ED 72O3 - MECHANICAL BEHAVIOUR OF ENGINEERING MATERIAL

1. ED 72O3
MECHANICAL BEHAVIOUR
OF ENGINEERING MATERIAL
SEM-II /YEAR- 2015-2016

2. UNIT I
BASIC CONCEPTS OF MATERIAL BEHAVIOR
Elasticity in metals and polymers
In physics, elasticity is the ability of a body to resist a distorting influence or stress and to return
to its original size and shape when the stress is removed. Solid objects will deform when forces
are applied on them. If the material is elastic, the object will return to its initial shape and size
when these forces are removed.
The physical reasons for elastic behavior can be quite different for different materials. In metals,
the atomic lattice changes size and shape when forces are applied (energy is added to the
system). When forces are removed, the lattice goes back to the original lower energy state. For
rubbers and other polymers, elasticity is caused by the stretching of polymer chains when forces
are applied.
Perfect elasticity is an approximation of the real world, and few materials remain purely elastic
even after very small deformations. In engineering, the amount of elasticity of a material is
determined by two types of material parameter. The first type of material parameter is called a
modulus, which measures the amount of force per unit area (stress) needed to achieve a given
amount of deformation. The units of modulus are pascals (Pa) or pounds of force per square inch
(psi, also lbf/in2
). A higher modulus typically indicates that the material is harder to deform. The
second type of parameter measures the elastic limit. The limit can be a stress beyond which the
material no longer behaves elastic and deformation of the material will take place. If the stress is
released, the material will elastically return to a permanent deformed shape instead of the
original shape.
When describing the relative elasticities of two materials, both the modulus and the elastic limit
have to be considered. Rubbers typically have a low modulus and tend to stretch a lot (that is,
they have a high elastic limit) and so appear more elastic than metals (high modulus and low
elastic limit) in everyday experience. Of two rubber materials with the same elastic limit, the one
with a lower modulus will appear to be more elastic.

3. Elasticity in metals and polymers
Strengthening mechanisms
Plastic deformation occurs when large numbers of dislocations move and multiply so as to result
in macroscopic deformation. In other words, it is the movement of dislocations in the material
which allows for deformation. If we want to enhance a material's mechanical properties (i.e.
increase the yield and tensile strength), we simply need to introduce a mechanism which
prohibits the mobility of these dislocations. Whatever the mechanism may be, (work hardening,
grain size reduction, etc.) they all hinder dislocation motion and render the material stronger than
previously.[1][2][3][4]
The stress required to cause dislocation motion is orders of magnitude lower than the theoretical
stress required to shift an entire plane of atoms, so this mode of stress relief is energetically
favorable. Hence, the hardness and strength (both yield and tensile) critically depend on the ease
with which dislocations move. Pinning points, or locations in the crystal that oppose the motion
of dislocations,[5]
can be introduced into the lattice to reduce dislocation mobility, thereby
increasing mechanical strength. Dislocations may be pinned due to stress field interactions with
other dislocations and solute particles, creating physical barriers from second phase precipitates
forming along grain boundaries. There are four main strengthening mechanisms for metals, each
is a method to prevent dislocation motion and propagation, or make it energetically unfavorable
for the dislocation to move. For a material that has been strengthened, by some processing
method, the amount of force required to start irreversible (plastic) deformation is greater than it
was for the original material.

