Ductile Fracture of Metals by Necking – Cup and Cone Failure

extensive plastic deformation and energy absorption (“toughness”) before fracture

Fracture: separation of a body into pieces due to stress, at temperatures below the melting point.

Steps in fracture:

• crack formation
• crack propagation

Major Fracture Mechanisms

Depending on the ability of material to undergo plastic deformation before the fracture two fracture modes can be defined – ductile or brittle

Ductile fracture – most metals (not too cold):

• Extensive plastic deformation ahead of crack
• Crack is “stable”: resists further extension unless applied stress is increased

Brittle fracture – ceramics, ice, cold metals:

• Relatively little plastic deformation
• Crack is “unstable”: propagates rapidly without increase in applied stress

Necking

Necking is local deformation that “begins at a tensile point or ultimate stress point”. After ultimate stress is reached, the cross-sectional area of a small

portion of the material decreases. This is a result of uniaxial tension or stretching. This newly smaller area has very large amounts of strain and is seen as an instability, called the “neck”. The act of necking can be shown on a stress-strain diagram. It is the range on the graph from the ultimate stress point to the point of fracture of the material. Necking takes place after a material passes through the elastic, yielding, and strain hardening region of a material test.

Necking is mostly associated with ductile materials, and is common during experimentation of steel in tension in many materials labs. The necking region can take on a cup or cone-like shape in ductile materials. In brittle materials, there is no necking region. The material will simply fracture with a relatively flat plane at the fracture area. Necking has criterion as determined by Considère in 1885:

1) During tensile deformation, the material has a decrease in cross-sectional area,
2) Strain hardening occurs during tensile deformation, and
3) All materials have flaws in their structure

(a) Necking
(b) Formation of microvoids
(c) Coalescence of microvoids to form a crack
(d) Crack propagation by shear deformation
(e) Fracture