Round Bar Research Articles

Study on buckling-restrained braces using multiple round steel core bars

This study proposes a novel buckling-restrained brace (BRB) using multiple round steel core bars to increase the feasible strength of BRBs utilizing steel bars with roll-threaded screw ends. The buckling restraining members of the proposed BRB consists of multiple primary tubes and a secondary tube. The primary tube directly restrains the buckling of each core bar, and the secondary tube restrains the buckling of the primary tube through spacers. This paper first describes the configuration of the proposed BRB and the design method of buckling restrainers, in which the spacer spacing is assumed to be the buckling length of the primary tube. Subsequently, cyclic loading tests of three specimens with different spacer spacing are presented to demonstrate the mechanical behavior of the proposed BRB, such as the hysteresis behavior, axial force, and bending moment of the primary tubes. The test results indicated that fulfilling the requirement of spacer spacing is essential for achieving stable behavior of the primary tubes and steel core bars. Finally, finite element analyses of buckling-restrained units of the proposed BRB with different combinations of primary tubes and spacer spacing are presented to verify the design method. The results of the parametric analyses indicated that the proposed design formula can provide a conservative evaluation of the stability of the primary tube.

Interfacial Microstructure and Properties of Clad Rebar Prepared by Clean-Interface Assembly and Vacuum Hot-Rolling

Stainless steel cladded rebars were successfully prepared by clean-interface assembly and vacuum hot-rolling process. The interfacial microstructure and properties of the clad rebars were investigated by scanning electron microscope (SVM), transmission electron microscope (TEM), and electron probe X-ray microanalyser (EPMA). The results demonstrated that owing to the diffusion of carbon, decarburised (roughly 50 μm) and composite zones (roughly 60 μm) formed on each side of the composite interface. The decarburized zone features a single ferrite texture, hence, a relatively low micro-hardness of 138HV while, due to the large amount of martensite formed within it, the composite zone has a relatively high micro-hardness of 218HV. The salt spray test showed that the corrosion rate of the clad rebars is close to that of the round stainless bars, and is approximately one-tenth that of the carbon rebars. In addition, a layered multipass welding process was used to produce a cladding joint, which was determined to have a tensile strength greater than the standard value of the parent material and excellent corrosion resistance.

Experimental and numerical study of micromechanical damage induced by MnS-based inclusions

The study is focused on the characterization of three microalloyed 38MnSiV6 steel variants in terms of microstructure, mechanical properties, and crack initiation. Conventional steel was used as reference material, while the two other variants with reduced sulfur content and Bi or Se addition were prepared. To determine the influence of the non-metallic inclusions present in the rolled round bars; hardness, tensile, impact toughness, and fatigue properties were examined longitudinally and transversally to the rolling direction. To describe the local strain and deformation around the inclusions during the microtensile tests, scanning electron microscope monitoring was adopted. Finite element analysis was applied to model strain/stress localization around inclusions during the microtensile tests. Modeling was performed in two solutions stages involving a macro-model (microtensile sample) and micro-model (inclusions). The comparison of the numerical analysis with the microstructural observations during microtensile tests showed a good agreement. The MnS and MnSeS inclusions were responsible for the intensive strain localization and subsequent crack initiation, while the Bi-MnS inclusions were not involved in this process.

Influence of the stress history and of the Lode angle on the determination of the ductile fracture locus for two steel alloys

The fracture locus of ductile materials is primarily related to the stress triaxiality. Recently, the Lode angle was also found to be relevant in the fracture locus determination. In order to characterize a material in terms of fracture locus, usually several tests, spanning a wide range of stress triaxiality and Lode angle values, have to be performed using different specimen geometries, obtained both from round bars and sheets.In this paper the fracture loci of two steel alloys (S500MC and 22MnB4) are obtained using a simplified procedure, based on the Bai–Wierzbicki model. The procedure considers, for experimental tests, only flat specimens, avoiding the use of round specimen of the same material which were not available. Experimental tests were performed on differently shaped specimens and the results were used to train finite element (FE) models. The triaxiality and Lode angle histories, computed for each test by the FE model, were used to obtain the fracture loci following two different approaches: the proportional loading approach and the non-proportional loading approach. The former does not consider how the stress triaxiality and Lode angle vary during the test, while the latter considers their history in the computation of the fracture locus.The results show how the Lode angle influences the fracture locus, especially for 22MnB4, and how non-proportional loading approach is more accurate to compute the damage for the different specimen geometries.

