Stress Relaxation Analysis Research Articles

Addition of external water improves the quality attributes of vacuum-frozen and thawed apple slices

Vacuum freezing is an innovative rapid freezing technique with various advantages and huge potential for diverse applications. This study aimed at investigating variations in the moisture, main quality, microstructure, and mechanical properties of vacuum-frozen and thawed apple slices resulting from the use of different amounts of external water added prior to freezing. Increased external water content (30%) contributed to lower frozen temperature and less water loss in vacuum-frozen apple samples, and higher drip loss, equivalent vitamin C content, lower relative electrical conductivity, lower soluble solids content and lower total color difference in thawed samples. Slices of thawed samples with more added external water before vacuum freezing displayed higher density of moisture distribution and reduced microstructure destruction. They also displayed higher elastomeric and less viscous behaviors, as assessed with stress-relaxation analysis, and increased hardness and chewiness and reduced cohesiveness, as assessed with texture profile analysis. The quality of thawed apple slices with 30% pre-added external water was closest to that of the fresh apples. The traditional contact plate freezing treatment produced the worst quality. Thus, vacuum freezing with 30% pre-added external water is recommended for the rapid freezing processing of apple slices.

A novel practical approach for monitoring the crosslink density of an ethylene propylene diene monomer compound: Complementary scanning acoustic microscopy and FIB-SEM-EDS analyses

Tuning of the crosslink density (CLD) in the rubber compounds is very crucial for optimizing the physical and mechanical properties of the ultimate rubber products. Conventionally, CLD can be measured via rheological methods such as moving die rheometer (MDR), via mechanical tests such as temperature scanning stress relaxation analysis (TSSR), or via direct swelling experiments using Flory–Rehner approach. In the current study, two novel techniques, focused ion beam - scanning electron microscopy (FIB-SEM) processing, with simultaneous energy dispersive X-ray spectrometry (EDS) mapping analysis and scanning acoustic microscopy (SAM) were combined and correlated to conventional methods on a model recipe of ethylene propylene diene monomer (EPDM) compound having different sulphur contents. Depending on the applied technique, the increase in the crosslink density with sulphur content was found to be 1.7 fold for the Flory–Rehner approach and 1.2 fold for both TSSR and MDR. It is directly monitored from the FIB-SEM-EDS analysis that the sulphur distribution and agglomeration behavior increased in line with ZnO content, which is an indirect indication of the rise in crosslink density. The impedance maps of the crosslinked samples obtained through SAM analysis revealed that the impedance of the samples increased with the increasing sulphur content, which can be attributed to higher level of crosslink density. A quantified correlation was obtained between SAM images and the crosslink density of the samples. It was shown that SAM is a promising tool for practical and non-destructive analysis for determining the formation of crosslink density of the rubbers.

Coupled effects of temperature and compressive strain on aging of silicone rubber foam

The coupled effects of temperature and compressive strain on aging behaviors and mechanisms of silicone rubber foam were investigated by conducting the accelerated aging tests. The silicone rubber foams subjected to various levels of temperature and compressive strain were characterized by Attenuated total reflection Fourier transform infrared (ATR-FTIR) spectroscopy, micro hardness, volume fraction, compression set, continuous stress relaxation, mechanical properties, and thermogravimetric analysis (TGA) etc. The ATR-FTIR results indicate that the complex oxidation reactions (crosslinking, chain scission, and oxidation of side chains) occurred simultaneously with the formation of oxidation products. The high temperature facilitated the increase in crosslinking density and enhanced the thermal stability of the foams, implying that the oxidation crosslinking predominated over chain scission leading to the formation of a denser network structure. The physical and mechanical properties of the foams underwent significant changes due to the oxidation reactions and the irreversible collapse of the porous microstructure. The elevated temperature and the compressive stress both have a direct effect on the degradation of the foam. The elevated temperature played a more important role in changes of the physical-mechanical properties and microstructure.

Analysis of residual stress relaxation of aluminum alloys EN AW 6061/‐82 T6 under cyclic loading

AbstractStress relaxation describes the reduction of stress under static or cyclic loading at a constant strain level. Several processes induce intentionally residual stresses, for example, autofrettage of thick‐walled pressurized tubes to improve their fatigue life. This well‐known process induces residual compressive stresses at the critical inner surface by using a single static but controlled overloading internal pressure. Relaxation of residual stresses due to cyclic loading in service would endanger the effectiveness of autofrettage and could finally lead to unexpected fatigue failure. In this study, strain‐controlled experiments up to 500,000 load cycles and amending nonlinear finite element simulations were done for the aluminum alloys EN AW 6061 T6 and EN AW 6082 T6 to study potential cyclic stress relaxation in four‐point bending tests after controlled single static plasticization for residual stress generation. This analysis identifies almost stable residual stresses for both materials under different cyclic strain‐controlled load levels.

