Stiffness Reduction Research Articles

Statistical damage model with strain softening for lime-stabilized rammed earth after elevated temperature

Many historical earthen buildings are damaged due to fire exposure in the past. It is important to understand the strength degradation of rammed earth after elevated temperature for guiding the strategy of building protection or rehabilitation. A total of 24 unconfined compression tests are conducted on lime-stabilized rammed earth specimens after elevated temperature up to 700°C. A quasi-linear reduction in strength and stiffness is found for rammed earth with the increase of temperature. At high temperature, the ductility of rammed earth is enhanced, e.g., strain at peak strength of 2.5% and 1.5% at 700°C and 20°C, respectively. Microstructural analyses demonstrate that with the increase of temperature, the specimen becomes more porous with reduced calcium carbonate precipitation, explaining the strength reduction. A new thermal damage model is proposed to describe the behavior of rammed earth after elevated temperature, in which the closure of pores is captured to show unrecoverable deformation, and the skeleton part is simulated using a thermal damage variable in a statistical manner to present the damage evolution (strain softening). By comparing with the measured stress-strain curves, one can confirm that the proposed method can provide effective prediction for the response of rammed earth after elevated temperature.

Collective mode across the BCS-BEC crossover in Holstein model

We investigate the emergence of the collective mode in the phonon spectra of the superconducting state within the Holstein model by varying the electron-phonon coupling. Using dynamical mean field theory combined with the numerical renormalization group technique, we calculate the phonon spectra. In the superconducting state with a pairing gap (ΔP), the peak position of the collective mode (ωcol) evolves from the Bardeen-Cooper-Schrieffer (BCS) regime, manifesting near 2ΔP and increasing with coupling, to the Bose-Einstein condensation (BEC) regime, where ωcol decreases with increasing coupling. The decrease of ωcol matches well with the reduction of superfluid stiffness, which originates from the increasing phase fluctuations of local pairs with coupling strength. In the crossover regime with intermediate coupling, ωcol aligns with the soft phonon mode (ωs) of the normal state and decreases with increasing coupling when ωs<2ΔP. Additionally, comparing the collective mode weight to ΔP suggests that the collective mode predominantly stems from U(1) gauge symmetry breaking across all coupling strengths. Published by the American Physical Society 2024

Optimizing Exercise Interval for Arterial Stiffness Improve-Ment in Middle-Aged Adults

By decreasing the interval periods and increasing the frequency of exercise bouts, we were able to confirm our hypothesis that middle-aged adults could achieve a greater reduction in arterial stiffness through intermittent exercise. Thirty middle-aged males were randomly divided into a control group (CON), continuous exercise group (CE), interval exercise long-long group (IELL), interval exercise long-short group (IELS), and interval exercise short-short group (IESS). The subjects performed moderate-intensity exercise on the treadmill. Cardio-ankle vascular index (CAVI) was assessed before exercise, right after the session ended, and at 30 and 60 minutes into the recovery period. Changes in values from the baseline (∆CAVI) during each measurement were used for analysis. The control group showed no significant change in ∆CAVI, while all exercise groups showed a significant decrease at 0, 30, and 60 minutes. The exercise groups had significantly lower ∆CAVI than the control group at 0 and 30 minutes. At 60 minutes, the IESS group had a significantly lower ∆CAVI than the control group. Additionally, at 0 and 30 minutes, the IELS and IESS groups had a significantly lower ∆CAVI than the CE group. Hence, interval exercise is more effective than continuous exercise regardless of the total duration, but the effectiveness depends on the interval duration and number of repetitions.

High Strength and Dynamically Tunable Adhesion Enabled by Composite Micropillar Arrays Fabricated via Solvent‐Assisted Molding

AbstractThe ability to control adhesion on demand is important for a broad range of applications, including the gripping and manipulation of objects in robotics and manufacturing, and the temporary attachment of wearable devices. Despite recent advances in tunable adhesive materials, most existing solutions have modest adhesion strength and are limited by a compromise between the maximum and minimum adhesion, where increased strength prevents the release of lighter objects. To overcome these challenges, thermally responsive polymers, which can exhibit both high stiffness and a large reduction in stiffness via heating, have the potential to enable strong and tunable adhesion. Here, a microstructured composite adhesive with high strength (>2 MPa) and dynamically tunable adhesion (16×) is realized using a solvent‐assisted molding technique. The adhesive consists of an array of composite micropillars whose small scale and material composition enable strong and tunable adhesion. While thermally actuated systems often have slow response times, it is shown that miniaturization allows response times to be reduced to <1s for heating and <10s for cooling. These strong, fast, and dynamically tunable adhesives offer advantages over existing solutions and can be manufactured for practical adoption through the scalable solvent‐assisted molding technique.

