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Поступила 29.12.2011
УДК 621.762; 691.002 (032)
ВЛИЯНИЕ МНОГОСЛОЙНЫХ НАНОТРУБОК НА РАЗРЫВНУЮ ПРОЧНОСТЬ
Акад. НАН Беларуси, докт. техн. наук, проф. ХРУСТАЛЕВ Б. М.11, докт. техн. наук, проф. ЛЕОНОВИЧ С. Н..1), докт. наук, проф. ЭБЕРХАРДШТАЙНЕРЙ.2), доктора техн. наук, профессора ЯКОВЛЕВ Г. И.3), ПЕРВУШИН Г. Н.3)
1 Белорусский национальный технический университет, Минск, Беларусь, 2) Институт механики материалов и конструкций, Венский технический университет, Вена, Австрия, 3) Ижевский технический университет, Ижевск, Россия
INFLUENCE OF MULTILAYER NANOTUBES ON FRACTURE TOUGHNESS
NASB Acad., Dr. Sc. (Engineering), Prof. KHROUSTALEVB.1), Dr. Sc. (Engineering), Prof. LEONOVICHS.1), Dr. Sc., Prof. EBERHARDSTEINER J.2), Dr. Sc. (Engineering), Prof. YAKOVLEV G.3), Dr. Sc. (Engineering), Prof. PERVUSHIN G.3)
1Belarusian National Technical University, Minsk, Belarus,
2Institute for Mechanics of Materials and Structures, Vienna University оf Technology, Vienna, Austria,
3Izhevsk State Technical University, Izhevsk, Russia
1. Introduction
Problems of carbon nanotubes dispersion during modification of cement pastes
When developing cement pastes with improved mechanical characteristics, application of carbon nanodispersed systems [1, 2] are very efficient. The introduction of carbon nanosystems into mineral binding matrix composition is established
[3-5] in order to result in micro-structures with forming new crystalline hydrate formations of higher density and strength.
The main purpose of this work is to establish an opportunity of dense cement concrete structure modification by means of multilayer carbon nano-
■■ Наука итехника, № 4, 2012
tubes Graphistrength by «Arkema» as a nanodis-persed additive and to evaluate their influence on the modified cement matrix structure.
According to the technical literature [6, 7], the modification with carbon nanotubes results in an improvement of 15-20 % of the mechanical characteristics of cement concretes. At the same time the mineral matrices are able to increase their strength up to 2-3 times [8] after introduction of carbon nanosystems into the composition of the binder in the amount of 0,0024 % of the binder mass, as it is shown by several investigations. According to [9, 10] carbon nanotubes are able to change the matrix microstructure «due to increased content of calcium hydrosilicates of high density and decreased nanoporosity».
The main reason for contradictions of results obtained by different investigators is an insufficient extent of carbon nanotubes dispergation because they originally become granules due to su-peractivity during their synthesis. In this context the proper use of suitable equipment in ethanol medium for carbon nanotubes dispergation, as mentioned in [11] is of great importance. Li et al. [12] implemented ultrasonic treatment of multimural carbon nanotubes in the solution of sulfuric and nitric to provide better bond between cement matrix. In [13] polyacrilic acids and ultrasound was used to achieve homogeneous dispersion of multimural carbon nanotubes in water solution. In general, the open literature reports on an (insignificant) increase of the mechanical strength of cement matrix modified with carbon nanotubes.
The main reason of insufficient influence of nanosystems on the structure and properties of modified cement matrix is incomplete dispergation of nanotubes. During synthesis they generate bells or granules up to a size of 400-900 ^m with high surface energy. At the same time nanoparticles hardly disperse into single nanostructures in water dispersive media and they require special technologies for their dispergation. The main goal when working with carbon nanotubes is to disintegrate bundles and large agglomerates arising during synthesis and to provide their stabilization in water suspension and steadiness of these nanotubes suspensions in storage.
