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Highlights: A relaxation test data set is reinterpreted with a focus on radial strain Specimens loaded below the crack damage stress contracted more than predicted by elasticity theory The rate of lateral contraction increased when the stress had dropped by between 50% and 80% of the total stress relaxation observed at the end of the test One specimen loaded just above crack damage changed from dilation to contraction at the transition from primary to secondary relaxation [ABSTRACT FROM AUTHOR]

Carbon fibre reinforced polymer (CFRP) laminates nano-enhanced with carbon nanofibers (CNFs): 0.75 wt% and 0.5 wt% of epoxies Sicomin and Ebalta, respectively, were manufactured and their static and viscoelastic behaviour analyzed. After 180 min, the bending stress decreases and creep displacement increases over time, with Ebalta nano-enhanced resin laminates with CNFs showing the best results. A strong dependence of creep behaviour and stress relaxation is obtained with the applied stress level. The results show that laminates produced with pure Sicomin resin increased creep displacement by 1%, for a bending stress of 200 MPa, and 2 times greater for the bending stress of 700 MPa. The viscoelastic behaviour of CFRP composites nano-enhanced with CNFs was accurately predicted by the Kohlrausch-Williams-Watts (KWW) model and Findley power law. [ABSTRACT FROM AUTHOR]

We present an analysis of magnitude clustering of microfractures inferred from acoustic emissions (AEs) during stick‐slip (SS) dynamics of faulted Westerly granite samples in frictional sliding experiments, with and without fluids, under triaxial loading with constant displacement rate. We investigate magnitude clustering in time across periods during, preceding and after macroscopic slip events on laboratory faults. Our findings reveal that magnitude clustering exists such that subsequent AEs tend to have more similar magnitudes than expected. Yet, this clustering only exists during macroscopic slip events and is strongest during major slip events in fluid‐saturated and dry samples. We demonstrate that robust magnitude clustering arises from variations in frequency‐magnitude distributions of AE events during macroscopic slip events. These temporal variations indicate a prevalence of larger AE events right after (0.3–3 s) the SS onset. Hence, magnitude clustering is a consequence of non‐stationarities. Plain Language Summary: Can we determine the size of a future earthquake based on the size of past earthquakes? This fundamental question has been controversially debated over the years, without an agreed‐upon answer. Here, we tackle this question under controlled conditions in a lab setting by studying the frictional stick‐slip dynamic of rough granite faults, which gives rise to mm‐scale seismic events. We find that the sizes of these seismic events are not independent during periods containing a macroscopic slip but instead are clustered such that larger seismic events tend to directly follow other large seismic events. We show that this can be explained by temporal changes in the frequency of occurrence of seismic events associated with the sliding motion of the fault. Our findings link the properties of mm‐scale seismic events with macroscopic slip on lab faults. Key Points: Magnitude clustering of acoustic emissions (AEs) is strongest during major slip events in fluid‐saturated samples of Westerly graniteVariations in the frequency‐magnitude distribution of AEs during slip events explain the strong magnitude clusteringThese variations indicate a prevalence of larger AEs right after the onset of slip events [ABSTRACT FROM AUTHOR]

The increasing use of glass‐fiber reinforced polypropylene (GFPP) composites in a wide range of applications requiring long‐term service in challenging environments underscores the importance of its long‐term durability. This study aimed to investigate the effect of thermal aging on the long‐term dynamic durability and stress relaxation of GFPP composites. The time–temperature equivalence principle (TTSP) was used to assess the dynamic modulus at various temperatures and frequencies, as well as the relaxation modulus at different temperatures and relaxation times. To better understand long‐term durability behavior, the study also examined the impact of molecular structure on the durability of GFPP. Changes in chemical composition, crystallinity, melting point, and melt flow index of GFPP due to thermal aging were measured and analyzed using Fourier transform infrared spectroscopy, and differential scanning calorimetry, respectively. The results revealed that oxidation led to a decrease in the crystallinity and molecular weight of GFPP. The destruction of GFPP's molecular structure due to oxidation resulted in reduced long‐term durability. While TTSP can predict the long‐term durability of GFPP for decades or even centuries, its application in predicting the long‐term durability of GFPP is more suitable for nonaging conditions. Highlights: Thermal aging reduces glass‐fiber reinforced polypropylene (GFPP's) long‐term durability.Time–temperature equivalence principle predicts GFPP durability under aging.GFPP's activation energy drops with thermal aging.Long‐term GFPP behavior aligns with Williams–Landel–Ferry model.Predictive models refine understanding of GFPP longevity. [ABSTRACT FROM AUTHOR]

The mechanical properties of sclera play an important role in ocular functions, protection, and disease. Modulating the sclera's properties by exogenous crosslinking offers a way to expand the tissue's range of properties for study of the possible influences on the eye's behavior and diseases such as glaucoma and myopia. The focus of this work was to evaluate the effects of genipin crosslinking targeting the porcine perilimbal sclera (PLS) since the stiffness of this tissue was previously found in a number of studies to influence the eye's intraocular pressure (IOP). The work includes experiments on tensile test specimens and whole globes. The specimen tests showed decreased strain-rate dependence and increased relaxation stress due to the cross-linker. Whole globe perfusion experiments demonstrated that eyes treated with genipin in the perilimbal region had increased IOPs compared to the control globes. Migration of the cross-linker from the target tissue to other tissues is a confounding factor in whole globe, biomechanical measurements, with crosslinking. A novel quantitative genipin assay of the trabecular meshwork (TM) was developed to exclude globes where the TM was inadvertently crosslinked. The perfusion study, therefore, suggests that elevated stiffness of the PLS can significantly increase IOP apart from effects of the TM in the porcine eye. These results demonstrate the importance of PLS biomechanics in aqueous outflow regulation and support additional investigations into the distal outflow pathways as a key source of outflow resistance. [ABSTRACT FROM AUTHOR]

