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During low‐temperature tempering of wear‐resistant steel, ε‐carbide precipitation can effectively relax residual stress, and its microscopic mechanism is currently unclear. To reveal the stress relaxation mechanism during ε‐carbide precipitation, density functional theory calculations are combined with experiments to explore the ε‐carbide growth behavior and the stress evolution. The results show that ε‐carbide precipitation includes two stages: ε‐precursor formation and ε‐carbide nucleation, and ε‐carbide is in situ nucleated from ε‐precursor matrix. The precipitation‐induced stress relaxation behavior mainly occurs in the early stage of ε‐precursor formation and ε‐carbide nucleation. The early stage of ε‐precursor formation and ε‐carbide precipitation can relax the residual stress to 35.73% and 27.07%, respectively. [ABSTRACT FROM AUTHOR]

This paper presents geometric analyses of welded frames after free relaxing and vibratory stress relief (VSR). The tested frames were components of a prototype packaging machine. Two types of relaxation were carried out to remove stresses introduced as a result of the welding process. One of the frames was subjected to free relaxation, while the other one was subjected to accelerated vibration relaxation. Detection of the frame geometry changes was performed using a photogrammetric system. In addition, an evaluation of the geometry change was conducted for fifteen variants of a steel frame support. A comparative analysis of the geometric deviations of the frames after free and vibratory stress relief confirmed the assumption that the frame post vibration stress relief better reproduces the nominal dimensions. Nevertheless, it should be emphasized that after vibratory stress relief, the frame is not subject to further deformation, which is a desirable effect. In the case of free relaxing, the frame undergoes dimensional changes in a random manner. In summary, carrying out accelerated vibratory stress relief allows for control of spontaneous dimensional changes in the designed frame of a packaging machine resulting from spontaneous relaxation of stresses arising from the welding process. The shortening of the relaxation process of the welded frame is also an unquestionable advantage. [ABSTRACT FROM AUTHOR]

To describe the evolution of residual stresses in epoxy resin during the curing process, a more detailed characterization of its viscoelastic properties is necessary. In this study, we have devised a simplified apparatus for assessing the viscoelastic properties of epoxy resin. This apparatus employs a confining cylinder to restrict the circumferential and radial deformations of the material. Following the application of load by the testing machine, the epoxy resin sample gradually reduces the gap between its surface and the inner wall of the confining cylinder, ultimately achieving full contact and establishing a continuous interface. By recording the circumferential stress–strain on the outer surface of the confining cylinder, we can deduce the variations in material bulk and shear moduli with time. This characterization spans eight temperature points surrounding the glass transition temperature, revealing the bulk and shear relaxation moduli of the epoxy resin. Throughout the experiments, the epoxy resin's viscoelastic response demonstrated a pronounced time-temperature dependency. Below the glass transition temperature, the stress relaxation response progressively accelerated with increasing temperature, while beyond the glass transition temperature, the stress relaxation time underwent a substantial reduction. By applying the time-temperature superposition principle, it is possible to construct the relaxation master curves for the bulk and shear moduli of the epoxy resin. By fitting the data, we can obtain expressions for the constitutive model describing the viscoelastic behavior of the epoxy resin. In order to validate the reliability of the test results, a uniaxial tensile relaxation test was conducted on the epoxy resin casting body. The results show good agreement between the obtained uniaxial relaxation modulus curves and those derived from the bulk and shear relaxation modulus equations, confirming the validity of both the device design and the testing methodology. [ABSTRACT FROM AUTHOR]

Changes in grain size of two firn (density from ∼550 to ∼820–840 kg m−3) samples, extracted at Summit Greenland during their stress relaxation over 3 days were observed using optical micrographs. These changes were confirmed by both 3-D microstructural reconstruction images and plots of microstructural parameters vs. time, viz., the grain size increased with the decrease of pore size, the specific surface area, and the index of prevalent ice curvature. Building on studies of both the viscoelastic behaviour of firn deformation and the measured changes in grain size, a quadratic-based model is constructed to describe the evolution of grain size during the stress relaxation stage, where the grain growth is in relation to the microstructure, the relaxation time, and the threshold stress (an equilibrium residual stress at the grain size maximum, which forces the grain growth before the arrival of the grain size maximum). This model can be scaled up from laboratory samples to the glaciers and ice sheets, e.g. the time at the grain size maximum spanning across days to thousands of days, even to a geological age. In addition to the firn samples which have undergone the stress relaxation prior to laboratory tests, especially sensitive to the effect of grain size, should be calibrated, this work also sheds light on learning more about the mechanical behaviours and changes in grain size during the deformation of firn or other porous materials under interrupted or cycle load, particularly from the stress relaxation, irrespective of the compression, tension, or shear. [ABSTRACT FROM AUTHOR]

• The symplectic approach is applied to solve the strip cross section temperature. • The mechanism of residual stress formation in hot strip rolling is studied by fiber rods model. • An analytical model for the coupling of temperature, crown and residual stress is established. • The strip critical exit crown ratio is obtained by using the residual stress analytical model. • Eliminating manifested and latent flatness defects by critical exit crown ratio of strip. Residual stress is introduced during hot-rolled strip production due to various factors, such as temperature distribution, crown change, and metal transverse flow. Excessive residual stress can cause flatness defects in the strip, while insufficient residual stress may result in deformation during subsequent cutting. To explore the mechanism of residual stress formation in hot-rolled strip and how to control it, an analytical model for the coupling of temperature, crown and residual stress is established. Firstly, based on the symplectic space Hamiltonian system, an analytical solution for the strip cross section temperature field in each inter-stand zone is obtained. Then the residual stress analytical model is derived based on the fiber rod model combined with the law of volume invariance, lateral metal flow function and high temperature stress relaxation phenomenon. Meanwhile, to verify the accuracy of the analytical model, a multi field coupled finite element model based on the measured data validation is established. The calculation results of the analytical model are in good agreement with the finite element model. Furthermore, the buckling model of strip in symplectic space Hamiltonian system is derived, and the critical buckling stress of different strip flatness defects is directly calculated. Finally, the residual stress analytical model is combined with critical buckling stress to calculate the critical exit crown ratio to keep the strip flat. [ABSTRACT FROM AUTHOR]

