Stress Relaxation Behavior Research Articles

Unexpected Slow Relaxation Dynamics in Pure Ring Polymers Arise from Intermolecular Interactions.

Ring polymers have fascinated scientists for decades, but experimental progress has been challenging due to the presence of linear chain contaminants that fundamentally alter dynamics. In this work, we report the unexpected slow stress relaxation behavior of concentrated ring polymers that arises due to ring-ring interactions and ring packing structure. Topologically pure, high molecular weight ring polymers are prepared without linear chain contaminants using cyclic poly(phthalaldehyde) (cPPA), a metastable polymer chemistry that rapidly depolymerizes from free ends at ambient temperatures. Linear viscoelastic measurements of highly concentrated cPPA show slow, non-power-law stress relaxation dynamics despite the lack of linear chain contaminants. Experiments are complemented by molecular dynamics (MD) simulations of unprecedentedly high molecular weight rings, which clearly show non-power-law stress relaxation in good agreement with experiments. MD simulations reveal substantial ring-ring interpenetrations upon increasing ring molecular weight or local backbone stiffness, despite the global collapsed nature of single ring conformation. A recently proposed microscopic theory for unconcatenated rings provides a qualitative physical mechanism associated with the emergence of strong inter-ring caging which slows down center-of-mass diffusion and long wavelength intramolecular relaxation modes originating from ring-ring interpenetrations, governed by the onset variable N/ND, where the crossover degree of polymerization ND is qualitatively predicted by theory. Our work overcomes challenges in achieving ring polymer purity and by characterizing dynamics for high molecular weight ring polymers. Overall, these results provide a new understanding of ring polymer physics.

Comparison of the stress relaxation and creep behavior of conventionally forged and additively manufactured René 65

Comparison of the stress relaxation and creep behavior of conventionally forged and additively manufactured René 65

Mechanical behavior of adhesively bonded butt‐joints under cyclic loading: Experiments and constitutive model

AbstractThe cyclic loading experiments on HP‐172B sealant bonded tubular butt‐joints were carried out with the strain‐controlled mode, and the effects of strain amplitude and mean strain on the cyclic relaxation stress and cyclic softening variable of the bonded butt‐joint were investigated. The experimental results show that the stress–strain response curve of the butt‐joint presents a corrugated shape under cyclic loading. The cyclic stress relaxation and cyclic softening of the bonded butt‐joint occur. The increase of mean strain and strain amplitude intensifies the cyclic stress relaxation and cyclic softening behaviors. According to the experimental observation, a nonlinear viscoelastic model is proposed to predict the mechanical behavior of butt‐joints under cyclic loading, which considers the cyclic stress relaxation and cyclic softening of the bonded butt‐joint. The comparisons between the model calculation and the experimental data show that the calculated stress–strain curve is in good agreement with the experimental data.

Stress relaxation behavior and its effect on the mechanical performance of a wire cable

Stress relaxation behavior and its effect on the mechanical performance of a wire cable

A Rheological Model with Integer and Non-Integer Orders Nonlinearities for Predicting Stress Relaxation Behavior in Viscoelastic Materials

Viscoelastic materials are widely used as devices for vibration control in modern engineering applications. They exhibit both viscous and elastic characteristic when undergoing deformation. They are mainly characterized by three time-dependent mechanical properties such as creep, stress relaxation and hysteresis. Among them, stress relaxation is one of the most important features in the characterization of viscoelastic materials. This phenomenon is defined as a time-dependent decrease in stress under a constant strain. Due to the inherent nonlinearity shown by the material response over a certain range of strain when viscoelastic materials are subjected to external loads, nonlinear rheological models are needed to better describe the experimental data. In this study, a singlenonlinear differential constitutive equation is derived froma nonlinear rheological model composed of a generalized nonlinear Maxwell fluid model in parallel with a nonlinear spring obeying a power law for the prediction of the stress relaxation behavior in viscoelastic materials. Under a constant strain-history, the time-dependent stress is analytically derived in the cases where the positive power law exponent, α ˂ 1 and α ˃1. The Trust Region Method available in MATLAB Optimization Toolbox is used to identify the material parameters. Significant correlations are found between the experimental relaxation data taken from literature and exact analytical predictions. The obtained results show that the developed rheological model with integer and non-integer orders nonlinearities accurately describes the experimental relaxation data of some viscoelastic materials.