4. In amorphous materials such as polymers, amorphous ceramics (glass), and amorphous metals,
the lack of long range order leads to yielding via mechanisms such as brittle fracture, crazing,
and shear band formation. In these systems, strengthening mechanisms do not involve
dislocations, but rather consist of modifications to the chemical structure and processing of the
constituent material.
The strength of materials cannot infinitely increase. Each of the mechanisms explained below
involves some trade-off by which other material properties are compromised in the process of
strengthening.
Strengthening mechanisms in metals
Work hardening
Main article: Work hardening
The primary species responsible for work hardening are dislocations. Dislocations interact with
each other by generating stress fields in the material. The interaction between the stress fields of
dislocations can impede dislocation motion by repulsive or attractive interactions. Additionally,
if two dislocations cross, dislocation line entanglement occurs, causing the formation of a jog
which opposes dislocation motion. These entanglements and jogs act as pinning points, which
oppose dislocation motion. As both of these processes are more likely to occur when more
dislocations are present, there is a correlation between dislocation density and yield strength,
where is the shear modulus, is the Burgers vector, and is the dislocation density.
Increasing the dislocation density increases the yield strength which results in a higher shear
stress required to move the dislocations. This process is easily observed while working a material
(in metals cold working of process). Theoretically, the strength of a material with no dislocations
will be extremely high (τ=G/2) because plastic deformation would require the breaking of many
bonds simultaneously. However, at moderate dislocation density values of around 107
-109
dislocations/m2
, the material will exhibit a significantly lower mechanical strength. Analogously,
it is easier to move a rubber rug across a surface by propagating a small ripple through it than by
dragging the whole rug. At dislocation densities of 1014
dislocations/m2
or higher, the strength of
the material becomes high once again. Also, the dislocation density cannot be infinitely high,
because then the material would lose its crystalline structure.

5. This is a schematic illustrating how the lattice is strained by the addition of interstitial solute.
Notice the strain in the lattice that the solute atoms cause. The interstitial solute could be carbon
in iron for example. The carbon atoms in the interstitial sites of the lattice creates a stress field
that impedes dislocation movement.
This is a schematic illustrating how the lattice is strained by the addition of substitutional solute.
Notice the strain in the lattice that the solute atom causes.
Solid solution strengthening and alloying
Main article: Solid solution strengthening
For this strengthening mechanism, solute atoms of one element are added to another, resulting in
either substitutional or interstitial point defects in the crystal (see Figure 1). The solute atoms
cause lattice distortions that impede dislocation motion, increasing the yield stress of the
material. Solute atoms have stress fields around them which can interact with those of
dislocations. The presence of solute atoms impart compressive or tensile stresses to the lattice,
depending on solute size, which interfere with nearby dislocations, causing the solute atoms to
act as potential barriers

6. The shear stress required to move dislocations in a material is:
where is the solute concentration and is the strain on the material caused by the solute.
Increasing the concentration of the solute atoms will increase the yield strength of a material, but
there is a limit to the amount of solute that can be added, and one should look at the phase
diagram for the material and the alloy to make sure that a second phase is not created.
In general, the solid solution strengthening depends on the concentration of the solute atoms,
shear modulus of the solute atoms, size of solute atoms, valency of solute atoms (for ionic
materials), and the symmetry of the solute stress field. The magnitude of strengthening is higher
for non-symmetric stress fields because these solutes can interact with both edge and screw
dislocations, whereas symmetric stress fields, which cause only volume change and not shape
change, can only interact with edge dislocations.
Figure 2: A schematic illustrating how the dislocations can interact with a particle. It can either
cut through the particle or bow around the particle and create a dislocation loop as it moves over
the particle.
Precipitation hardening
Main article: Precipitation strengthening
In most binary systems, alloying above a concentration given by the phase diagram will cause
the formation of a second phase. A second phase can also be created by mechanical or thermal
treatments. The particles that compose the second phase precipitates act as pinning points in a
similar manner to solutes, though the particles are not necessarily single atoms.

7. The dislocations in a material can interact with the precipitate atoms in one of two ways (see
Figure 2). If the precipitate atoms are small, the dislocations would cut through them. As a result,
new surfaces (b in Figure 2) of the particle would get exposed to the matrix and the particle-
matrix interfacial energy would increase. For larger precipitate particles, looping or bowing of
the dislocations would occur and result in dislocations getting longer. Hence, at a critical radius
of about 5 nm, dislocations will preferably cut across the obstacle, while for a radius of 30 nm,
the dislocations will readily bow or loop to overcome the obstacle.
The mathematical descriptions are as follows:
For particle bowing-
For particle cutting-
Figure 3: A schematic roughly illustrating the concept of dislocation pile up and how it effects
the strength of the material. A material with larger grain size is able to have more dislocation to
pile up leading to a bigger driving force for dislocations to move from one grain to another.
Thus, less force need be applied to move a dislocation from a larger, than from a smaller grain,
leading materials with smaller grains to exhibit higher yield stress.
poly phase mixture
A polyphase system is a means of distributing alternating-current electrical power. Polyphase systems
have three or more energized electrical conductors carrying alternating currents with a definite time
offset between the voltage waves in each conductor. Polyphase systems are particularly useful for
transmitting power to electric motors. The most common example is the three-phase power system
used for industrial applications and for power transmission. A major advantage of three phase power
transmission (using three conductors, as opposed to a single phase power transmission, which uses two
conductors), is that, since the remaining conductors act as the return path for any single conductor, the
power transmitted by a balanced three phase system is three times that of a single phase transmission

