Effect Of Stress Relaxation Research Articles

Time-dependent extracellular matrix alterations of young tendons in response to stress relaxation: a model for the Ponseti method.

The Ponseti method corrects a clubfoot by manipulation and casting which causes stress relaxation on the tendons. Here, we examined the effect of long-term stress relaxation on tendon extracellular matrix (ECM) by (1) an ex vivo stress relaxation test, (2) an in vitro tenocyte culture with stress relaxation and (3) an in vivo rabbit study. Time-dependent tendon lengthening and ECM alterations including crimp angle reduction and cleaved elastin were observed, which illustrated the mechanism of tissue lengthening behind the treatment-a material-based crimp angle reduction resulted from elastin cleavage. Additionally, in vitro and in vivo results observed restoration of these ECM alterations along with increased elastin level after 7 days of treatment, and the existence of neovascularization and inflammation, indicating the recovery and adaptation from the tendon in reaction to the treatment. Overall, this study provides the scientific background and information that helps explain the Ponseti method.

An elastoplastic phase-field study of the precipitation behaviors of Mg17Al12 phase in Mg-Al-Based alloys: Part I. Modeling and the role of plasticity

An elastoplastic phase-field model for typical diffusional transformations was developed, where plastic deformation was considered through a modified time-dependent Ginzburg-Landau dynamic equation. The model was applied to investigate the precipitation in Mg-Al-based alloys, with a focus on studying the effects of elastoplastic deformation associated with phase transformation on the morphology and growth kinetics of β-Mg17Al12 precipitates (with Burgers orientation). The elastoplastic modeling results show that the precipitates adopt a lath-shaped morphology with lozenge ends, consistent with experimental observations. Although the plastic strain accumulated in the transformation front has a moderate impact on the morphology of the precipitate, it exhibits a clear dual effect on the growth of the precipitate along [112̅0]α. Specifically, plasticity promotes the growth of the precipitate during the initial stage, owing to its stress relaxation effect, and induces a solute poor zone of atomic Al with a greater concentration gradient at the transformation front. However, the plastic strain tends to retard the growth of the precipitate afterward due to its limited ability to release the increasing compressive strain between the two phases. These results shed light on understanding the role of plasticity in general diffusional transformations and provide guidelines for tailoring the microstructure of alloys.

Analysis of spatial and temporal stress of waste concrete circular rock

Different proportions of waste concrete are mixed into the surrounding rock with crushing agent, and the boundary moisture content, the linear shrinkage rate, the expansion force test, the compaction test, the unlimited compressive strength and the CBR test were verified under different curing times. The surrounding rock stress state has significant spatial-temporal effects. Around the stress evolution process of the surrounding rock in the roadway excavation, the space-time effect of the surrounding rock stress distribution of the circular roadway under the uniform stress field is preliminarily analyzed, and the correlation of time and space and other related parameters is discussed. The study shows that the XiYuan model can fully reflect the stress relaxation effect of surrounding rock, and the three-way stress decreases with time and finally stabilizes in a certain balance state. In the early stage, the more obvious the stress concentration, the stress relaxation; the mechanical relaxation behavior of surrounding rock is a space-time function, the radial stress decreases with the rear distance of the excavation surface, and the annular stress is just opposite, while the axial stress remains unchanged. This study can provide a theoretical basis for the stability prediction and evaluation of rock mass roadway engineering.

Competitive effect of residual stress and surface roughness on the fatigue life of shot peened S42200 steel at room and elevated temperature

The effect of shot peening (SP) on the fatigue life of S42200 steel at room and elevated temperature is investigated. The fatigue test is carried out on a high temperature rotating bending tester. Surface integrity characterization is used to study the effect of SP on surface properties. Furthermore, Finite Element Method (FEM) combined with a fatigue damage model, namely Zheng Equation, is proposed to predict the fatigue life. The results show that SP effectively improves the fatigue life of S42200 steel under all temperatures. However, at lower temperature, higher strength SP leads to higher fatigue resistance, while at higher temperature, lower strength SP leads to higher fatigue resistance. This can be explained by the competitive effect of surface roughness and compressive residual stress (CRS) relaxation on the local stress concentration. This research can be a good guide to the application of SP at different temperatures.

