Compressive Behavior Research Articles

Mechanical characterization of 3D-Printed carbon fiber-reinforced polymer composites and pure polymers: Tensile and compressive behavior analysis

Mechanical characterization of 3D-Printed carbon fiber-reinforced polymer composites and pure polymers: Tensile and compressive behavior analysis

Compressive and Tensile Behavior of High-Ductility Alkali-Activated Composites with Polyethylene Terephthalate Powder

Researchers have been engaged in the study of high-ductility concrete (HDC) due to its excellent ductility and cracking control ability. This study combines the concepts of HDC and alkali-activated composites (AAC) to develop high-ductility alkali-activated composites (HDAAC) using polyethylene terephthalate (PET) powder. Experimental investigations were conducted to assess the compressive and tensile properties of HDAAC, focusing on the impact of varying PET powder content (0%, 15%, 30%, and 45%) and fly ash/slag ratios (FA/GGBS, 6:4, 7:3, and 8:2). The results indicated that the compressive strength of HDAAC ranged from approximately 30 MPa to about 100 MPa, with the specimens maintaining good integrity after axial compression failure due to the bridging action of PE fibers. The replacement of quartz powder (QP) with PET powder slightly decreased the compressive strength and elastic modulus of HDAAC, albeit mitigating its brittleness under compression. An increase in GGBS content enhanced the compressive strength and elastic modulus of HDAAC due to the increased formation of the C-A-S-H reaction products, leading to reduced porosity and a denser microstructure. Under axial tension, HDAAC exhibited typical multiple-cracking behavior with significant pseudo-strain hardening. Increases in the PET content and FA/GGBS ratio resulted in finer cracks, indicating excellent crack control and deformation capabilities. The initial cracking strength, tensile strength, and ultimate tensile strain ranged from 3.0 MPa to 4.6 MPa, 4.2 MPa to 8.2 MPa, and 4.1% to 7.2%, respectively. Despite a decrease in the initial cracking strength and tensile strength with higher PET content, the ultimate tensile strain of HDAAC slightly increased. Observations under a scanning electron microscope revealed a distinct interfacial transition zone near the PET powder, leading to poor bonding with the alkali-activated matrix. In contrast, QP dissolved on the surface in highly alkaline environments, forming better interface properties. These variations in interface properties can be used to interpret the variations in the mechanical performance of HDAAC.

Density dependent constitutive model for Bi-2212 powder compression deformation

Bi-2212 HTS materials are fabricated into multi-filamentary wires via powder-in-tube (PIT) method followed by proper heat treatment to obtain superconductivity, but how to predict the large compression deformation behaviors of the Bi-2212 powder is critical to design the processing of the Bi-2212 HTS wire. Drucker Prager/Cap (DPC) model was the most commonly used model for powders including Bi-2212 with soil-like mechanical behavior to consider its shear failure as well as hydrostatic compression. However, the parameters for DPC Cap evolve with densities change and the original model is inadequate to precisely describe the densification process of Bi-2212 powder with large strain. In this study, the modified DPC model with density dependent parameters was introduced for Bi-2212 powder compressions by measuring the failure strength and hydrostatic compressive behavior under different density states. The DPC yield surface was plotted with an evolution trend of non-linear outward expansion with density increased. FEM model of uniaxial compression based on the as-introduced model was built with subroutine VUSDFLD applied. The distribution of Mises stress and relative density were analyzed. The axial stress-density curve for FEM and experimental results were normalized and quantitively evaluated by Mean Square Error (MSE). The introduced model shows good convergence and could match the experimental results well with normalized MSE of 0.000207 and Root Mean Square Error (RMSE) of 0.0144, indicating the mean error percentage of 1.44%. The model introduced in this article provides supports toward large strain deformation simulation of Bi-2212 powder.