Inflammation and Arterial Stiffness as Drivers of Cardiovascular Risk in Kidney Disease.

Patients with chronic kidney disease (CKD) have an increased cardiovascular (CV) risk. The lower the glomerular filtration rate, the higher the CV risk. Current data suggest that several uremic toxins lead to vascular inflammation and oxidative stress that, in turn, lead to endothelial dysfunction, changes in smooth muscle cells' phenotype, and increased degradation of elastin and collagen fibres. These processes lead to both functional and structural arterial stiffening and explain part of the increased risk of acute myocardial infarction and stroke reported in patients with CKD. Considering that, at least in patients with end-stage kidney disease, the reduction of arterial stiffness is associated with a parallel decrease of the CV risk, vascular function is a potential target for therapy to reduce the CV risk. in this review, we explore mechanisms of vascular dysfunction in CKD, paying particular attention to inflammation, reporting current data in other models of mild and severe inflammation, and discussing the vascular effect of several drugs currently used in nephrology.

Deterioration Analysis of Pavement Structures Incorporating Polymer-Modified Asphalt

It is well-established that structural pavement deterioration is largely influenced by the frequency and magnitude of wheel loads. Furthermore, Polymer-Modified Asphalt (PMA) has gained a widespread application in pavement engineering, as the incorporation of polymers enhances the mechanical properties and improves the overall performance of the pavement, particularly with regard to fatigue resistance. In this study, an experimental program was conducted, comprising the Beam Wheel Tracker Fatigue Test (BWTFT), the Indirect Tensile Stiffness Modulus (ITSM) test, and the Indirect Tensile Fatigue Test (ITFT), on three types of asphalt mixtures: one conventional and two polymer-modified asphalt mixtures, under various conditions. Three distinct pavement structure scenarios were assumed/created/built to perform a deterioration analysis. Subsequently, an iterative approach was developed utilizing the average stiffness reduction and stiffness modulus data obtained from the laboratory results. The findings indicated that this method allows for a more accurate simulation of pavement behavior, confirming that strain levels fluctuate throughout the lifespan of the pavement. Furthermore, the study concluded that the use of PMA provides significantly greater benefits when a deterioration analysis is conducted, compared to traditional approaches.

Experimental and numerical investigations on the impact of openings on the stiffness of CLT shear walls

Cross-laminated timber (CLT) exhibits high in-plane strength and stiffness, positioning it as a viable material for shear walls in seismic regions. However, the influence of openings on the in-plane stiffness of CLT shear walls remains inadequately studied. In this study, 43 CLT shear walls with different opening sizes were tested under in-plane loading. The stiffness reduction was less than 29% for openings 800×800mm (53% of panel width), but exceeded 60% for openings 1200×1200mm (80% of panel width). When assessed with identical opening sizes, 139mm and 175mm thick CLT shear walls displayed consistent stiffness reductions. Panels with an aspect ratio of 3:1 exhibited a more pronounced drop than 2:1 aspect ratio panels. A validated 3D finite element model was used to investigate the influence of panel layup and opening shape and location on the in-plane stiffness reduction. For a distance between the opening and panel edge exceeding 300mm, the shape and location of openings exerted negligible effects. Finally, the Dujič’s model was shown to be a suitable predictor to evaluate the in-plane stiffness reduction of CLT shear walls with openings.

Effects of repeated isometric and eccentric contractions on active muscle stiffness.

