Stiffness Recovery Research Articles

Acoustic emission analysis of self-healing properties in cementitious materials with superabsorbent polymers (SAPs) during different environments

This paper studies the coupling effect of superabsorbent polymers (SAPs) and fly ash on the self-healing performance of mortar. The three-point bending test was used to evaluate the mechanical property recovery effect of mortar specimens after curing in different environments, and the relationship between crack closure rate, flexural strength and flexural stiffness recovery was established. The amplitude and energy in acoustic emission were used to evaluate the characteristics of self-healing products. The results show that compared with the control specimens, the addition of SAP improves the crack closure rate and mechanical property recovery effect of mortar specimens. With the increase of SAP particle size and content, the mechanical property recovery rate and crack closure rate increase accordingly. The order of self-healing effect of specimens in different healing environments is saturated calcium hydroxide > water >97 % RH > air. After reloading, the amplitude number and cumulative energy of samples with SAP are larger than those of the control specimens. The SEM results also prove this, and the self-healing products in the crack area of the mortar specimens with SAP are denser than those in the control specimens.

Energy Dissipation and Toughening of Covalent Networks via a Sacrificial Conformation Approach.

Covalent polymer networks find wide utility in diverse engineering applications owing to their desirable stiffness and resilience. However, the rigid covalent chemical structure between crosslinking points imposes limitations on enhancing their toughness. Although the incorporation of sacrificial chemical bonds has shown promise in improving toughness through energy dissipation, composite networks struggle to maintain both rapid recovery and stiffness. Consequently, a significant challenge persists in achieving a covalent network that combines high strength, stiffness, toughness, and fast recovery performance. To address this challenge, we propose a novel sacrificial structure termed "sacrificial conformation." In this approach, β-cyclodextrin is covalently embedded into the network skeleton as the sacrificial conformation element. Compared to traditional covalent networks (LCN), well-designed cyclodextrin-embedded covalent network (CCN) exhibit a 100-fold increase in Young's modulus and a 60-fold increase in toughness. Importantly, CCN maintains excellent elasticity, ensuring swift recovery after deformation. This sacrificial conformational strategy enables efficient energy dissipation without necessitating the rupture of chemical bonds, thereby overcoming the limitations of traditional approaches. This advancement holds great promise for the design and fabrication of advanced elastomers and hydrogels with superior mechanical properties and dynamic behavior.

Energy Dissipation and Toughening of Covalent Networks via a Sacrificial Conformation Approach

Covalent polymer networks find wide utility in diverse engineering applications owing to their desirable stiffness and resilience. However, the rigid covalent chemical structure between crosslinking points imposes limitations on enhancing their toughness. Although the incorporation of sacrificial chemical bonds has shown promise in improving toughness through energy dissipation, composite networks struggle to maintain both rapid recovery and stiffness. Consequently, a significant challenge persists in achieving a covalent network that combines high strength, stiffness, toughness, and fast recovery performance. To address this challenge, we propose a novel sacrificial structure termed "sacrificial conformation." In this approach, β‐cyclodextrin is covalently embedded into the network skeleton as the sacrificial conformation element. Compared to traditional covalent networks (LCN), well‐designed cyclodextrin‐embedded covalent network (CCN) exhibit a 100‐fold increase in Young's modulus and a 60‐fold increase in toughness. Importantly, CCN maintains excellent elasticity, ensuring swift recovery after deformation. This sacrificial conformational strategy enables efficient energy dissipation without necessitating the rupture of chemical bonds, thereby overcoming the limitations of traditional approaches. This advancement holds great promise for the design and fabrication of advanced elastomers and hydrogels with superior mechanical properties and dynamic behavior.