Absorbing capabilities of additively manufactured lattice structure specimens for crash applications: Damage tolerant design and simulations

AbstractIn the present work, the influence of defects on the compressive response of octet‐truss AlSi10Mg lattice structure specimens produced with a selective laser melting process is investigated. The defect population in one cell, in two cells, and cubic specimens composed of 27 cells has been assessed with micro‐computed tomography (micro‐CT) analyses. The statistical distributions of the characteristic defect sizes, i.e., the equivalent diameter, the volume, and the surface, assessed in the lattice structure specimens and in volumes randomly extracted from a rectangular bar have been compared. Finally, the compressive behavior of lattice structure specimens has been simulated with a simplified damage‐tolerant finite element model accounting for the influence of defects and compared with experimental results. The analyses have proven that the defect population in volumes extracted from a rectangular bar can provide reliable simulated results, even if micro‐CT inspections of a unit cell or specimens made of several cells are suggested.

Experimental studies on the mechanical behaviors of cold-formed thin-walled steel-foam concrete composite walls

This paper presents experimental studies to investigate the compressive and shear behaviors of cold-formed thin-walled steel-foam concrete composite walls. Three compressive walls and three shear walls were tested. The failure mode and load-displacement curve of the specimen were obtained from the compressive experiment. The compressive wall infilled with foam concrete mainly experienced distortion buckling of the end stud and local crushing failure of the concrete. The stud openings had a limited effect on the compressive bearing capacity of the composite wall. The failure mode and load-deformation curve were obtained in shear tests. The ductility index, shear stiffness, yield load, peak load, energy dissipation, and stiffness degradation were analyzed. The shear composite wall filled with foam concrete mainly occurred concrete crushing failure, local or distortional buckling failure of the end stud, and cracking failure of the calcium silicate board. The shear stiffness, shear capacity, ductility, and energy dissipation of the composite wall could be significantly improved after filling with foam concrete.

Cyclic compressive behavior of high-performance hybrid fiber reinforced recycled aggregate concrete: Degradation law and acoustic emission features

To tackle the issues of inadequate mechanical properties of recycled aggregate concrete and its limited application in load-bearing structural components, a high-performance hybrid fiber reinforced recycled aggregate concrete (HP-HFRAC) was developed in this study. Sixteen groups of HP-HFRAC specimens fabricated with varying replacement rates of recycled coarse aggregate and different fiber parameters were subjected to uniaxial compression, as well as acoustic emission (AE) tests, to investigate the cyclic behavior and uncover the underlying mechanisms. The findings demonstrated a strong correlation between the AE signals and the macroscopic phenomenon throughout the whole compression process. The AE amplitude distribution was found to be capable of identifying damage mechanisms, including matrix cracking (40–50 dB), fiber sliding (50–80 dB), and fiber pull-out (more than 80 dB). Additionally, the inclusion of hybrid fibers contributed to mitigating the degradation of stiffness and the accumulation of plastic strain, resulting in the ductile failure of the specimens and significantly enhancing the energy dissipation capacity. Specifically, the peak stress of specimens with 1.5% steel fibers increased by 19% compared to that of specimens without steel fibers, compensating for the strength loss due to the presence of recycled coarse aggregate. Finally, constitutive equations governing the skeleton curves and the unloading and reloading curves of HP-HFRAC subjected to cyclic compression were formulated, which were validated using both the test results and the predicted results of the physics-based data-driven model previously published in our companion paper. The findings of this study might contribute to the development of sustainable high performance recycled aggregate concrete for reliable application in load-bearing components.

Experimental analysis of the compressive behaviour of perforated brick masonry using digital image correlation

The paper presents an experimental program on the characterisation of the mechanical behaviour of perforated brick masonry (PBM) under uniaxial compression. The study is aimed to provide a better understanding on the compressive strength and elastic behaviour of a kind of masonry hardly dealt with in the literature. The mechanical response of the investigated masonry shows actually two complexities, linked respectively to the perforated nature of the bricks and the asymmetric cross-section of the units. The latter feature is particularly recurrent in exposed brickwork composed of perforated bricks, whenever one of the face shells has a larger thickness than the other one. For this purpose, an experimental campaign was carried out on units and PBM subjected to uniaxial compressive loading. The mechanical response of PBM is investigated to accurately determine the masonry Young’s modulus and the compressive strength. Measurements obtained with conventional sensors are compared with those derived from the Digital Image Correlation (DIC) technique. Lastly, the experimental findings in terms of compressive strength and Young’s modulus are cross-referenced with the estimations derived from empirical criteria available in Eurocode 6 and ACI 530.1–02.

