Stress Relaxation Curves Research Articles

Study of the stress relaxation resistance and microstructural evolution of a Cu–Ni–Si alloy strip

The stress relaxation resistance of a Cu–Ni–Si alloy strip after 48 h of initial loading stress of 80% σ0.2 at 125 °C was studied. The stress relaxation curve was fitted using a model. By employing EBSD, TEM, and other techniques, this study examined the microstructural characteristics of the Cu–Ni–Si alloy strip before and after stress relaxation. This study investigated the stress relaxation mechanism of the alloy and revealed that the stress relaxation rate of the Cu–Ni–Si alloy strip was 10.82% after 48 h at 125 °C. The stress relaxation process of the alloy strip was divided into two main stages. During the first stage, there was a significant rearrangement and movement of dislocations within the alloy strip, leading to a rapid decline in the remaining stress. In the second stage, the interaction among the Ni2Si phase, twins and dislocations led to a slow decrease in the remaining stress. After the stress relaxation of the alloy strip, the precipitated phase in the microstructure grew from 67 nm to 200 nm, the recrystallization microstructure increased from 57.91% to 62.42%, and the twin boundary content decreased from 22.1% to 18.5%. These observed changes in the microstructure are anticipated to further reduce the stress relaxation resistance of the alloy strip.

A Numerical Model to Predict the Relaxation Phenomena in Thermoset Polymers and Their Effects on Residual Stress during Curing-Part I: A Theoretical Formulation and Numerical Evaluation of Relaxation Phenomena.

This paper analyzes the effect of crosslinking reactions on a thermoset polymer's viscoelastic properties. In particular, a numerical model to predict the evolution of epoxy's mechanical properties during the curing process is proposed and implemented in an Ansys APDL environment. A linear viscoelastic behavior is assumed, and the scaling of viscoelastic properties in terms of the temperature and degree of conversion is modeled using a modified version of the TNM (Tool-Narayanaswamy-Mohynian) model. The effects of the degree of conversion and structural relaxation on epoxy's relaxation times are simultaneously examined for the first time. This formulation is based on the thermo-rheological and chemo-rheological simplicities hypothesis and can predict the evolution of epoxy's relaxation phenomena. The thermal-kinetic reactions of curing are implemented in a homemade routine written in APDL language, and the structural module of Ansys is used to predict the polymer's creep and stress relaxation curves at different temperatures and degrees of conversion.

Determining the Rheological Parameters of Polymers Using Machine Learning Techniques

Introduction. The paper investigates the methodology for determining the rheological parameters of materials based on the nonlinear Maxwell-Gurevich rheological model using the stress relaxation curves. The review of the main directions of the metaheuristic approaches (local search, evolutionary algorithms) to solving the combinatorial optimization problems is presented. The metaheuristic algorithms for solving some important combinatorial optimization problems with the special emphasis on building decision trees are described. The comparative analysis of the algorithms for solving the regression problem in CatBoost Regressor is carried out. The aim of the work is to determine the rheological properties of polymers using machine learning techniques.Materials and Methods. The objects of the study are the generated data sets obtained on the basis of the theoretical stress relaxation curves. The source data tables for model training across all samples are presented, and the statistical analysis of the source data sets characteristics is carried out. The total number of numerical experiments across all samples amounted to 346020 variations. To develop the models, the CatBoost artificial intelligence techniques were used; the regularization techniques (Weight Decay, Decoupled Weight Decay Regulation, Augmentation) were used to increase the model accuracy; and Z–Score technique was used for data normalization.Results. As a result of the research, the intelligent models for determining the rheological parameters of polymers (initial relaxation viscosity, velocity modulus) have been developed based on the generated data sets on the example of the epoxy binder EDT-10. Based on the testing results of the models with the best parameters, the quality assessments were carried out: for the parameter 𝜂∗0 the range of values MAPE 0.46 — 2.72, MSE 0.15 — 1.09, RMSE 0.19 — 0.44, MAPE 0.46 — 1.27; for the parameter 𝑚∗ — MAPE 0.07 — 0.32, MSE 0.01 — 0.13, RMSE 0.10 — 0.41, MAPE 0.58 — 2.72. The resulting metric values are permissible. The training graphs demonstrate the stability of the process.Discussion and Conclusion. The developed intelligent models are scalable and cross-platform, have practical applied significance that ensures their implementation in a wide range of the scientific and engineering apps.