Experimental and constitutive analyses of the stress relaxation behavior of glassy polymers

AbstractThe physical mechanism underlying the mechanical behavior of glassy polymers has been studied over decades but remains a long‐standing issue. A consensus view achieved is that the yield, flow, and stress relaxation behaviors are due to structural relaxation in the polymer mainly caused by chain conformation transitions. This is the key physical idea behind the many existing elastic–plastic constitutive models for glassy polymers. In this paper, such a constitutive model was employed for predicting and analyzing the stress relaxation of a glassy polymer. It is found that the model works well in predicting the pre‐yield stress relaxation but significantly underestimates the post‐yield stress relaxation. As considering the chain conformation transition alone leads to a dilemma for the model to concurrently represent the yield/flow and stress relaxation behaviors, the model was extended to incorporate an additional structural relaxation mechanism assumed to originate from the dissociation of weak linkages in the chain network. The extended model succeeds in concurrently representing the yield/flow, and stress relaxation behaviors in the whole deformation region, of which the reasons were analyzed. The knowledge revealed in this paper is instructive and may shed new light on understanding the structural relaxation and mechanical behavior of glassy polymers.

The influence of printing parameters on the mechanical properties of 3D printed TPU-based elastomers

Additive Manufacturing (AM) becomes more and more focus of studies in the scientific community. Nevertheless, elastomers in 3D printing are still a relatively understudied topic despite their extensive use in machine components. The further understanding of the technologies and knowledge acquirement are fundamental steps towards the improvement of the printing process and the broadening of feasible applications of 3D printed elastomers. This work focused on thermoplastic polyurethanes printed with Fused Filament Fabrication (FFF) and investigated the effect of infill deposition angle and contour lines on the tensile and the stress relaxation behaviour. Samples were printed in alternating as well as unidirectional infill orientations, the latter without and with outlines. Tensile tests revealed that alternating orientations of 0°–90° and 45°–135° have a similar behaviour and benefit the integrity of the part. The fully unidirectional orientation at 90° hindered the tensile strength due to the absence of outlines and consequent delamination. All comparative analyses displayed a low influence of the raster angle at lower strains. Stress relaxation results showed similar behaviour for samples with outlines, without a clear effect of the infill orientations. In summary, contour lines are essential and an alternating orientation is recommended for better part integrity.

Effect of Co/P micro-alloying on stress relaxation behaviour of Cu-15Ni-8Sn alloy

To improve the stress relaxation resistance of the Cu-5Ni-S8n alloy, the effects of Co or P addition on its stress relaxation resistance were investigated by scanning electron microscopy, transmission electron microscopy, electron backscatter diffraction), X-ray diffraction, and high-temperature creep durability tester. The results revealed that the relaxation rate increased dramatically as the temperature rose. The thermal stress relaxation resistance of Cu-15Ni-8Sn alloy was significantly improved with Co addition; After 100 h of stress relaxation tests at 200°C or 250°C, the residual stress value of the Cu-15Ni-8Sn alloy increased by 23 MPa or 22 MPa with Co addition, which was associated with the consumption of vacancies by Co atoms and the pinning effect of Co atoms on dislocation. However, at 250°C, the stress relaxation resistance of the Cu-15Ni-8Sn-0.2P alloy was lower than that of the Cu-15Ni-8Sn alloy. This difference was due to the inclusion of P, which promoted the recrystallization reaction. Article highlights Recrystallization and the decrease of dislocation density are the major reasons for the stress relaxation of the samples. The stress relaxation resistance of Cu-15Ni-8Sn alloy was significantly improved with Co addition. The addition of the P element is not conducive to the improvement of thermal stress relaxation resistance.