The Quasi-Plastic Deformation Mechanisms and Inverse Hall-Petch Relationship in Nanocrystalline Boron Carbide under Compression

Abstract Grain boundary (GB) plays a significant role in the deformation behaviors of nanocrystalline ceramics. Here, we investigate the compression behaviors of nanocrystalline boron carbide (n-B4C) with varying grain sizes, using molecular dynamics simulations with a machine-learning force field. The results reveal quasi-plastic deformation mechanisms in n-B4C: GB sliding, intergranular amorphization, and intragranular amorphization. GB sliding arises from the presence of the soft GBs, leading to intergranular amorphization. Intragranular amorphization arises from the interaction between grains with unfavorable orientations and the softened amorphous GBs, finally causing structural failure. Furthermore, the n-B4C models with varying grain sizes from 4.07 nm to 10.86 nm display an inverse Hall-Petch relationship due to the GB sliding mechanism. A higher strain rate in n-B4C often leads to higher yield strength, following a 2/3 power relationship. These deformation mechanisms are critical for the design of ceramics with superior mechanical properties.

Influence of Programming and Recovery Parameters on Compressive Behaviors of 4D‐Printed Biocompatible Polyvinyl Chloride or Vinyl–Poly(ε‐Caprolactone) Blends

In this article, a new class of biocompatible shape‐memory polymers (SMPs) through blending pcolyvinyl chloride or vinyl (PVC) and poly(ε‐caprolactone) (PCL) is introduced. The compressive shape‐memory behaviors of 4D‐printed SMP PVC with 5 and 10 wt% of PCL are studied in detail. In this respect, a set of experiments are carried out to understand thermomechanical responses of PVC–PCL blends under various shape‐memory parameters like programming temperature, load‐holding time, applied strain, and recovery temperature. Dynamic mechanical thermal analysis and scanning electron microscope imaging are also performed to provide thermal and morphological analyses. It is found that by raising the recovery temperature from 45 to 65 °C, the shape recovery ratio increases from 5.63 to 7.92 MPa when the PVC–PCL10 is programmed via the hot‐programming (HP) protocol. The highest level of shape fixity (100%) and the best performance of stress relaxation are achieved for HP sample, while the highest shape recovery ratio (100%) is obtained for cold programming. By applying the load‐holding time, the amount of shape fixity can reach from 88.14% to 100%. Results of this research are expected to provide an insightful understanding of the shape‐memory behaviors of PVC–PCL and be instrumental for 4D printing and programming of shape‐adaptive structures like shape‐memory intervertebral cages as spinal support devices.

Compressive behaviour and design of tapered lightweight concrete-filled double-skin stiffened steel tubular short columns with large hollow ratio

Tapered concrete-filled double-skin steel tubular (CFDST) members, due to their high strength-to-weight ratio and inherent spatial advantages, hold significant prospect in marine structures, such as ocean platforms and wind turbine tower. As the height and span of the column continue to increase, its weight and local buckling problems begin to emerge and become the main concerns of engineering design. To address this issue, lightweight concrete instead of normal concrete and tapered steel tubes with longitudinal stiffeners are adopted. As a result, a novel composite member, called tapered lightweight concrete-filled double-skin stiffened steel tubular short columns with large hollow ratio (LHRTL-CFDSST), is developed by the authors. In order to gain a more comprehensive understanding of the axial compression behavior of LHRTL-CFDSST members and provide an axial strength model, this study conducted axial compression tests on ten LHRTL-CFDSST short columns, investigating key parameters such as hollow ratio, longitudinal stiffeners and diameter-to-thickness ratio. The results indicated that both larger hollow ratio sections and diameter-to-thickness ratios tend to reduce the axial compressive strength and the ductility of such members, while the effect of the longitudinal stiffeners are opposite. Based on the test results, finite element (FE) model was developed and validated, and was then to conduct the mechanical analysis of the studied members, followed by a parametric study. Finally, a novel method for predicting the axial compressive strength of LHRTL-CFDSST short columns, applicable across a wide range of parameters, was proposed.