Self-healing performance of engineered geopolymer composites subjected to sodium sulphate

Engineered geopolymer composites (EGCs) have self-healing potential due to their multiple fine-cracking features. However, water immersion, as a self-healing condition for fibre-reinforced cement-based composites, may limit the self-healing ability of EGCs due to water would not directly participate in the precipitation reaction. By contrast, sodium sulphate is proved as an effective activator to promote the geopolymerisation process, while the self-healing performance of EGCs exposed to sodium sulphate has not been comprehensively demonstrated in the literature. Therefore, this paper investigates the self-healing performance of the EGCs subjected to sodium sulphate (Na2SO4) under wet-dry cycles. The results show that the EGCs preloaded at 3 days attain a higher increment of C-(A)-S-H gels at the cracks than those preloaded at 28 days, indicating the EGCs cracked at the early age have better self-healing performance under sulphate exposure. Besides, ettringite and calcium carbonate are detected as the self-healing products in the EGCs preloaded at 3 days, while the ettringite cannot be found in the EGCs preloaded at 28 days. The EGCs exposed to 5 % Na2SO4 solution and wet-dry cycles can achieve self-healing by forming more CaCO3. Increasing the Na2SO4 concentration to 10 % can accomplish self-healing by facilitating the C-(A)-S-H and CaCO3 formation, enhancing the tensile stiffness recovery and crack width reduction ratios for the EGCs with tensile strains of 2 % and 4 % at 3 days. Decreasing the preloading level also improves the tensile stiffness recovery and crack width reduction ratios regardless of the Na2SO4 concentration and preloading age, since the self-healing products are easier to fill the small cracks. Through a combination of Na2SO4 solution and wet-dry cycles, this study develops a new strategy for the accomplishment of self-healing of EGCs.

The Impact of Cardiopulmonary Rehabilitation on Ventriculoarterial Coupling in Post-Coronavirus Disease-2019 Patients.

Coronavirus disease-2019 (COVID-19) affects the cardiovascular system even after the acute phase of the disease. Cardiopulmonary rehabilitation may improve post-COVID-19 symptoms. This study aims to evaluate the impact of a cardiopulmonary rehabilitation program after acute COVID-19 on arterial stiffness, left ventricular function, and ventriculoarterial coupling (VAC). Forty-eight adults were examined 1 (T0) and 3-mo (T1) following recovery from COVID-19 and randomized 1:1 to participate or not in a 3-mo rehabilitation program. Matched subjects were enrolled as a non-COVID-19 group. Arterial stiffness was evaluated by carotid-femoral pulse wave velocity (PWV). Left ventricular (LV) systolic performance was evaluated with global longitudinal strain (GLS). The PWV/LV-GLS ratio was calculated as an index of VAC. High-sensitivity C reactive protein (hs-CRP) was measured. At T0, convalescent patients with COVID-19 had impaired PWV ( P =.001) and reduced VAC ( P =.001) compared to non-COVID-19 subjects. PWV (8.15±1.37 to 6.55±0.98 m/sec, P <.001) and LV-GLS (-19.67±1.98 to-21.3±1.93%, P <.001) improved only in convalescent patients with COVID-19 undergoing rehabilitation. Similarly, VAC was only improved in the rehabilitation group (-0.42±0.11 to-0.31±0.06 m·sec -1 ·% -1 , P <.001). A significant improvement in VO 2max was noted after rehabilitation (15.70 [13.05, 21.45] to 18.30 [13.95, 23.75] ml· kg -1 ·min -1 , P =.01). Finally, hs-CRP was improved in both groups with a significantly greater improvement in the rehabilitation group. A 3-mo rehabilitation program in convalesced patients with COVID-19 enhances the recovery of arterial stiffness, left ventricular function, and VAC, highlighting the beneficial mechanisms of rehabilitation in this patient population.

Comparison of autonomic nervous system dysfunction, arterial stiffness, and heart rate recovery according to spinal cord injury level.

To investigate the differences in autonomic nervous system (ANS) dysfunction, arterial stiffness, and the degree of delay in post-exercise heart rate recovery (HRR) according to the level of spinal cord injury (SCI), and propose preventive measures against cardiovascular diseases after SCI. This retrospective study included 51 patients with SCI. Based on the neurological level of injury (NLI), patients were divided into two groups: Group A (NLI at and above T6) and Group B (NLI below T6). To assess ANS dysfunction, the head-up tilt test and 24-hour ambulatory blood pressure monitoring were conducted. Arterial stiffness was measured using the pulse wave velocity test. The exercise tolerance test was conducted to measure post-exercise HRR. Group A had significantly higher values in the head-up tilt test and 24-hour ambulatory blood pressure monitoring. In the pulse wave velocity test, both sides (left and right) had significantly higher values in Group B. One minute after the exercise tolerance test, Group A had significantly slower HRR (18.8 ± 11.1 beats/minute) than Group B. Understanding the impact of ANS dysfunction and arterial stiffness on HRR in SCI according to NLI may provide insights for clinical management and preventative strategies for cardiovascular diseases.

