eng metallurgy 7 -5
TRANSCRIPT
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UNIT 5
Plastic Deformation
Slip and Twinning
Hot, Cold and Warm working
Recovery and Recrystallization concepts
Introduction to Fracture Mechanics
Ductile to Brittle transition
Creep and Fatigue failures Testing.
Text Books :1. Raghavan V, Physical Metallugy Principles and Practice, Prentice Hall India Pvt. Ltd., New Delhi,
2006
2. S.H.Avner, Introduction to Physical Metallurgy, Tata-McGraw Hill Publishing Co., New Delhi, 2000.
3. G.E.Dieter, Mechanical Metallurgy, McGraw Hill Publishing Co., New York, 1988.
Reference Books :
1. Donald R. Askeland, The Science and Engineering of Materials, Chapman and Hall,1990.2. Raghavan V, Materials Science and Engineering, Prentice Hall India Pvt. Ltd.,New Delhi, 20073. Budinski and Budinski, Engineering Materials Properties and Selection, Prentice Hall India
Pvt.Ltd.,2005
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DEFORMATION:
When force is applied on a metal piece, then the size and/or shape will be altered. Any
change in the size and/or shape of the metal is called as deformation of metal. Deformations may
be temporary or permanent depending on the existence of change. If change is there even after
removal of load then it is permanent; if change disappears after loading, then it is temporary
deformation.
.
Based on the nature of strain produced during deformation is classified into
(1) ELASTIC DEFORMATION
(2) PLASTIC DEFORMATION
1. ELASTIC DEFORMATION:
It is the deformation of a body which completely disappears as soon as the external load is
removed from the body. It is fully recoverable and time independent.
2. PLASTIC DEFORMATION:
It is the deformation of the body which remains even after removing the external load from
the body. The plastic deformation may occur under the tensile, compressive or torsional stresses.
Ex: forging, milling, turning.
Plastic deformation may occur by slip, twinning or a combination of both.Plastic deformation of a
body remains after removal of external load from the body. In crystalline materials, at
temperatures lower than 0.4Tm, the permanent deformation is called plastic deformation, where
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Tm is the melting point in degrees absolute. This may occur under tensile, compressive and
torsional stresses. Plastic deformation is a function of stress, temperature and rate of straining. It
occurs by the process of slip and twinning. In elastoplastic material, elastic deformation is
followed by plastic deformation before failure occurs. This property makes the metal suitable for
various forming processes. The operations such as turning, milling, sawing involve localized
plastic deformation in the region of tool activity and metal removal.
SLIP AND TWINNING:
SLIP:
Relative displacement of one part of the crystal with respect to the rest due to elongation
caused due to tensile stresses that crosses elastic limit is known as slip.Further increase in load
will cause movement of another parallel place. Each successive elongation requires higher stress
and results in the appearance of another step, which is actually the intersection of a slip plane with
the surface of the crystal. Progressive increase of the load leads to fracture of metal.
.
It is a flow that depends upon perfectly repetitive structure of the crystal which allows the
atoms in one face of a slip plane to shear away from their original neighbors in the other face, toslide in an organized way along with face, carrying their own half of the crystal with them, and
finally to join up again with new set of neighbors as nearly perfect as before.
The plane about which this sliding occurs is known as slip plane.
Resolution of axial tensile load F gives sin & cos components.
(Along slip plane)
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(Perpendicular to slip plane)
Shear stress = =
Normal stress =
Or is max when = 45
Differently oriented crystals of a given metal will begin to slip when different axial stresses
are applied but that the critical resolved shear stress, that is, the stress required to initiate slip, is
always same.
.
If the slip planes are either parallel or perpendicular to the direction of applied stress, slip
cannot occur, and either the material deforms by twinning or it fractures. As deformation proceeds
and the tensile load remains axial, both the plane of slip and the direction of slip tend to rotate into
axis of tension.