The rheological response of two commercial ferrofluids to transient shearing flows using a parallel disk rheometer device equipped with a magnetic cell is investigated. The basic difference between the ferrofluids is their volume fraction of magnetic particles. The first transient shear flow examined is a step-strain under the influence of a magnetic field, from which the stress relaxation functions for both magnetic fluids studied are obtained in terms of the magnetic field strength and the intensity of the step strain. The main relaxation times of both fluids are determined and shown to increase with the applied magnetic field parameter after some critical value. We also observed that the shear stress relaxes to a residual stress, which is strongly dependent on both magnetic field and strain strengths. This remarkable residual stress increases as the intensity of the magnetic field rises. In terms of the strain strength, this residual stress is found to have two interesting behaviors. First, for small values of strain, the residual stress increases linearly until a maximum is reached. Further increases in the strain strength lead to a nonlinear decrease in the residual stress. We conjecture that the linear regime is associated with a predominance of elastic deformation of the fluid microstructure while the nonlinear one to its plastic deformation or even to the structure breakup. The second experimental investigation of the magnetic fluids is carried out under the condition of oscillatory shear in a linear viscoelastic regime and in the presence of an applied magnetic field. The main viscoelastic moduli of the ferrofluids as functions of the non-dimensional frequency and the magnetic field intensity are presented. In addition, it is also shown, for both ferrofluids, that viscous and elastic characteristics are severely increased when the applied magnetic field intensity is enhanced. We also determine the shear elastic modulus for both magnetic fluids in the limit of low Deborah number as a function of the magnetic parameter. Compatibility checks between the viscous modulus and the apparent shear viscosity under conditions of the same frequency and shear rate are performed, and the first normal stress difference is calculated. [ABSTRACT FROM AUTHOR]

High‐strength steels with low carbon content have low stress relaxation during tempering and are prone to distortion during processing. To improve tempering stress relaxation, 10% compression predeformation is introduced. Carbide precipitation is characterized by dislocation density and dilatometry curves, and the evolution of residual stress during tempering is investigated. The results show that predeformation can inhibit cementite precipitation, increase the effective carbon concentration in the ferrite matrix, and induce the diffuse precipitation of alloy carbides. The dispersed alloy carbides provide more interfaces for Mn partitioning, strengthen the transformation plasticity, and enhance the stress relaxation. After tempering at 700 °C for 30 min, the average residual stress and the elastic strain energy of undeformed/predeformed specimens are 39.18/13.25 MPa and 25.40%/90.92%, respectively. [ABSTRACT FROM AUTHOR]

Large aluminum alloy ring forgings are the core components of heavy-duty rocket fuel storage tanks, and the large residual stress inside the rings leads to poor shape accuracy of large thin-walled parts. The initial stress of the 2219 aluminum alloy ring blank was tested using the drilling method, and the creep constitutive coefficient of the 2219 aluminum alloy was determined through stress relaxation tests. The numerical simulation processes of thermal stress relief (TSR), vibration stress relief (VSR), and thermal–vibration stress relief (TVSR) were compared and established. Through the correlation analysis between the actual measurement results of residual stress and the simulation results, it can be seen that the strong correlation in three directions at each measurement point accounts for over 37.5%, and the moderate correlation accounts for over 62.5%. This indicates that the numerical simulation model of 2219 aluminum alloy ring containing initial residual stress can accurately reflect the size and distribution of residual stress inside the actual ring. The simulation results show that the derived constitutive model can describe the stress relaxation process of TVSR by combining a single thermal time effect stress relaxation constitutive theory with a VSR plastic deformation material model. The simulation models established above were used to calculate the residual stress homogenization ability of three types of aging. The results showed that VSR, TSR, and TVSR can homogenize and reduce the residual stress field inside the ring, improve the distribution of residual stress inside the ring, and have a better overall homogenization ability of TVSR. The VSR control has a certain effect on reducing and homogenizing residual stress, but compared with TSR and TVSR, the reduction and homogenization ability of residual stress control is limited. The homogenization control effect TVSR > TSR > VSR, and the maximum equivalent stress homogenization rates of VSR, TSR, and TVSR are 52.8%, 80.6%, and 82.2%, respectively. Then, numerical simulation technology was used to study how the initial residual stress in the blank causes the deformation of the ring during the thin-walled machining process. The roundness error theory of the minimum containment area method was applied to evaluate the deformation degree during the thin-walled numerical machining process, and the TVSR method was used for stress regulation. The deformation law of the thin-walled machining of the ring under different aging parameters was studied. [ABSTRACT FROM AUTHOR]

In this study, a computer simulation technique is developed for numerically predicting the residual stress relaxation caused by bending-straightening in quenching-induction-hardened axle shafts. Both the strength and residual stress distributions inside the shaft before bending-straightening are taken into account in the simulation model. The calculated residual stress distribution is compared with that measured using the contour method to discuss the effectiveness of the developed technique. In addition, the generation of plastic strain due to bending-straightening is considered thorough a theoretical analysis based on material mechanics in addition to the numerical results obtained using the developed simulation technique. [ABSTRACT FROM AUTHOR]

In order to lower the level of residual stresses, welded joints of a number of structural steels are subjected to general or local post-weld heat treatment by the high-temperature tempering mode. Mathematical modeling methods are widely used, alongside the experimental investigation methods, to satisfy the continuously growing requirements to welded joint quality and to optimize the welding technology. Mathematical modeling of the process of welded joint heat treatment is often performed using a simplified creep function by Norton-Bailey law at a fixed soaking temperature, but here the processes of ductile deformation taking place during slow prolonged heating and cooling, are ignored. The effectiveness of application of different models of temperature creep for mathematical modeling of the processes of relaxation of residual stresses in welded joints was studied in this work with the purpose of developing recommendations for their application for various characteristic cases of post-weld heat treatment. Comparison of the results of modeling the process of stress relaxation performed on a number of examples, showed that the simplified creep function at short-term soaking during general furnace treatment can give an error compared to a more general creep model. Modeling of the local heat treatment technology revealed that the complex geometry of the component and poor choice of the heating element arrangement may lead to negative consequences, namely formation of new high residual stresses. Proper modeling of the processes of relaxation and redistribution of residual stresses in welded joints and structures as a result of general (furnace) or local post-weld heat treatment may optimize the process of furnace heat treatment and improve the quality and fatigue life of the welded structures. 16 Ref., 1 Tabl., 12 Fig. [ABSTRACT FROM AUTHOR]

