Wyniki wyszukiwania - BazTech

1

Zastosowanie techniki dzielonego pręta Hopkinsona do badania wpływu skrępowania bezwładnościowego na dynamiczną odpowiedź mechaniczną betonu

Pająk Małgorzata, Janiszewski Jacek, Baranowski Paweł, Pytlik Mateusz

Inżynieria i Budownictwo

|

2023

|

R. 79, nr 9-10

547--551

PL

W pracy przedstawiono wyniki dynamicznych badań wytrzymałościowych próbek betonowych skrępowanych tuleją ołowianą z wykorzystaniem techniki dzielonego pręta Hopkinsona. Przeanalizowany zakres dużych szybkości odkształcenia wynosił od 80 s-1 do 194 s-1. Wykazano przyrost wytrzymałości betonu wynikający nie tylko z wrażliwości badanego betonu na szybkość odkształcenia, ale także ze skrępowania, które w warunkach dynamicznego obciążenia wywołuje dodatkowe efekty bezwładnościowe.

EN

In the paper, the results from the SHPB tests of concrete specimens confined with the lead covers are presented. The analysed high strain rate was from 80 s-1 to 194 s-1. The increase of the compressive strength was noted due to strain rate sensitivity of concrete but also to confinement which caused additional inertial effects.

2

Effect of strain rate on static elastic response of glass-polyester composite. Part 1, Description of test method

Kozioł Mateusz

Composites Theory and Practice

|

2023

|

R. 23, nr 1

22--29

EN

This paper is the first of two parts of a pioneering study to evaluate the effect of the strain rate of a GFRP laminate on the stress response. The assessment concerns the elastic range of deformation only. The publication contains the assumptions and methodological description of the conducted experiments. The non-destructive bending tests and the methodology for determining the modulus of elasticity and the energy of the load-unload cycle are presented in detail. The full set of test results is presented in the appendix. The results and conclusions are discussed in the second part of the study, which is a separate publication.

3

Effect of strain rate on static elastic response of glass-polyester composite. Part 2, Analysis of experimental results

Kozioł Mateusz, Latusek Szymon

Composites Theory and Practice

|

2023

|

R. 23, nr 1

44--49

EN

This paper is the second part of a study aimed at evaluating the influence of the strain rate of a plain weave GFRP laminate in a non-destructive static three-point bending test on the stress response of the material. It was found that the stress level during the entire course of the deflection rises with the increase in the strain rate. The relative change in the stress level is comparable for the 0/90 and 45/-45 samples. As the loading speed increases, the elastic modulus of the material also grows. For an increment in the strain rate from 1.11·10–3 to 5.57·10–1 1/s, the increase is 10% for the 0/90 samples and 17.7% for the 45/-45 samples. The dependence of the modulus on the strain rate is logarithmic. Based on the theoretical analysis, the cause of the observed effects of the strain rate on the material response was attributed to the viscoelastic behawior of the matrix (cured polymer resin) and the viscoelastic behavior of the system of fibers at the level of the laminate mesostructure.

4

Experimental and numerical investigations on dynamic tensile behavior of a ZrB2-SiC ceramic

He R., Cao J., Yang K., Wang L., Kong D.

Archives of Mechanics

|

2023

|

Vol. 75, nr 1-2

53--73

EN

The dynamic tensile behavior of a ZrB2-SiC ceramic was investigated via split Hopkinson pressure bar tests and a bond-based peridynamic model. The experimental results showed that the peak dynamic splitting tensile stress increased linearly with the strain rate. The dynamic tensile stress history and fracture pattern exhibited a significant strain-rate dependence. In simulation, the strain-rate dependent on the critical energy release rate was introduced to predict the dynamic tensile behavior of the ZrB2-SiC ceramic. The numerical results were in good agreement with the experimental results, verifying the applicability of the peridynamic model. The maximum error of the peak dynamic splitting tensile stress between the experimental and numerical results was no greater than 6%. Moreover, the effect of the strain rate on the fracture patterns of the ZrB2-SiC ceramic composite can be well predicted by the peridynamics method. The ZrB2-SiC composite specimen split into two large fragments with additional small fragments under dynamic splitting tension. With the increase of strain rate, the main crack propagation and branching led to a larger fracture region in the middle of the specimen.

