Vol.1, No 7, 2000 pp. 825 - 833
UDC 539.421:621.039.542.3
MICROMECHANISM OF DUCTILE FRACTURE
INITIATION - VOID NUCLEATION AND GROWTH
Marko Rakin 1, Zorica Cvijović1, Vencislav
Grabulov2, Miloš Kojić3
1 Faculty of Technology and Metallurgy, University of Belgrade,
Karnegijeva 4, 11000 Belgrade, YU-Serbia
2 VTI, Kataniceva 15, 11000 Belgrade, YU-Serbia
3 Faculty of Mechanical Engineering, University of
Kragujevac, Sestre Janjic 6, 34000 Kragujevac, YU-Serbia
Abstract. Micromechanism of ductile fracture of most metals and
alloys includes void nucleation, growth and coalescence. The voids nucleate
at the second phase particles and non-metallic inclusions. Application
of so-called global criteria of fracture mechanics such as COD and J-integral
in characterization of ductile fracture onset does not provide satisfactory
results for all cases of external loading. The problems arising in solving
the phenomenon of severe plastic strain at crack tips and application of
the results obtained to describe behaviour of various structures of different
geometry are not insignificant. In present paper micromechanical model
based on a particular criterion of flow in a porous solid has been applied.
The model was initially established by Gurson, and later on modified by
Tvergaard and Needleman. Unlike traditional flow criteria (for instance,
with metals widely applied Von Mises criterion), established flow criterion
introduces volume fraction (f) variable. Through application of this model,
by combining experimental and numerical procedures, an effort is made to
predict ductile fracture of metals. In present paper fracture initiation
of smooth specimen has been analyzed; described model was incorporated
into finite element (FE) program, so that one of the results for each Gauss
point may be void volume fraction as well. Probably the most difficult
part of such a characterization of ductile fracture is to present physically
void nucleation as accurately as possible. An approach to void nucleation,
suggested by Chu and Needleman, has been discussed in this paper; the model
is based on hypothesis that void nucleation follows a normal distribution
of void formation predominantly around coarser non-metallic inclusions
in steel. It is particularly problematic to examine secondary voids nucleation
around smaller non-metallic inclusions and second phase particles, and
to realize their effects on further growth of the existing (primary) voids,
and especially on their coalescence resulting in fracture. This has been
accompanied by adequate metallographic analysis of non-metallic inclusions
and their volume fraction, which represents starting results for elastic-plastic
analysis of a porous solid using FE method. The results obtained suggest
that applied micromechanical model can be used for characterization of
initiation of ductile fracture in steel on geometries without precracks,
and that metallurgical analysis is necessary to describe physically the
first phase - void nucleation. Special contribution should represent application
of the results obtained with a simple geometry to the precracked structures,
which should be confirmed in work to follow.
MIKROMEHANIZAM INICIJALIZACIJE DAKTILNIH
FRAKTURA - NUKLEACIJA PRSKOTINA I NJIHOV RAST
U radu su, polazeći od rezultata dobijenih numeričkim putem, koristeći
pretpostavku o elastično-plastičnom telu, na standardnom kružnom uzorku
od nisko legiranog čelika za sudove pod pritiskom, izvedeni sledeći zaključci:
- razlika između GTN modela sa udelom zapreminske poroznosti uključenim
u kriterijum strujanja i tradicionalnog Von Mises-ovog je mala: oba proračuna
daju rezultate bliske eksperimentalnim
- FE proračun koji koristi elemente sa osam čvorova omogućuje da se
približno odredi dijagram zavisnosti tačke koalescencije pora od smanjenja
prečnika
- određivanje zapreminskog kritičnog udela poroznosti fc
- procedura bi se trebalo proveriti kod oblika sa predprskotinama.
