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1

Viscoelastic characterization of additively manufactured acrylonitrile butadiene styrene

Witek Szczepan

Computer Methods in Materials Science

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2023

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Vol. 23, No. 2

39--44

EN

The main objective of this work was to characterize the viscoelastic properties of additively manufactured Acrylonitrile Butadiene Styrene based on tensile stress relaxation tests. The stress relaxation measurements were conducted with a temperature range of 25–100°C. The two-layer viscoplastic constitutive model was adopted to describe the elastic and viscous behavior of the investigated material. The model parameters were calibrated using an inverse analysis and stress relaxation data. The model’s predictive capabilities were assessed by comparing the model predictions with experimental data not included in the calibration process.

2

Simulation of Stress Relaxation Behaviour of Composite Propellants with Varying Solid Loading Using the Generalized Maxwell Model

Bihari Bipin Kumar, Kumaraswamy Adepu, Jain Mukesh, Kurva Ramesh, Vipin Lisha

Central European Journal of Energetic Materials

|

2023

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Vol. 20, no. 2

221--235

EN

Hydroxyl-terminated polybutadiene (HTPB) based composite propellants possess viscoelastic behaviour and hence time and temperature dependent mechanical properties. The mathematical analysis of viscoelastic behaviour of composite propellants becomes complex due to the non-linearity involved under various loading conditions. In the present study, a linear viscoelasticity assumption was considered to simulate stresses related to storage conditions. In this paper, a study of stress relaxation behaviour of composite propellants was carried out using the Generalized Maxwell model to obtain the material viscoelastic characteristics. The relaxation behaviour of composite propellants having solid loading varying from 85% to 89% were studied at different temperatures, from –27 to +32 °C, using a Dynamic Mechanical Analyser (DMA). The generated relaxation curves were curve fitted using MATLAB (R2022a) with the Generalized Maxwell model. The simulation demonstrated that a maximum of four elemental parameters of the Generalized Maxwell model are sufficient and can represent a best fit of the relaxation behaviour of the studied composite propellants. The equilibrium modulus was also evaluated at different temperatures, along with other material constants that are essential parameters for performing the structure integrity analysis of a solid propellant rocket motor. It was observed that the equilibrium modulus decreases with an increase in temperature, but increases with an increase in solid loading in the propellant composition formulations.

3

The viscoelasticity, anisotropy and location-dependence of mechanical properties of rabbit iris investigated using uniaxial tensile tests

Li Tan, Qin Xiao, Liu Zhicheng, Zhang Haixia, Li Lin

Acta of Bioengineering and Biomechanics

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2023

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Vol. 25, no. 2

85--92

EN

Abnormal iris mechanical properties have been considered to be an important cause of pupillary-block and angle-closure glaucoma. In this research, viscoelasticity, anisotropy and location-dependence of mechanical properties of rabbit iris were investigated using uniaxial tensile test. Methods: Iris strips were taken along three directions: inner-circumferential direction (ICD), outer-circumferential direction (OCD) and radial direction (RD), respectively. Quasi-static tensile tests and stress–relaxation tests were applied on the iris strips. Then, the stress–stretch data was fitted with third order Ogden model; the stress–relaxation data was fitted with the third order Prony series model. Through comparing the tangent modulus and relaxation limit of the strips from different directions and locations, the viscoelasticity, anisotropy and location-dependence of mechanical properties of rabbit iris were explored. Results: The tangent moduli of iris at the stretch of 1.05 along ICD, OCD, and RD were 3.2 ± 1.4 kPa, 4.2 ± 2.6 kPa, 1.5 ± 0.8 kPa, respectively. Iris strips in ICD and OCD were found to have almost the same stress–relaxation behavior, and both relaxed slower than iris strips in RD. Conclusions: The mechanical properties of the iris were typically nonlinear, viscoelastic, anisotropic and location-dependent. The stress growth rate of the circumferential direction iris strip is significantly lower than that of RD and the stress–relaxation rate is significantly higher than that of the RD. That is, the iris is more prone to deformation in RD and the stress–retention ability after deformation in RD is weak, which is consistent with the fact that the iris bombe more likely happens in RD in vivo. The results of this study may also help us to establish a more accurate finite element model to simulate the flow field of humor aqueous and find the key factor of pupillary-block.