Factors Influencing Suffusion-Induced Delayed Displacement of River Bridge Piers

In recent years, local scour has occurred on the pier foundations of river bridges during heavy rain and river flooding, often resulting in bridge collapse. This study focused on the characteristic displacement, called “delayed displacement”, of the river bridge pier where the critical displacement of the piers was first observed several days after the flood when the train passed and not immediately after the flood. The authors hypothesized that one of the possible reasons for the delayed displacement is the suffusion of the supporting ground during the flood, followed by a compressive behavior due to the collapse of the soil skeleton under repeated traffic loads. Accordingly, this study performed erosion tests simulating flood and cyclic loading tests simulating train passage using a triaxial test apparatus to check the validity of this hypothesis. In some test cases, suffusion without any deformation occurred in the erosion test but deformed in the cyclic loading test just after the erosion test. This behavior matches the behavior of delayed displacement. It was also suggested that the risk of the delayed displacement becomes high when the soil skeleton was assumed to primarily comprise fine particles, and the void ratio and hydraulic gradient were high.

Mechanical recycling of foam from end-of-life mattresses by AIR-LAY process for the production of new mattresses with a fully circular approach

A novel method for the recycling of end-of-life mattresses foam, using an AIR-LAY process which employs a bi-component fiber as a binder for the polyurethane foam, have been developed and optimized. This method permits to obtain materials with the same density of the starting foam (25 kg/m3) and with a significantly lower density with respect to that obtainable with the rebonding process (above 70 kg/m3) that uses isocyanates to bond the foam particles. The obtained recycled foams have been tested by mechanical compression and recovery tests showing that compression values of 3.7 KPa, similar to that of the mattress foam (3.0 KPa), can be achieved using 20 % of bi-component fibers as binders and a density of 35 kg/m3. On the contrary, only 3 times stiffer and denser materials can be obtained using the rebonding technology, thus making them not suitable for application in mattresses. On the basis of the results of the optimization tests a set of mattresses containing a layer of the recycled foams have been prepared. The results of the tests on the prepared mattresses have shown that they have the same compression behavior, recovery time and durability (after 30,000 cycles of compression) of a standard mattress without the recycled layer, thus proving that a fully circular approach in the recycling of the foam of the mattresses is possible using the AIR-LAY process.

Rate-dependent damage and failure behavior of lithium-ion battery electrodes

The mechanical properties and failure behavior of composite electrodes are critical to understanding the internal short circuits and ensuring the crush safety of lithium-ion batteries used in electric transportation. This study provides a comprehensive experimental investigation into the strain rate–dependent tensile/compressive behavior and failure mechanism of both anode and cathode, covering a range from quasi-static to dynamic conditions. A universal testing machine equipped with an industrial camera was used to assess the mechanical properties and observe the deformation process of the electrodes under quasi-static conditions. Additionally, a split Hopkinson bar system, coupled with a high-speed camera, was utilized to evaluate the mechanical properties and capture the deformation process of the electrodes under dynamic loading. The detailed deformation and failure modes of the electrodes were revealed by a combination of post-mortem characterization and images from high-speed cameras. The experimental results demonstrate a significant strain rate effect on the tensile and compressive mechanical behavior and failure mechanism of both anode and cathode. The rate dependence of tensile/compressive strength and failure strain were discussed, and the underlying mechanism for strain rate sensitivity was analyzed. The results and conclusions from this study provide an important foundation for the detailed modeling and failure prediction of lithium-ion batteries under impact loading. Further, these findings facilitate the safety design of electric vehicles by informing to enhance crash safety.