MECHANISM OF SLIP:
Portions of the crystal on either side of a specific slip plane move in opposite directions
and come to rest with the atoms in nearly equilibrium positions, so that there is very little changein the lattice orientation. By application of the shear force, an extra plane of atoms (called
dislocation) has been formed above the slip plane. This dislocation moves across the slip plane and
leaves a step when it comes out at the surface of the crystal. Each time the dislocation moves
across the slip plane, the crystal moves one atom spacing. Since the atoms do not end up in
exactly normal positions after the passage of the dislocation, subsequent movement of the
dislocation across the same slip plane encounters greater resistance.
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Eventually, this resistance or distortion of the slip plane becomes great enough to lock the
dislocation in the crystal structure, and the movement stops. Further deformation will require
movement on another slip plane.Although the distortion is greatest on the active slip plane, its
effects is felt thought the lattice structure, and the applied load must be increased to cause
movement on another slip plane.
TWINNING:
Twinning is a movement of planes of atoms in the lattice parallel to a specific (twinning)
plane so that the lattice is divided into two symmetrical parts which are differently oriented.In
twinning, each atom moves by only a fraction of an inter atomic distance relative to its neighbors.
The orientation of the twinned region is different from the untwined region. Generally, twinning is
operative at lower temperatures and higher strain rates. Twinning happens in a region that involves
the movement of a large number of atoms, and usually it appears microscopically as a broad line
or band.
.
KINDS OF TWINS:
1) Mechanical twins: Happens by deformation (CPH, BCC metals)
2).Annealing twins: Happens by reheating. Change in normal growth mechanism.
SLIP Vs TWINNING:
Sl.
DESCRIPTION SLIP TWINNING
1. Amount of movement Moves whole number of inter atomic spacings
Moves fractional amountof inter atomic spacings
2. Microscopic appearance Broad lines or bands
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Thin line
3.
Lattice orientation
Very little change in
orientation, evidence is
temporary
Different lattice
orientation, evidence is
permanent
DIFFERENCE BETWEEN SLIP AND TWINNING:
SL.NO. SLIP TWINNING
1. Slip occurs along individual slip planesTwinning occurs over severalcrystallographic planes
2. Slip lines or bands disappear after grinding or
other surface finishing operations
Two lines run though the whole
depth of the material and hence
do not disappear on grinding etc.,
3.
Slip lines may be present in even or odd
numbers
Twin lines always occur is pairs.
4. Slip lines are open ended
Twin lines are close ended,
making boat shaped formation
5. Slip lines do not appear during any heat
treatment
Twin lines appear during the
annealing operation of some
materials
6. Atomic displacement in case of slip is a
multiple of atomic distance
In case of twinning atoms maymove a distance which is more or
less then the inter atomic spacing.
7. There is no change in the orientation of the
atoms after slip has occurred
Twinned atoms undergo a change
in their orientation and becomemirror of the untwined atom.
HOT COLD AND WARM WORKING:
HOT WORKING:
Hot working or metals takes place above the re-crystallization of work hardening range.
But in hot working process, the metals are given desired shape by subjecting them to forces
which cause them to undergo plastic deformation at the temperature above the re-
crystallization ranges. The re-crystallization temperature of steel is 800c. Although most hot
working on steel is done at temperatures above this temperature. Some metals such as lead, tin
and zinc have a low crystallization range and can be hot worked at room temperature. Most
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commercial metals require some heating. During all hot working operations, the metal is in a
plastic state and is readily formed by pressure. In hot working processes, due to the high
temperature of metals there is a rapid oxidation or sealing of the surface. Hot worked
components have poor surface finish and close tolerances on dimensions cannot be
maintained. The cost of tooling and handling is high but it is a rapid process.
ADVANTAGES:
1. Porosity in metal can be largely eliminated
2. Coarse grains are refined to obtain fine grain structure
3. Impurities in the form of inclusion are broken up and distributed though the metal
4. Physical properties are generally improved
5. Ductility and resistance to impact are improved
6. Strength increases and greater homogeneity is developed in the metal
7. Directional property resulting from the fiber structure is obtained
8. It is quick and economical process
DISADVANTAGES:
1. Tooling and handling costs are high
2. Poor surface finish due to rapid oxidation at high temperature
3. Close tolerances on dimensions cannot be maintained
4. The life of the tools used in this process is less because of high temperature
VARIOUS PROCESSES:
1. Forging
2. Welding
3. Rolling
4. Piercing
5. Spinning
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6. Extruding
7. Drawing/cupping
COLD WORKING:
If the mechanical work is done on the metal below its re-crystallization temperature then it
is known as cold working. There will not be any grain growth; but there is grain disintegrationand elongation. Cold working process is limited to some metals only.