4

Numerical simulation of single- and multi-step shear stress relaxations of isotropic magnetorheological elastomer using fractional derivative viscoelastic models

Nam T. H., Petríková I., Marvalová B.

Archives of Mechanics

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2022

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Vol. 74, nr 4

251--266

EN

The paper presents numerical simulations of single- and multi-step shear stress relaxations of isotropic magnetorheological elastomer (MRE) using fractional derivative Maxwell and Kelvin–Voigt viscoelastic models. The isotropic MRE has been fabricated by filling micro-sized carbonyl iron particles in silicone rubber. Fractional derivative Maxwell and Kelvin–Voigt viscoelastic models were used to fit the experimental data of the isotropic MRE measured by single- and multi-step relaxation tests at different constant strains and external magnetic fields. The fractional Maxwell viscoelastic model showed a relatively large difference between the measured and calculated results. The fractional Kelvin–Voigt model was fitted well with the experimental data of the isotropic MRE at various constant strain levels under different magnetic fields in both single- and multi-step shear stress relaxations. The calculated shear stress with the long-term prediction is in excellent agreement with the measured one. Therefore, the fractional derivative Kelvin–Voigt viscoelastic model is applicable to predict the long-term stress relaxation of the isotropic MRE.

5

Badanie relaksacji i pełzania wybranych papierów pakowych

Szmidt Karolina, Stanisławska Anna

Przegląd Papierniczy

|

2021

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R. 77, nr 12

661--667

PL

Celem pracy było opracowanie metody testowej umożliwiającej badanie pełzania i relaksacji naprężeń oraz zastosowanie tej metody do oceny właściwości reologicznych papierów pakowych. Wyznaczono zależność reologiczną obciążenie-odkształcenie, a także obciążenie zrywające i wydłużenie w momencie zerwania. Podczas badań relaksacji naprężeń i pełzania, zadane odkształcenia i obciążenia stanowiły 60% tych wartości dla granicy wytrzymałości danego materiału. Dla wszystkich przebadanych papierów relaksacja naprężeń była wyższa dla kierunku maszynowego (MD), a pełzanie było na ogół większe dla kierunku poprzecznego (CD). Najwyższą relaksacją naprężeń charakteryzował się liner-MD, a najniższą – clupak-CD. Z kolei najwyższe pełzanie zaobserwowano dla papieru clupak-CD, a najniższe dla papieru natron-MD. Najmniejszą anizotropię relaksacji naprężeń i pełzania zaobserwowano dla papieru clupak. Uzyskane wyniki pozwoliły na przeprowadzenie analizy zachowania się wybranych papierów pakowych podczas wystawienia ich na działanie obciążeń nieniszczących.

EN

The objective of the presented work was to develop a test method that enables the study of creep and stress relaxation and the use of this method to assess the rheological properties of packaging papers. The rheological dependence load – elongation, breaking load and elongation at break were determined. During the stress relaxation and creep experiments, the set elongation and load were equal to 60% of these values at break. For all tested papers, the stress relaxation was higher for the machine direction (MD), and the creep was generally greater for the cross direction (CD). The highest stress relaxation was found for the liner-MD, and the lowest – for the clupack-CD. The highest creep was observed for clupak-CD paper, and the lowest for natron-MD paper. The lowest anisotropy of stress relaxation and creep was for the clupack paper. The applied method made it possible to analyze the behavior of selected packaging papers when subjected to non-destructive loads.

6

Stress relaxation in samples made of acrylonitrile butadiene styrene material manufactured by fused deposition modeling

Szot Wiktor

Mechanik

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2021

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R. 94, nr 12

46--50

EN

Increased interest in fused deposition modeling (FDM) resulting, for example, from its use in the production of utility models determines the undertaking of research on mechanical and rheological properties of materials. Mechanical and rheological properties of models made of materials used in FDM technology depend on technological parameters. In this paper, the effect of 0° and 90° print orientation on stress relaxation was analyzed. Additionally, the usefulness of the rheological model to describe the relaxation curve was evaluated. Stress relaxation tests were performed by tensile testing. The five-parameter Maxwell-Wiechert model was used to describe stress relaxation. The tests showed little effect of print orientation on the rheological parameters of the five-parameter model. The Maxwell-Wiechert model showed a very good approximation to the stress relaxation curves.

