Local Stress Relaxation Research Articles

Local viscoelastic properties and shear stress propagation in bulk and confined polymer melts and low-molecular weight liquids

Through the analysis of the spatial correlations of local stress, we detect the propagation of long-ranged liquid-elasticity-mediated shear stress waves in polymeric and low-molecular weight liquids. The propagation of shear waves is effectively planar; i.e., σαβ propagates in the αβ plane. The autocorrelation functions of the local stress of a region are affected by both the relaxation of stress in that region and the propagation of stress from the region to the rest of the sample. However, due to the planar propagation of shear waves, the transfer of σαβ from those slices of the simulation box that are periodic in the αβ plane is negligible. This allows direct probing of the position-dependent local stress relaxation modulus of liquid in the vicinity of a confining surface. Published by the American Physical Society 2024

Local ratcheting and stress relaxation at notch root of CP-Ti specimen under cyclic loading: Experiment and simulation

Damage and crack always origin from notch, and it is necessary to understand the local ratcheting and stress relaxation at notch root for commercially pure titanium (CP-Ti). Symmetric strain-controlled cyclic test for smooth CP-Ti specimen is conducted to determine the cyclic constitutive relationship based on the combined hardening model. Asymmetric force-controlled cyclic test combined with Digital Image Correlation (DIC) technology is conducted to reveal local ratcheting strain distribution at notch root of CP-Ti specimen, while the distributions of stress, local stress ratio, and local hysteresis curve at notch root are obtained by finite element (FE) simulation. It is found that plastic zone, local ratcheting and stress relaxation at notch root area change significantly with cyclic load levels. In the low stress amplitude region with the mean nominal stress lower than 87 MPa, the small-scale yield occurs, and the notch root is in a state of plastic shakedown. With the mean nominal stress increasing to 96 MPa in the transition region, the notch root is in a state that exceeds the plastic shakedown limit but the continuous ratcheting progress is not found. In the high stress amplitude region with the mean nominal stress higher than 104 MPa, the plastic zone significantly expends, and the notch root is in a continuous ratcheting progress state, accompanied by stress relaxation. For a specific structural configuration, the local ratcheting at notch root is affected by both the load level and the cyclic deformation characteristics of the material. In the high stress amplitude region, the reverse yield occurs at the notch root during unloading, which promotes continuous ratcheting progress and stress redistribution, resulting in stress relaxation. The higher the load level, the larger the ratcheting strain rate, and the lower the stress relaxation rate.

Effects of strain and defects on hydrogen diffusion and trapping in [formula omitted]-iron using a developed Fe-H reactive force field

Strain and defects significantly affect the diffusion and trapping of hydrogen (H), which plays a critical role in the initiation of hydrogen embrittlement phenomena in α-iron. In this work, based on a newly developed Fe-H reactive force field, the molecular dynamic models with perfect lattice and defects across various strain regions are presented for studying diffusion and aggregation of H atoms in α-iron. At first, the effect of gradient strain on H diffusion and trapping are elucidated. Then, H diffusion and trapping behaviors at defects (including vacancies, edge dislocations, Σ3[11̄0](112) and Σ5[001](310) grain boundaries) undergoing various strain are investigated. The simulation results demonstrate that the gradient strain leads to a gradient distribution of H solution energies, which drives a directional diffusion and aggregation of H atoms from compressive to tensile strain region. The charge transfer from nearest neighbor Fe atoms to H atoms weakens the Fe-Fe interactions, consequently resulting in the local stress relaxation within the H-enrich region. Regarding the H diffusion and trapping at defects, the results show that tensile strain decreases the difference in H solution energies between the bulk structure and defects, which facilitates H diffusion and escape. Conversely, compressive strain amplifies the energy difference, leading to the enhancement of H trapping and inhibition of H transport. This study contributes to an enhanced comprehension of interactions between hydrogen and defects under strain from an atomic scale.

Effect of deformation on structural-phase state of structural steel and on formation of its yield strength and degree of hardening

The work shows that the values of the yield strength of steel 20 after long-term operation and deformation, obtained experimentally and by calculation, differ significantly. This is primarily due to the fact that all coefficients require experimental verification. However, the data obtained can be used to determine the role of each contribution (in %) in relation to the total received. Relaxation of such high local internal stresses (due to the appearance and intensive growth of the elastic component) can (and does) occur only through the destruction of the sample, which is what is observed. Therefore, it is the emergence and increase of local internal stresses that leads to the destruction of the sample.

