Stress Relaxation Mechanism Research Articles

Effects of pre-strain and stress level on stress relaxation ageing behaviour of 2195 Al–Li alloy: Experimental and constitutive modelling

In the present work, the stress relaxation behaviour, mechanical properties and microstructural evolution of 2195 Al–Li alloy with different pre-strains (0%, 2% and 4%) have been experimentally investigated under different initial stress levels at 180 °C for 16 h. It is found that pre-strain can extend the duration of the first stress relaxation stage and significantly promote the stress reduction. Meanwhile, a new “multi-step stage” stress relaxation feature, containing a particular linear relaxation stage, is observed for the 2195 Al–Li alloy during stress relaxation ageing (SRA) process. In addition, the tensile strength of the alloy increases with the increase of pre-strains and initial stresses, while the corresponding elongation is opposite. The evolution of precipitates, including G.P. Zones, θ″, θ′ and T1 phases, have been characterized by transmission electron microscope (TEM). The number of T1 phases increases with the increase of the pre-strains and initial stresses, while the number of θ′ phases gradually decreases. Based on precipitation kinetics, stress relaxation mechanism and dislocation theory, a physically-based SRA constitutive model considering the evolution of dislocation density and relative volume fraction of T1 and θ′ precipitates is established. The results calculated by the constitutive model are in good agreement with the experimental data.

Periodic array of misfit dislocation loops and stress relaxation in core-shell nanowires

In this paper, we consider the mechanism of misfit stress relaxation in lattice-mismatched core-shell nanowires (NWs) through the formation of a periodic array of misfit prismatic dislocation loops (PDLs). In doing so, we calculate and analyze the stress fields and strain energy of a PDL and the energy of pair elastic interaction between PDLs in an elastically isotropic cylinder with free surface. Based on these results, the critical parameters (misfit strain, ratio of core to shell radii) for the formation of a single misfit PDL and the equilibrium spacing of misfit PDLs in the periodic array of such PDLs in a core-shell NW are found. It is shown that this equilibrium spacing well corresponds to the experimental data available in the literature.

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

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.

Effect of Substrate Condition on the Structural and Tribotechnical Characteristics of Molybdenum Diselenide (MoSe2) Coatings

The results of studies on the influence of the substrate condition (stresses, roughness, grain size, texture) on the structure and tribotechnical properties of coatings applied onto compact molybdenum are presented. A difference is demonstrated for the structure of surface layers on cast and sintered molybdenum subjected to deformation associated with the relaxation of mechanical stresses induced in the course of manufacture thereof and with the transformation of grain size in the course of heat treatment.

Understanding and Control of Stress at Si-SiO<sub>2</sub> Interface

This paper presents the results of the investigation of stress relaxation at the Si-SiO2 interface using electron paramagnetic resonance (EPR) spectra, scanning electron microscopy (SEM) image technique, deflection analysis, X-ray photoelectron spectroscopy (XPS) and C-V characterisation of metal oxide semiconductor (MOS) structures. It has been shown, on the basis of EPR, XPS, C-V, and deflection data, that the mechanism of stress relaxation depends on the oxidation conditions: temperature, cooling rate and oxide thickness. In the Si-SiO2-Si3N4 system the stress relaxation occurs due to the difference in the thermal expansion coefficient of SiO2 and Si3N4 material. With an appropriate choice of oxidation conditions compressive stresses in SiO2 and tensile stresses in Si are almost equal and stress can be reduced considerably at the interface.

Stress in ion exchanged soda‐lime silicate and sodium aluminosilicate glasses: Experimental and theoretical comparison

AbstractGlass samples of two different chemical compositions were strengthened by ion exchange. Residual stress profiles were experimentally determined by differential surface refractometry. The experimental results were compared with values theoretically predicted by a formalized viscoelastic mathematical model incorporating both stress buildup and stress relaxation contributions. The comparison exhibited a remarkable agreement both in terms of strengthening characteristics and stress profiles curves between experimentally measured and theoretically predicted values. The mathematical model incorporated terms directly linked to selected relaxation mechanisms either isochoric shear stress driven and fast relaxation. The comparison demonstrated the relevance of the stress relaxation mechanisms for the different glass chemical compositions. The approach presented in this study will allow the prediction of the mechanical behavior of chemically strengthened glass starting from its chemical composition together with the ion exchange process parameters.