Prediction of Rheological Parameters of Polymers by Machine Learning Methods

Introduction. All polymer materials and composites based on them are characterized by pronounced rheological properties, the prediction of which is one of the most critical tasks of polymer mechanics. Machine learning methods open up great opportunities in predicting the rheological parameters of polymers. Previously, studies were conducted on the construction of predictive models using artificial neural networks and the CatBoost algorithm. Along with these methods, due to the capability to process data with highly nonlinear dependences between features, machine learning methods such as the k-nearest neighbor method, and the support vector machine (SVM) method, are widely used in related areas. However, these methods have not been applied to the problem discussed in this article before. The objective of the research was to develop a predictive model for evaluating the rheological parameters of polymers using artificial intelligence methods by the example of polyvinyl chloride.Materials and Methods. This paper used k-nearest neighbor method and the support vector machine to determine the rheological parameters of polymers based on stress relaxation curves. The models were trained on synthetic data generated from theoretical relaxation curves constructed using the nonlinear Maxwell-Gurevich equation. The input parameters of the models were the amount of deformation at which the experiment was performed, the initial stress, the stress at the end of the relaxation process, the relaxation time, and the conditional end time of the process. The output parameters included velocity modulus and initial relaxation viscosity coefficient. The models were developed in the Jupyter Notebook environment in Python.Results. New predictive models were built to determine the rheological parameters of polymers based on artificial intelligence methods. The proposed models provided high quality prediction. The model quality metrics in the SVR algorithm were: MAE – 1.67 and 0.72; MSE – 5.75 and 1.21; RMSE – 1.67 and 1.1; MAPE – 8.92 and 7.3 for the parameters of the initial relaxation viscosity and velocity modulus, respectively, with the coefficient of determination R2 – 0.98. The developed models showed an average absolute percentage error in the range of 5.9 – 8.9%. In addition to synthetic data, the developed models were also tested on real experimental data for polyvinyl chloride in the temperature range from 20° to 60°C.Discussion and Conclusion. The approbation of the developed models on real experimental curves showed a high quality of their approximation, comparable to other methods. Thus, the k-nearest neighbor algorithm and SVM can be used to predict the rheological parameters of polymers as an alternative to artificial neural networks and the CatBoost algorithm, requiring less effort to preset adjustment. At the same time, in this research, the SVM method turned out to be the most preferred method of machine learning, since it is more effective in processing a large number of features

Effect of ion distribution on stress relaxation in polyelectrolyte complex gels.

Solutions of polyelectrolytes consisting of polycations and polyanions in equal proportions were studied in the present work. Due to the physical cross-links formed by the charged groups, physical gels were formed in such systems. The mechanical properties and structure of the obtained gels depending on the charge arrangement along the backbone and the dimensionless Bjerrum length λ were investigated. The response of the systems to a uniaxial affine deformation was studied first. It was found that the systems can be divided into three groups depending on the charge arrangement: showing an almost elastic response; showing a viscoelastic response with a very long relaxation time; and showing a weak viscoelastic response with a short relaxation time. Interestingly, no stable aggregates were formed in the systems with the charges located on spacers, probably because of the increased mobility of the charges in such systems. The obtained stress relaxation curves had different functional forms, indicating that the relaxation has at least two characteristic times, which are different for different systems. In order to understand the molecular nature of the observed mechanical response, the temporal evolution of the network structure of a system showing a viscoelastic response with a very long relaxation time was studied; the aggregates were found to be dynamic, which leads to the relaxation of the "subchains" conformation.