Performance of Simplified Damage-Based Concrete Models in Seismic Applications

Significant progress in the finite-element (FE) modeling at the member-level of reinforced-concrete (RC) structures under seismic excitation has been achieved in the past decades; however, reliable and accurate analysis models for full-scale 3D system-level RC structures validated with experimental data are scarce. As the development of a complete nonlinear model is expensive and time consuming, simpler models, typically elastic or lumped-plastic in nature, are employed in practice with additional provisions prescribed to account for nonlinear behavior. Depending on the assumptions made by the analyst, there may be substantial uncertainties related to the response of a complete structure. Capturing global failure modes is challenging, and the assessment of strength and ductility capacities may be inaccurate, resulting in potentially unsafe designs. The objective of this research is to assess the performance of simplified damage-based concrete biaxial models in analyzing and capturing the structural behavior of full-scale RC systems. Damage-based models for concrete require minor input from the analyst, facilitating their use in a design setting, while their explicit, non-conditional convergence formulations allow for non-iterative solutions. This results in damage-based models featuring efficient computational analysis while accounting for complex phenomena such as the capacity to account for stiffness recovery in reversal loading (crack closing) and permanent strains in the concrete. A comparison between analytical and experimental data at both the element- and system-levels is conducted, and a viable damage-based model is proposed for a full-scale structure analysis. The results of the study show that damage-based models are a viable alternative to developing efficient analysis models for elements and whole structures.

A discrete-module-beam hydroelasticity method with finite element theory in analyzing VLFS in different engineering scenarios

A numerical tool is developed in the framework of the Discrete-Module-Beam method to analyze hydroelasticity of very large floating structures (VLFS) such as floating offshore photovoltaic platforms (FOPV). The proposed method utilizes finite element theory to perform structural analysis. The 3D potential flow theory is used to perform a hydrodynamic analysis after macro-submodule division. For the structural analysis part, the platform is divided into macro-submodules and each macro-submodule is represented by a beam, which is then further discretized into micro beam elements with the nodes categorized into three groups, that is, the lumped mass, the boundary nodes and the inner nodes. Derivation of the lumped-mass stiffness matrix and recovery of the displacement distribution are given. Procedures to deal with interconnections, complex boundary conditions and unsteady loads are elaborated. Good agreement is obtained in different loading scenarios to the numerical and experimental results.

Thermo-mechanical aging of carbon-black reinforced styrene-butadiene rubber under cyclic-loading

PurposeThe research aims to investigate the impact of thermo-mechanical aging on SBR under cyclic-loading. By conducting experimental analyses and developing a 3D finite element analysis (FEA) model, it seeks to understand chemical and physical changes during aging processes. This research provides insights into nonlinear mechanical behavior, stress softening and microstructural alterations in SBR compounds, improving material performance and guiding future strategies.Design/methodology/approachThis study combines experimental analyses, including cyclic tensile loading, attenuated total reflection (ATR), spectroscopy and energy-dispersive X-ray spectroscopy (EDS) line scans, to investigate the effects of thermo-mechanical aging (TMA) on carbon-black (CB) reinforced styrene-butadiene rubber (SBR). It employs a 3D FEA model using the Abaqus/Implicit code to comprehend the nonlinear behavior and stress softening response, offering a holistic understanding of aging processes and mechanical behavior under cyclic-loading.FindingsThis study reveals significant insights into SBR behavior during thermo-mechanical aging. Findings include surface roughness variations, chemical alterations and microstructural changes. Notably, a partial recovery of stiffness was observed as a function of CB volume fraction. The developed 3D FEA model accurately depicts nonlinear behavior, stress softening and strain fields around CB particles in unstressed states, predicting hysteresis and energy dissipation in aged SBRs.Originality/valueThis research offers novel insights by comprehensively investigating the impact of thermo-mechanical aging on CB-reinforced-SBR. The fusion of experimental techniques with FEA simulations reveals time-dependent mechanical behavior and microstructural changes in SBR materials. The model serves as a valuable tool for predicting material responses under various conditions, advancing the design and engineering of SBR-based products across industries.

