346 / 352
342
Crack size and tip region are very small when considered in a sample of macro- up to
the micro- scale (Fig. 13). This allows consider the material as continuous solid and
apply stresses, strains and crack analysis as defined in mathematical and numerical
models, in spite of series of accepted simplifications. It is also possible to assume that
crack tip exists as singular point in continuum solid, producing here described
consequences on loaded material behaviour. It is not easy to imagine crack tip as a point
in Fig. 4 and Fig. 6, but also in Fig. 13 at magnification higher than micro level. For that
the validity of applied formulae at macro scale is limited by the size of a structure, and
simplifications of models in structural integrity analysis must be respected.
5.3. Transition of properties at nanoscale
Presented approaches can be considered as further and very detailed extension of
previously developed crack significance assessment, including practical application, by
involved high level of theoretically based knowledge gathered at different scales, but
there are other influencing factors of high importance for material response and
behaviour under loading. One of them is transition of properties at nanoscale, indicating
that in fact material does not behave as expected, the problem similar to discovered nil-
ductility transition temperature of steel, mentioned in Section 2. ( in fact it does not treat
the situation at the crack tip.
In yet another strangeness of the nanoworld, researchers at the National Institute of
Standards and Technology (NIST) and the University of Maryland-College Park have
discovered that materials such as silica that are brittle in bulk form behave as ductile as
gold at the nanoscale /26/. Their results may affect the design of future nanostructures.
Pradeep Namboodiri, Doo-In Kim and colleagues from NIST first demonstrated the
latest incongruity between the macro and micro worlds with direct experimental evi-
dence for nanoscale ductility. Later on NIST researchers Takumi Hawa and Michael
Zachariah and guest researcher Brian Henz shared the insights they gained into the
phenomenon through their computer simulations of nanoparticle aggregates.
At the macroscale, the point at which a material will fail or break depends on its
ability to maintain its shape when stressed. The atoms of ductile substances are able to
shuffle around and remain cohesive for much longer than their brittle cousins, which
contain faint structural flaws that act as failure points under stress.
At the nanoscale, these structural flaws do not exist, and hence the materials regar-
ding the strength are nearly “ideal.” In addition, these objects are so small that most of
the atoms that comprise them reside on the surface. According to Namboodiri and Kim,
the properties of the surface atoms, which are more mobile because they are not bounded
on all sides, dominate at the nanoscale. This dominance gives an otherwise brittle mate-
rial such as silica its counterintuitive fracture characteristics. For that, they claim that
“the terms ‘brittle’ and ‘ductile’ are macroscopic terminology”, probably not applicable
at the nanoscale”. Accordingly, since crack size is defined at macro and micro level, its
definition for the application at lower levels should be modified, what is also necessary
for stress and strength, and many terms connected with them, like brittle and ductile,
including also toughness /26/.
Using an atomic force microscope (AFM), Kim and Namboodiri were able to look
more closely at interfacial fracture than had been done before at the nanoscale. They
found that the silica will stretch as much as gold or silver and will continue to deform