Compressive Bar Research Articles

Comparative study of strength-based damage evolution in ultra-high-performance concrete (UHPC) and conventional concrete (CC) under dynamic loading

The initiation and development of damage inside cementitious materials subjected to dynamic loadings, which is manifested by the cracking and fragmentation process, have a significant contribution to the load-bearing capacity of these materials. However, current popular concrete models hypothesize that damage only initiates after the specimen reaches its peak stress, and due in part to the lack of validation from sufficient experimental data, none of the models entertain the possibility of variation in damage evolution among different types of concretes. In this study, the evolution of damage in an ultra-high-performance concrete (UHPC) and a conventional concrete (CC), characterized by the degree of strength degradation, are comparatively investigated throughout the entire pre-/post-peak deformation process as a function of carefully measured plastic strain. Well-controlled intermittent dynamic loadings are achieved on a modified Kolsky compression bar apparatus to first introduce various degrees of damage in the concrete specimens, and then the carefully preserved specimens are tested for residual strength. Coupling with pulse shaping techniques, the incident wave profiles are tailored for UHPC and CC specimens to acquire dynamic stress equilibrium and constant strain-rate deformation. Further evaluation on the residual strength of damaged specimens reveals that damage initiates prior to the peak stress, and there is a distinct difference in damage evolution between these two types of concrete materials. This investigation demonstrates a more comprehensive view of damage evolution in concrete materials and offers novel experimental data to assist the damage model development.

Bond behavior of dual headed bars in concrete structures

The use of headed reinforcement and hooked bars to reduce development length is restricted to bars loaded in tension only and shall not be considered effective in developing bars in compression according to the ACI-318 code. However, there are situations where a small development length in compression is required. This paper presents results of experimental study for assessing the use of dual headed bars to develop bars in compression as well as in tension. A total of 35 pullout tests and bar compression tests were performed. Parameters affecting the anchorage behavior of dual headed bars such as spacing between heads, concrete cover, concrete compressive strength, head area and confining reinforcement were studied. The results showed that bars with dual heads loaded in compression exhibited remarkable improvement in bond capacity. The parameters studied affected bond strength in compression loading in the same way as they affect bond on tension loading. No improvement is obtained for such dual headed bars loaded in tension, and even slight decrease in anchorage capacity was recorded for some specimens loaded in tension. However, the loss in anchorage capacity for bars with dual heads loaded in tension was very small in comparison to the gain attained in anchorage capacity in compression loading.

Roof Stability of Rectangle Coal Roadway: In Light of Calculation of Compressive Bar Stability

The roadway roof is a key factor to the roadway stability. The analysis of roof stability is mainly based on numerical calculation and on-site observation, while the basic theory of the bearing mechanism is relatively weak. We have founded a critical pressure calculation model, on the theory of compressive bar, for the rectangle coal roadway stability. The model has been tested and verified on accuracy and feasibility while applied on a roadway case. The critical pressure for roof stability and roof bending moment and deflection under combined axial and lateral load was deduced using the theory of compressive bar stability. The numerical calculation verified the feasibility of numerical modeling of stability of compressive bar using FLAC3D, and the influence of the background ambient horizontal stress and the parameters of the contact surface to the roof stability were further studied. The result turns out that some factors lead to a higher instability tendency, including higher horizontal stress, higher cohesion force, and larger internal friction angle on the coal-rock interface and lower cohesion force and smaller friction angle on the rock-rock interface. The results contribute to bearing mechanisms of roadway roof stability, ground pressure and strata control theory and application, and design of bolting support.

Compressive stress–strain behaviour of stainless steel reinforcing bars with the effect of inelastic buckling

The corrosion of steel reinforcing bars due to chloride ingress is the most common cause of premature deterioration of reinforced concrete structures. Stainless steel reinforcement in concrete is a promising solution to address these durability and sustainability issues as it is highly resistant to corrosion from chloride ions, and it does not rely on the high alkalinity of concrete or the presence of concrete cover for protection. Modelling and analysis of the nonlinear behaviour of reinforced concrete structural components relies heavily on the existence of constitutive material models to simulate the stress–strain response of the reinforcing bars and concrete accurately. In this paper, the stress–strain behaviour of stainless steel reinforcing bars with the effect of inelastic buckling is examined experimentally and numerically for the first time. Stainless steel reinforcing bars of 12 mm diameter with various slenderness ratios of austenitic EN 1.4301 and duplex EN 1.4362 grades are tested under tension and compression. A validated finite element model of reinforcing bar in compression is developed which is used to conduct parametric study. A new compressive stress–strain constitutive model for stainless steel reinforcing bars is proposed which is calibrated against the numerical simulation data.

