Axial Compressive Strain Research Articles

Study on axial compressive behavior and stress–strain relationship of fiber-reinforced alkali-slag recycled concrete

Fiber-reinforced alkali-slag recycled concrete, a novel green building material with high-efficiency solid-waste utilization, can effectively utilize waste slag and recover coarse aggregates. However, no relevant research has been conducted on the mechanical properties under axial compression. Therefore, this study investigated the influence of fiber type, fiber content, and recycled coarse aggregate content on the axial compression mechanical properties of fiber-reinforced alkali-slag recycled concrete. The results show that an appropriate amount of steel, basalt, and polypropylene fibers can effectively improve the axial compressive strength, peak compressive strain, and elastic modulus of alkali-activated slag recycled concrete. The axial compressive strength, peak compressive strain, and elasticity can be enhanced by 10.6–31.9%, 26.0–29.3%, and 1.4–69.1%, respectively. Basalt fibers had the worst effect, while steel fibers had the best effect. A small proportion of recycled coarse aggregate can slightly increase the axial compressive strength and peak compressive strain while significantly reducing the elastic modulus. An axial compression stress–strain model of the fiber-reinforced alkali-slag recycled concrete was established. These results provide basic theoretical support for the comprehensive and effective exploitation of waste slag and building waste.

Testing of a Novel Induction Heat Treated Steel Brace with Enhanced Buckling Behaviour

AbstractSteel braces are widely used to stabilize steel structures forming horizontal or vertical truss structures; however, their relatively poor buckling behaviour is often lead to overdesigned structures. Induction Heat (IH) treatment technology is a novel contactless material transformation process to effectively strengthen local areas of steel sections. Utilizing IH to increase the strength of the middle length of steel brace sections, buckling behaviour can be improved. The present study experimentally investigates the compression behaviour of the novel braces fabricated by IH‐treated circular hollow steel sections. A Digital Image Correlation (DIC) system is used to measure the experimental deformation quantities, such as axial displacements, out‐of‐plane deformations, and strain distributions along the brace length. Four specimens were tested with two set of different slenderness ratios (λ). Each set includes one conventional steel brace (CSB) and one IH‐treated steel brace (IHSB) with a stronger section at its mid‐length. The specimens were subjected to a monotonic displacement‐controlled loading history until reaching an axial compressive strain of 2.5%. It was found that buckling load can increase up to 20% in IHSBs. IH treatment was also beneficial in improving the post‐buckling behaviour of the brace. The compression strength was found to be more than double in IHSBs than in CSBs at a ductility level of 5 (ductility is defined as the ratio of the target axial displacement to the yielding axial displacement). Moreover, out‐of‐plane displacements were reduced by 28% for the IHSBs at an axial strain of 1% showing a more evenly distribution of strain demands along the brace length.

Experimental and Numerical Analysis of Axial Behavior of Triaxial Woven Fabric Confined Concrete Columns

Continuous efforts are being made to improve plain concrete compressive strength and ductility by applying carbon, glass fiber, or hybrid-reinforced epoxy resin composites. The investigation centers on analyzing the axial compressive strength and strain, compressive stress–strain behavior, failure morphology, and crack evolution of the reinforced cylinders. Besides the experiments, non-linear finite element analysis was performed using the finite element (FE) package ABAQUS 2021. The test results indicate that carbon fiber triaxial woven fabric (TWF-C) confinement result in the most significant improvement of 118% in compressive stress than the concrete specimens. On the other hand, glass fiber triaxial woven fabric (TWF-G) confinement shows the highest enhancement of 161% in ductility. The mechanical properties of the sample utilizing glass fiber as the weft yarn and carbon fiber as the warp yarn (TWF-GC2) exhibit superior improvements of 22% in compressive stress and 8% in axial strain compared to the sample using glass fiber as the warp yarn and carbon fiber as the weft yarn (TWF-CG2). Samples with glass fiber as weft yarn show gradual cracks during loading, while carbon fiber as weft yarn show instantaneous damage. The numerical finite element models accurately predict the experimental results of the tested specimens in this study. There were 1.2~3% and 5~10% discrepancies for compressive stress and axial strain, respectively, between experimental and FE results. Overall, the results suggest that Triaxial woven fabric confinement is a valuable technique to improve the strength and strain of concrete and that the type of fibers used could be tailored for appropriate performance characteristics.

