Time-dependent Response Research Articles

A constitutive model considering creep damage of wood

AbstractThe serviceability of wooden structures involves multiphysical phenomena, notably the interactions among creep, plasticity, and damage. The influence of creep on the initialization of the damage and on its growth and spread can be adjusted by an additional alpha parameter in order to take into account the coupled effect between creep and damage more properly. We integrate an orthotropic viscoelastic model, based on the generalized Kelvin chain, with an orthotropic damage model, capturing both the immediate nonlinear elastic–plastic–damage response and the time-dependent viscous response of timber. The combination of these material models is important to obtain a realistic description of wood behavior, because the timber shows an immediate nonlinear elastic–plastic–damage response, but also the time-dependent viscous response. In this paper, we algorithmize, implement, and validate the concept of ‘creep damage’, a phenomenon observed in wooden structures. Benchmark tests reveal two distinct patterns of damage in beech wood, immediate postload damage that evolves over time and damage that occurs and spreads during the loading period.

Probing impact of allosteric (or distal) mutations on protein function through time-dependent linear response theory

Probing impact of allosteric (or distal) mutations on protein function through time-dependent linear response theory

Markers for Immunological Resilience: Effects of Moderate- and High-Intensity Endurance Exercise on the Kinetic Response of Leukocyte Subsets

The kinetic responses of leukocyte subsets to exercise and their recovery may serve as indicators of immunological resilience. These time-dependent responses were investigated in healthy young men using a bicycle ergometer test. Fifteen recreationally active male cyclists (20–35 years, VO2max 56.9 ± 3.9 mL kg−1 min−1) performed four exercise protocols with a 1 h duration in a cross-over design: at 70% of the maximal workload (Wmax) in a hydrated and a mildly dehydrated state, at 50% of the Wmax, and intermittently at 85/55% of the Wmax in blocks of 2 min. The numbers of lymphocytes, monocytes, neutrophils, eosinophils, basophils, thrombocytes, and NK cells (CD16 and CD56) were measured at different time points up to 24 h post-exercise. The total leukocyte counts and those of most subsets increased from the start of the exercise, peaking after 30–60 min of exercising. The neutrophil numbers, however, peaked 3 h post-exercise. The CD16brightCD56dim NK cells showed a 1.5-fold increase compared to the CD16brightCD56bright NK cells. Other than for MCP-1, no significant differences were found in the serum cytokine levels. Our results show that exercise intensity is reflected in different time-dependent changes in leukocyte subsets, which supports the concept that the exchange of immune cells between peripheral blood and tissues contributes to enhanced immune surveillance during strenuous exercise.

A Methodological Approach for Interpreting and Comparing the Viscoelastic Behaviors of Soft Biological Tissues and Hydrogels at the Cell-Length Scale

The behavior of a cell is strongly influenced by the physical properties and stimuli in its microenvironment. Furthermore, the activation and modulation of mechanotransduction pathways are involved in tissue development and homeostasis and even pathological processes. Thus, when developing materials aimed at mimicking the extracellular matrixes of healthy or pathological tissues, their mechanical features should be closely considered. In this context, nanoindentation represents a powerful technique for mechanically characterizing biological tissues and hydrogels at the cell-length scale. However, standardized experimental protocols and data analysis techniques are lacking. Here, we proposed a methodological approach based on the nanoindentation technique for quantitatively analyzing and comparing the time-dependent load relaxation responses of soft biological tissues and hydrogels. As this was an explanatory study, stress-relaxation nanoindentation tests were performed on samples of pig and human lung tissues and of a specific gelatin-methacryloyl (GelMA) hydrogel to quantify and compare their viscoelastic properties. The proposed method allowed for identifying the characteristic parameters needed for describing the behavior of each sample, permitting us to quantitatively compare their mechanical behaviors. All samples showed load relaxation at a defined indentation depth because of their intrinsic viscoelastic behaviors, and the GelMA samples showed the highest relaxation capabilities. The distribution of the characterization parameters showed that the biological samples presented similar time-dependent responses, while differences were observed in the GelMA samples. Overall, the proposed methodological approach allows for providing key insights into the time-dependent behaviors of soft biological tissues and hydrogels at the cell-length scale in view of supporting tissue engineering and pathophysiological investigations.

