Stress Relaxation Properties Research Articles

Introducing Dynamicity: Engineering Stress Relaxation Into Hydrogels Via Thiol-Ene Modified Alginate for Mechanobiological in vitro Modeling of the Cornea.

Developing biomaterials for corneal repair and regeneration is crucial for maintaining clear vision. The cornea, a specialized tissue, relies on corneal keratocytes, that respond to their mechanical environment. Altering stiffness affects keratocyte behavior, but static stiffness alone cannot capture the dynamic properties of in vivo tissue. This study proposes that the cornea exhibits time-dependent mechanical properties, similar to other tissues, and aims to replicate these properties in potential therapeutic matrices. First, the cornea's stress relaxation properties are investigated using nanoindentation, revealing 15% relaxation within 10 seconds. Hydrogel dynamicity is then modulated using a specially formulated alginate-PEG and alginate-norbornene mixture. The tuning of the hydrogel's dynamicity is achieved through a photoinitiated norbornene-norbornene dimerization reaction, resulting in relaxation times ranging from 30 seconds to 10 minutes. Human primary corneal keratocytes are cultured on these hydrogels, demonstrating reduced αSMA (alpha smooth muscle actin) expression and increased filopodia formation on slower relaxing hydrogels, resembling their native phenotype. This in vitro model can enable the optimization of stress relaxation for various cell types, including corneal keratocytes, to control tissue formation. Combining stress relaxation optimization with stiffness assessment provides a more accurate tool for studying cell behavior and reduces mechanical mismatch with native tissues in implanted constructs.

Fractional derivative modeling for characterizing stress relaxation properties of Hami melon during drying.

The viscoelastic properties of food materials will change significantly while drying is in progress, which greatly influences the food deformation caused by drying. This study aims to predict the viscoelastic mechanical behavior of Hami melon during drying using the fractional derivative model. To characterize the relaxation characteristics, based on the finite difference method, an improved Grünwald-Letnikov fractional stress relaxation model is proposed to derive an approximate discrete numerical solution of the relaxation modulus by applying the time fractional calculus. The Laplace transform method is used to verify the obtained results, and the equivalence of the two methods is proved. In addition, the stress relaxation tests prove that the fractional derivative model has a better prediction effect on the stress relaxation behavior of viscoelastic food than classical Zener model. The significant correlations between the fractional order and the stiffness coefficient and the moisture content is also studied. Which is negative correlation and positive correlation respectively.

Stress-relaxation properties of French fries as a function of moisture content, oil content, frying temperature, and testing temperature.

There is insufficient information on the effect of temperature, moisture content (MC), and oil concentration on the mechanical properties of fried potatoes. The static mechanical testing capabilities of a dynamic mechanical analyzer (DMA) were utilized to measure the stress relaxation function of fried potatoes as a function of frying time, temperature, and testing temperature. The French fries were prepared in oil heated at 177°C, 184°C, and 191°C, and their MC and oil content (OC) values were measured as a function of frying time. The samples loaded in DMA were subjected to 2% strain, and the stress relaxation was performed for 600 s. The DMA furnace temperature was set at 37°C, 60°C, and 90°C. A two-element generalized Maxwell model was fitted into the DMA data. French Fries lost more than 70% of their moisture and absorbed more than 80% of their final OC during the first 180 s of frying. During the same interval (up to 180 s), the relaxation modulus parameters (G0 , G1 , and G2 ) did not change significantly and then increased linearly. These parameters also did not change considerably for MC above 2 g/g solids. At lower MC values, G1 and G2 increased as the MC was reduced, but the elastic modulus G0 stayed almost constant, independent of the MC.

