IMAGE ANALYSIS OF THE FIBRE VOLUME
FRACTION AND THE POROSITY OF CARBON-CARBON COMPOSITE MATERIALS
Ing. Margit Žaloudková
Institute of Rock Structure and
Mechanics, Academy of Sciences of the Czech Republic, V Holešovičkách 41,
182 09 Prague 8
INTRODUCTION
The properties of carbon-carbon composite materials (C-C composites) are
depending on various factors like type and amount of fibre, pore size and
distribution, density and structure of the matrix, the adhesion between
fibres and matrix and so on.
The relative quantity of fibres usually expressed as the volume fraction of
fibres is a very important parameter of C-C composites, affecting
significantly their mechanical properties
C-C composites are candidate material for biomedical application and
overcome many of the common problems associated with metal implants in the
biomedical application. They exhibit a relatively sufficient value of
strength and low modulus of elasticity, which is comparable with that of
human bones. Carbon-carbon composites with pores about 50m
in diameter can be also favourable for tissue and bone ingrowth.
Measurements of the fibre volume fraction of 1D C-C composites and the
porosity of 2D C-C composites using image analysis method LUCIA are
described.
Fig 1. 2D C-C composite cross-section. The image
superposed from 20 image fields using Grab Large Image command of the system
Lucia G version 4.60 and the detailed image from the same source.
MATERIALS
Measurements of the fiber volume fraction were carried out on the series of
unidirectional (1D) C-C composites reinforced with carbon fibres by the wet
winding (prepreg) technology as the matrix precursor the phenolic resin was
used. Samples were cured under the pressure (C-P composite), then three
times reimpregnated and carbonised in N2 atmosphere (C-C carbonised
composite), and then graphitised in Ar atmosphere (C-C graphitised
composite).
The preparation of two-directional (2D) samples for the porosity measurement
was basically the same; instead of carbon fibres the plain-woven cloth from
carbon fibres was used.
For the measurement, polished sections were prepared. Sample was embedded in
epoxy resin and the exposed surface first smoothed, and then polished to a
scratch- and relief-free finish.
SOFTWARE AND HARDWARE
The fibre volume fraction and porosity were determined by the image analysis
method in the system LUCIA G version 4.60, Laboratory Imaging Ltd, Prague,
Czech Republic, using the metallurgical microscope Nikon Optiphot 100S,
maximal magnification 1000x, and the colour CCD camera Hitachi HV-C20
connected with the frame grabber FlashPoint Intigue Pro. The film scanner
Nikon COOLSCANII and the ink jet printer EPSON Stylus COLOR and digital
thermotransfer printers Mitsubishi CP-D1 and CP-800DW supplement the system.
FIBRE VOLUME FRACTION MEASUREMENT
The optical reflectance of fibres and matrix is not that distinct in C-C
composites, recognition of fibres in the embedding matrix in C-C composites,
especially in the graphitised sample, is quite difficult. For the maximal
differentiation of fibres and matrix the initiation macroprogram was
prepared that adjusted the intensity of illumination by gain setting and
white balance on camera and the aperture stop of the microscope. For
grabbing itself the shading correction was used. The quality of micrographs
significantly affects further measurement of fibre volume fraction.
Micrographs are shown at Fig 2a, 2b.
For the measurement of fibre volume fraction at magnification 1000x by the
image analysis method the main macroprogram was prepared.
The function of the main macro is shown on of Fig 3.
Fig 2. 1D C-C composite a) carbonised sample
b) graphitised sample
Fig 3. Main macro function
a) final micrograph grabbed after
initiation converted to grey levels
b) binary image after the first automatic tresholding
c) final binary image for the fibre volume fraction measurement
Fibre volume fraction was measured
on 50 fields at each sample using Area Fraction feature of the Field
Measurements. Area fraction value is defined as the ratio of the segmented
image area and the Measured Area. It has a strong stereological
interpretation: in the case of isotropic uniform random sections it is equal
to the volume fraction.
POROSITY MEASUREMENT
Porosity measurement on grabbed image is much easier due to the distinct
recognition of voids (pores and cracks) in the solid material (fibres and
matrix) of 2D C-C composite.
Contrast images grabbed with the initialisation macroprogram are shown on
Fig 4. For each material different initialisation macro was used according
to the best voids representation.
The colour images were processed
with the main measurement macroprogram. The final image was converted into
the grey scale using ExtractComponent command of Lucia Transform menu with
the further contrast enhancement using the Contrast command from the same
menu. Final grey scale colour image was via Define Treshold command
converted into the segmented binary image suitable for the measurement.
Object measurement was used, and each displayed void of the composite
material was characterised by the Area, MaxFeret, MinFeret, Elongation,
Circularity, and Orientation value.
The wide range of voids size had to be measured and was divided into four
measurements stages using four magnifications. For each magnification the
smallest measurable detail was calculated.