Brain oxygen responses induced by opioids: focus on heroin, fentanyl, and their adulterants.

Opioids are important tools for pain management, but abuse can result in serious health complications. Of these complications, respiratory depression that leads to brain hypoxia is the most dangerous, resulting in coma and death. Although all opioids at large doses induce brain hypoxia, danger is magnified with synthetic opioids such as fentanyl and structurally similar analogs. These drugs are highly potent, act rapidly, and are often not effectively treated by naloxone, the standard of care for opioid-induced respiratory depression. The goal of this review paper is to present and discuss brain oxygen responses induced by opioids, focusing on heroin and fentanyl. In contrast to studying drug-induced changes in respiratory activity, we used chronically implanted oxygen sensors coupled with high-speed amperometry to directly evaluate physiological and drug-induced fluctuations in brain oxygen levels in awake, freely moving rats. First, we provide an overview of brain oxygen responses to physiological stimuli and discuss the mechanisms regulating oxygen entry into brain tissue. Next, we present data on brain oxygen responses induced by heroin and fentanyl and review underlying mechanisms. These data allowed us to compare the effects of these drugs on brain oxygen in terms of their potency, time-dependent response pattern, and potentially lethal effect at high doses. Then, we present the interactive effects of opioids during polysubstance use (alcohol, ketamine, xylazine) on brain oxygenation. Finally, we consider factors that affect the therapeutic potential of naloxone, focusing on dosage, timing of drug delivery, and contamination of opioids by other neuroactive drugs. The latter issue is considered chiefly with respect to xylazine, which strongly potentiates the hypoxic effects of heroin and fentanyl. Although this work was done in rats, the data are human relevant and will aid in addressing the alarming rise in lethality associated with opioid misuse.

A numerical multi-scale method for analyzing the rate-dependent and inelastic response of short fiber reinforced polymers: Modeling framework and experimental validation

This research presents a numerical multi-scale approach that efficiently addresses the inelastic and time-dependent mechanical response of short fiber reinforced polymers (SFRPs) under monotonic loading conditions by linking the mechanical analysis from microscale analysis to a continuum model. To do so, first, the mechanical performance of a recently suggested unit cell, considering the intrinsic mechanical characteristics of both fiber and matrix, is studied to address the inelastic and rate-dependent mechanical behavior of completely aligned SFRPs. Then, the evaluated mechanical response is linked to the Hill's plasticity and two-layer viscoplastic (TLVP) models to represent the anisotropic mechanical response of SFRPs. Furthermore, an easy-to-use multi-step homogenization process is considered to numerically incorporate the influence of fiber misalignments. Finally, the suggested multi-scale technique is thoroughly validated at different strain rates, by using experimental observations of short fiber composites with high volume fraction and direct FE simulations of RVEs with complex microstructures.

Sustainability aggrandized performance of [Sm-Eu-Tm]:CsPbI1.8Br1.2 perovskite material in fully ambient solar cells, supercapacitors, and electro-catalysis

Present investigation reports the first investigation on cesium lead halide (CsPbI1.8Br1.2) semiconductor doped with lanthanides forming a [Sm-Eu-Tm]:CsPbI1.8Br1.2 system. This material was analyzed for time dependent response at the ambient conditions expressing stability reflected by minimal alterations in band energy spanning over 1.78–2.07 eV for an extended duration. With the 47.68 nm average crystallite size, the developed material expressed maximum contact when deposited in the form of thin films. [Sm-Eu-Tm]:CsPbI1.8Br1.2 used as an active photo-harvester in perovskite solar cells (PSCs) excelled in acquisition of 12.81 % efficiency. Fhe fabricated electrode accomplished specific capacitance of 429.3 F g−1. Impedance analysis indicated the excellent ionic diffusion with minimal resistance, which improved the electrical energy storage. Finally, the water splitting experiments were suggestive of the developed material as an effective H2 electro-catalyst with lower ηHER i.e. 146 mV and Tafel slope i.e. 120.4 mV dec−1 when equated with the O2 evolution.