Research on the Axial Compressive Behavior of Concrete-Filled Steel Tubular Column Reinforced with Annular Stiffener

Research on the Axial Compressive Behavior of Concrete-Filled Steel Tubular Column Reinforced with Annular Stiffener

Theoretical Modeling and Mechanical Characterization at Increasing Temperatures under Compressive Loads of Al Core and Honeycomb Sandwich

This work investigates the mechanical behavior under out-of-plane compression of the Al core and honeycomb sandwich at increasing temperatures of up to 300 °C. After the first introductive theoretical modeling on room-temperature compressive behavior, the experimental results at increasing temperatures up to 300 °C are presented and discussed. The analysis of the results shows that peak stress, plateau stress, and specific absorbed energy gradually decrease as the temperature increases. The final densification occurs always at the same strain level (around 75%). Sandwich honeycomb test temperatures have been limited to 200 °C for bonding problems of the skin to the sandwich due to the glue. The experimental and modeling results agree well at room temperature as well at increasing temperatures. The results can provide useful information to choose base materials for greater energy absorption at increasing temperatures.

Maximum Drift Demands of Earthquake Damaged Reinforced Concrete Columns Based on Residual Flexure Cracks

AbstractAssessment of existing reinforced concrete (RC) structures after an earthquake is a challenging task that must somehow relate qualitative and quantitative observations in the plastic hinge regions and the associated residual deformation capacity of damaged structures. Having an estimate available for the remaining drift capacity will result in more economical and informed decisions regarding demolition or strengthening options. This study aims to develop a practical methodology to estimate the maximum drift demand of an RC column based on the residual crack width. For this purpose, fiber-based frame elements are used to model the RC column considering appropriately concrete behavior in compression and tension stiffening effects. Afterwards, the accuracy and reliability of the proposed methodology are demonstrated by validating the computational approach with two cyclic experimental results from literature and new test data for a one-bay one-story RC frame conducted within the course of this study. A comprehensive parametric study is performed for RC columns with different axial loads, longitudinal and transverse reinforcement ratios, and ground motions to exhibit the stochastic behavior. The study identifies the axial load ratio as the predominant parameter. Key findings include strong correlations between maximum drift ratios and total residual crack widths, as well as maximum compressive strains, with regression analysis yielding equations for accurate drift ratio estimation. Simple predictive models are proposed to estimate the maximum deformation demands based on observed residual crack widths. Residual cracking exceeding 5 mm poses significant risk for the columns with axial load ratios above 0.4, with 90% probability of exceedance 2% drift ratio.

Nonlinear optimal control for robotic exoskeletons with electropneumatic actuators

