Subject: Machine Design -I

Topic: Machine Design consideration

Difficulty: Medium

Fracture: separation of a body into pieces due to stress, at temperatures below the melting point. Depending on the ability of material to undergo plastic deformation before the fracture two fracture modes can be defined

• Ductile Fracture
• Brittle Fracture

Ductile Fracture:

• Ductile fracture is a type of fracture characterized by extensive deformation of plastic or "necking."
• In ductile fracture, the crack grows at a slow pace and is accompanied by a great deal of plastic deformation. This usually occurs prior to the actual fracture.
• The term "ductile rupture" refers to the failure of highly ductile materials. In such cases, materials pull apart instead of cracking.
• In ductile fracture, there is absorption of massive amounts of energy and slow propagation before the fracture occurs.

• stages in ductile fracture are well explained by considering the tensile test. When a ductile material loaded gradually by the tensile load, steps in ductile fracture as follows.

(a)Necking

(b)Small cavity formation

(c) Formation of crack

(d) Cup and cone fracture

(a)Necking:

With elastic strain, the material becomes plastically deform and the neck form is shown in (a).

(b)Small cavity formation:

Within the neck, small cavities or voids are formed. These develops as a result of the stresses causing small particles of impurities or other discontinuities in the material to either fracture from the material matrix. More such nuclei are available to trigger the development of these cavities, less the material will extend before fracture and less ductile the material as shown (b). As the purity of the material increses the ductility increases and vice versa.

(c)Formation of crack:

These cavities then link up form an internal crack which spreads across the material in a direction at right angle to applied tensile stress. It is shown in (c).

(d)Cup and cone fracture:

The crack finally propagates to the material surface by sharing in a direction which is approximatrly 45° to the applied stress to give fracture as shown in (d)

Brittle Fracture:

• Brittle fracture is the failure of a material with minimum of plastic deformation.
• If the broken pieces of a brittle fracture are fitted together, the original shape & dimensions of the specimen are restored.
• Brittle fracture is defined as fracture which occurs at or below the elastic limit of a material.

The brittle fracture increases with

• Increasing strain rate.
• Decreasing temperature.
• Stress concentration conditions produced by a notch.