PL

Wzrost zainteresowania technologią osadzania topionego materiału (FDM), wynikający m.in. z zastosowania jej do produkcji modeli użytkowych, wymusza podjęcie badań nad właściwościami mechanicznymi w tym reologicznymi materiałów. Właściwości mechaniczne (i reologiczne) modeli wytwarzanych z materiałów stosowanych w technologii FDM zależą od parametrów technologicznych. W prezentowanym artykule poddano analizie wpływ orientacji wydruku 0° i 90° na relaksację naprężeń. Dodatkowo oceniono przydatność modelu reologicznego do opisu krzywej relaksacji. Testy relaksacji naprężeń wykonano poprzez próbę rozciągania. Do opisu relaksacji naprężeń zastosowano pięcioparametrowy model Maxwella-Wiecherta. Badania wykazały niewielki wpływ orientacji wydruku na parametry reologiczne modelu pięcioparametrowego. Model Maxwella-Wiecherta wykazał bardzo dobrą aproksymację do krzywych relaksacji naprężeń.

7

Stress relaxation phenomena in polymeric orthodontic ligatures

Milewski Grzegorz

Engineering of Biomaterials

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2021

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Vol. 24, no. 162

2--6

EN

Elastomeric products are applied in orthodontics mainly as elastic ligatures or chains and have become an alternative to wire ligation made of titanium alloy or stainless steel. Despite their popularity among the dentists and undoubtful advantages, some essential warnings are being raised regarding the degree of load loss. This relaxation phenomenon seems to be a dominant feature in the time-dependent behaviour of those elements in orthodontic procedures, such as dentition corrections or teeth extrusions. The aim of the paper was to examine and analyse the rheological properties of biocompatible orthodontic elastomeric ligatures. Five different polymeric orthodontic ligatures were examined in the following experiments: a simple relaxation test, relaxation simulating orthodontic extrusion and the two-steps relaxation process, which stands for so-called ‘secondary tightening’, resulting in the increase of the orthodontic force. The results of the relaxation experiments proved that among various descriptions of that phenomenon, the power-law descriptions fit the best time-dependent behaviour during orthodontic procedures. Power-law models give the most intensive initial relaxation, which is characteristic for elastomeric ligatures. The obtained results and analyses allow precise control of the treatment progress in the orthodontic extrusion procedure.

8

The influence of material aging on the structural behavior of a flexible roof with a polymer membrane shell

Chesnokov Andrei V., Mikhailov Vitalii V., Dolmatov Ivan V.

Computer Methods in Materials Science

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2021

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Vol. 21, No. 1

13--24

EN

The roof structure considered in the research consists of continuous cables and a number of spreaders forming a three-dimensional frame. The frame is covered with a polymer membrane made of flexible architectural fabrics. The elements of the roof are compact and suitable for transportation to remote construction sites. The roof also has advantages for developing areas with harsh climatic conditions. The flexible elements of the roof, however, only provide transmission of tensile forces. Under compression, cables slacken and the membrane becomes wrinkled. Pre-tension of the flexible elements, which is introduced to retain the operability of the roof, tends to gradually diminish due to material aging. The aging results in the alteration of strength properties and creep elongation of the structural elements. It induces membrane tearing on local areas. Force alteration in primary structural members is examined in the present study, with statistical methods used for data analysis. They include significance hypothesis testing and correlation coefficients estimation. The data are obtained by the Finite Element simulation of the roof using EASY-2020 software. The results of the work may be used for life expectancy assessment of flexible roof structures, providing important information for the preliminary design stage. The work contributes to the safety enhancement of cable-membrane structures and the expansion of their scope in permanent building constructions.

9

Comparative Study of Long- and Short-Stretch Woven Compression Bandages

Aboalasaad Abdelhamid R. R., Sirková Brigita Kolčavová, Bílá Pavlína, Khalil Amany A. S.