To stabilize suspensions with nanostructures different surface-active agents (surfactants) [14] are used. Their molecules are adsorbed on the sol-
id-liquid phase boundary surrounding separate nanotubes and their bundles [15, 16].
There are two ways of the process of synthesized nanoparticles dispergation i. e. obtaining particles with synthesized or ligand particles for material. In the first case there is an opportunity of surface modification of nanoparticles, e. g. substitution of ligands. And in the second case operations are performed with nanomaterial where collective properties of nanoparticles are of special importance. Hydrodynamic cavitation is the most optimal way of carbon nanotubes dispergation. Hyd-rodynamic cavitation has been used for about 50 years in industry. Although ultrasonic cavitation is widely used for intensification of technological processes, it requires higher energy expenditure in ultrasonic transmitters than in hydrodynamic cavi-tation devices. Due to this fact hydrodynamic devices are more prospective to be used for liquid medium cavitational treatment where interacting liquid flows produce cavitation. In general, the energy expenditure of hydrodynamic cavitation is 10-15 times less compared to ultrasonic cavitation.
Multilayer carbon nanotubes Graphistrength by «Arkema» consisting of layers of nanotubes with an outer diameter of 10 to 15 ^m and an average density of 50-150 kg/m3 were dispersed.
After the dispergation of the carbon nanosys-tems in the hydrodynamic device, carbon nanosys-tems with an effective diameter of 168,3 nm and with the a minimum diameter of 73,3 nm were obtained.
Sedimentation processes in suspensions are unavoidable due to the different densities of dispersion medium and discontinuous phase. In course of time, solid phase particles aggregate and precipitate. After 30 days storage the effective diameter of the nanosystems was 403,7 nm due to coagulation. However, sedimentation is a reversible process and the suspension can be dispergated again. The investigation of carbon nanotubes microstructure dispersions conformed polydispersity of carbon nanostruc-tures in surfactant Polyplast SP-1 medium.
Coagulation of nanostructures with formation of large agglomerates takes place after one month storage of the dispersions. At the same time separate nanotubes were observed.
2. Materials and research methods
Samples for mechanical tests were produced in compliance with standard technique. Properties of
Наука итехника, № 4, 2012
the fine cement concrete on the Portland cement «П400-Д0» (CEM II/A-S 32.5R) and on the glass sand with fineness modulus M = 3,08 were investigated.
Carboxymethyl cellulose in combination with superplasticizing admixture Polyplast SP-1 was used as surfactants for carbon nanotubes disperga-tion. Sodium carboxymathyl cellulose is an anionic polymer i. e. the product of cellulose and mono-chloracetic acid interaction. Super plasticizing admixture Polyplast SP-1 is a mixture of sodium salts of polymethilennaphthalenesulfuric acids.
The microstructure and microanalysis of the concrete cement matrix were studied on bitmapped electronic microscopes FEI Quanta 200, XL 30 ESEM-FEG by «PHILIPS» and JSM JC 25S by «JEOL».
The analysis of nanosystem sizes in suspensions was performed on BI-MAS/plus 90 device. Total heat generation and rate of heat generation change were studied in thermos calorimeter.
Carbon nanotubes dispersion results in cement matrix structures with formation of a compact defect-free cover on the solid phase surfaces including cement and aggregate particles. This cover provides better bond with surfaces of these particles. Spatial skeleton cells in the structure of the modified cement matrix are formed due to contact interactions of the structured boundary layers. Spot contacts provide ultimately filled system where collective transition to the bond in short range order causes harsh strengthening due to spatial packing formation.
3. Testing specimens
The testing specimens were made of concrete mix from local raw materials (cement, crushed stone, sand). The size of specimens was 100x100x100 mm.
Two types of specimens where applied to investigate the stress intensity factor on normal separation and cross-section shift (fig. 1). Initiators of the crack were made after cooling to ambient temperatures by means of a saw after full preheating.