8. but only one extra conductor is used. Thus, a 50% / 1.5x increase in the transmission costs achieves a
200% / 3.0x increase in the power transmitted.
The lowercase letters (a, b, p, and q) represent stoichiometric coefficients, while the capital
letters represent the reactants (A and B) and the products (P and Q).
According to IUPAC's Gold Book definition[1]
the reaction rate r for a chemical reaction
occurring in a closed system under isochoric conditions, without a build-up of reaction
intermediates, is defined as:
where [X] denotes the concentration of the substance X. (Note: The rate of a reaction is always
positive. A negative sign is present to indicate the reactant concentration is decreasing.) The
IUPAC[1]
recommends that the unit of time should always be the second. In such a case the rate
of reaction differs from the rate of increase of concentration of a product P by a constant factor
(the reciprocal of its stoichiometric number) and for a reactant A by minus the reciprocal of the
stoichiometric number. Reaction rate usually has the units of mol L−1
s−1
. It is important to bear
in mind that the previous definition is only valid for a single reaction, in a closed system of
constant volume. This usually implicit assumption must be stated explicitly, otherwise the
definition is incorrect: If water is added to a pot containing salty water, the concentration of salt
decreases, although there is no chemical reaction.
For any open system, the full mass balance must be taken into account: IN - OUT +
GENERATION - CONSUMPTION = ACCUMULATION
,
where is the inflow rate of A in molecules per second, the outflow, and is the
instantaneous reaction rate of A (in number concentration rather than molar) in a given
differential volume, integrated over the entire system volume at a given moment. When

9. applied to the closed system at constant volume considered previously, this equation reduces to:
, where the concentration is related to the number of molecules by
. Here is the Avogadro constant.
For a single reaction in a closed system of varying volume the so-called rate of conversion can
be used, in order to avoid handling concentrations. It is defined as the derivative of the extent of
reaction with respect to time.
Here is the stoichiometric coefficient for substance , equal to a, b, p, and q in the typical
reaction above. Also is the volume of reaction and is the concentration of substance .
When side products or reaction intermediates are formed, the IUPAC[1]
recommends the use of
the terms rate of appearance and rate of disappearance for products and reactants, properly.
Reaction rates may also be defined on a basis that is not the volume of the reactor. When a
catalyst is used the reaction rate may be stated on a catalyst weight (mol g−1
s−1
) or surface area
(mol m−2
s−1
) basis. If the basis is a specific catalyst site that may be rigorously counted by a
specified method, the rate is given in units of s−1
and is called a turnover frequency.
Factors influencing rate of reaction
 The nature of the reaction: Some reactions are naturally faster than others. The number of
reacting species, their physical state (the particles that form solids move much more
slowly than those of gases or those in solution), the complexity of the reaction and other
factors can greatly influence the rate of a reaction.
 Concentration: Reaction rate increases with concentration, as described by the rate law
and explained by collision theory. As reactant concentration increases, the frequency of
collision increases.
 Pressure: The rate of gaseous reactions increases with pressure, which is, in fact,
equivalent to an increase in concentration of the gas.The reaction rate increases in the
direction where there are fewer moles of gas and decreases in the reverse direction. For
condensed-phase reactions, the pressure dependence is weak.
 Order: The order of the reaction controls how the reactant concentration (or pressure)
affects reaction rate.
 Temperature: Usually conducting a reaction at a higher temperature delivers more energy
into the system and increases the reaction rate by causing more collisions between
particles, as explained by collision theory. However, the main reason that temperature
increases the rate of reaction is that more of the colliding particles will have the necessary