Infrared Laser Effect on Healthy and Ossified Costal Cartilage: The Development of Stable Load-Bearing Autoimplants.

The laser-induced stress relaxation provides new prospects to obtain stable long fragments of costal cartilage for autoimplantation avoiding the risk of spontaneous deformation and poor engraftment. However, the age-related alterations of cartilage may sufficiently influence its interaction with infrared (IR) laser radiation and disrupt the effectiveness and safety of the technique. The aim of the work is to study the influence of the structural quality of costal cartilage on its interaction with IR laser and efficiency of obtaining of curved implants for trachea surgery. Healthy costal cartilage was taken from pigs and human. Ossified costal cartilage was taken from humans of age 65 ± 7. The cartilage slices with a mean thickness of 3 mm were mechanically curved and processed to stress relaxation by laser irradiation with the wavelength 1.56 µm. The structure and mineral content were studied by X-ray microtomography and element analysis. The optical measurements included the study of the propagation of IR radiation, speckle interferometry, and IR radiometry. The aged cartilage demonstrates a high level of heterogeneity in structure and properties and decreased water content. The presence of dense inclusions consisting of amorphous calcined volumes makes the tissue more fragile and less elastic. The IR radiation propagation intensity for aged cartilage is at least twice higher than that for healthy cartilage. The thermal-induced motion of scatterers in aged cartilage is slower. X-ray microtomography showed the cartilage-like and the bone-like structures within the ossified samples. The main challenge for laser reshaping of aged cartilage is the presence of ossifications. However, the new stable curvature can be obtained with adjustment of laser power. To obtain the satisfying stable curvature of an implant the ossified volumes should be avoided The laser-induced stress-relaxation mechanism for aged cartilage can be particularly different from that of healthy tissue and the optimal laser regimes should be specified. Lasers Surg. Med. © 2020 Wiley Periodicals, Inc.

Theoretical study of post-shock stress relaxation and shock wave deformation around a pore in single and poly-crystalline α-RDX

The mechanical properties and shock dynamics of α-hexahydro-1,3,5-trinitro-1,3,5-triazine (α-RDX) are simulated by molecular dynamics (MD). Based on the simulation results, the equation of state, Hugoniot curve and isoentropic curve for α-RDX are calculated, and the microscopic structure of shock wave is investigated, including the wave profile and the shock front deformation. First, we prove that the shock wave profile is determined by the viscoelastic equation of α-RDX, and develop a method to inverse the viscoelastic coefficients from the velocity profile calculated by MD. Two stress relaxation mechanisms are obtained: the boundary scattering and molecular rotation. The boundary scattering mechanism shows that the damping time of stress relaxation is determined by the particle size of polycrystal. Second, we find that the pore collapse induces rarefaction wave after the shock front. The rarefaction wave function is evaluated by using the Riemann invariant method, and the shock front deformation is derived from the superposition rule of shock wave and rarefaction wave. A physical picture to describe the nanoscale pore collapse process for α-RDX is obtained.

Study on Springback Straightening after Bending of the U-Section of TC4 Material under High-Temperature Conditions.

The thin-walled structures of titanium alloys have peculiar characteristics involving thin curved surfaces, complicated structures, and a poor rigidity. Therefore, bending or twisting distortion frequently occurs in forging, extrusion, drawing, transportation, cooling, and manufacturing. Straightening theory focuses on the straightening curvature or bending moment at room temperature, and a unified analytical model of the straightening curvature, the straightening bending moment, and the straightening stroke, as well as a study on springback straightening under high-temperature conditions, have not been investigated comprehensively. In order to understand the inherent mechanism of springback straightening and quantitative prediction of springback under high-temperature conditions, uniaxial tension tests were carried out to obtain the true stress–strain model of material and stress relaxation under the stress relaxation model. This paper is based on the theory of elastic-plastic mechanics and combines this with the mechanism of stress relaxation to establish springback and residual relative curvature equations of springback. The law of springback straightening is further explored, and springback and residual deflection equations are provided. The results of the study showed that the relative errors of the theoretical residual deflection of the bending deformation and residual deflections obtained by the experiment were less than 20%, with an average absolute error of less than 10%. Therefore, the hardening models adopted can achieve an allowable relative error if hardening parameters are properly selected. The proposed research provides basic data for the prediction of springback straightening, and the design of springback compensation tools can be applied in practical applications.

