Nominal Yield Strength Research Articles

Micromechanical fracture model parameter influencing factor study of structural steels and welding materials

Abstract Micromechanical fracture models can be used to predict ductile fracture initiation for welded steel connections under strong earthquake. The parameter influencing factors of four such models including the void growth model (VGM), the stress modified critical strain model (SMCS), the Degraded Significant Plastic Strain (DSPS) model and Cyclic Void Growth model (CVGM) were analyzed in this paper. Firstly, the micromechanical fracture model parameters of three kinds of Chinese structural steels Q235, Q345, Q460 with nominal yield strength of 235 MPa, 345 MPa, 460 MPa respectively, and two kinds of welding materials ER 50, ER55-G were calibrated through the smooth notched tensile (SNT) tests, smooth notched cyclic (SNC) tests, their corresponding finite element analyses (FEA) and scanning electron microscope tests. Six kinds of loading histories were adopted in the SNC tests to study the influence of loading history on parameter calibration of Q460 steel. In order to investigate the influence of loading direction on the parameters of welding materials, two different loading directions including along and vertical to welding length direction were adopted in the tests on ER55-G welding material specimens. Subsequently, the parameter calibration results of different structural steels and welding materials were compared with that of previous studies to study the influence of material properties. The parameter calibration results of two batches of Q345 steel were compared to study the discreteness of the parameters between different batches of structural steels. Finally, the energy spectrum tests were carried out to obtain the chemical composition of each material, and the impacts of chemical composition on the parameters were analyzed. This paper is expected to contribute for the ductile fracture analysis of welded steel connections using micromechanical fracture models.

FRP-confined concrete core-encased rebar for RC columns: Concept and axial compressive behavior

In the ultimate failure mode of columns in RC structures under seismic load, the longitudinal reinforcement generally suffers from severe buckling due to the axial compressive load, spalling of concrete cover and dilation of concrete core. Proper configuration of RC columns to postpone or prevent the buckling phenomenon can enhance the seismic performance and make full use of the strength of steel reinforcement. In this paper, a new concept and configuration of FRP-confined concrete core-encased rebar (FCCC-R) for RC columns is proposed. The concrete surrounded the longitudinal rebar is confined by an FRP tube, which can prevent the rebar from premature buckling. The axial compressive behavior of the hybrid bar considering different parameters, including the slenderness ratio, FRP tube diameter, mortar strength, and bar position was studied experimentally. Furthermore, finite element (FE) simulation was carried out and calibrated by test results. Based on the FE model, additional discussion and a parametric study was performed. Finally, the minimum FRP tube diameter required for the FCCC-R considering the compressive behavior enhancement effect was calculated, and design curves for 3 types of steel bars with nominal yield strengths of 400 MPa, 800 MPa, and 1100 MPa were proposed.

Behaviour of cold-formed high strength steel RHS under localised bearing forces

The structural response of cold-formed high strength steel (CFHSS) rectangular hollow sections (RHS) under localised bearing forces is investigated. Web crippling tests were undertaken on cold-formed steel RHS that had nominal yield strengths (0.2% proof stresses) of 700 and 900 MPa. The experiments were conducted under end and interior load cases. The experimental results were utilised in a numerical investigation to verify finite element (FE) models. Upon verification, an extensive parametric study was conducted. The ultimate capacities obtained experimentally and numerically were compared with nominal resistances computed by the current North American Specification, European Code and Australian Standard. Overall, the codified resistance provisions gave either unconservative or overly conservative predictions. Therefore, improved design rules for CFHSS RHS under localised bearing forces were proposed. The reliability of the codified as well as modified design provisions were appraised. It is demonstrated that the modified design rules proposed in this paper can provide reliable predictions for the CFHSS RHS under localised bearing forces.

Mechanical properties of Galfan-coated steel cables at elevated temperatures

Investigating the mechanical properties of Galfan-coated steel cables at elevated temperatures is important for the fire-resistant design and fire-simulation analysis of pre-stressed structures. Unlike conventional structural types, Galfan-coated steel cables comprise several steel wires encircling a core wire in different layers. The overall mechanical property of a Galfan-coated steel cable is attributed to the individual properties and collaborative mechanism of Galfan-coated steel wires. The mechanical properties of a Galfan-coated steel cable at elevated temperatures are substantially different from those of conventional structural steel or the single steel wire. In this study, 42 Galfan-coated steel cables with 19, 37, and 61 wires were tested under steady state tension at ambient and elevated temperatures ranging from 100 to 600 °C. The test results show that the cables exhibit typical non-linear characteristics with a lower proportional limit and no clear yield plateau. The reduction factors in the mechanical properties of Galfan-coated steel cables at elevated temperatures are obtained and compared with values predicted in Eurocode 2 part 1–2 and ACI 216, and other studies. Equations for nominal yield strength, elastic modulus, ultimate strength, and ultimate strain for Galfan-coated steel cables at elevated temperatures are proposed in this study. Furthermore, a modified two-stage Ramberg-Osgood model for Galfan-coated steel cables at ambient and elevated temperatures is proposed.