10. activation energy resulting in more successful collisions (when bonds are formed
between reactants). The influence of temperature is described by the Arrhenius equation.
For example, coal burns in a fireplace in the presence of oxygen, but it does not when it is stored
at room temperature. The reaction is spontaneous at low and high temperatures but at room
temperature its rate is so slow that it is negligible. The increase in temperature, as created by a
match, allows the reaction to start and then it heats itself, because it is exothermic. That is valid
for many other fuels, such as methane, butane, and hydrogen.
Reaction rates can be independent of temperature (non-Arrhenius) or decrease with increasing
temperature (anti-Arrhenius). Reactions without an activation barrier (e.g., some radical
reactions), tend to have anti Arrhenius temperature dependence: the rate constant decreases with
increasing temperature.
 Solvent: Many reactions take place in solution and the properties of the solvent affect the
reaction rate. The ionic strength also has an effect on reaction rate.
 Electromagnetic radiation and intensity of light: Electromagnetic radiation is a form of
energy. As such, it may speed up the rate or even make a reaction spontaneous as it
provides the particles of the reactants with more energy. This energy is in one way or
another stored in the reacting particles (it may break bonds, promote molecules to
electronically or vibrationally excited states...) creating intermediate species that react
easily. As the intensity of light increases, the particles absorb more energy and hence the
rate of reaction increases.
For example, when methane reacts with chlorine in the dark, the reaction rate is very slow. It can
be sped up when the mixture is put under diffused light. In bright sunlight, the reaction is
explosive.
 A catalyst: The presence of a catalyst increases the reaction rate (in both the forward and
reverse reactions) by providing an alternative pathway with a lower activation energy.
For example, platinum catalyzes the combustion of hydrogen with oxygen at room temperature.
 Isotopes: The kinetic isotope effect consists in a different reaction rate for the same
molecule if it has different isotopes, usually hydrogen isotopes, because of the mass
difference between hydrogen and deuterium.
 Surface Area: In reactions on surfaces, which take place for example during
heterogeneous catalysis, the rate of reaction increases as the surface area does. That is
because more particles of the solid are exposed and can be hit by reactant molecules.
 Stirring: Stirring can have a strong effect on the rate of reaction for heterogeneous
reactions.
All the factors that affect a reaction rate, except for concentration and reaction order, are taken
into account in the reaction rate coefficient (the coefficient in the rate equation of the reaction).

11. Rate equation
Main article: Rate equation
For a chemical reaction a A + b B → p P + q Q, the rate equation or rate law is a mathematical
expression used in chemical kinetics to link the rate of a reaction to the concentration of each
reactant. It is of the kind:
For gas phase reaction the rate is often alternatively expressed by partial pressures.
In these equations is the reaction rate coefficient or rate constant, although it is not really
a constant, because it includes all the parameters that affect reaction rate, except for
concentration, which is explicitly taken into account. Of all the parameters influencing reaction
rates, temperature is normally the most important one and is accounted for by the Arrhenius
equation.
The exponents and are called reaction orders and depend on the reaction mechanism. For
elementary (single-step) reactions the order with respect to each reactant is equal to its
stoichiometric coefficient. For complex (multistep) reactions, however, this is often not true and
the rate equation is determined by the detailed mechanism, as illustrated below for the reaction of
H2 and NO.
For elementary reactions or reaction steps, the order and stoichiometric coefficient are both equal
to the molecularity or number of molecules participating. For a unimolecular reaction or step the
rate is proportional to the concentration of molecules of reactant, so that the rate law is first
order. For a bimolecular reaction or step, the number of collisions is proportional to the product
of the two reactant concentrations, or second order. A termolecular step is predicted to be third
order, but also very slow as simultaneous collisions of three molecules are rare.
By using the mass balance for the system in which the reaction occurs, an expression for the rate
of change in concentration can be derived. For a closed system with constant volume, such an
expression can look like
Example of a complex reaction: Reaction of hydrogen and nitric oxide
As for many reactions, the rate equation does not simply reflect the stoichiometric coefficients in
the overall reaction: It is third order overall: first order in H2 and second order in NO, although
the stoichiometric coefficients of both reactants are equal to 2.[2]