Tuning structural and thermal conductivity of carbon nanotubes under strain effects

Equilibrium molecular dynamics (EMD) computations have been performed to estimate the thermal conductivity (λ) of armchair and chiral single-walled carbon nanotubes (SWCNTs) under the influence of varying axial compressive and tensile strains ±γ(%). For the first time, the present work provides new outcomes of structural behavior of SWCNTs linked through λ, considering the combinations of nanotube length (L), system temperature (T) and effects of varying ±γ(%). A radial distribution function (RDF) test has performed in order to investigate the structure analysis of armchair and chiral SWCNTs with different compressive (−1% – −10%) and tensile (+1% – +10%) strain sequences at the extreme L (= 100 Å and 1900 Å) and T (=300 K). The RDF and visualization tests indicate that both configurations are more tunable under tensile strain but the buckling processes of armchair SWCNTs are less evident as compared to chiral configurations. The simulation data shows that the λ(T, L) increases as L extends and it also confirmed through slight increase in RDF peaks, while the λ(T, L) of chiral SWCNTs is high compared to armchair SWCNTs for a complete range of T at L (= 100 Å and 1900 Å). However, the calculations indicate that the delta change in λ(T, L) of armchair SWCNTs is more than the chiral one, as expected. The simulation data demonstrates that the positions of the highest and lowest values of the thermal conductivity of the chiral SWCNTs shift to a longer L as compared to the armchair SWCNTs. The λ(T, L) of SWCNTs remains high for -γ(%) range at short L (=100 Å) due to less broaden shift in RDF peaks but slightly higher for +γ(%) range at longer L (=1900 Å) due to boarder shift in RDF peaks. Moreover, the present simulation results obtained from EMD approach are found to be in reasonable agreement with those obtained via the previous known numerical results.

Test on Compressive Performance of Hollow Concrete-Filled Sandwich Circular Steel Tubes Connected by Thread

The connection method of lengthening the steel tube of hollow concrete-filled sandwich circular steel tubes and threaded connections is proposed. The length, depth and position are the basic parameters. Twelve hollow concrete sandwich circular steel pipes with threaded connections were designed and subjected to axial compression tests. The axial compressive loading–longitudinal compressive displacement curves, axial compressive loading strain of steel tube curves and failure mode of the specimens are analyzed, and the effects of different parameters on the axial compressive-bearing capacity and stiffness of the specimens are studied. The results showed that within the range of parameters studied, the axial compression load–longitudinal compression displacement curves of the specimens were the linear elastic stage and the elastic–plastic stage, which can be divided into a yield-strengthening stage and a decreasing stage. The bearing capacity and strength of the lined threaded connection specimen are not inferior to those of the ordinary specimen or the welded specimen. The bearing capacity and strength of the specimen increase with the increase of the thread length. The bearing capacity and strength of the specimens connected with inner liner screws at the ends are higher than those connected with inner liner bolts at the middle.

The effect of different strain on the structural and optical properties of multilayer γ-InSe

Strain engineering is one promising approach to tune electronic and optical properties of 2D materials. But which type mechanical strain is most efficient is unclear. Here we systematically investigate and compare the tuning effect from four different type strains for multilayer γ-InSe, including in-plane uniaxial tensile strain, in-plane biaxial tensile strain, hydrostatic compressive strain and out-of-plane axial compressive strain. The evolution of its phonon modes and luminescent properties upon strain were investigated using in situ Raman and photoluminescence (PL) spectroscopy. All phonon modes exhibit blue shift upon compression strain, while red shift upon tensile strain. Band gap reduction was observed for both compression strain and tensile strain. Strain type has great effect on the shift rate of Raman mode and band gap. Among four types strain, biaxial tensile strain exhibits the largest shift rates, being 8.49, 9.24 and 8.65 cm-1/% for three phonon modes and 85 meV/% for band gap. It suggests that biaxial tensile strain is more efficient for γ-InSe. This result is helpful for practical strain-based manipulation for 2D materials.