Thermal crack formation in TiCN/α-Al2O3 bilayer coatings grown by thermal CVD on WC-Co substrates with varied Co content

Within this work, the thermal stress build-up of chemically vapor deposited TiCN/α-Al2O3 bilayer coatings was controlled by tuning the coefficient of thermal expansion (CTE) of the substrate material. This was implemented through a Co content variation from 6 to 15 wt.% in WC-Co substrates, which exhibit higher CTEs with increasing Co contents and thereby approach the CTE values of TiCN and α-Al2O3. High temperature X-ray diffraction was employed to determine thermal expansion of an α-Al2O3 powder. Crystallographic texture of the α-Al2O3 coating layer was evaluated by electron backscatter diffraction and taken into consideration in order to assign the appropriate in-plane CTE. This consideration indicated a lower CTE mismatch of α-Al2O3 with WC-Co, compared to TiCN with WC-Co. X-ray diffraction was further utilized for the determination of residual stress in TiCN and α-Al2O3, demonstrating a decrease in both layers for Co contents below 12.5 wt.%. Decreasing stress signaled the formation of thermal crack networks confirmed by scanning electron microscopy surface images. Lower residual stresses were determined in TiCN compared to α-Al2O3 layers of bilayer coatings, contradicting finite element simulations of thermo-elastic stress, that were carried out to illustrate the stress relaxation effects caused by thermal cracks. Monolayer TiCN coatings were annealed at 1000 °C, to replicate stress relaxation taking place during α-Al2O3 deposition, exhibiting a similar residual stress state to TiCN base layers of bilayer coatings. Thermal crack formation was found to be the dominating stress relaxation mechanism in α-Al2O3, while TiCN undergoes further relaxation through secondary mechanisms.

Shear localization as a mesoscopic stress-relaxation mechanism in fused silica glass at high strain rates

Molecular dynamics (MD) simulations of fused silica glass deforming in pressure-shear, while revealing useful insights into processes unfolding at the atomic level, fail spectacularly in that they grossly overestimate the magnitude of the stresses relative to those observed, e. g., in plate-impact experiments. We interpret this gap as evidence of relaxation mechanisms that operate at mesoscopic lengthscales and which, therefore, are not taken into account in atomic-level calculations. We specifically hypothesize that the dominant mesoscopic relaxation mechanism is shear banding. We evaluate this hypothesis by first generating MD data over the relevant range of temperature and strain rate and then carrying out continuum shear-banding calculations in a plate-impact configuration using a critical-state plasticity model fitted to the MD data. The main outcome of the analysis is a knock-down factor due to shear banding that effectively brings the predicted level of stress into alignment with experimental observation, thus resolving the predictive gap of MD calculations.

Near-zero restrained shrinkage polymer concrete incorporating ceramsite and waste rubber powder

In this study, we reduced the restrained shrinkage of lightweight polymer concrete (LWPC) containing waste rubber powder (WRP) to develop a novel low-shrinkage LWPC for use in wet flue gas desulfurization (WFGD) or other high-temperature and strongly corrosive environments. We investigated the effects of ceramsite aggregate and WRP (0%, 2.5%, 5.0% and 7.5% of binder) on restrained shrinkage and the microstructure of polymer concrete (PC). The experimental results indicate that restrained shrinkage in PC with ceramsite aggregate and WRP is significantly less than that of PC with crushed stone aggregate. Unusual stress relaxation behavior of LWPC was observed. When WRP reached 7.5% of binder, the value of restrained shrinkage was nearly zero. Ultra-depth three-dimensional microscopy (UDTDM), X-ray computed tomography (XCT), and scanning electron microscopy (SEM) were used to characterize PC properties for the determination of stress relaxation mechanisms.

