Ultimate Strength Of Specimens Research Articles

Mechanical Behavior of High-Strength Bolted Joints Fabricated from Fire-Resistant Steel at Elevated Temperatures

The use of fire-resistant steel is an effective way to alleviate fire safety issues in steel structures. Considering the limited research on bolted joints fabricated from fire-resistant steel, this study presents the results of the experimental investigation on the mechanical behavior of 40 specimens of bolted joints at room temperature and elevated temperatures (20°C∼700°C). These specimens are assembled with fire-resistant steel plates and fire-resistant high-strength bolts (or ordinary high-strength bolts). The mechanical behavior of these specimens was investigated, which included the failure mode, deformation performance, and degradation of bearing capacity. The experimental results show that the treatment of the friction surface (specifically, impeller blasting and sprayed hard quartz sand) has a negligible effect on the ultimate strength of specimens and a relatively significant effect on the slip strength. In addition, reduction factors of the tensile strength (KN) and slip strength (KS) for both fire-resistant bolt specimens and ordinary high-strength bolt specimens decrease with the increase in temperature, especially in the case of elevated temperatures ranging from 400°C to 700°C. Prediction models with different reliability degrees are proposed to calculate the degradation of tensile strength under elevated temperatures, which has the potential to be used for the fire-resistant design of steel connections and structures.

Experiment Study on Residual Flexural Capacity of Prestressed Concrete Deck Slab Under Fatigue Loading

The deterioration of a concrete deck damaged by repeated traffic loads could directly affect its performance. Two full-scale specimens modified from a precast, prestressed concrete (PC) deck slab on a 12.65-m-wide composite bridge with twin I-girders spaced at 7.05 m were tested to study the degradation of mechanical performance of the slabs under fatigue loading. The depth of the slabs was 30 cm at the midspan and 40 cm at the two supports. One specimen was 3.0 m wide and conducted a three-stage fatigue loading before the static destructive loading, the other was 1.5 m wide and performed a static load test to failure only. The ultimate strength of specimens was obtained, and the variation of deflection, reinforcing strain, and cracks were discussed. The test results showed that the failure mode of the PC slabs was flexure subjected to static load. Fatigue loading significantly degenerated the stiffness of the slab, and the maximum stiffness reduction was about 35.2 %. The distribution scale of cracks was expanded about twice as much at the same load level under the fatigue loading. It could be deduced that the fatigue loading could enlarge the crack width, by comparing the measured results of concrete crack widths of the two specimens, however, the design provisions did not take this into account. A comparison between the transformed flexural capacity of the two specimens showed the residual flexural capacity was reduced by 7.8 %. Moreover, an analysis model was developed to predict the residual flexural capacity of PC slabs.

Static behavior of large-diameter stiffened steel tubes for wind turbine towers under combined compression-bending-torsion load

Thin-walled steel tubes have been widely used in wind turbine towers. These towers are commonly subjected to combined compression-bending-torsion loads; thus, the steel tubes are prone to local buckling. Installing stiffeners has proven an effective method to improve the local stability of the thin-walled members. This paper presents an experimental and analytical study of thin-walled steel tubes strengthened by longitudinal stiffeners. Six specimens with different types of stiffeners and various diameter-thickness ratios were tested under a combined compression-bending-torsion load. The failure modes, load–displacement relationships, ultimate strength, and deformability were revealed. The validated finite element model was used to analyze the structural mechanisms of the stiffened steel tubes. The results show that stiffeners can substantially alleviate the local buckling of steel tubes, significantly improving their ultimate strength and plastic deformability. In addition, T-type stiffeners perform better than I-type stiffeners. The ultimate strength of the specimens is preliminarily evaluated using the methods recommended in existing design guidelines. The calculated ultimate strength of the unstiffened specimens shows a good agreement with the test results, whereas the ultimate strength of the stiffened specimens is underestimated.

Experimental and analytical investigation of stiffened steel tubes for wind turbine towers under compression-bending load

Steel tubes with large diameter-thickness ratios are widely used in tubular steel towers of wind turbines. These steel tubes are susceptible to local buckling under compression-bending load, thus leading to a failure of the overall structures. In this paper, a new structural form of steel tubes with longitudinal stiffeners was proposed. Six specimens were tested to study the effects of stiffeners forms and diameter-thickness ratios of steel tubes. Finite element models were compared and validated by test results and used to further study the structural mechanisms. Lastly, the ultimate strength of specimens was evaluated by existing design guidelines preliminarily. According to the results, by setting stiffeners, especially the T-type stiffeners under the same consumption of the steel, the local buckling was prevented and the plastic deformability of the steel tubes was increased, resulting in the great enhancement of ultimate strength and ductility. The limits of quantity and width-thickness ratio of stiffeners to obtain a more stable and significant improvement effect were suggested through parametric analysis. The current methods agree well with the test results of unstiffened steel tubes but might be inaccurate for stiffened steel tubes.

