Download Ductile fracture

Parul Institute of Engineering & Technology
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Name of Unit
Topic
Name of Faculty
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Name of Studnets
: JOSHI ASHISH (130370119115)
Sub: Material Science & Metallurgy (131904)
2131904
Material Science And Metallurgy
Metallic Materials
Fracture of Metals
Devang Mahant
Topic: Fracture of Metal
Introduction
•
•
•
A fracture is the separation of an object or material
into two, or more, pieces under the action of stress.
Fracture is consider as the end result of plastic
deformation processes which results in creation of two
or more new surface.
Steps in fracture:
1. crack formation
2. crack propagation
Sub: Material Science & Metallurgy (131904)
Topic: Fracture of Metal
Introduction (cont.)
• There are five kinds of fracture in
metals based on the nature of process:
I.
Ductile,
II. Brittle,
III. Adiabatic shear,
IV. Creep,
V. Fatigue fracture.
• Three basics factors contribute to a
brittle-cleavage type of fracture:
I.
Triaxial state of stress
II. Low temperature
III. High strain rate or rapid rate of
loading
Sub: Material Science & Metallurgy (131904)
Topic: Fracture of Metal
3
Introduction (cont.)
• There are circumstances
under which certain ductile
materials behave as brittle.
• Two important cases of this
type of failure (i.e. brittle
failure of ductile materials)
are:
• Fatigue failure (which was
studied previously)
• Brittle fracture (which is
going to be treated here).
Sub: Material Science & Metallurgy (131904)
Topic: Fracture of Metal
Introduction (cont.)
• Common examples of catastrophic failures of structures
caused by brittle fracture are: Welded ships & tankers made
of mild steel (during World War II), Rails of railways during
cold winter periods.
Sub: Material Science & Metallurgy (131904)
Topic: Fracture of Metal
Introduction (cont.)
•
Brittle fractures in steel
structures usually occur without
visible or audible warnings at
stresses less than nominal Sy
value.
•
Such fractures usually initiate at
sharp notches and crack-like
defects, and may subsequently
propagate through a complete
structure at faster than speed of
sound.
Sub: Material Science & Metallurgy (131904)
Topic: Fracture of Metal
Ductile fracture
• Ductile fracture - most
metals (not too cold):
• Extensive
plastic
deformation ahead of crack
• Crack is “stable”: resists
further extension unless
applied stress is increased
Sub: Material Science & Metallurgy (131904)
Topic: Fracture of Metal
Ductile fracture (cont.)
Steps:
1. Necking
2. Cavity Formation,
3. Cavity coalescence to
form a crack,
4. Crack propagation,
5. Fracture
Sub: Material Science & Metallurgy (131904)
Topic: Fracture of Metal
Ductile fracture (cont.)
• Neck formation takes place at a point of plastic instability
under tensile load.
• The formation of the neck introduce a triaxial state of stress
in the region. Many fine cavities form in the neck region.
• Under continued straining cavities expand by plastic
deformation and coalesce into acentral crack.
• The crack grows perpendicular to the axial specimen until it
approaches the surface of specimen.
Sub: Material Science & Metallurgy (131904)
Topic: Fracture of Metal
Ductile fracture (cont.)
• Ductile materials - extensive
plastic
deformation
and
energy
absorption
(“toughness”) before fracture
• Brittle materials - little plastic
deformation and low energy
absorption before fracture
Sub: Material Science & Metallurgy (131904)
Topic: Fracture of Metal
Ductile fracture (cont.)
a. Very ductile, soft metals
(e.g. Pb, Au) at room
temperature, other metals,
polymers, glasses at high
temperature.
b. Moderately
ductile
fracture, typical for ductile
metals
c. Brittle
fracture,
cold
metals, ceramics.
(a)
Sub: Material Science & Metallurgy (131904)
Topic: Fracture of Metal
(b)
(c)
Ductile fracture (cont.)