Mechanical performance of fiber-reinforced polymer tank under road pavements for stormwater storage

Due to increasing precipitation intensity, rising sea level, and land development, urban flooding has become a major issue in many places around the world. This research aims to develop underground storage tanks (USTs) using fiber-reinforced polymer (FRP) materials for stormwater detention, storage, and reuse. The USTs are intended to be placed underneath highway or street pavements. FRP as one of the emerging structural materials possesses several advantages including high strength-to-weight ratio and corrosion resistance. The mechanical performance of USTs has rarely been visited in the literature, the majority of which focuses on leakage issues and associated environmental problems. This paper presents detailed analyses of the mechanical performance of FRP USTs, including a laboratory test on a small-scale FRP UST and a verified finite element (FE) analysis on full-scale FRP USTs. The influences of various factors on the mechanical performance of the USTs were examined, including wall thickness, water storage, interfacial roughness between FRP and soil, and tank sizes. Both results from experiments and FE studies agree satisfactorily and reveal that (i) underground FRP UST developed tensile axial strains at the top surface at the mid-span of the tank and the side surface at the mid-height; (ii) compressive axial strains developed in most of the cap region; and (iii) large compressive circumference strains developed in the FRP UST in most locations. Insights gained from the laboratory test and the FE analysis are expected to provide a general guidance on the design of FRP USTs under road pavements for urban flooding mitigation and stormwater harvesting.

Cyclic Compressive Behavior and Load-Strain Model of FRP-ECC-HSC Composite Columns

An innovative composite column, which consists of a high strength concrete (HSC) core, engineered cementitious composites (ECC) ring, and fiber-reinforced polymer (FRP) tube, has recently been developed and subjected to monotonic axial compression testing by the authors. In this study, the cyclic compressive behavior of this proposed FRP-ECC-HSC composite column was examined. Test parameters, including HSC core strength, FRP tube thickness, and ECC ring thickness, were investigated. Typical failure modes, dilation behavior, and axial load versus axial strain behavior were discussed and analyzed. It was found that the FRP-ECC-HSC composite columns exhibited improved deformability, compared with the counterpart traditional FRP-confined HSC columns, with the ultimate axial compressive strain increased by 0.7%–69.1% for the tested specimens. Meanwhile, the ultimate axial strain for cyclically loaded specimens is larger than that for monotonically loaded specimens in general, indicating a delayed column failure. Cyclic axial load-axial strain models, including the envelope model, unloading and reloading models, plastic strain equation, and stress deterioration equation, were proposed to predict the cyclic compressive behavior of the tested specimens. The proposed model was verified with the test results and exhibited good performance.

Fire-resistance behavior of concrete columns produced with recycled ceramic and silica aggregates: An experimental and numerical approach

This article aims to fully assess the fire-resistance behavior of fiber-reinforced concrete (FRC) columns made with recycled ceramic aggregates when subjected to severe thermal loading. In addition, the effects of fiber reinforcements and ceramic recycled aggregates (RAs) on the thermal behavior of concrete columns were addressed and compared with that of the conventional silica-made concrete columns. In this regard, twelve small-scale FRC columns containing recycled ceramic and silica aggregates are prepared and placed in a heating chamber for 90 min. Next, the columns were axially loaded until failure, and variations in compressive stress and axial strain of the specimens were measured, aiming to capture the thermal effects on the load-carrying capacity, axial strain, and failure modes of the concrete columns. The specimens were exposed to high temperatures of 20 °C, 600 °C, and 1000 °C according to ISO 834 Fire Standard, and were tested in humid and dry environmental conditions. Moreover, the failure modes of the columns (e.g., concrete spalling, crack pattern, etc.) are presented in terms of macroscopic imaging. In addition, in-depth analyses of the effects of fibers (steel and polypropylene) and moisture content on the thermal behavior of concrete columns is addresses utilizing scanning electronic microscope (SEM) technique. The results indicate that below the temperature of 600 °C, the axial behavior of the FRC columns with silica and ceramic aggregate are similar. However, as the temperature rises, the reduction in axial load-bearing capacity of the columns with silica aggregates is higher than that of the columns with ceramic aggregates. Also, it was observed that the environmental humidity negatively affects the overall fire-resisting behavior of the columns. Furthermore, a numerical procedure is presented in ABAQUS to estimate the behavior of FRC columns subjected to thermal loading and validated using the experimental results.

