A Fractional Creep Constitutive Model Considering the Viscoelastic-Viscoplastic Coexistence Mechanism

In order to improve the accuracy and universality of the nonlinear viscoelastic-plastic mechanical behavior characterization method of asphalt mixture, a new criterion for the division of the creep process of materials was established based on the strain yield characteristics, and the coexistence mechanism of Viscoelastic-Viscoplastic strain was proposed in the subsequent yield phase; then, a viscoelastic element was constructed in the form of a parallel connection of two fractional viscoelastic elements based on fractional calculus theory, and its mathematical equations were derived; with novel viscoelastic elements, a constitutive model characterizing the whole creep process of asphalt mixtures was developed and its analytical expression was derived. The laboratory short-term creep test of Cement and Asphalt Mortar (CA mortar) and the simulation test data of asphalt mixtures from the references were used to verify the constitutive model. The results show that the creep constitutive model of asphalt mixture established in this paper has excellent fitting accuracy for different phases of the creep process of asphalt mixture under different stress levels, where the minimum fitting correlation values R2 for CA mortar, asphalt mixture (applied to pavement engineering), and asphalt sand are 0.9976, 0.981, and 0.979, respectively. Therefore, this model can be used to provide a theoretical reference for the study of the characterization of the mechanical behavior of asphalt materials.

New Approach in the Determination of a Suitable Directionally Coarsened Microstructure for the Fabrication of Nanoporous Superalloy Membranes Based on CMSX-4

The pore size of nanoporous superalloy membranes produced by directional coarsening is directly related to the γ-channel width after creep deformation, since the γ-phase is removed subsequently by selective phase extraction. The continuous network of the γ'-phase thus remaining is based on complete crosslinking of the γ'-phase in the directionally coarsened state forming the subsequent membrane. In order to be able to achieve the smallest possible droplet size in the later application in premix membrane emulsification, a central aspect of this investigation is to minimize the γ-channel width. For this purpose, we use the 3w₀-criterion as a starting point and gradually increase the creep duration at constant stress and temperature. Stepped specimens with three different stress levels are used as creep specimens. Subsequently, the relevant characteristic values of the directionally coarsened microstructure are determined and evaluated using the line intersection method. We show that the approximation of an optimal creep duration via the 3w0-criterion is reasonable and that coarsening occurs at different rates in dendritic and interdendritic regions. The use of staged creep specimens shows significant material and time savings in determining the optimal microstructure. Optimization of the creep parameters results in a γ-channel width of 119 ± 43 nm in dendritic and 150 ± 66 nm in interdendritic regions while maintaining complete crosslinking. Furthermore, our investigations show that unfavorable stress and temperature combinations favor undirectional coarsening before the rafting process is completed.

Study on Shear Creep Characteristics of the Discontinuities with Different 3D Morphologies

The rheological phenomenon of rock mass affects the long-term safety of rock mass engineering. In this study, gneiss samples with different 3D morphologies are prepared by splitting tests and are tested through multi-step creep tests. The long-term strength of rock discontinuities is determined by using several methods. The test results show that as the 3D morphological parameter increases, the creep deformation, creep rate, and the duration of failure all decrease. The long-term strength of rock discontinuities is linearly related to the 3D morphological parameter. Based on the principle of damage mechanics for rock mass, a damage variable is introduced in the creep model, and an improved non-linear Burgers model is established. Research results are of great theoretical significance and practical value for the design, construction, and long-term safety of rock mass engineering.

Modeling of Thermo-Chemo-Mechanical Properties of Anode Mixture during the Baking Process

In the Hall–Héroult process, prebaked carbon anodes are utilized to produce primary aluminium. The quality of the anode plays a crucial role in the efficiency of electrowinning primary aluminium. In the production of anodes, the anode baking is considered as the stage most frequently causing anode problems. During the baking process, the anode undergoes complex physicochemical transformations. Moreover, the anode at a lower position, imposed by loading pressures from upper anodes, will creep during this process. Thus, the production of high-quality anodes demands efficient control of their baking process. This paper aims to investigate the thermo-chemo-mechanical properties of the anode paste mixture at high temperatures. These properties include kinetic parameters of pitch pyrolysis such as the activation energy and the pre-exponential factor, the thermal expansion coefficient (TEC) and relevant mechanical parameters related to the elastic, the viscoelastic and the viscoplastic behaviours of the anode. For this purpose, experiments consisting of the thermogravimetric analysis, the dilatometry and the creep test were carried out. Based on the obtained results, the forementioned parameters were identified. Relevant mechanical parameters were expressed as a function of a new variable, called the shrinking index, which is related to the volatile released in open and closed pores of the anode. This variable would be used to highlight the chemo-mechanical coupling effect of the anode mixture. New insights into the phenomena such as the expansion due to the increase of the pore pressure and the chemical shrinkage of the anode during the baking process were also gained in this work. These investigations pave the way for modeling the thermo-chemo-poromechanical behaviour of the anode during the baking process.

