Cross-sectional Phase Distribution Research Articles

Mechanical characteristics of Quenched and Self-Tempered (QST or TMT) steel reinforcing bars used in concrete structures

Quenched and Self-Tempered (QST) steel reinforcing bar (rebar herein) is widely used owing to its high strength and ductility over conventional hot-rolled or cold deformed bars. A typical QST rebar constitutes a composite microstructure of a hard ‘tempered martensite’ (TM) peripheral ring and a ductile ‘ferrite-pearlite’ (FP) core. TM and FP are predominantly responsible for the strength and ductility, respectively exhibited by a QST rebar. A good quality QST rebar cross-section shows a uniform, continuous, and concentric TM-ring around the FP core. However, recent studies reported that poor quality QST rebars with inadequate/defective cross-sections could influence mechanical and durability properties. This study evaluates the effects of inadequate cross-sectional phase distribution (CSPD) on the mechanical properties of QST rebars. In stage 1, tensile test on TM and FP extracted from a QST rebar clearly showed that FP is ductile with low strength, while TM had a brittle behaviour with high yield and ultimate strengths. Instage 2, the composite response of QST rebars collected from different manufactures were evaluated using tensile and bend tests to correlate the effect of CSPD. Tensile test results show a significant variability (up to 15%) in the stress–strain behavior and mechanical properties exhibited by rebars with inadequate CSPD while the failure pattern shows a composite response. Poor-quality rebar specimens in bend tests showed cracks which could induce crevice corrosion. Hence, CSPD has a significant influence on the mechanical and corrosion properties of a QST rebar. Acceptance criteria based on TM-ring percentage area and distribution in the CSPD is proposed to check the quality of the QST rebars.

Fire residual stress–strain curves of corroded thermo-mechanically treated (TMT) reinforcing steel bars

Thermo-mechanically treated (TMT) reinforcing steel bars are extensively employed in RC buildings these days because of their superior thermal- and seismic-resisting characteristics, which enable them to sustain most of their strengths during earthquakes. The distinctive cross-sectional phase distribution (CSPD) of martensite, bainite, and pearlite in TMT rebars makes them outstanding. The current study examines the extent to which TMT reinforcing steel bars' mechanical properties deteriorate as a result of exposure to a combination of corrosion-induced CSPD damage and high temperatures. The research attempted to model an accidental fire in an aging structure with corroded members as a result of exposure to extreme environmental conditions. Heated residual tensile tests on rebar specimens subjected to various combinations of corrosion-induced damage and high temperatures were used in the experiment. The results show a significant loss in the key mechanical characteristics of reinforcing steel due to the superimposition of elevated temperature exposure on corrosion-induced damage to CSPD.

Cross-sectional phase distribution measurement of slug flow in small channels

Cross-sectional phase distribution measurement of slug flow in small channels

Temperature-dependent properties of corroded super ductile TMT steel bars

Super ductile TMT reinforcing steel bars are commonly used these days for RC constructions, given their excellent thermal properties. SD TMT rebars are superior due to their distinct cross-sectional phase distribution (CSPD) of martensite, bainite, and pearlite. This paper investigates the influence of the superimposition of corrosion-induced damage by the exposure to elevated temperatures on properties of reinforcing steel simulating corroded structural members exposed to an accidental fire. The experimental program involved exposing bare bars to accelerated corrosion to achieve desired corrosion levels in terms of mass loss using the impressed current accelerated corrosion technique to a precision level of ± 1%. Subsequently, the identical specimens were heated to 250 °C, 550 °C, 800 °C, and 950 °C and allowed to achieve a steady-state at that temperature before being cooled down to ambient temperatures in the furnace itself and tested in tension to failure. A detailed analysis of the effect of various combinations of corrosion-induced mass loss and elevated temperatures on ductility characteristics of reinforcing steel using various prevalent methods in the available literature is presented. The results indicate a notable reduction in reinforcing steel's critical mechanical properties due to the superimposition of the two phenomena.

