Smooth Bar Research Articles

A method for determining material's equivalent stress-strain curve with any axisymmetric notched tensile specimens without Bridgman correction

Large deformation analyses of problems such as plastic forming, ductile fracture with finite element method need a full range of material's equivalent stress-strain curve or flow stress-strain curve. The equivalent stress-strain curve determined from the smooth round bar specimen should be corrected after diffuse necking, since tri-axial stress state occurs in the neck. The well-known Bridgman correction method is a candidate, however, it is not accurate as the strain increases. Furthermore, it is impossible to measure the equivalent stress-strain curve of each individual material zone in a weldment with cross weld tensile tests. To cope with these challenges, a correction function and an associated test procedure are proposed in this study. With the proposed procedure, the true stress-strain curve from any axisymmetric notched tensile specimen can be converted to the material's equivalent stress-strain curve accurately and no Bridgman correction is needed. The proposed procedure can be applied to both perfectly plastic and strain hardening materials. The equivalent stress-strain curve of each individual material zone in a weldment can also be measured with the proposed procedure.

Effects of rotational capacity and horizontal restraint on development of catenary action in 2-D RC frames

Research studies on 2-D reinforced concrete (RC) beams under middle column removal scenarios have found that catenary action (CA) can significantly increase structural resistance and deflection beyond flexural capacity. However, development of CA is highly dependent on the rotational capacity of RC beams and the degree of horizontal restraints from adjacent structures. Previous studies have identified the effect of varying beam details, (such as geometry and reinforcement ratio) on CA, but very few publications focused on the beneficial effect of rotational capacity on CA. To the authors’ best knowledge, there has not been any publication on quantifying the effect of rotational capacity on CA. Moreover, experimental studies of CA have so far been conducted on RC beams with full restraints at the two supporting ends. Hence, in this study, five RC frame specimens were tested to study the effects of rotational capacity and degree of horizontal restraint on CA. One frame served as a control specimen, two specimens for improvement in rotational capacity, and another two with inadequate horizontal restraint. Improvement in rotational capacity was engineered in one frame through provision of closer-spaced stirrups near to the joints and another frame through substitution of deformed bars with smooth round bars. Adverse effect of inadequate horizontal restraint was investigated under penultimate column removal scenarios. Through the testing and analysis of the five frames, the effects of improvement in rotational capacity and inadequate horizontal restraint on CA could be quantified and discussed. Finally, practical suggestions on enhancing CA through detailing were provided.

Laboratory pull-out tests on fully grouted rock bolts and cable bolts: Results and lessons learned

Laboratory pull-out tests were conducted on the following rock bolts and cable bolts: steel rebars, smooth steel bars, fiberglass reinforced polymer threaded bolts, flexible cable bolts, IR5/IN special cable bolts and Mini-cage cable bolts. The diameter of the tested bolts was between 16 mm and 26 mm. The bolts were grouted in a sandstone sample using resin or cement grouts. The tests were conducted under either constant radial stiffness or constant confining pressure boundary conditions applied on the outer surface of the rock sample. In most tests, the rate of displacement was about 0.02 mm/s. The tests were performed using a pull-out bench that allows testing a wide range of parameters. This paper provides an extensive database of laboratory pull-out test results and confirms the influence of the confining pressure and the embedment length on the pull-out response (rock bolts and cable bolts). It also highlights the sensitivity of the results to the operating conditions and to the behavior of the sample as a whole, which cannot be neglected when the test results are used to assess the bolt-grout or the grout–rock interface.