Joint stiffness endurance is considered essential in many sports events. We previously reported that reduced joint stiffness due to repetitive hopping was associated with reduced active muscle stiffness. However, the determinants of active muscle stiffness endurance were unknown. This study aimed to compare the effects of repeated isometric contractions (ISO), which induced metabolic muscle fatigue, and repeated eccentric contractions (ECC), which induced muscle damage, on active muscle stiffness endurance. Fourteen males performed two kinds of fatigue tasks (ISO and ECC) using only ankle joint. Before and after the fatigue tasks, changes in estimated muscle force and fascicle length during fast stretching were used to calculate the active muscle stiffness of the medial gastrocnemius muscle. In addition, the thickness of the plantar flexor muscles was measured before and after fatigue tasks. After fatigue tasks, no difference in the relative increase of muscle thickness was found between ISO and ECC. The increase in torque during fast stretching did not change after both ISO and ECC. The increase in fascicle length during fast stretching significantly increased after ECC but not ISO. Active muscle stiffness significantly decreased after ECC but not ISO. Active muscle stiffness decreased after repeated eccentric contractions damaging fascicles and did not change with repeated isometric contractions causing metabolic fatigue. These results implied that the joint stiffness reduction due to repetitive stretch-shortening cycle exercises shown in previous studies involved a reduction in active muscle stiffness due to repeated eccentric contractions.

Parametric analysis of self-excited aeroelastic instability of an isolated single-axis two-dimensional flat solar tracker

The escalating global energy demand is driving a transition towards sustainable energy sources, particularly solar power, which is experiencing significant growth. Single-axis, horizontally mounted photovoltaic (PV) solar trackers are emerging as one of the most cost-effective methods for solar energy generation. However, the reduction in structural stiffness has led to an increase in aeroelastic issues, some of which have resulted in catastrophic failures around the world, posing significant challenges. Recent aerodynamic studies have attributed these instabilities to stall flutter. The absence of a validated theory hampers early estimation of these aeroelastic phenomena. The primary aim in this work is to develop a semi-empirical framework to predict aeroelastic instabilities in isolated two-dimensional solar trackers, focusing on dynamic self-excited responses. This effort aims to improve the reliability and efficiency of solar tracker designs, optimizing energy utilization in the context of the evolving renewable energy landscape. The semi-empirical formulation developed here reveals the influence of geometric and mechanical characteristics on the critical wind speed at which aeroelastic instabilities set in. The effects of reduced inertia (m⁎) and mechanical damping coefficient (ξmech) of the structure on the critical wind speed at which aeroelastic instabilities arise are analyzed here. It is shown that increasing either inertia or damping has a minor impact on the critical speed when the initial values of these parameters are large, while the changes are significant when these initial values are small.

Role of sulfated GAGs in shear mechanical properties of human and porcine cornea

The corneal extracellular matrix is mainly composed of collagen fibers, proteoglycans, and glycosaminoglycans (GAGs). The present work was done to investigate the effect of GAGs on linear viscoelastic shear properties of human and porcine cornea. A clear understanding of structural functions of GAGs could result in the development of new intervention methods for diseased conditions that involve changes to the expression of GAGs/PGs. Here, we used keratanase II enzyme to deplete sulfated GAGs from porcine and human donor corneal disks. After quantifying the GAG content, collagen fiber diameter, and interfibrillar spacings of control and GAG-depleted specimens using the Blyscan assay and transmission electron microscopy, we performed torsional rheometry to determine their shear properties at different levels of axial strain. We found that the GAG content of control human (52.35 ± 3.40 μg/mg dry tissue) and porcine cornea (48.59 ± 7.79 μg/mg dry tissue) significantly reduced following keratanase II enzyme treatment. Moreover, we observed that the diameter of collagen fibers (28.78 ± 2.33 nm) and interfibrillar spacing (45.93 ± 2.33 nm) of human specimens were significantly smaller than the collagen fiber diameter (34.77 ± 21.90 nm) and interfibrillar spacing (54.28 ± 3.99 nm) of porcine corneal samples. Although GAG depletion did not have any significant effect on the collagen fiber diameter, it significantly increased the interfibrillar spacing in both porcine and human samples. Within the range of linear viscoelastic behavior, the shear stiffness of human and porcine corneal samples did not depend on the shear strain but significantly increased with increasing the applied axial strain. The average complex shear modulus was found to be between 1.0 KPa and 6.5 KPa and between 8.5 KPa and 31 KPa for control porcine and human discs, respectively. The GAG removal caused significant reduction of shear stiffness in both human and porcine corneal samples. Based on these findings, we conclude that sulfated GAGs are important in defining shear properties of porcine and human corneas and significant GAG content variation adversely affects corneal shear modulus.

