recent years, high strength steels (HSSs) with a minimum nominal yield stress of 460 MPa have been increasingly used in structural applications. An HSS may exhibit continuous or discontinuous yielding, depending on the chemical composition and manufacturing process and procedure. However, no criterion has been proposed in the literature to predict the occurrence of a yield plateau. Meanwhile, the ductility of HSSs is often limited, making it essential to predict their fracture for a safe design. However, existing stress-strain models used for HSSs cannot meet this need. To develop versatile stress-strain models for HSSs, 212 test curves are collected from open literature. In addition, four curves are obtained from tensile tests of two types of HSS produced in Australia (Bisalloy 80 and 400). By analysing the collected test data, a criterion is proposed to determine the occurrence of the yield plateau according to the yield stress of the steel and its carbon content. Two stress-strain models are then proposed for HSSs exhibiting continuous and discontinuous yielding, respectively. Moreover, predictions from the discontinuous model are also compared with the test data of conventional mild steel, proving that the model can also capture the stress-strain response of the conventional mild steel. Finally, the proposed models are used in the simulation of HSS coupons and beams, which proves the feasibility of the proposed HSS models for structural analysis.

Experimental evaluation and numerical simulation of low-yield-point steel shear panel dampers

Shear panel dampers are considered as a type of reliable passive energy dissipation device for seismic resistant structures with a damage-controlling concept. It has been revealed that the shear bulking of core plates and the premature failure of boundary plates are the main concerns for a shear panel damper that result in insufficient energy dissipation and undesirable ultra-low cycle fatigue performance. This study utilizes square steel tubes serving as out-of-plane stiffeners for the core plates made of a low-yield-point steel with a nominal yield stress of 225 MPa (named LYP225 steel). Meanwhile, a reduced flange plate section is adopted to mitigate the end fracture. An experimental study is carried out on three full-scale damper specimens with different flange plates and loading protocols. The test results show that the square tubes well prevent the bulking of the core plates and all the specimens exhibit plump hysteretic response and satisfactory energy dissipation capacity. The reduced plate sections reduce the likelihood of crack propagation occurring at the flange end, and thereby improve the ultra-low cycle fatigue performance. In order to develop the numerical model of shear panel dampers, a combined-hardening constitutive model with a memory surface is applied to represent the plasticity of LYP225 steel. Based on a plasticity analysis, a simplified calibration procedure is proposed associated with the three-dimensional constitutive model, where the plasticity parameters of LYP225 steel are determined using only the monotonic tensile test data. Eventually, the numerical results of uniaxial specimens and damper specimens are in good agreement with the test results, which validates the effectiveness of the constitutive model and the proposed calibration method.

Experimental investigation of CHS T/Y‐joints fabricated from high‐strength steel

AbstractIn this study, an experimental program on welded hollow section joints fabricated from high‐strength steel is presented. The experimental program tested a total of six T or Y cold‐formed circular hollow section (CHS) joints under brace axial compression. One mild steel with a nominal yield stress of 355 MPa and two high‐strength steels with nominal yield stresses of 460 and 650 MPa were included. The test results and existing test database were used to evaluate the relevant design provisions for high‐strength steel joints. The material factor – used in the current standards to reduce the design resistance of high‐strength steel tubular joints – was found to be adequate for CHS T/Y‐joints with steel grades up to 700 MPa. The chord stress effect resulting from a particular test setup needs to be considered when evaluating the experimental joint strength. The experimental load‐indentation relationships were analysed to appraise the behaviour of high‐strength steel CHS T/Y‐joints from the perspectives of ductility and serviceability. The ductility of high‐strength steel T/Y‐joints is slightly lower than that of mild steel joints. T/Y‐joints show relatively lower ductility and serviceability performance compared to CHS X‐joint counterparts that comprise identical members. Local strain distributions in tested T/Y‐joints of different steel grades were analysed to identify the location and intensity of strain concentration. The possibility of fracture failure near the weld seam of the cold‐formed chord member is discussed.

Experimental and numerical studies on stress concentration factors of high strength steel fabricated box X-joints

High strength steel (HSS) is commonly available in the form of steel plates. The built-up box tube with sharp corners which is welded from four steel plates is usually fabricated for construction. The stress concentration factor (SCF) is crucial for the determination of the hot spot stress range which governs the high-cycle fatigue resistance of tubular joints. This paper presents experimental and finite element (FE) investigations on the SCF of HSS fabricated box X-joints. Four X-joints which are fabricated from built-up box tubes with a nominal yield stress of 890 MPa are tested under brace axial compression. The SCFs at typical hot spot locations are obtained using strip strain gauges. A FE model which has been validated against the test results is employed to conduct the parametric study on totally 112 fabricated box X-joints. The loading cases of brace and chord axial loading are covered. The effects of brace to chord width ratio, chord width to wall thickness ratio and brace to chord wall thickness ratio on the SCFs at the hot spot locations are revealed. The CIDECT SCF formulae are shown to be not suitable for the fabricated box X-joints under brace and chord axial loading. More accurate SCF equations are proposed to calculate the SCFs at the hot spot locations of the box X-joints.