Models of the flexure-controlled strength, stiffness and cyclic deformation capacity of rectangular RC columns with smooth bars, including lap-splicing and FRP jackets

Cyclic tests of single concrete columns with smooth (plain) bars are not representative of building columns with lap splices at floor levels and story-long starter bars. Column specimens with fixity at top and bottom resemble building columns best, but few of those tested so far had smooth bars and even then without bar lap-splicing at floor level or FRP jackets at column ends. Empirical models based on single-column tests, especially the numerous ones with cantilever-type specimens, cannot be readily extended to columns with smooth bars in real-life buildings. Physical models of the Strut-and-Tie type are developed and are validated or calibrated through comparisons with laboratory tests. Their scope includes anchorage and splicing of bars with either 180° hooks or straight ends. Once validated, they are adapted to real-life multistory rectangular RC columns with smooth bars, in order to obtain the column properties of interest: the chord rotation at yielding and the cyclic ultimate chord rotation, with or without FRP jacketing. Different expressions apply to the top and bottom end of a column in a story, but a single one is used to estimate the column’s effective stiffness. Empirical alternatives fitted to the single-element test results have slightly less scatter than physical models, but caution is needed for their application to columns of real buildings. Simulations of the 3D seismic response of a plan wise asymmetric full size building, tested pseudo-dynamically before or after retrofitting all columns with FRPs or just two of them with RC jackets, provide certain confidence in the extension of the physical models for the estimation of the stiffness and ultimate deformation of columns with smooth bars in real-life buildings.

Experimental investigation of corrosion effect on bond behavior between reinforcing bar and concrete

An experimental study is conducted in this paper to investigate the effect of corrosion and corrosion-induced cracks on the bond behavior between steel bars and concrete. An accelerated corrosion test was used to corrode the steel bars embedded in concrete specimens under laboratory condition. Two series of pull-out tests were designed. Pull-out tests on the specimens with plain and deformed reinforcing bars are performed. The bond behavior of the two types of steel bar specimens including the ultimate bond strength, bond-slip relationships for different corrosion levels are discussed. An empirical bond degradation model is proposed combining a wide range of experimental data available in the literatures. The other pull-out tests on the bonding specimens with interior strain gauges are conducted and the bond stress at various positions are derived based on the observed reinforcement strains. The experimental results show that the corrosion influence on bond strength can be ignored when the corrosion loss is less than 2.4%. The bond behavior between smooth bar and concrete is more sensitive to the corrosion than that of specimen with deformed bar. The selection of steel bars is also an alternative way to improve the bond behavior especially for the deformed bar specimens. The distribution of bond stress is more uniform for the specimen with a large corrosion-induced crack width.

Smooth at one end and rough at the other: influence of object texture on grasping behaviour.

When picking up objects using a pinch grip, there are usually numerous places at which one could place the thumb and index finger. Yet, people seem to consistently place them at or close to the centre of mass (COM), presumably to minimize torque and therefore the required grip force. People also prefer to grasp objects by parallel surfaces and ones with higher friction coefficients (rough surfaces), to prevent the object from slipping when they lift it. Here, we examine the trade-off between friction and COM. Participants were asked to grasp and lift aluminium bars of which one end was polished and therefore smooth and the other was rough. Their finger positions were recorded to determine how they grasped the objects. The bars were oriented horizontally in the frontal plane, with the centre aligned with the participants' body midline. The bars varied in the horizontal offset between the COM and the edge of the rough region. The offset could be 0, 1 or 2cm. We expected participants to grasp closer to the rough area than the centre of the bar. Completely rough bars and completely smooth bars served as control conditions. The slipperiness of the surface that was grasped affected the height of the grasping points, indicating that participants adjusted their grasping behaviour to the slipperiness of the surface. However, the tendency to grasp closer to the rough area was minimal. This shows that the judged COM largely determines how an object is grasped. Friction has very limited influence.

Improving seismic performance of old-type RC frames using NSM technique and FRP jackets