Flexural Behavior of RC Beams Using Fe-Based Shape Memory Alloy Rebars as Tensile Reinforcement

Recently, various studies for the use of Fe-based shape memory alloy (Fe-SMA) in the construction field have been widely conducted. However, most of the studies for using Fe-SMA are carried out for applying Fe-SMA for strengthening deteriorated structures. However, if Fe-SMA is used as a reinforcement for new structures, the disadvantages of conventional prestressed concrete can be effectively solved. Therefore, in this work, an experimental study was conducted to evaluate the flexural behavior of concrete beams in which Fe-SMA rebars were used as tensile reinforcement. For the study, ten specimens were constructed with the consideration of the cross-sectional area and activation of Fe-SMA rebars as experimental variable. Activation of the Fe-SMA rebars by electrical resistance heating applied an eccentric compressive force to the specimen to induce camber. The camber increased by an average of 0.093 mm as the cross-sectional area of the Fe-SMA rebar increased by 100 mm2. It was also confirmed through the four-point bending tests that the initial crack loads of the activated specimens were 47.6~112.8% greater than those of the nonactivated specimens. However, the ultimate strength of the activated specimens showed a slight difference of 3% to those of the nonactivated specimens. Therefore, it was confirmed that the effect of Fe-SMA activation on the ultimate strength of specimens was negligible.

Restoring force model of high‐strength concrete‐encased CFST composite columns

SummaryTo investigate the seismic behavior of high‐strength concrete‐encased concrete‐filled steel tube (HSC‐encased CFST) composite column, eight specimens with a high axial compression ratio were tested under lateral cyclic loading and constant axial compressive loading in this paper. Key points on the load–displacement skeleton curve of the composite columns were obtained based on the experimental results. The test parameters include the axial compression ratio, volume stirrup ratio, stirrup form, and the diameter‐width ratio. All specimens failed in the flexural mode, with the crushing of compressive concrete and losing of vertical load‐carrying capacity. The ductility and deformation capacity decrease with the axial compression ratio increasing, and enhancing the volume stirrup ratio within 1.33–1.63% can provide an effective confinement of composite column. The ultimate strength of specimen with the octagonal stirrup is 23.8% higher than that with well stirrup, and the ductility and load‐carrying capacity are enhanced due to the increase of diameter‐width ratio. A degrading trilinear restoring force model of HSC‐encased CFST composite column with a high compression ratio is proposed. The comparisons made between calculated results and test results show such a high agreement, which indicates the proposed restoring force model is applicable to such HSC‐encased CFST composite column. It provides a theoretical basement for further research on the nonlinear dynamic analysis of such CFST composite column.

Применение технологий 3D-печати при изготовлении мастер-моделей в машиностроении

Analysis of 3D-printing methods used in the molding production to manufacture master-models has been carried out. The technology was selected, which allowed one to make high-precision parts, combining the molding and the 3D-printing. Factors effecting on the quality of 3D-models printed by this technology were analyzed. Experimental studied for determination of the printing parameter influence (layer thickness, filling percentage, printing velocity) on ultimate strength of specimens made of ABS-plastic were carried out.

Compressive behaviour of tapered concrete-filled double skin steel tubular stub columns

Tapered Concrete-Filled Double Skin Steel Tubular (CFDST) columns have been commonly used in structures. To report further information, it is necessary to perform more studies on the structural behaviour of tapered CFDST columns, especially for those with high hollow ratios. This paper thus conducted an investigation on the tapered CFDST stub columns under axial compression. Specimens with varied tapered angle, hollow ratio and tubes' thickness were tested, and then typical failure modes, load versus displacement relations as well as strain development were revealed. Validated finite element models were given to further analyse the columns' structural mechanisms including the stress distributions, the interaction between the steel tubes and the sandwiched concrete, and the influence of main parameters. The ultimate strength of specimens was calculated tentatively using the methods in the existing design guidelines, and their accuracy and applicability for the tapered CFDST stub columns with high hollow ratios were preliminarily evaluated. A simplified method was suggested to predict the ultimate strength based on one of the guidelines, and the predictions show reasonable agreement with the test results.

