ENT 487 FRACTURE MECHANISMS IN METALS DR. HAFTIRMAN LECTURE 10 Thu, 18 SEPTEMBER 2008

DUCTILE FRACTURE The commonly observed stages in ductile fracture are as follows: Formation of a free surface at an inclusion or second phase particle cracking. Growth of the void around the particle, by means of plastic strain and hydrostatic stress. Coalescence of the growing void with adjacent voids.

DUCTILE FRACTURE In materials where the second-phase particles and inclusions are well-bonded to the matrix, void nucleation is often the critical step; fracture occurs soon after the void form. When void nucleation occurs with little difficulty, the fracture properties are controlled by the growth and coalescence of voids. The growing void reach a critical size, relative to their spacing, and a local plastic instability develops between voids, resulting in failure.

VOID NUCLEATION Figure shows schematically illustrates three of the most common fracture mechanisms in metals and alloys. Ductile Fracture: usually fails as the result of nucleation, growth, and the coalescence of microscopic void that initiate specific crystallographic planes. Cleavage fracture (transgranular): Figure involves separation along specific crystallographic planes. It can be preceded by large-scale plasticity and ductile crack growth. Intergranular fracture : cleavage is intergranular fracture. Its occurs when the grain boundaries are the preferred fracture path in the material.

THE MICROMECHANISMS OF FRACTURE IN METALS.

Uniaxial tensile deformation of ductile materials

SEM Fractograph which shows ductile fracture in a low carbon steel

B). the spherical inclusion which nucleated a microvoid

Void Growth and Coalescence Figure A and B are scanning electron microscope (SEM) fractographs that show dimpled fracture surface that are typical of microvoid coalescence. Figure B shows an inclusion that nucleated a void.

Void Growth and Coalescence

Void Growth and Coalescence Void nucleation, growth, and coalescence in ductile metals. Inclusions in ductile matrix. Void nucleation Void growth Strain localization between voids Necking between voids Void coalescence (tautan lompang)

Void Growth and Coalescence

Void Growth and Coalescence

Void Growth and Coalescence Once voids form, further plastic strain and hydrostatic stress cause the voids to grow and eventually coalesce. Figure shows schematically illustrates the growth and coalescence of microvoids (lompang). If the initial volume fraction of voids is low (<10%), each void can be assumed to grow independently; upon further growth, neighboring void interact. Plain strain is concentrated along a sheet of voids, and local necking instabilities develop.

Void Growth and Coalescence Formation of the cup and cone fracture surface in uniaxial tension. Void growth in a triaxial stress state. Crack and deformation band formation Nucleation at smaller particles along the deformation bands. Cup and cone fracture.

Void Growth and Coalescence Figure illustrates the formation of the cup and cone fracture surface that commonly observed in uniaxial tensile tests. The neck produces a triaxial stress state in the center of the specimen, which promotes void nucleation and growth in the larger particles. Upon further strain, the voids coalesce, resulting in a penny-shape flaw. The outer ring of the specimen contains relatively few voids, because hydrostatic stress is lower than in the center. The penny-shaped flaw produces deformation bands at 45° from the tensile axis.

Void Growth and Coalescence Figure is a photograph of the cross-section of a fractured tensile specimen; note the high concentration of microvoids in the center on the necked region, compared with the edges of the necked region.

Void Growth and Coalescence A cup and cone fracture surface. a) The central portion of the specimen exhibits a typical dimpled appearance, but the outer region appears to be relatively smooth, particularly at low magnification. b) A few widely spaced voids are evident in the outer region (higher magnification).

Void Growth and Coalescence A representative fractograph at higher magnification of the 45° shear surface. Note the dimple appearance, which is charactiristic of microvoid coalescence. The average void size and spacing, however, are much smaller than in the central region of the specimen.