•
•
Ductile fracture: high plastic
deformation & slow crack
propagation
Three steps:
– Specimen forms neck and
cavities within neck
– Cavities form crack and
crack propagates towards
surface, perpendicular to
stress
– Direction of crack changes
to 450 resulting in cup-cone
fracture
Sub: Material Science & Metallurgy (131904)
Topic: Fracture of Metal
Scanning electron micrograph
showing conical equaxial
features produced during the
fracture of a steel sample
Brittle fracture
• No significant plastic deformation before fracture
• Common at high strain rates and low T
• Three stages of brittle fracture:
1. Plastic deformation concentrates dislocations along slip planes
2. Microcracks nucleate due to shear stress where dislocations are
blocked
3. Crack propagates to fracture
Ex.: hcp Zn single crystal under high stress along {0001} plane
• Brittle fracture mostly occur due to defects like:
– porosity
– tears and cracks
– corrosion damage
– embrittlement due to atomic hydrogen
• Most brittle fractures are transgranular
Sub: Material Science & Metallurgy (131904)
Topic: Fracture of Metal
Brittle fracture (cont.)
• Brittle fracture - ceramics, ice, cold
metals:
– Relatively little plastic deformation
– Crack is “unstable”: propagates
rapidly without increase in applied
stress
– No appreciable plastic deformation
– Crack propagation is very fast
– Crack propagates nearly
perpendicular to the direction of
the applied stress
– Crack often propagates by cleavage
- breaking of atomic bonds along
specific crystallographic planes
(cleavage planes).
Sub: Material Science & Metallurgy (131904)
Topic: Fracture of Metal
Brittle fracture (cont.)
• A great deal of attention was directed to the
brittle failure of welded ships and tankers.
• Failures occured during winter months and
when the are in heavy seas and anchored at
dock.
• This fact focussed on that normally ductile
mild steel can become brittle under certain
conditions.
• Therefore, researches aimed to understand
the mechanism of brittle fracture and fracture
in general.
• While the brittle failure of ships concentrated
great attention to brittle failure in mild steel.
• Brittle failures in tanks, pressure vessels,
pipelines, and bridges have been noticed.
Sub: Material Science & Metallurgy (131904)
Topic: Fracture of Metal
Difference: Ductile & Brittle
Ductile failure
1)
2)
3)
4)
5)
6)
Brittle failure
It
involves
large
plastic
deformation.
It is always preceded by the
localized deformation called
“necking”.
Ductile
fracture
normally
occurs in F.C.C metals.
Ductile fracture normally occurs
through the grains.
A complete ductile fracture
presents a rough dirty surface
.it has rough dirty contour.
It occurs by slow tearing of the
metal with expenditure of
considerable energy.
Sub: Material Science & Metallurgy (131904)
1) It is associated with minimum
plastic deformation.
2) It does not involve “necking”.
3) Brittle fracture is normally
observed in B.C.C and H.C.P
metal but not in F.C.C metal.
4) Brittle fracture normally follows
the grain boundaries.
5) A complete brittle fracture
shows sharp facets which reflect
light.
6) It occurs suddenly without any
warning.
Topic: Fracture of Metal
Ductile-to-Brittle Transition (Embrittlement)
1.
Hydrogen Embrittlement: Hydrogen produces severe embrittlement in
many metals. Even very small amount of hydrogen can cause cracking in
steel and titanium. It may be introduced during melting and entrapped
during solidification, or it may be picked up during heat treatment, acid
pickling, electroplating or welding.
2.
Temper Embrittlement: Tempering some steels within 450 - 590 °C
results in temper brittlement, which is manifested by increase in impact
transition temperature. It is due to segregation of certain elements to
grain boundaries, giving local hardening to fracture.
3.
Blue Brittleness: Low-carbon steels exhibit two types of aging which
causes an increase in transition temperature: quench aging & strain
aging. Strain aging is the slow increase in hardness in steels finished by
cold work (mainly cold rolling). Blue brittleness is attributed to strain
aging caused by heating cold worked steel to around 205 °C.