Cyclic loading test for shear-deficient reinforced concrete exterior beam-column joints with high-strength bars

The seismic behavior of exterior beam-column joints is significantly affected by the beam rebar yield strength and joint hoop ratio. In the present study, high-strength bars were used for beam longitudinal bars and stirrups in exterior beam-column joints with the joint hoop ratio lower than the requirement of ACI 318–14. In an attempt to distribute the damage from the joint panel toward the beam in the shear deficient exterior beam-column joints, the beam shear resistance was intentionally decreased by removing the beam cross-ties or by using the high-strength beam stirrups at a large spacing. Cyclic loading tests were performed on 9 exterior beam-column joints. The test parameters were the amount of joint hoop bars, the yield strength and diameter of the beam longitudinal bars, the yield strength and spacing of the beam stirrups, and use of beam cross-ties. The test results showed that the seismic performance was improved by increasing the beam flexural damage, preventing a shear failure, through a reduced contribution of the beam stirrups to the shear strength of the beam or weakening the beam by removing the beam cross-ties that increased vulnerability of buckling of the beam longitudinal bars. As the damage concentration was transferred from the joint panel to the weakened beam, the energy dissipation capacity and damping ratio increased by 30% and 54%, respectively. It was found that 33% decrease in the amount of joint hoop bars compared to that of prescribed in ACI 318, equal to one leg of joint stirrup, was enough to change the failure mode from the beam flexural failure to severe joint shear failure. The allowable development length of beam compression bars was discussed to prevent the cover concrete spalling at the back face of the column. Particle image velocimetry (PIV) was conducted to accurately measure the joint shear deformation and the distribution of the local flexural deformation of the beam.

Experimental and theoretical studies on lateral buckling of submarine pipelines

Global buckling of a submarine pipeline during high pressure/high temperature (HP/HT) operation results in a loss of pipeline stability that is similar to a bar in compression; this phenomenon constitutes one of the key factors affecting pipeline integrity and design. To intuitively study the buckling response, a test system was designed that can account for thermal loading and pipe-soil interactions, and this system was used to perform a series of small-scale model tests on the lateral buckling of submarine pipelines with different initial imperfections. Based on the hat-shaped buckling profiles of the test pipelines, a new buckling mode called "hat-shaped buckling" was proposed. In an attempt to study the conditions under which the pipeline exhibits this hat-shaped buckling mode, the changing law of the buckling mode was investigated through finite element analyses of pipelines with different parameters, including the length of the pipeline and the amplitude and wavelength of the initial imperfection. Subsequently, an analytical solution for calculating the buckling amplitude of a pipeline with a hat-shaped buckling profile was proposed. The theoretical solution was compared to the experimental data, which verified the feasibility of the model in calculating pipeline buckling deformation. The experimental data, the buckling mode based on these data and the corresponding analytical model discussed herein may provide a reference for future experimental studies of pipeline buckling.

Material Characterization of GFRP Bars in Compression Using a New Test Method

Abstract This article presents a new test method for determining the mechanical properties of glass fiber–reinforced polymer (GFRP) composite bars in compression, namely the compressive strength, compressive modulus of elasticity, ultimate crushing strain, and compressive stress-strain curves of the bars. The contribution of GFRP bars in compression is currently neglected by major design guidelines related to GFRP-reinforced concrete columns. However, the demand for using GFRP bars is increasing because multiple researchers have shown the effectiveness of the bars in concrete columns. Thus, the need for characterization of the mechanical properties of GFRP bars is increasing, while there is no standardized test method to evaluate the compressive properties of these bars. Therefore, in this article, a new test method is proposed for evaluating the compressive characteristics of GFRP bars. The proposed test method was examined through testing a total of 35 specimens. It was observed that the test method was able to evaluate the compressive characteristics of the GFRP bars successfully. Three different modes of compressive failure were observed, which were related to the crushing of GFRP bars in different locations in the bar, but no premature failure or bar buckling was observed. Moreover, a comparison between tensile and compression characteristics of the GFRP bars showed that the tensile test results are not sufficient to estimate the compressive characteristics, and performing a compression test is necessary.

Identification of the Model of Nonlinear Elasticity in Dynamic Experiments

Dynamic methods for identifying a model of a nonlinearly elastic deformable body are considered. By the effective phase velocities of longitudinal and transverse waves propagating along and across the axis of the compressed bar, it is possible to determine five elastic constants of the second and third orders included in the model relations. Calculation formulae are obtained and an example of determining the dependence of phase velocities on the preliminary deformation for polyamide 6 is given. The influence of preliminary deformations on polar diagrams of wave velocities is investigated.