Comparative analysis of the viscoelastic properties of collagen-like proteins by virtual creep test

Viscoelasticity of collagen is essential for the integrity of connective tissue and its aberrancy may result in collagen dysfunction and the emergence of connective tissue diseases. Precise identification of viscoelastic properties of collagens, and affecting factors are necessary to understand collagen behavior in the extracellular matrix as well as the mechanism of collagen-related diseases. The aim of this study is to investigate the mechanical and viscoelastic properties and time-lapse changes of protein-protein and protein-solvent hydrogen bonds of proline-rich and hydroxyproline-rich collagens by molecular dynamics simulation applying a virtual creep test. To this end, ten different collagen-like protein structures including [(Gly-Pro-Ala)₇]₃, [(Gly-Pro-Arg)₇]₃, [(Gly-Pro-Asp)₇]₃, [(Gly-Pro-Lys)₇]₃, [(Gly-Pro-Ser)₇]₃, [(Gly-Pro-Hyp)₇]₃, [(Gly-Ala-Hyp)₇]₃, [(Gly-Glu-Hyp)₇]₃, [(Gly-Leu-Hyp)₇]₃ and [(Gly-Val-Hyp)₇]₃ were virtually built and the viscoelastic properties of the structures were determined by virtual creep test according to Kelvin-Voigt model with various constant pulling forces. Different pulling forces ranged from 500 piconewton (pN) to 5000 pN were applied. As a result, Young's modulus of the collagens was found positively correlated with the pulling force. The viscosity values and relaxation times were negatively correlated. Results also revealed a decreased number of intramolecular hydrogen bonds in hydroxyproline-rich collagens (but not in proline-rich collagens) and an increased number of protein-solvent hydrogen bonds in response to the increasing pulling force. Our results also confirmed that proline and hydroxyproline are the most critical amino acids in determining the collagen viscoelasticity. We suggest that collagen length and mechanical force may be additional important factors for biomechanical properties and behavior of collagen in the extracellular matrix.Communicated by Ramaswamy H. Sarma.

Short- and Long-Term Structural Characterization of Cured-in-Place Pipe Liner with Reinforced Glass Fiber Material

Cured-in-place pipe (CIPP), as a kind of trenchless sewer rehabilitation technology, is a method to repair sewer pipe using unsaturated polyester resin. This study develops a CIPP liner using hot water or steam curing as well as glass fiber, in contrast to traditional methods, which use nonwoven fabric. Composite material samples were fabricated by combining liner materials using various methods, and the structural characteristics of the liners were compared and analyzed through short- and long-term flexural strength tests. A long-term test was conducted for 10,000 h, and the results revealed 13.3 times higher flexural strength and 8 times higher flexural modulus than the American Society for Testing Materials minimum criteria for CIPP short-term properties. The maximum creep retention factor was 0.64, thereby reducing the design thickness of the CIPP by up to 54%. The structural characteristics also improved when glass fibers were mixed with traditional CIPP liner, making it possible to reduce the thickness by 30%. Glass fibers result in high structural strength when combined with unsaturated polyester resin. Structural strength increased, even when glass fibers were mixed with traditional CIPP liner. The main contribution of this research is the development of a high strength CIPP liner and improvement of the structural properties of CIPP lining without using the specially formulated resin or lining materials.

Viscoelastic Parameters of Asphalt Mixtures Identified in Static and Dynamic Tests

We present two methods used in the identification of viscoelastic parameters of asphalt mixtures used in pavements. The static creep test and the dynamic test, with a frequency of 10 Hz, were carried out based on the four-point bending beam (4BP). In the method identifying viscoelastic parameters for the Brugers' model, we included the course of a creeping curve (for static creep) and fatigue hysteresis (for dynamic test). It was shown that these parameters depend significantly on the load time, method used, and temperature and asphalt content. A similar variation of parameters depending on temperature was found for the two tests, but different absolute values were obtained. Additionally, the share of viscous deformations in relation to total deformations is presented, on the basis of back calculations and finite element methods. We obtained a significant contribution of viscous deformations (about 93% for the static test and 25% for the dynamic test) for the temperature 25 °C. The received rheological parameters from both methods appeared to be sensitive to a change in asphalt content, which means that these methods can be used to design an optimal asphalt mixture composition-e.g., due to the permanent deformation of pavement. We also found that the parameters should be determined using the creep curve for the static analyses with persistent load, whereas in the case of the dynamic studies, the hysteresis is more appropriate. The 4BP static creep and dynamic tests are sufficient methods for determining the rheological parameters for materials designed for flexible pavements. In the 4BP dynamic test, we determined relationships between damping and viscosity coefficients, showing material variability depending on the test temperature.