The investigation of the stress cracking behavior of PBT by acoustic emission

AbstractDespite being a major cause of failure of polymer products under service, environmental stress cracking (ESC) is still a phenomenon poorly investigated and, as consequence, not fully understood. This article aims to contribute to this field in a study of the stress cracking behavior of polybutylene terephthalate (PBT) exposed to ethanol (a common fluid, used in several daily situations), by means of two types of mechanical tests (tensile and stress relaxation). During mechanical testing the technique of acoustic emission (AE) was applied to identify certain mechanisms of internal failure. As a non‐destructive technique, AE is widely used with metals to monitor the integrity of components but it is very scarcely applied to polymers. The article, therefore, innovates by using AE as a tool to better understand the ESC behavior of an engineering polymer produced under different conditions. The PBT samples were prepared by injection molding, using different mold temperatures (20°C, 40°C, and 80°C), resulting in different crystallinities. The results showed that the tensile properties were highly dependent on the crosshead speed and that ethanol affected the mechanical performance of the materials, anticipating the necking, but its evaporation during the experiment reduced the actual temperature on the sample bar. This latter effect caused an unexpected behavior, with an increase in tensile strength and reduction of stress relaxation rate, the opposite trends that would be expected from a stress cracking agent. The crystallinity of PBT had some influence on the results, with the more crystalline samples being more resistant to ESC, as observed by both stress relaxation and AE analyses. The technique of AE showed to be a very useful tool to understand the mechanical behavior and to differentiate the various testing conditions, with the detection and quantification of hits and released energy, related to the several stages of deformation and failure, including the formation of shear bands, nucleation and growth of crazes and cracks, and so forth. The fracture surface analyses by scanning electron microscopy were very consistent with the AE experiments, in which rough topography were related to high acoustic activities.

Analysis of stress relaxation of paper tube

Roll a long paper strip into a tight tube and put it vertically on a table. Why does it often unwind in jerks? What determines the period of the jerks? In the following work, we explain the fact of why the paper tubes unwind in jerks by researching mechanical structure of paper tube theoretically and analyze the data of experiments. Finding that the basic cause of paper tube motion in jerks is stress relaxation. Successfully prove our theory by our experiments. Then we investigate the influence of several physical quantities on period of jerks, such as humidity, friction factor of paper layers. All of these factors determine the period of jerks.

Predicting the Relaxation Modulus for the Study of the Delayed Behaviour of Kenaf Fibres in Stress Relaxation

Plant fibres (PFs) are preferred reinforcements of bio-composites. Knowledge of their lifespan requires a study of their viscoelastic behaviour. In this paper, a stress relaxation analysis of kenaf fibres was performed at a constant rate of deformation at room temperature. A method for extracting the relaxation modulus in the deferred zone was proposed. This method was compared, using simulation, with the Zapas-Phillips method and experimental data via three predictive models: the stretched exponential function or KWW, the inverse power law of Nutting and the prony series. The results indicate that the relaxation modulus obtained by the method proposed is in good agreement with the experimental modulus. In addition, the estimated error is of the same order of magnitude as in the case of the Zapas-Phillips method. The parameters estimated from the KWW function (β = 0.4) and prony series model showed an important contribution in the study of the delayed response of kenaf fibres. These results can have a significant impact on the use of kenaf fibres in midterm and long-term loading applications.

SERA: An ISIP analysis approach to estimate fracture height and correlate stress escalation and relaxation in multifrac horizontal wells

In hydraulic fracturing of horizontal wells in unconventional reservoirs, every fracturing stage by increasing the minimum horizontal stress modifies the vertical stress contrast as well as reduces the stress anisotropy. The induced stress gradually declines thereafter due to fracture closure caused by fluid leak-off. The time-dependent stress behavior with localized spatial effects generated by fixed but dynamic sources offers an incentive to fracturing optimization studies. Here, drawing upon the stress superposition principle, an analytical model is derived for Stress Escalation and Relaxation Analysis (SERA) to correlate the stress state at the wellbore. The SERA's model possesses a set of tuning parameters that are adaptively adjusted based on perforation design, waiting time between stage treatments, and ISIP of all completion stages. As a computationally efficient ISIP analysis technique, SERA estimates the average height of wet fracture and sets an upper limit to the critical stress state. In case of a dominant vertical stress contrast, SERA allows to determine explicitly the conditions under which the onset of wet fractures into the bounding layers of lower stress takes place, and when the stress re-orientation controls the fracturing regime, it narrows down the range of estimates of the horizontal stress anisotropy. Furthermore, it provides a tool to design optimal timing and sequencing strategies for alternating fracturing by correlating post-treatment stress relaxation parameters. We validate the model with numerical simulations and perform a sensitivity analysis on the influential parameters that can be carefully designed to give more control over stress shadowing.