Bearing-strength of high strength steel plates in two-bolt connections

This paper presents an experimental and numerical study on the bearing behavior of two-bolt connections arranged in the direction perpendicular to load between high-strength steel members. A series of 36 connections are fabricated from steels with nominal yield strength of 550 MPa, 690 MPa and 890 MPa and tested to failure in double shear. The effect of steel grades, end distance, edge distance and bolt spacing on the failure mode, bearing resistance and deformation capacity are investigated with the supplementary data from the validated numerical model. To explain the reduced bearing resistance compared to tearout failure, the mechanism of splitting failure in two-bolt connections is analyzed with the assistance of numerical simulation. The boundaries to identify tearout failure, splitting failure and mixed failure with the combined features of necking and tearout are derived. A formula to predict the ultimate resistance of mixed failure is proposed. To achieve an optimum use of high strength materials, an optimal range of edge distance to bolt spacing distance ratio is suggested based on the parametric analysis. Comparison with test results show that Eurocode3 method can be extended to bolted connections between high strength steel members with considerable margin of safety.

Low temperature impact toughness of high strength structural steel

Impact toughness properties of four kinds of high strength steel (HSS) were investigated with the nominal yield strength 460, 690, 800 and 960 MPa, respectively. The specialty of impact toughness among base metal, heat affected zone (HAZ) and weld metal were also compared and discussed. The correlations between the impact toughness of HSS and their nominal yield strength, plate thicknesses, testing temperature, welding methods were further studied. Combined with the scanning electron microscope (SEM) fracture surface observation, the microstructural features and corresponding fracture mechanisms have been analyzed. The results show that the impact toughness for HSS base metal deteriorated with the increasing nominal yield strength. The impact toughness of weld metal for HSS is much lower than that of the corresponding base metal. The impact performance of HAZ is discretized due to the welding thermal cycling on the HAZ where significant changes in grain structure and properties occur. The microcosmic fracture surface of HSS is examined to explain the reduction in impact performance under low temperature.

Technology for Modifying Aluminum Alloys with Ultrafine Silicon

Results are provided for a study of the structure and mechanical properties of alloy AK12 modified with tabletted ultra-finely dispersed modifier based on silicon carbide, introduced “beneath a metal stream” without additional mixing, and in a rotary device. It is established that the optimum method for introducing modifier is that of “beneath a metal stream.” In this case the ultimate breaking strength increases from 110 to 150 MPa, nominal yield strength from 85 to 94 MPa, and relative elongation from 2.3 to 6.2% (metal pouring temperature into a mold 750 °C). Metallographic examination is used to determine the reduction in macro-grain size in test pigs (ingots) by a factor of four compared with control alloys, absence of porosity, and an increase in ingot density from 2.59 to 2.66 g/cm3. Introduction of ultra-finely dispersed modifier based on silicon carbide increases the number of crystallization centers in a 1 kg ingot to 2·1011 units.

Flexural behavior of high-strength steel hybrid composite beams

In this study, hybrid utilization of high-strength steel in composite beams was proposed in order to maximize their flexural capacity and full-scale testing was conducted in two phases to investigate their flexural behavior. In fabricating specimens, high-strength steels were utilized for the bottom flange while normal-strength steels were used for the top flange and the web. In Phase I testing, however, all the high-strength steel bottom flange specimens were not able to reach their plastic moment by about 10–15% due to unexpected longitudinal shear failure along the beam axis, although sufficient shear studs were provided for full composite behavior and the plastic neutral axis location was limited within 15% of the total depth of the composite beam section. The specimens in Phase II testing designed with additional shear reinforcements showed no longitudinal shear cracking and developed their plastic capacity with reasonable deformability as intended in design. This implies that, different from the design of conventional composite beams, checking the longitudinal shear strength of composite concrete slab is crucial when designing hybrid composite beams utilizing high-strength steels. The nominal longitudinal shear strength was well predicted by the shear-friction based models in ACI 318-14 and AASHTO LRFD. The test results of this study show that when the limitation on the depth of the plastic neutral axis set forth by Eurocode 4 is satisfied along with sufficient longitudinal shear strength, the plastic stress design method can still be applied to the design of hybrid composite beams utilizing high-strength steels whose nominal yield strength is as high as 650 MPa.