Functional PCA and deep neural networks-based Bayesian inverse uncertainty quantification with transient experimental data

This work focuses on developing an inverse uncertainty quantification (IUQ) process for time-dependent responses, using dimensionality reduction by functional principal component analysis (PCA) and deep neural network (DNN)-based surrogate models. The demonstration is based on the IUQ of TRACE physical model parameters using the FEBA benchmark transient experimental data on peak cladding temperatures during reflooding. The quantity-of-interest (QoI) is time-dependent peak cladding temperature (PCT) profiles. Conventional PCA can hardly represent the data precisely due to the sudden temperature drop at the time of quenching. As a result, a functional alignment method is used to separate the phase and amplitude information in the PCT profiles before conventional PCA is applied for dimensionality reduction. The resulting PC scores are then used to build DNN-based surrogate models to significantly reduce the computational cost in Markov Chain Monte Carlo sampling, while the code/interpolation uncertainty is accounted for using Bayesian neural networks. We compared four IUQ processes with different dimensionality reduction methods and surrogate models. The proposed approach has demonstrated the best performance in reducing the dimensionality of the transient PCT profiles. This approach also produces the posterior distributions of the physical model parameters with which the model predictions have the best agreement with the experimental data.

Quantification of mechanical behavior of rat tail under compression.

The development of vibration-induced finger disorders is likely associated with combined static and dynamic responses of the fingers to vibration exposure. To study the mechanism of the disorders, a new rat-tail model has been established to mimic the finger vibration and pressure exposures. However, the mechanical behavior of the tail during compression needs to be better understood to improve the model and its applications. To investigate the static and time-dependent force responses of the rat tail during compression. Compression tests were conducted on Sprague-Dawley cadaver rat tails using a micromechanical system at three deformation velocities and three deformation magnitudes. Contact-width and the time-histories of force and deformation were measured. Additionally, force-relaxation tests were conducted and a Prony series was used to model the force-relaxation behavior of the tail. The rat tails' force-deformation and stiffness-deformation relationships were strongly nonlinear and time-dependent. Force/stiffness increased with an increase in deformation and deformation velocity. The time-dependent force-relaxation characteristics of the tails can be well described using a Prony series. We successfully quantified the static and time-dependent force responses of rat tails under compression. The identified mechanical behavior of the tail can help improve the rat-tail model and its applications.

Advancing viscoelastic material modeling : Tackling time-dependent behavior with fractional calculus

A combination of inherent unique stress-strain response features, Viscoelastic Materials (VM) provide an integral part in many different fields of engineering. Although practical, these materials’ highly complex time-dependent methods according to non-linear loading scenarios are impossible to model using conventional viscoelastic models such as Maxwell and Kelvin-Voigt. The present study introduces a framework that pushes over traditional Maxwell and Kelvin-Voigt approaches by employing fractional calculus in order to enhance the prediction of VM performance. The mathematical representation makes use of the Caputo fractional derivative for expressing an artificial viscoelastic polymer’s non-linear and time-dependent responses. Dynamic Mechanical Analysis (DMA) and Stress Relaxation Tests (SRT) proved the polymer possessed 1500 MPa fractional modulus and 0.65 fractional order, respectively. The resulting model involved more significant computing resources, but contrasting testing indicated that it accurately depicted stress relaxation and dynamic responses. As mentioned, the technique integrates mathematical and actual viscoelasticity for industrial uses while offering a precise basis for advanced material analysis.

Enhancing nanoscale viscoelasticity characterization in bimodal atomic force microscopy.