A predicting model for three-dimensional crack growth in power-law creeping solids

Creep fracture mechanics has been extensively studied in the past half a century, but the gap between the creep crack-tip asymptotic field and crack growth prediction has not been effectively bridged so far, hindering the development of high temperature damage tolerance design. Here a predicting model is developed for three-dimensional crack growth in power-law creeping solids with the C(t)PC-Tz asymptotic solution being applied to quantify the creep crack growth driving force. The local resistance for creep crack growth in three-dimensional cracked structures is then described and evaluated based on the micro-cavity growth and constraint theories. Validation against forty-four crack growth experimental data for eleven creeping materials with deferent properties and specimen geometries and subjected to different load level in the literature shows that the proposed model can effectively predict the creep crack growth rate and provide the upper and lower limits. In all cases, the predicted crack growth life is within a triple error band around the experimental data. All the needed material parameters can be obtained from tension tests of standard uniaxial round bar specimens. Therefore, the proposed model can bridge the C(t)PC-Tz asymptotic solution with creep crack growth in practical materials and specimens, paving a road to predict damage tolerance of three-dimensional cracked structures at high temperature.

Investigations on the Specifics of Laser Power Modulation in Laser Beam Welding of Round Bars

Welding round bars of large diameters in a rotational laser beam welding process corresponds with weld pool bulging and the risk of weld defects. Power modulation is a promising approach for bulge reduction and for keyhole stabilisation to achieve superior weld quality. The following investigations are about the specific effects of power modulation for round bars with a diameter of 30 mm. The welding speed is 0.95 m/min and argon is used as shielding and process gas. Triangle shaped power modulation at 8 kW average laser beam power, 0/2/4/6 kW amplitude power and 2/10/50 Hz modulation frequency is used for the round bar welding of a 1.4301 steel alloy. The welds are evaluated by visual inspection, metallographic cross sections and scanning acoustic microscopy. The amount of weld defects increases at medium and high power modulation, but weld pool bulging is already reduced at low power modulation. Weld pool bulging can be impeded by a low normalised power modulation frequency of 0.05 and a high modulation depth of 0.86. The power modulation’s advantages of weld mixing and degassing do not apply to rotational round bar welding because of the linear welding speed’s gradient from the specimen surface to the centre.

Influence of Extrusion Rate on Microstructure and Mechanical Properties of Magnesium Alloy AM60 and an AM60-Based Metal Matrix Nanocomposite

Metal matrix nanocomposites are attracting attention because of their great potential for improved mechanical properties and possible functionalization. These hybrid materials are often produced by casting processes, but they can also develop their property profile after hot working, e.g., by forging or extrusion. In this study, a commercial cast magnesium alloy AM60 was enriched with 1 wt.% AlN nanoparticles and extruded into round bars with varied extrusion rates. The same process was carried out with unreinforced AM60 in order to determine the influences of the AlN nanoparticles in direct comparison. The influence of extrusion speed on the recrystallization behavior as well the effect of nanoparticles on the microstructure evolution and the particle-related strengthening are discussed and assessed with respect to the resulting mechanical performance.

