TYPES OF FRACTURE

Ductile

Brittle (Transgranular) – clevage

Intergranular

MICRO-MECHANISM OF DUCTILE FRACTURE

i) Formation of macro-crackStagesa)Nucleation of void (Breaking of interfacial bond or de-cohesion between second phase particles/ inclusions and parent matrix) - Continuum model, Argon et al. Decohesion stress is due to combined effect of von-Mises and hydrostatic stress Independent of particle size - Dislocation model, Goods and Brown Considers particle size The models fail at higher particle size because origination of micro-crack then occurs due to particle cracking and not by debonding as assumed by these models

b) Void growth by plastic strain and hydrostatic stress - Rice and Tracey model The circular void changes to elliptical type

c) Strain localization and necking between voids

d) Coalescence of voids to form macro-crack

ii) Macro-crack growth Stages a) Nucleation of new voids ahead of crack tip b) Their coalescence with parent crack resulting in crack growth

iii) Fracture of the structure

Stages in formation of macro-crack

(a) - Inclusions (b) - Void nucleation (c) - Void growth (d) - Strain localization(e) - Necking between voids (f) - Void coalescence

Stages in cup and cone fracture under uni-axial tension

(a) - Void growth at centre of necked region (b) - Formation of penny crack which leads to deformation bands (c) - Void nucleation along deformation bands due to high strain concentration and plasticity (d) - Cup and cone fracture

cd

Stress tri-axiality Penny shaped flaw or crack

Deformation bands at 45 deg.

Cup

Cone