The metals are crystalline in nature and are made up or irregularly shaped grains of
different sizes. Each grain is made up of atoms in an orderly arrangement. The orientation of
the atoms in a particular grain is uniform but differs from adjacent grains. In cold working of
metals, the grain structure changes resulting in the grain fragmentation, movement of atoms
and lattice distortion. Slip planes develop this lattice structure at points where atomic bonds of
attraction are the weakest, thus the whole block of atoms is displaced. But the orientation of
the atoms is not changed when slip occurs. Twinning occurs where the atoms are reoriented. In
twinning, the lattice on one side of the plane is oriented in a different fashion from the other.
Cold working requires greater pressure than hot working. As the metal is in a move rigid
state, it is not permanent by deformed until the stress exceeds the elastic limit. In cold working
range, there is no re-crystallization of grains and hence there is no recovery from grain
distortion or fragmentation. Tensile strength and hardness increase with corresponding
decrease in ductility by the phenomenon known as work hardening or strain hardening. This is
due to resistance built up in the grains by atomic dislocation of fragmentation or lattice
distortion. During cold working, the metal is deformed which gives rise to severe stresses,
known as residual stresses inside the metal. These stresses are undesirable and can be removed
by suitable heat treatment process. This heating of metal will not have any appreciable changein physical properties or grain structure of the metal. Further heating in the crystallization
range will eliminate the effect of cold working and restore the metal to its original condition.At
room temperature most cold working processes can be performed. The cold working merely
distorts the grains and does little towards reducing its size.
In cold working, use of soaking pits and furnaces as well as the handling of heated material
are avoided and hence it results in to faster production. The deformation of metals is brought
about by the method of slip of planes in the process of cold working. Also the force required
for hot working of metals, because in cold working the metals are not deformed permanently
till the elastic limit is exceeded. Cold working produces an improved surface finish and closertolerance on dimensions. For metals that do not respond to heat treatment, cold working is
possible method used to increase hardness. It is also useful in the forming of many articles by
extension of the ductile materials. As shaping process, it is limited to ductile materials.
COLD WORKING PROCESSES:
1. Drawing
2. Squeezing
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3. Bending
4. Shearing
5. Extruding
6. Hobbing
EFFECT OF COLD WORKING ON METALS:
1. Ductility, tensile strength, hardness increases
2. Residual stresses are formed
3. A distorted grain structure is formed
4. Surface finish is improved
5. Re-crystallization temperature for steel is raised
DIFFERENCE BETWEEN HOT WORKING AND COLD WORKING:
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WARM
WORKING:
Warm working is plastic deformation of a metal at temperatures below the temperature
range for re-crystallization and above room temperature. It attempts to combine the advantage
of both hot and cold working into one operation. Warm working has been applied most
extensively to steel forging process, where it offers the potential of fewer forging steps,
reduced forging loads and energy savings compared with cold forging.
ADVANTAGES:
1. Consumes less mechanical energy
2. Improved dimensional control
3. High surface finish
LIMITATIONS:
1. Special die required
2. Lubricant required
3. Cant be used for all metals
Sl.No.
Point Hot working Cold working
1. Working temperatureAlways above re-crystallization
temperature
Always below
2. Residual stresses Not produced Always produced
3. Hardness Not affected Increases
4. Strain hardening Not produced Produced
5. Energy required Less More
6. Directional properties
Produced in the direction
of flow
Not produced
7. Oxidation
Produced hence not
suitable for reactivematerials
Prevented and
suitable forreactive metals
8. Surface finish Not so good Better
9. Applicability
To all metals that
becomes plastic onheating
To all ductile
materials only
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RECOVERY AND RECRYSTALLISATION:
RECOVERY:
Recovery has been defined as the process of releasing internal stresses in metal at relatively low
temperature which are below its re-crystallization temperature. Recovery hardly affects the
structure and mechanical properties of the metal. It reduces internal stresses and increases tensilestrength. Electrical conductivity is also slightly increases. Recovery is an important method for
relieving internal stresses in castings; forgings welded and fabricated equipments, cartridge cases,
and boiler tubes without lowering the strength acquired during cold working. The recovery is a
process of stress relieving treatment.