Creep and stress relaxation experiments of 1960-grade high-strength steel wire at elevated temperatures

With the increasing spacing of large-span structures, the application of ultra-high-strength steel wire with a strength approaching or exceeding 2000 MPa is rapidly expanding. However, the related research on its creep and stress relaxation behaviour is still limited. In this paper, a total of 30 high-temperature creep tests and 31 high-temperature stress relaxation tests were conducted on 1960-grade high-strength steel wire (1960-HSSW) with a diameter of 5.0 mm. The full process creep strain curves were obtained by reinserting and resetting the high-temperature extensometer. The maximum creep strain measured was 50.67 %. The creep and stress relaxation curves of different high-strength steel wires (steel strands) were compared, revealing significant variations in mechanical properties among different steel wires. The steady-state creep rate and high-temperature creep rupture critical stress ratio of the 1960-HSSW were given. Especially, the critical stress ratio became lower than the yield strength reduction coefficient at temperatures exceeding 400 °C and it indicated that the 1960-HSSW could undergo creep rupture before yield. The stress relaxation coefficient was introduced to consider the stress relaxation of 1960-HSSW. Stress relaxation curves for the 1960-HSSW were provided at temperatures ranging from 100 to 550 °C. Moreover, based on the microstructure observed under transmission electron microscopy, it was found that damage occurred in the 1960-HSSW after exposure to 300 °C. Therefore, a critical temperature of 300 °C was recommended for the fire protection of 1960-HSSW, which could be the theoretical support for its fire protection design. Additionally, a high-temperature creep prediction model, which includes expressions for the termination time, was established for the 1960-HSSW. As a result, a conservative prediction of the high-temperature creep for the 1960-HSSW was provided. Moreover, A stress relaxation coefficient model was also provided. The predicted results of the aforementioned models were in good agreement with experimental results.

Processing of Polymers Stress Relaxation Curves Using Machine Learning Methods

Currently, one of the topical areas of application of machine learning methods is the prediction of material characteristics. The aim of this work is to develop machine learning models for determining the rheological properties of polymers from experimental stress relaxation curves. The paper presents an overview of the main directions of metaheuristic approaches (local search, evolutionary algorithms) to solving combinatorial optimization problems. Metaheuristic algorithms for solving some important combinatorial optimization problems are described, with special emphasis on the construction of decision trees. A comparative analysis of algorithms for solving the regression problem in CatBoost Regressor has been carried out. The object of the study is the generated data sets obtained on the basis of theoretical stress relaxation curves. Tables of initial data for training models for all samples are presented, a statistical analysis of the characteristics of the initial data sets is carried out. The total number of numerical experiments for all samples was 346020 variations. When developing the models, CatBoost artificial intelligence methods were used, regularization methods (Weight Decay, Decoupled Weight Decay Regularization, Augmentation) were used to improve the accuracy of the model, and the Z-Score method was used to normalize the data. As a result of the study, intelligent models were developed to determine the rheological parameters of polymers included in the generalized non-linear Maxwell-Gurevich equation (initial relaxation viscosity, velocity modulus) using generated data sets for the EDT-10 epoxy binder as an example. Based on the results of testing the models, the quality of the models was assessed, graphs of forecasts for trainees and test samples, graphs of forecast errors were plotted. Intelligent models are based on the CatBoost algorithm and implemented in the Jupyter Notebook environment in Python. The constructed models have passed the quality assessment according to the following metrics: MAE, MSE, RMSE, MAPE. The maximum value of model error predictions was 0.86 for the MAPE metric, and the minimum value of model error predictions was 0.001 for the MSE metric. Model performance estimates obtained during testing are valid.

Material characterization of EN AC-Al Si12CuNiMg alloy in stress relaxation and creep conditions

Al–Cu–Mg alloys are widely used in various applications, especially engine components, where it is subjected to elevated temperatures during operation. Prolonged exposure of these components to elevated temperatures gradually leads to creep and relaxation phenomena. This study aims to investigate the effects of temperature on stress relaxation and creep behavior of the aluminum-silicon alloy EN AC-Al Si12CuNiMg. Furthermore, this study evaluates the effect of temperature on the rheological model parameters of this alloy. Experimental tests were conducted to investigate the stress relaxation and creep behavior of this alloy at different temperatures of 150°C, 250°C, and 350°C. The rheological model parameters were then determined from the obtained experimental data. The Standard Linear Solid Model (SLSM) was used to fit the stress relaxation and creep curves obtained from the experiment. Overall, the results indicate that increasing temperature significantly affects the stress relaxation and creep behavior of the investigated alloy, with higher temperatures leading to increased relaxation and creep rates. Additionally, the rheological model parameters of the alloy were found to vary with temperature.