Mechanism of Pressure-Sensitive Adhesion in Nematic Elastomers.

Nematic liquid crystal elastomers (LCEs) have anomalously high vibration damping, and it has been assumed that this is the cause of their anomalously high-pressure-sensitive adhesion (PSA). Here, we investigate the mechanism behind this enhanced PSA by first preparing thin adhesive tapes with LCE of varying cross-linking densities, characterizing their material and surface properties, and then studying the adhesion characteristics with a standard set of 90° peel, lap shear, and probe tack tests. The study confirms that the enhanced PSA is only present in (and due to) the nematic phase of the elastomer, and the strength of bonding takes over 24 h to fully reach its maximum value. Such a long saturation time is caused by the slow relaxation of local stress and director orientation in the nematic domains after pressing against the surface. We confirm this mechanism by showing that freshly pressed and annealed tape reaches the same maximum bonding strength on cooling, when the returning nematic order is forming in its optimal configuration in the pressed film.

Geometric theory of mechanical screening in two-dimensional solids.

Holes in mechanical metamaterials, quasilocalized plastic events in amorphous solids, and bound dislocations in a hexatic matter are different mechanisms of generic stress relaxation in solids. Regardless of the specific mechanism, these and other local stress relaxation modes are quadrupolar in nature, forming the foundation for stress screening in solids, similar to polarization fields in electrostatic media. We propose a geometric theory for stress screening in generalized solids based on this observation. The theory includes a hierarchy of screening modes, each characterized by internal length scales, and is partially analogous to theories of electrostatic screening such as dielectrics and Debye-Hückel theory. Additionally, our formalism suggests that the hexatic phase, traditionally defined by structural properties, can also be defined by mechanical properties and may exist in amorphous materials.

Multiscale computational and experimental analysis of slip-GB reactions: In situ high-resolution electron backscattered diffraction and concurrent atomistic-continuum simulations

In this paper, in situ high-resolution electron backscattered diffraction (EBSD) is combined with concurrent atomistic-continuum (CAC) simulations to study the interactions between dislocation-mediated slip and grain boundaries (GBs) in Ni. It is found that the local stress associated with slip-GB intersections first increases upon the pileup of dislocations, then remains high even after the nucleation of dislocations in the neighboring grain, only relaxing after the nucleated dislocations propagate away from the GB due to more incoming dislocations participating in the pileup. The local stress relaxation is accompanied by an atomic-scale GB structure reconfiguration, which affects not only the subsequent dislocation transmission, but also the configuration of those dislocations away from the GB. These findings demonstrate the importance of incorporating local stress history at higher length scale models, such as crystal plasticity finite element.

Dual precipitates and heterogeneous fine-grain structure induced strength-ductility synergy in a CoCrNi-based medium-entropy alloy

A novel (CoCrNi)93.5Al3Ti3C0.5 medium-entropy alloy was designed, in this work, based on the equiatomic surplus-ductility carrier to achieve multi-coupling strengthening. Dual precipitates of M23C6 carbides and L12 ordered phase were co-introduced to not only play strengtheners, but also play constructors for the grain architecture. A heterogeneous fine-grain structure (HGS) was produced under the interactions of the simultaneous precipitation and recrystallization behaviors. It consisted of the core-containing heterogeneous microdomains (CHMs) which acted as the functional motif with a continuous, yet uniform, spatial distribution. An excellent mechanical property combination of a yield strength of 1425 MPa, ultimate tensile strength of 1516 MPa, and a total elongation of 16% was achieved in the current alloy. It was revealed that the hetero-deformation induced (HDI) hardening and twofold precipitation hardening were primarily responsible for the high strength. Furthermore, the coupling deformation mechanisms of the studied alloy were unveiled, involving of the micro-scaled continuous hetero-deformation and the nano-scaled stacking-fault-mediated planar slip combining with coherency-induced shearing mechanism. These jointly contributed to the continuous strain partition, local stress relaxation, extra strain-hardening, and thereby the appreciable elongation. In addition, we demonstrated that the higher HDI hardenability dependent closely on the favorable CHM-HGS configuration factors. This study provides meaningful insights into the collaborative architecture between intermetallic compounds and grain structure, diversifying the microstructural design strategy of metallic materials.