A bi-phasic numerical approach for non-linear response and stiffness recovery related to residual thermal stress in UHTCMCs

The tensile properties of sintered Cf-ZrB2/SiC UHTCMCs were investigated for different lamination sequences, revealing ultimate strengths of 570 MPa (0°/0°), 120 MPa (0°/90°), and 40 MPa (90°/90°). The results facilitated modeling the material’s non-linear tensile response, characterized by a remarkably prolonged plateau with pseudo-plastic behavior, followed by stiffness recovery before ultimate failure. A simplified analytical model was developed to predict this behavior originated by residual thermal stresses and inelastic phenomena. A complete constitutive law was then developed and implemented in a FE model, utilizing a bi-phasic approach including Drucker-Prager plasticity and orthotropic ductile damage. Two-step analyses were performed, starting with a thermal step to represent the buildup of a self-equilibrating RTS state in the material phases, followed by a mechanical simulation. This demonstrated the model's efficacy in capturing the non-linear response in both homogeneous and cross-ply lay-ups, contributing to advancements in materials engineering and the design of UHTCMCs-based hot structures.

Study on structural performance of repaired reinforced concrete beams with partially debonded longitudinal rebars

In order to evaluate the structural performance of repaired RC beams with partially debonded longitudinal rebars, structural experiments were conducted. In the experiments, real-scale and half-scale RC beam specimens underwent cyclic loading of 1.0% or less in terms of deformation angle, repaired with epoxy resin injection method, and multiple cyclic loading again. As a result of the experiments, damage was suppressed in specimens with partially debonded longitudinal rebars even after repairs. Further, the specimens with partially debonded longitudinal rebars exhibited high recovery of initial stiffness after repair compared to the specimen with bonded longitudinal rebars. In addition, from study of the costs required to repair cracks, it was confirmed that RC beams with partially debonded longitudinal rebars used smaller quantities of materials used for repairs and could potentially reduce repair cost by approximately 50% compared to RC beams having standard bonding properties.

A Finite Element Modeling Approach for Analyzing the Cyclic Behavior of RC Frames

Numerous approaches and inputs for nonlinear finite element analysis used for modeling reinforced concrete systems under cyclic loads have been outlined. Practitioners usually take into account a number of significant factors when modeling, for example deciding upon the constitutive relations, the connection among concrete and steel reinforcement, and the mesh sensitivities, in order to accurately simulate the nonlinear response of reinforced concrete under cyclic loads. Therefore, it is essential to thoroughly assess popular modeling approaches and develop a reliable modeling approach with reliability, stability, and consistency. A modeling technique and practical guidelines for nonlinear finite element analysis of reinforced concrete frames subjected to cyclic loading are described in this study, taking into account the influence of parameters such as dilation angle, stiffness recovery, friction coefficient, and mesh sizes. The hysteretic force–displacement behavior and load-carrying capacity of reinforced concrete frames subjected to cyclic loading have been compared using numerical and experimental curves. Discrete fracture paths are specified in the model’s geometry to achieve an accurate depiction of the stress distribution and an adequate fit of the hysteretic behavior. The proposed modeling approach provides a precise representation of the stress distribution and hysteretic behavior while keeping computational costs reasonable.

Evolution of self-healing performance of UHPC exposed to aggressive environments and cracking/healing cycles

This paper investigates the resilience of UHPC's self-healing capabilities under aggressive environmental conditions and cracking/healing cycles. UHPC specimens ‘with a double-edged wedge splitting geometry were made, incorporating a commercial crystalline admixture (Penetron Admix®). The evaluation of UHPC's healing capacity involved subjecting pre-cracked samples to three different water immersion conditions: tap water, saltwater, and geothermal water. The closure of cracks during different curing periods was meticulously recorded using optical microscopy. Furthermore, specialized tests, including ultrasonic pulse velocity (UPV) measurements and splitting tensile tests, were conducted to quantify the recovery of mechanical properties. The results reveal that extended exposure results in a gradual closure of cracks, where salt water and geothermal water exhibit lower self-healing capabilities. Self-healing improves after the 1st crack/self-healing cycle but decline rapidly after the 2nd cycle. Mechanical property is strongly correlated with the extent of self-healing, and all samples display varying degrees of stiffness recovery, with the most pronounced recovery occurring after the 1st cycle. However, following the 2nd cycle, the stiffness recovery values decrease due to repeated loading, resulting in increased damage and a reduced number of reactive particles, thereby compromising self-healing and stiffness recovery. Despite enduring multiple instances of crack damage, UHPC samples still exhibit notable toughness recovery, underscoring the enduring efficacy of the self-healing mechanism even in challenging conditions.

