This document discusses different types of material fractures: brittle, ductile, fatigue, and creep. Brittle fractures occur with little plastic deformation, while ductile fractures involve significant plastic deformation. Fatigue fractures result from repeated cyclic stresses below the material's tensile strength. Creep fractures happen due to excessive deformation over time under steady loads. The mechanisms of each type are also described, such as Griffith's theory of stress concentrations leading to brittle fractures or dislocation movement causing creep fractures.

1. FRACTURE
BY
Dr. K. SENTHILARASAN
ASSISTANT PROFESSOR
DEPARTMENT OF PHYSICS
E.G.S.PILLAY ARTS & SCIENCE COLLEGE
NAGAPATTINAM-611002

2. Fracture
Fracture refers to failure of a solid body under load by
breakage into two or more pieces. Thus the separation or
fragmentation of a metal into two or more parts under the action
of load is called fracture.

3. Four Types Fractures
• Brittle fracture
• Ductile fracture
• Fatigue fracture
• Creep fracture

4. Brittle fracture:
A brittle fracture may be defined as a fracture which takes
place by the rapid propagation of crack with a quite negligible plastic
deformation.
Ductile fracture:
It is defined as the fracture which takes place by a slow
propagation of crack with appreciable plastic deformation.
Fatigue fracture:
Fatigue fracture is defined as the fracture which takes place
under repeatedly applied fatigue stress. It occurs at stress well below
the tensile strength of the materials. The fatigue stresses are found in
the machine parts such as axles, shafts and crank shafts.
Creep fracture:
Creep fracture is defined as the fracture which takes place due
to excessive creeping of materials under steady loading.

5. Brittle fracture Ductile fracture
Brittle fracture is the one which has the
movement of crack with a negligible
plastic deformation adjacent to crack.
Ductile fracture is the one which is accompanied with
large plastic deformation and is a result of intense
localised plastic deformation adjacent to crack.
Rapid rate of crack propagation. Slow rate of crack propagation.
Failure is on account of direct stress. Failure is on account of shear stress developed at 45°
Surface obtained at the fracture is dull and
accompanied with hills and valleys.
Surface obtained at the fracture is shifting and
accompanied with the formation of slip planes.
It is characterised by separation of normal
to tensile stress.
It is characterised by the formation of cap and cone.
It occurs when the material is in elastic
condition.
It occurs when the material is in plastic condition.
The tendency of brittle fracture is
increased by decreasing temperature,
increasing strain rate and work hardening.
The tendency of ductile fracture is increased by
dislocations and other defects in metals.

6. Mechanism of brittle fracture:
Griffth’s theory in glass.
Normally the stress at which a material fracture is far lower than the
value of the ideal breaking stress calculated from the atomic bond strength.
The ideal or theoretical fracture strengths of engineering materials are
very high. But actual fracture strengths observed in these materials are much
lower.
Griffth postulated that in a brittle material there are always presences
of micro cracks which act to concentrate the stress the stress at their tips.
There is a High stress concentration at the tip of the cracks even when
the applied stress is quite low. When the applied stress is below a critical level,
it is impossible for a micro crack of the given size to grow.
The value of the critical stress for the growth of a micro crack
decreases with increasing crack size is reached; the growth of the crack is
likely to be very rapid.
Thus the fracture strength is inversely proportional to the square root
of the crack length. For ductile materials there is always some plastic
deformation before fracture. This involves an additional energy term.

7. Fracture strength
The graphite flakes in cast irons act as
micro cracks and lower the fracture strength.
From the above formula, one can get the size of
the largest flaw or crack which can be allowed in
service.

8. • OA is fairly along straight line
indicates to stress to strain is
constant and Hooke’s law
holds well from O to A.
• A is called proportional limit.
• AB indicates stress to strain
not constant so metal continues
to behave perfectly elastic.
• B is called elastic limit
• C is called lower yield point
• D is called upper yield point.
• E is tendency of the material or
ultimate tensile strength.
• DE upward curve.
• EF downward curve
• F Fracture.

9. Mechanism of ductile fracture
• The tensile stress applied across the specimen
is increased beyond the elastic limit, there is a
uniform reduction in its cross sectional area. The
process continues till the ultimate tensile strength
is reached.
• It observed that during the formation of neck
small micro cracks are formed due to the
combination of dislocations which are formed
during the manufacture of the specimen.
• The continued plastic deformation increases
the size of the cracks into bigger cracks

10. Fatigue
Fatigue:
• The failure of a material under repeated
applied stress is called fatigue. Total number
of cycles required to bring about final fracture
under a given condition of use is called fatigue
life.
• The maximum stress withstand by a material
undergoing repeated cycle of loading before
fracture is called fatigue strength.

11. Mechanism of fatigue growth
Fatigue fracture is not taking place in an instantaneous
manner.
It is followed by three stages: Nucleation of crack
Crack growth
Fracture
The material undergoing repeated cycles of loading and
unloading, tensile strength and compressive stress are
developed in alternating manner.
The cycles load continue, the micro cracks is grown in
its size. These types of micro cracks create stress concentration
at some areas.
Finally the growth of micro cracks leads to fracture it is
called fatigue fracture.

12. Creep fracture
Creep fracture is defined as the fracture which
takes place due to excessive creeping of materials under
steady loading.
• Mechanism of creep fracture:
Creep fracture takes place due to sliding of grain
boundaries at moderate stresses and temperature and
movement of dislocation from one slip to another by
climbing at higher stresses and temperature. The crack
initiates along the grain boundaries which act as points of
high stress concentration or due to growth of voids along
the grain boundaries.

13. Mechanism of creep fracture
Creep fracture takes place due to sliding of
grain boundaries at moderate stresses and
temperature and movement of dislocation from one
slip to another by climbing at higher stresses and
temperature.
The crack initiates along the grain boundaries
which act as points of high stress concentration or
due to growth of voids along the grain boundaries.