Intrinsically self-healing and stretchy poly(urethane-urea) elastomer based on dynamic urea bonds and thiol-ene click reaction

The development of self-healing elastomers with superior physical properties is a challenging task. Herein, we present a design concept for a self-healing, highly stretchable, and robust poly(urethane–urea) elastomer prepared via a thiol-ene click reaction by a facile one-pot in situ photopolymerization method. The objective of the utilized approach is to incorporate weak noncovalent H-bonds, dynamic covalent urea bonds, and products of a thiol-ene click reaction into a polymeric backbone. This study also applies polytetramethylene ether glycol domains as soft segments to facilitate the polymeric chain diffusion and dynamic exchange reaction. Owing to these unique molecular characteristics, the resultant elastomer possesses a uniform structure and versatile properties, such as high stretchability corresponding to a maximum strain of 464%, ultimate tensile strength of 10.9 MPa, self-recoverability, high self-healing efficiency of 99%, high recyclability, and good weldability. Upon rupture, the as-prepared elastomer completely restores its original mechanical properties after heating to 80 °C for 5 h. The dynamic reversibility of the elastomer is evidenced by stress relaxation analysis and rheological testing. In addition, it can be effectively encapsulated and potentially used for fabricating self-healing stretchable conductive devices.

The role of branching architecture in shape memory semi‐IPNs: Shape memory effect and tube model analysis

AbstractThe impact of branching architecture of one continuous uncrosslinked phase on properties of classic shape memory semi‐interpenetrating polymer networks (semi‐IPNs) was explored. Crosslinked poly (methyl methacrylate) (PMMA)/star‐shaped polyethylene glycol (PEG) (PMMA/SPEG) semi‐IPNs and PMMA/linear PEG (PMMA/LPEG) semi‐IPNs were synthesized with the same PEG content. Mechanical properties, phase structure, thermal properties, dynamic mechanical properties, and shape memory properties of these two semi‐IPNs systems were compared. Due to the better compatibility of SPEG in the PMMA network, which was derived from little crystallization compared with PMMA/LPEG semi‐IPNs, PMMA/SPEG semi‐IPNs exhibited a combination of large tensile strength and high elongation at break. PMMA/SPEG semi‐IPNs, which had little crystallization exhibited superior shape recovery versus PMMA/LPEG semi‐IPNs, which had more crystallization. Moreover, the higher the crystallinity in PMMA/PEG semi‐IPNs was the worse long‐term temporary shape retention. Based on tube model theory, the high shape recovery capacity of PMMA/SPEG semi‐IPNs is mainly ascribed to the retraction of free PEG arms, which is entropically favorable and thermally activated due to the fluctuations of the path length. This result is supported by stress relaxation analysis and the influence of long shape fixity time on shape fixity ratio for these two systems.

Covalent adaptable networks of polydimethylsiloxane elastomer for selective laser sintering 3D printing

Polydimethylsiloxane (PDMS) elastomer is one of most investigated and pioneering 3D printing materials. However, the powder-based 3D printing remains a big challenge due to the thermoset intrinsic character of PDMS. Here we realized the powder-based selective laser sintering (SLS) 3D printing of PDMS elastomer based on one kind of novel self-designed PDMS covalent adaptable networks (CANs) containing hindered pyrazole urea dynamic bonds. The dynamic chemical mechanism of the hindered pyrazole urea bonds is confirmed by small molecule models experiments, density functional theory calculation, and also stress relaxation analysis. The electron-donating large steric hindrance substituent in the pyrazole ring was found to effectively promote the dynamic character of pyrazole urea bond. The PDMS CANs show excellent self-healing and reprocessing properties. Furthermore, kilo-scale PDMS powders were prepared by cryogenic grinding and the powder morphology was optimized for SLS 3D printing. A PDMS insole with complex structure designed based on the personalized foot pressure distribution data was printed, and the foot orthotic and self-healing functions for the insole were demonstrated.