Development of Ti-Mo-Fe alloys combining different plastic deformation mechanisms for improved strength-ductility trade-off and high work hardening rate

Titanium-based biomaterials are the gold standard for orthopedic implants; however, they are not generally suitable for the manufacture of intravascular stents. Their low strength-ductility trade-off and low work hardening rate are their main limitations. However, Ni-free alloys are desirable for such application in order to avoid allergic reactions caused by the high Ni-content materials currently applied. Therefore, in this study, three alloys of the Ti-Mo-Fe system (Ti-8Mo-2Fe, Ti-9Mo-1Fe and Ti-10.5Mo-1Fe) were designed to present high strength-ductility compromise and high work hardening rate. Their microstructures, mechanical properties and plastic deformation mechanism were investigated. Athermal ω precipitates were observed in the β matrix of all solution-treated alloys. In the solution-treated β matrix of the Ti-9Mo-1Fe alloy, additional nanometer-sized α" particles were detected by transmission electron microscopy (TEM). Although the combined TWIP/TRIP effects were expected by the design method on the Ti-8Mo-2Fe and Ti-9Mo-1Fe alloys, no TRIP effect was actually observed. In fact, stress-induced martensitic (SIM) transformation occurred mainly at the {332}<113> twins/matrix interfaces for all the strained microstructures and acted as a localized stress-relaxation mechanism, delaying the fracture. Based on the electron backscatter diffraction (EBSD) analyses, in the Ti-8Mo-2Fe and Ti-10.5Mo-1Fe alloys, the formation of a dense network of {332}<113> twins was responsible for their high and steady work hardening rates (1370 and 1120 MPa) and large uniform elongations (22% and 34%). The absence of SIM α" as the primary mechanism of plastic deformation and solid solution hardening of Fe resulted in their high strengths (yield strength of 772 and 523 MPa). In Ti-9Mo-1Fe, the formation of mechanical twinning was hindered, resulting in limited strain-hardening capability and low uniform elongation (6%). The nanometer-sized α" particles in its β matrix along with the athermal ω precipitates are thought to impair the mechanical twinning and the ductility of this alloy.

On tailoring deployable mechanism of a bistable composite tape-spring structure

A bistable composite tape-spring (CTS) structure is a thin-walled open slit tube with fibres oriented at ±45°, which is stable at both its extended and coiled configurations. The traditionally manufactured CTS is effectively self-deploying. Here, we devise a novel manufacturing method to tailor the deployable mechanism of a CTS. This is achieved through adjusting the internal stress levels by subjecting the fully coiled as-manufactured CTS samples to additional heat treatment process. Their deploying performance were then characterised through a bespoke axial load monitoring rig, in order to reveal the underlying mechanics. It is demonstrated three different deployable mechanisms can be manufactured based on the same CTS, namely self-deploying, neutrally stable and self-coiling CTS structures. The processed CTS samples also show specialised stabilities, varying from mono-stability to multi-stability. The tailoring mechanism is then proposed. Since the CTS sample is in a highly strained state when coiled, the polymeric composite material would slowly deform due to viscoelasticity. According to the time-temperature superposition principle, the custom heat treatment of a fully coiled tape is equivalent to acceleration of the viscoelastic deformation, where local stress relaxation occurs on both the outer and inner tape surfaces, which in turn altering the internal stress levels. Thereby, the heat treatment processing is able to tailor the deployable mechanism of the CTS structure. These enrich the deployment diversity of the CTS to better fit the requirements for various deployable mechanisms in order to further promote the applications of CTS structures to aerospace explorations.

Experimental and Numerical Evaluations of Localized Stress Relaxation for Vulcanized Rubber.

Vulcanized rubbers are commonly used to provide the energy absorption under compressive deformation from other engineering components. However, if a constant compressive deformation is maintained on rubber, the load response is not constant but decreases with time; i.e., the stress relaxation. A decrease in force response with time of rubber can be experimentally evaluated by the stress relaxation test. In the present work, the localized stress of vulcanized rubber during a compressive stress relaxation test (i.e., ASTM D6147) was evaluated. Hyperelastic behavior was assumed during rapid application of strain, while the viscoelastic behavior was assumed during stress relaxation. Hyperelastic and viscoelastic parameters were experimentally evaluated using a standard specimen. Finite element analysis (FEA) models were applied for the predictions of stress relaxations of rubbers with various geometries and applied strains. FEA results were in good agreement with results of the stress relaxation tests. Localized stresses in rubber during rapid application of compressive strain and stress relaxation were successfully evaluated. The findings can give the localized phenomena of vulcanized rubber during a stress relaxation test, which can be used as a guideline for the design, usage, and improvement of rubber and viscoelastic polymeric components.