The performance of lap splices in RC beams under inelastic reversed cyclic loading

A number of design codes such as ACI 318-14 have prohibited the use of lap splices in critical zones of special moment frames where flexural yielding is anticipated. This restriction is due to the unreliable performance of lap splices under cyclic loading. This article is concerned with the flexural performance of RC beams with lap splices under reversed quasi-static cyclic loading. For the calculation of lap splice length two procedures were adopted here; i.e. conventional design procedure recommended in ACI 318-14 and modified splice length which assumes 1.25fy in design relations instead of the nominal yield strength. For this purpose, a total of nine full-scale RC beams with different splice and reinforcement configurations for two grades of steel were designed and fabricated. Four-point bending tests were carried out on test specimens to evaluate the performance of lap splices under pure bending. Test results indicated that the specimen with splice designed according to ACI318-14 provisions could not survive reverse cyclic loading up to flexural failure. However, the specimens with a modified splice length were able to survive cyclic loading with adequate flexural ductility.

Deformation mechanisms of Al0.1CoCrFeNi high entropy alloy at ambient and cryogenic temperatures

The deformation mechanisms of an Al0.1CoCrFeNi high entropy alloy (HEA) produced by the vacuum levitation melting (VLM) method were studied in the compression test at temperatures range from 298 K to 77 K. The nominal yield strength was 120, 168, 275 MPa for the alloy tested at 298, 200, and 77 K, respectively. Note that all the compression tests were done only up to the ~ 50% plastic strains. In addition, the remarkable strain-hardening ability and the serration phenomena were observed during the deformation of the alloy at 77 K. Transmission electron microscopy (TEM) and high-resolution TEM (HRTEM) characterizations revealed that at 298 and 200 K, the deformation in the alloy occurs on the normal FCC {111}< 110 > slip systems by planar dislocation glide. In contrast, at 77 K, nano-twinning becomes an additional deformation mechanism. Overall, the significant increase in the nominal yield strength, extensive strain-hardening ability, and the serration phenomena are attributed to the capability of extensive nano-twinnings at 77 K.

Nanoindentation of high-purity vapor deposited lithium films: A mechanistic rationalization of diffusion-mediated flow

Abstract

Nanoindentation of high-purity vapor deposited lithium films: A mechanistic rationalization of the transition from diffusion to dislocation-mediated flow

Abstract

Finite element modelling of concrete-filled double-skin short compression members with CHS outer and SHS inner tubes

Composite steel-concrete construction utilising the benefits of both materials has extensively been used in structural engineering with the ability to be used in onshore and offshore applications. One of these elements is the concrete-filled double-skin tubes (CFDST) which is a concept that has been developed in recent years. In this column, the steel tubes can support considerable amounts of loads while providing permanent formwork to the concrete. Likewise, the steel tubes add confinement to the concrete whereas the concrete infill delays the local buckling of the steel tubes. This paper provides the behaviour of CFDST short columns under concentric compressive loads. The specimens studied consist of an outer skin, which is a circular hollow section (CHS), and an inner skin, which is a square hollow section (SHS), with the annulus filled with concrete while the inner tube is completely empty. A finite element (FE) analysis is generated in order to analyse the performance of such columns. Therefore, to assure the accuracy of the modelling of these specimens, FE models with concentric axial loads are developed and compared against results from past experiments. In view of this, different stress-strain curves for structural steel and concrete infill are identified, and those that provided the best curve fittings were selected for the parametric study. Accordingly, the best combination of the constitutive models of both the steel (suiting the cold-formed tubes) and the concrete (filled in double-skin tubes) is found based on previous research and considered herein. The study aims at examining the effect of various diameter-to-thickness (Do/to) ratios, width-to-thickness (Bi/ti) ratios and material properties such as nominal compressive strength and nominal yield strength on the fundamental behaviour of CFDST. In the parametric study, high nominal compressive strengths are tested, and the steel tubes are cold-formed from different design yield strengths. Overall, this paper provides important conclusions for current circular CFDST short columns with an inner SHS and an outer CHS.