Prediction of stress triaxiality dependency of critical strain using mechanical properties for metallic materials

The main purpose of this study is to propose the prediction approach of stress triaxiality dependency of critical strain at ductile crack initiation obtained using axisymmetric smooth and notched round bar specimens. From parametric FE-analyses, it was found that the stress triaxiality history up to ductile crack initiation was dependent on strain hardening exponent. The possibility that strain hardening exponent was one of the dominant parameters to critical strain could be also derived from “Mohr-Coulomb fracture criterion”. In addition, taking into account ductility after uniform elongation, prediction approach of the stress triaxiality dependency of critical strain using mechanical properties, which could be obtained by tensile test with smooth specimen, was proposed. Then, using some metallic materials with different mechanical properties, it was confirmed that the stress triaxiality dependencies of critical strain could be predicted using the proposed approach and mechanical properties of the material.

A physically-based representative stress methodology for predicting stress rupture life of Ni-based DS superalloy

Smooth and three types of U-shape single-edge notched plate specimens adopted to experimentally investigate stress rupture behavior of Ni-based Directionally Solidified (DS) superalloy at 850 °C exhibit notch weakening effect and multi-source cracking initiation near the notch root. However, stress rupture behavior of smooth and V-shape notched round bars at 1040 °C revealed by Li et al indicates notch strengthening effect and creep micro-holes originating mostly from the central portion. A combined creep-viscoplastic constitutive model is employed to analyze the distribution of stress, strain and stress Triaxial Factor (TF) near the notch root. The different stress distribution and creep restraint between asymmetric notched plate specimens and symmetric notched round bars are the main reasons for the corresponding failure mechanism. Meanwhile, a good qualitative relationship exists between TF value and stress rupture life of notched specimen. Especially, the area with maximum TF value (TFmax) is highly consistent with creep damage initiation region. Hence, based on the distribution characteristics of the initial tensile loading, a representative stress method independent of time -changing creep load at the location of TFmax is conducted for life prediction. The predicted results of both smooth and notched plate specimens and round bars agrees well with the experimental results.

Research on fracture of steel structure welded joint based on micro-mechanism

Fracture failure of welding joints is one of the primary forms in steel structure, and fracture prediction can provide suggestions and guidance for engineering applications. To predict the ductile fracture of welded joints, the Void Growth Model (VGM) and the Stress Modified Critical Strain Model (SMCS) based on the microscopic mechanism were studied. The tensile tests of Q355 steel, Q235 steel, ER50-3 and ER49-1 consumables commonly used in engineering were carried out. The constitutive models of four materials were established through the tensile tests of standard round bar specimens. The fracture parameters ηmon and γ of base metal, weld metal, and heat affected zone were calculated through the tensile tests of notched round bar specimens and the UVARM (User subroutine to generate element output). Finally, the fracture prediction model was used to predict the fracture of the thin-walled steel tube beam-column joints, and the applicability of the VGM and SMCS models was discussed. The function fθ¯ on the Lode Angle parameter is introduced to modify the VGM model. The modified fracture prediction model was used to predict the initial fracture of beam-column joints.

High-Efficiency of PVD Coating Process by Applying an Additional Rotation

This article analyzes PVD coatings (physical vapor deposition—the coating material is vaporized and deposited by sputtering or arc evaporation, and PVD coatings are applied at lower substrate temperatures and thus can be applied to a wider range of substrates) applied to samples which are located in a fixture. This fixture enables additional rotation of the sample via the coating process. The fixture allows an increase of coated tools in one batch, and therefore an increase of the current capacity of the coating machine. The introductory section of the article describes the process of product design, including its modifications. The experimental section is focused on the functionality checking of the proposed design. The coating process was carried out on a machine named INNOVA. To guarantee the correct coating application during the process, it is necessary to research the coating thickness and the chemical composition of the samples and compare these results with the results of samples which were coated without using a designed fixture. Round bars with a diameter of 10 mm were chosen as test samples. On these samples, a FUTURA monolayer was applied on a TiAlN base. Chemical composition and coating thickness were evaluated using a scanning electron microscope (SEM). Using a fixture with a fourth rotation, the same chemical composition and coating thickness were achieved as those samples which were coated in a process without the use of a fourth rotation. Therefore, it was possible to confirm a capacity increase of the coating machine.