Щодо розрахунку релаксації напружень в тонкостінних циліндричних оболонках із лінійно-в'язкопружних матеріалів

The problems of stress relaxation analysis in thin-walled cylindrical shells made of linear viscoelastic materials under uniaxial and biaxial loading have been solved. The analysis is based on a there-dimensional model of viscoelasticity starting from the hypothesis of the deviators proportionality. The viscoelastic properties of a material are given with relationships that establish the dependence between stress and strain intensities as well as between the mean stress and volumetric strain by the Bolzmann-Volterra type equation. The kernels of relaxation intensity and volumetric relaxation are given with the Rabotnov exponential-fractional functions. The parameters of relaxation kernels are determined from creep test result using the relationships between creep kernels under the complex stress state and creep kernels under the one- dimensional stress state. The problems of the analysis of normal and tangential stresses relaxation in thin-walled cylindrical shells made of high density polyethylene “ПЭВП” under uniaxial tension, pure torsion and combined tension with torsion loading have been solved and experimentally approved.

Numerical and experimental analysis of creep deformation and stress-relaxation in Sn-5Sb lead-free alloy

In-service performance of solder alloys is highly dependent on its high temperature behaviors like creep and stress relaxation. In this investigation, creep behavior of Sn-5Sb alloy is studied using three integrated creep equations, i.e., combination of power-law and omega model (PO), exponential and omega model (EO), and logarithmic and omega model (LO). In addition, stress relaxation (SR) curves are derived using two approaches of spring-dashpot (SD) and Peleg (P) model to establish the best fit to experimental data. Required creep data were attained using creep tests at several stresses at 500 K while relaxation ones were carried out under a constant strain of 0.5% at 300, 330, 360 and 400 K. Nonlinear least-square fitting (NLSF) analysis was carried out to deliver the best fit of the experimental data and recognition of constants for each equation. Results showed that in the lower stress level of 160 MPa, the CEO model presents a better fitting of experimental creep data with respect to CPO and CLO, whereas, CPO is with superior accuracy in the higher stress regions. In SR mode, SD model did not indicate good performance at 500 and 550 °C while showing good accordance with the experimental data at 600 and 650 °C. P model on the other hand, was superior to SD model in overall temperature range as obtained parameter values followed temperature dependence quite well. It is concluded that the EO and P models can accurately predict the creep and relaxation behavior of Sn-5Sb solder alloy, respectively.

Cold creep of titanium: Analysis of stress relaxation using synchrotron diffraction and crystal plasticity simulations

It is well known that titanium and some titanium alloys creep at ambient temperature, resulting in a significant fatigue life reduction when a stress dwell is included in the fatigue cycle. It is thought that localised time dependent plasticity in ‘soft’ grains oriented for easy plastic slip leads to load shedding and an increase in stress within a neighbouring ‘hard’ grain that is poorly oriented for easy slip. Quantifying this time dependent plasticity process is key to successfully predicting the complex cold dwell fatigue problem. In this work, synchrotron X-ray diffraction during stress relaxation experiments was performed to characterise the time dependent plastic behaviour of commercially pure titanium (grade 4). Lattice strains were measured by tracking the diffraction peak shifts from multiple plane families (21 diffraction rings) as a function of their orientation with respect to the loading direction. The critical resolved shear stress, activation energy and activation volume were established for both prismatic and basal slip modes by fitting a crystal plasticity finite element model to the lattice strain relaxation responses measured along the loading axis for three strong reflections. Prismatic slip was the easier mode having both a lower critical resolved shear stress (τcbasal = 252 MPa and τcprism = 154 MPa) and activation energy (ΔFbasal= 10.5×10−20J = 0.65 eV andΔFprism = 9.0×10−20J = 0.56 eV). The prism slip parameters correspond to a stronger strain rate sensitivity compared to basal slip. This slip system dependence on strain rate has a significant effect on stress redistribution to ‘hard’ grain orientations during cold dwell fatigue.

Rheology of fiber-enriched steamed bread: Stress relaxation and texture profile analysis

Wheat bran is one of the major dietary fiber sources widely used in the food industry in order to produce fiber-enriched foods. The effects of wheat bran substitution (0-30%), water absorption (46-62%) and electric power (4-12 kW) of steamer on stress relaxation and textural parameters of steamed breads were evaluated by the Texture Analyzer. The results showed that mechanical stress relaxation data of steamed breads were fitted well by both the Peleg-Normand and three-element Maxwell models. At 10-40% strains tested, common and bran-enriched steamed breads were more elastic measured at low strain. It was suitable to perform the stress relaxation test of steamed bread at 10-30% strains. Generally, increasing the substitution of flour by bran resulted in less elasticity and higher hardness of steamed bread. Results of sensory evaluation indicated no significant difference in textural and overall acceptability among various steamed breads with 0, 10 and 20% of wheat bran. Medium and high water absorptions (54-62%) produced fiber-enriched steamed bread with better elasticity and texture. The elasticity of the steamed bread was the lowest at 4 kW electric power of steamer. Significant correlations were found between textural characteristics and stress relaxation parameters. This study suggests that 20% bran-enriched steamed bread, with better elasticity and sensory acceptability, can be produced at 54% of water absorption and 12 kW of electric power of the steamer.