Mechanical response of ultra-high strength (Grade 1200) steel under extreme cooling conditions

Ultra-high strength steels (UHSS) with nominal yield strength of up to 1200 MPa have many potential applications in engineering fields. However, due to inadequate knowledge on the mechanical behaviour of these materials under extreme loading conditions (e.g. severe temperature changes), their extent of application in civil construction has remained restricted. This paper characterises the mechanical properties of Grade 1200 UHSS under extreme cooling conditions, which is defined as cooling from fire temperatures with an extreme cooling rate or cooling from ambient state to sub-zero temperatures. In order to simulate the extreme cooling rate, UHSS standard dog-bone specimens are heated up to different fire temperatures and cooled to room temperature using water-quenching (WQ) technique. To evaluate the effect of extreme cooling rate on the post-fire mechanical response of the tested materials, the residual mechanical properties of water-quenched specimens are compared to the air-cooled (AC) ones for different fire temperature exposures. For simulation of extreme cooling temperatures, the test specimens are cooled to sub-zero temperatures using Liquid Nitrogen (LN) injection. The changes in the mechanical properties of the tested specimens are then quantified and discussed. In both set of experiments, the mechanical behaviour of UHSS under extreme cooling conditions is compared to the common structural steels. Microstructural evaluation of UHSS tested specimens are also presented using optical and scanning electron microscopy (SEM) techniques.

Experimental and model investigation on residual stresses in Q460GJ thick-walled I-shaped sections

Residual stresses can occur through a variety of mechanisms, and welding is one of mechanisms. This kind of stress variation through materials can be very complex and can vary between compressive and tensile stresses from layer to layer. A new type of high performance structural steel named by Q460GJ has been increasingly utilized in many large scale steel constructions in China, such as the National Stadium of China in Beijing (Birds Nest) and Canton Tower in Guangzhou. Q460GJ structural steel has a nominal yield strength of 460 MPa, which does not decrease significantly with the increase of steel plate thickness. Thus, Q460GJ structural steel is normally used in thick-walled welded sections. Nevertheless, the residual stresses on thick-walled sections may be more complex. Currently, only limited knowledge is available on the residual stress distribution of the members fabricated from high performance structural steel plates, especially for thick-walled sections. In this paper, the residual stresses in four welded medium-walled and four thick-walled H-section specimens fabricated from Q460GJ structural steel plates were studied by experimental tests. Residual stresses were measured by sectioning method, and the variation of residual stress through plate thickness was determined by slicing method. This study provides the magnitude and distribution of the longitudinal residual stresses. This paper also explores the effect of plate thickness on residual stress distribution and the interaction between flanges and webs. Finally, based on the statistical evaluation of the experimental results, a simplified multilayer residual stress model is proposed for predicting residual stress.

Fatigue strength of repaired welded connections made of very high strength steels

Modern steel manufacturing techniques make it possible to produce steel with the nominal yield strength up to 1300 MPa for structural purposes. However, the application of very high strength steels is still limited in the civil engineering structures due to lack of knowledge about the effects of the manufacturing process and experimental results regarding the structural behaviour of the material. Moreover, in a fatigue loaded very high strength steel structure absolute and relative stress variations will be higher compared to stresses in structures made of lower steel grades. Accordingly, the fatigue issue will be one of the most important design criteria for very high strength steel structures. In this current study, V-shape welded specimens were manufactured from S690 and S890 rolled steels and cast steels with similar yield strengths. Fatigue cracks were created in the weld toe of the specimen under a fluctuated loading and subsequently the fatigue damaged specimens were repaired by the removal of the cracks with subsequent welding. The fatigue strength curves of repaired specimens are compared with the detail categories of EN 1993-1-9 [8] and the fatigue strength curves of the test results in the as-welded condition from literature. The fatigue strength of the fatigue damaged connections was completely recovered by the established repair procedure.

Residual mechanical properties of stainless steels S30408 and S31608 after fire exposure

