Showing papers on "Stress relaxation published in 2021"

Enhanced substrate stress relaxation promotes filopodia-mediated cell migration.

Abstract: Cell migration on two-dimensional substrates is typically characterized by lamellipodia at the leading edge, mature focal adhesions and spread morphologies. These observations result from adherent cell migration studies on stiff, elastic substrates, because most cells do not migrate on soft, elastic substrates. However, many biological tissues are soft and viscoelastic, exhibiting stress relaxation over time in response to a deformation. Here, we have systematically investigated the impact of substrate stress relaxation on cell migration on soft substrates. We observed that cells migrate minimally on substrates with an elastic modulus of 2 kPa that are elastic or exhibit slow stress relaxation, but migrate robustly on 2-kPa substrates that exhibit fast stress relaxation. Strikingly, migrating cells were not spread out and did not extend lamellipodial protrusions, but were instead rounded, with filopodia protrusions extending at the leading edge, and exhibited small nascent adhesions. Computational models of cell migration based on a motor–clutch framework predict the observed impact of substrate stress relaxation on cell migration and filopodia dynamics. Our findings establish substrate stress relaxation as a key requirement for robust cell migration on soft substrates and uncover a mode of two-dimensional cell migration marked by round morphologies, filopodia protrusions and weak adhesions. It is now shown that cells migrate robustly on soft, viscoelastic substrates with fast stress relaxation using a migration mode marked by a rounded cell morphology and filopodia protrusions extending at the leading edge.

Importance of Broad Temperature Windows and Multiple Rheological Approaches for Probing Viscoelasticity and Entropic Elasticity in Vitrimers

Abstract: Model polydimethylsiloxane telechelic vitrimers with dynamic boronic ester bonds were synthesized to investigate the viscoelastic properties of dynamic networks with extremely low Tg via multiple rheological approaches. Frequency sweeps and stress relaxation tests, conducted at more than 120 °C above Tg, show the anticipated Arrhenius behavior of relaxation time with inverse temperature and give the same activation energy for a fixed molecular weight, obtained using a variety of analysis methods. Time–temperature superposition demonstrates that the flow regime is thermorheologically simple, while the modulus of the plateau regime increases with increasing temperature, consistent with a conserved network topology and associative bond exchange. As relaxation times decrease, the rubbery plateau modulus increases, indicating a decoupling of terminal dynamics from mechanics. Below 40 °C, a second Arrhenius regime with lower activation energy emerges, which is attributed to a transition from relaxation dominated by reaction exchange kinetics to relaxation dictated by local polymer dynamics. Our work points to the importance of assessing a broad temperature window and using multiple approaches in probing vitrimers and dynamic networks.

Effects of Conventional and Severe Shot Peening on Residual Stress and Fatigue Strength of Steel AISI 1060 and Residual Stress Relaxation Due to Fatigue Loading: Experimental and Numerical Simulation

Abstract: This study investigates and compares the effects of different shot peening treatments including conventional and severe shot peening on microstructure, mechanical properties, fatigue behavior, and residual stress relaxation of AISI 1060 steel. Shot peening treatments were applied with two Almen intensities of 17 and 21 A and a wide ranges of coverage (100%–1500%). Various microstructural observations were carried out to analyze the evolution of microstructure. Microhardness, residual stress and surface roughness measurements and also axial fatigue test were performed. Moreover, the extent of the residual stress relaxation during cyclic loading was investigated by means of XRD measurements. Furthermore, numerical simulation of residual stress relaxation due to fatigue loading was carried out and validated against experimental investigations. The comparison indicated a good agreement for the surface residual stress relaxation up to 100 cycles. The experimental results indicated the efficiency of severe shot peening processes in obtaining nanostructured surface layer and achieving superior mechanical properties and fatigue behavior. Also, residual stress measurements revealed that stress relaxation started with a high rate at the initial stages of loading and gradually increased at higher number of cycles which was lower in the case of severely shot peened samples compared to the conventionally treated ones.