In order to lower the level of residual stresses, welded joints of a number of structural steels are subjected to general or local postweld heat treatment by the high-temperature tempering mode. mathematical modeling methods are widely used, alongside the experimental investigation methods, to satisfy the continuously growing requirements to welded joint quality and to optimize the welding technology. mathematical modeling of the process of welded joint heat treatment is often performed using a simplified creep function by Norton–Bailey law at a fixed soaking temperature, but here the processes of ductile deformation taking place during slow prolonged heating and cooling, are ignored. the effectiveness of application of different models of temperature creep for mathematical modeling of the processes of relaxation of residual stresses in welded joints was studied in this work with the purpose of developing recommendations for their application for various characteristic cases of postweld heat treatment. comparison of the results of modeling the process of stress relaxation performed on a number of examples, showed that the simplified creep function at short-term soaking during general furnace treatment can give an error compared to a more general creep model. modeling of the local heat treatment technology revealed that the complex geometry of the component and poor choice of the heating element arrangement may lead to negative consequences, namely formation of new high residual stresses. Proper modeling of the processes of relaxation and redistribution of residual stresses in welded joints and structures as a result of general (furnace) or local post-weld heat treatment may optimize the process of furnace heat treatment and improve the quality and fatigue life of the welded structures. [ABSTRACT FROM AUTHOR]

The effect of heat treatment on the relaxation properties of austenitic steels is studied with the aim to choose the treatment modes providing durability of bolts, flange connectors and other fasteners. The experimental data obtained show different intensities of stress relaxation after different modes of quenching and tempering. The most efficient and economically expedient treatment for the steels studied is suggested. [ABSTRACT FROM AUTHOR]

• Long life 500,000 cycle residual stress relaxation affects. • Cyclic mean stress relaxation, various plasticity levels. • Predictive methods for inclusion of residual stress relaxation. • High cycle residual stress measurements, XRD. • Neuber stress–strain notch correction, residual stress. • Smith Watson Topper mean stress life model. The presence of residual stress in welded components can have a significant impact on fatigue response. Residual stresses were assessed in thick welded A36 cantilever specimens (T-specimens) at the fatigue critical area. Testing involved subjecting the T-specimens to varying bending stresses and load cycles, leading to different levels of local plasticity at the weldtoe. Extensive X-ray diffraction (XRD) measurements were conducted to analyze the residual stress distribution near the weldtoe. Residual stress states were evaluated in both as-welded specimens and those subjected to fatigue loading with strain amplitudes up to 0.001 at the weldtoe stress concentration zone. The findings suggest that fatigue loading can induce changes in the welding-induced residual stress field, ranging from moderate to significant alterations. Therefore, fatigue life predictions should consider the potential evolution of residual stresses due to cyclic loading, including the effects of cyclic stress relaxation in fatigue life calculations. [ABSTRACT FROM AUTHOR]

The rolling correction process can eliminate machining distortions of aluminum alloy 7075-T651 structural parts. The thermal stability of the corrected structural parts under the action of temperature loading, especially the macroscopic shape stability, is key to ensure the safe service of mechanical equipment. In this study, different thermal loads were used to postprocess aluminum alloy 7075-T651 structural parts after rolling correction. The thermal stability of the rolled samples was analyzed by characterizing the microstructure and physical and mechanical properties. The results show no obvious change compared to rolling in the distortion of the parts after temperature treatments at 120 °C, 160 °C, 230 °C and 300 °C; the distortion changes were only 10.48%, 2.74%, 8.13% and 8.70%, respectively. The residual stresses in the rolling areas of the samples decreased by 35.58%, 26.08%, 75.97% and 83.13%, respectively. The microhardness also showed a decreasing trend. There was no obvious change after treatment at 120 °C, but the hardness decreased by approximately 5%, 23% and 56%, respectively, after treatments at other temperatures. However, the rolling stress relaxed under thermal stress. The microstructure change analysis shows that the material microstructure is mainly dominated by static reversion at lower thermal loads. With increasing thermal load, the samples are mainly affected by the static recrystallization effect, the microstructure is gradually blurred, and the hardness decreases significantly. In conclusion, although the residual stresses introduced by rolling would occur in different degrees of stress relaxation under the thermal load, the microstructure changes caused by thermal load did not significantly affect the macroscopic distortion of the samples, and the macroscopic shape of the structural parts after rolling correction had good thermal stability. [ABSTRACT FROM AUTHOR]

Stress relaxation properties have a significant impact on the performance of polyvinyl chloride (PVC) geomembranes (GMBs) at the peripheral joints of the membrane faced rockfill dam (MFRD). This paper presents a fractional order viscoelastic model (FOVM) to measure the relaxation stress as a function of time. Model parameters were obtained by best fit to results from wide-width strip tensile tests conducted at three tensile rates and three initial strains for 48 h. The results of a 90 d stress relaxation test demonstrate the applicability of the model to describe the stress relaxation behavior of PVC GMBs. The tensile rate and initial strain marginally influenced the relaxation modulus rate, while having no effects on the fractional derivative order. Residual stress could account for the difference in relaxation stress between the longitudinal and transverse specimens. Finally, the FOVM could be used for predicting the service cycle under specifying failure stress criteria. Furthermore, it has great potential for applications in predicting the long-term deformation of PVC GMBs at the peripheral joints of MFRD. Furthermore, it has great potential for applications in predicting the long-term deformation of PVC GMBs at the peripheral joints of MFRD. [ABSTRACT FROM AUTHOR]

The method of calculating the vibration treatment conditions for welded metallic structure elements based on the complex cycle stress limit diagram is proposed to reduce residual stresses by a preset value without any danger of fatigue damage at the process stage. The method is based on the relation between the quantitative reduction of residual stresses and accumulated plastic strains. The method was tried out in the tests of 20 steel box beams and St3 steel plates clad lengthwise. The tests of 45 steel specimens were used as an example to experimentally and analytically establish that inspective of means that caused plastic strains, viz in static tension or cyclic loading at stress relaxation and different external loading amplitudes, residual stresses decrease practically by the same value. [ABSTRACT FROM AUTHOR]

The processing of polymers and polymeric materials always leaves some residual stresses in the final product, which can have a positive but usually a negative effect on the properties of the final product. Polycarbonate was injection molded into the mold, which was heated to 50, 60, or 70°C to produce test specimens with different amounts of residual stresses. They were then annealed at various temperatures below Tg to partially remove the stresses or heated to 145°C (above Tg) to completely remove them. Comparison between annealed and as‐molded samples showed a strong influence of residual stresses on mechanical properties. Dynamic mechanical analysis (DMA) analysis showed that the loss modulus can be divided into the intrinsic loss modulus, which is a material property, and the residual stress, which can be relaxed at a given temperature. The stress relaxation curve, obtained by subtracting the loss modulus of the annealed sample from the loss modulus of the as‐molded sample gives us the ability to estimate the residual stresses that have been or can still be relaxed by heating to a given temperature. [ABSTRACT FROM AUTHOR]