Determination accuracy enhancement of viscoelastic constitutive model of optical glass

This study proposes an effective method for improving the determination accuracy of the viscoelastic constitutive model via the combination of uniaxial creep with stress relaxation and the use of creep data to correct stress relaxation curves, known as “stress relaxation and creep correction (SRCC).” A typical molding glass was selected as the study material to fit and correct the constitutive model using the new method, and glass was regarded as a thermorheologically simple material to describe the temperature-dependent viscoelasticity. Glass cylinders were used in uniaxial compression stress relaxation experiments, which were conducted on a molding machine, and the experimental results were fitted with the generalized Maxwell constitutive model. The constitutive model was corrected further by optimization of the long-term modulus based on the SRCC method, and friction coefficients were calibrated under different molding conditions to improve the simulation accuracy. To verify the accuracy of the corrected constitutive model, we constructed a two-dimensional axisymmetric numerical model of molding biconvex aspheric lens and extracted and compared the molding displacement curves with experimental data. The results indicate that more accurate guidance for the optimization of the glass molding process can be provided based on the corrected model.

A numerical model for stress relaxation analysis of sealing systems in nonbonded pipe end fittings

Nonbonded flexible pipes consist of polymer materials that experience stress relaxation at high temperatures, which ultimately affects the sealing performance of end sealing systems. To assess stress relaxation curves under varying temperatures and pre-strains, this paper conducts both tensile and stress relaxation tests on PVDF materials. To describe the stress relaxation characteristics of these materials, the Prony series is utilized. In order to identify the parameters of the series, the paper employs the Levenberg-Marquardt method of nonlinear regression. A finite element model is established to verify the accuracy of the parameter identification method. Subsequently, the paper establishes a two-dimensional axisymmetric finite element model of the sealing system in the end fitting, while taking into account fluid pressure inside the pipeline using a pressure penetration method. The impact of stress relaxation on the sealing performance is then discussed. Finally, the paper alters the coefficients of the Prony series to explore their significance on the sealing performance of the sealing system.

Tensile stress relaxation behavior of a novel wide‐angle fabric/EPDM composite at room temperature

AbstractThe main purpose of this study is to analyze the fundamental deformation and relaxation mechanisms responsible for the stress relaxation behavior of a novel wide‐angle fabric/ethylene propylene diene monomer (EPDM) composite (W‐60 composite) at room temperature with different initial strain. The designed and constructed stress relaxation test device can test five samples simultaneously. The stress relaxation curves of the W‐60 composite are in general agreement with those of the pure rubber. It is proved again that the wide‐angle fabric has good formability. Through the slope calculation, the stress relaxation curves of the W‐60 composite can be divided into two stages: rapid relaxation stage (RRS) and slow relaxation stage (SRS). The cross‐sections are observed using photographic tracking techniques and CT scans. The stress relaxation mechanism of the W‐60 composite is dominated by the internal physical relaxation. It shows many more correlations under the experiment results for the three‐component Maxwell's model with parallel‐connect nonlinear spring (NS‐TCM model). However, in the mechanical models with more parameters, the computational problem also increases due to the accumulation of local minima, which makes it harder to find global minima.Highlights This study builds stress relaxation testing equipment for flexible composite. This study uses a 7‐day stress relaxation pre‐experiment. The stress retrogradation and elastic recovery have occurred. The deformation and strengthening mechanisms of stress relaxation are revealed. The NS‐TCM model shows a positive correlation with the experimental results.

Is the mechanical function of meniscal tissue altered in osteoarthritic knees?