Molecular dynamic simulations of plasticity and phase transition in Mg polycrystalline under shock compression

We have investigated the shock-induced plasticity and phase transition in the hexagonal columnar nanocrystalline Mg by large-scale nonequilibrium molecular dynamics simulations. The preexisting grain boundaries (GBs) induce the nucleation of the {10–12} twins for the local stress relaxation. The twins grow up in grains leading to the orientation rotation. The phase transition from the hexagonal close-packed phase to the body-centered cubic phase begins when the migrating twin GBs meet in A- and C-type grains, and continues in the plastic deformation regions. The phase-transition pathway involves two steps: the reorientation and phase transformation.

Ion irradiation-induced localized stress relaxation in W thin film revealed by cross-sectional X-ray nanodiffraction

The influence of ion irradiation on residual stress and microstructure of thin films is not fully understood. Here, 5 MeV Si2+ ions were used to irradiate a 7 µm thick tungsten film prepared by magnetron sputtering. Cross-sectional X-ray nanodiffraction and electron microscopy analyses revealed a depth-localized relaxation of in-plane compressive residual stresses from −2.5 to −0.75 GPa after the irradiation, which is correlated with the calculated displacements per atom within a ~2 µm thick film region. The relaxation can be explained by the irradiation-induced removal of point defects from the crystal lattice, resulting in a reduction of strains of the 3rd order, manifested by a decrease of X-ray diffraction peak broadening, an increase of peak intensities and a decrease of lattice parameter. The results indicate that ion irradiation enables control over the residual stress state at distinct depths in the material.

Coupling effect of stretch-bending deformation and electric pulse treatment on phase transformation behavior and superelasticity of a Ti-50.8 at.% Ni alloy

In order to improve the functional stability of Ti-50.8 at.% Ni alloy, repeated stretch-bending deformation (SBD), electric pulse (EP), and age-hardening were combined. Phase transformation behavior and superelastic properties of NiTi alloy subjected to different treatments were compared and analyzed in terms of average grain size, dislocation morphology, and nanoscaled Ni4Ti3 precipitates. The results indicate that the average grain size was refined from the annealing state of 9.1 μm down to 5.4 μm, and that a certain amount of dislocations or dislocation networks were introduced via 7 SBD passes. As EP-processed, both local stress relaxation and the non-uniform temperature field facilitated the microstructure to be a more homogeneous and stable state. With the increase of pulse frequency from 150 Hz to 400 Hz, a two-stage martensitic transformation occurred to replace the one-stage phase transformation and the combined multi-process induced multi-stage martensitic transformation behavior. Nanoscaled Ni4Ti3 precipitates with sizes in the range of 20–40 nm were homogeneously distributed in the matrix of NiTi alloy after aging at 250 °C for 48 h. As a result, based on the coupling effect of “Annealing-SBD-150 Hz EP-aging”, excellent functional stability was obtained. The accumulative residual strain was reduced from the annealing state of 3.6%–1.1%.

Effect of Pulse Current on Residual Stresses in AMg6 Aluminum Alloy in Electrodynamic Treatment

The development of energy-efficient methods of controlling the stress state of metallic materials and welded joints is topical for modern production. One of such methods is electrodynamic treatment (EDT). It is based on the passage of electric current pulses (ECP) through the metal, time-synchronized with an impact load initiated by electrodynamic forces. In this case, an electrodynamic deformation process and a process of formation of elastic strain waves occur simultaneously in the material being treated; they are defined as “electrodynamic impact”, the result of which is a local stress relaxation in the metal. The developed mathematical model of the formation of stress and strain fields during EDT, which is based on the motion of an elastoplastic medium, does not take into account the effect of ECPs on dynamic relaxation processes. A computational and experimental assessment of the effect of ECPs, which is based on the experimental assessment of the stress state and microindentation of AMg6 aluminum alloy plates after EDT without and with the passage of ECPs through the metal being treated, has been performed by the mathematical modeling of the EDT process taking into account microindentation results. The passage of ECPs contributes to an increase in tensile plastic strains and hence to an increase in residual compressive stresses (provided that the specimens to be treated are rigidly fixed). The reliability of the procedure has been confirmed by the results of evaluating the distribution of residual welding stresses in AMg6 alloy butt joints after EDT.