Readily Recyclable, Auto-Catalyzed Tung Oil-Derived Vitrimers and Carbon Fiber-Reinforced Composites

The high strength and durability of carbon fiber-reinforced composites (CFRCs) have made them a popular choice in aerospace, civil engineering, and sports equipment. However, the non-degradable resin matrix and non-destructive recycling of carbon fibers present a challenge for their sustainability. To address this issue, we have developed a new type of CFRCs that are readily recyclable and high-performing, using an auto-catalyzed tung oil-derived vitrimer and acid curing agents (TTMA) in combination with bisphenol A epoxy resin (DGEBA) or 4,4′-methylene bis (N,N-di glycidylaniline) (TGDOM). The DGEBA/TGDOM-TTMA networks contain abundant ester bonds, hydroxyl groups, and tertiary amine, which can undergo dynamic transesterification reactions (DTER) at high temperatures, leading to topological rearrangement of cross-linked networks. This results in an epoxy resin matrix with excellent mechanical properties, thermostability, and easy reprocessing, self-healing, and degradation at elevated temperatures. Moreover, the TGDOM-TTMA exhibits good stress relaxation properties due to the high concentrations of hydroxyl and tertiary amine. Additionally, we have found that carbon fibers can be completely recycled from CFRCs using ethanolamine via amidation reaction, with the recycled carbon fibers maintaining nearly 100% of the mechanical properties of the virgin samples. This study offers a sustainable and convenient strategy for designing readily recyclable and high-performance CFRCs, with important implications for the future of composite materials.

Mathematical modelling on peristaltic motion and temperature distribution of dusty Jeffrey fluid under the influence of variable viscosity

ABSTRACT This study describes the behavior of peristaltic motion and temperature distribution of dusty Jeffrey fluid with variable viscosity. The flow-anchored equations are computed by assuming a low Reynolds number and long wavelength deliberations. The solution of the mathematical framework is heeded analytically by using Bernoulli’s rule of integration technique for velocity and temperature distribution, volume flow rate, and pressure rise. Pumping characteristics are studied based on different pressure ranges. The dynamics of various coherent parameters are deliberated through graphical elucidation. In fluid transmission, an interesting phenomenon is trapping in which a bolus is transported with the speed of the wave and this trapped bolus is pushed forward along the peristaltic wave. This trapping mechanism is visualized with the aid of contour plots and reveals that the size of the trapped bolus shows opposite behavior in both cases by increasing the viscosity parameter. The current inquiry is also designated for Newtonian fluid as a special case. The novelty of this work is to explain the Jeffrey fluid model which is capable of describing stress relaxation properties that normally lag in viscous fluids, thus this article clearly demonstrated the importance of choosing variable viscosity according to the requirement of the concerned fields. The outcome of this investigation can as well aid in understanding the transport phenomena in the human physiological system.

Engineering viscoelastic alginate hydrogels for hiPSC cardiomyocyte culture.

Heart disease is the leading cause of death worldwide, however research progress of heritable cardiomyopathies has been limited by the lack of cost effective and accurate human models. Human induced pluripotent stem cells (hiPSCs) are a renewable cell source that can be differentiated into human cardiomyocytes (CMs), presenting a reliable tool for in vitro cellular models. The physiological environmental cues that CMs are exposed to are extraordinarily complex and in vitro systems typically fail to mimic key aspects such as viscoelasticity and adhesion ligand type. Viscoelasticity in particular is important to model as it is exhibited by virtually all tissues in the body and can influence several cell functions including spreading, differentiation and proliferation. However, the majority of experiments with CMs to date have used covalently crosslinked hydrogels that do not exhibit any stress relaxing properties. Here, we engineered biomaterials to recapitulate these viscoelastic heart tissue characteristics. Since the stress relaxation properties of heart tissue have not been directly measured, we first mechanically characterized left ventricular porcine tissue. The elastic modulus was around 7 kPa, within range of reported values. However, the tissue exhibited much faster stress relaxation values than we anticipated; relaxing to half its max value in less than 10 s while skeletal muscle tissue is reported to relax closer to 100 s. We synthesized an alginate hydrogel with conjugated PEG chains to mimic these fast-relaxing properties while maintaining similar elastic modulus values. We then seeded CMs on the hydrogels, imaged and analyzed the structure and function via confocal microscopy to assess cell adherence and morphology. These studies have validated our engineered hydrogel system as a hiPSC-CM culture platform and model system to investigate how viscoelasticity influences iPSC-CM function in wild-type and disease cell lines.