Abstract Joint strengthening of the existing RC frames with smooth bars (plain bars) built before the 1970s, protects beam-column joints against shear failure and increases the lateral load resistance of these frames; but due to the inherent flexural weakness of the columns, and the lack of strong column-weak beam mechanism, the occurrence of soft storey is probable in these frames under lateral loads, thereby making them vulnerable. In order to find a suitable solution for the seismic retrofitting of these frames, different retrofit strategies were investigated. NSM technique was utilized for the flexural strengthening of columns as an effective and practicable method, and for the seismic retrofitting of joints, FRP jackets were considered due to their compatibility with the NSM technique. Consequently, by combining these techniques, different retrofit strategies were proposed. To evaluate these strategies, first the proposed physical models for columns, beams, and joints were extracted based on the experimental results, and the analytical model of the frames was created by assembling these physical models. Afterwards, nonlinear static and incremental dynamic analyses were conducted. The results indicate that in the frames with strengthened joints and columns, strong column-weak beam concept is achieved, soft storey mechanism is eliminated, and the lateral load capacity and the dissipated energy increase considerably. Therefore, the seismic performance of the frames improves. Also, the flexural strengthening of columns is adequate to be performed up to the storey in which soft storey mechanism may occur, and the flexural strengthening of the columns at other storeys does not have a major effect.

Behaviour and modelling of hybrid SMA-steel reinforced concrete slender shear wall

Superelastic Shape Memory Alloys (SMAs) possess unique mechanical characteristics that make them appealing as alternative reinforcement for seismic applications; specifically, the capacity to recover large strains upon unloading, levels that would result in permanent deformations in steel reinforcement. An experimental study was conducted to assess the performance of a ductile, hybrid SMA-deformed steel reinforced concrete shear wall. A companion wall with deformed steel reinforcement only was also investigated. The walls were subjected to reverse cyclic displacements to failure. The results of the experimental program demonstrate that the hybrid SMA wall was significantly more effective at restoring resulting in marginal residual displacements after being subjected to drifts exceeding 4%. The hybrid SMA wall experienced similar lateral strength and displacement capacities to the steel reinforced wall. The hybrid SMA wall provided substantial, albeit less, energy dissipation and lower stiffness at yielding; the influence of these performance parameters on seismic behaviour is noted in the paper. A hysteretic constitutive model based on a trilinear envelope response, and linear unloading and reloading rules provided satisfactory simulations of behaviour of the SMA wall. Discrepancies in response are attributed to bond modelling of the smooth SMA bars. Furthermore, finite element analysis is also used to illustrate the effect of axial load on the re-centering capacity of the shear walls.

Voiding Mechanisms in Deformed Polyamide 6 Observed at the Nanometric Scale

Micromechanisms of deformation and cavitation within the spherulitic microstructure of a polyamide 6 (PA6) material have been observed with a true spatial resolution of 50 nm by magnified synchrotron radiation holotomography. Smooth round bars in PA6 were submitted to interrupted monotonic tensile tests. Before the engineering peak stress, only pre-existing nanometric voids were identified. At the peak stress, void growth and coalescence have been observed in the neck and 2 mm apart from the neck along the revolution axis. Two kinds of cavity arrangements have also been identified, studied quantitatively, and attributed to specific regions of the deformed spherulites. Voiding appeared in the equatorial plane of the spherulites as diffuse annular voided zones, and polar fans were characteristically penny-shaped voids stacked in columns parallel to the loading axis in the spherulite poles. A cylindrical volume located at the center of these spherulites remained intact (no voids).

Nondestructive assessment of corrosion of reinforcing bars through surface concrete cracks

Degradation of reinforced concrete (RC) structures is a serious safety problem affecting all industrialized countries, and economical aspect of this problem cannot be neglected. One of the main reasons for the degradation of RC is the corrosion of steel reinforcing bars as a result of concrete cover cracking and bar cross section reduction. As a consequence, the structural capacity of reinforced concrete elements decreases progressively as degradation develops. The use of nondestructive testing (NDT) and/or analytical formulation can represent high-quality and rapid methods to evaluate the corrosion penetration of bars, and provide a useful parameter to design a retrofit. This paper presents an additional tool that can be used to evaluate and assess the vulnerability of existing structures in terms of determination of bar cross section lost. This can be accomplished by calculating reinforcing bar cross section loss due to corrosion by measuring external crack widths of the concrete cover by using simple charts and formulas. Predictions were obtained by using two analytical models, developed by the authors. Their predictions have been satisfactorily compared to both the literature and novel experimental results, and compared to previous empirical models available in scientific literature. The experimental results were obtained by corroding real scale concrete specimens reinforced by using smooth and ribbed bars (according to old and modern building codes) using 3.5%wt sodium chloride solution and an imposed current.