Aluminum alloys are commonly used in the aerospace industry because of their good specific strength and corrosion resistance. However, reducing residual stresses and machining deformation of aluminum alloy components are the focus of researches. In this paper, a material model of 2A14 aluminum alloy during solution treatment and aging process is established with thermal expansion, high temperature tensile and stress relaxation experiments. The stress relaxation during aging is described with the Zener-Wert-Avrami model based on the experimental data. Quenching and aging experiments of 2A14 aluminum alloy C-ring samples were carried out. Temperature changes of C-rings were measured during quenching, residual stresses in the samples after solution, and aging are measured by XRD, respectively. Based on measured temperature variations, surface heat transfer coefficients of a C-ring during quenching were calculated by inverse heat transfer calculation and a novel optimization method. The changes of temperature, stresses, and notch opening distance of a C-ring during solution and aging process were calculated, and calculated results were in good agreement with the measured results. [ABSTRACT FROM AUTHOR]

Shear cutting processes have a detrimental effect on fatigue of high strength metal components. The effect tends to increase with material grade, counteracting the task of reducing weight in chassis components using higher strength materials. Base material fatigue data are often available, but assessment of components with cut edges often require additional costly and time-consuming testing. This paper provides a methodology for estimation of fatigue life reduction by using residual stresses obtained from process simulations, and measured surface roughness in the cut edge. A stress relaxation criterion is applied to handle reduction of the initial local residual stresses. Two complex phase steels and one aluminum alloy are studied for validating the approach. Polished fatigue data is reduced to estimate S–N curves of trimmed and punched specimens at different load ratios. Good agreement between the model and test results are found for all cases. The needed data for the predictions are only a high cycle S–N relationship for polished material, uniaxial tensile properties, and the cut edge fracture surface residual stress and roughness without any parameter fitting, making it a convenient tool for estimating the reduction in fatigue life and for parameter studies. • A method to estimate HCF life of shear cut specimens. • Surface roughness, residual stresses and load ratio are considered. • No further calibration is needed if the S–N curve for the base material is known. • Cyclic stress relaxation is accounted for. • Validated with fatigue testing of polished and shear cut specimens. [ABSTRACT FROM AUTHOR]

A comparative study on the mechanical properties and residual stress profiles of ceramic coatings produced using plasma spraying under comparable conditions are presented. Alumina, titania, alumina-13%titania, zirconia and chromia coatings were deposited, for the first time, under similar particle velocity and a comparable particle temperature to melting temperature ratio. The coatings were characterized by porosity, hardness, indentation modulus, and fracture toughness, and full residual stress depth profiles of the coated specimens. The degree of particle melting and associated coating properties relied on the latent heat of fusion of the feedstock. The top coatings were found to possess different residual stress field depending on their coefficient of thermal expansion. The effect of tensile quenching stress is perceived when the difference in thermal expansion coefficients between the topcoats and the bond coat is low, e.g., titania and zirconia coatings. On the other hand, for a higher differential thermal expansion coefficient, effect of compressive thermal stress is prevalent, e.g., alumina, alumina-titania, and chromia coatings. Moreover, the highly compressive residual stress in the grit blasted substrate was relaxed after coating deposition, and the extent of stress relaxation was dependent on plasma power. Hence, the full residual stress state in a coated component relied on the combined influence of the difference in thermal expansion coefficients between the topcoat and bond coat, degree of feedstock melting, and the extent of stress relaxation owing to annealing effect by the plasma jet. [ABSTRACT FROM AUTHOR]

After casting and solidification, Al cast cylinder heads undergo a sequence of heat treatments to achieve the desired material properties. This sequence comprises solution heat treatment (SHT), quenching, and artificial aging. Internal stresses are formed due to temperature gradients in the complex geometry of the cylinder heads during quenching from the SHT temperature to room temperature. Especially high tensile stresses can produce damage during service. However, part of these internal stresses relaxes during the aging treatment. This work aims to systematically measure the relaxation of the stresses, as well as to phenomenologically model the amount and rate of relaxation. Cast specimens of AlSi7Cu0.5Mg are heat-treated in a furnace before relaxation in a creep testing machine. SHT and SHT plus aging at 180, 200, and 230 °C for 0 h, 0.5 h, 1 h, 4 h, and 6 h are carried out before testing. The relaxation of the stress at constant temperature and strain over 5 h is recorded at three different testing temperatures (180 °C, 200 °C, and 230 °C). The relaxation process is strongly dependent on the testing temperature: at 180 °C and 200 °C the equilibrium stress was already reached after around 1 h, and at 230 °C the equilibrium was reached after 3.5 h. The initial stress values do not influence the relaxation rate. A phenomenological relaxation model is developed to calculate the stress decrement over time and the equilibrium stress in the AlSi7Cu0.5Mg-alloy. The model allows for calculating the levels of residual stress at any time during artificial aging as a function of the thermal history of the alloy, the relaxation temperature, and the initial stress level. Complete relaxation of the initial stress is not reached within 5 h. [ABSTRACT FROM AUTHOR]

Materials with residual stress can experience irreversible plastic strains as carbides precipitate during the tempering process, thereby relaxing the residual stress. In previous studies, this plastic behavior is named "precipitation plasticity." Although precipitation plasticity plays a key role in the relaxation of the residual stress, the micromechanism involved is not yet elucidated. Here in this study, experiments are performed on low‐carbon steel to study precipitation plasticity and the residual stress during tempering. The results show that precipitation plasticity is mainly produced by the generation of interfacial vacancies at the carbide–matrix interface and the diffusion and evolution of these vacancies under temperature and stress fields. Two precipitation plasticity regimes are observed, namely, Precipitation Plasticity I, which is associated with vacancy diffusion, and Precipitation Plasticity II, which is associated with dislocation proliferation. Precipitation Plasticity I is found to be controlled by the volume mismatch and carbide size and is the main cause of residual stress relaxation. The finer and more dispersed the carbide precipitates are, the more significant the precipitation plasticity is. Precipitation Plasticity II causes a slight increase in the residual stress, which, however, is considerably smaller than the stress relaxation corresponding to Precipitation Plasticity I. [ABSTRACT FROM AUTHOR]