Effect of solid-state phase transformation on residual stress of selective laser melting Ti6Al4V

Abstract: Residual stress in selective laser melting (SLM) is one of the key challenges in terms of precision control, success rate and the performance of deposited components. Ti6Al4V belongs to α+β titanium alloy, the residual stress of selective laser melted (SLMed) Ti6Al4V component maybe affected by solid-state phase transformation result from complex thermal history of SLM. In the present study, effect of solid-state phase transformation on residual stress of SLM Ti6Al4V was investigated. A coupled modeling method of thermo-metallurgical-mechanical considering solid-state phase transition is provided and validated by microstructure observation and residual stress measurements. Then the solid-state phase transformation strain was neglected in the validated model, the computed residual stress is used to investigate the effect of solid-state phase transformation strain on residual stress by comparing with experimental measurements. In addition, the influence of the difference in yield strength and thermal expansion coefficient between the original phase and the transition phase on the residual stress is discussed. It has been found that the residual stress of SLMed Ti6Al4V is related to the direction, the longitudinal residual stress is about twice transverse, and they are all tensile stresses. The microstructure and residual stress predicted by the simulation are in good agreement with the experimental measurements. The microstructure of SLM Ti6Al4V is mainly composed of martensite α', and the average error between the predicted longitudinal stress and measurement is 2.1%. The solid-state phase transformation has a stress relaxation effect during the SLM Ti6Al4V, predicted longitudinal and transverse residual stress exceeds the experimental measurement by up to 80.7% and 53.9%, when neglecting the solid-state phase transformation strain. The influence of the solid-state phase transformation of SLM Ti6Al4V on the residual stress is mainly determined by the volume change between the solid-state phases.

Study of the in situ stress field in a deep valley and its influence on rock slope stability in Southwest China

Abstract: The in situ stress field in valley areas is an important factor to consider in the construction of hydropower projects in southwest China. Due to intense tectonic movement, the action of gravity, and rapid river erosion, the resulting stress has a complex distribution and a large magnitude. The availability of stress measurements is insufficient to provide adequate understanding of the regional stress state. In this research, inversion analysis for the in situ stress field of a valley in the upper reaches of the Lancang River is carried out by using stepwise regression analysis and the FLAC3D numerical simulation, and the temporal and spatial distribution characteristics of the in situ stress are obtained. Based on those results, the influence of stress unloading effects on slope stability is discussed. The results show that the formation of the modern in situ stress field is the comprehensive result of interactions among gravity, tectonic movement, and river erosion. The reliability of the inverse result can be increased by improving the mesh density of the calculated model and considering multistep river erosion. Under the influence of the unloading effects, tensile fractures were formed and the rock mass loosened near the slope surface. With the presence of bedding dipping into the slope, toppling failure was triggered by the unloading rebound that mainly occurred in the stress relaxation zone.

Characterization of the strain-rate–dependent mechanical response of single cell–cell junctions

Abstract: Cell–cell adhesions are often subjected to mechanical strains of different rates and magnitudes in normal tissue function However, the rate-dependent mechanical behavior of individual cell–cell adhesions has not been fully characterized due to the lack of proper experimental techniques and therefore remains elusive This is particularly true under large strain conditions, which may potentially lead to cell–cell adhesion dissociation and ultimately tissue fracture In this study, we designed and fabricated a single-cell adhesion micro tensile tester (SCAµTT) using two-photon polymerization and performed displacement-controlled tensile tests of individual pairs of adherent epithelial cells with a mature cell–cell adhesion Straining the cytoskeleton–cell adhesion complex system reveals a passive shear-thinning viscoelastic behavior and a rate-dependent active stress-relaxation mechanism mediated by cytoskeleton growth Under low strain rates, stress relaxation mediated by the cytoskeleton can effectively relax junctional stress buildup and prevent adhesion bond rupture Cadherin bond dissociation also exhibits rate-dependent strengthening, in which increased strain rate results in elevated stress levels at which cadherin bonds fail This bond dissociation becomes a synchronized catastrophic event that leads to junction fracture at high strain rates Even at high strain rates, a single cell–cell junction displays a remarkable tensile strength to sustain a strain as much as 200% before complete junction rupture Collectively, the platform and the biophysical understandings in this study are expected to build a foundation for the mechanistic investigation of the adaptive viscoelasticity of the cell–cell junction