5

Influence of Filler Surface Modification on Static and Dynamic Mechanical Responses of Rice Husk Reinforced Linear Low-Density Polyethylene Composites

Omar Mohd Firdaus, Al Bakri Abdullah Mohd Mustafa, Ting Sam Sung, Jeż Bartłomiej, Nabiałek Marcin, Md Akil Hazizan, Zulkepli Nik Noriman, Abd Rahim Shayfull Zamree, Azmi Azida

Archives of Metallurgy and Materials

|

2022

|

Vol. 67, iss. 2

507--519

EN

Filler surface modification has become an essential approach to improve the compatibility problem between natural fillers and polymer matrices. However, there is limited work that concerns on this particular effect under dynamic loading conditions. Therefore, in this study, both untreated and treated low linear density polyethylene/rice husk composites were tested under static (0.001 s-1, 0.01 s-1 and 0.1 s-1) and dynamic loading rates (650 s-1, 900 s-1 and 1100 s-1) using universal testing machine and split Hopkinson pressure bar equipment, respectively. Rice husk filler was modified using silane coupling agents at four different concentrations (1, 3, 5 and 7% weight percentage of silane) at room temperature. This surface modification was experimentally proven by Fourier transform infrared and Field emission scanning electron microscopy. Results show that strength properties, stiffness properties and yield behaviour of treated composites were higher than untreated composites. Among the treated composites, the 5% silane weight percentage composite shows the optimum mechanical properties. Besides, the rate of sensitivity of both untreated and treated composites also shows great dependency on strain rate sensitivity with increasing strain rate. On the other hand, the thermal activation volume shows contrary trend. For fracture surface analysis, the results show that the treated LLDPE/RH composites experienced less permanent deformation as compared to untreated LLDPE/RH composites. Besides, at dynamic loading, the fracture surface analysis of the treated composites showed good attachment between RH and LLDPE.

6

Processing routes, resulting microstructures, and strain rate dependent deformation behaviour of advanced high strength steels for automotive applications

Nanda Tarun, Singh Vishal, Singh Gurpreet, Singh Manpreet, Kumar B. Ravi

Archives of Civil and Mechanical Engineering

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2021

|

Vol. 21, no. 1

112--135

EN

Automobile industry is continuously striving to obtain light body-in-white structures to meet tightened regulations on flue-gas emissions/crash-testing parameters. ‘Advanced high strength steels (AHSS)’ find increased applications in the automotive industry because of improved crashworthiness/formability at reasonably low costs. AHSS category mainly includes transformation induced plasticity (TRIP) steels, twinning induced plasticity (TWIP) steels, dual phase (DP) steels, complex-phase (CP) steels, and quenching-partitioning (Q&P) steels. AHSSs provide superior strength-ductility combination than conventional high-strength steels by virtue of their multi-phase microstructures. Mechanical properties of AHSSs are greatly influenced by processing routes/derived microstructures. Furthermore, mechanical properties/tensile deformation behavior are also strain rate dependent. During an automobile crash, deformation occurs at strain rates which are exceedingly higher than quasi-static conditions. So, investigation of AHSS properties under both quasi-static as well as high strain rates conditions is important to check applicability for superior crash-resistance. The present work critically reviews details of processing routes, room temperature microstructures, mechanical properties, and finally strain rate dependence of tensile deformation behaviour of AHSSs. Finally, main gaps in existing literature/scope for future research with regards to high strain rate deformation dependent properties of this steel category are presented.

7

Fracture behavior of 7075-T6 aluminum alloy under electromagnetic forming and traditional stamping

Yan Ziqin, Xiao Ang, Cui Xiaohui, Lin Yuhong, Yu Hailiang, Chen Baoguo

Archives of Civil and Mechanical Engineering

|

2021

|

Vol. 21, no. 3

821--835

EN

7075-T6 aluminum alloy is a high strength aluminum alloy that is widely used in automotive and aerospace manufacturing. Nevertheless, its plastic deformation ability is poor at room temperature. Electromagnetic forming (EMF) is a method dedicated to forming materials at high speed using magnetic force, which can significantly improve their plastic deformation ability. However, it is difficult to understand the dynamic processes of high-speed forming. Herein, finite element models of EMF and quasi-static stamping were developed using the ANSYS and ABAQUS software. The Johnson–Cook constitutive model was used to describe the stress–strain behavior of the material, while the Johnson–Cook damage model was used to describe the fracture behavior of the material and the distribution of the fracture strain. The sheet exhibited a small temperature rise before fracture, which has little effect on the material fracture strain by EMF. After fracture, the temperature increased sharply. Scanning electron microscopy images revealed that the material underwent a melting phenomenon at the fracture location. Numerical simulation results demonstrated that the fracture energy of EMF was three times that of traditional stamping, and the high strain rate is the main factor affecting the fracture strain of the material.