RECOVERY MECHANISM:
1. AT LOW TEMPERATURE: (vacancy motion)
(i) Migration of point defects to grain boundaries and dislocations
(ii) Combination of point defects
2. AT INTERMEDIATE TEMPERATURE: (dislocation movement)
(i) Arrangement of dislocation with in tangle
(ii) Annihilation of dislocations
(iii) Grain growth
3. AT HIGH TEMPERATURE: (dislocation climb)
(i) Disappearance of boundary between sub grains
(ii) Polygonisation
(iii) Dislocation climb
DISAPPEARANCE OF BOUNDARY:
In cold worked metals, for doing recovery process, the heating is at low temperature. By
this, the vacancies, interstitials and dislocations undergo rearrangement in the lattice vacancies and
interstitials are eliminated first and then some dislocations of opposite sign are annihilated.However, majority of dislocations are not removed by usual recovery treatments. The minor
structural changes during recovery have pronounced effect on residual stresses and on electrical
properties.
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RECRYSTALLISATION:
As the upper temperature of the recovery range is reached, minute new crystals appear in
the microstructure. These new crystals have the same composition and lattice structure as the
original undeformed grains and are not elongated but are equiaxed. The new crystals generally
appear at the most drastically deformed portions of the grain, usually the grain boundaries and slip
planes. The duster of the atoms from which the new grains are formed is called a nucleus. Re-crystallization takes place by a combination of nucleation of strain free grain and the growth of
these nuclei to absorb the entire cold worked material.
Formation of new grains and thus the metal is said to re-crystallize. This is usually
followed by grain growth during which growth of the larger re-crystallized grains occurs at the
expense of smaller ones, obeying the tendency of polycrystalline material to reduce its total
interfacial surface energy. The re-crystallization process causes a rapid change in the mechanical
properties and microstructure of the metal. The strength and hardness are greatly reduced, ductility
increases and complete stress relief takes place distorted, elongated grains disappear and new
grains are formed. Re-crystallization temperature depends on the degree of deformation of coldworked metal. The greater the degree of work hardening, the lower will be the temperature.
COMPARISON BETWEEN RECOVERY AND RECRYSTALLISATION:
FACTORS THAT CONTROL RE-CRYSTALLIZATION TEMPERATURE:
1. Temperature of heating
2. Heating time
3. Amount of previous cold work
MECHANISM OF RE-CRYSTALLIZATION:
1. Pre-existing grain boundaries
2. Sub grain boundaries resulting from deformation
Sl.No.
Metal Re-crystallizationtemperature (c)
Melting temperature(c)
1. Aluminum(Al) 150 660
2. Copper(cu) 200 1083
3. Iron(Fe) 450 1535
4. Nickel(Ni) 620 14525. Magnesium(Mg) 150 651
6. Tungsten(w) 1210 3400
7. Silver(Ag) 100 960.5
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GROWTH OF PRE-EXISTING GRAIN BOUNDRIES:
The boundary between a grain of high dislocation density and a grain of low dislocation
density suddenly grows. Thus the nucleation is essentially a growth phenomenon. We can observe
through the electron microscope that this mobile boundary is pinned at two points as it bulges out.
The nucleation by this growth mechanism will occur at boundaries having grain boundary
mobility. Ex. High angle boundaries.
GROWTH OF SUB-GRAIN BOUNDRIES:
The sudden growth may be due to either by coalescence mechanism or by grain boundary
migration. High mobility boundary forms a high angle boundary, which suddenly grows out.
Adjustment between sub-boundaries occurs on atomic scale. These adjustments modify high
mobility boundary and thus nucleation occurs. This mechanism is more common in highly
deformed metals large deformation produces large micro orientations between sub-grains and high
angle sub boundaries.