Effect of Mg on the stress relaxation resistance of Cu–Cr alloys

Abstract: Cu–Cr and Cu–Cr–Mg alloys were prepared to determine the stress relaxation mechanism and the effect of Mg on the stress relaxation resistance of Cu–Cr alloys by analyzing the microstructure, morphological characteristics of the precipitates and dislocation densities before and after the stress relaxation test using electron backscatter diffraction (EBSD), transmission electron microscopy (TEM) and X-ray diffraction (XRD). The formation of Cottrell atmospheres in the Cu–Cr–Mg was verified by theoretical calculations and the serrations in the stress-strain curves; thus, the way that Mg improved the stress relaxation resistance of the Cu–Cr alloy was identified. The stress decreased rapidly at first, dropped slowly and then gradually stabilized during the stress relaxation of the two alloys. The kinetic equations of both alloys were obtained. The stress relaxation behavior was very sensitive to the temperature, and an increase in the temperature accelerated the rate of stress reduction. Recrystallization and a decrease in the dislocation density were accompanied by the stress relaxation of both alloys, but the changes in the morphological characteristics of the precipitates were negligible. The recrystallization was the main factor behind the stress relaxation of both alloys. Mg obviously improved the stress relaxation resistance of the Cu–Cr alloy. There were no Cottrell atmospheres at room temperature, and the Mg and mobile dislocations formed Cottrell atmospheres only at 100 °C and above. The increase in the stress relaxation resistance of the Cu–Cr–Mg alloy was attributed to the interactions among the fine grains, precipitates, Mg atoms and dislocations.

Predictably Engineering the Viscoelastic Behavior of Dynamic Hydrogels via Correlation with Molecular Parameters

Abstract: Rational design of dynamic hydrogels with desirable viscoelastic behaviors relies on an in-depth understanding of the principles correlating molecular parameters and macroscopic properties. To quantitatively elucidate such principles, a series of dynamic covalent hydrogels crosslinked via hydrazone bonds is designed. The exchange rate of the hydrazone bond is tuned by varying the concentration of an organic catalyst, while maintaining the crosslinking density unchanged. This strategy of independently tuning exchange dynamics of crosslinks and crosslinking density allows unambiguous analysis of the viscoelastic response of the dynamic hydrogels as a function of their network parameters. It is found that the terminal relaxation time of the dynamic hydrogels is primarily determined by two factors: the exchange rate of crosslinks and the number of effective crosslinks per polymer chain, and is independent of the network architecture. Furthermore, a universal correlation is identified between the terminal relaxation time determined from stress relaxation and the exchange rate determined via reaction kinetics, which can be generalized to any viscoelastic hydrogel network, in principle. This quantitative correlation facilitates the development of dynamic hydrogels with a variable desired viscoelastic response based on molecular design.

On the influence of alloy composition on the additive manufacturability of Ni-based superalloys

Abstract: The susceptibility of nickel-based superalloys to processing-induced crack formation during laser powder-bed additive manufacturing is studied. Twelve different alloys -- some of existing (heritage) type but also other newly-designed ones -- are considered. A strong inter-dependence of alloy composition and processability is demonstrated. Stereological procedures are developed to enable the two dominant defect types found -- solidification cracks and solid-state ductility dip cracks -- to be distinguished and quantified. Differential scanning calorimetry, creep stress relaxation tests at 1000$^\circ$C and measurements of tensile ductility at 800$^\circ$C are used to interpret the effects of alloy composition. A model for solid-state cracking is proposed, based on an incapacity to relax the thermal stress arising from constrained differential thermal contraction; its development is supported by experimental measurements using a constrained bar cooling test. A modified solidification cracking criterion is proposed based upon solidification range but including also a contribution from the stress relaxation effect. This work provides fundamental insights into the role of composition on the additive manufacturability of these materials.

In-situ XRD study on the effects of stress relaxation and phase transformation heat treatments on mechanical and microstructural behaviour of additively manufactured Ti-6Al-4V

Abstract: Additively Manufactured (AM) titanium (Ti) components are routinely post-thermal heat treated (HT), to reduce internal stresses, as well as to obtain more desirable microstructural features, yielding improved mechanical performance. Currently, there is no consensus on the optimum HT method for AM Ti-6Al-4V, as the mechanism for the main phase transformation ( α ′ (martensite) → α + β (equilibrium)) is still ambiguous. In this study, stress relaxation and phase transformation in the alloy are investigated in detail, via isothermal heat treatments and in situ high temperature X-ray Diffraction (XRD). The latter was carried out at heating rates of 5 and 200 °C/min. The relationship between crystallographic evolution during isothermal treatments and mechanical behaviour was determined. Isothermal holding at 400 °C resulted in an increase in ultimate tensile strength (UTS) and yield strength (YS) by 3.4% and 2.1%, respectively, due to the relief of tensile microstrain. It was found that isothermal treatment conducted between 550 and 700 °C promotes martensitic decomposition, resulting in the formation of a transitional - α t r phase, which has an asymmetrical hexagonal crystal lattice. The formation of this α t r phase was determined to be the main factor contributing to a major decrease in ductility.