Autex Research Journal

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2021

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Vol. 21, no. 4

435--444

EN

Compression therapy using bandages or socks is the most common treatment for venous leg ulcers and edema. This article aims to compare the compression between long- and short-stretch bandages (LSB and SSB, respectively). Load-elongation curves, cyclic loading, and elastic recovery are investigated for both Cotton/Polyamide/Polyurethane and 100% bleached cotton bandages as LSB and SSB, respectively. Static (resting) and dynamic (working) pressures are measured on seven male legs, 31 ± 3.6 years old, using both two and three layers bandaging. Picopress pressure tests are performed on the ankle and mid-calf positions at gradual decreasing compression from the ankle to the knee. The deviation percentage between the experimental results by Picopress and theoretical calculations using Laplace's law and Al-Khaburi equations is compared. LSB recovered approximately 99% of its original length after stress-relaxation whereas SSB recovered only 93% of its original length after 5 days of cyclic load-relaxation. Moreover, SSB lost approximately 28.6% of its activity after wearing on the human leg for 5 days.

10

Stress relaxation of porcine tendon under simulated biological environment: experiment and modeling

Łagan Sylwia D., Liber-Kneć Aneta

Acta of Bioengineering and Biomechanics

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2021

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Vol. 23, no. 1

59--68

EN

Purpose: The aim of the study was to investigate the viscoelastic response in the low and high physiological strain with the use of experimental and modeling approach. Methods: Viscoelastic response in the low, transition and high physiologic strain (3, 6 and 9%) with consideration of simulated biological environment (0.9% saline solution, 37 °C) was measured in relaxation tests. Preconditioning of tendons was considered in the testing protocol and the applied range of load was obtained from tensile testing. The quasi-linear viscoelasticity theory was used to fit experimental data to obtain constants (moduli and times of relaxation), which can be used for description of the viscoelastic behavior of tendons. The exponential non-linear elastic representation of the stress response in ramp strain was also estimated. Results: Differences between stress relaxation process can be seen between tendons stretched to the physiological strain range (3%) and exceeding this range (6 and 9%). The strains of 6% and 9% showed a similar stress relaxation trend displaying relatively rapid relaxation for the first 70 seconds, whereas the lowest strain of 3% displayed relatively slow relaxation. Conclusions: Results of the model fitting showed that the quasi-linear viscoelastic model gives the best fit in the range of low physiological strain level.

11

Verification of the Peleg and Normand equation at varied loads and moisture during stress relaxation of wheat

Łysiak Grzegorz, Kulig Ryszard, Wójcik Monika

Agricultural Engineering

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2019

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Vol. 23, No. 3

89--99

EN

Wheat kernel is made up of structures of different apparent viscosities with varied ability to withstand stress and dissipate strain energy. Its complex mechanical behaviour determines technological susceptibility and is important for wheat quality assessment. The aim of the study was the examination of the Peleg and Normand model to characterize the overall stress relaxation behaviour of wheat kernel at varying loading conditions. The relaxation experiments were made with a help of a universal machine Zwick Z020 in compression at four distinct initial load levels, i.e., 20, 30, 40, and 50 N. The measurements were made for intact wheat kernels at seven levels of moisture content. Relaxation characteristics were approximated with the help of Peleg and Normand formula. An interactive influence of the load level and moisture on Peleg and Normand constants have been confirmed. The initial loading level had none or only slight effect on the model coefficients (Y(t), k1 and k2). The parameters of the Peleg and Normand model decreased with the increase of water content in kernels. For moist kernels, a higher amount of absorbed compression energy was relaxed, since less energy was necessary to keep the deformation at a constant level.