TYPES OF RE-CRYSTALLIZATION:
1. Primary re-crystallization
2. Secondary re-crystallization
PRIMARY RE-CRYSTALLIZATION:
It occurs when cold worked metal is heated. It is defined as the nucleation and growth of
strain-free grains, from the matrix of cold worked metal. When primary re-crystallization occurs,
there is some degree of recovery and sub grain formation. Primary re-crystallization is of much
importance because the properties of an alloy, after properties it has before cold working. Consider
a cold working operation of an alloy. (Example: Deep drawing).
During cold working, it becomes hard and less ductile. It becomes hard and less ductile. It
becomes difficult to continue the forming operation. A partial forming operation is done first and
then the alloy is given a re-crystallization (annealing) treatment. This partially formed alloy
regains its original ductility and hardness (of easy deformation). Now the job can be given for next
forming operation for deep drawing process.
SECONDARY RE-CRYSTALLIZATION (OR) COARSENING:
When annealing of a deformed sample is continued, beyond that stage of primary re-
crystallization, or sample is heated at higher temperature, after primary re-crystallization is
complete; the usual grain growth is interrupted. Some grains suddenly experience very rapid
growth. The dimensions of these rapidly grown grains may be of the order of few centimeters
while the rest of the grains remain small. These grains expand at the cost of other grains. This is
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called secondary re-crystallization. The large grains are not freshly nucleated; there are only
particular grains of primary structure which are grown large. (At temperature
(i) It shows the grain that rapidly grows in size. It is not clear that which primary grain
will grow in size, but the grains that grow is larger than mean primary grain size.
(At
(ii) Further growth in size of grains. (At
LAWS OF RE-CRYSTALLIZATION:
(i) A minimum of 2 to 8 % deformation is necessary for re-crystallization.
(ii) The smaller the degree of deformation, the higher the temperature required to initiate re-
crystallization
(iii) Increase in annealing time decreases the temperature required for re-crystallization
(iv)When degree of deformation is more and annealing temperature is less, the crystallized
grain size will be smaller
(v) New grains do not grow into deformed grains
(vi)Continued heating after primary re-crystallization causes grain size to increase
TEMPERATURE OF RE-CRYSTALLIZATION AND %DEFORMATION ARE HAVING INVERSE RELATIONS
CHANGE IN VARIOUS PROPERTIES PRODUCED BY HEATING OF COLD WORKED METAL
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DIFFERENCE BETWEEN RECOVERY AND RE-CRYSTALLIZATION:
Sl.
No. Point Recovery Re-crystallization
1.Working temperature range Low (between room and High (at
2.Mechanical properties Not affected Affected
3.
Process Relieve internal stresses Rapid change in
microstructure and
properties
4. Enhanced property Tensile strength increases Ductility increases
DIRECTIONAL PROPERTIES:
In rolling and forging operation, the crystals of the metals get elongated along the direction
of plastic flow. The polyhedral grains are converted in to fibers at low temperature. The structure
is known as fibrous structure. The direction of the plastic flow is determined always in the
direction in which the material elongates during mechanical shaping. For example, if we take two
hooks, one made by rolling process and another one by forging process. The plastic flow lineshows that Forged hook is stronger than rolled hook because flow lines are perpendicular to stress
direction. But rolled hook shear stress acts in the direction parallel to fibers, weak, easily fails
during service.
WORK HARDENING OR STRAIN HARDENING:
When metals are deformed at room temperature by the application of load, they offer more
and more resistance to further deformation. The tensile strength and hardness of the metal
increases but ductility decreases. The phenomena are known as the work hardening or strain
hardening.
FRACTURE MECHANICS
FRACTURE:
Fracture is the mechanical failure of the material which will produce the separation or
fragmentation of a solid into two or more parts under the action of stresses. Crack initiation and
crack propagation are components of fracture
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Fracture: Types
1.Brittle 2.Ductile 3.Creep 4.Fatigue 5.Shearing
BRITTLE FRACTURE: (CLEAVAGE FRACTURE)
It is defined as a fracture which takes place by the rapid propagation of crack with a
negligible deformation.Example: glass
In crystallography, the fracture takes place normal to the specific crystallographic planes,called cleavage planes. In polycrystalline materials the fracture takes place along the grain
boundaries. The tendency of the brittle fracture is increased with decreasing temperature and
increasing strain rate.
