This document discusses mechanisms of fracture in materials under monotonic loading. It describes how materials can fail in either a brittle or ductile manner. Brittle materials like ceramics exhibit little plasticity and fracture along crystallographic planes or grain boundaries. Ductile materials like metals undergo extensive plastic deformation, necking, and leave dimpled fracture surfaces. Ductile failure occurs through void nucleation, growth and coalescence while brittle failure involves a single crack propagating through the material. Fatigue failure results from fluctuating stresses below the material's tensile strength, causing localized structural damage and eventual crack formation over many loading cycles.

1. Fracture Mechanics
Lecture Three

2. Mechanisms of fracture
In order to understand the
various approaches to
modeling fracture, fatigue
and failure, it is helpful to
review briefly the features
and mechanisms of failure in
solids.

3. Failure under monotonic loading
If you test a sample of
any material under
uniaxial tension it will
eventually fail. The
features of the failure
depend on several
factors, including:

4. 1-The materials involved
and their microsctructure
2-The applied stress state
3- Loading rate
4-Temperature
5-Ambient environment
(water vapor; or presence
of corrosive environments)

5. Materials are normally
classified loosely as
either `brittle’ or
`ductile’ depending
on the characteristic
features of the
failure.

6. Brittle Materials
Examples of `brittle’
materials include ceramics
(Oxides, Carbides &
Nitrides) and inter-
metallics, as well as BCC
metals at low temperature
(below about ¼ of the
melting point).

7. Features of a brittle material are
 Very little plastic flow
occurs in the specimen
prior to failure;
 The two sides of the
fracture surface fit together
very well after failure.

8.  The fracture surface appears
faceted – you can make out
individual grains and atomic
planes.
 In many materials, fracture
occurs along certain
crystallographic planes. In
other materials, fracture
occurs along grain boundaries

9. Ductile Materials
Examples of `ductile’
materials include FCC metals
at all temperatures; BCC
metals at high temperatures;
polymers at relatively high
temperature.

10. Features of a `ductile’
fracture are
Extensive plastic flow occurs
in the material prior to
fracture.
There is usually evidence of
considerable necking in the
specimen

11. Fracture surfaces don’t fit
together.
The fracture surface has a
dimpled appearance – you
can see little holes, often
with second phase particles
inside them.

12. Complex Materials
Of course, some materials
have such a complex
microstructure (especially
composites) that it’s hard
to classify them as
entirely brittle or entirely
ductile.

13. How Brittle Fracture occurs
Brittle fracture occurs as a result
of a single crack, propagating
through the specimen. Most
materials contain pre-existing
cracks, in which case fracture is
initiated when a large crack in a
region of high tensile stress
starts to grow.

14. How ductile fracture occurs
Ductile fracture occurs as a
result of the nucleation,
growth and coalescence of
voids in the
material. Failure is
controlled by the rate of
nucleation of the voids and
their rate of growth.

15. High tensile stress
promotes rapid void
nucleation and
growth, but void
growth generally
also requires
significant bulk
plastic strain.

16. Another Suggestion for ductile failure
A ductile material may also fail
as a result of plastic instability –
such as necking, or the
formation of a shear band. This
is analogous to buckling – at a
critical strain, the component no
longer deforms uniformly, and
the deformation localizes to a
small region of the solid.

17. This is normally
accompanied by a loss of
load bearing capacity and
a large increase in plastic
strain rate in the localized
region, which normally
results in failure

18. Schematic Representation for
Ductile & Brittle Fracture

20. What is Fatigue of Materials
fatigue is the progressive and
localised structural damage that
occurs when a material is subjected
to cyclic or fluctuating strains at
nominal stresses that cause
structural failure. The maximum
values are often significantly less
than the ultimate tensile stress, and
may be below the yield stress of the
material.

31. Important Point

32. Where Y depends on the specific
specimen geometry

40. Striations

41. Striation Micrograph

43. Fatigue Striations

47. Fatigue Design
Safe Life Approach

48. Fatigue Design
Fail-Safe Approach