In articular cartilage, the pericellular matrix acting as a specialized mechanical microenvironment modulates environmental signals to chondrocytes through mechanotransduction. Matrix viscoelastic alterations during cartilage development and osteoarthritis (OA) degeneration play an important role in regulating chondrocyte fate and cartilage matrix homeostasis. In recent years, scientists are gradually realizing the importance of matrix viscoelasticity in regulating chondrocyte function and phenotype. Notably, this is an emerging field, and this review summarizes the existing literatures to the best of our knowledge. This review provides an overview of the viscoelastic properties of hydrogels and the role of matrix viscoelasticity in directing chondrocyte behavior. In this review, we elaborated the mechanotransuction mechanisms by which cells sense and respond to the viscoelastic environment and also discussed the underlying signaling pathways. Moreover, emerging insights into the role of matrix viscoelasticity in regulating chondrocyte function and cartilage formation shed light into designing cell‐instructive biomaterial. We also describe the potential use of viscoelastic biomaterials in cartilage tissue engineering and regenerative medicine. Future perspectives on mechanobiological comprehension of the viscoelastic behaviors involved in tissue homeostasis, cellular responses, and biomaterial design are highlighted. Finally, this review also highlights recent strategies utilizing viscoelastic hydrogels for designing cartilage‐on‐a‐chip. Summary: It is well known that viscoelasticity has been found to be a universal characteristic of living tissues, cells, and molecules.Matrix viscoelasticity, as a critical physical cues of cartilage matrix microenvironment, regulates collective chondrocyte fate and cartilage formation. But, how matrix viscoelasticity affects the chondrocyte behavior and function remains unclear.Growing evidence has demonstrated that matrix viscoelasticity regulates chondrocytes in some ways not anticipated from previous mechanisms of mechanotransduction, which was based on purely substrate stiffness.In this review, we introduce recent work elucidating the effect of matrix viscoelasticity on chondrocytes morphology, phenotype, function, inflammatory response, and signaling pathways.These findings have provided insights into viscoelasticity as a design parameter for cartilage tissue engineering, regenerative medicine, and cartilage‐on‐a‐chip, even shed light into the pathological mechanism of joint growth and degeneration. [ABSTRACT FROM AUTHOR]

Springback of sheet metals consists of two components: springback caused by elastic recovery during unloading and time-dependent springback caused by viscoplastic phenomena over time after unloading. While springback can be reduced by punch holding (or bend holding), the influences of bend holding on each of these two springback components are unclear. This study aims to elucidate the effects of bend holding on these two springback components and total springback by conducting L-bending tests on an advanced high-strength steel (SPCN118Y) sheet and an aluminum (A1050-O) sheet. The total springback two days after unloading decreased when bending was held because the springback during unloading decreased due to stress relaxation while holding. The time-dependent springback also tended to decrease with bend holding in most cases, but its amount and reduction were considerably smaller than those in the springback during unloading. The percentage of timedependent springback to total springback was larger for A1050-O than for SPCN118Y. The percentage of SPCN118Y was not much affected by bend holding, while that of A1050-O became larger when bending was held. [ABSTRACT FROM AUTHOR]

Laser Hot Wire (LHW) Directed Energy Deposition (DED) Additive Manufacturing (AM) processes are capable of manufacturing parts with a high deposition rate. There is a growing research interest in replacing large cast Nickel Aluminum Bronze (NAB) components using LHW DED processes for maritime applications. Understanding thermomechanical behavior during LHW DED of NAB is a critical step towards the production of high-quality NAB parts with desired performance and properties. In this paper, finite element simulations are first used to predict the thermomechanical time histories during LHW DED of NAB test coupons with an increasing geometric complexity, including single-layer and multilayer depositions. Simulation results are experimentally validated through in situ measurements of temperatures at multiple locations in the substrate as well as displacement at the free end of the substrate during and immediately following the deposition process. The results in this paper demonstrate that the finite element predictions have good agreement with the experimental measurements of both temperature and distortion history. The maximum prediction error for temperature is 5% for single-layer samples and 6% for multilayer samples, while the distortion prediction error is about 12% for single-layer samples and less than 4% for multilayer samples. In addition, this study shows the effectiveness of including a stress relaxation temperature at 500 °C during FE modeling to allow for better prediction of the low cross-layer accumulation of distortion in multilayer deposition of NAB. [ABSTRACT FROM AUTHOR]

Biomaterials that integrate multiple functionalities, mimic the extracellular matrix (ECM) microenvironment to support cellular growth, and adhere robustly to damaged tissues are highly needed to advance tissue engineering. Protein‐based biomaterials are promising due to their inherent biocompatibility, biomimicry, biodegradation, and cell‐supportive properties. Herein, by leveraging the unique ability of dithiolanes to generate on‐demand in situ thiols, a new class of dithiolane‐modified, protein‐based biomaterial that combines unique, seemingly opposing functions for tissue engineering is developed. Dithiolane‐modified gelatin, a model protein used herein, enabled photoinitiator‐free photo‐crosslinking to form multi‐functional gelatin‐dithiolane (GelDT) hydrogels, which displayed exceptional long‐term stability in cell culture media (>28 days) to support the growth of both surface‐seeded and encapsulated cells. GelDT hydrogels allowed pre‐gelation tuning of biomechanical properties and biodegradation via introducing physical crosslinks, and post‐gelation tuning of matrix stress‐relaxation rate, via responding to exogenous thiols, independently of other parameters. Furthermore, GelDT enabled covalent immobilization of bio‐active molecules, glutathione‐responsive drug release, supported efficient 3D bioprinting due to its shear‐thinning ability, and demonstrated robust tissue adhesion in various contexts (bare skin, ex‐vivo, in‐vivo) due to covalent disulfide coupling with endogenous tissue thiols. Together, this study presents a novel multi‐responsive and multi‐functional protein‐based biomaterial, anticipated to advance tissue engineering and regeneration. [ABSTRACT FROM AUTHOR]