PL

Ziarniak pszenicy składa się ze struktur o różnych własnościach lepko-sprężystych, różnej wytrzymałości i zdolności do rozpraszania energii odkształcenia. Jego złożone zachowanie mechaniczne determinuje podatność technologiczną i jest ważne dla oceny jakości ziarna. Celem badań była analiza zmienności parametrów modelu Pelega i Normanda w różnych warunkach obciążenia początkowego. Pomiary przeprowadzono przy wykorzystaniu uniwersalnej maszyny Zwick Z020. Zastosowano test jednoosiowego ściskania pojedynczych ziarniaków dla czterech początkowych poziomów obciążenia, tj. 20, 30, 40 i 50 N. Badania wykonano dla siedmiu poziomów wilgotności. Charakterystyki relaksacyjne aproksymowano za pomocą równania Pelega i Normanda. Potwierdzono interaktywny wpływ poziomu obciążenia i wilgotności na stałe modelu. Początkowy poziom obciążenia nie miał żadnego lub wpływał tylko nieznacznie na współczynniki modelu (Y(t), k1 i k2). Parametry modelu Pelega i Normanda spadały wraz ze wzrostem zawartości wody w ziarniakach. W przypadku wilgotnych ziarniaków relaksacji podlegała istotnie większa wartość zaabsorbowanej energii, potrzeba było też mniej energii aby utrzymać odkształcenie na stałym poziomie.

12

Analiza wpływu przebiegu procesu odkształcenia na nawrót sprężysty asfaltów modyfikowanych polimerami – część I

Jurczak R.

Magazyn Autostrady

|

2018

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Nr 10

56--60

PL

Ze względu na coraz trudniejsze warunki pracy nawierzchni pożądana jest zdolność mieszanek mineralno-asfaltowych do rozpraszania naprężeń (relaksacji), która wiąże się przede wszystkim z obecnością lepiszcza. Zatem użyty w mieszance asfalt powinien wykazywać zdolność do relaksacji w możliwie jak najszerszym zakresie temperatur. Przydatnych informacji o podatności lepiszcza do większej relaksacji naprężeń powstających w mieszance może dostarczyć pomiar nawrotu sprężystego.

EN

Because of more and more difficult working conditions of the surface, the ability of asphalt mixtures to dissipate stresses (relaxation) is desired, which is mainly related to the presence of a binder. Therefore, the asphalt used in the mixture should have the ability to relax in the widest possible temperature range. A test of elastic recovery can provide useful information about the binder’s susceptibility to greater stress relaxation in the mixture.

13

Effect of cyclic hardening on stress relaxation in SUS316HTP under creep-fatigue loading at 700ºC: experiments and simulations nobutada

Ohno N., Sasaki T., Shimada T., Tokuda K., Yoshida K., Okumura D.

Journal of Theoretical and Applied Mechanics

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2018

|

Vol. 56 nr 2

497--510

EN

Cyclic hardening and stress relaxation experiments of SUS316HTP were performed under creep-fatigue loading with tensile strain holding at 700ºC. Experiments revealed that under strain holding, the slow stress-relaxation stage satisfying Norton’s law with slight cyclic hardening followed a rapid stress-relaxation stage that was noticeably affected by cyclic hardening. This suggests that in the slow stress-relaxation stage, inelastic deformation mechanisms different from that of viscoplasticity occurred. Experiments were simulated using a cyclic viscoplastic-creep model in which the inelastic strain-rate was decomposed into viscoplastic and creep components that were affected differently by cyclic hardening. The simulation accurately reproduced the experiments.

14

Nieliniowo lepkosprężyste równanie konstytutywne gąbczastej tkanki kostnej i identyfikacja stałych materiałowych

Pawlikowski M., Jankowski K., Kokot G., Skalski K., Makuch A.

Inżynieria Powierzchni

|

2018

|

nr 1

12--17

PL

W pracy sformułowano nowe równanie konstytutywne dla gąbczastej tkanki kostnej człowieka. Zapostulowano funkcję energii odkształcenia, która opisuje sprężystą odpowiedź tkanki na obciążenie. W modelu konstytutywnym uwzględniono także nieliniową lepkosprężystość. Stałe materiałowe w równaniu konstytutywnym zidentyfikowano na podstawie wyników testów relaksacji naprężeń i monotonicznego ściskania metodą dopasowania krzywych. Wykorzystano tutaj algorytm Levenberga-Marquardta metody najmniejszych kwadratów. Na podstawie testów relaksacji zidentyfikowano wartości czasów relaksacji, natomiast na podstawie testów monotonicznego ściskania określono stałe sprężyste i lepkosprężyste. Testy były wykonane w temperaturze pokojowej na prostopadłościennych próbkach kości gąbczastej uzyskanych z główek kości udowej podczas operacji wszczepienia endoprotezy stawu biodrowego.