Associations between folate intake and knee pain, inflammation mediators and comorbid conditions in patients with symptomatic knee osteoarthritis

BackgroundTo investigate the associations between folate intake and changes in knee pain, inflammation mediators and comorbid conditions over 2 years in patients with symptomatic knee osteoarthritis (OA).MethodsA post-hoc analysis was performed based on data from the VIDEO study, a multicenter, randomized, double-blind, placebo-controlled clinical trial aimed at assessing the impact of vitamin D supplementation on patients with knee OA who were also vitamin D deficient. The original trial’s design and inclusion and exclusion criteria were integrated into this subsequent post-hoc analysis. The average daily folate intake was evaluated using the Dietary Questionnaire for Epidemiological Studies version 2 over two years. The progression of knee symptoms was monitored at the baseline and then at months 3, 6, 12, and 24, utilizing the Western Ontario and McMaster Universities Index alongside a 100-mm visual analog scale. Levels of serum inflammatory mediators were quantified using ELISA techniques. Assessments of knee joint structures, leg muscle strength, depressive symptoms, feet pain, and low back pain were treated at both baseline and follow-up intervals.ResultsFolate intake was correlated with reductions in overall knee pain, dysfunction, and stiffness, as well as decreased levels of Leptin and Apelin. Additionally, it was associated with enhanced leg muscle strength and diminished feet and low back pain. However, there is no association between folate intake and alterations in serum cytokine levels or knee joint structural changes. Within the subsets of overall knee pain, a significant relationship was identified between folate intake and the reduction of pain experienced when ascending or descending stairs and standing for two years.ConclusionsFolate intake was linked with reduced knee pain, lower levels of adipokines, and a decreased prevalence of comorbid conditions in individuals with knee OA, implying that folate consumption may be associated with an improvement in knee OA symptoms, but further research is needed to verify this association.

Experimental investigation of ice resistance in jacket foundations for large offshore wind turbines

ABSTRACT Each winter, the Bohai Sea in China freezes, forming a thick ice layer that significantly affects the safety of ship and offshore structures. Offshore wind turbines, particularly high-rise structures, are especially vulnerable to collapse under the pressure of sea ice. This paper presents an experimental study on the ice resistance of the steel jacket foundation of a 10 MW offshore wind turbine. The study examines the failure modes, hysteresis curves, stiffness, and energy dissipation capacity of the jacket foundation under ice loading. Results indicate that the branch tube joints within the sea ice zone are prone to fracture, leading to a reduction in the overall stiffness and load-bearing capacity of the foundation. These findings offer a valuable experimental basis for improving the ice resistance performance of offshore wind turbines and enhancing their structural safety in ice-prone regions.

Mechanical characterization and impact damage assessment of Al/SiC functionally graded coating under elevated temperatures

Functionally graded materials are a class of composite materials that finds widespread use in aerospace, defense and automobile applications due to their tailored material properties for the specific need. In the present research, impact dynamics and the damage behavior of functionally graded plasma spray coating (FGPS) on an aluminium 6061-T6 substrate under high velocity impact at various temperatures were studied. The FGPS coating consists of four layers having various proportions of Al and SiC (50/50, 40/60, 30/70 and 20/80 weight percentages) and the coating thickness was measured to be 232.8 μm. Experiments were carried out at 260 J of impact energy on the FGPS samples at various temperatures using a single-stage gas gun along with the thermal setup. A finite element model was developed with the Johnson-Cook damage model and piecewise linear plasticity material model, along with suitable contact algorithms. Impact simulations at various temperatures reveal that the stiffness reduction at higher temperatures results in decreased peak contact force, decreased rebound velocity and increased central deflection. The molten splats observed through the micro-morphological study have spread several micrometers, which indicates excellent bonding between the particles and the porosity content was found to be 1.35%. The research findings on the impact dynamics and damage behavior of FGPS can significantly improve material design for a wide range of applications.