We investigate the viscoelastic relaxation to equilibrium of an isolated disordered planar epithelium described using the cell vertex model. In its standard form, the model is formulated as coupled evolution equations for the locations of vertices of confluent polygonal cells. Exploiting the model's gradient-flow structure, we use singular-value decomposition to project modes of deformation of vertices onto modes of deformation of cells. Expressing the dynamics in terms of operators of a discrete calculus, we show how eigenmodes of discrete Laplacian operators (specified by constitutive assumptions related to dissipation and mechanical energy) provide a spatial basis for evolving fields, and demonstrate how the operators can incorporate approximations of conventional spatial derivatives. We relate the spectrum of relaxation times to the eigenvalues of the Laplacians, modified by corrections that account for the fact that the cell network (and therefore the Laplacians) evolve during relaxation to an equilibrium prestressed state, providing the monolayer with geometric stiffness. While dilational modes of the Laplacians capture rapid relaxation in some circumstances, showing diffusive dynamics, geometric stiffness is typically a dominant source of monolayer rigidity, as we illustrate for monolayers exposed to unsteady stretching deformations. [ABSTRACT FROM AUTHOR]

The paper investigates the short- and long-term compressive stress relaxation behavior of isotropic and anisotropic magneto-sensitive elastomers (MSEs) prepared by incorporating carbonyl iron microparticles into a silicone rubber. The effects of applied compressive strain, magnetic field, and temperature on the short-term stress relaxation behavior of the isotropic and anisotropic MSEs were determined up to 1200 s. The stress relaxation behavior of the MSEs considerably depended on the applied compressive strain, magnetic field, and temperature. The stress of the MSEs increased with increasing compressive strain and magnetic-field intensity, but decreased with increasing temperature. The isotropic MSE exhibited approximately linear elastic behavior, while the anisotropic MSE revealed nonlinear elastic characteristics. The compressive stress and the relaxation modulus of the anisotropic MSE are considerably higher than those of the isotropic MSE. The compressive stress relaxation behavior of the isotropic and anisotropic MSEs was simulated using a fractional derivative viscoelastic Kelvin–Voigt model. The model parameters were identified by fitting the relaxation modulus to the short-term measured data of the MSEs. The compressive stress estimated from the studied model with fitted parameters was in excellent agreement with the measured data of the MSEs at different compressive strains, magnetic fields, and temperatures. The model was then used to estimate the long-term stress relaxation of the MSEs. An excellent agreement between long-term predicted results and experimental data of the MSEs has been reached when fitting the model to the medium-term experimental data. [ABSTRACT FROM AUTHOR]

In this work, we investigate the viscoelastic properties of hydrogels through stress relaxation experiments to better understand the force-dependent dynamics of these materials with the aspiration of expanding their application envelope within the biomedical field and beyond. We experimentally studied the viscoelastic behavior of 4 different types of hydrogels: covalently crosslinked polyacrylamide (PAAm), covalently crosslinked PAAm network immersed in a viscous alginate solution, ionically crosslinked alginate along with crosslinked PAAm-alginate double network. Through our investigations, we demonstrate that we can tailor the viscoelasticity of a covalently bonded PAAm network by tuning the viscosity of the solution in the gel. Moreover, based on the stress relaxation test of ionically crosslinked alginate gel and the double network gel, we have revealed the quantitative correlation between the ionic bond dissociation and force-dependent viscoelastic behavior of gels containing ionic crosslinks. [ABSTRACT FROM AUTHOR]

Cell interactions with the extracellular matrix (ECM) influence intracellular signaling pathways related to proliferation, differentiation, and secretion, amongst other functions. Herein, bone-marrow derived mesenchymal stromal cells (MSCs) are encapsulated in a hydrazone crosslinked hyaluronic acid (HA) hydrogel, and the extent of stress relaxation is controlled by systemic introduction of irreversible triazole crosslinks. MSCs form elongated multicellular structures within hydrogels containing RGD peptide and formulated with elastic composition slightly higher than the hydrogel percolation threshold (12 % triazole, 88 % hydrazone). A scaling analysis is presented (12 ∼Nα) to quantify cell-material interactions within these structures with the scaling exponent (α) describing either elongated (0.66) or globular (0.33) structures. Cellular interactions with the material were controlled through peptides to present integrin binding ECM cues (RGD) or cadherin binding cell-cell cues (HAVDI) and MSCs were observed to form highly elongated structures in RGD containing hydrogels (α=0.56±0.05), whereases collapsed structures were observed within HAVDI containing hydrogels (α=0.39±0.04). Finally, cytokine secretion was investigated, and a global increase in secreted cytokines was observed for collapsed structures compared to elongated. Taken together, this study presents a novel method to characterize cellular interactions within a stress relaxing hydrogel where altered cluster morphology imparts changes to cluster secretory profiles.

Bioactive hydrogel materials have great potential for applications in bone tissue engineering. However, fabrication of functional hydrogels that mimic the natural bone extracellular matrix (ECM) remains a challenge, because they need to provide mechanical support and embody physiological cues for angiogenesis and osteogenesis. Inspired by the features of ECM, we constructed a dual-component composite hydrogel comprising interpenetrating polymer networks of gelatin methacryloyl (GelMA) and deoxyribonucleic acid (DNA). Within the composite hydrogel, the GelMA network serves as the backbone for mechanical and biological stability, whereas the DNA network realizes dynamic capabilities (e.g., stress relaxation), thereby promoting cell proliferation and osteogenic differentiation. Furthermore, functional aptamers (Apt19S and AptV) are readily attached to the DNA network to recruit bone marrow mesenchymal stem cells (BMSCs) and achieve sustained release of loaded vascular endothelial growth factor towards angiogenesis. Our results showed that the composite hydrogel could facilitate the adhesion of BMSCs, promote osteogenic differentiation by activating focal adhesion kinase (FAK)/phosphatidylinositol 3-kinase (PI3K)/protein kinase B (Akt)/β-Catenin signaling pathway, and eventually enhance vascularized bone regeneration. This study shows that the multifunctional composite hydrogel of GelMA and DNA can successfully simulate the biological functions of natural bone ECM and has great potential for repairing bone defects.