Interval borders and the size of the smallest measurable detail were used as
a Restriction from the LUCIA Measure menu.
Porous system of 2D C-C composite is formed from pores and cracks. The pores
and cracks are shown on Fig 5. Pores and cracks can be differenced using its
shape character. Pores are predominantly elliptical to circular. This factor
was used for Circularity and Elongation restriction from LUCIA Measure menu.
Circularity in LUCIA system is a derived shape measure, calculated from the
area and perimeter. Elongation is determined as a ratio of MaxFeret and
MinFeret features.
It was impossible with the lowest
magnification (objective 5x) we have to see the whole cross-section and
measure big holes and delamination cracks. The new feature of LUCIA G
version 4.60 was used to superpose big picture of cross-section from several
image fields. All cross-section of C-C composite is superposed from up to 38
image fields grabbed with objective 5x. The colour image is converted into
the grey level image and segmented into binary image used for the
measurement.
CONCLUSION
It may be concluded, that the main
problem of image analysis method for the measurement of fibre volume
fraction of 1D C-C composites, especially of graphitised samples, is to
acquire sharp micrograph with the distinct recognition of fibres in the
composite matrix. Measurement of coarse pores and cracks is easier due the
distinct recognition of voids and the substance of composite. Initial
macroprogrammes for grabbing image and main macros for measurements itself
make measurements faster, and enable us to process really large amount of
images, and to measure the statistically significant amount of data.
RELATED PUBLICATIONS
Burešová, M.: Image analysis -
fibre volume fraction and mutual resolution of fibres and matrix in C-C
composites (1998). TEXTSCI´98, Liberec: p. 576 – 578, grant No. 383/1998 Burešová, M., Černý, M.: Relation between fibre volume fraction and
mechanical properties of C-C composites (1998). EUROCARBON´98, Strasbourg,
France: p. 491- 492, grant No. 383/1998 Glogar, P., Černý, M., Krula, M., Burešová, M.: Elastic properties of
carbon-carbon composite cylindrical shells with braided reinforcement
(1999), 3-rd International Conference on New Products and Production
Technologies for a New Textile Industry, Gent, Belgium: p. 122-129, grant
No. 106/99/0096 Burešová, M., Balík, K.: Study of the surface properties of C-C composites
as biomaterials, Biomaterialy w medycynie i weterynarii, Rytro 2000, Poland,
p. 15, grant No.106/99/0491 Glogar, P., Burešová, M., Manocha, L.: Studies on change in void structures
in ceramic impregnated CFRC with heat treatment using Image analysis,
Eurocarbon 2000, Berlin, SRN, p. 209-210, grant No. 106/99/0626 Burešová, M., Balík, K.: Study of the surface properties of C-C composites
as biomaterials. Engineering of Biomaterials, No.12, 2000 Poland, p. 28-30,
grant No.106/99/0491 Balík, K., Burešová, M., Machovič, V., Novotná, M., Pešáková, V., Sochor,
M.: Biocompatibility of C-C composites covered with PyC and pHEMA,
Engineering of Biomaterials, No.17 - 19, p. 9, Poland 2001, grant No.
106/00/1407.
Informace z konference
ENGINEERING OF BIOMATERIALS
POLAND 2001
BIOCOMPATIBILITY OF C-C COMPOSITES
COVERED WITH PYC
AND pHEMA
K. Balík*, M. Burešová*, V.
Machovič**, M. Novotná**,
V. Pešáková***, M. Sochor****
*Institute of Rock Structure and
Mechanics, Academy of Sciences of the Czech Republic, V Holešovičkách 41,
182 09 Prague 8
**Institute of Chemical Technology, Technická 5, 162 09 Prague 6
***Rheumatism Institute, Na Slupi 4, 128 50 Prague 2
****Czech Technical University, Technická 4, 166 07 Prague 6
The application of carbon-carbon
composite materials as a biomaterial is mainly limited by its cost and
brittleness of the matrix. The brittleness leads very often to the formation
of microparticles in the tissue, which may cause inflammations around
implants then. To prevent the releasing of carbon particles, C-C composites
have been covered by different layers. In our work we have studied the
biocompatibility of C-C composite surface covered with pyrolytic carbon and
pHEMA (Poly-Hydroxy-Ethyl- Methyl-Acrylate), synthetic polymeric hydrogel
utilized for biomedical applications.
The specimens were reinforced with the carbon plain wave fabric from Torayca
T800H fibres with the phenolic resin Umaform LE as a matrix precursor. The
specimens were three times impregnated, recarbonized and infiltrated and
covered with pyrolytic carbon. Final carbon-carbon samples were impregnated
and covered with pHEMA solution in the autoclave.