Ti-6Al-4V profiles prepared by hot extrusion are usually accompanied by bending and twisting. The hot twist-stretch straightening is an effective strategy such that the bending deflection and twisting angle can be simultaneously decreased by a single straightening process. In addition, utilizing stress relaxation effect, the residual stress and springback can be greatly reduced by holding the straightening temperature and strain constant for a period after twist-stretch straightening. In this study, the hot deformation behaviors of the Ti-6Al-4V profile were revealed by experiments. The tensile model was obtained by uniaxial tensile tests within ranges of temperatures (500–700 °C) and strain rates (5 × 10−5–1 × 10−3 s−1). The creep constitutive model was acquired with stress relaxation experiments in ranges of temperatures (500–700 °C) and pre-strain of 1.5%. Then, the coupled thermo-mechanical model of hot twist-stretch straightening was established. Based on orthogonal experiment strategy, the effects of straightening temperature, stretch strain, and holding time on the bending deflection and torsion angle of profile were investigated systematically and the process was optimized. The straightening accuracy is significantly affected by straightening temperature and holding time. By using optimized process parameters in practical straightening experiments, the deflection/length and angle/length after straightening does not exceed 2‰ and 2.5‰°/mm, respectively, which is basically consistent with the numerical simulation result. [ABSTRACT FROM AUTHOR]

Integral panel is an important structure to reduce aircraft weight. Nanosecond laser beams can be used for the forming and surface modification in panels. To explore the propagation of laser-induced stress waves and their influence on surface stress relaxation, a simulation model is used to capture the propagation process of stress waves. The results show that the stress wave first decays rapidly, and then the attenuation rate decreases gradually. In the initial propagation stage, the attenuation of the stress wave can be fitted by σx=Ee-kx; then, after propagating a certain distance, the stress wave amplitude is more suitable to σx= Ex-k. During the propagation processing, the stress wave is reflected on the shocked and back surface. A tensile stress wave is formed by the reflection of the incident compressive stress wave, and the value of the stress wave is improved. The stress wave has a great influence on the residual stress distribution in 1mm thin panels. When the stress wave is reflected on the back surface, a stress is induced into the surface. When the stress wave is reflected by the shocked surface, the stress on the surface is relaxed because the incident wave is a tensile wave. Therefore, when a nanosecond laser is used in thin panels, the shock wave has an important influence on the stress distribution, which needs to be considered in engineering applications. [ABSTRACT FROM AUTHOR]

Thermally-induced distortion and residual stresses in parts fabricated by the additive manufacturing (AM) process can lead to part rejection and failure. Still, the understanding of thermo–mechanical behavior induced due to the process physics in AM process is a complex task that depends upon process and material parameters. In this work, a 3D thermo-elasto-plastic model is proposed to predict the thermo–mechanical behavior (thermal and distortion field) in the laser-directed energy deposition (LDED) process using the finite element method (FEM). The predicted thermo–mechanical responses are compared to stainless steel 316L (SS 316L) deposition, with single and double bead 42-layer wall samples subject to different inter-layer dwell times, which govern the thermal response of deposited parts in LDED. In this work, the inter-layer dwell times used in experiments vary from 0 to 10 s. Based on past research into the LDED process, it is assumed that fusion and thermal cycle-induced annealing leads to stress relaxation in the material, and is accounted for in the model by instantaneously removing stresses beyond an inversely calibrated relaxation temperature. The model predicts that, for SS 316L, an increase in dwell time leads to a decrease in in situ and post-process distortion values. Moreover, increasing the number of beads leads to an increase in in situ and post-process distortion values. The calibrated numerical model's predictions are accurate when compared with in situ and post-process experimental measurements. Finally, an elongated ellipsoid heat source model is proposed to speed up the simulation. [ABSTRACT FROM AUTHOR]

The special mechanical properties of cemented carbide with high strength and hardness will cause complex stress due to excessive force and heat in the process of precision manufacturing, which will affect precision retention and endurance limit. Given the health and environmental threat of conventional flood cooling and the harsh processing environment of dry grinding, minimum quantity lubrication (MQL) has become an irreplaceable method to machining cemented carbide. However, the addition of nanoparticles changes the force and heat during grinding, which makes the influence on the residual stress of cemented carbide complicated. Therefore, based on the single abrasive grinding force model, the effective abrasive particle number was obtained by simulating the distribution of abrasive particles on the grinding wheel surface, and the mechanical stress model was established, which was loaded onto the workpiece in iterative attenuation mode. The thermal stress model was established based on the temperature field model. The final residual stress prediction model was obtained by determining whether the grinding process yields results and carrying out stress loading and stress relaxation. Experimental verification of the model was carried out under four different grinding conditions of YG8. The minimum friction coefficient of 0.385 was obtained under nanofluid minimum quantity lubrication (NMQL). In the precision analysis of the model, the minimum error value was 5.9% in the direction perpendicular to the feed direction of the workpiece in the dry grinding condition, which proved that the residual stress model had certain reliability. [ABSTRACT FROM AUTHOR]

Vibration treatment of welded structures is a tool for relaxation of residual welding stresses, used to stabilize the dimensions over time. The use of this advanced technology in different periods of production has its own characteristics. The traditional performance of vibration action after welding is carried out at resonant or near-resonant frequencies; the result of this action can be estimated by the change in the amplitude-frequency characteristic that occurs due to deformations during relaxation of residual stresses. Another method involves combining welding with vibration, performed at frequencies that ensure the formation of high-quality penetration, the efficiency of this processing is determined, among other things, by reducing residual stresses; in addition, the values of residual deformations are reduced, the shape of the penetration is optimized. The problem of choosing the optimal period of exposure to welded stainless steel structures manufactured using argon arc welding is reduced to comparing the efficiency according to various criteria: redistribution of residual stresses, influence on residual deformations, suitability for control, change in mechanical properties. [ABSTRACT FROM AUTHOR]