Purpose To provide high torques needed to move a robot’s links, electric actuators are followed by a transmission system with a high transmission rate. For instance, gear ratios of 100:1 are often used in the joints of a robotic manipulator. This results into an actuator with large mechanical impedance (also known as nonback-drivable actuator). This in turn generates high contact forces when collision of the robotic mechanism occur and can cause humans’ injury. Another disadvantage of electric actuators is that they can exhibit overheating when constant torques have to be provided. Comparing to electric actuators, pneumatic actuators have promising properties for robotic applications, due to their low weight, simple mechanical design, low cost and good power-to-weight ratio. Electropneumatically actuated robots usually have better friction properties. Moreover, because of low mechanical impedance, pneumatic robots can provide moderate interaction forces which is important for robotic surgery and rehabilitation tasks. Pneumatic actuators are also well suited for exoskeleton robots. Actuation in exoskeletons should have a fast and accurate response. While electric motors come against high mechanical impedance and the risk of causing injuries, pneumatic actuators exhibit forces and torques which stay within moderate variation ranges. Besides, unlike direct current electric motors, pneumatic actuators have an improved weight-to-power ratio and avoid overheating problems. Design/methodology/approach The aim of this paper is to analyze a nonlinear optimal control method for electropneumatically actuated robots. A two-link robotic exoskeleton with electropneumatic actuators is considered as a case study. The associated nonlinear and multivariable state-space model is formulated and its differential flatness properties are proven. The dynamic model of the electropneumatic robot is linearized at each sampling instance with the use of first-order Taylor series expansion and through the computation of the associated Jacobian matrices. Within each sampling period, the time-varying linearization point is defined by the present value of the robot’s state vector and by the last sampled value of the control inputs vector. An H-infinity controller is designed for the linearized model of the robot aiming at solving the related optimal control problem under model uncertainties and external perturbations. An algebraic Riccati equation is solved at each time-step of the control method to obtain the stabilizing feedback gains of the H-infinity controller. Through Lyapunov stability analysis, it is proven that the robot’s control scheme satisfies the H-infinity tracking performance conditions which indicate the robustness properties of the control method. Moreover, global asymptotic stability is proven for the control loop. The method achieves fast convergence of the robot’s state variables to the associated reference trajectories, and despite strong nonlinearities in the robot’s dynamics, it keeps moderate the variations of the control inputs. Findings In this paper, a novel solution has been proposed for the nonlinear optimal control problem of robotic exoskeletons with electropneumatic actuators. As a case study, the dynamic model of a two-link lower-limb robotic exoskeleton with electropneumatic actuators has been considered. The dynamic model of this robotic system undergoes first approximate linearization at each iteration of the control algorithm around a temporary operating point. Within each sampling period, this linearization point is defined by the present value of the robot’s state vector and by the last sampled value of the control inputs vector. The linearization process relies on first-order Taylor series expansion and on the computation of the associated Jacobian matrices. The modeling error which is due to the truncation of higher-order terms from the Taylor series is considered to be a perturbation which is asymptotically compensated by the robustness of the control algorithm. To stabilize the dynamics of the electropneumatically actuated robot and to achieve precise tracking of reference setpoints, an H-infinity (optimal) feedback controller is designed. Actually, the proposed H-infinity controller for the model of the two-link electropneumatically actuated exoskeleton achieves the solution of the associated optimal control problem under model uncertainty and external disturbances. This controller implements a min-max differential game taking place between: (i) the control inputs which try to minimize a cost function which comprises a quadratic term of the state vector’s tracking error and (ii) the model uncertainty and perturbation inputs which try to maximize this cost function. To select the stabilizing feedback gains of this H-infinity controller, an algebraic Riccati equation is being repetitively solved at each time-step of the control method. The global stability properties of the H-infinity control scheme are proven through Lyapunov analysis. Research limitations/implications Pneumatic actuators are characterized by high nonlinearities which are due to air compressibility, thermodynamics and valves behavior and thus pneumatic robots require elaborated nonlinear control schemes to ensure their fast and precise positioning. Among the control methods which have been applied to pneumatic robots, one can distinguish differential geometric approaches (Lie algebra-based control, differential flatness theory-based control, nonlinear model predictive control [NMPC], sliding-mode control, backstepping control and multiple models-based fuzzy control). Treating nonlinearities and fault tolerance issues in the control problem of robotic manipulators with electropneumatic actuators has been a nontrivial task. Practical implications The novelty of the proposed control method is outlined as follows: preceding results on the use of H-infinity control to nonlinear dynamical systems were limited to the case of affine-in-the-input systems with drift-only dynamics. These results considered that the control inputs gain matrix is not dependent on the values of the system’s state vector. Moreover, in these approaches the linearization was performed around points of the desirable trajectory, whereas in the present paper’s control method the linearization points are related with the value of the state vector at each sampling instance as well as with the last sampled value of the control inputs vector. The Riccati equation which has been proposed for computing the feedback gains of the controller is novel, so is the presented global stability proof through Lyapunov analysis. This paper’s scientific contribution is summarized as follows: (i) the presented nonlinear optimal control method has improved or equally satisfactory performance when compared against other nonlinear control schemes that one can consider for the dynamic model of robots with electropneumatic actuators (such as Lie algebra-based control, differential flatness theory-based control, nonlinear model-based predictive control, sliding-mode control and backstepping control), (ii) it achieves fast and accurate tracking of all reference setpoints, (iii) despite strong nonlinearities in the dynamic model of the robot, it keeps moderate the variations of the control inputs and (iv) unlike the aforementioned alternative control approaches, this paper’s method is the only one that achieves solution of the optimal control problem for electropneumatic robots. Social implications The use of electropneumatic actuation in robots exhibits certain advantages. These can be the improved weight-to-power ratio, the lower mechanical impedance and the avoidance of overheating. At the same time, precise positioning and accurate execution of tasks by electropneumatic robots requires the application of elaborated nonlinear control methods. In this paper, a new nonlinear optimal control method has been developed for electropneumatically actuated robots and has been specifically applied to the dynamic model of a two-link robotic exoskeleton. The benefit from using this paper’s results in industrial and biomedical applications is apparent. Originality/value A comparison of the proposed nonlinear optimal (H-infinity) control method against other linear and nonlinear control schemes for electropneumatically actuated robots shows the following: (1) Unlike global linearization-based control approaches, such as Lie algebra-based control and differential flatness theory-based control, the optimal control approach does not rely on complicated transformations (diffeomorphisms) of the system’s state variables. Besides, the computed control inputs are applied directly on the initial nonlinear model of the electropneumatic robot and not on its linearized equivalent. The inverse transformations which are met in global linearization-based control are avoided and consequently one does not come against the related singularity problems. (2) Unlike model predictive control (MPC) and NMPC, the proposed control method is of proven global stability. It is known that MPC is a linear control approach that if applied to the nonlinear dynamics of the electropneumatic robot, the stability of the control loop will be lost. Besides, in NMPC the convergence of its iterative search for an optimum depends on initialization and parameter values selection and consequently the global stability of this control method cannot be always assured. (3) Unlike sliding-mode control and backstepping control, the proposed optimal control method does not require the state-space description of the system to be found in a specific form. About sliding-mode control, it is known that when the controlled system is not found in the input-output linearized form the definition of the sliding surface can be an intuitive procedure. About backstepping control, it is known that it cannot be directly applied to a dynamical system if the related state-space model is not found in the triangular (backstepping integral) form. (4) Unlike PID control, the proposed nonlinear optimal control method is of proven global stability, the selection of the controller’s parameters does not rely on a heuristic tuning procedure, and the stability of the control loop is assured in the case of changes of operating points. (5) Unlike multiple local models-based control, the nonlinear optimal control method uses only one linearization point and needs the solution of only one Riccati equation so as to compute the stabilizing feedback gains of the controller. Consequently, in terms of computation load the proposed control method for the electropneumatic actuator’s dynamics is much more efficient.