Experimental response of an existing RC bridge with smooth bars and preliminary numerical simulations

The paper discusses the experimental seismic performance assessment of a structural prototype simulating typical existing highway bridges which were designed with poor seismic details. The structural system is a reinforced concrete (RC) 1:3 scale single span bridge which has been tested dynamically with shaking tables by considering strong motions of the 1980 Irpinia (Italy) earthquake. Low-strength concrete and smooth bars were used for the RC circular bridge piers of the tested prototype. Comprehensive modal response analysis has been carried out for the single piers and for the bridge sub-assemblage to estimate period elongations and variations of equivalent viscous damping coefficients at increasing levels of strong motions. The effects of cumulative damage induced by earthquake sequences have also been investigated experimentally. The sample bridge pier possesses low aspect ratio; nevertheless the piers exhibited a flexural response. Fixed-end rotations, with significant longitudinal reinforcement slip, were observed at the base of the bridge pier at about 0.40% lateral drift ratio. As a consequence, a rocking mechanism of the bridge pier initiated; such mechanism affected significantly the pier response at large shaking intensities. An in-depth analysis of local and global response quantities has been provided in order to accurately identify the structural behaviour. Finally, available analytical formulations and refined numerical models were considered to simulate the earthquake response of the tested bridge. The pier lateral response has been investigated by means of push-over analysis and nonlinear time histories. The accuracy of the proposed models, the differences and recommendations for numerical modelling of bridge piers with smooth internal reinforcements are also discussed.

直径1mm丸棒引張型ミニチュア試験片を用いたCrMoV鍛鋼のクリープ損傷評価

Creep damage assessment of high temperature components in aged thermal power plants is necessary to maintain reliable operation. Although an accurate creep damage assessment result is obtained by a destructive assessment method in which creep tests are performed, it is difficult to cut a standard size specimen from actual components. Therefore, development of testing and damage assessment methods by using a miniature specimen are expected. In this study, creep tests of a CrMoV forging steel were conducted by using a solid bar tensile type miniature creep specimen with 1mm diameter at 600℃. It was demonstrated that equivalent creep deformation and rupture properties to those obtained from the standard size specimen are obtained by the miniature specimen under stress level below 200MPa. Creep damage materials were produced by interrupting creep tests performed by using a standard sized smooth bar specimen and two types of round notch bar specimen with radiuses at notch root of 0.5mm(R0.5) and 2.0mm(R2.0). Uniaxial tensile creep tests were performed by using the miniature creep specimens taken from the plain specimens and the notch roots of the notch specimens. Although accurate creep damage assessment results were obtained from the creep tests with the miniature creep specimens taken from the plain specimens, it should be noted that the results may give under estimation for higher creep damaged condition. Creep damage assessment results based on the creep tests with miniature specimens also gave under estimation for the creep damaged material of R0.5 due to lower axial creep strain accumulation under multiaxial stress states before interrupting the creep tests. On the other hand, accurate creep damage assessment results were obtained for the creep damage materials of R2.0 indicating that creep tests with the miniature specimens may be applicable to creep damage assessment of components under certain range of stress multiaxiality.