Brittle fracture is characterized by rapid rate of crack propagation with minimal energy
absorption. A brittle fracture occurs by separation normal to tensile stress.Brittle fractures have
been observed in BCC and HCP crystals but not in FCC metals. There is very little mass
deformation but there is no gross deformation. Brittle fracture normally follows grain boundaries.
Tendency of brittle fracture is increased with
(i) Decreasing temperature
(ii) Increasing strain rate
(iii) Tri axial stress condition
Brittle fracture is to be avoided because it may cause heavy damage because it does not give any
prior warning.
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Example: pressure vessels, bridges, ships
DUCTILE FRACTURE:
It is the fracture which takes place by a slow propagation of crack with appreciable plastic
deformation. It always preceded by localized deformation called NECKING.
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Specimen of very ductile metals (Au, Pb...) may actually be drawn down to a point before
they rupture. In moderately ductile metals, fracture takes place through local necking which
caused after appreciable plastic deformation. The failure of most polycrystalline materials occurswith cup and cone ductile fracture.
(Fig) ductile fracture with cup-cone formation
(Fig) completely ductile fracture
PREVENTION OF DUCTILE FRACTURE:
1. High hardness metals
2. Fine grained structure
3. High YMand cohesive energy
4. Defect/dislocation free
COMPARISON BETWEEN BRITTLE AND DUCTILE FRACTURE:
Sl.no. Brittle fracture Ductile fracture
1. It occurs with negligible plastic deformation It occurs with large plastic deformation
2. It occurs at the point where micro cracks is
more
It occurs in some localized region where
deformation is large
3. The rate of crack propagation is rapid Slow
4. Failure is due to the direct stress Failure is due to shear stress
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5. It is characterized by separation of normal totensile stress
It is characterized by the formation of cupand core
6. Fractured surface shows a sharp planar face Fractured surface is rough dirty grey contour
7. Increasing factors:
1. Increasing temperature2. Increasing strain rate
3. Work hardening
Increasing factor:
1. Dislocations2. Other defects
MECHANISM OF FRACTURE:
1. Brittle fracture : (i) Griffith crack theory (ii) cleavage
2. Ductile fracture : (i) cup and core formation
BRITTLE FRACTURE: (a) GRIFFITH THEORY
It is proved that the stress at which a material fractures is far below the lower value of the
ideal breaking strength calculated from the atomic strength. In other words, the fracture strength of
real materials is far lower than ( the theoretical minimum value for an ideal solid.
According to Griffith, the discrepancy between the strengths of real and ideal materials is
due to many fine cracks which act to concentrate the stress at their tips or ends. The micro cracks
in the metals that cause local concentration of stress to values high enough to propagate the crack
and eventually to fracture of metals.
GRIFFITH THEORY:
A crack will propagate when the decrease in elastic strain energy is at least equal to the
energy required to create the new crack surface.ie. in a brittle fracture, there are many fine cracks.
These cracks concentrate the applied stress at their tips or ends. When the stress at the tips of acrack exceeds the theoretical stress values, the crack expands and fracture occurs.
EXPLANATION OF MECHANISM OF BRITTLE FRACTURE:
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Let us consider a crack of elliptical cross-section in a rectangular specimen
Let,
Radius of curvature at the ends of the ellipse
Maximum stress at the tip of the crack
Tensile stress applied to the specimen
C Half length of the crack
It is observed that when a tensile stress is applied to the specimen, then the applied stress is
distributed about the crack in such a way that the maximum stress occurs at its tips. The maximum
stress ( ) at the tip of the crack is given by
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It is understood that when an elastic material is stressed, potential energy is stored in thematerial before crack occurs. This stored energy is known as elastic strain energy. When a crack
begins propagating, elastic energy is released. It is also understood that as the crack propagates,
new surfaces are created and a certain amount of energy, called surface energy, must be provided
to create them. Griffith supposed that the crack propagates when the released strain energy is just
sufficient to provide the surface energy necessary for the creation of the new surface.