Deformation and fracture of lithosphere-inspired polymeric multi-layer composites

Inspired by the diversity of structures and patterns inherent in the earth's lithosphere, this study endeavors to enhance the interplay between stiffness and toughness through the introduction of a new class of polymeric multi-layer composite materials termed by the authors as "lithomers". Structured single-edge notched bending specimens were fabricated using a combination of additive manufacturing and casting, employing two different methacrylate-thiol resins. The outer layers exhibit a stiff and brittle characteristic, while the layer in between is compliant in nature. Three types of lithomers with wave-like structures and one with a rectilinear structure were investigated regarding their stiffness and toughness in a 3-point bending setup. The results were compared with those of a pure stiff matrix material. The findings revealed that fracture toughness increased regardless of the interlayer's shape compared to the pure matrix material. Correspondingly, this enhancement in fracture toughness correlated with a reduction in stiffness. The most balanced results in terms of stiffness and fracture toughness were achieved, with the lithomer having a wave-like structure in its initial stage. It exhibited a roughly 27 times improvement in fracture toughness with a moderate decrease in stiffness of approx. 1/5 compared to the pure matrix material.

Statistical safety evaluation method for composite high-pressure hydrogen vessels considering stacking sequence effect and fiber strength development rate

We developed a method to evaluate the failure rate (FR) for the cylindrical part of a composite high-pressure hydrogen vessel with arbitrary dimensions, number of layers, and stacking sequences by giving the mean and standard deviation of the fiber strength development rate and the stiffness reduction rate under various damage types. The FR is the rate at which the residual strength of a vessel is below the maximum service pressure (125%NWP). Assuming a case in which the vessel is designed with the damage-tolerance design, the FR was calculated for the case of all-layer matrix cracking (assuming beginning-of-life) and fiber failure in two layers in addition to the matrix cracking (assuming the most severe damage that is tolerable during service). The observations demonstrate the importance of improving the probability of detection for non-destructive testing in ensuring the safety of damage-tolerance-designed vessels after fiber failure. The mean strength of the fibers mainly affected the mean residual strength of the composite high-pressure hydrogen vessels when the dimensions and stacking sequence remained unchanged.

Modal analysis of concrete gravity dam incorporating pre-stress condition along with soil–structure interaction

Purpose The fundamental period of the structure plays an important role in the seismic analysis. This study aims to analyze the modal response of dam, the two-dimensional (2D) FEM model is developed by using ANSYS 2022 R1 software. Design/methodology/approach To examine the optimized mesh size to achieve grid independence, the variable element size has been considered, and its optimal value is calculated using the technique of response surface optimization. Further, the effect of damping ratios of 5%, 8% and 10% is also considered for the free vibration analysis of the dam structure. Findings The results show that the natural frequencies of the dam decrease with a reduction in stiffness of the whole structure. Further, the effect of pre-stress conditions is analyzed and the study has proved that the natural frequency increases after considering the pre-stress as initial condition during modal analysis. Further, it is found that the damping has a substantial effect on frequency for higher modes of vibrations. Research limitations/implications The study only focused on modal analysis of the gravity dam, and this study’s results can be used further to evaluate the dynamic behavior of the dam including hydrodynamic conditions. Originality/value The finite element tool is used to evaluate the modal response of gravity dam incorporating pre-stress and damping ratio along with soil–structure interaction. Moreover, to the best of the authors’ knowledge, no earlier study has been conducted to evaluate the effect of damping and pre-stress conditions on the stability and natural frequency of the system.

Fatigue Properties of Carbon Fiber–Reinforced Foams and Experimental Observation of the Damage Growth Mechanism

ABSTRACTCarbon fiber–reinforced foams (CFRFs) are expanded thermoplastic composite materials reinforced with three‐dimensional discontinuous carbon fibers. Herein, the effects of their unique internal structure on fatigue properties were investigated. Through tension‐tension fatigue tests and the digital image correlation (DIC) method, distinct stiffness reduction behavior was observed across the entire specimen and at the fracture points. The results suggest that local stiffness reduction behavior affects the fatigue properties. From the DIC method, damage was observed by scanning electron microscopy and the fiber tortuosity, and the void fraction were quantified using X‐ray computed tomography scans. In the case of three‐dimensional oriented fibers, stress was concentrated at fiber ends, fiber intersections, and bent fibers, resulting in fiber pull‐outs and matrix cracks. In the case of voids, the void size affected damage development, and the stress concentration around the voids caused fiber fracture and matrix cracks.