[Display omitted] Clear aligners undergo rapid stress relaxation in warm, moist oral environments, compromising therapeutic effectiveness and longevity of treatment. To develop an innovative multilayer composite material with improved stability and reduced stress release, we have engineered an innovative coating characterized by the surface aggregation of polydimethylsiloxane (PDMS), which imparts a pronounced hydrophobic effect. In addition, the chemically and physically cross-linked structure of the coating reduces the free volume created by molecular chain rearrangement owing to the presence of water molecules, thereby minimizing water penetration into the coating. Concurrently, the coating's internal structure is enriched with numerous polar functional groups to capture water molecules that penetrate into the inside of the coating. Through combination of these mechanisms, water molecules are effectively sequestered, thereby impeding their penetration into the polyethylene terephthalate glycol (PETG) substrate. The impact of the polydimethylsiloxane content on the triple-action water-resistance mechanisms was thoroughly examined using attenuated total reflection (ATR)-Fourier transform infrared (FTIR), water absorption rate, water swelling rate, and X-ray photoelectron spectroscopy. The low surface energy cross-linked polyurethane coating is applied to the polyethylene terephthalate glycol (PETG) substrate to create a novel composite material with specific mechanical properties and reduced stress relaxation. The composite material remains stable in simulated oral environment with linear swelling rate of 0.58 % upon water absorption. Additionally, the stress release rate of the composite material within 336 h is notably lower (23.64 %) than that of PETG (62.29 %). [ABSTRACT FROM AUTHOR]

Abstract The diverse applications of magnetorheological elastomer (MRE) drive efforts to understand consistent performance and resistance to failure. Stress relaxation can lead to molecular chain deterioration, degradation in stiffness and rheological properties, and ultimately affect the life cycle of MRE. However, quantifying the energy and molecular dynamics during stress relaxation is challenging due to the difficulty of obtaining atomic-level insights experimentally. This study employs molecular dynamics (MD) simulation to elucidate the stress relaxation in MRE during constant strain. Magnetorheological elastomer models incorporating silicone rubber filled with varying magnetic iron particles (50–80 wt%) were constructed. Experimental results from an oscillatory shear rheometer showed the linear viscoelastic region of MRE to be within 0.001–0.01% strain. The simulation results indicated that stress relaxation has occurred, with stored energies decreased by 8.63–52.7% in all MRE models. Monitoring changes in energy components, the highest final stored energy (12,045 kJ) of the MRE model with 80 wt% Fe particles was primarily attributed to stronger intramolecular and intermolecular interactions, revealed by higher potential energy (3262 kJ) and van der Waals energy (− 2717.29 kJ). Stress relaxation also altered the molecular dynamics of this MRE model as evidenced by a decrease in kinetic energy (9362 kJ) and mean square displacement value (20,318 Å2). The MD simulation provides a promising quantitative tool for elucidating stress relaxation, preventing material failure and offering insights for the design of MRE in the nanotechnology industry.

The diverse applications of magnetorheological elastomer (MRE) drive efforts to understand consistent performance and resistance to failure. Stress relaxation can lead to molecular chain deterioration, degradation in stiffness and rheological properties, and ultimately affect the life cycle of MRE. However, quantifying the energy and molecular dynamics during stress relaxation is challenging due to the difficulty of obtaining atomic-level insights experimentally. This study employs molecular dynamics (MD) simulation to elucidate the stress relaxation in MRE during constant strain. Magnetorheological elastomer models incorporating silicone rubber filled with varying magnetic iron particles (50–80 wt%) were constructed. Experimental results from an oscillatory shear rheometer showed the linear viscoelastic region of MRE to be within 0.001–0.01% strain. The simulation results indicated that stress relaxation has occurred, with stored energies decreased by 8.63–52.7% in all MRE models. Monitoring changes in energy components, the highest final stored energy (12,045 kJ) of the MRE model with 80 wt% Fe particles was primarily attributed to stronger intramolecular and intermolecular interactions, revealed by higher potential energy (3262 kJ) and van der Waals energy (− 2717.29 kJ). Stress relaxation also altered the molecular dynamics of this MRE model as evidenced by a decrease in kinetic energy (9362 kJ) and mean square displacement value (20,318 Å2). The MD simulation provides a promising quantitative tool for elucidating stress relaxation, preventing material failure and offering insights for the design of MRE in the nanotechnology industry. [ABSTRACT FROM AUTHOR]

Conjugated polymers consist of complex backbone structures and side‐chain moieties to meet various optoelectronic and processing requirements. Recent work on conjugated polymers has been devoted to studying the mechanical properties and developing new conjugated polymers with low modulus and high‐crack onset strain, while the thin film mechanical stability under long‐term external tensile strain is less investigated. Here we performed direct mechanical stress relaxation tests for both free‐standing and thin film floated on water surface on both high‐Tg and low‐Tg conjugated polymers, as well as a reference nonconjugated sample, polystyrene. We measured thin films with a range of film thickness from 38 to 179 nm to study the temperature and thickness effect on thin film relaxation, where an apparent enthalpy–entropy compensation effect for glassy polymer PS and PM6 thin films was observed. We also compared relaxation times across three different conjugated polymers and showed that both crystalline morphology and higher modulus reduce the relaxation rate besides higher glass transition temperature. Our work provides insights into the mechanical creep behavior of conjugated polymers, which will have an impact on the future design of stable functional organic electronics. [ABSTRACT FROM AUTHOR]

Thermomechanical devulcanization of butyl rubber (BR1), bromobutyl rubber (BR2) vulcanizates, both prepared in laboratory, and a waste commercial butyl rubber (BR3) using a twin-screw extruder is reported. The commercial butyl rubber vulcanizate (BR3) was used as a tire curing bladder and the waste rubber received from a tire manufacturing company. The devulcanization process was carried out at various barrel temperatures and screw speeds (40, 80 and 120 r/min). The lowest sol fraction was obtained for devulcanized waste commercial butyl rubber (D-BR3). According to Horikx theory, devulcanized butyl rubber (D-BR1) presented mainly crosslink breakup while devulcanized bromobutyl rubber (D-BR2) and devulcanized waste commercial butyl rubber (D-BR3) showed a mixture of main chain and crosslink scission. As cure rheographs at 190°C indicated, the addition of devulcanized rubbers to the corresponding virgin ones influenced both scorch and optimal cure times. The tensile properties of devulcanized/virgin rubber blends changed depending on the blend type and virgin rubber. The stress relaxation experiment revealed that the addition of devulcanized rubber to virgin one mostly influenced short relaxation times and also decreased G0. BR1/D-BR3 blends exhibited higher elastic component than BR1/D-BR1 and BR2/D-BR2 samples. The Han plots revealed higher compatibility for BR2/D-BR2 blends. [ABSTRACT FROM AUTHOR]