Influence of Ni4Ti3 precipitate on pseudoelasticity of austenitic NiTi shape memory alloys deformed at high strain rate

The influence of heat treatment on mechanical properties of austenitic NiTi alloys during impact loading is studied in this paper. Hot-rolled rod samples were annealed at 800 °C for 5 h to obtain homogenized texture. After homogenization, samples were aged at 500 °C and 550 °C for 1–4.5 h. Fine and/or coarse Ni4Ti3 precipitates disperse homogeneously or heterogeneously in grain interior and at grain boundaries depending on aging temperatures and time. The high strain rate compressive behaviors of austenitic NiTi SMAs after various heat treatments were investigated using a Kolsky (split Hopkinson) compression bar. The phase transformation temperatures were determined by differential scanning calorimeter (DSC). The microstructures of the samples were studied by scanning electron microscopy (SEM) and electron backscattered diffraction (EBSD). It is observed that the initial stress for phase transformation, work hardening during phase transformation, and transformation temperatures vary with heat treatments. The initial stress for martensitic transformation increases with increasing heating temperature and time. Ni4Ti3 precipitates are shown to improve the pseudoelasticity of NiTi SMAs during impact loading.

Dynamic compressive behaviour of hybrid basalt-polypropylene fibre-reinforced concrete under confining pressure: Experimental characterisation and strength criterion

The dynamic compressive behaviour of hybrid basalt-polypropylene fibre-reinforced concrete (HBPRC) under confining pressure was investigated using a Hopkinson compression bar device with active confining pressure. The relationship between the pore structure and the dynamic compressive behaviour of HBPRC was discussed. The results indicate that the dynamic compressive behaviour of HBPRC under confining pressure shows evidently the strain rate dependence and the confining pressure dependence. The addition of basalt fibre (BF) and polypropylene fibre (PF) reduces the failure degree of concrete and improves the shear failure feature. The relationship between the pore surface fractal dimension and the strain rate effect of HBPRC indicates that the dynamic mechanical properties of HBPRC are affected not only by the fibre itself but also by the effect of fibre on the pore structure. In view of strengthening and toughening, BF is conducive to increasing the strength of HBPRC, whereas the PF is conducive to improving the toughness. Finally, based on the Hoek-Brown strength criterion, an empirical dynamic strength criterion for HBPRC accounting for the effect of fibre and pore structure is established and verified.

Deployable double layer tensegrity grid platforms for sea accessibility

Tensegrity systems are a class of reticulated space structures composed of compressed bars maintained in equilibrium by a network of tensioned cables. Their stiffness depends both on elements’ mechanical properties and their internal self-stress state. Taking advantage of their structural properties, we respond to the challenge of accessibility for everybody to the sea with a new concept of modular lightweight and deployable platforms. Variable configurations are developed to fit ecologically into the marine environment thanks to the transparency of double layer tensegrity structures. Moreover, allowing practical assembly and disassembly is considered in the design to respect the coastal law. Through a numerical study, we demonstrate in this paper the capability of this solution under various representative load cases and support conditions.After the structural and design optimization of elements constrained by weight and stiffness, we detail the design of the nodes, which are the key components ensuring geometry and foldability of the structure. Finally, on-site setting and interfacing with ground supports is experimented in marine conditions to proof the feasibility of this concept.

Concrete Columns Reinforced with GFRP and BFRP Bars under Concentric and Eccentric Loads: Experimental Testing and Analytical Investigation

Twelve concrete columns reinforced longitudinally with fiber-reinforced polymer (FRP) bars were tested under both concentric and eccentric loadings. The investigated parameters were the type of the FRP bar, the longitudinal reinforcement ratio, and the load eccentricity-to-width ratio. The test results showed that the columns reinforced with basalt-FRP (BFRP) and glass-FRP (GFRP) experienced similar load-carrying capacity with a difference of less than 5%. Both types of columns attained lower ultimate capacity than their steel-reinforced counterparts. The contribution of the GFRP and BFRP bars to the ultimate capacity of the columns was similar, approximately 11% of the capacity, as compared to 31% for the steel bars. The effect of increasing the reinforcement ratio on the capacity was more pronounced in the eccentric FRP-reinforced concrete (FRP-RC) columns than the concentric ones. The analytical investigation showed that ignoring the strength contribution of the FRP bars, as recommended by most of the current codes and design guidelines, would result in conservative predictions. It also showed that current Canadian design code recommendation limiting the strains in FRP bars in compression to 2,000 µɛ yielded reasonable predictions of the column capacity.