Vibration of damaged bio-composite beams reinforced with random short Alfa fibers: Experimental and analytical investigations

This paper describes an investigation of the vibration of a cracked bio-composite beam reinforced with random short Alfa fibers using both analytical and experimental methods. The main novelty is the incorporation of local natural short fibers in the dynamic study of bio-based beams in the presence of a transverse crack. In addition, damping coefficient was predicted versus the crack length, crack position and fibers content. In the experimental model, tensile tests were made to predict Young's modulus and ultimate strength of specimens. After that, vibration tests were made to predict natural frequencies and damping coefficients versus crack depths, crack positions and fibers content. In the absence of similar experimental works on Alfa fibers, a simplified analytical model of flexural vibration has been developed to compare the results of experimental measurements. For different boundaries conditions, the linear fracture mechanics combined with Castigliano's theorem were used to estimate the local flexibility matrix at the cracked zone. For the natural frequencies, close agreement was found between the experimentally measured values and those given by the analytical model. From obtained results, we showed the increase in fiber content tends to reduce the strength and the natural frequencies of Alfa reinforced composite beams. Finally, we concluded that depth and position of the crack had a significant effect on the natural frequencies and damping coefficients of the bio-composite beam.

Test on mechanical behavior of pultruded concrete-filled GFRP tubular short columns after elevated temperatures

This paper presents an experimental investigation on the mechanical behavior of pultruded concrete-filled GFRP tubular (CFGT) short columns exposed to elevated temperatures under axial compressive loading. A total of 117 pultruded CFGT short columns subjected to different elevated temperatures and temperature duration were axially tested, including 108 specimens after elevated temperature and 9 specimens at ambient temperature served as the control group. The main variables explored in this paper included the GFRP tube thickness, concrete compressive strength, elevated temperature, and temperature duration. The results obtained from the experiments indicate that the ultimate strength and initial stiffness of specimens decrease remarkably with the increase of the elevated temperature, while the ductility of specimens is enhanced accordingly. Especially, the elevated temperature causes an obvious reduction (about 37%) in the ultimate strength of specimens when it exceeds 200 °C. Among the four main variables, the elevated temperature exerts the most significant impacts on the ultimate strength of specimens, the GFRP tube thickness and concrete compressive strength have certain positive effects, while the influence of temperature duration is almost negligible. Based on the experimental results, a parameter formula for the calculation of the ultimate strength of pultruded CFGT short columns after elevated temperatures is proposed, which shows great accuracy and rationality compared with the test results.

Tests on axial strength of circle CFST stub columns under marine atmosphere in cold region

This paper aims to study the compressive behavior of circular concrete-filled steel tubular stub columns under marine atmosphere in cold region. The specimens after salt spray corrosion and freeze-thaw cycles were tested under axial compressive load where steel grade, corrosion rate and freeze-thaw cycles were considered. The experimental phenomena and results are discussed in detail. Current design methods for circular concrete-filled steel tubular stub columns were modified to consider the effect of atmospheric corrosion and freeze-thaw cycles. The results show that the corrosion rate, the cycle number and steel strength have little effect on the failure pattern of circular CFST stub columns, neither on the failure pattern of core concrete. The ultimate strength of specimens is reduced approximately linearly with the increase of corrosion rate and freeze-thaw cycles regardless of steel strength. Strength reductions of the specimens using Q235 steel are smaller than those of the counterparts using Q345 steel. The ultimate displacement and ductility indexes of the specimens are decreased by corrosion rate and freeze-thaw cycle. The enhancement factor φ tends to increase with the increase of test period which indicates that confinement effect contributes to resist the strength reduction due to corrosion and freeze-thaw cycle. Various design standards combined with the reduction factor proposed based on experimental results are adopted to predict the ultimate strength of CFST stub columns under atmospheric corrosion and freeze-thaw cycle.

Effects of Crimp and Textile Architecture on the Stiffness and Strength of Composites with 3D Reinforcement

The aim of this study is to experimentally determine how the weave architecture and yarn crimp affect the measured tensile stiffness and strength of composites containing 3D textile reinforcement. It is shown that both the stiffness and strength decrease nonlinearly with increasing 3D crimp. The ultimate strength of specimens containing nominally straight yarns and specimens containing crimped yarns can differ more than a factor of 3, and the stress causing onset of damage can be affected even more. Adding nominally straight stuffer yarns into a 3D-woven reinforcement significantly increases the fibre volume fraction, the stiffness, and the strength of the composite. However, since the stuffer yarns are virtually straight and thus stiffer than the warp yarns, they attract the load and reach their strength at relatively lower strain than the warp yarns. The reinforcement architecture varies between the surfaces and the interior of the studied textiles, which has corresponding influence on the local stiffness. The onset of failure is predicted satisfactorily accurate with relatively simple estimations. The ultimate strength is a result of extensive damage progression and thus more dubious to predict.