Fatigue evaluation of the API specification 12F shop welded flat bottom tanks

Shop-built storage tanks are widely used in several industries all over the world. These equipment are fabricated with relatively small dimensions and capacities to facilitate their transportation to production fields. Particularly, API 12F shop-welded, flat bottom tanks are a group of standard equipment with specific sizes and capacities that are commonly used in the upstream, exploration, and production segments of oil and gas projects. The extensive utilization of this equipment has raised the need to investigate their behavior under different load cases and determine their service life due to cyclic loading.Throughout this investigation, a fatigue evaluation was performed following the guidelines of the ASME BPVC Section VIII, Division 2, design-by-analysis rules. The thirteen API 12F tanks were separated in three different groups according to their diameters. Also, different thicknesses as well as pressure cycles including internal pressure and vacuum were considered for the evaluation of each group.An elastic stress analysis using finite elements was conducted on shell models, axisymmetric models and solid submodels to determine the stress components and stress tensor range as well as obtain the effective alternating equivalent stress. Moreover, the fatigue penalty factor and a fatigue strength reduction factor were defined in accordance with the ASME code specifications. Hence, the protection against failure from cyclic loading of these equipment was determined using smooth bar design fatigue curves and the permissible number of operational cycles and location of the most critical joint were computed for each API 12F tank.

Modeling of gravity-designed RC sub-assemblages subjected to lateral loads

This paper aims to propose numerical models for seismic assessment of old existing RC beam-column sub-assemblages. In particular, two modeling approaches are described: for internal (usually named “C joints”) and for external (named “T joints”) beam-column sub-assemblages, both designed only for vertical loads without reinforcing details rules (such as inadequate bars lap splice, absence of hoops within the joint panel). They were developed starting from the failure mechanisms observed during an experimental campaign carried out on specimens reproducing old existing RC sub-assemblages cast in full and reduced scale, as well as reinforced with smooth longitudinal bars. The proposed models are not time-consuming with respect to both modeling and computational efforts, and do not require any calibration procedure from an experimental data set. They may be easily implemented in a general-purpose finite element program since merely based on geometrical and mechanical properties of RC elements. The numerical predictions show a good agreement with the experimental ones demonstrating that the proposed models are able of reproducing the lateral response of the two considered sub-assemblages typologies, in terms of both failure mechanism and hysteretic dissipative capacity.

An improved ductile fracture model for structural steels considering effect of high stress triaxiality

This study presents results of experimental investigation and ductile fracture model for ductile crack initiation, propagation and final failure in structural steels subjected to high stress triaxiality. To this end, uniaxial tension tests on smooth flat bars, U-notched specimens and V-notched specimens are conducted. Nonlinear finite element analysis of the notched specimens is carried out to obtain the stress triaxiality distributions and histories of notch tip and center of specimens. Based on the previous three-stage and two-parameter ductile fracture model proposed by the authors, the equivalent plastic displacement at element failure during simulations is obtained by notched specimen tests, and an improved ductile fracture model is presented considering the effect of high stress triaxiality during both the plastic stage and the softening stage. The relationship between the equivalent plastic displacement at element failure and nonuniform ratio (nonuniform ratio is the ratio of the average value of stress triaxiality of notch tip and center) is determined by a series of tests and analyses. Detailed finite element analyses that employ the improved ductile fracture model are shown to predict ductile fracture behavior under high stress triaxiality with good accuracy across the mesh sizes, notch radii, and notch degree in terms of ductile crack initiation point, ultimate load point and load-displacement curve.

FEM predictions of damage in continous fiber ceramic matrix composites under transverse tension using the crack band method

This paper develops a numerical micromechanics fracture model based on the crack band methodology for the prediction of damage in continuous fiber ceramic matrix composites (CMCs). The energy based crack band formulation offers mesh objective predictions in finite element simulations. Detailed finite element models of CMCs are created based on the actual microstructure and compared with randomly created representative volume elements (RVE). The influence of geometrical inhomogeneities, such as fiber clustering and variations of the fiber coating thickness, on the stress-strain response and the crack development are investigated. Furthermore, FEM models of multi-layer cross-ply laminates are created including thousands of explicitly modeled fibers and fiber coatings. Crack density predictions are compared with well established analytical models by Laws and Dvorak [1] and Nairn [2]. An eight layer FEM model is used to predict damage propagation within each layer. The Numerical prediction of the stress-strain response is compared with experimental results of smooth bar tensile coupons.