12. In chemical kinetics, the overall reaction rate is often explained using a mechanism consisting of
a number of elementary steps. Not all of these steps affect the rate of reaction; normally the
slowest elementary step controls the reaction rate. For this example, a possible mechanism is:
1. (fast equilibrium)
2. (slow)
3. (fast)
Reactions 1 and 3 are very rapid compared to the second, so the slow reaction 2 is the rate
determining step. This is a bimolecular elementary reaction whose rate is given by the second
order equation : , where k2 is the rate constant for the second step.
However N2O2 is an unstable intermediate whose concentration is determined by the fact that the
first step is in equilibrium, so that : , where K1 is the equilibrium
constant of the first step. Substitution of this equation in the previous equation leads to a rate
equation expressed in terms of the original reactants
This agrees with the form of the observed rate equation if it is assumed that . In
practice the rate equation is used to suggest possible mechanisms which predict a rate equation in
agreement with experiment.
The second molecule of H2 does not appear in the rate equation because it reacts in the third step,
which is a rapid step after the rate-determining step, so that it does not affect the overall reaction
rate.
In materials science, superplasticity is a state in which solid crystalline material is deformed well
beyond its usual breaking point, usually over about 200% during tensile deformation. Such a
state is usually achieved at high homologous temperature. Examples of superplastic materials are
some fine-grained metals and ceramics. Other non-crystalline materials (amorphous) such as
silica glass ("molten glass") and polymers also deform similarly, but are not called superplastic,
because they are not crystalline; rather, their deformation is often described as Newtonian fluid.
Superplastically deformed material gets thinner in a very uniform manner, rather than forming a
"neck" (a local narrowing) that leads to fracture.[1]
Also, the formation of microvoids, which is
another cause of early fracture, is inhibited.[citation needed]
In metals and ceramics, requirements for it being superplastic include a fine grain size (less than
approximately 20 micrometres) and a fine dispersion of thermally stable particles, which act to
pin the grain boundaries and maintain the fine grain structure at the high temperatures and
existence of two phases required for superplastic deformation. Those materials that meet these
parameters must still have a strain rate sensitivity (a measurement of the way the stress on a
material reacts to changes in strain rate) of >0.3 to be considered superplastic.

13. The mechanisms of superplasticity in metals are still under debate—many believe it relies on
atomic diffusion and the sliding of grains past each other. Also, when metals are cycled around
their phase transformation, internal stresses are produced and superplastic-like behaviour
develops. Recently high-temperature superplastic behaviour has also been observed in iron
aluminides with coarse grain structures. It is claimed that this is due to recovery and dynamic
recrystallization.
In materials science, superplasticity is a state in which solid crystalline material is deformed well
beyond its usual breaking point, usually over about 200% during tensile deformation. Such a
state is usually achieved at high homologous temperature. Examples of superplastic materials are
some fine-grained metals and ceramics. Other non-crystalline materials (amorphous) such as
silica glass ("molten glass") and polymers also deform similarly, but are not called superplastic,
because they are not crystalline; rather, their deformation is often described as Newtonian fluid.
Superplastically deformed material gets thinner in a very uniform manner, rather than forming a
"neck" (a local narrowing) that leads to fracture.[1]
Also, the formation of microvoids, which is
another cause of early fracture, is inhibited.[citation needed]
In metals and ceramics, requirements for it being superplastic include a fine grain size (less than
approximately 20 micrometres) and a fine dispersion of thermally stable particles, which act to
pin the grain boundaries and maintain the fine grain structure at the high temperatures and
existence of two phases required for superplastic deformation. Those materials that meet these
parameters must still have a strain rate sensitivity (a measurement of the way the stress on a
material reacts to changes in strain rate) of >0.3 to be considered superplastic.
The mechanisms of superplasticity in metals are still under debate—many believe it relies on
atomic diffusion and the sliding of grains past each other. Also, when metals are cycled around
their phase transformation, internal stresses are produced and superplastic-like behaviour
develops. Recently high-temperature superplastic behaviour has also been observed in iron
aluminides with coarse grain structures. It is claimed that this is due to recovery and dynamic
recrystallization.
Fracture mechanics was developed during World War I by English aeronautical engineer, A. A.
Griffith, to explain the failure of brittle materials.[1]
Griffith's work was motivated by two
contradictory facts:
 The stress needed to fracture bulk glass is around 100 MPa (15,000 psi).
 The theoretical stress needed for breaking atomic bonds is approximately 10,000 MPa
(1,500,000 psi).
A theory was needed to reconcile these conflicting observations. Also, experiments on glass
fibers that Griffith himself conducted suggested that the fracture stress increases as the fiber
diameter decreases. Hence the uniaxial tensile strength, which had been used extensively to
predict material failure before Griffith, could not be a specimen-independent material property.