8

Dynamic non-linear Mohr-Coulomb strength criterion for hybrid basalt-polypropylene fibre-reinforced concrete under impact loading

Fu Qiang, Xu Wenrui, Huang Daguan, He Jiaqi, Zhang Lu, Kou Hailei, Niu Ditao

Archives of Civil and Mechanical Engineering

|

2021

|

Vol. 21, no. 3

95--113

EN

Variations in the dynamic triaxial strength of hybrid basalt–polypropylene fibre-reinforced concrete (HBPRC) with strain rate and confining pressure were investigated, and a dynamic non-linear Mohr–Coulomb (M–C) strength criterion for HBPRC was established. The results showed that the dynamic strength of HBPRC increased non-linearly with the strain rate and confining pressure; however, the strain rate effect decreased with an increase in the confining pressure. The restraint effect of the basalt fibre and polypropylene fibre on the cracks enhanced the strain rate effect of the dynamic strength of concrete. The cohesion of HBPRC increased with the strain rate and confining pressure but decreased with an increase in the amount of fibre monofilaments. However, the internal friction angle showed a reverse trend. The established dynamic non-linear M–C strength criterion reflected the relationship of the dynamic strength of HBPRC with the confining pressure and strain rate, as well as the effect of fibre content on dynamic strength. The less average standard deviation and the tangential relationship between the strength envelope and the Mohr’s circle of stress demonstrated the applicability of the established dynamic non-linear M–C strength criterion.

9

Correlation between Tensile Deformation Behavior and Microstructural Morphology of Nuclear Grade Austenitic Stainless Steel Welded Joints using Infrared Thermography Technique

Rajasekaran R., Lakshminarayanan A. K., Menaka M.

Welding Technology Review

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2020

|

R. 92, nr 1

7--15

EN

Tensile deformation behavior of nuclear grade Austenitic Stainless Steel (SS) and its welded joints fabricated by Gas Tungsten Arc Welding (GTAW) and Activated Flux Gas Tungsten Arc Welding (AGTAW) processes were studied and correlated with relevant microstructural morphologies using Infrared Thermography (IRT) technique. The microstructure of base metal showed a complete austenite phase. GTAW Fusion Zone (FZ) exhibited both primary ferrite and primary austenite mode of solidification. Meantime, AGTAW FZ exhibited only primary austenite mode of solidification. A strain rate of 4.4x10-4 s-1 was used during the tensile test of the base metal and welded joints. The failure locations of the base metal, GTAW and AGTAW samples were noticed at the center of the gauge portion, the base metal side away from Fusion Line (FL) and Heat Affected Zone (HAZ) respectively. The temperature variations of the base metal and weld zones were recorded in the form of thermograms using the IR camera at the different stages of the tensile deformation. During deformation study, peak temperature of 39.2 °C, 38.8 °C and 34 °C were observed at the base metal, GTAW and AGTAW samples respectively. The lesser peak temperature of the AGTAW sample compared to the base metal and GTAW samples indicated that the AGTAW sample undergone lesser deformation. Moreover, tensile deformation behaviours of the base metal and welded joints were correlated with their microstructural morphologies using corresponding temperature curves.