Prediction of machining-induced residual stress in orthogonal cutting of Ti6Al4V

In addition to having significant effects on the service life of machined parts, residual stress can cause large deformations during machining of thin-walled parts in aerospace field. The residual stress in machined part surface layer is induced by mechanical and thermal stress in the machining process of parts. Therefore, it is essential to accurately predict the generated residual stress due to the machining process. To address this issue, an improved prediction model of the residual stress for orthogonal cutting process based on analytical method is presented by taking the mechanical and thermal stress into consideration. Oxley’s cutting force model and Waldorf’s plowing force model are used to calculate the chip formation force and plowing force based on the contact mechanics, Johnson-Cook constitutive model, and slip line theory. The cutting temperature is predicted according to our previous work based on the Huang-Liang model and the Komanduri-Hou model. The final residual stress is predicted by calculating the loading and relaxation of mechanical and thermal stress. The presented prediction method of residual stress is validated by orthogonal cutting of a Ti6Al4V pipe. It is found that the prediction values of the presented model show a good agreement with the experimental results, which indicates that the presented model can be adopted to predict the residual stress of Ti6Al4V during orthogonal cutting.

Effect of grain size on high-temperature stress relaxation behavior of fine-grained TC4 titanium alloy

Abstract In order to analyze the effect of grain size on stress relaxation (SR) mechanism, the SR tests of TC4 alloy with three kinds of grain size were performed in a temperature range of 650–750 °C. A modified cubic delay function was used to establish SR model for each grain size. A simplified algorithm was proposed for calculating the deformation activation energy based on classical Arrhenius equation. The grain size distribution and variation were observed by microstructural methods. The experimental results indicate that smaller grains are earlier to reach the relaxation limit at the same temperature due to lower initial stress and faster relaxation rate. The SR limit at 650 °C reduces with decreasing grain size. While the effect of grain size on SR limit is not evident at 700 and 750 °C since the relaxation is fully completed. With the increase of grain size, the deformation activation energy is improved and SR mechanism at 700 °C changes from grain rotation and grain boundary sliding to dislocation movement and dynamic recovery.

Effects of an interfacial layer on stress relaxation mechanisms active in the Cu-Si thin film system during thermal cycling

The authors report effects of placing a very thin metallic interlayer, such as W and Ni, in between the Cu film and the Si substrate on cyclic thermal stress-induced interfacial sliding and hillock growth in Cu. Cu–Si samples with no interlayer were the most prone to both interfacial sliding and hillock growth, whereas samples with the Ni interlayer were the most resistant against these deleterious phenomena. While the rate of interfacial sliding decreased with each consecutive thermal cycle, hillocks continued to grow undeterred. The obtained experimental results are discussed, considering the compressive stress field generated in the Cu film.

Mechanism of stress relaxation and phase transformation in additively manufactured Ti-6Al-4V via in situ high temperature XRD and TEM analyses

Additive manufacturing is being increasingly used in the fabrication of Ti-6Al-4V parts to combine excellent mechanical properties and biocompatibility with high precision. Unfortunately, due to the build-up of thermal residual stresses and the formation of martensitic structure across a wide range of typical processing conditions, it is generally necessary to use a post-thermal treatment to achieve superior mechanical performance. This investigation aims to obtain a deeper understanding of the micro/nanostructural evolution (α′ martensite phase decomposition), accounting for the kinetics of phase transformation during the heat treatment of 3D-printed Ti-6Al-4V alloy. As the mechanism of phase transformation and stress relaxation is still ambiguous, in this study the changes in crystal lattice, phase, composition and lattice strain were investigated up to 1000°C using both in situ high temperature X-ray diffraction (XRD) and transmission electron microscopy (TEM). Based on the result a mechanism of phase transformation is proposed, via the accommodation/substitution of Al, V and Ti atoms in the crystal lattice. The proposed mechanism is supported based on elemental concentration changes during heat treatment, in combination with changes in crystal structure observed using the high temperature XRD and TEM measurements. This study provides a deeper understanding on the mechanism of phase transformation through martensitic decomposition, as well as a deeper understanding of the influence of post-thermal treatment conditions on the alloy's crystal structure.