Experimental Investigation on the Axially Loaded Performance of Notched Hexagonal Concrete‐Filled Steel Tube (CFST) Column

This study aimed to investigate the static performance of notched hexagonal concrete‐filled steel tube (CFST) stub columns through axial loading. Notch length, notch location, and notch direction in 14 CFST stub columns were experimentally studied. Stress process, failure mechanism, and ultimate strength in the notched CFST columns were analyzed. Results show that notches in steel tubes can weaken the restraining effect of steel pipes on core concrete and induce a decrease in the ultimate strength of specimens. The failure mode of components is greatly affected by notch orientation. The notch is closed under axial compression in the horizontally notched specimen, and the slotting indicates outward buckling in the vertically notched specimen. Based on the test results, a method for calculating the ultimate strength of notched hexagonal CFST columns was established. This research encourages the extensive application of these structures in civil engineering.

Finite Element Modeling of Axially Loaded CFRP-Confined Rectangular Reinforced Concrete Columns

This paper investigates numerically the behaviour of rectangular RC columns strengthened with carbon fiber reinforced polymer (CFRP) composites under uniaxial loading. For this a reason, a parametric study is conducted and the effects of CFRP layers number, compressive strength of unconfined concrete, and fiber orientation on the behaviour of such columns have been studied. The number of CFRP layers has been changed from one to five layers while the fibers are oriented transversely. Compressive strength of unconfined concrete has been increased from 26 MPa to 45 MPa. In addition, three different fiber orientations are considered. The results show that an increase in the number of CFRP layers would enhance the ultimate strength of specimens. Although increasing the number of layers would not increase the ultimate strength of specimens exponentially, but the rate of strength gain would also decrease. Moreover, it is shown that lateral strains increase as the layer number increases. The effect of unconfined concrete strength on the ultimate strength is less for low strength concrete than high strength concrete. Evaluating the effect of fiber orientation shows that the maximum ultimate strength is obtained from transverse orientation and as the angle of orientation increases, the ultimate strength decreases.

Mechanical properties of steel-CFRP composite specimen under uniaxial tension

This paper introduces new specimens of Steel-Carbon Fibre Reinforced Polymer composite developed in accordance with standard test method and definition for mechanical testing of steel (ASTM-A370). The main purpose of this research is to study the behaviour of steel-CFRP composite specimen under uniaxial tension to use it in beams in lieu of traditional steel bar reinforcement. Eighteen specimens were prepared and divided into six groups, depending upon the number of the layers of CFRP. Uniaxial tensile tests were conducted to determine yield strength and ultimate strength of specimens. Test results showed that the stress-strain curve of the composite specimen was bilinear prior to the fracture of CFRP laminate. The tested composite specimens displayed a large difference in strength with remarkable ductility. The ultimate load for Steel-Carbon Fibre Reinforced Polymer composite specimens was found using the model proposed by Wu et al. (2010) and nonlinear FE analysis. The ultimate loads obtained from FE analysis are found to be in good agreement with experimental ones. However, ultimate loads obtained applying Wu model are significantly different from experimental/FE ones. This suggested modification of Wu model. Modified Wu's model which gives a better estimate for the ultimate load of Steel-Carbon Fibre Reinforced Polymer (SCFRP) composite specimen is presented in this paper.

Research on Behavior of Rectangular CFST Stub Columns with Binding Bars under Axial Compression

The constitutive model of core concrete was proposed, which was suitable for finite element (FE) analysis of rectangular concrete-filled steel tubular (CFST) columns with binding bars. Based on the ABAQUS FE platform, the USDFLD subroutine was compiled, which could consider the changing of Poisson ratio of many materials in loading process. A FE model was developed and used to simulate the rectangular CFST columns with binding bars. The mechanical behavior were analyzed for stress versus strain relationship curves, confined effect among steel tube, core concrete and binding bars, longitudinal stress distributions of core concrete. The results show that the FE results simulated by using constitutive relationship of core concrete of this paper are in good agreement with test results. The local buckling of steel tube is delayed which can make the yield of steel tube occur before ultimate strength of specimens, the concrete confinement of the corner regions and central regions is improved, the longitudinal stress of concrete is increased and the capacity and ductility of specimens are improved, by setting binding bars and decreasing the binding bars spacing.