Fractional Order Viscoelastic Model for Stress Relaxation of Polyvinyl Chloride Geomembranes

Stress relaxation properties have a significant impact on the performance of polyvinyl chloride (PVC) geomembranes (GMBs) at the peripheral joints of the membrane faced rockfill dam (MFRD). This paper presents a fractional order viscoelastic model (FOVM) to measure the relaxation stress as a function of time. Model parameters were obtained by best fit to results from wide-width strip tensile tests conducted at three tensile rates and three initial strains for 48 h. The results of a 90 d stress relaxation test demonstrate the applicability of the model to describe the stress relaxation behavior of PVC GMBs. The tensile rate and initial strain marginally influenced the relaxation modulus rate, while having no effects on the fractional derivative order. Residual stress could account for the difference in relaxation stress between the longitudinal and transverse specimens. Finally, the FOVM could be used for predicting the service cycle under specifying failure stress criteria. Furthermore, it has great potential for applications in predicting the long-term deformation of PVC GMBs at the peripheral joints of MFRD. Furthermore, it has great potential for applications in predicting the long-term deformation of PVC GMBs at the peripheral joints of MFRD.

Stress relaxation behavior of 3D printed silicone rubber foams with different topologies under uniaxial compressive load

3D printed elastomeric foams present good potential for mechanical application and their stress relaxation behaviors are critical for engineering performance. In this work, polydimethylsiloxane (PDMS) foams with simple tetragonal (ST) and face-centered tetragonal (FCT) structures were printed and present different stress relaxation behaviors. The ST foam had a similar stress relaxation rate to that of the stochastic foam (15–16%), whereas the FCT foam had a lower stress relaxation rate (10%). The ST foam had high local stress concentrations under compression, because it deformed by uniaxial buckling deformation, leading to high stress relaxation of the PDMS matrix. In contrast, the stress distribution was more uniform in the FCT foam due to its bending deformation under compression. Thus, the compressive deformation mechanism of the printed topology greatly affects the stress relaxation properties. This finding provides an important guide for the design of elastomeric foams with low stress relaxation for high performance applications.

Reprocessable and degradable bio-based polyurethane by molecular design engineering with extraordinary mechanical properties for recycling carbon fiber

In recent years, bio-based covalent adaptable networks (CANs) and vitrimers have developed rapidly in order to meet the renewable and sustainable requirements. It should be emphasized that the performance of the network need be upgraded and optimized to substitute petroleum-based polymers. Herein, a series of high-performance bio-based polyurethane were synthesized containing castor oil, isosorbide, vanillin derivative and 4,4′-Dicyclohexylmethane diisocyanate. By fine-tuning the bio-based diols, the hydrogen bond distribution and the crosslink density of the network were adjustable. The thermal mechanical properties and stress relaxation properties of the bio-based polyurethane were optimized. The bio-based polyurethane CANs shows tensile strength of 38.1 MPa, elongation at break of 243% and toughness of 55.7 MJ m −3 , which is outstanding among most reported bio-based CANs. In addition, the polymer has excellent water resistance, shape memory and re-deformability. Due to the dissociation of imine bonds under mild acid conditions, the carbon fiber composite can readily degrade in mixed solvent for 3 h at 60 °C and the properties of recycled carbon fibers are highly preserved. Fabrication of extraordinary mechanical properties, reprocessably, and degradable polyurethanes through network structure design. • Nonplanar rigid ring of isosorbide brings extraordinary mechanical properties to bio-based polyurethane CANs. • Adjusting the ratio of diols can tune the degree of cross-linking and distribution of hydrogen bonds in the networks. • Isosorbide can also improve fluidity of the network and lower activation energy. • CF composites can be chemically degraded under mild conditions for non-destructive recycling of CF.

Fabrication and Evaluation of Gellan Gum/Hyaluronic Acid Hydrogel for Retinal Tissue Engineering Biomaterial and the Influence of Substrate Stress Relaxation on Retinal Pigment Epithelial Cells.