Cyclic elastoplastic constitutive model for stainless steels compatible with multiple strengths

The application of stainless steel materials in civil engineering has begun to attract attention over the last decade. Nevertheless, most studies of stainless steel structures still use the oversimplified Chaboche model. Therefore a more accurate constitutive model is needed to describe the cyclic elastoplastic behavior of stainless steel. The model should be able to simulate stainless steel of varying strengths and should be easily calibrated for practical engineering purposes. This paper proposes a compatible constitutive model that meets these objectives. The constitutive model combines two forms of hardening, is dominated by isotropic and kinematic hardening, and accounts for the strain memory effect and the average stress relaxation effect. A complete numerical implementation scheme for the constitutive model is proposed and coded in the user material subroutine (UMAT) based on the implicit return mapping algorithm. Furthermore, simplified methods and recommendations for the calibration of material parameters are given. The simulation results of a detailed finite element model are compared with the results of the cyclic loading tests, and good agreements are obtained. The proposed constitutive model can be further used in seismic or other dynamic nonlinear analyses of stainless steel structures to correctly predict their strength, deformation, and energy dissipation capacity.

A novel approach to evaluate the mechanical responses of elastin-like bioresorbable poly(glycolide-co-caprolactone) (PGCL) suture

Poly(glycolide-co-caprolactone) (PGCL) has become a novice to the bioresorbable suture owing to the synergistic properties taken from the homo-polyglycolide (PGA) and polycaprolactone (PCL) such as excellent bioresorption and flexibility. In addition to under conventional monotonic loading, the understanding of mechanical responses of PGCL copolymers under complex loading conditions such as cyclic and stress relaxation is crucial for its application as a surgical suture. Consequently, the present work focuses on evaluating the mechanical responses of PGCL sutures under monotonic, cyclic, and stress relaxation loading conditions. Under monotonic loading, the stress-strain behavior of the PGCL suture was found to be non-linear with noticeable strain-rate dependence. Under cyclic loading, inelastic responses including stress-softening, hysteresis and permanent set were observed. During cyclic loading, both stress-softening and hysteresis were found to increase with the maximum strain. In multi-step stress relaxation, the PGCL sutures were observed to exhibit a strong viscoelastic response. In an attempt to describe the relationship between the stress-relaxation and strain-induced crystallization (SIC) occurring during the loading and relaxation processes, a schematic illustration of the conformational change of polymer chains in PGCL sutures was proposed in this work. Results showed that SIC was dependent on the strain level as well as the loading and relaxation durations. The inelastic phenomena observed in PGCL sutures can be thus correlated to the combined effect of stress relaxation and SIC.

Bauschinger effect in Gd micro-alloying metastable high-entropy alloy

To investigate Bauschinger effect in metastable high-entropy alloys with strain-induced FCC → HCP phase transformation, we designed a series of Gd micro-alloying non-equiatomic metastable high-entropy alloys. The results show that all tested alloys have asymmetrical tension-compression properties exhibiting the obvious Bauschinger effect, which is related to the different levels of phase transformation in compression and tension owing to different deformation textures. Excessive phase transformation suppresses the increase in pile-up density of geometrically necessary dislocations (GNDs), resulting in a back stress relaxation effect. The inverse phase transformation observed by quasi in-situ EBSD proves the instability of the HCP phase. The established kinetic model points out limitation of the classical back stress measurement method in metastable high-entropy alloys.

Determination of in-plane residual stress and eigenstrain in laser peened thin sheet using unit pulse function and equilibrium constraint