PL

W pracy zbadano zachowanie deformacji podczas rozciągania austenitycznej stali nierdzewnej i jej połączeń spawanych wykonanych metodą GTAW (Gas Tungsten Arc Welding) oraz AGTAW (Activated Flux Gas Tungsten Arc Welding), a następnie skorelowano je z odpowiednimi morfologiami mikrostrukturalnymi za pomocą termografii w podczerwieni (ang. lnfrared Thermography). Mikrostruktura materiału bazowego wykazała całkowitą fazę austenitu. Spoina GTAW wykazywała zarówno ferryt, jak i austenit, podczas gdy spoina AGTAW wykazywała jedynie austenit. Podczas próby rozciągania materiału bazowego i złączy spawanych zastosowano prędkość odkształcania o wartości 4,4x10-4 s-1. Do zerwania poszczególnych próbek doszło odpowiednio na środku próbki materiału bazowego, w linii wtopienia złącza spawanego GTAW i w strefie wpływu ciepła (SWC) złącza spawanego AGTAW. Zmiany temperatury w materiale rodzimym i poszczególnych obszarach złączy spawanych rejestrowano w formie termogramów za pomocą kamery na podczerwień, przy różnych etapach deformacji podczas rozciągania. Podczas badań odkształceń zaobserwowano maksymalne wartości temperatury: 39,2 °C, 38,8 °C i 34 °C odpowiednio w próbkach z materiału bazowego, spawanych GTAW i spawanych AGTAW. Niższa maksymalna temperatura próbki spawanej metodą AGTAW w porównaniu z pozostałymi próbkami wskazała, że uległa ona mniejszemu odkształceniu. Ponadto zachowania deformacji przy rozciąganiu materiału rodzimego i złączy spawanych zostały skorelowane z obrazami ich mikrostruktur przy użyciu odpowiednich krzywych temperatur.

10

Experimental study of compressive failure of concrete under static and dynamic loads

Liu L., An H. M.

Archives of Civil Engineering

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2020

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Vol. 66, nr 3

427--441

EN

The fracture and fragmentation of concrete under static and dynamic loads are studied. The uniaxial compressive strength test is employed to study the concrete behavior under static loads while the split Hopkinson pressure bar is used to study the dynamic behavior of the concrete under static loads. The theories for acquiring the stress, strain and strain rate of the concrete in the dynamic test by Hopkinson pressure bar has been introduced. The fracture patterns of the concrete in the uniaxial compressive test have been obtained and the static concrete compressive strengths have been calculated. The fracture patterns of the concrete in the uniaxial compressive test have been obtained and the static concrete compressive strengths have been calculated. The fracture and fragmentation of the specimen under dynamic loads have been acquired and the stress-strain curves of concrete under various impact loads are obtained. The stress-strain curve indicates a typical brittle material failure process which includes existing micro-fracture closure stage, linear-elastic stage, nonlinear-elastic stage, and post-failure stages. The influence of the loading rate for the compressive strength of the concrete has compared. Compared with the concrete under static loads, the dynamic loads can produce more fractures and fragments. The concrete strength is influenced by the strain rate and the strength increases almost linearly with the increase of the strain rate.

11

Technological Development of a Yarn Grip System for High-Speed Tensile Testing of High-Performance Fibers

Unger Reimar, Schegner Philipp, Nocke Andreas, Cherif Chokri

Autex Research Journal

|

2019

|

Vol. 19, no. 4

347--354

EN

Particularly in terms of carbon fiber (CF) rovings and further high performance fibers, it is a highly demanding task to clamp technical yarns with low elongations at break during high-speed tensile tests due to their sensitivity to shear stress. For fibers to be tested, a low elongation at break results in short testing times and requires high acceleration. In this paper, four different yarn grips that can be applied with various test machines will be introduced and compared to a wedge screw grip. By using most sensitive CF rovings, advantages and disadvantages of these gripping devices will be qualitatively evaluated by means of testing machines with test speeds of up to 20 m/s and strain rates of up to 200 s-1, respectively. Hence, the reproducibility and precision of test results were considerably enhanced by optimizing the geometry and mass of yarn grips. Moreover, theoretical approaches and calculations for the design of yarn grips suitable for test speeds of up to 100 m/s will be presented.

12

Zjawisko formowania płaszczyzn adiabatycznego ścinania w problematyce penetracji

Magier M.

Problemy Techniki Uzbrojenia

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2018

|

R. 47, z. 147

7--17

PL

W artykule przeanalizowano zjawisko formowania płaszczyzn adiabatycznego ścinania, zachodzące w procesach charakteryzujących się bardzo dużymi deformacjami struktury rozpatrywanego materiału. Na przykładzie analizy porównawczej zjawiska wnikania pocisków z zubożonego uranu i spieku na osnowie wolframowej, określono wpływ płaszczyzn adiabatycznego ścinania na proces penetracji i przedstawiono możliwości w zakresie modyfikacji struktury spieków na osnowie wolframowej, które mogą mieć decydujący wpływ na zwiększenie zdolności penetracji. Niniejsze analizy przeprowadzono na podstawie przeglądu literaturowego.