Stress–strain curves and mechanical properties of corrosion damaged super ductile reinforcing steel

Super ductile thermo-mechanically treated (SD TMT) reinforcing steel bars are commonly used these days for RC constructions given their excellent thermal and seismic resistant properties. The superior performance of SD TMT rebars is due to their distinct cross-sectional phase distribution (CSPD) of martensite, bainite, and pearlite, which impart better combinations of strength and ductility than conventional classes of steel. This paper presents an investigation carried out to study the influence of corrosion-induced damage to cross-sectional phase distribution (CSPD) on mechanical properties of SD TMT bars in comparison with the performance of conventional mild steel and ordinary TMT steel bars exposed to an aggressive environment. The influence of corrosion-induced damage to cross-sectional phase distribution (CSPD) on the stress–strain response of SD TMT bars has been investigated through tensile tests in a displacement controlled UTM. The study involved tensile tests carried out on SD TMT bars corroded to 25 different levels ranging from (0–45%) on five different rebar diameters (8mmØ, 10mmØ, 12mmØ, 16mmØ and 20mmØ). The results suggest better characteristic performance of SD TMT steel bars over conventional hot-rolled rebars in mechanical behaviour's critical aspects. A correlation predicting the reduction of mechanical properties of corroded TMT bars' residual capacity concerning corrosion percentage has been suggested.

Post corrosion bond-slip models for super ductile steel with concrete

Super ductile thermo-mechanically treated (SD TMT) reinforcing steel bars are commonly used these days for RC constructions given their excellent thermal as well as seismic resistant properties. SD TMT rebars are superior due to their distinct cross-sectional phase distribution (CSPD) of martensite, bainite, and pearlite which impart better combinations of strength and ductility compared to conventional classes of hot rolled and TMT steels. The present study investigates the bond-slip response of super ductile reinforcing steel bars with concrete following corrosion-induced damage. Cylindrical pullout specimens of diameter 80 mm, 100 mm, 120 mm, 160 mm, and 200 mm were prepared as per RILEM specifications with centrally placed SD TMT bars steel bars of sizes (8mmØ, 10mmØ, 12mmØ, 16mmØ, and 20mmØ). Pull out specimens were immersed in NaCl solution before exposure to impressed current in an accelerated corrosion setup to achieve desired levels of corrosion 10 different levels of corrosion. The corroded specimens were then tested to failure in pull-out mode using displacement controlled UTM to investigate the residual bond strength and bond-slip behavior. The slip of the rebar, at the loaded as well as the unloaded end, was recorded through a set of linear variable displacement transducers (LVDT) arrangement mounted to the projected reinforcing bar on each side of the specimen. The study presents unique bond-slip response curves obtained in displacement controlled set up to depict the overall behavior of the bond mechanism. The results indicate a notable reduction in bond strength as a result of corrosion-induced damage at the interface of steel and concrete. A detailed account of the test results, analysis, discussion, and comparisons with various models in the existing literature has been presented. A normalized bond-slip relationship is proposed to model the bond-slip response of SD TMT rebars with various levels of corrosion.

Flow morphology and heat transfer analysis during high-pressure steam condensation in an inclined tube part I: Experimental investigations

In this paper, experimental investigations on the flow morphology and heat transfer in a single steam condenser tube are presented, which were performed at the thermal hydraulic test facility COSMEA (COndensation test rig for flow Morphology and hEAt transfer studies). This facility was setup to study the interrelation of condensation heat transfer with two-phase flow in a single condenser tube that is cooled by forced convection. Studies were performed for elevated pressures up to 65 bar at saturation conditions and for inlet steam mass flow of up to 1 kg/s and different inlet steam qualities. The wall heat flux is measured with distributed heat flux probe and global condensation rates were obtained from integral heat and mass balances. As a unique feature the cross-sectional phase distribution was studied via X-ray computed tomography. The data is going to be used for the validation of numerical simulations with 1D ATHLET and 3D CFD codes as presented in the second part of this paper.

Microstructural and corrosion characteristics of Quenched and Self-Tempered (QST) steel reinforcing bars