The residual stress field is a critical physical quantity for the predicting fatigue life and adjusting forming shape in the peening industry. It is an important engineering requirement to accurately obtain the reliable residual stress of treated coupons and components. To improve the spatial resolution and accuracy of stress measurements in laser peened targets, the slitting method via unit pulse function was carried out in the present study. A 4 mm thick rectangular sheet subjected to uniform laser peening was studied thoroughly via theoretical analysis and experimental tests. First, the physical interpretation and basic implementation of the unit pulse function were described. The relevant regularization, error reduction, and analysis methods were expounded. Then, a safe distance of multiple slitting measurements was proposed and further investigated through numerical simulation. The effect of stress relaxation and the positions of the strain gauges employed were also verified. Experimentally, a biaxial through-thickness distribution of residual stress was calculated using the unit pulse approach. Then, a cross-method validation procedure was performed through an incremental X-ray diffraction (XRD). Finally, the distribution patterns of the bending and elongating stress components were analyzed. The unbalanced laser peening-induced stress was obtained through a backward calculation using the equilibrium constraint. The results indicated that general stress field was in a biaxial state, which possessed a roughly equivalent amplitude and uniform distribution. Along the depth direction, both the peened and unpeened faces were under compression, while the sheet center was in tension for global compensation. Further research revealed that the residual stress introduced by laser peening was fundamental to the process, which can provide a test-oriented solution for eigenstrain and reliable data support for deformation prediction.

Synergistic Effects of Interfacial Energy Level Regulation and Stress Relaxation via a Buried Interface for Highly Efficient Perovskite Solar Cells

An electron-transport layer with appropriate energy alignment and enhanced charge transfer is critical for perovskite solar cells (PSCs). In addition, interface stress and lattice distortion are inevitable during the crystallization process of perovskite. Herein, IT-4F is introduced into PSCs at the buried SnO2 and perovskite interface, which assists in releasing the residual stress in the perovskite layer. Meanwhile, the work function of SnO2/IT-4F is lower than that of SnO2, which facilitates charge transfer from perovskite to ETL and consequently leads to a significant improvement in the power conversion efficiency (PCE) to 23.73%. The VOC obtained is as high as 1.17 V, corresponding to a low voltage deficit of 0.38 V for a 1.55 eV bandgap. Consequently, the device based on IT-4F maintains 94% of the initial PCE over 2700 h when stored in N2 and retains 87% of the initial PCE after operation for 1000 h.

Does friction contribute to formability improvement using servo press?

Servo press forming machines are advanced forming systems that are capable of imparting interrupted punch motion, resulting in enhanced room temperature formability. The exact mechanism of the formability improvement is not yet established. The contribution of interrupted motion in the ductility improvement has been studied through stress relaxation phenomena in uniaxial tensile (UT) tests. However, the reason for improved formability observed when employing servo press is complicated due to the additional contribution from frictional effects. In the present work, an attempt is made to decouple the friction effect on formability improvement numerically. The improved formability is studied using a hole expansion test (HET). The limit of forming during hole expansion is modeled using the Hosford-Coulomb (HC) damage criteria, which is implemented as a user subroutine in a commercial explicit finite element (FE) software. Only the contribution of stress relaxation is accounted for in the evolution of the damage variable during interrupted loading. Therefore, the difference between simulation and experimental hole expansion ratio (HER) can be used to decouple the friction effect from the overall formability improvement during hole expansion. The improvement in HER due to stress relaxation and friction effect is different. The study showed that the model effectively captures the hole expansion deformation process in both monotonic and interrupted loading conditions. Compared to stress relaxation, friction effect played a major role during interrupted HET.

Effect of stress relaxation on soft magnetic properties of Fe76Si9B10P5 metallic glass

Stress relief can improve the soft magnetic properties of Fe-based metallic glasses (MGs) and is vital for industrial applications. In this work, we investigated the evolution of soft magnetic properties, relaxation dynamics, and mechanical properties of Fe-based MGs under different applied tensile strains and stress relaxation times. We found that stress relaxation can significantly reduce coercivity Hc by 95% compared with as-quenched state. Furthermore, the coercivity Hc, apparent activation volume Vact and time constant tr all show analogous two-stage variation with annealing time, accompanied by approximate crossovers. This suggests that the microstructure change emerges, further verified by the domain wall motion and the transition from elastic to plastic. These results are helpful in preparing Fe-based MGs with excellent soft magnetic and mechanical properties by controlling the stress relaxation condition.