EN

The phenomenon of creation of adiabatic shear bands occurring at extreme strains of investigated material is analysed in the paper. The effects of penetration for projectiles made of depleted uranium and tungsten sinters were compared to analyse an influence of the adiabatic shear bands into the process of penetration and to show some possibilities for structural modification of tungsten sinters increasing the penetration capabilities. The comparative analyses were conducted on the base of literature review.

13

Densification behaviour of carob powder

Akangbe O. L., Adamovský R., Blahovec J., Rutkowski K.

Infrastruktura i Ekologia Terenów Wiejskich

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2018

|

nr IV/1

1099--1100

EN

The behaviour of carob (Ceratonia siliqua L.) powder relevant to its densification at low pressure was determined given different equipment aspect ratios (0.5, 1.0 and 1.5) and varying rates of deformation (5.5, 10 and 14.5 mm min-1). The effects of both parameters were examined through the analysis of variance and trends were fitted to observed mechanical responses using standard regression technique. Both aspect ratio and the time rate of deformation had highly significant effects on the material’s response to load. Strain rate and specific power requirement were power functions of the equipment’s aspect ratio, at all rates of deformation. These findings furnish insights on mechanical response in compressed food powders which are applicable in modelling related food handling and processing systems.

14

Effect of Strain Rate on the Microstructure of Warm-Deformed Ultrafined Medium-Carbon Steel

Yuan Q., Xu G., Liu S., Liu M., Hu H.

Archives of Metallurgy and Materials

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2018

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Vol. 63, iss. 4

1805--1813

EN

In this study, medium-carbon steel was subjected to warm deformation experiments on a Gleeble 3500 thermosimulator machine at temperatures of 550°C and 650°C and strain rates of 0.001 s-1 to 1 s-1. The warm deformation behavior of martensite and the effects of strain rate on the microstructure of ultrafine grained medium-carbon steel were investigated. The precipitation behavior of Fe3C during deformation was analyzed and the results showed that recrystallization occurred at a low strain rate. The average ultrafine ferrite grains of 500 ± 58 nm were fabricated at 550°C and a strain rate of 0.001 s-1. In addition, the size of Fe3C particles in the ferrite grains did not show any apparent change, while that of the Fe3C particles at the grain boundaries was mainly affected by the deformation temperature. The size of Fe3C particles increased with the increasing deformation temperature, while the strain rate had no significant effect on Fe3C particles. Moreover, the grain size of recrystallized ferrite decreased with an increase in the strain rate. The effects of the strain rate on the grain size of recrystallized ferrite depended on the deformation temperature and the strain rate had a prominent effect on the grain size at 550°C deformation temperature. Finally, the deformation resistance apparently decreased at 550°C and strain rate of 1 s-1 due to the maximum adiabatic heating in the material.

15

Experimental study temperature evolution of pseudoelastic TiNi alloys during shock-induced phase transformation

Shen L., Liu Y., Shan J.

Archives of Mechanics

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2018

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Vol. 70, nr 3

191--205

EN

The temperature evolution and the mechanical characteristics of pseudoelasticity TiNi alloys have been studied experimentally at different strain rates. During SHPB testing, the temperature changes were in situ measured by an infrared system recording infrared radiation emitted from the surface of the specimen. It was found that the temperature evolution and the mechanical behavior has a remarkable strain rate effect. With the strain rate increasing, both phase transition subsequent stress and modulus of loading the phase transition stage were higher, exhibiting significant strain and the strain rate hardening characteristic. They were accompanied by the temperature increasing, which suggest that the stress increments result from the temperature change, independently of the strain rate. Calculation analysis results show that latent heat and the dissipated energy in the form of the hysteresis loops, are mainly the sources of the temperature change.

16

Wpływ przemieszczenia niszczącego na wyniki symulacji numerycznej MES

Löschner P., Niesłony P., Jarosz K.