14. Griffith suggested that the low fracture strength observed in experiments, as well as the size-
dependence of strength, was due to the presence of microscopic flaws in the bulk material.
To verify the flaw hypothesis, Griffith introduced an artificial flaw in his experimental glass
specimens. The artificial flaw was in the form of a surface crack which was much larger than
other flaws in a specimen. The experiments showed that the product of the square root of the
flaw length (a) and the stress at fracture (σf) was nearly constant, which is expressed by the
equation:
An explanation of this relation in terms of linear elasticity theory is problematic. Linear elasticity
theory predicts that stress (and hence the strain) at the tip of a sharp flaw in a linear elastic
material is infinite. To avoid that problem, Griffith developed a thermodynamic approach to
explain the relation that he observed.
The growth of a crack requires the creation of two new surfaces and hence an increase in the
surface energy. Griffith found an expression for the constant C in terms of the surface energy of
the crack by solving the elasticity problem of a finite crack in an elastic plate. Briefly, the
approach was:
 Compute the potential energy stored in a perfect specimen under a uniaxial tensile load.
 Fix the boundary so that the applied load does no work and then introduce a crack into
the specimen. The crack relaxes the stress and hence reduces the elastic energy near the
crack faces. On the other hand, the crack increases the total surface energy of the
specimen.
 Compute the change in the free energy (surface energy − elastic energy) as a function of
the crack length. Failure occurs when the free energy attains a peak value at a critical
crack length, beyond which the free energy decreases by increasing the crack length, i.e.
by causing fracture. Using this procedure, Griffith found that
where E is the Young's modulus of the material and γ is the surface energy density of the
material. Assuming E = 62 GPa and γ = 1 J/m2
gives excellent agreement of Griffith's predicted
fracture stress with experimental results for glass.
Irwin's modification

15. The plastic zone around a crack tip in a ductile material
Griffith's work was largely ignored by the engineering community until the early 1950s. The
reasons for this appear to be (a) in the actual structural materials the level of energy needed to
cause fracture is orders of magnitude higher than the corresponding surface energy, and (b) in
structural materials there are always some inelastic deformations around the crack front that
would make the assumption of linear elastic medium with infinite stresses at the crack tip highly
unrealistic. [2]
Griffith's theory provides excellent agreement with experimental data for brittle materials such as
glass. For ductile materials such as steel, though the relation still holds, the surface
energy (γ) predicted by Griffith's theory is usually unrealistically high. A group working under
G. R. Irwin[3]
at the U.S. Naval Research Laboratory (NRL) during World War II realized that
plasticity must play a significant role in the fracture of ductile materials.
In ductile materials (and even in materials that appear to be brittle[4]
), a plastic zone develops at
the tip of the crack. As the applied load increases, the plastic zone increases in size until the
crack grows and the material behind the crack tip unloads. The plastic loading and unloading
cycle near the crack tip leads to the dissipation of energy as heat. Hence, a dissipative term has to
be added to the energy balance relation devised by Griffith for brittle materials. In physical
terms, additional energy is needed for crack growth in ductile materials when compared to brittle
materials.
Irwin's strategy was to partition the energy into two parts:
 the stored elastic strain energy which is released as a crack grows. This is the
thermodynamic driving force for fracture.
 the dissipated energy which includes plastic dissipation and the surface energy (and any
other dissipative forces that may be at work). The dissipated energy provides the
thermodynamic resistance to fracture. Then the total energy is
where γ is the surface energy and Gp is the plastic dissipation (and dissipation from other
sources) per unit area of crack growth.