Polymer based composites are widely used for treatment, for example as biofilm resistant seals of root canal fillings. Such clinical use, however, fails more frequently than other dental composite restorations, due to stress-related misfits. The reason for this is that the biomaterials used are inserted as viscous masses that may bond to the substrate, yet shrinkage stresses arising during setting of the cross-linking polymer, work against durable adhesion. Here we combine phase contrast enhanced time-lapse radiography (radioscopy), digital image correlation (DIC) and submicrometer resolution phase-contrast enhanced microtomography (PCE-CT), to reveal the spatial and temporal dynamics of composite polymerization and strain evolution. Radioscopy of cavities located in the upper part of human root canals demonstrates how the composite post-gelation "densification" is dominated by viscous flow with quantifiable motion of both particles and entrapped voids. Thereafter, these composites enter a "stress-relaxation" stage and exhibit several structural adaptations, induced by residual shrinkage stresses. Consequently critical alterations to the final biomaterial geometry emerge: (i) entrapped bubbles expand; (ii) microscopic root filling pull-out occurs; (iii) the cavity walls deform inwards, and (iv) occasionally delamination ensues, propagating out from the root canal filling along buried restoration-substrate interfaces. Our findings shed new light on the interactions between confined spaces and biomedical composites that cross-link in situ , highlighting the crucial role of geometry in channeling residual stresses. They further provide new insights into the emergence of structural flaws, calling attention to the need to find new treatment options. This work quantifies recurring spatial and temporal material redistribution in composites used clinically to fill internal spaces in teeth. This knowledge is important for both promoting biomaterial resistance against potentially pathologic biofilms and for improving structural capacity to endure years of mechanical function. Our study demonstrates the significant role of geometry and the need for improved control over stress raisers to develop better treatment protocols and new space filling materials. The use of high-brilliance X-rays for time-lapse imaging at submicrometer resolution provides dynamic information about the damaging effects of stress relaxation due to polymerization shrinkage. [Display omitted] [ABSTRACT FROM AUTHOR]

Background: Mechanical constitutive models of metals can be difficult to validate without loss of generality. Creep-induced stress relaxation in stainless steels is one such example: stress triaxiality may play a role in the deformation rate but is challenging to measure experimentally. Objectives: We aimed to determine whether the accuracy of constitutive laws for multiaxial deformation, particularly creep deformation, can be verified by measuring the evolution of the complete stress tensor at a point within a structure. Methods: Creep stress relaxation specimens of Type 316H stainless steel were exposed to 550 °C for different durations. We used time-of-flight neutron diffraction and finite element analysis to determine the complete stress tensors at points within the specimens, tracking their development as the residual stress field relaxed. Results: Multiaxial stress relaxation of 316H occurs at 550 °C due to plasticity and creep, and can follow a non-proportional deformation path. In our measurements, over-determination of the elastic strain tensor using neutron diffraction was found to reduce stress uncertainty. A popular creep constitutive model for 316H, when used with finite element analysis, predicts slightly higher strain rates than were observed experimentally. Conclusions: Sequential neutron diffraction stress measurements can be used to validate material constitutive laws in situations involving multiaxial deformation. This could be used to substantiate models of plasticity, viscoelasticity and creep. The RCC-MR primary/secondary creep law for 316H stainless steel is conservative for cases involving a complex initial stress field. [ABSTRACT FROM AUTHOR]

Residual stress formation during cold spraying process may result in deteriorative effects on the performance of coating materials. The objective of this investigation is to characterize residual stress built-up in two well-known nickel-based superalloys (Inconel 625 and Inconel 718) deposited using cold spraying technique. To this end, the residual stress was precisely measured using x-ray diffraction method. Here, residual stress in the subsurface regions was only studied because the surface properties may alter during sample preparation. The average residual stress was slightly higher in Inconel 625 compared to the Inconel 718 sample. Heat treatment at 800 °C helped in the reduction of porosities which exerted tensile stress in subsurface regions of both coatings. Stresses with opposite signs could cancel each other and result in reduction of residual stress after heat treatment. However, the recovery of residual stress was higher for Inconel 718 coating. In the next step as-sprayed and heat-treated coating samples were subjected to microindentation test to measure their hardness and study the crack formation in the samples. The as-sprayed Inconel 625 exhibited higher hardness than Inconel 718, but the hardness of Inconel 625 decreased more drastically after heat treatment. While the cracks were formed on both as-sprayed samples around indents, no cracks were found in the heat-treated samples. The results from this study will contribute to better understanding the performance of cold spray deposited superalloys under service conditions and the effect of stress relaxation heat treatment on elimination of residual stress. [ABSTRACT FROM AUTHOR]

Stress 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. [ABSTRACT FROM AUTHOR]

A mathematical model was developed to calculate the relaxation of residual stresses in a surface-hardened prismatic sample subjected to biaxial loading under creep conditions. The calculation results were validated against experimental data for a surface-hardened sample of EP742 alloy after ultrasonic hardening under creep conditions at a temperature of 650 °C for 100 h. A detailed theoretical analysis was performed to investigate the effect of the type of stress state on residual stress relaxation under thermal exposure (temperature exposure in the absence of mechanical stress) and under biaxial loading at a constant intensity of external stresses. The results of the study show that in a plane stress state under uniform tension, there is a slowdown of residual stress relaxation, and under compression, there is an increase in the relaxation rate compared to the case of thermal exposure. [ABSTRACT FROM AUTHOR]

This paper presents an analysis of the leaning angle in relation to the aspect ratio of the block array in 3D NAND flash memory with a high aspect ratio using TCAD simulation. The simulation data were validated through cross-verification with the Stoney equation, a standard tool for diagnosing film stress. The study confirmed a linear relationship between intrinsic stress and film deformation in single-layer structures. In multi-layer structures, the deformation of the film changes according to the intrinsic stress of the most recently deposited layer. TCAD simulation used a block array structure comprising nine strings between each common source line and a layer range of 10 to 70. The residual stress from the mold stacking process leads to leaning because of stress relaxation after slit etching. Because of the accumulation of lateral stress at each deposition stage, the maximum lateral stress increases with the number of layers. • Leaning of a Block Array in 3D NAND Flash Memory was investigated using TCAD • The simulation data were validated through cross-verification with the Stoney equation • we observed uneven lateral stress between the top and bottom areas of the block array structure's left surface, suggesting that the resultant torque is the primary cause. • The residual stress from the mold stacking process leads to leaning because of stress relaxation after slit etching. [ABSTRACT FROM AUTHOR]

To investigate the spatial distribution of welding residual stress (WRS) and effect of post welding heat treatment (PWHT) on WRS relaxation in the welded components of steel bridges, a 45 mm-thick Q345qD butt-welded specimen was manufactured. Experimental measurement and numerical simulation methods were employed to examine specimen surface and internal WRSs. The simulated results exhibited good agreement with the measured ones, confirming the rationality of the numerical simulation process. Subsequently, through the PWHT simulation in ABAQUS, the evolution of WRS during PWHT was comprehensively analyzed and the effect of PWHT parameters on the stress relaxation was studied. The measured and simulated results show that the longitudinal welding residual stresses (LWRSs) are tensile in the weld region and compressive away from the weld. High-value stresses are mainly concentrated on specimen surfaces and the mid-thickness. Transverse welding residual stresses (TWRSs) present multi-layered distribution, alternating between tension and compression within the weld region. WRS reduction mainly occurs during the heating stage of PWHT, with slight decrease during the holding stage and an increase during the cooling stage. Optimizing PWHT parameters, including temperature, and holding time, heating/cooling rates, proved effective in achieving substantial WRS relaxation. [Display omitted] • The WRS distribution in 45 mm butt-welded joint was investigated by experiment measurement and numerical simulation. • The WRS evolution behavior during PWHT was simulated and analyzed. • The effects of PWHT temperature, holding time, heating and cooling rates on WRS relaxation are discussed. [ABSTRACT FROM AUTHOR]