BackgroundDeteriorating meniscal function is thought to play a role in knee osteoarthritis. Meniscal proteoglycans maintain mechanical stiffness of the tissue through electrostatic effects. This study aimed to investigate whether the mechanical properties of macroscopically intact meniscus are preserved in osteoarthritis. MethodsDiscs of lateral meniscal tissue two millimetres thick and of five millimetres diameter from osteoarthritic knees and from healthy donors were placed within a confined compression chamber, mounted in a materials testing machine and bathed in isotonic 0.14M PBS, hypotonic deionised water or hypertonic 3M PBS. Following equilibrium, a 10% ramp compressive strain was applied followed by a 7200 second hold. Resultant stress relaxation curves were fitted to a nonlinear poroviscoelastic model with strain dependent permeability using finite element modelling to determine mechanical parameters. All samples were assayed for proteoglycan content. Comparison of results was undertaken using multivariate ANOVA. ResultsThirty samples from osteoarthritic knees and 18 samples from healthy donors were tested. No significant differences in mechanical parameters or proteoglycan content was observed between groups. In both groups Young’s modulus (E) was significantly greater, and zero-strain permeability significantly reduced, in samples tested in deionised water compared to samples tested in 0.14M or 3M PBS (all p < 0.05). ConclusionMechanical parameters of intact lateral meniscus in osteoarthritic knees are similar to those found in healthy knees. Proteoglycan concentration and their electrostatic contribution to mechanical stiffness of the meniscus is maintained in menisci derived from osteoarthritic knees. Whilst macroscopic tears in the meniscal ultrastructure may contribute to osteoarthritis, intact meniscal tissue maintains its function.

Investigation on relaxation properties of maize kernels based on the multicomponent structure

The damage to maize (Zea mays) during processing is closely related to its viscoelastic properties, but the viscoelastic properties of the components of the maize kernel are unknown. This work aimed to measure the stress relaxation of components, evaluate the relaxation behavior, and observe the microstructure for interpreting relaxation properties. It was found that the stress relaxation curves of the components shifted downward with increasing moisture content. The two-element Maxwell model could successfully express the stress relaxation curves. There was a decreasing trend in the model parameters of the components as moisture content increased. The differences in the degree of the elastic behavior would result in a difference in the residual stress percentage and relaxation rate, which could be better interpreted by the microstructure analysis. The results could help optimize maize processing strategies to maintain product quality.

Long-term stress relaxation behaviors and mechanisms of 2219 Al–Cu alloy under various temperatures and initial stresses

Large 2219 Al–Cu alloy aerospace integral components suffer from long-term stress relaxation aging (SRA) due to complex temperature and stress loads during aging treatment/forming and service process, which makes it difficult to ensure their appropriate residual stress and excellent mechanical and service properties. However, the research is limited to a thorough understanding of macroscopic and microscopic features and underlying mechanisms of the long-term SRA under multivariable aging conditions. Therefore, this study investigated macroscopic and microscopic features of long-term SRA under different temperatures (120 °C to 190 °C), initial stress levels (100 MPa to 250 MPa) and durations (0 h to 50 h) through stress relaxation curves, metallographic traits, Vickers hardness, tensile performance, dislocations and phases of precipitation. On the basis of experimental outcomes, the comprehensive mechanisms beneath SRA were unraveled through dislocation theory, multiphase strengthening mechanisms and thermodynamics, where the interplays of stress relaxation behavior with age-hardening response were taken into consideration. The results showed elevations in the rate of stress reduction as the temperature and initial stress rose. At an initial stress greater than the yield stress of alloy, a marked increase in stress relaxation was found, and the mechanisms transform from the intragranular motion of dislocations and diffusion of grain boundaries to the intragranular and intergranular motion of dislocations and migration of grain boundaries. The stress reduction rate rose sharply when the temperature exceeded 175 °C, and the dislocation movement mechanisms transform from gliding to climbing of dislocations. Stress relaxation is in nature progressive transformation of strain from elastic into a permanently inelastic state via the motion of dislocations, leading to the decrease of movable dislocations and the increase of immovable dislocations with more stable configurations. The age hardening is mainly determined by precipitation strengthening, supplementarily by dislocation strengthening, and obvious stress orientation effect (SOE) of G.P. zones and θ" phases degenerates strengthening effect. The interplay between stress relaxation behavior and age-hardening response influences the thermal–mechanical coupling SRA of 2219 Al–Cu alloy, which depends fundamentally on the motion of dislocations and their interplay with precipitated phases. This is a thermal activation process concerning the interplay between internal (age-hardening resistance) stress and external (initial) stress. The initial energy of elastic strain offers Gibbs free energy as the SRA driver, and a steady state of stress relaxation is attained with the lowest energy of elastic strain. These findings provide valuable insights into exploring innovative aging treatment/forming for optimizing residual stress, mechanical performance and service property in a synergistic manner.

