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CRACK ANALYSIS AT NANO LEVEL
Sedmak Stojan
Faculty of Technology and Metallurgy, University of Belgrade, Serbia
sedmak@divk.org.rs1. INTRODUCTION
Two main contributions of fracture mechanics (FM) in the development of engine-
ering structures are connected with fracture itself (prevention and analysis) and develop-
ment of new materials based on specification with defined FM parameters. In this way
structural security, safety and reliability have been significantly improved.
The introduction of nano structures can be considered as an important extension in the
area of engineering structures, and the benefits of this extension are well recognized. The
application of experience gathered with FM parameters is natural, since the requirements
regarding security, safety and reliability are very important for nano structure /1/.
Some retrospect of FM development and review of its parameters /2, 3/ can help to
understand the dilemmas and limitations in FM application to nanostructures, but the size
effect should be recognized first, Fig. 1 /4/. Although this figure might request extended
description, one also must have in mind the size of non-included huge structure, for
which is FM of high interest, like sea platforms, ships, bridges, TV towers, new designed
buildings, which extend the scheme in Fig. 1 at least for three orders of magnitude.
Depending on the application, crack and its tip can be considered at different levels.
In a structure of macro size deeper knowledge of structure composition, up to the nano
level, gathered by the application of modern testing devices and modelling can help to
obtain more accurate assessment of crack significance, assuring in this way not only the
security but also the economy. Introduced materials and structures of nano size appeal to
assess their integrity and reliability in service, and this is next request to analyse crack
behaviour at nano level. This is the purpose of this lecture.
2. REVIEW OF FRACTURE MECHANICS PARAMETERS DEVELOPMENT
Extensive investigation and experimental testing performed after the incident with
Liberty ship (Fig. 2) /5/ which fractured in 1943 in an inexplicable way for that time, are
the fundamentals of FM developments, although FM is not established as a direct result.
This is one illustrative and well documented case study. Studying this failure the experts
turned from experience aspect to experimental and theoretical aspects. From the
standpoint of here considered topic, the most important finding was that unknown
material properties, impact toughness and nil-ductility transition temperature were iden-
tified as responsible. Gathered new experience was of prime interest for improvement in
design of structure and materials, extended the problem to practical solution, which
allowed prescribing more convenient codes and standards regarding the security. But still
this was not fracture mechanics approach.
It is to recall that the ship was a welded structure and that time it was allowed to
strike the arc on a plate (Fig. 3) /6/. The plasticity of parent metal was sufficient to arrest
crack growth through different microstructures in the heat-affected zone (HAZ).