Tempering stress relaxation behavior and microstructure evolution of 300 M steel

Tempering stress relaxation behavior and microstructure evolution of 300 M steel

Stress Relaxation and Grain Growth Behaviors of (111)-Preferred Nanotwinned Copper during Annealing

Highly (111)-oriented nanotwinned Cu (nt-Cu) films were fabricated on silicon wafers for thermal-stress characterization. We tailored the microstructural features (grain scale and orientation) of the films by tuning the electroplating parameters. The films were heat-treated and the relaxation behaviors of thermal stresses in the films were explored using a bending beam system. Focused ion beam (FIB) and electron back-scattered diffraction (EBSD) were then employed to characterize the transformations of the microstructure, grain size, and orientation degree of the films. The results indicated that the degree of (111)-preferred orientation and grain size significantly decrease with increasing the current density. The nt-Cu films with a higher degree of (111)-preferred orientation and larger grains exhibit the slower rates of stress relaxation. The film with larger grains possesses a smaller grain boundary area; thus, the grain boundary diffusion for the thermal-stress release is suppressed. In addition, the induced tensile stress in the films with larger grains is smaller leading to the difference in microstructural changes under annealing.

A comparison on isothermal and thermomechanical fatigue behavior of 316LN stainless steel with various tension dwell time

Isothermal and thermomechanical fatigue tests with various tension dwell time (designated as creep-IF and creep-TMF respectively) were conducted. The cyclic stress response and stress relaxation behavior were qualitatively and quantitatively discussed based on TEM and EBSD characterization. The fatigue dominated transgranular crack initiation and propagation led to specimen fracture in all creep-IF tests. The transition of dominant damage mechanism from fatigue to creep-fatigue interaction occurred when dwell time increased to 1800 s in creep-TMF test, which caused the intergranular fracture mode and a much shorter life than creep-IF. Moreover, the life prediction model based on tension strain energy was proposed.

Influence of residual stress and its relaxation on the corrosion bending fatigue resistance of EA4T axle steel treated by ultrasonic surface rolling

The fatigue properties and residual stress relaxation behavior of EA4T axle steel treated by ultrasonic surface rolling process (USRP) are investigated in air and corrosive environments. The result reveals that the surface roughness (Ra) is decreased from 1.42 to 0.17 μm after the USRP. A surface layer with ultrafine grains, work hardening, and compressive residual stress (CRS) is achieved by the USRP. Compared with the as-machined specimens, the USRP specimens exhibit slightly lower electrochemical corrosion resistance but superior fatigue performance in air and corrosive environments. The dislocation density is significantly decreased in the corrosive environment during fatigue loads, accompanied by a rapid relaxation of surface CRS. The influence of USRP on the corrosion fatigue performance is elucidated. The residual stress relaxation mechanism and its influence on fatigue resistance are also analyzed.

Effect of Carbon Nanofibers on the Viscoelastic Response of Epoxy Resins.

Two epoxy resins with different viscosities were enhanced up to 1 wt.%, applying a simple method with carbon nanofibers (CNFs). These were characterized in terms of static bending stress, stress relaxation, and creep tests. In bending, the contents of 0.5 wt.% and 0.75 wt.% of CNFs on Ebalta and Sicomin epoxies, respectively, promote higher relative bending stress (above 11.5% for both) and elastic modulus (13.1% for Sicomin and 16.2% for Ebalta). This highest bending stress and modulus occurs for the lower viscosity resin (Ebalta) due to its interfacial strength and dispersibility of the fillers. Creep behaviour and stress relaxation for three stress levels (20, 50, and 80 MPa) show the benefits obtained with the addition of CNFs, which act as a network that contributes to the immobility of the polymer chains. A long-term experiment of up to 100 h was successfully applied to fit the Kohlrausch-Williams-Watts (KWW) and Findley models to stress relaxation and creep behaviour with very good accuracy.

Orchestrating Chemical and Physical Cross-Linking in Protein Hydrogels to Regulate Embryonic Stem Cell Growth.

Protein hydrogels are ideal candidates for next-generation biomaterials due to their genetically programmable properties. Herein, we report an entirely protein-based hydrogel as an artificial extracellular matrix (ECM) for regulating the embryonic stem cell growth. A synergy between chemical and physical cross-linking was achieved in one step by SpyTag/SpyCatcher reaction and P zipper association at 37 °C. The hydrogels' stress relaxation behaviors can be tuned across a broad spectrum by single-point mutation on a P zipper. It has been found that faster relaxation can promote the growth of HeLa tumor spheroids and embryonic stem cells, and mechanical regulation of embryonic stem cells occurs via retention of the cells at the G1 phase. The results highlight the promise of genetically encoded protein materials as a platform of artificial ECM for understanding and controlling the complex cell-matrix interactions in a 3D cell culture.

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.