With excellent corrosion resistance and appearance, stainless steels have been widely used in building structures. Although fire accidents in steel buildings have happened occasionally in recent years, only few steel structures have collapsed entirely and most can still be used after reasonable assessment, repair, and reinforcement, thus reducing economic losses of fire accidents. The post-fire mechanical properties of stainless steels are the key parameters in the process of structural assessment after a fire. In this paper, the effects of exposure temperature, exposure duration, and cooling mode on mechanical properties of common stainless steels S30408 and S31608 were studied through experiments. The following conclusions were obtained: 1) when the exposure temperature is lower than 1000 °C, the post-fire ultimate tensile strength, rupture stress, and post-fracture elongation of S30408 and S31608 have little weakening; 2) When the temperature is lower than 1000 °C, the post-fire elasticity modulus of S30408 and S31608 increased to some extent, initially increasing then decreasing with exposure temperature increase, reaching the peak at 800 °C with an increase coefficient of about 50%. 3) The post-fire nominal yield strength of S30408 and S31608 decreased significantly, and when exposure temperature reached 1100 °C, the post-fire nominal yield strength of S30408 and S31608 decreased by about 30% and 20%, respectively. 4) Cooling mode (natural cooling in air and water cooling) and high temperature exposure duration (30 min and 180 min) had slight effects on the post-fire elasticity modulus, nominal yield strength, ultimate tensile strength, rupture stress, and post-fracture elongation of stainless steels.

Experimental Investigation into Corrosion Effect on Mechanical Properties of High Strength Steel Bars under Dynamic Loadings

The tensile behaviors of corroded steel bars are important in the capacity evaluation of corroded reinforced concrete structures. The present paper studies the mechanical behavior of the corroded high strength reinforcing steel bars under static and dynamic loading. High strength reinforcing steel bars were corroded by using accelerated corrosion methods and the tensile tests were carried out under different strain rates. The results showed that the mechanical properties of corroded high strength steel bars were strain rate dependent, and the strain rate effect decreased with the increase of corrosion degree. The decreased nominal yield and ultimate strengths were mainly caused by the reduction of cross-sectional areas, and the decreased ultimate deformation and the shortened yield plateau resulted from the intensified stress concentration at the nonuniform reduction. Based on the test results, reduction factors were proposed to relate the tensile behaviors with the corrosion degree and strain rate for corroded bars. A modified Johnson-Cook strength model of corroded high strength steel bars under dynamic loading was proposed by taking into account the influence of corrosion degree. Comparison between the model and test results showed that proposed model properly describes the dynamic response of the corroded high strength rebars.

Constitutive model for full-range cyclic behavior of high strength steels without yield plateau

In order to characterize static and cyclic behavior of high strength structural steels, coupons made of two typical kinds of such steels in China, i.e. Q550 and Q690 steels with nominal yield strength of 550 MPa and 690 MPa respectively, were designed and tested under monotonic tension and a variety of cyclic loading protocols. Uniaxial stress-strain curves from the test show that for both steels no obvious yield plateau was observed, and short-range isotropic softening occurred right after initial yielding, while the subsequent strain hardening was contributed by both kinematic hardening and long-range isotropic hardening. Based on these phenomena, a constitutive model was developed specifically for those high strength steels and implemented in numerical analysis through user-defined subroutines. Calibration method for material parameters in this constitutive model was also established that mainly depends on tension coupon test result. After that, simulation was conducted by using the developed model with its stress-strain curves compared with those from cyclic loading test. Good agreement was obtained. Therefore, the constitutive model and its calibration method can be used for further seismic or dynamic nonlinear analysis of high strength steel structures and will help to develop reasonable and reliable ductile design method of those structures.

Q690 high strength steel T-stub tensile behavior: Experimental and numerical analysis

To evaluate the tension behavior of welded high strength steel (HSS) T-stub joint accurately, an experimental study on the mechanical properties of welded T-stub joints with nominal yield strength of 690MPa under tensile load was conducted. This experimental program includes twelve welded T-stub joints varying with bolt diameter, bolt strength grade, and flange size. Plastic bearing capacity and initial stiffness were obtained from the test and compared with the EC3 Part 1-8 predicted ones. A simplified finite element model considering the nonlinearity of material, geometry and contact was developed and verified in order to perform an extensively parametric study. The effects of flange thickness, bolt pretension and stiffener on the mechanical properties of joints were investigated and discussed. The test results show that the methods proposed in EC3 Part 1-8 for plastic bearing capacity calculation and identifying the failure mode can be applicable to HSS T-stub joints, but the methods evaluated with initial stiffness overestimate the initial stiffness of the joint. The parameter analysis results show that when the flange boundary condition close to simply supported or fixedly supported, the effect of which on the initial stiffness of the joint should be taken into account. The initial stiffness of joint increases linearly as the bolt pretension increases. However, when the pretension exceeds the standard value, an increase in pretension exerts a weak influence on the initial stiffness. When the length-height ratio of stiffener is lower than 1.0, the post-limit stiffness degenerates remarkably, and thus the ductility gets worse.