Cell therapies for age-related macular degeneration (AMD) treatment have been developed by integrating hydrogel-based biomaterials. Until now, cell activity has been observed only in terms of the modulus of the hydrogel. In addition, cell behavior has only been observed in the 2D environment of the hydrogel and the 3D matrix. As time-dependent stress relaxation is considered a significant mechanical cue for the control of cellular activities, it is important to optimize hydrogels for retinal tissue engineering (TE) by applying this viewpoint. Herein, a gellan Gum (GG)/Hyaluronic acid (HA) hydrogel was fabricated using a facile physical crosslinking method. The physicochemical and mechanical properties were controlled by forming a different composition of GG and HA. The characterization was performed by conducting a mass swelling study, a sol fraction study, a weight loss test, a viscosity test, an injection force study, a compression test, and a stress relaxation analysis. The biological activity of the cells encapsulated in 3D constructs was evaluated by conducting a morphological study, a proliferation test, a live/dead analysis, histology, immunofluorescence staining, and a gene expression study to determine the most appropriate material for retinal TE biomaterial. Hydrogels with moderate amounts of HA showed improved physicochemical and mechanical properties suitable for injection into the retina. Moreover, the time-dependent stress relaxation property of the GG/HA hydrogel was enhanced when the appropriate amount of HA was loaded. In addition, the cellular compatibility of the GG/HA hydrogel in in vitro experiments was significantly improved in the fast-relaxing hydrogel. Overall, these results demonstrate the remarkable potential of GG/HA hydrogel as an injectable hydrogel for retinal TE and the importance of the stress relaxation property when designing retinal TE hydrogels. Therefore, we believe that GG/HA hydrogel is a prospective candidate for retinal TE biomaterial.

Stress Relaxation Properties of Five Orthodontic Aligner Materials: A 14-Day In-Vitro Study.

We aimed to investigate the stress relaxation properties of five different thermoplastic aligner materials subjected to 14 days of constant deflection. Five different thermoplastic aligner materials were selected, whose elastic properties varied: F22 Evoflex, F22 Aligner, Durasoft, Erkoloc-Pro and Duran. The static properties of these materials—in particular, stiffness, stress–strain curve and yield stress—were measured with a three-point bending test. For all the tests that were performed, a minimum of three samples per material were tested. The yield load, yield strength, deformation and particularly the stiffness of each material were found to be similar in the single-layer samples, while the double-layer samples showed far lower stiffness values and were similar one to another. F22 Evoflex and Erkoloc-Pro maintained the highest percentages of stress, 39.2% and 36.9%, respectively, during the 15-day period. Duran and Durasoft obtained the lowest final stress values, 0.5 MPa and 0.4 MPa, respectively, and the lowest percentage of normalized stress, 4.6% and 3.9%, respectively, during the 15-day period. All the materials that we tested showed a rapidity of stress decay during the first few hours of application, before reaching a plateau phase. The F22 Evoflex material showed the greatest level of final stress, with relatively constant stress release during the entire 15-day period. Further research after in vivo aging is necessary in order to study the real aligners’ behavior during orthodontic treatment.

An Experimental Study on Stress Relaxation of Yunnan Sandstone

Abstract This study presents the results from five relaxation tests conducted under uniaxial compression at room temperature. Test specimens were from sandstone formations at a mine, in Yunnan, and exhibited brittle failure. The test apparatus was valid for conducting stress relaxation experiments on sandstones by theoretical model and deformation measurements. Multiple load levels were applied in steps to each specimen, with each displacement level maintained for 12 d or longer. As soon as axial stress was decreased, it stabilized at an almost constant value after 12–24 h. Three distinct relaxation phases were observed to exist at all displacement levels except the specimen 3 in all tests. Based on the LM-NLSF method, model parameters under different strain levels were obtained. Comparison of test curves with theoretical curves showed that the Burgers model, which considers damage, was suitable for describing the stress relaxation properties of sandstones and the obtained parameters had a certain practical value.

Effect of trace alloying elements on the stress relaxation properties of high strength Cu–Ti alloys

Modern electronic components put forward higher requirements for copper-based alloys, hoping to have high elastic properties and stress relaxation resistance. The addition of Fe increased the dislocation density, tensile strength, and the number of deformed bands after cold rolling. After cold rolled by 94% and aged at 450 °C for 5 min, the hardness, tensile strength, and elongation of the CuTiCrMg–Fe alloy was 448 HV, 1610 MPa and 4.75%, respectively. Adding Fe can improve the stress relaxation resistance of samples, and the stress relaxation rate of the CuTiCrMg–Fe alloy was 14.7% after loading at 200 °C for 100 h. The stress relaxation process of Cu–Ti alloys is mainly divided into the dislocation's reaction stage and dislocations and precipitates' interaction stage. These findings have important implications for the development of Cu–Ti alloys with high strength and high-stress relaxation resistance.