Stress relaxation behavior and effects of accelerated weathering on properties of silane-treated rubberwood/recycled polypropylene wood-plastic composites

Stress relaxation behavior and effects of accelerated weathering on properties of silane-treated rubberwood/recycled polypropylene wood-plastic composites

Experimental validation of numerical model for thermomechanical performance of material extrusion additive manufacturing process: Effect of infill design & density

The optimum selection of process parameters, materials, and product design is essential to achieve the desired response of 3D-printed structures, especially in functional components. The current practices of the experimental optimization process require significant resources, which can be limited through numerical modeling and simulation techniques. In this study, a thermomechanical numerical model is used to predict the performance of the additive manufacturing (AM) process, i.e., fused filament fabrication (FFF). 3D printing (3DP) process simulations were performed for tensile testing coupons using carbon fiber-reinforced polyamide-6 (PA6-CF) material. The numerical model predicted the effect of infill patterns and densities on the deflections and distortions during the FFF process. The numerical model predictions were validated via experiments performed under similar conditions. The results conclude that the numerical model can adequately predict the process-induced deflections and distortions during the FFF process. Generally, higher dimensional control was observed for rectangular infill patterns and increased infill density. However, the numerical model overestimates the shrinkage as the stress-relaxation effect is not considered in the numerical model and underestimates the warpages as perfect build plate adhesion is assumed.

INFLUENCE OF HIGH TEMPERATURE TREATMENT ON THE MECHANICAL CHARACTERISTICS OF CARBON-CARBON COMPOSITE MATERIALS WITHPYROCARBON MATRIX

Pyrolytic compaction of porous substrates is one of the methods for obtaining carbon-carbon composite materials. When using such materials at high temperatures, for example, as elements of heaters, it is necessary to take into account the effect of high temperatures on their mechanical characteristics. In this work, the influence of high-temperature treatment on me-chanical characteristics was studied and the mechanism of destruction of samples of the mate-rial "Argolon GR" produced by JSC "Composite" was considered. It is shown that with an in-crease in the processing temperature from 1800 to 2400 °C, the open porosity of the samples increases in proportion to the increase in the number and size of cracks in the samples. The compressive failure stress depends very weakly on temperature, however, the graph of the corresponding dependence clearly shows its slight decrease with an increase in the processing temperature from 2000 to 2400 °C, which corresponds to the accumulation of damage in the material matrix that reduces its strength. Attention is drawn to a significant increase in the ten-sile strength of the material after high-temperature treatment, which is not typical for carbon-carbon composite materials. The dependence of the breaking stress in tension with an increase in the processing temperature for material samples has a pronounced maximum at 2000 °C. The analysis of the change in impact strength, the roughness parameter of the fracture surface of the samples, and the breaking stress in tension showed that the dominant mechanism during fracture is the mechanism of strength increase associated with the violation of the compatibility condition of deformations in the components of the material, while the effect of thermal stress relaxation is very small.

Towards understanding and restraining the mechanical relaxation effect in polishing silicon carbide with a detachable bonnet tool

Bonnet polishing has shown promising shape adaptability and processing quality in the surface finishing of glass-ceramic optical components with complex surfaces. This study proposed a new robotic bonnet polishing device with a detachable polishing cloth to realize relatively high polishing accuracy on sintered silicon carbide (S-SiC) with low cost and pollution. During polishing, the contact state between the secondary-thermoformed polishing cloth and workpiece was found to follow a complex force curve, including a short-term force drop caused by quasi-stress relaxation and a long-term force rise caused by creep. Therefore, a predictive model was established to modify the removal efficiency error induced by force fluctuation and to estimate the remaining service time. On this basis, a new approach was proposed to accelerate the initial-stage creep and restrain the force drop during polishing via temperature control. The results obtained in tool influence function and form correction experiments proved that the temperature threshold to suppress the stress relaxation effect of the polishing pad is around 45 °C. Meanwhile, the pre-treatment time of the pad can be compressed by more than half at this temperature to maintain the high determinacy and efficiency of the polishing process. After polishing, almost no subsurface damage was detected near the surface of the S-SiC block. However, due to the impact of original micropores and the scratches caused by the abrasive aggregation effect, the roughness is challenging to continue decreasing after Ra reaches 3∼4 nm.

A Manufacturing Process Simulation of Toughened Cyanate-Ester-Based Composite Structures with Respect to Stress Relaxation.