The improvement of computing power provides a support for finite element simulation analysis, which can solve more complex structures and nonlinear problems more efficiently. In this study, based on the finite element simulation analysis technology, the complex mechanical properties of Mn-Cu alloy are simulated. Prony series constitutive model and parallel rheological constitutive model are respectively adopted to simulate the stress relaxation test of Mn-Cu alloy under different deformation conditions. The stress relaxation experimental data are extracted and converted into the corresponding model parameters and the finite element simulation calculation is carried out. The simulation model parameters which can accurately describe the nonlinear mechanical properties of Mn-Cu alloy are obtained. By comparing the simulation and experimental results, it is concluded that the parallel rheological framework can more accurately characterize the stress relaxation behavior of Mn-Cu alloy. Under different initial strain conditions, the relative error of the simulation results and the test results under corresponding working conditions is less than 1%. Compared with Prony series constitutive model, parallel rheological framework is more suitable for the simulation of Mn-Cu alloy under complex working conditions. [ABSTRACT FROM AUTHOR]

Disulfide‐contained vitrimers that possess unprecedented functions, such as reprocessing and self‐healing, are considered to be one of the most promising materials since they also exhibit mechanochromic behaviors. A few reports have given the phenomenon of mechanochromism, but the relationship between their structure and mechanochromic behavior needed to further research. Herein, three aromatic disulfide‐contained epoxy vitrimers with different degree of unsaturation are prepared by changing the epoxy monomer. The effect of network structure on the dynamic and mechanochromic properties are investigated. With increasing the degree of unsaturation from the aliphatic to aromatic epoxy vitrimers, the Tg‐value increases and the chain mobility decreases, so the aliphatic epoxy vitrimers shows the lowest activation energy, the lowest temperature of malleability Tv, and the shortest relaxation times. In addition, these epoxy vitrimers all showes reversible mechanochromic behaviors due to the generation and coupling of sulfenyl radicals, which can be accelerated by the high temperature and solvent. Meanwhile, the epoxy structure has a great influence on the mechanochromic behavior, including the absorption wavelength and fading kinetics of grinding powders. These results can be utilized to design desired vitrimer materials for various applications that requires both the intrinsically self‐report damage and the dynamic properties such as recycling. [ABSTRACT FROM AUTHOR]

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The static recrystallization and strain-induced precipitation behavior of Ti and Ti-Zr low carbon microalloyed steels were studied through isothermal stress relaxation experiments at different deformation temperatures (875–1025 °C). The precipitation kinetic curves of microalloyed carbides were obtained, and the effect of Zr on the precipitation kinetics of microalloyed carbides was analyzed. Two static recrystallization kinetic models were established, and the activation energies for static recrystallization of Ti steel and Ti-Zr steel were calculated to be 312.44 and 246.59 kJ/mol, respectively. The results indicate that the addition of Zr lowers the nose point temperature of the precipitation–time–temperature curve, shortens the incubation period for the nucleation of precipitates, promotes the nucleation of strain-induced precipitates in Ti microalloyed steel, and reduces the coarsening rate of precipitates particles by an order of magnitude. In addition, the addition of Zr also advances the end time of static recrystallization of deformed austenite, inhibits the growth of static recrystallization austenite grains in Ti steel during the isothermal process, and makes the austenite grains finer and more uniform. [ABSTRACT FROM AUTHOR]

This study employed nano-indentation technology, molecular dynamics simulation, and experimental investigation to examine the stress relaxation behaviour of a polycrystalline γ-TiAl alloy. The simulation enabled the generation of a load-time curve, the visualisation of internal defect evolution, and the mapping of stress distribution across each grain during the stress relaxation stage. The findings indicate that the load remains stable following an initial decline, thereby elucidating the underlying mechanism of load change during stress relaxation. Furthermore, a nano-indentation test was conducted on the alloy, providing insight into the load variation and stress relaxation behaviour under different loading conditions. By comparing the simulation and experimental results, this study aims to guide the theoretical research and practical application of γ-TiAl alloys. [ABSTRACT FROM AUTHOR]

Tomatoes are prone to mechanical damage due to improper gripping forces during automated harvest and postharvest processes. To reduce this damage, a dynamic viscoelastic model based on long short-term memory (LSTM) is proposed to fit the dynamic compression stress relaxation characteristics of the individual fruit. Furthermore, the classical stress relaxation models involved, the triple-element Maxwell and Caputo fractional derivative models, are compared with the LSTM model to validate its performance. Meanwhile, the LSTM and classical stress relaxation models are used to predict the stress relaxation characteristics of tomato fruit with different fruit sizes and compression positions. The results for the whole test dataset show that the LSTM model achieves a RMSE of 2.829 × 10 − 5 Mpa and a MAPE of 0.228%. It significantly outperforms the Caputo fractional derivative model by demonstrating a substantial enhancement with a 37% decrease in RMSE and a 36% reduction in MAPE. Further analysis of individual tomato fruit reveals the LSTM model's performance, with the minimum RMSE recorded at the septum position being 3.438 × 10 − 5 Mpa, 31% higher than the maximum RMSE at the locule position. Similarly, the lowest MAPE at the septum stands at 0.375%, outperforming the highest MAPE at the locule position by a significant margin of 90%. Moreover, the LSTM model consistently reports the smallest discrepancies between the predicted and observed values compared to classical stress relaxation models. This accuracy suggests that the LSTM model could effectively supplant classical stress relaxation models for predicting stress relaxation changes in individual tomato fruit. [ABSTRACT FROM AUTHOR]