Relaxation behavior of biaxially stretched PLA film during the heat setting stage

In this paper, the relaxation behavior of polylactic acid (PLA) film in the heat-setting stage of biaxial stretching was studied. Firstly, the polylactic acid casting films were stretched synchronously in different ratios. We found that the Machine direction (MD) and Transverse direction (TD) stress relaxation curves exhibited a separation trend with the increase in the stretching ratio, and the relaxation amplitude increased gradually. Then, the stress relaxation curves were fitted by the expansion exponential equation (KWW equation). The results showed that the coefficient used to characterize the homogeneity of stress relaxation increased with the increase in the stretching ratio, and the homogeneity in Machine direction was better than that in Transverse direction. Finally, we analyzed the evolution of rheological units and the activation energy spectrum during stress relaxation. We found that the volume of rheological units gradually decreased with the increase in the stretching ratio. The activation energy spectrum exhibited a Gaussian distribution, and the symmetry axis of distribution curves shifted to the high energy. The above results would be of great significance in further understanding the deformation mechanism of polylactic acid film during biaxial stretching and providing theoretical guidance for the preparation of high-performance BOPLA films.

Comparative Study of Creep and Stress Relaxation Behaviour during Ageing of 7050 Aluminum Alloy

This work focuses on quantitatively investigating the relationship between creep and stress relaxation of 7050 aluminum alloy at ageing temperatures. The creep strain rates, creep exponent, n, and activation energy were extracted from both the creep and stress relaxation curves to explore the creep mechanisms. Results showed that higher applied stress promotes the creep and stress relaxation extents. The calculated creep strain rates from creep and stress relaxation curves located in similar ranges and share similar n values (n = 3.7 for creep and n = 3.5 for stress relaxation), indicating that both tests are dominated by the dislocation slip mechanism. Additionally, the n value for creep tests is a constant throughout the tests, indicating that the dominant creep mechanism is unchanged. However, the activation energy changes from 123 kJ/mol to 139 kJ/mol, possibly due to evolved dislocations. Such a new finding could provide a supporting mechanism for building constitutive equations depending on the evolved A value. Furthermore, compared to the single-stage creep ageing, no primary creep was observed in T74 multi-stage creep ageing while similar amount of total creep strains was achieved. This suggests an alternative loading sequence for creep age forming of 7050 aluminum alloy, i.e., loading at the second stage to avoid prolonged exposure to high temperatures, extend the tool life, and achieve similar amounts of inelastic deformations. Based on all the above studies, creep and stress relaxation have a certain relationship. This paper provides a theoretical basis for future research on creep ageing forming of 7050 aluminum alloy.

Norepinephrine as an Enhancer Promoting Corneal Penetration of Riboflavin for Transepithelial Corneal Crosslinking.

Previously, we found norepinephrine (NE) could affect the corneal epithelial integrity, herein we investigated the feasibility and safety of NE serving as a chemical enhancer to promote corneal penetration of riboflavin during transepithelial corneal crosslinking (CXL). The dosage of NE that could promote riboflavin diffusion through the healthy epithelial barrier without inducing epithelial damage in C57BL/6 mice was determined. The safety of NE treatment was confirmed by morphological and histological examinations of the whole cornea. The efficacy of NE in promoting riboflavin penetration was verified by slit lamp and scanning electron microscope (SEM), and corneal biomechanical measurement after CXL. To better fit the clinical scenario, increased NE dosage and shortened riboflavin infiltration time were further evaluated. The lowest dosage of NE (1 mg/mL) that facilitated transepithelial riboflavin permeation was 2 µL. No visible corneal structure alteration was observed after NE treatment. SEM indicated dissociation of intercellular junctions among corneal epithelial cells. Homogenous distribution of riboflavin throughout corneal stroma was observed. NE-treated corneas reached comparable biomechanical properties after CXL, including stress-relaxation curve and elastic modulus, with corneas treated with the commercially available transepithelial drug Peschke TE. To better fit the clinical scenario, increasing NE up to 5.5 µL helped riboflavin infiltrate the corneal stroma within 30 minutes. After CXL with 9 mW/cm2 ultraviolet-A (UVA) for 2.5 minutes, the cornea showed significantly enhanced corneal biomechanical properties with undisturbed corneal endothelium. NE serves as an effective enhancer in increasing riboflavin diffusion with limited impairment on corneal epithelium and has great potential for clinical application. NE serves as an effective enhancer for riboflavin penetration and clinical transepithelial CXL.