Polymeric, soft, and biological materials exhibit viscoelasticity, which is a time dependent mechanical response to deformation. Material viscoelasticity emerges from the movement of a material's constituent molecules at the nano- and microscale in response to applied deformation. Therefore, viscoelastic properties depend on the speed at which a material is deformed. Recent technological advances, especially in atomic force microscopy (AFM), have provided tools to measure and map material viscoelasticity with nanoscale resolution. However, to obtain additional information about the viscoelastic behavior of a material from such measurements, theoretical grounding during data analysis is required. For example, commercially available bimodal AFM imaging maps two different viscoelastic properties of a sample, the storage modulus, E', and loss tangent, tan δ, with each property being measured by a different resonance frequency of the AFM cantilever. While such techniques provide high resolution maps of E' and tan δ, the different measurement frequencies make it difficult to calculate key viscoelastic properties of the sample such as: the model of viscoelasticity that describes the sample, the loss modulus, E'', at either frequency, elasticity E, viscosity η, and characteristic response times τ. To overcome this difficulty, we present a new data analysis procedure derived from linear viscoelasticity theory. This procedure is applied and validated by performing amplitude modulation-frequency modulation (AM-FM) AFM, a commercially available bimodal imaging technique, on a styrene-butadiene rubber (SBR) with known mechanical behavior. The new analysis procedure correctly identified the type of viscoelasticity exhibited by the SBR and accurately calculated SBR E, η, and τ, providing a useful means of enhancing the amount of information gained about a sample's nanoscale viscoelastic properties from bimodal AFM measurements. Additionally, being derived from fundamental models of linear viscoelasticity, the procedure can be employed for any technique where different viscoelastic properties are measured at different and discrete frequencies with applied deformations in the linear viscoelastic regime of a sample.

A martingale approach to time-dependent and time-periodic linear response in Markov jump processes

A martingale approach to time-dependent and time-periodic linear response in Markov jump processes

Solar cycle as a distinct line of evidence constraining Earth’s transient climate response

Severity of warming predicted by climate models depends on their Transient Climate Response (TCR). Inter-model spread of TCR has persisted at ~ 100% of its mean for decades. Existing observational constraints of TCR are based on observed historical warming response to historical forcing and their uncertainty spread is just as wide, mainly due to forcing uncertainty, and especially that of aerosols. Contrary, no aerosols are involved in solar-cycle forcing, providing an independent, tighter, constraint. Here, we define a climate sensitivity metric: time-dependent response regressed against time-dependent forcing, allowing phenomena with dissimilar time variations, such as the solar cycle with 11-year cyclic forcing, to be used to constrain TCR, which has a linear time-dependent forcing. We find a theoretical linear relationship between the two. The latest coupled atmosphere-ocean climate models obey the same linear relationship statistically. The proposed observational constraint on TCR is about 1/3\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$${{1}}/{{3}}$$\\end{document} as narrow as existing constraints. The central estimate, 2.2 oC, is at the midpoint of the spread of the latest generation of climate models, which are more sensitive than those of the previous generations.

Time-Scaling Creep in Salt Rocks for Underground Storage

Salt is an elastoviscoplastic material that exhibits time-dependent deformation (creep). Experimental measurements of salt creep behavior help predict underground gas repositories’ long-term geomechanical behavior. Previous time-scaling creep experiments have focused on the axial strain of unconsolidated sands. i.e., time-scaling creep effects under zero lateral strain conditions without describing the creep behavior of the radial strain. In addition, the time-scaling creep of the radial and axial strain has not been investigated in salts. A comparative testing procedure and analysis method was conducted on Spindletop salt plugs using triaxial tests for multistage triaxial tests (MST) and different holding time durations and stress regimes, resulting in time-dependent strain responses (creep tests). The MST showed evolving deformational mechanisms under the mapped yield surface based on the irrecoverable to recoverable strain ratio beginning with crack closure or conformance, plasticity, and ending at early crystal surface failure. Unlike unconsolidated sands, salts showed both time and strain amplitude scaling. The axial and radial strain data show scaling behavior under low and high levels of deviatoric stress separated by a transitional period. The salt showed only an axial creep response at low deviatoric stress distally from the yield surface (one-dimesional (1D) response or zero lateral strain), which indicates negative dilatant deformation or uniaxial compaction. In contrast, the salts showed equal strain amplitude scaling factors both axially and radially at high deviatoric stress proximal to the yield surface (two-dimensional (2D) response or unconstrained boundary condition), which suggests positive dilatant deformation. Microstructural images showed accumulated creep damage under high deviatoric stress associated with parallel planes of dislocation-intergranular slip, microcracking, and compaction-induced dilational strains. The period of scaling is interpreted as regions where a single mechanism is dominating. Strain amplitude scaling for both low and high deviatoric creep stress tests provides inputs for a constitutive model of creep response in understanding the magnitude of mechanical damage associated with time-independent stress-strain curves in salts for the structural integrity of salt caverns during cyclic fluid injection and depletion.