Mechanical behavior of multiscale hybrid fiber reinforced recycled aggregate concrete subject to uniaxial compression

This paper presents a multiscale hybrid fiber reinforcing strategy that can remarkably compensate for the adverse effects of recycled coarse aggregate (RA) on the performance of concrete. The hybrid fibers including macro steel fiber (SF), micro polypropylene fiber (PPF), and cellulose nanofiber (CNF) were utilized for crack arresting at multi-levels and improving the mechanical properties of RA concrete (RAC). This work aims to investigate the mechanical performance of multiscale hybrid fiber reinforced RAC (MFRRAC) under uniaxial compression and unveil the multiscale hybrid fiber reinforcement mechanism. A total of 34 groups of prismatic specimens were tested, with an emphasis on the effects of RA replacement rate and fiber parameters on the failure mode, compressive strength, deformation ability, and stress-strain curve. The results showed that the incorporation of multiscale hybrid fibers has a marked impact on the compressive properties of RAC. Specifically, the addition of multiscale hybrid fibers could significantly alter the failure pattern of RAC. In addition, compared with RAC without fibers, the axial compressive strength and peak strain of MFRRAC could be increased by up to 39.34% and 24.85%, respectively. Furthermore, the hybrid fiber reinforcement mechanism on the mechanical enhancement in a physical-chemical manner was revealed, in particular, multiscale crack resistance and hybrid fiber synergetic effect. Finally, a stress-strain model was developed with RA replacement rates and fiber parameters taken into account, and the predictions can correlate well with available experimental results with wide applicability. This research work lays a foundation for the accurate design and analysis of MFRRAC structures.

Predicting ultimate condition and transition point on axial stress–strain curve of FRP-confined concrete using a meta-heuristic algorithm

Accurately predicting key reference points on the axial stress-strain curve of fiber-reinforced polymer (FRP)-confined concrete is of great importance for the pre-design and modeling of structures manufactured with this composite system. This paper presents a detailed study on the development of accurate and practical expressions for predicting the ultimate condition and transition point, as key reference points, on axial stress-strain curves of FRP-confined concrete using generic programming (GP). A comprehensive data tuning and cross-validation analysis was firstly performed to develop prediction models. Afterwards, the accuracy and performance of the developed empirical expressions were examined by sensitivity analysis, parametric analysis and model validation. Finally, a comparison was made between the performance of these proposed expressions and that of the existing best-performing expressions in the literature using statistical analysis. Based on the sensitivity and parametric analysis of the database, it is shown that: compressive strength ( f’ cc ) and axial transition strain ( ɛ c1 ) are more sensitive to FRP lateral stiffness ( K l ); ultimate axial strain ( ɛ cu ) is more sensitive to K l -to-unconfined compressive strength ( f’ co ) ratio and fiber ultimate tensile strain ( ɛ fu ); hoop rupture strain ( ɛ h,rup ) is more sensitive to fiber elastic modulus ( E f ); and axial transition strength ( f’ c1 ) is more sensitive to f’ co . It is also shown that the proposed expressions provided more accurate predictions of the ultimate condition and transition point on the axial stress-strain curve of FRP-confined concrete than the existing expressions. This was achieved by using a larger number of datasets and accurately capturing the effects of the most influential input parameters in the proposed expressions.

