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Tesi etd-12162010-182202

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Tipo di tesi
Tesi di dottorato di ricerca
A New Energetic Failure Criterion and Constitutive Models of Porous Materials into the Fracture Process Zone, in the Framework of Elastic-Plastic Fracture Mechanics
Settore scientifico disciplinare
Corso di studi
tutor Prof. Aldo Frediani
relatore Prof. Massimiliano Lucchesi
Parole chiave
  • void growth
  • void coalescence
  • fracture mechanics
  • finite deformation
Data inizio appello
Data di rilascio
Riassunto analitico
In the last fifty years many e fforts have been made to extend the issues of
Fracture Mechanics from linear elastic to elastic{plastic behaviour of material.
In this sense, one of the most pursued approach was to adopt the
Griffith-like energy balance for elastic-plastic materials as energy input rate
for crack growth. However, for ductile materials, during crack growth the
energy release rate tends to zero and, consequently, crack propagation is not
possible. This conclusion is known as the paradox of Rice. Hence, a Griffith-like
energy balance stops being (identified as) a generalized crack driving force.
In addition, it is known that the paradox reflects the inadequacy of the
continuum mechanics to describe the ductile fracture process in a small region
close to the crack tip, called Fracture Process Zone; there, material undergoes
nucleation, growth and coalescence of micro-defects, which strongly a ffect the
overall macroscopic fracture process. In order to find a criterion to predict
the fracture toughness and crack propagation, it is reasonable to de fine an
energy input rate based on microstructural behaviour of crack, rather than
using a continuum approach.
The aim of this work is to find a di fferent approach to predict the residual
static strength of cracked structures, which includes the continuum mechanics
problem of microstructural damage phenomena. For this, an in-depth
study of the damage evolution in the fracture process zone is required to
determine the highly non-linear coupling between the FPZ and the plastic
dissipation in the background material. In the process of damage evolution,
work-hardening behaviour of material, stress triaxiality, large strain conditions
and the microstructural aspects, such as void-size and void-shape, play
an important role. The microscale eff ect of plastic flow localization is the
macroscale softening of the material. We start by revisiting the inadequacy
of the Griffith-like energy balance as crack driving force; meanwhile, we suggest
how it is possible to find, from the point of view of continuum mechanics,
a di fferent criterion able to predict the crack growth resistance curve. In the
second part of this work we de fine a constitutive model which take the damage
evolution in the FPZ into account. Hence, the effects of stress triaxiality,
void-size and void-shape on the plastic
ow localization are envisaged.