Experimental Study on Residual Load Bearing Capacity of SRC Eccentric Columns after Exposure to Fire

This paper provided three test data pertaining to the mechanical properties of steel reinforced concrete (SRC) eccentric columns after exposure to fire and one comparative test data pertaining to the mechanical properties of steel reinforced concrete columns at room temperature. The influence of eccentricity on failure mode, distortion performance and ultimate load bearing capacity are mainly studied. Test results show that the failure modes of steel reinforced concrete eccentric columns after exposure to fire are similar to that at room temperature. Strain along the section height at mid-span section of eccentric columns before loaded to 90% ultimate load bearing capacity is linearly distributed and well agree with the plane section supposition. After exposure to fire, the flexural rigidity and load bearing capacity of specimens are all declined compared with that at normal temperature. In various loading stages from the initial loading to 80% ultimate strength, the ratio of flexural rigidity of SRC eccentric columns after exposure to fire and at normal temperature is ranged from 0.30 to 0.59. With the same concrete strength and heating condition, the ultimate strength of specimens decreases with the increasing of eccentricity. The ultimate bearing capacity of all specimens at normal room temperature is calculated on the method proposed by Chinese regulation JGJ 138-2001. The compared results of experimental values and calculating values show that the residual load bearing capacity of SRC eccentric columns after exposure to fire is about 69% to 81% of that at room temperature.

Effects of Duct Types and Emulsifiable Oils on Bond and Friction Losses in Posttensioned Concrete

Emulsifiable oils are often used in posttensioned construction to reduce friction losses and provide temporary corrosion protection for tendons prior to grouting. This paper addresses the effects of two emulsifiable oils and three duct types on bond and friction losses. Bond test results indicate that corrugated galvanized steel ducts provide better anchorage than corrugated HDPE ducts. Rigid steel pipes performed poorly because of failure at the duct-concrete or grout-duct interface. Bond test results also indicate that the ultimate strength of posttensioned specimens with oiled tendons is similar to or better than the ultimate strength of specimens with unoiled tendons. However, specimens with oiled tendons experienced greater slip at a given load than specimens with unoiled tendons. Friction test results indicate that current recommended design values for the coefficient of friction for steel pipes and galvanized ducts are accurate. However, the measured coefficient for HDPE ducts is significantly less than the AASHTO-recommended value. Friction tests also indicate that lubrication of the tendon reduces the friction coefficient by 15% in rigid steel pipes and HDPE ducts if stressing occurs while the oil is fresh.

Fatigue Behavior of RC Slab Retrofitted with Carbon Fiber Mesh after Different Levels of Pre-Damage

Three RC slab specimens were designed and manufactured to investigate the fatigue behavior of RC slab retrofitted with carbon fiber mesh (CFM) and mortar. Among three slab specimens, two slabs were pre-loaded to introduce damage prior to the fatigue loading tests. Fatigue loading was applied to the top of slab maintaining 40% of the ultimate strength of specimen at 2Hz loading speed. The fatigue test was terminated when the loading cycle exceeded 1,000,000 cycles. From the test results, the effectiveness of the retrofitting using CFM was evaluated. Test result showed that the specimens retrofitted after either severe or light damages had very similar strength and stiffness to those of the specimens retrofitted without damage. It was found that the strength was even slightly improved in the specimen retrofitted with CFM.

Strength of Alloys Based on Iron, Nickel, and Titanium in High-Pressure Hydrogen

The influence of high-pressure hydrogen at room temperature on the static and fatigue properties of corrosion-resistant materials based on iron and nickel is compared for different types of loads. The sequence of characteristics can be arranged in the order of increase in the influence of hydrogen as follows: fatigue limit, ultimate strength of specimens with concentrators, relative elongation of smooth specimens, fracture pressure for a membrane under biaxial tension, percentage reduction of the area of smooth specimens, percentage reduction of the area of specimens with concentrators, and low-cycle durability. The dependences of the intensities of true stresses on the intensities of true strains (in uniaxial and biaxial tension) reveal the difference between the curves plotted in air and in hydrogen. The diagrams of hydrogen resistance taking into account the strain rates are presented. The sequence of investigated types of steel and alloys can be arranged in order of decrease in the low-cycle hydrogen resistance as follows: annealed stable austenitic and iron-nickel alloys, nitrided Cr-Ni-Mn steels, titanium alloys of the Ti-Al-Sn and Ti - Al -V systems, steels with nonstabilized austenite, high nickel alloys, maraging steels, and ferritic steels. The micromechanism of hydrogen fracture is characterized by the presence of a great number of microstructural fracture sites (bulk damage) and the completeness of local plastic relaxation. New procedures aimed at increasing the hydrogen resistance of steels and alloys are proposed, namely, the thermocyclic annealing with shot-term overheating, additional compression (with an optimal value of about 20%), and changing the temperature of aging.