Stress Relaxing Simulation on Digital Rock: Characterize Attenuation Due To Wave‐Induced Fluid Flow and Scattering

AbstractDigital rock physics (DRP) is essential to understand the wave elastic and attenuation characteristics of complex porous rocks, offering insights into interpreting seismic and sonic logging data. Most of the existing DRP technology can only simulate wave‐induced fluid flow (WIFF) or scattering effects separately in a relatively narrow or single‐frequency band. To incorporate both the effects of WIFF and elastic scattering simultaneously, we develop a stress relaxing simulation (SRS) technique on digital rocks to characterize the P wave dispersion and attenuation signatures in a broad frequency band. We propose to use the combined methodology of curve resampling and wavelet‐packet denoising to suppress the high‐frequency noise of the stress relaxation curve. For a digital rock containing pore‐crack elements, we demonstrate that SRS is capable of characterizing the attenuation due to squirt flow and scattering as well as their interactions. The simulation results of wave dispersion and attenuation using SRS are overall consistent with other well‐developed DRP technology of transmitted wave simulation (high‐frequency band), forced oscillating simulation (intermediate‐frequency band), and static simulation (low‐frequency band), suggesting the reliability and robustness of the developed methodology. We also apply the SRS on a realistic 2D digital rock of porous sandstone. It is found that the attenuation due to elastic scattering of the microscopic heterogeneities is significantly higher than that caused by squirt flow, which might be caused by the lack of pore connectivity impeding the fluid exchange at the microscopic scale due to the limitation of 2D digital porous rocks.

Fused Deposition Modeling and Characterization of Heat Shape Memory Poly(lactic) Acid-Based Porous Vascular Scaffold.

Shape memory polymers have received widespread attention from researchers because of their low density, shape variety, responsiveness to the environment, and transparency. This study deals with heat-shape memory polymers (SMPs) based on polylactic acid (PLA) for designing and fabricating a novel porous vascular scaffold to treat vascular restenosis. The solid isotropic material penalization method (SIMP) was applied to optimize the vascular scaffolds. Based on the torsional torque loading of Hyperworks Optistruct and the boundary conditions, the topological optimization model of a vascular scaffold unit was established. Forward and reverse hybrid modeling technology was applied to complete the final stent structure's assembly. The glass transition temperature for the present SMPs is 42.15 °C. With the increase in temperature, the ultimate tensile strength of the SMPs is reduced from 29.5 MPa to 11.6 MPa. The maximum modulus at room temperature was around 34 MPa. Stress relaxation curves show that the material classification is a "thermoset" polymer. The superb mechanical properties, the transition temperature of the SMPs, and the recovery ratio made it a feasible candidate for a vascular scaffold. A circular tube based on the shape memory polymers was presented as an example for analyzing the recovery ratio in an unfolding state. A higher recovery ratio was obtained at a temperature of 65 °C with a tube thickness of 2 mm. Finally, the proposed porous vascular scaffold was successfully fabricated, assessed, and compared with the original and previously developed vascular scaffolds. The proposed scaffold structure regains its initial shape with a recovery ratio of 98% (recovery temperature of 47 °C) in 16 s. The tensile strength, Young's modulus, and bending strength of the proposed scaffold were 29.5 MPa, 695.4 MPa, and 6.02 MPa, respectively. The results showed that the proposed scaffold could be regarded as a potential candidate for a vascular implantation.

Prediction of rheological parameters of polymers, using gradient boosting algorithm CatBoost

The article deals with the problem of determining the rheological parameters of polymers from stress relaxation curves using the CatBoost machine learning algorithm. The model is trained on theoretical curves constructed using the non-linear Maxwell-Gurevich equation. A comparison is made with other methods, including the classical algorithm, non-linear optimization methods and artificial neural networks.