Experimental and Numerical Simulation to Study the Reduction of Welding Residual Stress by Ultrasonic Impact Treatment.

In this study, the effects of ultrasonic impact treatment (UIT) on the residual stress in a repair welding joint are investigated by experimental and finite element methods. A three-dimensional numerical analysis approach including a thermomechanical-coupled welding simulation and dynamic elastic-plastic UIT simulation is developed, which has been validated by X-ray diffraction measurement and indentation strain method. The results show that longitudinal residual stresses basically turned into the small tensile stress state from the large tensile stress state, and transverse residual stresses have mainly turned into compressive stresses from large tensile stress after the UIT. In the thickness direction, the average decrease of longitudinal residual stress is 259.9 MPa, which is larger than the 149.1 MPa of transverse residual stress. The calculated residual stress distribution after the UIT of the thin plate is compared with that of the thick plate in the literature, with the results showing the stress accumulation layer inside the thick plate. The simulation results show that the elastic strains are decreased slightly and the equivalent plastic strain is increased markedly after UIT, which explains the mechanism of residual stress relaxation.

Grain growth stagnation in thin films due to shear-coupled grain boundary migration

Normal grain growth in thin films attached to a substrate has been considered. It has been shown that shear-coupled grain boundary migration results in the build-up of elastic stresses in the film. A semi-quantitative model of grain growth combining the elements of disclinations theory with the Burke-Turnbull model of normal grain growth has been proposed. It has been shown that shear-coupled grain boundary migration may lead to stagnation of normal grain growth, whereas for high coupling factor values, the grains in the film do not grow at all. Finally, the mechanisms of stress relaxation enabling some grain growth are discussed.

Measurement of the silica glass fatigue limit

AbstractThe static fatigue limit, or the threshold stress intensity factor, Ko, for first subcritical crack growth has been measured directly in silica glass for T ≥ 600°C using the double cantilever beam (DCB) crack growth technique. Values measured ranged from 0.48 to 0.61 MPa·m1/2 for a temperature range of 600°C‐850°C, respectively. Cracks growing near the static fatigue limit had a time‐dependence, where the crack growth decreased and appeared to stop at K ≈ Ko. Slow crack growth curves (K‐v) have been measured from room temperature, 50% RH, up to 850°C with subcritical crack growth not measurable for T > 900°C. Increasing temperature was found to first increase, and then decrease the slope of Region I, and a peak in fatigue resistance was found around 150°C‐300°C. At T > 600°C subcritical crack growth was observed for K higher than previously measured KIC values. This observation and the static fatigue limit in silica are explained by a water‐assisted stress relaxation mechanism at the crack tip.

Strain, Stress and Stress Relaxation in Oxidized Zrcual-Based Bulk Metallic Glass

Surface engineering of Zr51.3Al8.5Cu31.3Ni4Ti4.9 bulk metallic glass (BMG) by gaseous oxidizing below the glass-transition temperature is investigated as a means to introduce compressive residual stress in the surface region. The ZrCuAl-based BMG was exposed to an extremely low oxygen partial pressure of 10-41 bar at 600 K for 60 h. The oxidizing treatment led to the formation of an internal oxidation zone, consisting of finely dispersed crystalline cubic ZrO2 (c-ZrO2), metallic regions inclined with the surface and Cu-hillocks at the surface. The stresses introduced by the volume expansion associated with oxidation were evaluated from i) the lattice strains within c-ZrO2, as determined with an X-ray diffraction (XRD) based method, and ii) strain-relaxation as a response to annular focused ion beam (FIB) milling, as monitored with digital image correlation (DIC). XRD analysis yielded -1.5 GPa (compressive stress) in the crystalline c-ZrO2, while the strain relaxation monitored with FIB-DIC analysis indicated compressive residual stresses of -1.4 GPa in the internal oxidation zone. The strain and stresses determined with the independent measurement methods are discussed. The quantitative macro-strains are discussed in relation to the microstructural features and stress relaxation mechanisms during evolution of the internal oxidation zone.