Correlation between crack length and load drop for low-cycle fatigue crack growth in Ti-6242

For low-cycle fatigue tests with smooth bars the number of cycles to initiation is commonly defined from a measured relative drop in maximum load. This criterion cannot be directly related to the crack length, which is the actual measure of interest. In order to establish a relation between load drop and crack length for the high strength titanium alloy Ti-6242, this investigation compares data from controlled low-cycle fatigue crack growth tests and numerical simulations of these tests. To achieve sufficient accuracy in this relation, focus is given to modelling of mean stress relaxation. Three constitutive models, the Chaboche, the Ohno–Wang and the Chaboche with threshold, are evaluated with respect to experiments. Furthermore, a straightforward method with cycle-scaling of the material parameters are used to efficiently reduce calculation cost. It is shown that it is possible to determine the relationship between load drop and crack length from numerical simulations, provided that care is taken to relevant aspects of the materials stress–strain response. These results are also used to numerically evaluate the effect on load drop of the extensometer position relative to the crack.

Implementation and application of Dung’s model to analyze ductile fracture of metallic material

In this paper, the Dung’s microscopic damage model which depicts void growth under plastic deformation is applied to predict ductile fractures in high strength steel API X65. The model is implemented as a vectorized userdefined material subroutine (VUMAT) in the ABAQUS/Explicit commercial finite element code. Notched and smooth round bars under uniaxial tension loading are simulated to show the effect of equivalent plastic strain versus the void volume fraction growth of the material at and after crack initiation. Predictions of the ductile behavior from void nucleation to final failure stage are compared with the built-in Gurson – Tvergaard – Needleman (GTN) model in ABAQUS. Also, comparison with experimental results from the literature is discussed.

FEM analysis of push-out test response of Hybrid Steel Trussed Concrete Beams (HSTCBs)

Aiming to investigate the steel truss–concrete stress transfer mechanism in Hybrid Steel Trussed–Concrete Beams (HSTCBs), a three-dimensional (3D) nonlinear Finite Element (FE) model is developed. The constitutive relationship of the steel composing the plates and the rebars is modeled by means of a quadri-linear law, while the concrete behavior is defined by means of a Concrete Damaged Plasticity (CDP) model, suitable for modeling concrete and brittle materials. Two main failure mechanisms are considered, namely the tensile cracking and the compressive crushing. In order to accurately grasp the complicate dowel and bond phenomena arising at the steel–concrete interface, a 3D solid model is realized to account for the actual contact surfaces between the truss and the concrete. The behavior of HSTCBs constituted by either deformed or smooth steel diagonal bars is investigated, and four models for steel–concrete interface are analyzed. The model is calibrated on some of the existing experimental push-out tests available in the literature and, afterwards, it is used to simulate the slip–load curve of a number of push-out tests in order to individuate the main geometrical and mechanical parameters influencing the maximum load that can be transferred from the steel plate to the concrete core. The FE simulations are also used to develop a first interpretation of the maximum load carried by the steel-to-concrete connection by means of analytical regression of the results numerically obtained, taking into account the geometrical configuration of the beam and the range of the geometrical and mechanical parameters.

Bond strength of glass fibre-reinforced polymer ground anchors with helically wound ribs in different bonding agents

Bond strength of glass fibre-reinforced polymer (GFRP) bars with helically wound ribs was experimentally investigated in this study and compared to that of smooth glass fibre-reinforced polymer bars and steel bars. A total of 63 pullout specimens are tested. Variables of the tests involved four types of bars, namely, glass fibre-reinforced polymer bar with helically wound ribs, smooth glass fibre-reinforced polymer bar, ribbed steel bar and smooth steel bar, as well as different bonding agents, including normal concrete, high-performance concrete, grout and epoxy resin. Test results showed that bond failure of glass fibre-reinforced polymer bars with helically wound ribs is because of the shearing off of the ribs with small slips, and the ribs helically wound can improve the bond strength of glass fibre-reinforced polymer bars by 3∼14 times, which is around 13, 9·31 and 15·7 MPa in concrete, grout and epoxy resin, respectively. In addition, polypropylene fibres in high-performance concrete can improve the bond behaviour of anchors.