Micromechanism of ductile fracture initiation: Void nucleation and growth

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 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 precrached structures, which should be confirmed in work to follow. .

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. .