Robustness of an airport double layer space truss roof

Abstract Robustness analyses are very well referenced for concrete or steel frame structures but less for spatial structures; in particular for truss roofs. Here, we present a robustness analysis of an existing airport space structure. A finite element model was implemented based on the original design documents, where the structure is modeled as a spatial truss composed of elastic, perfectly hinged bars. With respect to five main design loading conditions, the most stressed bars among the top layer, the bottom layer and the diagonals, were alternately removed, and the stress variations in the remaining bars monitored. A total of fifteen analyses with removal of either the most stretched or the most compressed bar were run. Also, reductions of the most stressed bars’ axial stiffness were considered to investigate the effects of such local reductions on the global structural flexibility. Linearized global buckling analyses were also conducted to point out again the effect of a global loss of stiffness, due to local losses. The study gives basic information about the general behavior of the structure in case of failure, or damage, of a key element. Results show that this kind of lightweight and efficient structures are very sensitive to local losses, since their redistribution capabilities are not large.

The nonclassical problem of the compressed bar stability

The non-classical problem of the stability loss of a straight vertical bar with elastic flexible elements at the ends is considered. The mathematical model for the study of stability loss consists of a linearized differential equation for buckling of a constant cross section bar, supplemented by boundary conditions. Critical force is determined by analytical and numerical methods. Two versions of computational algorithms are presented. For one design scheme of the rod in the Matlab computing environment, the critical forces are calculated using two methods. The found critical forces differ insignificantly, which confirms the reliability and efficiency of the proposed algorithms.

DYNAMIC DUCTILE-BRITTLE TRANSITION AND FRACTURE TOUGHNESS MEASUREMENT OF METAL UNDER INTERMEDIATE-LOW LOADING VELOCITIES

The phenomenon of ductile-brittle transition and the measurement of dynamic fracture toughness of metallic materials under impact loading are important parts of the research on dynamic mechanical properties of metal materials. In view of the lack of understanding of ductile-brittle transition of metallic materials under impact loading and the difficulty in measuring the dynamic $J$-resistance curve of ductile materials at relative low loading rate, a method is proposed to measure the ductile-brittle transition process of 15MnTi and 11MnNiMo steels at different loading rates, and the effect of crack tip constraint on the rate change of dynamic ductile-brittle transition of the two materials, by using high-speed material testing machine and its corresponding special fixtures. The dynamic fracture toughness of 15MnTi steel under low loading rate was measured by adjusting the length of compression bar and changing the crack propagation by means of the brake of upper roller. The experimental results indicate that the CT specimen of 15MnTi steel is characterized as ductile fracture when loading velocity is lower than 0.025~m/s, and the fracture character of CT specimen of 15MnTi steel is ductile-brittle combination when loading velocity is between 0.1~m/s and 0.5~m/s, and brittle fracture of the CT specimen of 15MnTi steel starts from 0.5~m/s. The phenomenon of brittle fracture followed by ductile fracture for the CT specimen of 11MnNiMo steel occurs when the loading rate is greater than 1.5~m/s. The dynamic brittle fracture rate of 15MnTi and 11MnNiMo steels is significantly affected by crack tip constraint, and the dynamic brittle fracture rate of the material decreases obviously with the increase of in-plane constraint and out of plane constraint. It is also found that in the three-point bending tests, the fracture toughness of 15MnTi steel decreases slowly with the increase of loading rate, when the loading rate is lower than 8788~MPa$\cdot$mm/s.

Energy Dissipation Analysis of FRP Concrete Steel Tube under Multiple Impact

In order to study the effects of wall thickness and impact times on the splitting tensile properties of FRP pipe concrete steel pipe, the dynamic impact test of FRP pipe concrete steel pipe is carried out by using the separated Hopkinson compression bar system with a diameter of 74mm. By changing the wall thickness and impact times, the variation laws of FRP concrete dynamic characteristics under different conditions are analyzed. The test results show that the increase of FRP pipe thickness greatly improves the load resistance of the specimen, the increase of FRP pipe thickness greatly improves the toughness and fatigue resistance of the specimen, and the energy absorption capacity of FRP pipe material is better than that of concrete; In terms of failure mode, the increase of FRP pipe thickness significantly enhances the protection ability of the specimen.