Directional dependence on concomitant pressure and volume increases during left ventricular filling

Myocardial infarction continues to be a leading cause of mortality and morbidity globally. A major challenge post-myocardial infarction is scar tissue growth, which eventually can lead to heart failure. Cardiovascular regenerative strategies to minimize scar tissue growth and promote cardiac tissue formation are currently being actively pursued via the development of cardiac patches. However, the patch must have viscoelastic properties that mimic healthy cardiac tissues to facilitate proper cardiac patch-to-cell communications. To this end, we investigated the tissue microstructure and the stress relaxation properties of the porcine left ventricle (LV) along its long and short axes using a nanoindentation technique. We found significantly higher collagen density along the long axis than the short axis (p < 0.05). We then identified a much more rapid stress relaxation along the porcine LV's short axis compared to its long axis during the diastolic filling timeframe. Therefore, these findings show that concomitant LV pressure and volume increases from blood filling during diastole are directional dependent, with its short axis responsible for increase in LV volume and the long axis responsible for increase in LV pressure. These directional-dependent stress relaxation properties are essential in the design of structurally, bio-mimetic cardiac patches to support cardiac function and regeneration.

Gradual Stress-Relaxation of Hydrogel Regulates Cell Spreading.

There is growing evidence that the mechanical properties of extracellular matrices (ECMs), including elasticity and stress-relaxation, greatly influence the function and form of the residing cells. However, the effects of elasticity and stress-relaxation are often correlated, making the study of the effect of stress-relaxation on cellular behaviors difficult. Here, we designed a hybrid network hydrogel with a controllable stress-relaxation gradient and a constant elasticity. The hydrogel is crosslinked by covalent bonds and dynamic peptide-metal ion coordination interactions. The stress-relaxation gradient is controlled by spatially controlling the coordination and covalent crosslinker ratios. The different parts of the hydrogel exhibit distinct stress-relaxation amplitudes but the have same stress-relaxation timescale. Based on this hydrogel, we investigate the influence of hydrogel stress-relaxation on cell spreading. Our results show that the spreading of cells is suppressed at an increasing stress-relaxation amplitude with a fixed elasticity and stress-relaxation timescale. Our study provides a universal route to tune the stress-relaxation of hydrogels without changing their components and elasticity, which may be valuable for systematic investigations of the stress-relaxation gradient in cell cultures and organoid constructions.

Investigation on the properties of epoxy asphalt mixture containing crumb rubber for bridge expansion joint

The purpose of this research is to propose epoxy asphalt materials for bridge expansion joints. Epoxy asphalt was modified by adding 0%, 20%, 30% and, 40% crumb rubber (CR). The viscosity, tensile properties, adhesion, high-temperature stability, moisture resistance, low-temperature, and stress relaxation properties of rubber-modified epoxy asphalt (REA) were investigated comparing with the commercial BJ200-red. Thermal stress of epoxy asphalt expansion joint was also simulated by the finite element analysis platform. The results indicated that with the increase of CR content, the viscosity and deformation of REA binder increase, while operation time, the tensile strength, and adhesive decreases. CR has a slight negative effect on the high-temperature performance of epoxy asphalt mixture, and has a positive effect on low temperature deformation. When the rubber powder content is 30%, epoxy asphalt mixture has excellent mechanical properties and its high-temperature stability is higher than BJ200 mixture, but the low-temperature performance of the two is equivalent. Thermal stress analysis shows that epoxy asphalt containing 30% CR cannot fracture above −15 ℃, so it can be recommended for bridge expansion joints.