A Near-Infrared Fluorogenic Probe for Rapid, Specific, and Ultrasensitive Detection of Sphingosine in Living Cells and In Vivo.

Sphingosine (Sph) plays important roles in various complex biological processes. Abnormalities in Sph metabolism can result in various diseases, including neurodegenerative disorders. However, due to the lack of rapid and accurate detection methods, understanding sph metabolic in related diseases is limited. Herein, a series of near-infrared fluorogenic probes DMS-X (X = 2F, F, Cl, Br, and I) are designed and synthesized. The fast oxazolidinone ring formation enables the DMS-2F to detect Sph selectively and ultrasensitively, and the detection limit reaches 9.33 ± 0.41nm. Moreover, it is demonstrated that DMS-2F exhibited a dose- and time-dependent response to Sph and can detect sph in living cells. Importantly, for the first time, the changes in Sph levels induced by Aβ42 oligomers and H2 O2 are assessed through a fluorescent imaging approach, and further validated the physiological processes by which Aβ42 oligomers and reactive oxygen species (ROS)-induce changes in intracellular Sph levels. Additionally, the distribution of Sph in living zebrafish is successfully mapped by in vivo imaging of a zebrafish model. This work provides a simple and efficient method for probing Sph in living cells and in vivo, which will facilitate investigation into the metabolic process of Sph and the connection between Sph and disease pathologies.

Time-dependent response of continuous steel-concrete composite beams under sustained loading

Although the long-term behavior of composite beams raises critical concerns in their application, only a few investigations have been conducted on the time-dependent response of continuous composite beams (CCBs). Herein, long-term tests were conducted on four two-span CCBs with different loading ages. The shear stiffness and time-slip relationship of flexible shear connectors were evaluated by carrying out short-term and long-term push-out tests. The experimental results revealed that loading age substantially affected the development of deflection, interface slip, crack, and internal force redistribution of CCBs under sustained loads. A finite element model (FEM-2) of test specimens was developed, considering the damage evolution, creep and shrinkage of concrete slab, and stiffness degradation of connectors. After the FEM results were verified against the experimental data, the developed FEM-2 model was implemented to further investigate the influences of loading age and loading level on the long-term performance of CCBs.

Effect of Silica in the Manufacture of Autologous Serum Eye Drops on Corneal Stromal Cells.

The aim of this study was to evaluate the effect of serum clot activator, silica (SiO 2 ), which may be used for making autologous serum eye drops, on human corneal fibroblasts. Cultured human corneal fibroblasts were exposed to 10%, 20%, and 30% silica for 1, 6, and 24 hours; methyl thiazolyl tetrazolium-based colorimetric assay was performed to determine the survival rate of fibroblasts and lactate dehydrogenase leakage assay to assess the cytotoxicity. The apoptotic response was evaluated by flow cytometric analysis and fluorescence staining with Annexin V and propidium iodide. Cellular morphology was evaluated by inverted phase-contrast light microscopy and electron microscopy. The survival rate of human corneal fibroblasts and cytotoxicity showed both dose-dependent and time-dependent responses. The fluorescent micrograph and flow cytometry showed that as the exposure time increased, more cells underwent apoptosis or necrosis after treatment with 30% silica. When observed with light and electron microscopy, the number of corneal fibroblasts decreased and they were more detached from the dish. In addition, damaged corneal fibroblasts showed degenerative changes after exposure to 30% silica. Silica showed dose-dependent and time-dependent toxicity in human corneal fibroblasts. It is safer to keep the blood in tubes without a clot activator when manufacturing autologous serum eye drops to prevent possible corneal cytotoxicity.