Stimuli‐responsive surfaces have garnered intensive research attention in addressing the challenges of severe information leakage problems, but they exhibit limitations in terms of intricate preparation and compatibility. Herein, the study reports a time‐dependent wrinkled surface controlled by photo‐induced stress relaxation of azobenzene‐ containing crosslinked polymer network comprised of furan‐containing polymer (PEA‐Fu) and maleimide‐substituted azobenzene (AZO‐2MI) as skin layer. Two kinds of wrinkled surfaces induced by two crosslinked networks (thermal‐ and UV‐induced) can be prepared by Diels‐Alder reaction and UV‐induced radical coupling reaction, exhibiting distinct amplitude decrease rate resulted from different stress relaxation rate induced by photo‐isomerization of azobenzene. By UV preprogramming with photomasks, various time‐dependent wrinkle patterns are obtained and can be identified at specific times under 450 nm irradiation. The time window for information read out can be preset through different UV preprogramming time and boundary conditions to achieve multi‐level information encryption. Furthermore, the written information can be camouflaged as another “false” information by regional‐selective irradiation under 450 nm irradiation, which offers a new method for information protection. Based on distinctive time‐dependent characteristics and excellent stability, this responsive wrinkled surface offers bright prospects in multi‐level information encryption. [ABSTRACT FROM AUTHOR]

Adult neural stem cells (NSCs) reside in the dentate gyrus of the hippocampus, and their capacity to generate neurons and glia plays a role in learning and memory. In addition, neurodegenerative diseases are known to be caused by a loss of neurons and glial cells, resulting in a need to better understand stem cell fate commitment processes. We previously showed that NSC fate commitment toward a neuronal or glial lineage is strongly influenced by extracellular matrix stiffness, a property of elastic materials. However, tissues in vivo are not purely elastic and have varying degrees of viscous character. Relatively little is known about how the viscoelastic properties of the substrate impact NSC fate commitment. Here, we introduce a polyacrylamide-based cell culture platform that incorporates mismatched DNA oligonucleotide-based cross-links as well as covalent cross-links. This platform allows for tunable viscous stress relaxation properties via variation in the number of mismatched base pairs. We find that NSCs exhibit increased astrocytic differentiation as the degree of stress relaxation is increased. Furthermore, culturing NSCs on increasingly stress-relaxing substrates impacts cytoskeletal dynamics by decreasing intracellular actin flow rates and stimulating cyclic activation of the mechanosensitive protein RhoA. Additionally, inhibition of motor-clutch model components such as myosin II and focal adhesion kinase partially or completely reverts cells to lineage distributions observed on elastic substrates. Collectively, our results introduce a unique system for controlling matrix stress relaxation properties and offer insight into how NSCs integrate viscoelastic cues to direct fate commitment. [ABSTRACT FROM AUTHOR]

The Cu–Cr–Ni alloy is a key material for the manufacturing of connectors, which requires excellent resistance to stress relaxation. However, the inherent correlation among microstructure, texture, and properties is still unclear. In this study, we investigated the influence of calefaction and stress relaxation on the grain boundaries (GBs), textures, and properties of the Cu–Cr–Ni alloy. The results showed that calefaction and stress relaxation had opposite effects on GBs and textures. Calefaction led to a decrease in the proportion of low-angle grain boundaries (LAGBs), an increase in the Schmidt factor (SF) value of the grains, and a transition of texture from <111> to <113>. The grains with higher SF values were more susceptible to plastic deformation, which deteriorated the stress relaxation resistance. By comparison, stress relaxation led to an increase in the proportion of LAGBs, a decrease in SF values of the grains, and a transition of texture from <113> to <111> and <001>. After stress relaxation, the variation trends of the GBs and textures were consistent with those of other plastic deformations, indicating that stress relaxation can be verified by the variations in GBs and textures. Our findings provide a theoretical basis for improvements in stress relaxation resistance of the Cu-based alloys used in connector industry. [ABSTRACT FROM AUTHOR]

Motivated by the enormous potential of hydrogels in regenerative medicine, new biocompatible gelatin-based hybrid hydrogels were developed through a green process using poly(ethylene glycol) diglycidyl ether as a cross-linking agent, adding carrageenan and chitosan polysaccharides to the network to better mimic the hybrid composition of native extracellular matrix. Overall, the hydrogels show suitable structural stability, high porosity and pore interconnectivity, good swellability, and finally, biocompatibility. Their mechanical behavior, investigated by tensile and compression tests, appears to be characterized by nonlinear elasticity with high compliance values, fast stress-relaxation, and good strain reversibility with no sign of mechanical failure for compressive loading–unloading cycles at relatively high deformation levels of 50%. Degradation tests confirm the hydrogel bioresorbability by gradual hydrolysis, during which the structural integrity of both materials is maintained, while their mechanical behavior becomes more and more compliant. Human Umbilical Cord-derived Mesenchymal Stem Cells (hUC-MSCs) were used to test the hydrogels as potential carriers for cell delivery in tissue engineering. hUC-MSCs cultured inside the hydrogels show a homogenous distribution and maintain their growth and viability for at least 21 days of culture, with an increasing proliferation trend. Hence, this study contributes to a further understanding of the potential use of hybrid hydrogels and hUC-MSCs for a wide range of biomedical applications, particularly in soft tissue engineering. [ABSTRACT FROM AUTHOR]

Copper alloys used in connectors rely significantly on stress relaxation resistance as a key property. In this study, a heavily deformed Cu-Cr-Ag-Si alloy underwent aging at varying temperatures, with a subsequent analysis of its mechanical properties and microstructure, with a particular emphasis on understanding the mechanism of improving stress relaxation resistance. As the aging temperature rose, the Cr precipitated into a Cr-Si composite element precipitated phase. Both work hardening and precipitation strengthening played vital roles in enhancing the stress relaxation resistance of the Cu-Cr-Ag-Si alloy, with the latter exerting a more pronounced impact. The notable performance enhancement observed after aging at 450 °C can be attributed to the synergistic effects of work hardening and precipitation strengthening. Following aging at 450 °C, the alloy demonstrated optimal performance, boasting a tensile strength of 495.25 MPa, an electrical conductivity of 84.2% IACS, and a level of 91.12%. These exceptional properties position the alloy as a highly suitable material for connector contacts. [ABSTRACT FROM AUTHOR]