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E Youngs modulus
Surface energy per unit area (J/
C Half length of crack
This is known as GRIFFITH FRACTURE EQUATION.
CLEAVAGE:
At low temperatures, brittle fractures takes place when a knife edge is hammered as shown
in fig. below, then the crystal will split into two parts with reference to a plane called cleavage
plane. This operation is called as cleavage.
Example: Fe (BCC) {110}
FCC metal do not develop cleavage
Zinc crystals are capable of being cleaved at room temperature but only with somedifficulty. Very nice cleavages are possible at -196c.
Cleavage of zinc produces distorted surfaces.
Alkali metals like NA, K, etc. do not cleave although they are BCC in structure.
DUCTILE FRACTURE: MECHANISM
(Cup and cone fracture)
The various stages at which ductile fracture takes place is mentioned in fig.
Fig:
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(a) Formation of neck: when the tensile stress is increased beyond the ultimate tensile stress, a
neck is formed in the specimen
(b) Fine cavities: it happens when the plastic deformation continues.
(c) Central necking: cavities grow in size if the straining is continuous. A central neck is also
formed.
(d) Crack propagation: the crack grows in a direction perpendicular to the axis of the specimen
until it approaches the surface of the specimen. It then propagates to form the cone part of
fracture.
(e) Cup and cone: the central cup region of the fracture has very fibrous appearance.
(f)
DUCTILE TO BRITTLE TRANSITION:
When BCC metals (ex. Steels) are subjected to impact loads at comparatively low
temperatures, a transition occurs from ductile fracture (requiring high energies) to non ductile
(brittle) fracture (requiring lower energy). This transition can become quite important to the
engineer who is designing a structure which will be subjected to impact stresses. When the
transition temperature is below operating temperatures, brittle type fractures will not occur. Eachfracture crack originated at same point of stress concentration, probably a sharp corner or
fabrication defect, which crack then propagated around the entire structure of split.
Below a certain temperature, the energy to break a specimen (ex. Iron) under impact
loading decreases abruptly which corresponds to transition from ductile to brittle behaviors. This
temperature is known as transition temperature.
Example: BCC and HCP metals, ceramics, polymers
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Titanic ship fracture
FCC dont show transition behavior
The ductile brittle transition temperature depends on
(i) Crystal structure
(ii) Alloying elements (ex. Carbon content in steel)
(iii) Rates of strain
(iv)Micro structure of material
(v) Smaller grain size
(vi)Impurities
(vii) Size and shape of specimen
(viii) Stress distribution
CREEP AND FATIGUE FAILURES:
CREEP:
It is defined as the property of a material by virtue of which it deforms continuously under
a steady load.
It is the permanent deformation of a material under a steady load as a function of time,
usually at higher temperature when a material is subjected to a constant loading, then the timedependant strain occurring under the constant stress is known as creep
Example: zinc, lead (at room temperature)
Fe, Ni, Cu (at elevated temperature)
FACTORS AFFECTING CREEP:
1).Grain size
2).Thermal stability of the microstructure
3).Chemical reactions
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4).Prior strain
PREVENTION OF CREEP:
1. Use of coarse grained materials
2. Strain hardening
3. Free from residual stresses
4. Precipitation hardening
5. Proper heat treatment
CREEP CURVE:
1. Instantaneous elongation
2. Primary creep
3. Secondary creep
4. Tertiary creep
A creep test curve under constant nominal stress at constant temperature is shown above. Different
stages are
1. Primary creep
2. Secondary creep
3. Tertiary creep.
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I. PRIMARY CREEP:
In this stage, the creep is mainly due to dislocation movement. The creep rate decreases
with time, during this stage, the recovery effect is less than the work hardening effect. Hence the
creep rate decreases logarithmically.
II. SECONDAY CREEP:
During this stage, the rates of work hardening and recovery are equal, so the material
creeps at steady rate. For the above reason, secondary creep is usually termed as steady state
creep. Steady state creep may be viscous or plastic in character, depending upon the state level
and temperature. It is the important part of the creep curve which is used to estimate the service
life of the alloy.