Application of Peleg's equation to describe creep responses of potatoes under constant and variable storage conditions

The application of Peleg's equation to characterize creep behavior of potatoes during storage was investigated. Potatoes were stored at 25, 15, 5C, and variable (fluctuating) temperature for 16 or 26 weeks. The Peleg equation adequately described the creep response of potatoes during storage at all storage conditions (R²  = .97to .99). Peleg constant k₁ exhibited a significant (p < .05) decreasing trend with storage time (t_s ) for samples stored under the experimental conditions whereas the constant k₂ appeared to be unaffected much by t_s except for samples stored at 25C. Under constant temperature storage, k₁ was found to be a linear function of t_s (R²  = .87 to .97). Also, the rate of change of k₁ appeared to be temperature dependent described by a linear relationship between the degradation rate constant for k₁ (α) and storage temperature (T). For the variable storage condition, a bulk mean temperature (T_bm ) was calculated to account for a series combination of storage time and temperature which the potatoes were subjected to. It was possible to describe the changes in k₁ due to variable storage temperature in terms of T_bm and t_s using stepwise multiple regression (R²  = . 94).

PRACTICAL APPLICATIONS

Precise description of the changes in the rheological properties of raw potatoes during storage could help predict the associated effect on the texture of cooked potatoes. Easy and simple methods of describing creep responses during storage or processing will be potentially helpful to better understand the phenomenon. The model parameters from such model could be used to relate rheological properties of raw and cooked potatoes. Moreover, the model parameters could be used to establish relationship between instrumental and sensory attributes which will help in the prediction of sensory attributes from instrumental data.

Mechanical Properties, Short Time Creep, and Fatigue of an Austenitic Steel

The correct choice of a material in the process of structural design is the most important task. This study deals with determining and analyzing the mechanical properties of the material, and the material resistance to short-time creep and fatigue. The material under consideration in this investigation is austenitic stainless steel X6CrNiTi18-10. The results presenting ultimate tensile strength and 0.2 offset yield strength at room and elevated temperatures are displayed in the form of engineering stress-strain diagrams. Besides, the creep behavior of the steel is presented in the form of creep curves. The material is consequently considered to be creep resistant at temperatures of 400 °C and 500 °C when subjected to a stress which is less than 0.9 of the yield strength at the mentioned temperatures. Even when the applied stress at a temperature of 600 °C is less than 0.5 of the yield strength, the steel may be considered as resistant to creep. Cyclic tensile fatigue tests were carried out at stress ratio R = 0.25 using a servo-pulser machine and the results were recorded. The analysis shows that the stress level of 434.33 MPa can be adopted as a fatigue limit. The impact energy was also determined and the fracture toughness assessed.

Xyloglucan endotransglucosylase/hydrolase (XTH) overexpression affects growth and cell wall mechanics in etiolated Arabidopsis hypocotyls

Growth and biomechanics of etiolated hypocotyls from Arabidopsis thaliana lines overexpressing xyloglucan endotransglucosylase/hydrolase AtXTH18, AtXTH19, AtXTH20, and PttXET16-34 were studied. Overexpression of AtXTH18, AtXTH19, and AtXTH20 stimulated growth of hypocotyls, while PttXET16-34 overexpression did not show this effect. In vitro extension of frozen/thawed hypocotyls measured by a constant-load extensiometer started from a high-amplitude initial deformation followed by a slow time-dependent creep. Creep of growing XTH-overexpressing (OE) hypocotyls was more linear in time compared with the wild type at pH 5.0, reflecting their higher potential for long-term extension. XTH-OE plants deposited 65-84% more cell wall material per hypocotyl cross-sectional area than wild-type plants. As a result, their wall stress under each external load was lower than in the wild-type. Growing XTH-OE hypocotyls had higher values of initial deformation·stress(-1) compared with the wild type. Plotting creep rates for each line under different loads against the respective wall stress values gave straight lines. Their slopes and intercepts with the abscissa correspond to ϕ (in vitro cell wall extensibility) and y (in vitro cell wall yield threshold) values characterizing cell wall material properties. The wall material in XTH-OE lines was more pliant than in the wild type due to lower y values. In contrast, the acid-induced wall extension in vitro resulted from increasing ϕ values. Thus, three factors contributed to the XTH-OE-stimulated growth in Arabidopsis hypocotyls: their more linear creep, higher values of initial deformation·stress(-1), and lower y values.