Thermal Guanidine Metathesis for Covalent Adaptable Networks.

We demonstrate that a dynamic chemical reaction that we term thermal guanidine metathesis (TGM) can serve as the basis for covalent adaptable network (CAN) materials. CANs are a class of cross-linked polymers that transition from thermoset to thermoplastic-like rheological behavior upon significant activation of reversible exchange reactions within the network and thus can be reprocessed. Small molecule studies indicate the TGM reaction proceeds by a dissociative mechanism, and guanidine-cross-linked network polymers can be reprocessed at elevated temperature. These TGM-based CANs exhibit dynamic behavior, such as dissolution in the presence of monofunctional exchange partners and stress relaxation above Tg. Additionally, differences in the activation energies obtained by small molecule kinetic studies and stress relaxation analysis are consistent with key predictions of the Semenov-Rubinstein model of thermoreversible gelation of highly cross-linked networks.

Analysis of Creep Strains and Stress Relaxation in Thin-Walled Tubular Members Made of Linear Viscoelastic Materials. 1. Superposition of Shear and Volume Creep

The problem of creep strains and stress relaxation analysis in thin-walled tubular members made of linear viscoelastic materials under combined tension and torsion loading is solved. The solution is obtained using viscoelastic models in the form of superposition of shear and volume creep. The creep and relaxation kernels are represented by fractional-exponential functions. The solutions are validated experimentally by analyzing longitudinal, transverse, and shear creep strains as well as the relaxation of normal and tangential stresses in thin-walled tubular members made of organic glass and high-density polyethylene.

Preparation of multi-temperature responsive elastomers by generating ionic networks in 1,2-polybutadiene using an anionic melting method.

The development of reversible networks in elastomers provided unique inspiration for the design of advanced polymers with excellent properties. In this paper, we adopted an anionic melting method to introduce carboxylate groups into 1,2-polybutadiene (1,2-PB), using maleic anhydride as a modifier, and sodium hydride (NaH), calcium hydride (CaH2), and lithium aluminum hydride (LiAlH4) as metallization reagents. Na-Based, Ca-based, and Li/Al-based ionic bond networks were constructed in the covalently crosslinked 1,2-PB. The effects of the electronegativity and valence of the metal ions on the strength and reversible temperature of the ionic network were studied. Payne effect was shown by rheological tests, demonstrating the interactions between the ionic networks and rubber chains. The reforming temperature for these ionic networks was studied by stress-relaxation analysis, and shape memory experiments were performed based on these temperatures. This concept provides novel inspiration for the design of high-performance and temperature-adaptive elastomers.

Tensile characteristics and stress relaxation analysis of woven fabrics in terms of fabric direction

Woven fabrics in various end uses are subjected to tensile loads in different directions, so investigation of the effect of fabric direction on the tensile behaviour and the stress relaxation performance of fabrics is important and needs to be considered. In this study, the tensile and stress relaxation properties of woven fabrics with five various weave structures have been analysed, in different directions. It was concluded that the tensile properties of fabrics such as Young’s modulus, breaking load and elongation and also the work of rupture were significantly affected by the fabric direction and weave structure. Moreover, it was determined that the fabric tensile stress relaxation (%) was considerably affected by the applied strain level, fabric direction and weave structure in the confidence range of 95% and it might well be expressed as a Gaussian function of sample direction.

Composites based on high-density polyethylene, polylactide and calcium carbonate: effect of calcium carbonate nanoparticles as co-compatibilizers

Investigating the compatibility mechanism of hybrid composites based on two polymers and one mineral nanofiller is a challenge that needs to be better understood. This study evaluates the effect of calcium carbonate (nCaCO3) nanoparticles as co-compatibilizers in a blend based on poly(lactic acid) (PLA), polyethylene from renewable sources (HDPE) and maleic anhydride grafting polyethylene (HDPE-g-MA or MA) using rheological and morphological analyses. Morphological assessment by scanning electron microscopy showed the formation of co-continuous-like phase morphology when nCaCO3 was added to the PLA/MA/HDPE blend, indicating that nCaCO3 affected the dispersion of PLA domains in the HDPE matrix. Rheological characterization carried out by stress relaxation and sweep frequency response analysis demonstrated that the addition of nCaCO3 improved the solid-like behavior of the blend, corroborating the morphological results. Additionally, an interaction mechanism of nCaCO3 as a co-compatibilizer was also proposed.