Zeszyty Naukowe Politechniki Rzeszowskiej. Mechanika

|

2017

|

z. 89 [295], nr 4

487--495

PL

W artykule przedstawiono wpływ parametru przemieszczenia niszczącego Uf na wyniki symulacji numerycznej procesu skrawania w programie Abaqus/CAE. W symulacji wykorzystano model konstytutywny Johnsona-Cooka, opracowany dla stopu tytanu Ti6Al4V. Omówiono warunki brzegowe oraz wpływ zmiękczenia termicznego stopu tytanu przy różnych prędkościach skrawania na wyniki symulacji MES. Wykazano znaczący wpływ przemieszczenia niszczącego Uf na przebieg dekohezji i kształtowanie wióra.

EN

In the article the effect of displacement at failure Uf on the simulation results of machining process conducted in Abaqus/CAE program is presented. A Johnson-Cook constitutive model with parameters adequate for Ti6Al4V titanium alloy was used in the simulation. Boundary conditions and the effects of thermal softening at different cutting speeds on the simulation results were described. The results shows a substantial effect of displacement at failure value on decohesion and chip formation.

17

Wpływ temperatury i szybkości odkształcania na charakterystyki wytrzymałościowe materiałów metalicznych

Kyzioł L.

Zeszyty Naukowe Akademii Morskiej w Gdyni

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2017

|

nr 100

109--119

PL

Przedstawiono różnicę pomiędzy odkształcaniem quasi-statycznym a dynamicznym materiałów metalicznych. Dla dużych szybkości odkształceń naprężenie (odkształcenie) w ciałach przemieszcza się z określonymi prędkościami jako fala. Odkształcanie dynamiczne związane jest z rozchodzeniem się fali, natomiast statyczne można rozpatrywać jako okresowe stany równowagi. Przy niedużych szybkościach odkształceń ciało pozostaje w warunkach izotermicznych, a dla dynamicznych procesów odkształcenia jest to proces adiabatyczny. Wprowadzono pojęcie naprężenia progowego ?^ (ang. mechanical threshold stress), które jest maksymalną wartością naprężenia dla danego rodzaju materiału metalicznego w temperaturze 0 K lub, ekwiwalentnie, przy nieskończenie dużej szybkości odkształcenia. Z obniżeniem temperatury następuje zmniejszenie się ruchliwości dyslokacji. Zjawisko to jest związane ze spadkiem zdolności poruszania się atomów w sieci w obniżonych wartościach temperatury, których ruchy drgające całkowicie ustają w temperaturze 0 K. Pojęcie naprężenia progowego zostało zobrazowane za pomocą przykładu obliczeniowego.

EN

The article presents the difference between the quasi-static and dynamic strain of metallic materials. At the high strain rate, stress (strain) in metallic materials moves with specified velocities as a wave. The dynamic deformation is related to the propagation of the wave while the static deformation can be seen as temporary states of equilibrium. At small speed deformations, the body remains in isothermal conditions, while the dynamic strain processes are the adiabatic processes. The concept of the mechanical threshold stress ?^ has been introduced, which is the maximum stress value for the particular type of metallic material for 0 K or, equivalently, infinitely high speed deformation. With the decrease of the temperature, there is a reduction of the dislocation mobility. This phenomenon is associated with a decrease in mobility of atoms in the network at reduced temperatures, which vibrating movement completely stops at 0 K. The concept of the mechanical threshold stress has been illustrated using the calculation example.

18

Eksperymentalna metodyka oceny wrażliwości stopów na szybkość odkształcania

Moćko W.

Transport Samochodowy

|

2017

|

z. 2

69--86

PL

W pracy omówiono podstawowe metody badawcze, wykorzystujące koncepcję pręta Hopkinsona, wykorzystywane do wyznaczania krzywych ściskania materiałów konstrukcyjnych w warunkach obciążeń udarowych. Ponadto zaprezentowano opracowane i wykonane w Instytucie Transportu Samochodowego stanowisko stosowane do badania charakterystyk mechanicznych materiałów przy dużych szybkościach odkształcania. Opisana aparatura posłużyła do wyznaczenia charakterystyk naprężenia w funkcji odkształcenia wybranych stopów. Na podstawie otrzymanych wykresów można ocenić wrażliwość badanych materiałów na szybkość odkształcania.