Rapid rehabilitation of damaged UHPC-NSC composite pier after earthquake

Resilience-oriented earthquake engineering requires that engineering structures can recover rapidly after earthquakes. However, the rehabilitation time of bridge piers is strongly dependent on the structure and retrofit plan. Although an ‘ultra-high-performance concrete (UHPC) – normal strength concrete (NSC)’ composite pier, with a cross-section of an outer UHPC tube and inner NSC core, can deliver satisfactory damage control, its post-earthquake rehabilitation has not been reported. This study investigated the rehabilitation performance of damaged UHPC-NSC composite piers. Six damaged composite piers were retrofitted, and cyclic tests were conducted to compare the performance of the retrofitted and original specimens. The actual compressive strength of the re-poured UHPC significantly affected the performance of the retrofitted composite piers. Parametric analysis was conducted based on the validated numerical model to investigate the impact of the rehabilitation time on the retrofitted performance quantitatively. The functions of the critical rehabilitation time were derived with respect to the strength and thickness of the re-poured UHPC, considering the full recovery of the initial stiffness and peak strength as the criteria.

Analysis of adhesive scarf repairs on composite sandwich beams under three-point bending loading

The efficiency of a repair strategy applied to a honeycomb/carbon-epoxy sandwich beam was analyzed in this work. Experimental tests were performed considering three-point bending loading of undamaged, damaged and repaired specimens. In the damaged case, the central part of the core and of one of the skins were removed. The repair scheme consists of bonding a plug of honeycomb into the core and repairing the carbon-epoxy skin considering a scarf patch. A finite element analysis incorporating cohesive zone mixed-mode I + II analysis was performed. The resultant load-displacement numerical curves were compared with the experimental ones. It was verified that the repair procedure is efficient regarding recovery of the initial stiffness and strength of the undamaged case, and it was concluded that the proposed numerical model is a valid tool to deal with repair of sandwich beams.

Investigation of Crack Repairing Technique to Delay Fracture Initiation of Steel Members Subjected to Low Cycle Fatigue

Stress concentrations have become a common phenomenon in steel elements when arresting a fracture by implementing the crack stop hole (CSH) technique. Embedding the CSH with Carbon Fibre-Reinforced Polymer (CFRP) enhances the fatigue life by delaying fractures while achieving stiffness recovery due to the superior mechanical characteristics of the CFRP material. Hence, the low cyclic fatigue (LCF) behaviour of 90 strengthened and non-strengthened CSH specimens was examined in this context. These specimens were subjected to a range of 0 to 10,000 fatigue load cycles at a frequency of 5 Hz. At the end of fatigue exposure, the average tensile strength was measured in each case. The application of a CFRP patch on the CSH effectively recovered the strength losses while enhancing the strength in the range of 32% to 45% with respect to the non-strengthened specimens. The developed numerical model based on the cyclic J-integral technique agrees with the test results. This study introduced geometry-related design guidelines for this novel CSH hybrid technique.

Liquefaction and post-liquefaction behaviors of unreinforced and geogrid reinforced calcareous sand