EN

In the paper a new constitutive equation for human cancellous bone tissue was formulated. A strain energy function was postulate, which describes the elastic response of the tissue to the load. The nonlinear viscoelasticity was also taken into account in the constitutive model. The material constants in the constitutive equation were identified on the basis of the results of stress relaxation tests and monotonic compression tests using the curve fitting method. The Levenberg-Marquardt algorithm of the least square method was used here. On the basis of the relaxation tests, the values of relaxation times were identified, while the elastic and viscoelastic constants were determined on the basis of monotonic compression tests. The tests were performed at room temperature. On the cuboidal samples of cancellous bone obtained from femoral heads during the surgeries of hip joint prosthesis implantation.

15

Relaxation of process-induced stresses in composite parts – mixed experimental-numerical approach

Galińska A.

Engineering Transactions

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2018

|

Vol. 66, iss. 2

111-128

EN

The main source of the process-induced deformations are stresses which occur in composite parts during cure. Those stresses cause unexpected reduction of the part strength which may bring about catastrophic consequences. The measurements of the process-induced stresses in composite parts are very difficult, so the problem has been neglected so far. Therefore, in the scope of the present work an attempt is made to evaluate the stresses in composite parts made of carbon/epoxy unidirectional prepreg by a numerical simulation. The results of the simulation are verified by the comparison of the calculated and measured deformations of a composite part. Since the composite materials exhibit viscoelastic behaviour, some of the process-induced stresses in composite parts may be relaxed during exploitation of the part, which may lead to the decrease of the undesired process-induced stresses. Therefore, a user defined viscoelastic-orthotropic material model was developed in order to take into account the stress relaxation. Such an approach allowed to evaluate the process-induced stress level in composite parts during exploitation without difficult and time-consuming experiments.

16

Evaluation of relaxation properties of digital materials obtained by means of PolyJet Matrix technology

Bochnia J.

Bulletin of the Polish Academy of Sciences. Technical Sciences

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2018

|

Vol. 66, nr 6

891--897

EN

The paper presents the results of research on the stress relaxation of selected digital materials obtained by means of additive technology. A 5-parameter Maxwell-Wiechert model was used to describe the stress relaxation curves, allowing to obtain a very close fit. Anisotropy of properties was found due to the direction of sample printouts.

17

Własności reologiczne nowych stopów na bazie aluminium dedykowanych do zastosowań na cele elektryczne

Mamala A., Grzebinoga J., Kowal R., Ściężor W., Jabłoński M., Gniełczyk M.

Rudy i Metale Nieżelazne Recykling

|

2017

|

R. 62, nr 7

7--10

PL

Współczesny przemysł elektroenergetyczny oczekuje nowych dedykowanych stopów na osnowie aluminium, posiadających ponadstandardowe właściwości eksploatacyjne, a w tym podwyższoną odporność na działanie długotrwałej siły obciążającej przewód elektroenergetyczny. Długoczasowa eksploatacja przewodów wykazuje znaczny wpływ czynników zewnętrznych na stale postępujące procesy reologiczne. Zachodzące w materiale procesy reologiczne determinowane są przez naprężenie oraz temperaturę, których wzrost znacznie podwyższa szybkości wielkość zachodzących zmian. Reologiczny przyrost długości przewodu może skutkować spadkiem naprężenia naciągu, przyrostem zwisu przewodu i w konsekwencji niebezpieczeństwem elektrycznego przebicia do ziemi. W artykule przedstawiono analizę wpływu dodatku srebra na poziomie -0,1% na własności reologiczne drutów AlAg poddanych trzem różnym zadanym naprężeniom początkowym (UO, 80, 120 MPa), w porównaniu do aluminium jako materiału referencyjnego. Materiały uzyskano na drodze ciągłego odlewania.