Quenched and Self-Tempered (QST) steel reinforcing bar (rebar, herein) is widely used in the reinforced concrete (RC) systems across the world. A typical QST steel rebar cross-section consists of a ductile ‘ferrite-pearlite’ (FP) core and a hard ‘tempered-martensite’ (TM) periphery. A good quality QST steel rebar is expected to have an FP core encircled by a uniformly thick, continuous, and concentric TM ring/phase. This study assessed the cross-sectional phase distribution (CSPD) of QST steel rebars produced in various countries using macroetching. These countries include Australia, Bahrain, Germany, Hungary, India, Italy, Japan, Malaysia, Russia, Singapore, SriLanka and Switzerland. Unfortunately, many rebars with 8, 12, and 16 mm diameter revealed inadequate CSPD with discontinuous and/or non-uniform TMphase. Also, the adverse effects of this inadequate CSPD on the corrosion resistance were evaluated using Cyclic Potentiodynamic Polarization (CPP) tests. For this, the FP and TM plate/coupon specimens (15 × 15 × 3 mm) were extracted from a QST steel rebar and metallographic type specimens were prepared. Then,thespecimens were immersed in chloride (Cl−) contaminated simulated concrete pore solution (denoted as SCPS). Then, the CPPtests were done after incremental addition of Cl− in SCPS to identify the Cl− threshold (Clth) of TM and FP. The FP exhibited about 10–15% lesser Clth than TM. An analytical case study showed that the use of such QST steel rebars with inadequate CSPD can result in about 20% reduction in service life. Then, bend tests were conducted on 8 mm good quality and poor quality QST steel rebars used for stirrups. Poorquality rebars with inadequate CSPD showed visible surface-cracks in some products, which could lead to severe crevice corrosion, even without the presence of chlorides. Hence,better quality control in the quenching and self-tempering process is required to improve the corrosion resistance of QST steel rebars.

Combat molten aluminum corrosion of AISI H13 steel by low-temperature liquid nitrocarburizing

Possibility of improving the resistance of AISI H13 steel to molten aluminum corrosion by liquid nitrocarburizing (LNC) was explored. The effects of the LNC parameters in terms of temperatures (703/723/743 K) and soaking time (4/8/12 h) on phase transformation, microstructure, and resistance to molten aluminum were fully studied. The surface phase compositions and the cross-sectional phase distribution of the LNC treated specimens were studied by implementable X-ray diffraction analysis. Microstructure, element distribution, microhardness, and the kinetics of the nitrocarburized case formation were fully researched. Immersion test of corrosion resistance to molten aluminum was carried out at 1023 K for 30min. It is observed that an oxide layer can be produced on the top of the nitrocarburized case during LNC treatment, which cannot be regularly produced by other nitriding methods. The nitrocarburized case consists of a compound layer, a diffusion layer, and a transition layer. The growth of the nitrocarburized case is proportional to the squared treatment time and follows the Arrhenius law for the treatment temperature. The activation energy is estimated to be 195.4 kJ mol−1. While the nitrocarburized case provided limited resistance to molten aluminum, the oxide layer formed on the top of the nitrocarburized case conferred significantly improved molten aluminum corrosion resistance, especially a duplex oxide layer produced at 743 K.

Numerical simulation and experimental study of liquid–liquid flow dispersion in conical spiral pipes

This paper presents the numerical simulation and experimental study of an immiscible liquid–liquid flow dispersion in conical spiral pipes for oil–water separation. Flow patterns of oil–water flow in the pipe are identified. The flow characteristics such as pressure drop, cross sectional phase distribution, and outlet flow rate are obtained. In addition to flow behavior, separation performance of the conical spiral pipes is examined under different operating conditions. The effects of geometric parameters such as conical angle, pipe diameter, pitch height, and outlet split ratio, on oil–water separation are revealed. Moreover, the effects of inlet velocity, inlet oil concentration, and operating temperature on the separation are obtained. In addition, it has shown that surface treatment for the conical spiral pipes has an impact on the oil–water separation. In particular, the V-shaped sawtooth surface microstructure can enhance the oil–water separation, and Graphene-coated surface exhibits good separation capacity under high flow velocity.

Measurement of Liquid Distributions in Particle Packings Using Wire-Mesh Sensor versus Transmission Tomographic Imaging

The paper compares two different imaging techniques for the investigation of the hydrodynamics in a laboratory packed bed reactor operating at different stationary states and with two different particle packings. The wire-mesh sensor offers cross-sectional liquid holdup distribution imaging at an ultra high speed of 10000 frames/s and good spatial resolution of 6 mm. It is therefore a very useful imaging tool for transient and periodic flow conditions. Up to now its influence on the flow in a packed bed was never analyzed. γ-ray computed tomography uses 662 keV γ photons to obtain cross-sectional phase distribution images. It offers as well a good spatial resolution of 2 mm and does not influence the flow but needs rather long scanning times. As a noninvasive technique γ-ray tomography has been used as a reference modality to evaluate the wire-mesh sensor measurements. Data from both imaging modalities are compared utilizing different analyzing models and showed good agreement. For the wire-mesh sensor on...