д- 50 - 50 .,¡>5
s s 1 1 к)
■ — s s in
100 i 100 i "7c_lffl_j i. 50 j
Type 'a' Type 'b'
Fig. 1. Testing specimens
54
4. The definition method of stress intensity factor on normal separation and cross-section shift
Regularity of crack resistance is investigated by fracture mechanical methods. In this work the so-called nonequilibrium test is accepted as the basic method to experimentally determine crack resistance and toughness of destruction [17-22].
The nonequilibrium test is characterized by loosing the deformation stability of the specimen at the localization moment of deformation up to maximum loading with dynamic activating of crack.
The crack resistance characteristics are applied to:
• compare different concrete mixes, and the technological process of manufacturing and concrete quality control;
• assessments of concretes enabling a reasonable selection for construction;
• structural calculations with taking into account defects and application conditions;
• cause studies of structure destructions.
Type 'a' specimen cubes with two initial cracks were used for determining Kic. The tests were performed under the scheme of central compression on a press by means of two supports made of metal bars (fig. 2a, b). The cube destruction occurs unstably within a plane of a moving crack between the two oppositely cuts.
Fig. 2b. Specimens before and after Kic tests
итехника, № 4, 2012
The value of Kic was determined according to the following equation
Kic —-
P
Bd1/2
18,3| -Г -430(-f + 3445ia
d I I d I I d
5/2
-110761 -
7/2
129671 -
9/2
(1)
where P - the load, destroying the specimen, in MN; B - the thickness of the specimen; d - height or width of the specimen; a - depth of a cut (all dimensions in meter).
The crack resistance on cross-section shift was determined in tests with type 'b' specimens by means of a loaded plate between the two parallel cuttings (fig. 3a, b). The big advantage of this test is that the type 'b' specimen can be gained from the already tested type 'a' specimen (the same cube). Thus, the factor of normal separation Kic and cross-section shift Kiic were defined on one and the same concrete fragment.
Fig. 3a. Scheme of test specimens of type 'b'
Fig. 3b. Specimens before and after Kiic tests 5. Experimental data Kic, Kiic, E, Gic
Table 1
Experimental data of concrete cube tests on fracture toughness. Determination of stress intensity factor at normal separation Kic
Samples Date of fabrication Date of test Mass of sample, g Size of ample, cm Ultimate load P, kN Average value of load P, kN ktc , TCcp MN-m-3'2 Specific energy of quasi-static destruction qc = kic /e , N/m
К-2 2390 10х10хх10 9,87
К-3 2410 10x10x10 9,70 10,64 0,623 11,25
К-4 2406 10x10,1x10 12,34
В-2 2402 10x10x10 8,49
В-3 08.08.2011 06.09.2011 2386 10x10x10 9,18 10,08 0,59 10,09
В-4 2384 10x10x10 12,56
2F-2 2388 10x10x10,1 6,47
2F-3 2396 10x10,1x10 11,74 8,99 0,526 8,02
2F-4 2402 10x10,1x10 8,76
K-1 2422 10x10,1x10,1 11,16
K-3 2410 10x10x10 6,73 10,52 0,616 10,99
K-4 2418 10x10,1x10 13,67
O-1 2414 10x10x10 10,17
O-3 09.08.2011 06.09.2011 2416 10x10x10,1 8,85 9,54 0,558 9,03
O-4 2406 10x10x10 9,61
Ю-1 2398 10x10x10 11,67
Ю-2 2396 10x10,1x10 10,38 11,20 0,656 12,47
Ю-3 2423 10x10x10 11,54
Наука итехника, № 4, 2012
The stress intensity factor on cross-section shift was determined according to the following equation
P
Ending mable 2
kiic =
2aB
JY (l, b),
(2)
where P - the load, destroying the specimen, in MH; Y(l, b) - correction index = 0,97; B - width of the specimen; a - depth of a cut (all dimensions in meter).