Viscosity and structure relaxation in Zr60Cu20Al10Ni10 metallic glass

This work reports the estimation of the viscosity, relaxation time and free volume of Zr60Cu20Al10Ni10 metallic glass (MG) by extrapolating differential scanning calorimetry (DSC), thermo-mechanical analyzer (TMA), and nanoindentation measurement data. The structural relaxation process of Zr60Cu20Al10Ni10 MG is shown through the study of viscosity and relaxation behavior also displayed by these measurements. It has the ability to exhibit strong fluid behavior near the glass transition temperature and exhibits a large strength parameter of 26.9 and a Vogel-Fulcher-Tamman temperature of 378 K. At the same time its stress relaxation exhibits sensitivity to strain rate and paused load. Finally, we estimated the residual free volume from melt freezing inside the Zr60Cu20Al10Ni10 MG by DSC technique as 0.041. These results indicate that the Zr60Cu20Al10Ni10 MG has a large free volume and exhibits both strong fluidic properties and significant stress relaxation behavior.

Effect of annealing treatment on mechanical properties of 3D-Printed composites

This study aims to analyse the annealing effect on the mechanical properties of PETG, carbon fibre reinforced PETG (PETG + CF) and Kevlar fibre reinforced PETG (PETG + KF). Regardless of the material, the final dimensions vary with temperature and exposure time, showing that the addition of fibres had no significant influence. On the other hand, the presence of fibres affects the radius of curvature, but this parameter shows to be very dependent on the type of fibre. In terms of hardness, higher temperatures and exposure times generally lead to higher values, achieving improvements of about 20% over untreated material. Regarding the bending properties, while they increase with exposure time and temperature for composites, in the case of neat PETG they increase with temperature and decrease with exposure time. Compared to untreated specimens, the maximum bending strength observed after the annealing treatment was 10.2%, 31.8% and 11.1% for PETG, PETG + CF and PETG + KF, respectively, while the maximum bending modulus was 17.6%, 61.1% and 62.6% higher, respectively. Benefits were also observed in terms of impact strength, stress relaxation and creep behaviour of all materials. For example, after annealing, lower stress relaxation and creep displacement values were observed.

Flame-Retardant and Recyclable Soybean Oil-Based Thermosets Enabled by the Dynamic Phosphate Ester and Tannic Acid

The demands of safety and sustainability have driven the development of intrinsic flame-retardant biobased polymers from renewable materials. Herein, a mechanically robust, good flame-retardant, and recyclable thermoset was developed from renewable epoxidized soybean oil (ESO) by using 2-hydroxyethyl methacrylate phosphate (HEMAP) as the reactive flame retardant and tannic acid (TA) as the charring agent. The flame resistance of the obtained ESO-based thermoset achieved the highest UL-94 of V-0 rating and a limited oxygen index value of 26.7% due to the synergistic flame-retardant effect of phosphate and TA. The flame-retardant mechanisms of the gaseous phase and condensed phase were fully investigated by thermogravimetric infrared, scanning electron microscopy-energy-dispersive spectrometry, X-ray photoelectron spectroscopy, and Raman spectra. It is confirmed that the incorporation of phosphate and TA could effectively promote the formation of dense carbon layers and delay the pyrolysis of long aliphatic chains. The ternary crosslinking of ESO, HEMAP, and TA via free-radical polymerization and epoxy-ring opening reaction resulted in a rigid network with a high crosslink density, bestowing the thermoset with superior tensile strength (20.0 MPa), flexural strength (36.3 MPa), and bonding strength (16.7 MPa on steel). Moreover, the ESO-based thermoset exhibited a fast stress relaxation behavior due to the transesterification of dynamic β-hydroxyl phosphate esters, which enables the network with thermal-healing ability and recyclability. This study explores a feasible method to prepare an intrinsic flame-retardant polymer from commercially available and renewable vegetable oils and natural polyphenols.

Effect of the temperature and grain direction on the stress relaxation behavior of PCM-impregnated and nonimpregnated wood under tensile and bending stresses

Effect of the temperature and grain direction on the stress relaxation behavior of PCM-impregnated and nonimpregnated wood under tensile and bending stresses

Creep and stress relaxation behavior of functionalized microcrystalline cellulose/epoxy composites

Creep and stress relaxation behavior of functionalized microcrystalline cellulose/epoxy composites