Real-time damage characterization for GFRCs using high-speed synchrotron X-ray phase contrast imaging

We report the application of high-speed synchrotron X-ray phase contrast imaging (PCI) in real-time damage characterization for glass fiber reinforced composites (GFRCs) subjected to dynamic loading. Dynamic single-edge notched bending (DSENB) experiments on pre-notched S-2 GFRCs were performed on a modified Kolsky compression bar. During loading, the synchrotron X-ray beam penetrated through the composite specimen from the side to detect damage evolution inside the material. Entire dynamic events were recorded by a high-speed camera as image sequences.0° and 90° unidirectional and cross-ply composites were investigated. An optical imaging technique was also employed to capture similar dynamic events in comparison with the radiographic imaging. It is demonstrated that high-speed X-ray PCI had sufficient phase contrast to characterize a crack initiation at a 20-μm spatial resolution within 920 ns and track the crack geometry during propagation, thereby providing reliable data to quantify the dynamic damage resistance of GFRCs. Furthermore, being capable of recognizing microscopic damage-related features at a sub-10-μm resolution, high-speed X-ray PCI provided fundamental material failure mechanisms to reveal the essential of macroscale structural failure of composites. It can also track the damage evolution inside and between individual plies of laminated composites. However, current high-speed X-ray PCI technique only supports in-situ observation and the high timing and spatial resolutions are limited within a field of view of ~2.5 mm in square, preventing its application in the three-dimensional and larger-area damage detection for GFRC structures.

A method for characterization of multiple dynamic constitutive parameters of FRCs

We propose a method to measure multiple dynamic material constitutive parameters of unidirectional fiber reinforced composites (FRCs) in a single experiment. Dynamic short-beam shear (DSBS) experiments were performed on a modified Kolsky compression bar, with integration of high-speed imaging and digital image correlation (DIC). The unidirectional FRCs investigated were S-2 glass fiber reinforced matrix of TGDDM-Jeffamine® D230 with monoamine functionalized partially reacted substructures (mPRS) and commercially available SC-15. Analytical solutions of normal and shear strains of a composite beam were derived based on Timoshenko beam theory, assuming material to be transversely isotropic and have different moduli in tension and compression in each principle material orientation. Tensile and compressive moduli were inversely computed through monitoring normal strain slope when specimen was constantly loaded at a speed of ~7.3 m/s within a small deflection. Non-linear shear stress-strain behavior of the composite was described via Ramberg–Osgood equation. Finite element (FE) analysis was conducted in ABAQUS, simultaneously defining via user subroutine UMAT the transverse isotropy of material, bi-modulus constitutive model, and non-linear shear stress-strain relation. The method proposed in this work was validated by comparing strain distributions computed by FE model and DIC measurements. Comparing with traditional dynamic tensile, compressive, and shear experiments on FRCs, this method significantly simplifies the specimen preparation and design of complicated gripping fixtures for multiple experiments. Furthermore, systematic errors resulting from variations of specimen geometry and dimension, loading direction, and instrumentation are reduced, thereby providing compatible data for numerical studies on impact behavior of composites.

A Methodology for Optimal Design and Simulation of Deployable Structures

Deployable structures with bars have been used in different technological fields since the beginning of the 20th century. It was as of 1930 when their use in construction became widespread, especially in roofs with large spans. The research group at San Pablo University in Madrid, along with Keene State College has developed a new type of deployable structure that minimizes the weight and maximizes the span. The structural efficiency is based on the fact that the tensioned elements of the modules are made of cables. In addition to contributing to the overall stiffness and strength of the structure, these cables are designed to guide the folding and unfolding process of the structure. In each module the tension cable is extended until it is connected to the midpoint of the compression bar, thus regulating its position and avoiding the possibility of unforeseen folding while the structure is fully deployed. This article shows a method to optimize the geometry and to test the static behavior of the structure.

Determination of the material model and damage parameters of a carbon fiber reinforced laminated epoxy composite for high strain rate planar compression

The progressive failure of a 0°/90° laminated carbon fiber reinforced epoxy composite was modeled in LS-DYNA using the MAT_162 material model, including the strain rate, damage progression and anisotropy effects. In addition to conventional standard and non-standard tests, double-shear and Brazilian tests were applied to determine the through-thickness shear modulus and the through-thickness tensile strength of the composite, respectively. The modulus reduction and strain softening for shear and delamination parameters were calibrated by low velocity drop-weight impact tests. The rate sensitivities of the modulus and strength of in-plane and through-thickness direction were determined by the compression tests at quasi-static and high strain rates. The fidelity of the determined model parameters was finally verified in the in-plane and through-thickness direction by the 3D numerical models of the Split Hopkinson Pressure Bar compression tests. The numerical bar stresses and damage progressions modes showed acceptable correlations with those of the experiments in both directions. The composite failed both numerically and experimentally by the fiber buckling induced fiber-matrix axial splitting in the in-plane and the matrix shear fracture in the through-thickness direction.