Effect of twinning on the orientation dependence of mechanical behaviour and fracture in single crystals of the equiatomic CoCrFeMnNi high-entropy alloy at 77K

The plastic flow behaviour of single crystals of the CoCrFeMnNi high-entropy alloy is shown to be greatly influenced by the twinning multiplicity on the stress-strain (σ(ε)) curves, the strain hardening coefficient Θ = dσ/dε, plasticity and fracture. It is shown that a maximum effect of strain hardening is obtained during the onset of twinning in two systems in the [1¯11]- and [1¯44]-oriented crystals, while a transition to the development of twinning predominantly in one system simultaneously with slip, is accompanied by a decrease in strain hardening. At 77K in the [1¯11]-, [1¯23]- and [011]-oriented crystals, when twinning develops in one system with slip, the extension of the linear hardening stage and plasticity increase as a result of the suppression cross-slip and shifting the neck formation according to the Considère condition to higher stress levels. The development of deformation by twinning only in several systems and the suppression of the relaxation of local stresses during twin-twin intersection by slip or twinning, leads to the failure of quasi-cleavage in the [1¯44]-oriented crystals at 77K, after 50% strain. With the development of twinning predominantly in one system and a high density of dislocations, the [1¯11]-, [1¯23]- and [011]-oriented crystals show ductile failure, since the stresses for neck formation according to the Considère condition are reached before the stresses for brittle fracture.

Ratcheting Prediction at the Notch Root of Steel Samples Over Asymmetric Loading Cycles

Abstract The present study intends to evaluate local ratcheting and stress relaxation of medium carbon steel samples under various asymmetric load levels by means of two kinematic hardening rules of Chaboche (CH) and Ahmadzadeh-Varvani (A-V). The Neuber's rule was coupled with the hardening rules to predict ratcheting and stress relaxation at the vicinity of the notch root. Stress-strain hysteresis loops generated by the CH and A-V models were employed to simultaneously control ratcheting progress over stress cycles and stress relaxation at notch root while strain range kept constant in each cycle. The higher cyclic load levels applied at the notch root accelerated shakedown over smaller number of cycles and resulted in lower relaxation rate. The larger notch diameter of 9 mm on the other hand induced lower stress concentration and smaller plastic zone at the notch root promoting ratcheting progress with less materials constraint over loading cycles compared with notch diameter d = 3 mm. Predicted ratcheting results through the A-V and CH models as coupled with the Neuber's rule were found in good agreements with the experimental data. The choice of the A-V and CH hardening rules in assessing ratcheting of materials was attributed to the number of terms/coefficients and complexity of their frameworks and computational time/central processing unit (CPU) required to run a ratcheting program.

Influence of cyclic loading on the relaxation behavior of compressive residual stress induced by UIT

Fatigue strength of welded joints is improved by inducing compressive residual stress at weld toes. The ultrasonic impact treatment (UIT) is regarded as an effective method for inducing compressive residual stress at weld toes. However, excessive loads that cause plastic deformation at weld toes relieve the induced compressive residual stress. In addition, stress fluctuation affects the relaxation of residual stress even it is smaller than the yield strength. The relaxation of compressive residual stress decreases the fatigue strength improvement. Therefore, in order to evaluate the fatigue strength of welded joints enhanced by UIT, it is necessary to quantify the relaxation of local residual stresses at weld toes under cyclic loading. In this study, we measured a residual stress at the UIT groove using the X-ray diffraction method with a small-diameter collimator and an irradiation angle that was corrected based on the shape effect of the UIT groove, and we investigated its relaxation behavior under cyclic loading. In addition, we estimated the fatigue test results of the out-of-plane gusset joints improved by UIT using the modified Goodman diagram considering the relaxation of residual stress.

On the mechanisms of stress relaxation and intensification at the lithium/solid-state electrolyte interface

Abstract

Subgrain dynamics during recovery of partly recrystallized aluminum

The recovery dynamics of 498 individual subgrains within the non-recrystallized matrix of a 50% recrystallized aluminum sample is studied using high resolution X-ray diffraction at a synchrotron source. The subgrains’ change in size, orientation, axial strain and internal disorder are monitored as functions of temperature. The subgrains are surprisingly active, and the individual subgrains exhibit markedly different growth kinetics. Contradictory to common knowledge we find that the individual subgrains’ sizes change significantly even though the average size remain constant. Furthermore we observe no correlation between subgrain size and growth rate. An analysis of the volume changes of the individual subgrains indicates a broad spectrum of critical temperatures above which the subgrains grow or shrink. Possible mechanisms are discussed and related to relaxation of local stresses.