Influence of multi-scale fiber on residual compressive properties of a novel rubberized concrete subjected to elevated temperatures

Green and environment-friendly rubberized concrete has attracted extensive attention from scholars worldwide. However, the incorporation of crumb rubber can weaken the mechanical properties of concrete, particularly when it is subjected to elevated temperatures. To improve the residual compressive performance of rubberized concrete, a novel multi-scale fiber reinforced rubberized concrete (MSFRRC) is developed by adding calcium carbonate whiskers, polyvinyl alcohol (PVA) fibers, and steel fibers. In this study, 10%, 20%, and 30% of the sands are substituted by the same volume of crumb rubber with sizes between 0.4 mm and 0.8 mm. The impacts of the crumb rubber content and multi-scale fiber on the residual Young's modulus, axial compressive strength, peak strain, stress-strain curve, and compressive toughness of MSFRRC after being heated up to various temperature levels (25 °C, 200 °C, 400 °C, 600 °C, and 800 °C) are discussed. Also, the failure mode combined with the acoustic emission technology and the thermal effect on the microstructure of rubberized concrete specimens, are investigated. Results reveal that incorporating multi-scale fiber can effectively restrain the development of cracks and explosive spalling under high temperatures. Besides, it can cover the strength loss caused by crumb rubber and impose a positive effect on resisting thermal damage. The compressive strength of MSFRRC can be enhanced by about 5.4%–19% with the incorporation of different fibers. The results show that specimens without fibers suddenly fail once the peak stress is reached and exhibit brittle fracture. However, the brittleness can be improved by adding multi-scale fibers. It can be observed that, after exposure to 800 °C, the damage of MSFRRC can develop at the early loading stage and develop in the whole loading process, showing a relatively ductile fracture. Meanwhile, the pore structure can be refined by adding CaCO3 whiskers and the addition of multi-scale fibers can slightly increase the porosity and cannot restrain the heating-induced pore coarsening. Furthermore, the empirical formulas to predict the residual stress-strain relation of MSFRRC under uniaxial compression are proposed by considering crumb rubber, multi-scale fibers, and high temperature.

Study on axial compressive stress-strain relationship of alkali-activated slag lightweight aggregate concrete

Alkali slag lightweight aggregate concrete (AAS-LWAC) is a new type of green concrete prepared by alkali excited slag as cementing material and lightweight aggregate as coarse aggregate. It can effectively reduce the amount of Polandite cement and reduce carbon dioxide emission. However, there are no relevant studies on its axial compression constitutive properties. The effects of the type of lightweight aggregate, fiber type, and fiber content on the elastic modulus Ec, peak compressive strain εc, and axial compressive strength fcc of AAS-LWAC were studied in this paper. A suitable AAS-LWAC axial compression constitutive model is proposed. Scanning Electron Microscope (SEM) and Computed Tomography (CT) experiments were carried out to reveal the influence mechanism of the microstructure on the axial compressive properties of AAS-LWAC. The results show that AAS-LWAC with pulverized coal ash ceramsite as coarse aggregate has the highest axial compressive strength and peak compressive strain, and AAS-LWAC with shale ceramsite as coarse aggregate has the highest elastic modulus. Steel fiber can significantly enhance the axial compressive strength and peak compressive strain of AAS-LWAC, and the improvement effect is the best when the volume content of steel fiber is 0.6%. Three kinds of fibers can increase the elastic modulus of AAS-LWAC, and the effect of steel fiber is the most obvious.

Axial compression behaviour of rectangular recycled aggregate concrete-filled steel tubular stub columns

Recycled aggregate concrete-filled steel tubular (RACFST) column is a type of composite column that combines the mechanical efficiencies of concrete-filled steel tubular (CFST) columns and the economic and environmental advantages of recycled aggregate concrete (RAC). Till now, experimental and numerical investigations on the axial compressive behaviour of rectangular RACFST columns are not sufficient and design methods for axial compressive stiffness, peak strain and steel tube local stability are still lacking. This study presents experiments on 16 rectangular RACFST stub columns subject to axial compression, investigating the effects of replacement ratio of recycled coarse aggregate (RCA), aspect ratio and depth to thickness ratio of steel tube. The full-range axial load vs. deformation behaviour, working mechanism and failure mode of rectangular RACFST columns are found to be similar to those of natural CFST counterparts, while the occurrence of the first local buckling of steel tube is advanced due to the incorporation of RCA. The increase in replacement ratio negatively affects the axial stiffness of rectangular RACFST columns more than the load-bearing capacity. Nonlinear finite element analysis is subsequentially conducted, and the influences of replacement ratio and aspect ratio on the axial compressive performance and confinement effect of rectangular RACFST columns are numerically studied. On the basis of extensive test data and parametric studies, practical recommendations are finally given for designing the ultimate capacity, axial compressive stiffness, peak strain and tube local buckling of rectangular RACFST stub columns.

