Failure Analysis

1. Course: Materials Science
Failure Analysis

2. Motivation
Brittle ceramics and glasses typically break following elastic deformation, without
plastic deformation
With continuous service at relatively high temperatures, any engineering material can
fracture when creep deformation reaches its limit.
Structural materials
with preexisting flaws
The inherent brittleness of ceramics and glasses combined with their common use at
high temperatures make thermal shock a major concern.
Fracture Mechanics
Cyclic loading
conditions
Fatigue
Failure analysis can be defined as the systematic study of the nature of the various
modes of material failure.
Apply the understanding provided by failure analysis to avoid future disasters.

3. Impact Energy
Impact energy, the energy necessary to fracture a piece under an impact load, is a
similar analog of toughness
Hardness was seen to be the analog of strength measured by tensile test!
Charpy Test
The impact energy from the Charpy test correlates with the area under
the total stress–strain curve (i.e., toughness).
We expect alloys with large values
of both Strength and Ductility to
have large-impact fracture energies.
• The fcc alloys generally show ductile fracture modes in Charpy testing, and hcp
alloys are generally brittle.
• Bcc alloys fail in a brittle mode at relatively low temperatures and in a ductile mode
at relatively high temperatures.

4. Impact Energy
Increasing yield strength combined with decreasing dislocation velocities at decreasing
temperatures eventually leads to brittle fracture.
(a) Very ductile, soft metals (e.g. Pb, Au) at room
temperature (b) Moderately ductile fracture, typical
metals (c) Brittle fracture, cold metals and
ceramics.
• For a ductile surface, the fracture originates near the center and spreads outward with
a dimpled texture. Near the surface the fracture continuing at approximately 45°.
• For a brittle fracture, appears a typical cleavage texture, almost a horizontal surface.
Brittle fractures occur with no apparent
deformation before fracture; ductile fractures
occur when visible deformation does occur
before separation
Ceramics and glasses have fracturing behavior
that differ those of metallic materials.
Due to how ceramics are processed, there are
often preexisting defects in the material,
introducing preexisting cracks.

5. Fracture Toughness
Fracture mechanics has come to mean the general analysis of failure of structural materials with preexisting flaws.
Fracture toughness is represented by the symbol KIC, and is the critical value of the stress intensity at
a crack tip necessary to produce catastrophic failure under loading.
𝐾𝐼𝐶 = 𝑌𝜎𝐹 𝑝𝑎 Y is a dimensionless
geometry factor on the
order of 1, σf is the
applied stress at failure,
and a is the length of a
surface crack.
• Highly brittle materials, with little or no ability to deform plastically
in the vicinity of a crack tip, have low KIC values, and are susceptible
to catastrophic failures.
• Highly ductile alloys can undergo substantial plastic deformation on
both a microscopic and a macroscopic scale prior to fracture, and have
moderate to high KIC values.
Major use of fracture mechanics is to characterize those alloys of intermediate ductility
that can undergo catastrophic failure below their yield strength due to the stress-
concentrating effect of structural flaws.

6. Fatigue
Fatigue is the general phenomenon of material failure after several cycles of loading to a stress level below the ultimate
tensile stress
• Repeated stress applications can create localized plastic deformation at the metal surface, eventually manifesting as
sharp discontinuities.
• These intrusions, once formed, continue to grow into cracks, reducing the load-carrying ability of the material and
serving as stress concentrators
Crack growth with number of stress cycles, N, at two
different stress levels. The crack growth rate, increases
with increasing crack length. For a given crack length, the
rate of crack growth is significantly increased with a
higher stress.
• At low stress levels or for small crack sizes, preexisting cracks do not
grow during cyclic loading.
• If the loads are above a certain threshold, microscopic cracks will
begin to initiate at stress concentrations such as holes, persistent slip
bands.
• Nominal maximum stress values that cause such damage may be
much less than the strength of the material.