EN

Contemporary electricity industry expects new alloy on aluminum matrix with above-standard operating properties, including increased resistance to long-term loading force of a conductor. Long term operation of conductor shows a significant influence of external factors on constantly progres¬sive rheological processes. Rheological processes are determined by the tension and temperature, which growth significantly increases the speed and size of occurring changes in material. Rheological increase of the length of the conductor may result in a decrease of tension stress, increase of sag of conductor and consequently the danger of electrical breakthrough to the ground. The paper shows the analysis of the effect of a silver addition in an amount of 0.1% to aluminum on the rheological properties of the wire subjected to three different initial stress (UO, 80, 120 MPa), the rheological properties was also examined for reference aluminum. Materials were obtained in continuous casting process.

18

Behaviour of Long-Lasting Stress Relaxation of Various Types of Yarns

Laureckienė G., Milašius R.

Autex Research Journal

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2017

|

Vol. 17, no. 4

379--385

EN

The main goal of this researcher is estimating of the possibility of long-lasting (even until 200,000 s) stress relaxation by empirical investigation, which was performed for a few thousands of seconds. The empirical investigations of longlasting stress relaxation of different types of yarns (multifilament polyester, cotton and woollen) at different levels of elongation, i.e. at 3%, 5%, 7% and 10%, were carried out. The method of long-lasting relaxation behaviour prediction by the break-point of relaxation rate as well as the linear dependence of second part of relaxation were used. It was found that the behaviour of relaxation can be described using time logarithmic scale by two straight lines, and the value of stress relaxation in long time period could be estimated by the second line. The break-point of relaxation rate of all kinds of yarns occurs in the area of 100-200 s after relaxations started. The obtained results showed that the place of relaxation break-point depends on the level of elongation but does not depend on the type of yarns.

19

Prediction model of long-term prestress loss interaction for prestressed concrete containment vessels

Shokoohfar A., Rahai A.

Archives of Civil and Mechanical Engineering

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2017

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Vol. 17, no. 1

132--144

EN

Conventional methods disregard the interaction effects between concrete creep and tendon relaxation to evaluate the long-term prestressed losses. The main contribution of this paper is to present a new method to compute this interaction for prestressed concrete containment vessels. A time-dependent numerical study is conducted to compare long-term prestress losses in different prediction models. A prestress loss test program is conducted to validate the numerical analysis method for a concrete containment vessel. To perform a more comprehensive numerical study, various variables such as radius to water height ratios (R/H), tendon material (improved or normal relaxation), and different constant temperature conditions are considered in the analysis models. The obtained analytical results are categorized into three classes: creep and shrinkage losses, stress relaxation losses and simultaneous long-term losses. The analytical results show that the (R/H) ratios have no significant effects on long-term prestress loss. Nevertheless, the other investigated variables have impressive effects on the long-term loss evaluation. The analytical results are used to calculate the interaction coefficients, which demonstrate the interaction between concrete creep losses and tendon relaxation. Eventually, the interaction coefficient data are expressed as a function of stress relaxation loss to creep and shrinkage loss ratio.

20

Analiza kinetyki rekrystalizacji po odkształceniu za pomocą metody relaksacji naprężenia

Pidvysots'kyy V., Kuziak R.

Prace Instytutu Metalurgii Żelaza

|

2016

|

T. 68, nr 2

10--16

PL

Głównym celem niniejszej pracy było poddanie weryfikacji metody relaksacji naprężenia do analizy procesów rekrystalizacji zachodzących po odkształceniu. W artykule przedstawiono wyniki badań relaksacji naprężeń oraz ilościowego pomiaru struktury. W oparciu o uzyskane dane stwierdzono że metoda relaksacji naprężeń pozwala określić kinetykę rekrystalizacji po odkształceniu i wyeliminować wpływ zdrowienia w procesie odbudowy struktury.

EN

The main objective of this work was to verify the stress relaxation method for the analysis of recrystallisation processes occurring after deformation. The article presents the results of stress relaxation and quantitative measurement of the structure. Based on the obtained data, it was found that the stress relaxation method allows to determine the kinetics of recrystallisation after deformation and to eliminate the impact of recovery in the process of structure rebuilding.