Axial compressive behaviours of concrete-filled square GFRP tubular columns at low temperatures

This study reported experimental and theoretical investigations on low-temperature compression behaviours of concrete-filled square GFRP tubular (CFSFT) columns. The influences of low temperatures (T = 20, −30, −60, and −80 °C), thickness of GFRP tubular (tf = 3.66, 5.26, and 6.49 mm), and spacing of spiral GFRP bars (S = 40, 60, and 80 mm) on axial compressive behaviours of CFSFT columns were investigated through eighteen axial compression tests. Failure modes, load-axial strain behaviours, dilation behaviours of CFSFT columns, and load–strain behaviours of GFRP bars at low temperatures have been detailed reported. Test results indicated that reducing temperature from 20 to −80 °C decreased the confinement effectiveness of GFRP tubular on concrete core, but improved the compressive strength of GFRP tubular, concrete core and GFRP bars. These improved compressive strengths increased the ultimate compressive capacity of CFSFT columns. Prediction equations have been developed to estimate the ultimate compression capacity and its corresponding axial compression strain of CFSFT columns, which have considered the contribution of longitudinal GFRP bars and spiral GFRP bars. Validations of predictions against test results and comparisons with code predictions proved the improved accuracy of these developed prediction equations.

Numerical Assessment of the Structural Damage of a Composite Lining Water Conveyance Tunnel Subjected to Reverse Fault Conditions

In this paper, the structural responses and failure characteristics of a new type of water conveyance tunnel lining structure subjected to reverse fault conditions were numerically investigated by considering multiple loads and interaction separation modes between different structural layers. This study proposes a new evaluation standard for the safety level of the damage state of the composite lining water conveyance tunnel. It also discusses the influences of fault dislocation displacement (Δf), dip angle (β), and the mechanical properties of the surrounding rock in the fault fracture zone on the water conveyance tunnel response and damage. The results indicate that the buckling failure of the steel tube under axial compression is the dominant failure mode of the composite lining structure. With increasing fault dislocation displacement, the axial compressive strain and circumferential shear strain of the composite lining are most severely damaged on the sliding plane. With decreasing fault dip angle, the axial compressive strain of the composite lining weakens, while the bending and shear strains increase. The increase in rock stiffness in the fault fracture zone reduces the damage scope but increases the composite lining structural damage severity. Overall, the numerical results of this study provide a better understanding of the failure mode and damage process of composite lining water conveyance tunnels under reverse fault conditions; therefore, this study can serve as a reference for composite lining structure disaster assessments.

Strain Response and Buckling Behavior of Composite Cylindrical Shells Subjected to External Pressure with One End Fixed and the Other under Free Boundary Conditions.

This study aims to reveal the buckling behavior of filament-wound composite cylindrical shells subjected to external pressure. The boundary conditions of the cylindrical shells were one end fixed and the other free. The carbon fiber stacking sequences were [±90]2/([±20]/[±90]/[±40]/[±90]/[±60]/[±90])2/[±90]. Finite element software ANSYS 16.2 was used for the numerical simulation to predict the critical buckling pressure and buckling behavior of composite cylindrical shell. External hydrostatic pressure tests were conducted, where the buckling behavior and strain response were observed. Numerical simulation accurately predicted the critical buckling pressure of carbon fiber/epoxy filament composite cylindrical shells under external pressure with 3.5% deviation from the experimental results. The buckling modes simulated by the finite element method agreed well with the deformed shape observed in the experiment, which was characterized by the uniform distribution of the three hoop waves. Comparing the axial compressive strain and hoop compressive strain of the composite shell, it was found that the circumferential stiffness of the shell was weaker than the axial stiffness. In addition, a comparative study of the strains of the fixed-end and free-end metal control sleeves was carried out. The results show that the boundary conditions have a significant influence on the strain response of control sleeves.