The stiffness properties of the investigated concrete were determined using ultrasonic measurements. Knowing the path length, the measured travel time (t) can be used to calculate the complex modulus of elasticity (E) as follows
С =
1
E(l -V)
p(l + v)(l - 2v)'
(3)
where ci - ultrasonic pulse velocity; v - Poisson's ratio; E - complex modulus of elasticity; p - density of concrete specimens.
Fig. 4. Ultrasonic pulse velocity method
Consideration of the elastic deformation of concrete enables to define the specific energy of quasi-static destruction (Gi) through the quasi-static stress intensity factor
K =4&Дъ.
(4)
Table 2
Experimental data of concrete cube tests
on fracture toughness. Determination of stress intensity factor at cross-section shift Kiic
Samples Date of fabrication Date of tests Ultimate load Р, kN Average value of load Р, kN KIICCp j MN-m-3'2
1 2 3 4 5 6
К-2(1) 73,92
К-2(2) 72,11
К-3(1) 08.08.2011 07.09.2011 55,23 75,48 4,7
К-4(1) 77,81
К-4(2) 78,06
1 2 3 4 5 6
В-2(1) 85,32
В-2(2) 96,32
В-3(1) 100,90 92,45 5,75
В-3(2) 83,55
В-4(1) 87,25
2F-2(1) 08.08.2011 07.09.2011 76,12
2F-2(2) 86,51
2F-3(1) 83,99 83,93 5,22
2F-3(2) 83,44
2F-4(1) 89,61
2F-4(2) 64,65
K-1(1) 87,40
K-1(2) 87,44
K-3(1) 95,36 88,83 5,532
K-3(2) 75,94
K-4(1) 93,60
K-4(2) 08.08.2011 07.09.2011 80,37
O-1(1) 85,21
O-1(2) 97,60
O-3(1) 93,36 93,94 5,851
O-3(2) 82,83
0-4(1) 98,58
O-4(2) 94,95
Ю-1(1) 102,2
Ю-2(1) 100,7
Ю-2(2) 08.08.2011 07.09.2011 92,58 98,55 6,1377
Ю-3(1) 98,72
Ю-3(2) 88,73
Table 3
Experimental data of concrete cube tests on fracture toughness. Determination of Kic and Kiic
Samples Ultimate load (scheme 1), kN Kic , ICcp MN-m-3'2 Ultimate load (scheme 2), kN KiiCcp, MN-m-3'2
К-1 9,13 0,534 29,0 45,38 2,826
К-2 6,89 0,403 - -
К-3 5,93 0,347 44,51 41,70 2,772 2,597
К-4 7,53 0,441 43,40 49,36 2,702 3,075
О-2 7,55 0,442 57,39 53,89 3,574 3,356
О-3 5,70 0,334 48,45 48,08 3,017 2,994
О-4 7,44 0,435 57,59 59,36 3,586 3,697
О-2* 0 - 43,20 40,35 2,69 2,513
6. Conclusions
The presented results allow to conclude, that the method of nonequilibrium tests of cubes with initiated cuts delivers a quite adequate picture of
■■ Наука итехника, № 4, 2012
the crack resistance and toughness of destruction of usual and high-strength concrete.
From non-destructive stiffness tests using ultrasonic pulse velocity measurements, the value of the complex modulus of elasticity was determined.
The experimental results of the stress intensity factors Kic and Kiic allow to analyze cracking and concrete destruction, and to develop new high strength materials and to perfect designing.
Based on own experimental results the energy of quasi-static destruction Gi were measured in addition.
Cement concretes modified with multilayer carbon nanotubes change its morphology of crystalline hydrates with formation of the contact zones of increased density of the aggregate surface according to microstructural analysis of new formations. Such structures increase the strength resistance of the concrete, which is proved by results of physical and mechanical tests of the concrete modified with carbon nanotubes.
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Поступила 26.04.2012
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