Despite the common use of the standard linear solid model (SLSM) in viscoelasticity, the physical significance as well as the difference between the Maxwell and Kelvin forms of SLSM are still not clear. This paper demonstrates that each parameter of those two models has its specific physical meaning, and introduces the relationships allowing the transformations between those parameters. Regardless of their physical significance, those two models are equivalent in terms of their mathematical properties. Hence, no matter which model is chosen, consistent analysis results can always be obtained as long as the physical meaning of each parameter is accurately interpreted. [ABSTRACT FROM AUTHOR]

The article considers assessment methods and criteria of damage inflicted to turbine blades under the effect of static loads in carrying out 3D structural analyses of modern foreign and domestically produced high-capacity power units. Factors that should be considered in performing strength and lifetime analyses of the rotor blades of high-capacity turbines when subjected to short- and long-term static loading are pointed out. The article also describes 3D techniques for carrying out elastoplastic assessment of short-term static strength using a procedure for determining the limit rotation speed to blade fracture, airfoil residual displacements and strains, shank ultimate strength and displacement, root tearing-off, shear, flexural strength, etc. The article presents mutually complementary techniques for determining the bearing capacity as well as global and local long-term strength with using cumulative strain predictions by creep curves. Criteria used in different lifetime assessment methods are described, including those applied at different design stages and in using thermal protection coatings. Cases are considered in which creep strains are determined in the absence of data on creep curves by carrying out elastoplastic analyses by isochronous curves and lifetime analysis using the Larson–Miller curves. The need to take multiaxiality into account in estimating local creep in places of stress concentration is shown, and the applicability limits and criteria of such assessment that make it possible to increase the predicted lifetime by up to two times are described. Examples of tensile and compressive stress relaxation in estimating cumulative creep strain are given. Matters of creep interaction with other types of damage, including high-cycle and low-cycle (thermal cycling) fatigue, and various turbine loading kinds are considered. [ABSTRACT FROM AUTHOR]

In this research, the phenomenon of stress relaxation in unreinforced and reinforced sandy soil with geotextile layer has been studied using a large-scale direct shear test. To investigate the effect of shear speed and soil density on the amount of resistance loss due to stress relaxation, two shear speeds of 0.5 and 5 mm/min and two relative densities of 35% and 60%, respectively, loose and medium density, have been used. All samples have been tested in direct shear test under vertical stress of 100 kPa. The results show that the amount of resistance loss due to stress relaxation depends on the shear stress level, soil density, presence of a geotextile layer in the soil and shear speed. In such a way that by increasing the level of shear stress and shear speed and by decreasing the density of sand and also with the presence of a geotextile layer in soil, the amount of resistance loss due to stress relaxation increases. [ABSTRACT FROM AUTHOR]

Aiming at solving the failure problems of a wire cable caused by stress relaxation, the stress relaxation behavior of a wire cable under axial tensile load is investigated based on the elastic theory and creep theory. The Poisson's ratio effect is considered in the multilayer contact analysis between the neighboring wires of the rope. A finite element model for the axial mechanical property and stress relaxation performance between the wires of the cable is established. The model is verified and validated by comparison with published data. The results show that the stress relaxation leads to low contact stress which leads to reduced rate of relaxation in the wire cable. With the increase of the lay angles of the spiral wires, the contact pressures between different layers of the cable increase, while the stress relaxation behavior reduces the effect to a certain extent. Compared with the lay angle of the intermediate layer, the lay angle of the outer layer has more pronounced effect on the stress relaxation of the rope. The decreasing of the elastic modulus and increasing of the Poisson's ratio of the wire material help to reduce the stress relaxation and creep strain of the wire cable. [ABSTRACT FROM AUTHOR]

Bottlebrush polymer surfactants (BPSs), formed by the interfacial interactions between bottlebrush polymers (BPs) with poly(acrylic acid) side chains dissolved in an aqueous phase and amine-functionalized ligands dissolved in the oil phase, assemble and bind strongly to the fluid-fluid interface. The ratio between NBB (backbone degree of polymerization) and NSC (side chain degree of polymerization) defines the initial assembly kinetics, interface packing efficiency, and stress relaxation. The equilibrium interfacial tension (γ) increases when NBB < NSC, but decreases when NBB ≫ NSC, correlating to a pronounced change in the effective shape of the BPs from being spherical to worm-like structures. The apparent surface coverage (ASC), i.e., the interfacial packing efficiency, decreases as NBB increases. The dripping-to-jetting transition of an injected polymer solution, as well as fluorescence recovery after photobleaching experiments, revealed faster initial assembly kinetics for BPs with higher NBB. Euler buckling of BPS assemblies with different NBB values was used to characterize the stress relaxation behavior and bending modulus. The stress relaxation behavior was directly related to the ASC, reflecting the strong influence of macromolecular shape on packing efficiency. The bending modulus of BPSs decreases for NBB < NSC, but increased when NBB ≫ NSC, showing the effect of molecular architecture and multisite anchoring. All-liquid printed constructs with lower NBB BPs yielded more stable structured liquids, underscoring the importance of macromolecular packing efficiency at fluid interfaces. Overall, this work elucidates fundamental relationships between nanoscopic structures and macroscopic properties associated with various bottlebrush polymer architectures, which translate to the stabilization of all-fluidic printed constructs.