Sub: Material Science & Metallurgy (131904)
Topic: Fracture of Metal
17
Ductile-to-Brittle Transition (Embrittlement)
4. Temperature Embrittlement:
• This phenomena in which ductile
materials transformer to brittle is
called ductile to brittle transition
(DBT). At low T, high stress levels
or fast loading rates gives ductile
to
brittle
transition.
This
temperature is called ductile to
brittle transition temperature
(DBTT)
• E.g., Sinking of Titanic: Titanic was
made up of steel which has low
DBT temperature. On the day of
sinking, sea temperature was –20
C which made the structure highly
brittle and susceptible to more
damage
Sub: Material Science & Metallurgy (131904)
Topic: Fracture of Metal
Fatigue fracture
• Fatigue: the phenomenon leading to
fracture under repeated stresses
having the maximum value less than
the ultimate strength of the material
• Metals often fail at much lower stress
at cyclic loading compared to static
loading
• Crack nucleates at region of stress
concentration and propagates due to
cyclic loading
• Failure occurs when cross sectional
area of the metal too small to
withstand load
Sub: Material Science & Metallurgy (131904)
Fatigue-fractures surface of steel shaft
Topic: Fracture of Metal
Factors Affecting Type of Fracture
1. Temperature:
•
The notched-bar impact test has
the greatest importance in
determining “ductile-to-brittle
transition” of a metal.
•
This transition occurs at a
temperature below which the
material is brittle and fractures
with a low energy absorption &
low ductility, and above which it
is ductile.
Sub: Material Science & Metallurgy (131904)
Topic: Fracture of Metal
Factors Affecting Type of Fracture
(cont.)
•
The transition actually covers a range of temperatures in
which degree of brittleness increases gradually as
temperature falls.
•
It is very difficult to make a universal definition of transition
temperature as two different materials having the same
transition temperature may have different failures.
•
Therefore, there are many definitions of this temperature.
Sub: Material Science & Metallurgy (131904)
Topic: Fracture of Metal
Factors Affecting Type of Fracture
(cont.)
2. Composition: In metals, the impact
testing is mostly applied to steel,
testing of nonferrous materials is
seldom.
• The main factors influencing brittleness
of steels are: Composition, heat
treatment and section size.
• The greater is the hardness, the higher
is the transition temperature.
• Considering the effect of composition
in steels, carbon content plays
important role
• The
optimum
combination
of
properties in quenched and tempered
low alloy steel occurs for 0.3 - 0.4 % C.
Sub: Material Science & Metallurgy (131904)
Figure 8
The effect of other elements on
impact properties can be found in
the textbook.
Topic: Fracture of Metal
Factors Affecting Type of Fracture
(cont.)
3. Grain Size: As the grain size increases, transition
temperature increases and fracture stress
decreases. Thereby, it is possible to improve
ductility and toughness of steel by obtaining
ultrafine grain size.
4. Microstructure: The shape of carbide precipitates
in steel has a great effect on impact toughness.
•
A tempered martensitic structure has the best
combination of strength and fracture toughness.
Tensile properties of such structures of the same
carbon content and the same hardness are alike,
but great variations in their impact toughness with
temperature
Sub: Material Science & Metallurgy (131904)
Topic: Fracture of Metal
Factors Affecting Type of Fracture
(cont.)
5. Orientation: The orientation of
test bar in a formed product
affects both the impact energy
and the value of Fracture
Appearance
Transition
Temperature,FATT, as well as the
tensile ductility. For rolled
products, orientation does not
have a great influence on FATT.
Effect of specimen orientation of
Charpy transition-temperature curves
Sub: Material Science & Metallurgy (131904)
Topic: Fracture of Metal
THANK YOU
Sub: Material Science & Metallurgy (131904)
Topic: Fracture of Metal