16. The modified version of Griffith's energy criterion can then be written as
For brittle materials such as glass, the surface energy term dominates and
. For ductile materials such as steel, the plastic dissipation term
dominates and . For polymers close to the glass transition
temperature, we have intermediate values of .
Stress intensity factor
Main article: Stress intensity factor
Another significant achievement of Irwin and his colleagues was to find a method of calculating
the amount of energy available for fracture in terms of the asymptotic stress and displacement
fields around a crack front in a linear elastic solid.[3]
This asymptotic expression for the stress
field around a crack tip is
where σij are the Cauchy stresses, r is the distance from the crack tip, θ is the angle with respect
to the plane of the crack, and fij are functions that depend on the crack geometry and loading
conditions. Irwin called the quantity K the stress intensity factor. Since the quantity fij is
dimensionless, the stress intensity factor can be expressed in units of .
When a rigid line inclusion is considered, a similar asymptotic expression for the stress fields is
obtained.
Strain energy release
Main article: Strain energy release rate
Irwin was the first to observe that if the size of the plastic zone around a crack is small compared
to the size of the crack, the energy required to grow the crack will not be critically dependent on
the state of stress at the crack tip.[2]
In other words, a purely elastic solution may be used to
calculate the amount of energy available for fracture.
The energy release rate for crack growth or strain energy release rate may then be calculated as
the change in elastic strain energy per unit area of crack growth, i.e.,

17. where U is the elastic energy of the system and a is the crack length. Either the load P or the
displacement u can be kept fixed while evaluating the above expressions.
Irwin showed that for a mode I crack (opening mode) the strain energy release rate and the stress
intensity factor are related by:
where E is the Young's modulus, ν is Poisson's ratio, and KI is the stress intensity factor in mode
I. Irwin also showed that the strain energy release rate of a planar crack in a linear elastic body
can be expressed in terms of the mode I, mode II (sliding mode), and mode III (tearing mode)
stress intensity factors for the most general loading conditions.
Next, Irwin adopted the additional assumption that the size and shape of the energy dissipation
zone remains approximately constant during brittle fracture. This assumption suggests that the
energy needed to create a unit fracture surface is a constant that depends only on the material.
This new material property was given the name fracture toughness and designated GIc. Today, it
is the critical stress intensity factor KIc, found in the plane strain condition, which is accepted as
the defining property in linear elastic fracture mechanics.
Larson-Miller parameter
is a means of predicting the lifetime of material vs. time and temperature using a correlative
approach based on the Arrhenius rate equation. The value of the parameter is usually expressed
as LMP=T(C + log t) where C is a material specific constant often approximated as 20, t is the
time in hours and T is the temperature in Kelvin.
Creep-stress rupture data for high-temperature creep-resistant alloys are often plotted as log
stress to rupture versus a combination of log time to rupture and temperature. One of the most
common time–temperature parameters used to present this kind of data is the Larson-Miller
(L.M.) parameter, which in generalized form is
T = temperature, K or °R
= stress-rupture time, h
C = constant usually of order 20

18. According to the L.M. parameter, at a given stress level the log time to stress rupture plus a
constant of the order of 20 multiplied by the temperature in kelvins or degrees Rankine remains
constant for a given material.
A deformation mechanism map is a way of representing the dominant deformation mechanism
in a material loaded under a given set of conditions and thereby its likely failure mode.
Deformation mechanism maps usually consist of some kind of stress plotted against some kind
of temperature axis, typically stress normalised using the shear modulus versus homologous
temperature with contours of strain rate.[1] [2]
For a given set of operating conditions calculations
are undergone and experiments performed to determine the predominant mechanism operative
for a given material.
Deformation Maps can also be constructed using any two of stress (normalised), temperature
(normalised) and strain rate, with contours of the third variable. A stress/strain rate plot is useful
because power-law mechanisms then have contours of temperature which are straight lines.