Prediction of properties on large diameter welded pipe: case study on 32″ × 16 mm X65 HSAW pipe

Large diameter welded pipes are amongst the most cost-effective transportation means for oil and gas. The production of those pipes involves different cold forming steps, as a result of which the mechanical properties on pipe will be different from the plate or coil properties. The steel manufacturer has several parameters at hand to control the properties of his final product. However, the pipe manufacturer only has a narrow process window, but eventually he is responsible for the properties of his product, i.e. the pipe. Furthermore, for some pipeline applications, the properties in both the transverse and longitudinal pipe direction must be within certain limits. This paper presents a Finite Element model which allows simulating pipe forming and subsequent mechanical testing and thus could be adopted to predict pipe properties from coil/plate properties. The complex hardening behaviour exhibited by pipeline grades is described by an extended version of the Levkovitch-Svendsen model, a constitutive model which accounts for isotropic, kinematic and distortional hardening. To validate the model, numerical predictions were compared to experimental results obtained from mechanical tests conducted on 32″ × 16 mm X65 HSAW (Helical Submerged Arc-Welding) pipes. The properties on pipe were evaluated by means of ring expansion tests and tensile tests on (flattened) full-thickness dog-bone samples and non-flattened round bar samples. Furthermore, tensile tests were performed in the transverse and longitudinal pipe direction and tests were conducted before and after hydrotesting. In general, the numerical predictions are in good agreement with the experimental data.

Research Paper on Analysis and Design of Steel Truss by using Angle and Tube Section

Now a days many structures such as railway station roof, aircraft hangers, airport terminals, parking garage, industrial building roof, large factory roof, multi-story roof truss, parking garages roof truss are for the best truss section which will suited for that selected steel roof truss structures because of the large spacing required heavy concreting and large quantity of steel for beam, column and slab casting so that is why, it is uneconomical and deflection in member is more. That’s why most probable solution for this condition tubular steel truss is unique solution within the economical point of view and various parameter are high in strength to weight ratio, low maintenance cost, long life span, no angularity, easy to manufacture and install. In today’s construction industry most of the steel building are constructed with critical sections of steel which is analysis and designed also built by regular practices approaches, therefore result of such structures are under in heavy loads or highly expensive structures. So tubular trusses are the most effective and best possible replacement for regular steel angle section. As we compare various section it is best construction industry as one of the types of steel section is most light weigh, with less economy, and other technical parameters. For low-cost economy point of view which is the main objective of the research work includes comparison of regular steel angle section structures with tube section structure for selected steel roof truss conditions. After reading literature review it is found that 15-20% cost is saved by using tube section. Analysis and design of steel roof truss elements was carried out by STADD PRO V8i computer software, by manually applying Indian Standards codes of practices. The advantage of tube section buildings is under the economy of roof truss design. There are totally different ten types of the steel roof truss section available in market. As we comparing two of them which are angle and tubular section. Other are I- section, T-section, Channel Section, Round bars, square bars, Corrugated Sheet, expanded metals, Flat sheets. This research study is carried out to provides in comparing that which method is low in economy, more load capacity of load transport of roof to purlins and purlins to members then member to column and further column to RCC block then RCC block to soil subgrade. Comparing both the section on the economical parameters by STAAD PRO V8i Software by calculating steel take off and price comparison of both the section. Also comparing parameters like node displacement, node reaction, beam reaction, min. & max axial forces and beam stresses. Structural analysis and design of both section by STADD PRO V8i computer software. After reviewing literature review papers final research paper is based on optimum and accurate steel roof truss design. Manually calculate dead load, dead load and wind load by using Indian Standard Code of Practice IS 800:2007 and IS 875(Part 3)-1987. The STAAD PRO OUTPUT method is used for determining the quantity of steel (weight) & other parameter like max. vertical displacement, max. & min. vertical force, moment at X-axis, moment at Z- axis. After analyzing fully deciding and comparing least value of quantity of steel take off in software considered as most economical truss. Key Words: Structural Analysis, Conventional/Traditional Steel Truss, Angle Section, Tube Section, AutoCAD, STAAD PRO V8i, Steel Roof Truss, IS 800: 2007, IS 806: 1968, IS 875 : 1987 for Tube Section, etc.