Physical and thermo-mechanical properties of shape memory polyurethane containing reversible chemical cross-links.

Abstract: Orthodontic chains are one of the main parts of orthodontic braces In this study, in order to obtain a suitable polymer for this application as well as troubleshoot the main drawbacks such as stress relaxation and water absorption, thermoplastic polyurethane (TPU) elastomers with various compositions were synthesized and characterized by Fourier-transform infrared spectroscopy (FTIR), differential scanning calorimetry (DSC), and X-ray diffraction (XRD) analyses Mechanical properties of samples were evaluated by tensile, elasticity, and stress relaxation tests According to the required properties for orthodontic chain application, PU2000-311 which contains reversible crosslinks, was selected as the most favorable sample among other pure samples Moreover, to reduce the water absorption content of PU2000-311, its nanocomposite containing 1 wt% of silica nanoparticles was prepared via solution casting method As water content angle and field emission scanning electron microscopy (FESEM) images illustrate, incorporation of 1 wt% of modified silica nanoparticles has increased PU2000-311-1S hydrophobicity In vitro oral environment study showed crystability of samples has recovered great portion of relaxed force Stress relaxation study indicated samples are applicable in oral temperature range and temperature changes have assisted recovery of relaxed force and reduced treatment period Finally, shape memory study showed that optimum samples could recover 100% of their original shape

The role of grain boundary ferrite evolution and thermal aging on creep cavitation of type 316H austenitic stainless steel

Abstract: To understand the interaction between microstructural evolution and creep cavitation during stress relaxation at an elevated temperature, an ex-service AISI type 316H stainless steel sample containing both weld metal and heat affected zone (HAZ) from an advanced gas-cooled reactor was studied. Multiple techniques that include secondary electron microscopy, electron backscatter diffraction (EBSD), transmission electron microscopy (TEM) and plasma focused ion beam tomograph were used for microstructure and creep cavities characterisation. Although no creep cavities were observed in the weld metal, the HAZ was extensively creep cavitated. At randomly oriented grain boundaries, creep cavities are present and closely linked with M23C6 and ferrite precipitates formed during thermal aging. Less precipitation (e.g. absence of ferrite) and less creep cavitation were observed at Σ3 coincidence site lattice boundaries. During in-service aging, at random grain boundaries, M23C6 formation and growth cause the local elemental depletion of γ stabilisers and promote a phase transformation from austenite to ferrite. The crystallographic relationship between ferrite and austenite were also studied by EBSD and TEM. Ferrite precipitates formed during aging often grow into the austenite grain not expected by traditional nucleation and growth theory, likely due to physical constraints by the existing carbides at the grain boundaries. The formation and growth of creep cavities is closely associated with the M23C6 and ferrite formed on grain boundaries. This study highlights the importance of considering the effect of thermal aging in accelerating creep cavitation.

Thermo-mechanical modelling of stress relief heat treatments after laser-based powder bed fusion

Abstract: Laser-based powder bed fusion, due to its layer-by-layer nature, results in a unique stress profile in a part after the primary production process. The residual stresses are typically tensile near the top, while they are compressive near the bottom of the part. When it is removed without proper precautions, the part will bend excessively. In order to alleviate this deformation, a stress relief heat treatment can be applied. In this paper, such a stress relaxation heat treatment is modelled to investigate the effect of the post-processing parameters. The model uses an Arrhenius-type creep equation to simulate the influence of the heat treatment temperature and dwell time on the stress field in a relatively simple cantilever beam produced in Ti-6Al-4V. Via validation of the simulations, the effect of the heat treatment is shown to be represented accurately. The validated model is used to predict the deformation that results from the residual stresses after heat treating the part under various conditions. The results from the simulations ultimately allow choosing the optimal heat treatment conditions to obtain a given reduction in the residual stress level, while reducing the need for extensive experimental investigations.

On the interplay of friction and stress relaxation to improve stretch-flangeability of dual phase (DP600) steel.

