‘’FAILURE’’
Part 1
IE-114 Materials Science and General Chemistry
Lecture-8
OUTLINE
- Mechanisms of crack propogation for both ductile and
brittle modes of fracture
- Impact fracture testing techniques
- Strengths of Brittle Materials
- Fracture toughness and Plain Strain Fracture Toughness
Failure
The inability of a material to;
(1) Perform the intended function
(2) Meet performance criteria although it may still be operational
(3) Perform safely and reliably even after deterioration
Examples of Failure: Yielding, wear, buckling, corrosion and fracture
 Fracture: The seperation of solid under stress into two or more parts.
Ductile
Brittle
 Mixture of ductile and brittle fracture may also be seen
 Most of the metal alloys are ductile, but ceramics are brittle. Polymers
exhibits the two types of fracture.
(1) Ductile Fracture

 Ductile fracture of a metal occurs after
extensive plastic deformation.
Fracture point
 Characterized by slow crack propogation

(a) For extremely soft metals:
 There is about 100% reduction in area.
(b) Most common tensile fracture profile:
Necking is followed by frature.
 Pure gold or lead at RT or some metals
 Polymers and inorganic glasses at
high T.
 A typical fracture process has two steps:
1) Crack formation
2) Propagation (this is the step determining the mode of the
fracture)
Stages of cup-and-cone DUCTILE fracture
Initial necking
Small cavity
formation
Coalescence of cavities
to form a crack
Crack
propagation
Final shear fracture at a
45o angle relative to
tensile direction
Ductile fracture proceeds slowly as the crack
length is extended. This type of crack is called
stable.
Cup-and-cone fracture of an aluminum alloy
Characteristic Features of Ductile
Fracture Surfaces
 Dimples are characteristics features of ductile failure
 Each dimple is one half of a microvoid formed and then seperated during the
fracture process
Equiaxed dimples formed during
microvoid coalescence
Shear dimples resulting from shear
loading
(2) Brittle Fracture
 Ceramics fracture in a brittle manner with little or no plastic deformation at room
temperature
 Many metals with the HCP crystal structure (i.e. Zn) fractures in a brittle
manner (limited number of slip systems)
 BCC metals such as -iron, Mo, W fracture in brittle manner at low
temperatures and high strain rates

Fracture Point

 Cracks spread rapidly with very little accompanying plastic
deformation. This type of cracks are called unstable.
 The motion of the crack is perpendicular to applied stress.
Characteristic Features of Brittle
Fracture Surfaces
 Brittle crystalline materials have successive and repeated breaking
of atomic bonds along specific crystallographic planes (cleavage
planes) and fracture cracks pass through grains. The process is
called cleavage and type of fracture is called transgranular.
Transgranular:
Intergranular:

Most brittle fractures in polycrystalline metals
are transgranular

When grain boundary contains brittle
film or segregated detrimental
particles

Cracks propagate across the matrix of
grains

Cracks propogates along the grain
boundaries
Brittle Fracture in Metals
 In many cases, brittle fracture in metals occurs due to existance of defects,
low operating temperatures, or high strain rates
 Defects may be formed either in manufacturing stage or develop during service
 Manufacturing ( forging, rolling, extrusion and casting) defects;
- Large inclusions
- Poor microstructure
- Porosity
- Tears
- Cracks
- Voids
- Sharp corners
 Brittle fracture initiates at the defect location (stress risers)
- The fracture of a metal starts at a place where the stress-concentration is highest (which
may be at the top of a crack)
Brittle Fracture Surfaces seen in Steels
 Fracture surfaces of materials that failed in a brittle manner will
have their own characteristics
 Cracks initiated at sharp corner
V-shaped ‘chevron’ marking
 Cracks initiated at surface crack
Radial fan shaped ridges
Very Moderately Brittle
• B is most common mode.
• Ductile fracture is desired
Why?
Soft metals at RT (Au, Pb)
Metals, polymers,
inorganic glasses at high T.
A
B
crack + plastic
Note:
Remnant of
microvoid
formation
and
coalescence.
C
Brittle: crack failure
Plastic
region
Brittle fracture:
no warning.
Cup-cone fracture in Al
Brittle fracture: mild Steel
Toughness and Impact Testing
 The test is used to measure the impact energy (amount of energy a
material can absorb before fracturing) , which is also called notch
toughness.
 Standardized
tests:
 Charpy
 Izod
- Used to measure the impact energy (or notch toughness)
- Standart V-notch specimens are used
Standart test specimen
Impact Testing
IZOD
CHARPY
IZOD
The load is applied as an impact blow
from a weighted pendulum hammer. It is
released from a fixed height (h).
Pendulum strikes and fractures the
specimen at the notch. The pendulum
continues its swing rising to maximum
height h’. The
energy absorption,
computed from the difference between h
and h’ is a measure of the impact energy.
initial height
final height
IMPACT TESTING APPARATUS
Ductile to Brittle Transition Temperature(DBT)
 Low temperatures, high stress values, and fast loading rates may
all cause a ductile material to behave in a brittle manner
 Important in material selection for components that operate in cold
environment
Impact Energy
 Ductile-to-brittle trasnsition temperature is determined by
conducting Charpy or Izod test at various temperatures
FCC metals (e.g., Cu, Ni)
BCC metals (e.g., iron at T < 914°C)
polymers
Brittle
More Ductile
High strength materials (  y > E/150)
Temperature
Ductile-to-brittle
transition temperature
 Ductile to brittle transition temperature of Steel used in Titanic was 32oC, the seawater
temperature at the time of accident was -2oC
 This temperature is often defined as:
-The temperature at which the absorbed energy assumes some value (e.g. 20 J)
or
- The temperature corresponding to some given fracture appearance (e.g. %50
fibrous)
Fracture surfaces of V-notched charpy impact specimens
brittle
(shiny)
ductile
(fibrous)
 Metal alloys with FCC structures (Al and Cu) remain ductile even at extremely low T
 BCC and HCP alloys experience this transition.
 Factors that influence the ductile to brittle transition:
Composition
Heat treatment
Processing
 Decreasing the grain size lowers the transition T and increasing C
content raises the CVN transition of the steels.
Fracture Mechanics
 This research area is about the relationships between material
properties, stress level, the presence of crack producing flaws and
crack propagation mechanisms.
 The measured fracture strengths for brittle materials are lower than
those predicted by theoretical calculations.
Microscopic flaws or cracks already existing within the material.
SURFACE CRACK
INTERNAL CRACK
The flaws are sometimes called
stress raisers due to their ability to
amplify the applied stress in their locale.
 The fracture of a material starts at a place where the stress (or stress
concentration) is maximum
Maximum Stress at the crack tip, m
 If the crack has an elliptical shape and is oriented perpendicular to the
applied stress;
m = 2o(a/t)1/2
σm
σ0
ρt
a
: maximum stress at the crack tip
: magnitude of the nominal applied tensile stress
: radius of curvature of the crack tip
: length of a surface crack or half of the length of an internal crac
Stress concentration factor, Kt
The ratio of maximum stress at the crack tip to nominal applied stress:σm/ σno
Kt = m/ o = 2(a/t)1/2
This factor shows the degree to which an external stress is amplified at the tip of a
crack.
* The effect of stress raiser in brittle fracture is more significant than in ductile fracture of the
materials. In ductile material, the plastic deformation indicates the point when maximum
stress exceeds the yield strength. This causes a more uniform distribution of stress in the
vicinity of the stress raiser. But this does not occur in brittle materials.
The critical stress (σc) required for crack
propagation in a brittle material:
c=(2Es/a)1/2
E: Modulus of elasticity
s : specific surface energy
a : one half of the length of an internal crack
Griffith Theory of Brittle Fracture
Energy balance between release of elastic
strain energy during propogation of crack and
surface energy
Condition for Crack Propogation in brittle material
If the magnitude of a tensile stress at the tip of a flaw exceeds the value of this
critical stress, then a crack forms and then propagates, which results in fracture.
(m> c)
 Above equation applies only to completely brittle materials. But, some metals
which fail in a brittle manner will experience some plastic deformation. So, s in
above equation is replaced by p + s
p : plastic deformation energy associated with crack extension
Fracture Toughness, Kc
 Fracture Toughness (Kc) is a measure of materials resistance to brittle
fracture when a crack is present.
 Fracture of a metarial starts at a place where the stress concentration is highest (e.g. at
the top of a sharp crack)
The critical value of stress concentration is Kc and depends on applied load and width of
the crack
SURFACE CRACK
INTERNAL CRACK
Kc = Yca
σc : critical stress for crack propagation
a : crack length
Y : dimensionless parameter and its value depends
on both crack and specimen sizes and geometries,
and load application.
Y=1 for aplate of infinite width having a throughthickness crack
Y= 1.1 for aplate of semi-infinite width having an
edge crack lentgh of a.
Plain Strain Fracture Toughness, KIc
 For relatively thin specimens, the value of Kc depends on the thickness.
Plain Strain Condition:
KIc = Ya
B  2.5 (KIC/y)2
B: specimen thickness
KIC: plain strain fracture toughness
 If the thickness is much greater than the crack dimensions, Kc becomes
independent of thickness and plain strain conditions exists. This means
that when a load operates on a crack in the manner shown in there is no
strain component perpendicular to the front and back faces.
 KIc is a fundamental material property
and depends on:
 Temperature
 Strain rate
 Microstructure
 The magnitude of KIC decreases with increasing strain rate and
decreasing temperature
 KIC decreases as yield strength is improved by solid solution or
dispersion additions or by strain hardening.
 KIC increases with reduction in grain size.
Room Temperature Yield Strength and Plane Strain
Fracture Toughness Data for Some Materials
 Brittle materials have relatively low fracture tougnhness (Kıc)
 Ductile materials have high Kıc values