Time-dependent transient response of piles partially embedded in layered viscoelastic saturated cross-anisotropic soils using a fractional-order Merchant model

A 3D analytical method is proposed for the lateral time-dependent response of monopiles partially embedded in stratified saturated viscoelastic cross-anisotropic soils under horizontal transient loads. Nevertheless, previous studies on ocean engineering have rarely considered the rheological properties of seabed materials. In this study, depending upon Biot's dynamical formulation, the linear elastic solution of the layered cross-anisotropic saturated soils due to lateral transient and vertical loads is solved using the analytical layer-element method (ALEM). The fractional-order Merchant model is adopted to describe the rheological deformation of the seabed, and the viscoelastic solution of the laminar cross-anisotropic saturated soils is derived by combining the elastic-viscoelastic transformation theory. Using the consolidation solution of the seabed as the kernel function, the pile-soil boundary problem is analyzed by the boundary element method (BEM) to obtain the BE equation of the viscoelastic saturated soils. The Timoshenko beam is employed to simulate the monopile, and based on the finite element method (FEM), the monopile is discretized into pile units to derive the FE matrix equation. Finally, the dynamic response formulation of the partially embedded offshore monopile is obtained by coupling the BE and FE equations through the coordination conditions of close contact between the Timoshenko beam and the seabed. The accuracy of the methodology is verified by comparing with the existing solutions and the numerical results from ABAQUS. And the effects of the transient load type, seabed viscosity, and pile properties on the horizontal response of partially embedded monopiles are explored.

Dynamic multicolor luminescent anti-counterfeiting based on spiropyran-engineered gold nanoclusters

Multicolor luminescent materials with time-dependent optical responses are attractive for developing advanced anti-counterfeiting techniques, but it still remains challenging to rationally design and fabricate these emerging materials. Herein, we report the design of novel dynamic luminescent materials based on spiropyran-engineered gold nanoclusters (AuNCs) for anti-counterfeiting applications. Encapsulating fluorescent AuNCs with spiropyran-labeled bovine serum albumin enables the construction of dynamic multicolor luminescent nanoparticles (DML NPs) based on fluorescence resonance energy transfer. These AuNCs-based DML NPs exhibit photochromism and dynamic fluorescence color change from green to yellow to orange-red with the extension of UV irradiation time. Moreover, the photochromism and dynamic fluorescent behavior is highly reversible when exposed to the irradiation of visible light. By taking advantage of photochromic and time-dependent dynamic fluorescence, potential application in optical information storage, dynamic anti-counterfeiting authentication and multi-mode anti-counterfeiting are successfully demonstrated based on DML NPs-deposited polyvinyl alcohol film. This study provides a new strategy of designing novel dynamic multicolor luminescent materials with promising optical properties, which can further advance the practical application of AuNCs-based luminescent materials in the field of information encryption and anti-counterfeiting.

Effect of Shear History on Solid-Liquid Transition of Particulate Gel Fuels.

Investigating the structural evolution of particulate gels is a very challenging task due to their vulnerability and true flow characteristics. In this work, deeper insight into the rheological properties of gel fuels filled with fumed silica (FS) and aluminum microparticles (Al MPs) was gained by changing shear procedures. Firstly, the flow curves were found to no longer follow the monotonic power law and exhibited subtle thixotropic responses. As the shear rate increased, the gel structure underwent a transition from local shear to bulk shear in the nonlinear region after yielding. This finding reveals the prevalence of nonideal local shear in industry. Secondly, the time-dependent rheological responses demonstrated that the strength spectrum of gel fuels depends on the applied shear rate, with stress relaxation more easily observed at lower shear rates. Those results involved the structural disruption, recovery, and equilibrium of particulate gels from two scales of shear rate and shear time.