III. TERTIARY CREEP:
In this stage creep rate increases with time until fracture occurs. Generally the tertiary
creep occurs due to necking of the specimen or grain boundary sliding.
MECHANISM OF CREEP FAILURE:
1. DISLOCATION CLIMB
(i) At high temperature
(ii) Atomic climb
2. VACCANCY DIFFUSION
(i) Movement of vacancy
3. GRAIN BOUNDARY SLIDING
(i) At low temperature
CREEP TESTS:
The purpose of creep tests is to determine the creep limit. The creep limit or the limiting
creep stress is defined as the stress that will not break the specimen when applied for an infinite
period at a specific constant temperature. In creep test, we measure stress, strain, temperature and
time. It is the tension test that is done at constant load and constant temperature value of strain of
the test piece is noted as a function of time. The test arrangement is as shown:
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FATIGUE:
The behavior of the materials subjected to fluctuating or repeated loads is called fatigue.
FATIGUE FRACTURE:
The fatigue fracture is defined as the fracture which takes place under repeatedly applied
fatigue stresses.The fatigue fracture occurs at stresses well below the tensile stresses of the
materials.
Example: high speed machines, motor shafts, bolts, springs, gear teeth valves,turbine blades, air planes, automobile and gas engine parts, wire rope, suspension bridges
STRESS CYCLES:
(i) Reversed stress
(ii) Fluctuation stress
(iii) Irregular stress
FACTORS AFFECTING FARIGUE STRENGTH:
(i) Chemical composition, grain size, amount of cold working
(ii) Rise in temperature decreases fatigue strength
(iii) Corrosion decreases fatigue strength
(iv)Residual stress, stress gradients can be induce problem to fatigue strength. Proper design to
avoid those factors is important
http://www3.ntu.edu.sg/mae/research/labs/materials/_creep_test.jpg -
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PREVENTION OF FATIGUE FAILURE:
(i) Avoid sharp recesses and stress risers
(ii) Maintain good surface finish
(iii) Reduce corrosive environments
(iv)Fine grain materials
S-N DIAGRAM:
S-N diagram can be obtained by plotting the number of cycles of stress reversals (N)
required to cause fracture against the applied stress level(S).
Fatigue strength of steels in more compared to non-ferrous metals.
Fatigue stress (or) fatigue strength
Fatigue limit (or) endurance limit
Fatigue life
In high-cycle fatigue situations, materials performance is commonly characterized by an S-N
curve, also known as aWohlercurve. This is a graph of the magnitude of a cyclical stress (S)
against the logarithmic scale of cycles to failure (N).
FATIGUE TEST:
http://en.wikipedia.org/wiki/August_W%C3%B6hlerhttp://en.wikipedia.org/wiki/August_W%C3%B6hlerhttp://en.wikipedia.org/wiki/Logarithmic_scalehttp://en.wikipedia.org/wiki/August_W%C3%B6hlerhttp://en.wikipedia.org/wiki/Logarithmic_scale -
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Fatigue tests determine the resistance of material to repeated pulsating (or) fluctuating
loads.Fatigue tests are done using rotating beam fatigue testing machine as shown in fig.
below:
TESTING PROCEDURE:
The step by step procedure for fatigue testing is given below:
(i) The test specimen is placed on the machine
(ii) Now the specimen is rotated using an electric motor
(iii) When the specimen is rotating, it can be noted that the upper surface of the specimen is
subjected to tension and its lower surface experiences compression.
(iv)As the specimen rotates, there is sinusoidal variation of stress between a state of maximum
tensile stress and a state of maximum compressive stress
(v) The cycles of stress are applied until the specimen fractures. A reduction counter records
this number of stress cycles
(vi)Now a number of specimen of the same material( at least six specimens) are tested in the
same manner under different stress levels and the results are plotted on S-N graph
(vii) S-N graph is drawn and S-N curve is obtained
INFERENCES MADE:
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(i) Life time of part
(ii) Maximum allowable load
(iii) To fix endurance limit
(iv)Fatigue life obtained (stress, time, cycles)
(v) Endurance ratio = endurance limit / tensile strength 0.5