The Effect of Section Enlargement with Cementitious Grout on the Eccentric Compression Behavior of RC Columns

The Effect of Section Enlargement with Cementitious Grout on the Eccentric Compression Behavior of RC Columns

Uniaxial Compressive Behaviors of Sandstone-concrete Binary with Rough Interface after High Temperatures

Uniaxial Compressive Behaviors of Sandstone-concrete Binary with Rough Interface after High Temperatures

Mechanical properties of an improved flexible auxetic metamaterial and its application in self-expanding vascular stents

Auxetic metamaterials have shown great significance in the field of cardiovascular stents, but improving their applicability remains an issue. Here, we develop a type of improved auxetic metamaterial called the Re-entrant-Arrow-Snake (RAS) structure by adding arrow-shaped and snake-shaped structures to the Re-entrant (RE) structure. We first systematically study its in-plane elastic properties and compression behaviors. It is observed that the RAS structure exhibits improved vertical stiffness, horizontal flexibility, and clear auxetic effect in the vertical direction. The influence of the additional structures on the deformation pattern and the stability is also discussed by the in-plane compression analysis. Furthermore, we propose a new self-expanding (SE) vascular stent named RAS-SE based on the RAS structure. Numerical analysis demonstrates that the RAS-SE stent possesses enhanced dual-plateau radial force, negative foreshortening behavior, and improved bending flexibility, indicating better deployment performance and support in clinical applications. Biomechanical studies also show that axial deformation caused by the auxetic effect of the RAS-SE stent only slightly increases stress damage to plaque without affecting stress damage of the intima. This study further expands the application of metamaterials in SE vascular stents, and the proposed RAS structure shows high applicability on the SE stent, thus warranting further attention.