A comparative study of tensile fatigue life in various flexible-oriented three-dimensional woven process structures based on finite element models

To achieve a comparative study of tensile fatigue life in various three-dimensional (3D) structures, 3D orthogonal woven composite (3DOWC), off-axis 3D orthogonal woven composite (OA-3DOWC) and multiaxial 3D woven composite (M3DWC), were designed and manufactured. Tensile and fatigue tests were performed, and microcomputed tomography (Micro-CT) was utilized to observe the mesoscale structural characteristics and the fatigue fracture morphology. Finite element models were established based on the observation. Then a modified power-law fatigue damage model was used for reduction of stiffness and strength during fatigue loading process, and a normalized life model was used to predict the fatigue life at different stress levels. Comparisons were conducted with existing models to analyze their predictive accuracy. Results showed that the tensile strength of M3DWC decreased by 54.6 % compared to 3DOWC, while its fatigue performance of M3DWC was significantly improved. Additionally, OA-3DOWC had the worst tensile strength but slightly improved fatigue performance compared to 3DOWC. And for the maximum error between the prediction and experimental results for the fatigue life is 11.16 %. For the fatigue life at the same stress level, M3DWC > OA-3DOWC > 3DOWC, due to the complex crack propagation paths in the staggered fiber arrangement.

Seismic performance of perforated-steel plate shear wall with different perforation layouts

The Perforated-Steel Plate Shear Wall in ANSI/AISC 341-16 requires the plate to have a regular hole pattern with parallel holes and a uniform diameter. There are also rules regarding the minimum number of holes. It becomes a challenge if the opening requirements do not comply with these provisions. This study proposes the percentage value of perforation as a parameter to obtain the seismic capacity of shear wall plates. Experimental studies were conducted on six shear wall plates to achieve this goal. Three specimens were made with a straight perforation layout, and the rest were staggered. All plates had a thickness of 2 mm and were given circular perforations with a diameter of 65 mm. Each group of specimens was assigned a perforation percentage ranging from 10.25%-49.59%. The ultimate load, a measure of material strength, decreases as the percentage of perforations increases. Based on the results of experimental testing with cyclic load, the impact of the perforation layout on seismic behavior shows that the staggered configuration can accept greater average loads than the straight layout by 12.5%-40.3%. Specimens with the same percentage of perforations show no significant difference in dissipation energy when observed from the start of loading to a drift ratio of 6%. The difference in total energy dissipation between specimens with the same perforation percentage is less than 30%. However, both show similar patterns of stiffness reduction and pinching effects.

A new inerter-based acoustic metamaterial MRE isolator with low-frequency bandgap

Abstract Acoustic metamaterials are capable of generating bandgaps at specific frequency ranges, which makes them have good applications in the field of vibration isolation. The bandgaps can be further broadened with active control, nonlinear components and graded structures, such as: controllable stiffness by magnetorheological elastomer (MRE) and graded stiffness. However, the current approaches to reducing the bandgaps have limitations. Both the reduction in structural stiffness and the increase in mass will reduce the overall stability of the acoustic metamaterial. In this research, a novel inerter-based acoustic metamaterial MRE isolator (IAM-MREI) was designed and prototyped to lower the bandgap. Inerters can generate a large equivalent mass with very light weight. Moreover, it is discovered that elements containing quadratic frequency terms are added to the dispersion matrix of the IAM-MREI due to the frequency-independent force applied to the resonators, which is generated by the inerters. By this way, the bandgap calculated by this dispersion matrix is greatly lowered and broadened, which cannot be achieved only with extra equivalent mass. The effects of the inerters on the overall performance of the IAM-MREI was thoroughly investigated and validated both theoretically and experimentally. The evaluation experiments confirmed that the IAM-MREI possesses a low-frequency bandgap and can provide great vibration isolation performance.