The presence of pHEMA on the surface and in inner pores of composite was
indicated with optical microscope and infrared microspectroscopy. The volume
fraction of pHEMA in inner pores of composite was detected from polished
cross-sections using image analysis method, 57% of all open coarse pores was
penetrated with pHEMA.
Embryonal human lung fibroblasts were cultured on composites. Plastic Petri
dishes for tissue culture were taken as a control surface. The metabolic
activity of cultured cells, and the level of some cytokines were determined.
The cells cultured on C-C carbons coated with pHEMA exhibited several times
higher metabolic activity in comparison with the uncoated C-C composite. The
cytokines were estimated by immunoreaction in medium after the cell
cultivation. The levels of both inflammatory cytokines were higher in
comparison with the control surface.
KINETIKA KARBONIZACE PRYSKYŘICE
EBOLIT FF
František Kolář a Jaroslava Svítilová
Ústav struktury a mechaniky hornin, Akademie věd ČR
V Holešovičkách 41, 182 00 Praha 8, tel.+420 266 009 343; email: kolar@irsm.cas.cz
Skelný uhlík lze připravit
karbonizací fenolických a furfuralových pryskyřic. Při nevhodném vedení
karbonizace vznikají v produktu defekty, které se projeví zhoršením jeho
mechanických vlastností. Tyto defekty souvisí s migrací pyrolyzních plynů
uvolňovaných při karbonizaci. Znalost pyrolyzní kinetiky je proto důležitá
pro návrh optimálního teplotního režimu karbonizace. Bylo zjištěno, že
stupeň karbonizace studované fenolformaldehydové pryskyřice příliš nezávisí
na teplotní historii procesu, ale jen na okamžité teplotě. Závislosti stupně
karbonizace na teplotě pro různé rychlosti ohřevu se prakticky překrývají.
Kinetické rovnice n-tého řádu nepopisují takovýto systém s dostatečnou
přesností. Na základě experimentálních dat TGA byl navržen kinetický model,
který vystihuje chování fenolických pryskyřic při karbonizaci s dostatečnou
přesností pro technologické aplikace (Kolář, Svítilová 2001):
1)
resp.
2)
kde t je čas, T abs.
teplota, R univerzální plynová konstanta, E aktivační energie,
A* předexponenciální faktor a
rychlost ohřevu, obecně nekonstantní funkce.
Jde pochopitelně jen o jisté
přiblížení, model popisuje chování karbonizačních systémů, které jsou po
celou dobu procesu v blízkosti rovnováhy. Platí proto tím přesněji, čím je
rychlost ohřevu nižší. Jelikož skutečné rychlosti ohřevu při přípravě
skelného uhlíku jsou nižší než námi studované, dá se předpokládat, že
uvedený model bude dobře použitelný pro optimalizaci teplotního režimu
karbonizace.
Byly provedeny experimenty, které měly za účel zjistit, jak je model schopen
predikce chování studovaného polymeru při zahřívání obecnými stupňovitými
cykly, kdy je lineární teplotní vzrůst střídán s úseky o konstantní teplotě.
Tyto experimentální závislosti pak byly porovnány s funkcemi
(t)
vypočtenými integrací odvozené kinetické rovnice. Typická experimentální
závislosti (t)
je na obrázku proložena vypočtenou křivkou. Pro srovnání je i zde znázorněn
průběh úbytku hmotnosti vypočtený integrací arheniovské rovnice prvního
řádu.
Závislost stupně konverze na
čase při stupňovitém karbonizačním cyklu. Experimentální body jsou zde
proloženy funkcí (t)
vypočtenou integrací rovnice (1) pro stupňovitý teplotní režim:
1- rovnice 1. řádu, 2 – kinetická rovnice (1), 3 - exp. -
experimentální body
byl instalován v odd. uhlíkových
materiálů ÚSMH AVČR. Umožňuje provádět tahové a ohybové zkoušky vzorků
studovaných materiálů při zatížení do 50 kN. V peci do 1500°C lze
uskutečňovat ohybové a tlakové zkoušky tepelně odolných materiálů na vzduchu
nebo v proudu dusíku při zatížení do 2 kN. Pomocí vysokoteplotních
extenzometrů je možno měřit deformaci ohybově nebo tahově namáhaných vzorků,
např. vláknových kompozitů nebo drátů ze speciálních slitin, umístěných v
peci.
Na fotografii je měřicí rám a
příprava pokusu v tahovém uspořádání (pootevřená pec s válcovým pracovním
prostorem, v popředí vpravo držák vysokoteplotního extenzometru Maytek
zasahujícího při měření do pece). Přístroj byl pořízen v rámci projektu
P2046110 „Analýza chování komplexních systémů“, zařazeného do Programu
podpory rozvoje přístrojového vybavení progresivních vědních oborů AVČR.
Bude využíván při řešení projektů zaměřených na výzkum teplotně stálých
kompozitů a jiných materiálů.