339 / 352
335
ration about its validity should be necessary. First of all is how to define crack and its tip
at nano level. It is visible in Fig. 1 that CNT is about 1.3 nm wide, the value not clearly
contained in Fig. 13. Eventual defects are, of course, of lower size. It is difficult to find
exact position for defects at nanoscale in Fig. 1.
It is clear that further consideration of structural integrity and reliability of nano mate-
rial and eventual defects in it is necessary. This should include not only the state of stress
and strain around defect boundary, but also many fundamental notions, such as strength
and deformation parameters and properties.
4. NANO MATERALS AND THEIR FRACTURE PROPERTIES
The development of nano materials is impressive, what is confirmed by the number of
published papers devoted to performed research and achieved application, overpassing
2000 till the year 2006 /1/, extended eventually by exponential law. Today a spectrum of
nano materials is available for different applications, exhibiting different properties.
First nano material obtained in controlled manufacturing in year 1991, carbon nano
tube (CNT), attracted an enormous interest for the application in production of elec-
tronics and mechanics components. Single and multi wall CNT are accepted as super
strong and stiff carbon fibres in developing new generation of composite materials.
Monolayer of carbon atoms (graphene) and carbon nano tube are presented in Fig. 15 /1/.
A cylindrical single layer of graphene or single-wall nanotubes (SWNT) represents the
most elementary CNT. In cylindrical configuration the properties of graphene are
conserved, especially those concerning the electrical and mechanical properties. A
SWNT has the diameter of an order of 1-2 nm, capped with ends of, usually six,
pentagonal cells. In this uniform arrangement of atoms the strength of structure is close
to ideal since the full capacity of atomic bonds is seized. It reflects to fracture strength as
well. As an illustrative comparison of modulus of elasticity and fracture properties of
different materials are given in Table 2. The values in Table 2 should be taken only as
informative, since the conditions for their determination can be very different. This
should have in mind when the data given in Table 2 are used and compared /1/. In
graphite, each carbon atom in a basal plane of compact packed hexagonal cell is strongly
bonded (covalent sp2) to the three neighbouring atoms while adjacent basal planes are
bonded by weak van der Waals forces (π bond). For that, the elastic modulus of the
graphite in the basal plane is very high while the strength of the graphite perpendicular
on the basal planes is comparatively poor. In curved graphene sheets of CNT, the bonds
are not purely of sp2 or π type, but an admixture, hence, variation in mechanical
properties may be a result.
Typical
nano materials
are ranged from 1 do 100 nm in size, Fig. 15d. The area to
volume ratio is much higher than at micro and macro level. In addition to quantum
effect, expressed already for several nm, this contributes to unexpected useful properties
of nano materials. Nano crystalline materials are of extremely high strength and
hardness, but they can also be very ductile and tough, resistant to fatigue, creep, erosion
and corrosion, combined with high degree of electrical and thermal conductivity.
To use the benefits of obtained nano structures properties some comparison with
macro scaled structures should be done.
Design, manufacturing and exploitation of structures on macro scale are all based on
stress and strain parameters, comparing materials properties and corresponding values