The objectives of this study were to experimentally determine the effects of the stress relaxation of a cyanate-ester-based composite, derive and integrate constitutive equations into commercial FEM software, and apply this approach to understand the formation of residual stress in a typical aerospace structure—namely, a stiffened panel. In preliminary studies, a cyanate-ester-based composite with increased fracture toughness for high-temperature applications was developed. High curing temperatures up to 260 °C will inevitably lead to high process-induced stresses. To assess the magnitude of impact on the development of internal stresses, the relaxation behavior of the neat resin was measured and characterized. The system was toughened, and the effect of stress relaxation increased as the temperature got closer to the glass transition temperature of the toughener, which was approximately 240 °C. With the use of an incremental linear viscoelastic model, the relaxation behavior was integrated into a process model with a holistic approach. A stiffened panel was manufactured and used as the validation use case. The displacement field was validated with an optical 3D measuring system, and good agreement was found between the simulated and experimental results. The maximum difference between the elastic and the viscoelastic solution was found to be 15%. Furthermore, the stress magnitude in the transverse material direction resulted in a more critical value higher than the material strength.

Effect of cyclic softening and mean stress relaxation on fatigue crack initiation in a hemispherical notch

AbstractCorroded surfaces of offshore mooring chains typically show a very irregular shape with different pit geometries. Fatigue tests were performed in three‐point bending on specimens with hemispherical notches, representing an idealized geometry of a corrosion pit. Digital image correlation was used to detect crack initiation lives and locations. The aim of the study was to quantify mean stress relaxation (MSR) and its influence on fatigue initiation life. MSR data were obtained from strain‐controlled fatigue tests on R4‐grade offshore mooring chain steel. Interestingly, MSR was observed even at a stress amplitude below the cyclic yield stress. For initiation lives below 100,000 cycles, significant improvement in predictions was obtained if cyclic softening and MSR were accounted for. Using only the monotonic stress‐strain curve led to less accurate predictions. For longer lives, scatter in the base material data and residual stresses from machining of the notch made predictions less accurate.

A unified framework for linear thermo-visco-elastic wave propagation including the effects of stress-relaxation

We present a unified framework for the study of wave propagation in homogeneous linear thermo-visco-elastic (TVE) continua, starting from conservation laws. In free-space such media admit two thermo-compressional modes and a shear mode. We provide asymptotic approximations to the corresponding wavenumbers which facilitate the understanding of dispersion of these modes, and consider common solids and fluids as well as soft materials where creep compliance and stress relaxation are important. We further illustrate how commonly used simpler acoustic/elastic dissipative theories can be derived via particular limits of this framework. Consequently, our framework allows us to: (i) simultaneously model interfaces involving both fluids and solids and (ii) easily quantify the influence of thermal or viscous losses in a given configuration of interest. As an example, the general framework is appliedto the canonical problem of scattering from an interface between two TVE half spaces in perfect contact. To illustrate, we provide results for fluid–solid interfaces involving air, water, steel and rubber, paying particular attention to the effects of stress relaxation.

Numerical Simulation and Process Optimization on Hot Twist-Stretch Straightening of Ti-6Al-4V Alloy Profile.

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.

Effect of substrate condition on wire fed electron beam additive deposition

Electron beam wire fed (EB-WF) additive manufacturing (AM) can be utilised for cost-effective part repair in the aerospace industry and, especially for titanium alloys, the vacuum processing effectively mitigates high temperature contamination for enabling reliable high-performance. This research advanced EB-WF additive processing and simulation for depositing Ti–6Al–4V thin walls (3 mm in thickness) that emulates repair of damaged fan and compressor blades. The main focus of this research was to understand the effect of the initial substrate microstructure and residual stress profile on the final deposit properties. The results revealed that the EB-WF additive repair process for depositing Ti–6Al–4V yielded minimal distortion (<350 μm) and residual stresses (<150 MPa (0.15σys)). The initial residual stress states of the substrate were found to have a negligible effect on the final residual stress profiles, from the stress relaxation effect during EB-WF AM. Although significant variance in the microstructure for each substrate condition was present after deposition, their mechanical properties were similar. Deposited test specimens had tensile yield and ultimate strength values ranging between 800-830 MPa and 860–880 MPa, respectively. The similar mechanical properties of the interface were correlated with the microstructural features such as layer bands and titanium alpha (α) colonies.