EN

Basic experimental methods based on the Hopkinson bar concept were discussed in this paper. The methods are used to determine compressive curves of materials under impact loading conditions. Moreover, the testing stand designed and fabricated in the Motor Transport Institute was introduced. The testing stand is used to analyse mechanical behavior of a materials at a high strain rates. Described equipment was applied to estimate stress-strain curves of selected structural materials. On the basis of the obtained characteristics strain rate sensitivity may be investigated.

19

Preliminary investigations of creep strain of Neogene clay from Warsaw in drained triaxial tests assisted by computed microtomography

Kaczmarek Ł. D., Dobak P. J., Kiełbasiński K.

Studia Geotechnica et Mechanica

|

2017

|

Vol. 39, nr 2

35--49

EN

The study concerns soil creep deformation in multistage triaxial stress tests under drained conditions. High resolution X-ray computed microtomography (XμCT) was involved in structure recognition before and after triaxial tests. Undisturbed Neogene clay samples, which are widespread in central Poland, were used in this study. XμCT was used to identify representative sample series and informed the detection and rejection of unreliable ones. Maximum deviatoric stress for in situ stress confining condition was equal 95.1 kPa. This result helped in the design of further multistage investigations. The study identified the rheological strain course, which can be broken down into three characterizations: decreasing creep strain rate, transitional constant creep velocity, and accelerating creep deformation. The study found that due to multistage creep loading, the samples were strengthened. Furthermore, there is a visibly “brittle” character of failure, which may be the consequence of the microstructure transformation as a function of time as well as collapse of voids. Due to the glacial tectonic history of the analyzed samples, the reactivation of microcracks might also serve as an explanation. The number of the various sizes of shear planes after failure is confirmed by XμCT overexposure.

20

Wyznaczanie krzywych naprężeń rzeczywistych z uwzględnieniem sztywności maszyny wytrzymałościowej i prędkości przesuwu trawersy

Godzimirski J.

Przegląd Mechaniczny

|

2017

|

nr 5

33--37

PL

Podstawowymi właściwościami wytrzymałościowymi materiałów konstrukcyjnych są: granica plastyczności, wytrzymałość doraźna oraz moduł sprężystości wzdłużnej. Współczesne maszyny wytrzymałościowe umożliwiają określenie granicy plastyczności i wytrzymałości doraźnej z dużą dokładnością, lecz określenie modułu sprężystości wzdłużnej wymaga zastosowania ekstensometru, ponieważ maszyna wytrzymałościowa mierzy jedynie przemieszczenia ruchomej trawersy, a nie wydłużenie badanego materiału. W statycznej próbie rozciągania nie uwzględnia się zmiany przekroju próbki, co powoduje, że uzyskiwana krzywa σ = σ (ɛ) jest tzw. krzywą inżynierską, wystarczająco dokładną do obliczeń wytrzymałości konstrukcji, ale nie do obliczeń parametrów przeróbki plastycznej metali. Zaproponowano metodykę uwzględniania sztywności maszyny wytrzymałościowej przy określaniu charakterystyki σ = σ (ɛ) badanych materiałów oraz prosty sposób opracowania krzywej naprężeń rzeczywistych na podstawie krzywej inżynierskiej. Oszacowano, jakiego rzędu błędy można popełnić nie uwzględniając w badaniach wytrzymałościowych sztywności maszyny wytrzymałościowej oraz odkształceń próbki w trakcie badań. Wykazano również istotny wpływ szybkości odkształceń materiałów, które pełzają w temperaturze otoczenia na ich charakterystyki σ = σ (ɛ).

EN

The principal strength properties of construction materials are: yield point, ultimate strength and Young's modulus. The modern testing machines make possible to carry out the yield point and the ultimate strength with high accuracy of measurement. The determination of Young's modulus needs to use an extensometer because the testing machine takes the displacement of testing machine cross-bar but not extension of the tested material. The change of cross-sectional area is not taking into account in static tensile test. It causes that deter-mined σ = σ (ɛ)curve is so called engineering curve that is sufficiently accurate for calculations of construction strength but isn't for calculations parameters of plastic working. The simple method was proposed to take into consideration the stiffness of testing machine in order to the determination of stress-strain curves of tested materials and simple method for determination true stress-strain curves on the basis of engineering curves. It was estimated how big error is made when testing machine stiffness and change of cross-sectional area is not take into account in static tensile test. It was shown essential influence of strain rate of creeping at ambient temperature materials on σ = σ (ɛ) curves.