Iron-based shape memory alloys (Fe-SMAs) are e-merging materials with extensive application in civil structures owing to their unique properties, including the shape memory effect. However, it is crucial to understand the time dependent behavior of Fe-SMAs for their effective application as pre-stressing element. In particular, behavior at individual stress, the underlying mechanism, and the transformation kinetics have not been investigated yet. To address these important fundamental research gaps, in-situ compression creep and stress relaxation experiments with high-energy X-ray diffraction (HEXRD) of a Fe-17Mn-5Si-10Cr-4Ni-1(V,C) Fe-SMAs were conducted. The time-dependent behavior of the Fe-SMA was investigated at different stress levels with respect to the yield strength (YS) at room temperature. The experimental result showed that the material exhibits a creep strain of up to 1.84 % and 56 MPa relaxed stress at test stress of 769 MPa (1.6 σYS) within one hour of holding. Stacking fault probability and phase volume fraction quantification provide an understanding of the mechanisms based on different stress levels. The transformation kinetics traced from the characteristics of HEXRD peaks offer further insights on creep depending on the contribution of {hkl} families. The paper concludes with an evaluation of the existing models for predicting creep and stress relaxation of Fe-SMA.

Abstract We present an analysis of magnitude clustering of microfractures inferred from acoustic emissions (AEs) during stick‐slip (SS) dynamics of faulted Westerly granite samples in frictional sliding experiments, with and without fluids, under triaxial loading with constant displacement rate. We investigate magnitude clustering in time across periods during, preceding and after macroscopic slip events on laboratory faults. Our findings reveal that magnitude clustering exists such that subsequent AEs tend to have more similar magnitudes than expected. Yet, this clustering only exists during macroscopic slip events and is strongest during major slip events in fluid‐saturated and dry samples. We demonstrate that robust magnitude clustering arises from variations in frequency‐magnitude distributions of AE events during macroscopic slip events. These temporal variations indicate a prevalence of larger AE events right after (0.3–3 s) the SS onset. Hence, magnitude clustering is a consequence of non‐stationarities.

This study investigates the stress relaxation behavior and associated microstructure evolution in laser melted deposited Ti–6Al–4V alloy under various temperature and pre-strain conditions for the first time. A set of stress relaxation and tensile tests has been performed to characterize the stress and strength evolution properties of the as-deposited alloy, and microstructural analysis has also carried out for mechanisms identification. It is found that the as-deposited alloy exhibits a lower threshold stress (less than 10 MPa), faster relaxation rate, and a more significant decrease in strength (9.9%∼15.8%) when compared with the same testing conditions of conventionally extruded alloy. Based on the microstructural evolutions, as well as the stress exponent analysis, the stress relaxation mechanisms have been identified: at low pre-strain and temperature (700 °C and 0.5%), diffusion creep plays the dominant role, while with the increasing pre-strains and temperatures, the mechanism transits to dislocation climb, and eventually to grain boundary sliding, at 750 °C and 10%. It is observed that the fine lamellae in the as-deposited alloy are susceptible to morphological changes during relaxation, including dislocation rearrangement, lamellae spheroidization and coarsening. These microstructural evolutions lead to the creep mechanism transitions, as well as the noticeable reduction in yield strength after relaxation. The results could provide insights into designing appropriate stress relaxation parameters for deposited alloys in hybrid additive manufacturing processes.

In-situ stress is a core factor controlling dynamic gas disasters in coal mines. Existing in-situ stress models are primarily based on the elastic properties of coal-rock formations and are not suitable for viscoelastic coal media. There is a need to establish an in-situ stress distribution model suitable for viscoelastic coal properties. Coal seams, especially tectonic coal, exhibit significant creep and stress relaxation characteristics. In this study, a Fractional Maxwell stress relaxation mathematical model was developed for viscoelastic coal reservoirs. The stress component of the extended Eaton model was improved to account for the stress relaxation in viscoelastic coal seams and an in-situ stress distribution model was established for viscoelastic coal seams. The results indicate that over geological time scales, viscoelasticity significantly affects the differential stress in coal seams. The degree of coal body creep and stress relaxation increases with the increase in viscosity coefficient and fractional order factor. As the elastic modulus decreases, the Poisson’s ratio, tectonic ratio, and gas pressure increase. The stress characteristics of coal seams exhibit a trend of hydrostatic pressure state distribution. Among them, the elastic modulus and Poisson's ratio contribute significantly to the difference between vertical principal stress and horizontal stress. The tectonic ratio contributes significantly to the difference between horizontal stresses. Gas pressure contributes equally to all three principal stresses. The stress relaxation effect can alter the relative magnitudes of maximum and minimum horizontal principal stresses and reduce stress anisotropy. Based on the critical stress model, a reduction in friction coefficient and an increase in gas pressure were similarly observed in coal seams under different in-situ stress conditions, leading to an approach towards hydrostatic pressure distribution. The mechanical properties of tectonic coal are more conducive to approaching hydrostatic pressure states over geological time scales. The application of the in-situ stress model established in this study validated the field measurements and provided new insights into gas enrichment and permeability reduction in tectonic coal.

Residual stress is an important surface integrity index to evaluate the crack initiation and failure of die surface. The efficient prediction of cutting residual stress can guide the high-quality machining of die and improve its service life. The existing cutting residual stress prediction models are complex, time-consuming and inefficient. In this paper, a hybrid prediction method of cutting residual stress based on finite element-analytical-neural network is proposed. Firstly, the stress, strain and temperature of the cutting surface are obtained based on the orthogonal cutting finite element model. Then, the stress relaxation analytical algorithm considering the elastic-plastic state of the material is used to replace the stress release process of the finite element, and the residual stress distribution data were obtained based on the joint model of finite element and analytical algorithm. Secondly, the surrogate model of BP neural network (SSA-BP) is improved based on SSA algorithm to realize the rapid prediction of characteristic value of residual stress. The effectiveness of the finite element-analytical-neural network hybrid model was verified by the cutting residual stress test of H13 steel. Finally, the effects of tool structure parameters and cutting parameters on the residual stress distribution and the maximum compressive stress and maximum tensile stress of H13 steel were studied. This method can provide a flexible and efficient basic model for obtaining the optimal cutting conditions for controlling the residual stress of H13 steel and other metals.