To explore the feasibility of geogrid reinforcement as a promising countermeasure to improve the liquefaction and post-liquefaction resistance of calcareous sand, extensive undrained monotonic and multi-stage triaxial tests were performed on unreinforced and geogrid reinforced calcareous sand with different relative densities. The test results illustrate that pore pressure generation curves of unreinforced and reinforced calcareous sand gradually evolve from S-shaped to hyperbolic-shaped with the increase in relative density, cyclic stress ratio, and effective confining pressure. Following this, a pore pressure model applicable to both unreinforced and reinforced calcareous sand is proposed. The liquefaction resistance of calcareous sand increases with the increase in relative density, whereas an elevated cyclic stress ratio increases its liquefaction susceptibility. A virtually unique relationship can be observed between the liquefaction resistance normalized to the product of phase transformation strength ratio and relative density against the number of cycles for triggering liquefaction, providing an effective means of early assessing sand liquefaction resistance. Moreover, the geogrid exhibits excellent reinforcement efficiency in enhancing the liquefaction resistance of calcareous sand at relative densities of 50% and 70%. During the post-liquefaction stage, increasing relative density and geogrid reinforcement can accelerate the recovery of stiffness and strength for liquefied calcareous sand and improve the post-liquefaction strength. In general, geogrid reinforcement is considered a good alternative to densification for improving the engineering properties of calcareous sand and offers great application prospects in marine engineering construction.

Formation and properties of zein Particle-GMS Co-stabilized pickering emulsions: Insights into the complex interface

The interplay between colloidal particles and surfactant co-adsorption plays a pivotal role in determining interface properties and emulsion stability. This study delves into the influence of the Zp/GMS ratio on the physical stability of Zein particle-GMS co-stabilized emulsions (CPEs) and how co-adsorption and interfacial interactions impact this stability. Results indicated that the preferential adsorption of GMS facilitated the diffusion (Kdiff increased from 0.083 mNm−1 s−0.5 to 0.206 mNm−1 s−0.5) and penetration of Zp (KP increased from −17.0 × 104 s−1 to −23.6 × 104 s−1), triggering interface rearrangement. Consequently, intricate interfaces formed, inducing a synergistic reduction in interfacial tension. Moreover, the decreased Zp/GMS ratio bolstered the lateral molecular interactions between Zp and GMS at the interface, while also enhancing the longitudinal relaxation of GMS during interface deformation. Therefore, the Zp/GMS ratio played a pivotal role in controlling the stiffness, elasticity, and deformability recovery of complex interfaces in both lateral and longitudinal dimensions. Furthermore, in both O/W and W/O CPEs, a decreased Zp/GMS ratio improved the physical stability of CPEs by reducing the rate of droplet coalescence. To summarize, the enhanced stability of CPEs can be attributed to the reduction in droplet aggregation. This reduction is influenced by a combination of improved mechanical properties resulting from lateral molecular interactions at the interface and enhanced deformability recovery due to the longitudinal relaxation of GMS. These findings not only deepen our understanding of particle-surfactant interactions in Pickering emulsions but also pave the way for formulating emulsions with specific textures and stabilities.

Coupled effects of simultaneous autogenous self-healing and sustained flexural loading in cementitious materials

The self-healing efficiency of cementitious materials might be impaired by external loading. This study investigates the coupled effects of sustained flexural loading and self-healing mechanisms. To this end, prismatic mortar specimens were pre-cracked at two days with 10-μm wide cracks under bending. Some beams were then immersed for healing and simultaneously loaded for about 4 weeks using an innovative three-point bending test device allowing bending creep displacement measurement. At the same time, reference beams were immersed without load. After the healing period, the specimens were reloaded to assess the effect of creep and healing on the stiffness and strength recovery. It was found that, during immersion, the creep deformation gradually decreased with time, probably due to the precipitation of healing products. However, after immersion, the mechanical properties recovery of specimens undergoing creep during self-healing decreased by 12 % compared to the reference specimens. Microscopic observations were consistent with the mechanical recovery measurements, suggesting partial healing of the loaded specimens.

Advanced Self-Healing Ceramics with Controlled Degradation and Repair by Chemical Reaction.

Controlling the chemical reaction rate concerning degradation and repair is found to be important to design advanced self-healing ceramics. The recovery and degradation behaviors of strength and stiffness were investigated by exposing aqueous solutions of different pH and calcium ion concentrations to the introduced crack on typical self-healing ceramics dispersed with alumina cement as a self-healing agent. The chemical reaction of cement undergoes the following three stages: dissolution of components such as calcium ions, formation of a gel, and formation of final products. Experimental and thermodynamic assessments revealed that even under conditions where the final products are identical (thermodynamic equilibrium), kinetic effects (excessive dissolution of components or insufficient crystal formation) result in strength degradation rather than repair. It was also suggested that the repair function could be enhanced by controlling the nucleation site of the crystals.