Formation Process and Mechanical Deformation Behavior of a Novel Laser-Printed Compression-Induced Twisting-Compliant Mechanism

A novel compression-induced twisting (CIT)-compliant mechanism was designed based on the freedom and constraint topology (FACT) method and manufactured by means of laser powder bed fusion (LPBF). The effects of LPBF printing parameters on the formability and compressive properties of the laser-printed CIT-compliant mechanism were studied. Within the range of optimized laser powers from 375 to 450 W and with the densification level of the samples maintained at above 98%, changes in the obtained relative densities of the LPBF-fabricated CIT-compliant mechanism with the applied laser powers were not apparent. Increased laser power led to the elimination of residual metallurgical pores within the inclined struts of the CIT mechanism. The highest dimensional accuracy of 0.2% and the lowest surface roughness of 20 μm were achieved at a laser power of 450 W. The deformation behavior of the CIT-compliant mechanism fabricated by means of LPBF exhibited four typical stages: an elastic stage, a heterogeneous plastic deformation stage, a strength-destroying stage, and a deformation-destroying stage (or instable deformation stage). The accumulated compressive strain of the optimally printed CIT mechanism using a laser power of 450 W went up to 20% before fracturing, demonstrating a large deformation capacity. The twisting behavior and mechanical properties were investigated via a combination of finite-element simulation and experimental verification. An approximately linear relationship between the axial compressive strain and rotation angle was achieved before the strain reached 15% for the LPBF-processed CIT-compliant mechanism.

Enhancement of compressive strength and strain ductility of SMA fiber reinforced concrete considering fiber’s aspect ratios

In this study, the effect of recovery stress on the toughness of cementitious materials was investigated using randomly distributed SMA fibers in concrete. The composite was reinforced by cold drawn crimped SMA fibers due to their great potential in developing a prestressing effect as well as providing sufficient bond resistance. To this end, considering the same volumetric fraction, different aspect ratios of the SMA fibers were embedded into the matrix and then molded into different reinforced concrete cylinders. Compression tests were performed where several parameters, including compressive strength, ductility, axial, and volumetric strain were examined before and after inducing the prestressing effect. Furthermore, the present study dealt with fracture modes, as well as internal and surface crack analysis via X-ray and digital image correlation (DIC) equipment to investigate the effect of prestressing with different aspect ratios. The results showed that not only the current geometry of SMA fibers in concrete could improve the ductility of the composite, but also elevate the compressive strength demonstrating a considerable rise in energy absorption capacity.

Analytical solution for predicting the interaction stress of axially loaded concrete-filled double-tube columns

Concrete-filled double-tube (CFDT) members are promising steel-concrete composite structural forms. Existing studies mainly focus on members’ load-deformation performance. The interaction stress between components, which is a fundamental cause for change in load-deformation performance, receives much less attention. This study presents an analytical solution for interaction stress between components of axially loaded CFDT columns. The stress–strain relationships and force equilibrium equations of the inner concrete, inner steel tube, sandwiched concrete, and outer steel tube were analysed using elasticity method first. Subsequently, the deformation compatibility equations were established concerning the interactions between the components. Based on the above analysis, an analytical solution was derived. Equivalent lateral deformation coefficients were used to substitute Poisson’s ratio of concrete to account for the nonlinear behaviour of materials during loading. The proposed model shows reasonable agreement with experimental results. Finite element analysis further shows the accuracy of the interaction stress model till an axial compressive strain of 0.01 of the composite column. The derived interaction stress model was successfully applied in predicting the load-bearing capacities of CFDT columns through validation of 39 specimens from five independent studies. The average ratio of the predicted-to-experimental bearing capacities was 1.054, with a coefficient of variation of 0.077. The accuracy of models in Eurocode 4, ACI 318-11, and GB 50936 were also evaluated. Thereby the proposed model provides insights into the interaction analysis between components and mechanism-based load-bearing capacity prediction of CFDT columns.