Closed-form equations for the calculation of stress intensity factors for embedded cracks in round bars subjected to tensile load

Fatigue crack propagation initiated from internal defects is a typical mechanism observed in high cycle fatigue (HCF) and very high cycle fatigue (VHCF) of cylindrical tensile specimens subjected to uniaxial cyclic loads. To study the fatigue crack propagation of these embedded cracks by means of a fracture mechanics approach, solutions for the Stress Intensity Factor (SIF) for different crack configurations are needed. In this paper, a set of closed-form equations for the calculation of the SIF of embedded cracks in round bars subjected to tensile load is presented. Two sets of solutions are provided, which allow for different levels of approach to be considered. The first solution provides the SIF for the vertex points of an internal elliptical crack as a function of three dimensionless parameters related to crack size, crack position and crack aspect ratio. The second solution is a simplification for eccentric circular cracks located at any position of the cross section. The methodology necessary for the application to the study of fatigue crack propagation is also presented, and a comparison with those obtained from experimental tests is included, which exhibits a very good capacity for prediction.

Accurate experimental determination of rock fracture toughness under simulated reservoir confining pressures

Rock toughness under confining pressure is recognized as a key mechanical property to understand brittle fracture in oil and gas projects. This article discusses a linear elastic fracture mechanics method to assess the critical stress intensity factor (KIC) of shale rocks subjected to confining pressure. Linear elastic fracture toughness (KIC) is determined in Round Bar Straight-Notch (RBSN) samples by a novel experimental set up. The device uses a primary chamber to apply a fluid-driven load to induce Mode-I opening. A secondary and independent chamber ensures variable hydraulic confinement pressures, up to 80 MPa, and both systems are isolated by O-rings. This allows analyzing the influence of fluid composition, temperature and confinement pressure. Early testing and finite element modelling allowed validating experimental results and identifying possible sources of experimental errors. Experimental assessment is discussed in terms of KIC tests. The effects of confinement on KIC in terms of crack closure mechanisms are recognized, pointing out a plausible cause for increased toughness as confinement rises.

On the link of the embrittlement mechanisms and microcrack nucleation and propagation properties for RPV steels. Part II. Fracture properties and modelling

Further development of local approach models is considered from viewpoint of links of local brittle fracture properties with embrittlement mechanisms and fracture modes for RPV steels. The experimental and numerical investigations are performed for 2Cr-Ni-Mo-V steel and A533 steel used for RPVs of WWER and PWR types. RPV steels are studied in the following states: (1) the initial (as-produced) state; (2) the thermally-embrittled state modelling hardening mechanism of radiation embrittlement; (3) the thermally-embrittled state modelling non-hardening mechanism of radiation embrittlement; (4) the irradiated state. Experimental studies include testing smooth and notched round bars and cracked compact tension specimens. Microcrack nucleation and propagation properties are obtained for all the investigated states of the materials on the basis of the experimental data and the predicted results obtained with the Prometey model.

On the applicability of the cumulative strain energy density for notch fatigue analysis under multiaxial loading

In this paper, the cumulative strain energy density, also called fatigue toughness, was used to predict the fatigue life of notched members subjected to multiaxial loading. The relationship between the total energy absorbed to fracture and the number of cycles to failure was established from uniaxial strain-controlled fatigue tests. Multiaxial fatigue tests were performed on notched round bars made of high-strength steel under proportional bending-torsion considering different normal stress to shear stress ratios and different loading orientations. The fatigue lives predicted with the proposed model were compared to those obtained with other stress-based and energy-based models. Overall, the proposed model led to good predictions, and showed better predictive capabilities than the other tested models.