Water Local Volume Fraction on Oil in Water Dispersion

The phase distribution of water-oil flows was studied experimentally from a separated flow without mixer to a oil in water or water in oil dispersed in horizontal tubes. Under most conditions the pattern was oil continuous in water dispersed or water continuous in oil dispersed flow continuously and there is entrainment in the form of drops of phase into the other. The investigations were carried out through the cross-sectional phase distribution in the flow of mixtures of water and kerosene such as EXXSOL-D80 in a horizontal 25.4 mm bore stainless steel section. The phase fraction distribution was determined using a traversing beam gamma densitometer, with the beam being traversed in three directions (0 0 , 45 0 and 90 0 of the vertical line passing through the axis of the tube). Measurements were made at three positions spaced along the 9.7 m test section length (1.0 m, 5.85 m and 7.72 m along the horizontal tube). The measurements were done in the Two-phase Oil Water Experimental Rig (TOWER) facility. This facility allows the two fluids to be fed to the test section before they are separated and returned once more to the test line. The flow developed naturally from an initial stratified flow in which the oil and water were introduced separately at the top and the bottom of the test section respectively. It was found that the liquids were fully inter-dispersed by the end of the test section. The results were also used to define the flow patterns in water-oil liquid-liquid flow system. The phase fraction distribution was shown to be homogeneously mixed near to the outlet of the test section.

Effect of Gravity on Flow Characteristics of Developing Vertical Upward Bubbly Flow (2nd Report, Cross-Sectional Phase Distribution Pattern)

Local flow measurements were performed for vertical upward bubbly flows in a 9 mm-diameter pipe at normal- and micro-gravity conditions. A stereo image-processing method was used for measuring void fraction profile, interfacial area concentration and Sauter mean diameter. The flow measurements were performed at four axial locations (axial distance from the inlet normalized by the pipe diameter, z/D=5, 20, 40 and 60) at various flow conditions of superficial gas velocity (0.00840-0.0298 m/s) and superficial liquid velocity (0.138-0.914 m/s). By taking into account the normalized parameters on void peak fraction and void peak intensity in pipe cross-section, the phase distribution pattern was quantitatively-classified into three patterns : a wall peak, a core peak and an intermediate peak. The phase distribution pattern maps were newly presented for vertical upward bubbly flows at normal-and micro-gravity conditions.

A hybrid dielectric slab-beam waveguide for the sub-millimeter wave region

A hybrid dielectric-slab-beam waveguide is suggested which should be well suited as a transmission medium for the design of planar quasi-optical integrated circuits and devices operating in the millimeter- and sub-millimeter-wave regions. The guide consists of a grounded dielectric slab in which a sequence of equally spaced cylindrical lenses is fabricated. The center line of the slab guide is the axis of the lenses. The structure uses two distinct waveguiding principles in conjunction to guide electromagnetic waves. In the direction normal to the slab surface, the guided fields behave as surface waves of the slab guide; their energy is largely confined to the interior of the dielectric and they are guided by total reflection at the slab surface. In the lateral direction the waves behave as Gauss-Hermite-beam modes that are guided by the lenses, which periodically reconstitute their cross sectional phase distribution, resulting in a wave beam that is iterated with the lens spacing. The guided fields are in effect TE and TM modes. The analysis of the new guiding structure is analyzed, the mode spectrum is calculated, and the iteration loss due to the finite size of the lenses is estimated. >

On the guided propagation of electromagnetic wave beams

Any field in a half-space can be described by a continuous spectrum of cylindrical waves. If this spectrum comprises substantially only waves whose propagation constant is very close to the plane wave propagation constant, the field can be resolved into a set of elementary wave beams which are characterized by Laguerre polynomials. They satisfy orthogonality relations like the wave modes in a waveguide. The elementary beams or "beam modes" can be reiterated and guided by reconstituting the cross-sectional phase distribution at certain intervals. Reiterative beams are utilized in the beam waveguide. The finite size of the phase resetting devices effects a modification of the reiterative beam modes and causes diffraction losses. These losses decrease very rapidly with increasing diameter of the phasing devices.