Abstract: Industrial servo presses have been used to successfully demonstrate improved formability when deforming sheet metals. While the time dependent viscoplastic behavior of material is attributed to the observed formability improvement, much less effort has been devoted to understand and quantify the underlying mechanisms. In this context, the hole expansion test (HET) of a dual phase steel was interrupted at pre-defined punch travel heights to understand the time-dependent effects on stretch-flangeability. The effect of pre-strain, hold time and edge quality on hole expansion ratio (HER) improvement was studied. The present study shows that the HER improves significantly in interrupted HET. This improved HER is due to the combined effects of stress relaxation and friction on deformation behavior. The ductility improvement estimated from uniaxial stress relaxation tests was used to estimate the contribution of stress relaxation and friction, respectively, in HET. This study shows that friction plays a significant role in improving HER at high pre-strain. It was also demonstrated that frictional effects are largely influenced by edge quality.

Low cycle fatigue and stress relaxation behaviours of powder metallurgy Ni-based superalloy FGH4098

Abstract: In this study, the effects of microstructure, temperature and dwell time in a loading cycle on low cycle fatigue (LCF) and stress relaxation behaviours of a powder metallurgy Ni-based superalloy (i.e. FGH4098) for aeroengine turbine disc application were studied along with detailed microscopic characterisation of fracture features and deformation substructures. The results indicate that the LCF failure mode of FGH4098 transits from transgranular to intergranular with the increase of temperature and dwell time, and the propensity of intergranular fatigue cracking is higher in fine-grained FGH4098. FGH4098 predominantly shows cyclic hardening behaviour which is mainly related to dislocation interactions. With the increase of temperature and/or dwell time, the cyclic hardening behaviour diminishes to some extent due to the increasing shearing of secondary γ′ precipitates by stacking fault and partial dislocation pairs. Similarly, stress relaxation in FGH4098 is also associated with the shearing of γ′ precipitates and becomes more prominent at high temperature with long dwell period. Micro-twinning may also contribute to stress relaxation at 750 °C.

The nonequilibrium behaviors of covalent adaptable network polymers during the topology transition

Abstract: Vitrimers with bond exchange reactions (BERs) are a class of covalent adaptable network (CAN) polymers at the forefront of recent polymer research. They exhibit malleable and self-healable behaviors and combine the advantages of easy processability of thermoplastics and excellent mechanical properties of thermosets. For thermally sensitive vitrimers, a molecular topology melting/frozen transition is triggered when the BERs are activated to rearrange the network architecture. Notable volume expansion and stress relaxation are accompanied, which can be used to identify the BER activation temperature and rate as well as to determine the malleability and interfacial welding kinetics of vitrimers. Existing works on vitrimers reveal the rate-dependent behaviors of the nonequilibrium network during the topology transition. However, it remains unclear what the quantitative relationship with heating rate is, and how it will affect the macroscopic stress relaxation. In this paper, we study the responses of an epoxy-based vitrimer subjected to a change in temperature and mechanical loading during the topology transition. Using thermal expansion tests, the thermal strain evolution is shown to depend on the temperature-changing rate, which reveals the nonequilibrium states with rate-dependent structural relaxation. The influences of structural relaxation on the stress relaxation behaviors are examined in both uniaxial tension and compression modes. Assisted by a theoretical model, the study reveals how to tune the material and thermal-temporal conditions to promote the contribution of BERs during the reprocessing of vitrimers.

Effect of geometrical structure variations on the viscoelastic and anisotropic behaviour of cortical bone using multi-scale finite element modelling.

Abstract: Multi-scale finite element analysis is performed to ascertain the effect of geometrical changes at multiple structural scales on the mechanical properties of cortical bone. Finite element models are developed, with reference to experimental data from existing literature, to account for bone's viscoelastic behaviour and anisotropic structure from the most fundamental level of bone consisting of mineralised collagen fibrils, up to the macroscopic level consisting of osteons and the Haversian canals. A statistical approach is incorporated to perform sensitivity analyses on the effects of different geometrical parameters on the effective material properties of cortical bone at each length scale. Numerical results indicate that there is an exponential correlation between the mineral volume fraction and the effective stiffness constants at each length scale. This contributes to the exponential behaviour of the instantaneous moduli describing cortical bone's two-phase stress relaxation process: a fast and slow response relaxation behaviour. Results indicate that the fast response relaxation time is independent of bone's structural anisotropy, whilst being dependent on variations in the global mineral volume fraction between length scales. However, the slow response relaxation time is independent of the changes in mineral volume fraction. It is also observed that the slow response relaxation time varies with bone's anisotropic structure, and therefore, contributes to the anisotropic properties of bone.