Annealing effects on fracture process and tensile strength of non-combustible Mg products fabricated by selective laser melting

This study systematically investigates the effects of annealing heat treatment on the microstructure, plastic strain manifestation, and the consequent fracture process in products fabricated via selective laser melting (SLM). Several Mg-9%Al-2%Ca alloy round bars and cubic specimens were fabricated and heat-treated subsequently. The round bar specimens were subjected to tensile testing and subsequent fracture surface and electron backscatter diffraction analyses for the observation of plastic strain. The microstructural and microhardness analyses of the cut cubic specimens showed that the coarse microstructure at the melt pool borders dissipated owing to the annealing effect, and the overall hardness of the material was significantly reduced. Consequently, when tensile stresses were applied, stable crack propagation was enabled by microstructure homogenization instead of the sudden interruption of propagation, a phenomenon exclusively responsible for fracture under as-built conditions. Accordingly, the annealed specimens showed an increase in ductility but a reduction in the ultimate tensile strength. Furthermore, the fracture toughness value, which is a critical parameter used to predict unstable fractures under as-built conditions, can no longer be considered for the annealed specimens due to the elimination of the crack propagation interruption. Therefore, this study discusses the validity of applying an R-curve. Accordingly, a concrete analysis of the fracture process in annealed specimens can provide valuable insights for determining the conditions under which annealing would be an optimal process for the end product.

Experimental Investigation of Machining Time and Optimization of Machining Parameters Using RSM and Genetic Algorithm (GA) on 2205-Duplex Stainless Steel

Duplex stainless steel has become one of the fastest-growing materials in the stainless steel family due to pitting resistance, stress-corrosion cracking, the combination of excellent mechanical properties, production features, and the area of applications such as oil and gas, nuclear and thermal power plants, chemical processing industries, saltwater processing industries, and pipeline systems. However, it is more difficult to machine due to its high toughness, low thermal conductivity, and ductility. The experiment has conducted using 2205- Duplex Stainless steel round bar material considering carbide cutting tools using Computer Numerical Control lathe to estimate machining time to address and meet the industrial need. Using Central Composite Designed by using Response Surface Methodology technique develops a second-order mathematical model based on the machining parameters. The Analysis of Variance technique was used to investigate the material's performance characteristics, and the impact of cutting parameters on the work piece was analyzed using the Design Expert-V12 software. Cutting speed is the most crucial determining factor compared to other factors. The Genetic Algorithm is trained and tested in MATLAB to evaluate the best possible solutions. The genetic Algorithm recommends the most outstanding lowest predicted value of 1.2204 mm. The confirmatory analysis shows the experimental values, and their error percentage is within ±2%; these shows indicated predicted values are very close to the Genetic Algorithm results. The conclusions were in good agreement with the experimental machining time values.

Evaluation of cyclic bond-slip behavior of smooth bars in low strength concrete: An experimental and stochastic study

The vulnerability of non-engineered/substandard reinforced concrete (RC) members could arise from the bond-slip failure due to low interaction between the smooth bar and poor-quality (low-strength) concrete. Therefore, the cyclic bond-slip response of plain round bars in poor-quality concrete is characterized by experimental and stochastic approaches. To this end, beam-type bond-slip specimens are first exposed to quasi-static cyclic loading. Then, mathematical models are derived from the experimental hysteric responses. Inherent uncertainties in nominally identical tests reveal the high scatter in the response parameters. Therefore, finite element (FE) models generated by implementing the proposed mathematical expressions for bond-slip response are combined with a suitable stochastic sampling technique, which is Latin Hypercube Sampling (LHS). The apparent outcome is the set of load–displacement curves, which formed an envelope around the deterministic model. The peak capacity is captured satisfactorily by the stochastic-numerical models whereas the overall cyclic response is reproduced less accurately. The evaluated bond stress values and corresponding slips in the confined concrete are higher than the ones in the unconfined concrete. Moreover, the cyclic excitation impacts on the bond strength are more recognizable than monotonic loading. The sensitivity analysis, which grades the contribution of the material parameters on the global response, points out the tensile strength of concrete fct as the most significant parameter while the remaining ones have a weak or no influence.