In order to reveal the variation trend of stress in T9A complex helical structure during long period of service, the stress relaxation of T9A helical compression spring was investigated via experiment and finite element analysis. The stress relaxation experiments of the T9A helical compression spring were carried out for up to 30 h at 90°C, 110°C, 130°C and 150°C, and a modified constitutive relation was established on the basis of the experimental data. As a result, it proves that the model is effective to predict the whole relaxation process. Meanwhile, the finite element analysis of the helical structure spring was carried out. It is found that the steel wire in the spring is relaxed from outside to inside in the process of relaxation. [ABSTRACT FROM AUTHOR]

With molecular dynamics simulations, we systematically investigate melting of a set of ∑3(110)70.53° tilt grain boundaries (GB) in Cu bicrystals, including coherent twin boundaries (CTBs), 12 asymmetric tilt grain boundaries (ATGBs), and symmetric incoherent twin boundaries (SITBs), in the order of increasing length weight of SITB or GB energy. ATGBs decompose into CTBs and SITBs, which migrate and coalesce as a result of internal stress relaxation. GBs can be superheated or premelted, and GB melting temperature decreases exponentially with increasing SITB weight, owing to the systematics in GB microstructure. GB melting nucleates at disordered CTB-SITB junctions, and grows along SITBs and then into grain interiors, with the solid-liquid interfaces preferentially aligned with {111}. [ABSTRACT FROM AUTHOR]

We report in situ measurements of irradiation-induced creep on amorphous (a-) Cu56Ti38Ag6, Zr52Ni48, Si, and SiO2. Micropillars 1 μm in diameter and 2 μm in height were irradiated with ∼2MeV heavy ions during uniaxial compression at room temperature. The creep measurements were performed using a custom mechanical testing apparatus utilizing a nanopositioner, a silicon beam transducer, and an interferometric laser displacement sensor. We observed Newtonian flow in all tested materials. For a-Cu56Ti38Ag6, a-Zr52Ni48, a-Si, and Kr+ irradiated a-SiO2 irradiationinduced fluidities were found to be nearly the same, ≈3 GPa-1 dpa-1, whereas for Ne+ irradiated a-SiO2 the fluidity was much higher, 83 GPa-1 dpa-1. A fluidity of 3 GPa-1 dpa-1 can be explained by point-defect mediated plastic flow induced by nuclear collisions. The fluidity of a-SiO2 can also be explained by this model when nuclear stopping dominates the energy loss, but when the electronic stopping exceeds 1 keV/nm, stress relaxation in thermal spikes also contributes to the fluidity. [ABSTRACT FROM AUTHOR]

We report in situ measurements of irradiation-induced creep on amorphous (a-) Cu56Ti38Ag6, Zr52Ni48, Si, and SiO2. Micropillars 1 μm in diameter and 2 μm in height were irradiated with ∼2MeV heavy ions during uniaxial compression at room temperature. The creep measurements were performed using a custom mechanical testing apparatus utilizing a nanopositioner, a silicon beam transducer, and an interferometric laser displacement sensor. We observed Newtonian flow in all tested materials. For a-Cu56Ti38Ag6, a-Zr52Ni48, a-Si, and Kr+ irradiated a-SiO2 irradiationinduced fluidities were found to be nearly the same, ≈3 GPa-1 dpa-1, whereas for Ne+ irradiated a-SiO2 the fluidity was much higher, 83 GPa-1 dpa-1. A fluidity of 3 GPa-1 dpa-1 can be explained by point-defect mediated plastic flow induced by nuclear collisions. The fluidity of a-SiO2 can also be explained by this model when nuclear stopping dominates the energy loss, but when the electronic stopping exceeds 1 keV/nm, stress relaxation in thermal spikes also contributes to the fluidity. [ABSTRACT FROM AUTHOR]

We characterize the evolution of flow units associated with the flow 'defects' in metallic glass by monitoring the fictive temperature change of a typical metallic glass upon isothermal annealing below its glass transition temperature. The correlations between the fictive temperature Tf, enthalpy change, and the concentration of flow units have been obtained. Such correlations help in understanding the evolution process of flow units, structural feature, and structural relaxation behaviors in metallic glasses, and can rationalize effects of the cooling rate, aging, and annealing on properties and structure of metallic glasses. [ABSTRACT FROM AUTHOR]

An orthotropic steel deck (OSD) has a complicated structure, and its fatigue life is mainly determined by various welding details. Fatigue assessment of deck-to-rib welding details (DRWDs) under long-term train loads is an important concern for engineers. Properly assessing the initial residual stress and the mechanism of stress relaxation in DRWDs under long-term external loading is a prerequisite for predicting the fatigue damage and service life of OSDs. In this paper, a finite element analysis method is proposed to calculate the residual stress relaxation in DRWDs of OSDs under constant/variable amplitude cyclic loading. First, experiments on full-size OSD specimens were carried out using the hole drilling strain-gauge method, and the multi-axial distribution characteristics of residual stress on the sub-surface of the deck were obtained. On this basis, a refined residual stress analysis model of DRWDs using thermal-structural sequence coupling analysis and life and death unit technology is established, and the accuracy of the model is verified by the test data. Second, a coupling stress analysis model that considers the welding residual stress and mechanical stress using cyclic plastic constitutive model is established. The combined influence of number of cycles, stress amplitude, and stress ratio on multi-axial residual stress relaxation effect under constant/variable amplitude cyclic loading is investigated. Finally, a release formula of welding residual stress relaxation coefficient is proposed based on the external loading stress amplitude, stress ratio, and material yield stress. The results show that (1) with the increase in the number of loading cycles, the stress decreases until it is stabilized, while the global distribution of welding residual stress remains unchanged. Most of the welding residual stress release (about 95%) occurs in the first cycle; (2) the residual stress relaxation decreases with the increase in stress amplitude and increases linearly with the stress ratio; (3) the residual stress release is controlled by the maximum amplitude stress in the variable amplitude cyclic loading. After the residual stress is released, the stress will not continue to be released if the DRWDs have the same or smaller amplitude loading. [ABSTRACT FROM AUTHOR]