Effect of silane acrylate on the surface properties, adhesive performance, and rheological behavior of acrylic pressure sensitive adhesives for flexible displays

In this study, novel silane acrylates, urethane silane acrylate (USA) and urethane silane linear acrylate (USLA), comprised of urethane and silane groups were designed and synthesized, and their effect on the surface properties, adhesion, and rheological behavior of acrylic PSA was evaluated. The introduction of the synthesized USA and USLA affected the chain arrangement, and at least 3 wt% of the silane acrylates was required to alter the surface properties (roughness and wettability). Compared to a commercial silane acrylate (SA), USA and USLA showed improved cohesion of the acrylic PSA aided by the urethane bond; however, the length of the terminal silane groups produced some variations. USA improved the cohesive properties of acrylic PSA due to its short chain, which further increased the peel strength of the acrylic PSA as compared to USLA (USA: 1363 ± 173 gf/25 mm, USLA: 1258 ± 132 gf//25 mm, based on 1 wt% content). However, at 3 wt% or more, excessive cohesion led to a decrease in the interaction with the substrate, thereby decreasing the peel strength. On the other hand, SA gradually increased the peel strength of the acrylic PSA, but cohesive failure occurred due to the insufficient increase in the cohesion. With respect to the flexibility of the acrylic PSAs, SA slightly improved the modulus of the high-temperature region aided by the silane group, and the recovery properties were increased from 10.8% to a maximum of 31% at 100% strain. In addition, the low cohesive properties resulted in very high stress relaxation properties (>99%). The effects of USA and USLA on flexibility were similar, but the high cohesive force further improved the elastic properties, resulting in better recovery than those imparted by SA (USA: 87 ∼ 89.4%, USLA: 56.6 ∼ 91.7%). Comparing the two urethane-based silane acrylates, the stress relaxation property of USA was lower than that of USLA, which is related to the cohesive property of the acrylic PSA. In addition, all of the synthesized acrylic PSAs showed excellent optical properties with transmittance >98.1% and haze <2.23%, indicating their potential for application in flexible displays.

Influence of high content crumb rubber and different preparation methods on properties of asphalt under different aging conditions: Chemical properties, rheological properties, and fatigue performance

Asphalt pavements are prone to aging in construction and during service, which can affect the properties of the asphalt. The aging behaviors of high content crumb rubber modified asphalt (HCRMA) were investigated in this paper. The binders were aged in the laboratory and the performances before and after aging were tested using the attenuated total reflection-Fourier transform infrared spectroscopy (ATR-FTIR) test, temperature sweep (TS) test, frequency sweep tests, multiple stress creep recovery (MSCR) test, zero shear viscosity (ZSV) test, bending beam rheometer (BBR) test, and linear amplitude sweep (LAS) test. The FTIR test result shows that as the aging progresses, the crumb rubber (CR) continues to undergo desulphurization and degradation reactions in the HCRMA, with the release of substances such as silica and S-C bonded material from the CR into the bituminous phase. The elasticity of HCRMA decreases and then increases as the degree of aging increases. Besides, the low temperature performance of HCRMA is controlled by the stiffness value, and the low temperature stress relaxation properties of HCRMA in the aging process are better than that of 20% content crumb rubber modified asphalt. Moreover, aging can reduce the integrity and fatigue resistance of HCRMA while increasing the compatibility of HCRMA.

Fabrication of highly elastic and optically transparent adhesive films for flexible displays using multifunctional photocrosslinker

As the potential form factor of flexible optoelectronic devices broadens, the strain recovery and stress relaxation performance of transparent adhesive materials should be improved to endure repetitive deformation. In this study, it was demonstrated that the introduction of a multifunctional photocrosslinker into acrylate adhesive resin meets the elastic properties of optically clear adhesive films. The more the crosslinking agents were embedded, the more the recovery and stress relaxation properties improved without light distortions. Compared with pristine adhesive film, the proposed adhesive films containing optimal concentration of photocrosslinker exhibited excellent relaxation and recovery characteristics as well as high optical transmittance.

Thermomechanical constitutive equations for glass and numerical simulation on automobile glass forming technology

To establish a comprehensive numerical model for automotive glass forming, firstly, this paper conducts material tests of 3·2 SG glass to obtain detailed material properties. Through thermal expansion experiments, the thermal expansion coefficients including glassy and liquid states are obtained; then by using three-point bending stress relaxation and differential scanning calorimetry experiments, the stress relaxation and structural relaxation properties of the glass are obtained. Finally, a comparison analysis of the simulation and the actual spherical deviation for an actual automobile glass product is carried out. The result shows that the product simulation and the actual product spherical deviation correspond. The maximum value of the spherical deviation is within 0·94 mm between simulation and the actual product spherical deviation. The accuracy can meet the design requirements, and the established numerical model is reliable.