Surface engineering of Zr 51.3 Al 8.5 Cu 31.3 Ni 4 Ti 4.9 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 nano-crystalline cubic ZrO 2 (c-ZrO 2), 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-ZrO 2 , 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 nano-crystalline c-ZrO 2 , while the strain relaxation monitored with FIB-DIC analysis indicated compressive residual stresses of −1.4 GPa in the internal oxidation zone. The strains 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. Image, graphical abstract [ABSTRACT FROM AUTHOR]

Glass 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. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

The prediction of residual stress relaxation is essential to assess the safety of welded components. This paper aims to study the influence of various effective parameters on residual stress relaxation under cyclic loading. In this regard, a 3D finite element modeling is performed to determine the residual stress in welded aluminum plates. The accuracy of this analysis is verified through experiment. To study the plasticity effect on stress relaxation, two plasticity models are implemented: perfect plasticity and combined isotropic-kinematic hardening. Hence, cyclic plasticity characterization of the material is specified by low cycle fatigue tests. It is found that the perfect plasticity leads to greater stress relaxation. In order to propose an accurate model to compute the residual stress relaxation, the Taguchi L18 array with four 3-level factors and one 6-level is employed. Using statistical analysis, the order of factors based on their effect on stress relaxation is determined as mean stress, stress amplitude, initial residual stress, and number of cycles. In addition, the stress relaxation increases with an increase in mean stress and stress amplitude. [ABSTRACT FROM AUTHOR]

Bitumen ageing has a significant impact on the mechanical performance of asphalt concrete. This study utilises dynamic mechanical tests (frequency sweep, fatigue, and relaxation) to investigate the effect of ageing on the viscoelastic characteristics of bitumen. As ageing progressed, a shift of the Black diagrams curves towards lower phase angles was observed; at the same time the shape of the curves changed to a straight line and the curvature reduced. Interestingly, based on the standard fatigue analysis criteria, ageing appears to have a positive effect on bitumen fatigue life. Consequently, it occurs that studying only the stiffness and fatigue behaviour of bitumen cannot directly characterise ageing-induced degradation. The stress relaxation test was found to be more suitable to characterise ageing of bitumen. Aged bitumen had higher residual stresses and longer relaxation times. Hence, aged bitumen was found to be more susceptible to stress accumulation and thus cracking. A linear relationship was established between the crossover modulus and the stress relaxation ageing indices for the studied bitumen. This relationship suggests that crossover modulus has the potential to be used as a parameter to characterise the bitumen relaxation properties by means of routine tests such as the DSR frequency sweep tests. [ABSTRACT FROM AUTHOR]

A model for the kinetics of residual stress evolution is described that can be applied to both the relaxation during a growth interruption and the recovery following resumption of growth. The stress is attributed to simple atomistic processes based on reversible diffusion of atoms in and out of the grain boundary. Each of these processes is considered separately to understand how the rates differ during relaxation and recovery and how they depend on grain size, film thickness, temperature, deposition rate, and initial stress. The model is compared with experimental data obtained by others for several materials (Fe, Ni and Au). Image, graphical abstract [ABSTRACT FROM AUTHOR]

Due to the layer-by-layer nature of the process, parts produced by laser-based powder bed fusion (LPBF) have high residual stresses, causing excessive deformations. To avoid this, parts are often post-processed by subjecting them to specially designed heat treatment cycles before or after their removal from the base plate. In order to investigate the effects of the choice of post-processing steps, in this work the entire LPBF process chain is modelled in a commercial software package. The developed model illustrates the possibilities of implementing and tailoring the process chain model for metal additive manufacturing using a general purpose finite element (FE) solver. The provided simplified computational example presents an idealised model to analyse the validity of implementing the LPBF process chain in FE software. The model is used to evaluate the effect of the order of the process chain, the heat treatment temperature and the duration of the heat treatment. The results show that the model is capable of qualitatively capturing the effect of the stress relaxation that occurs during a heat treatment at elevated temperature. Due to its implementation, the model is relatively insensitive to duration and heat treatment temperature, at least as long as it is above the relaxation temperature. Furthermore, the simulations suggest that, when post-processing, it is necessary to perform the stress relaxation before the part is removed from the base plate, in order to avoid a significant increase of the deformation. The paper demonstrates the capability of the simulation tool to evaluate the effects of variations in the process chain steps and highlights its potential usage in directing decision-making for LPBF process chain design. [ABSTRACT FROM AUTHOR]

We report a study of strained Si metal-oxide-semiconductor field-effect transistors (MOSFET’s) fabricated using a high thermal budget. The impact of Si channel strain on MOSFET performance, leakage current, and yield is investigated for Si1-xGex virtual substrates having Ge compositions varying from 0% to 30%. Increasing the Ge fraction in the SiGe virtual substrate increases the amount of tensile strain in the Si layer and consequently increases the electron mobility. High levels of strain, however, reduce the critical thickness of strained Si, above which Si becomes metastable and susceptible to relaxation during high-temperature device fabrication. Increasing the Ge composition in the virtual substrate up to 30% is shown to result in significant enhancements in MOSFET drain current and transconductance due to increased strain in the device channels. However, cross-wafer electrical yield data as a function of Ge composition are reported and show that increasing Ge compositions above 15% simultaneously reduces device yield. Off-state leakage current and gate oxide interface trap density are also shown to increase significantly when the Ge content in the virtual substrate is raised above 25%. Trade-offs between device performance and wafer yield are thus presented. The results identify the appropriate parameter window for virtual substrate Ge composition if acceptable MOSFET on-state performance, off-state performance, device yield, and reliability are to be achieved using a high thermal budget process. Detailed physical and electrical analyses have been carried out in order to understand the causes of the degraded performance for high Ge content virtual substrates. The reduction in yield with increasing Ge composition is shown to be related to a combination of strain relaxation and as-grown material quality. The strain state has been studied using Raman spectroscopy, Schimmel etching, and transmission electron microscopy, in conjunction with electrical measurements. The ability of techniques commonly used to assess strain relaxation is critically examined. The degraded electrical performance for strained Si/Si0.70Ge0.30 devices is shown to correlate well with the presence of surface threading dislocations resulting from strain relaxation. The relative effects of as-grown material quality and strain relaxation on device performance have also been investigated. The impact of device operating conditions on performance enhancements has further been analyzed and implications for the design of both n- and p-channel strained Si/SiGe MOSFET’s are discussed. [ABSTRACT FROM AUTHOR]