19. UNIT II
BEHAVIOUR UNDER DYNAMIC LOADS AND DESIGN APPROACHES
The stress intensity factor, , is used in fracture mechanics to predict the stress state ("stress
intensity") near the tip of a crack caused by a remote load or residual stresses.[1]
It is a theoretical
construct usually applied to a homogeneous, linear elastic material and is useful for providing a
failure criterion for brittle materials, and is a critical technique in the discipline of damage
tolerance. The concept can also be applied to materials that exhibit small-scale yielding at a
crack tip.
The magnitude of depends on sample geometry, the size and location of the crack, and the
magnitude and the modal distribution of loads on the material.
Linear elastic theory predicts that the stress distribution ( ) near the crack tip, in polar
coordinates ( ) with origin at the crack tip, has the form [2]
where is the stress intensity factor (with units of stress length1/2
) and is a dimensionless
quantity that varies with the load and geometry. This relation breaks down very close to the tip
(small ) because as goes to 0, the stress goes to . Plastic distortion typically occurs at
high stresses and the linear elastic solution is no longer applicable close to the crack tip.
However, if the crack-tip plastic zone is small, it can be assumed that the stress distribution near
the crack is still given by the above relation.

20. Fatigue, low and high cycle fatigue test, crack initiation and propagation
ASTM defines fatigue life, Nf, as the number of stress cycles of a specified character that a
specimen sustains before failure of a specified nature occurs.[2]
For some materials, notably steel
and titanium, there is a theoretical value for stress amplitude below which the material will not
fail for any number of cycles, called a fatigue limit, endurance limit, or fatigue strength.[3]
Engineers have used any of three methods to determine the fatigue life of a material: the stress-
life method, the strain-life method, and the linear-elastic fracture mechanics method.[4]
One
method to predict fatigue life of materials is the Uniform Material Law (UML).[5]
UML was
developed for fatigue life prediction of aluminium and titanium alloys by the end of 20th century
and extended to high-strength steels,[6]
and cast iron.[7]
Characteristics of fatigue
Fracture of an aluminium crank arm. Dark area of striations: slow crack growth. Bright granular
area: sudden fracture.
 In metal alloys, when there are no macroscopic or microscopic discontinuities, the
process starts with dislocation movements, which eventually form persistent slip bands
that become the nucleus of short cracks.
 Macroscopic and microscopic discontinuities as well as component design features which
cause stress concentrations (holes, keyways, sharp changes of direction etc.) are common
locations at which the fatigue process begins.
 Fatigue is a process that has a degree of randomness (stochastic), often showing
considerable scatter even in well controlled environments.
 Fatigue is usually associated with tensile stresses but fatigue cracks have been reported
due to compressive loads.[8]
 The greater the applied stress range, the shorter the life.
 Fatigue life scatter tends to increase for longer fatigue lives.
 Damage is cumulative. Materials do not recover when rested.
 Fatigue life is influenced by a variety of factors, such as temperature, surface finish,
metallurgical microstructure, presence of oxidizing or inert chemicals, residual stresses,
scuffing contact (fretting), etc.

21.  Some materials (e.g., some steel and titanium alloys) exhibit a theoretical fatigue limit
below which continued loading does not lead to fatigue failure.
 High cycle fatigue strength (about 104
to 108
cycles) can be described by stress-based
parameters. A load-controlled servo-hydraulic test rig is commonly used in these tests,
with frequencies of around 20–50 Hz. Other sorts of machines—like resonant magnetic
machines—can also be used, to achieve frequencies up to 250 Hz.
 Low cycle fatigue (loading that typically causes failure in less than 104
cycles) is
associated with localized plastic behavior in metals; thus, a strain-based parameter should
be used for fatigue life prediction in metals. Testing is conducted with constant strain
amplitudes typically at 0.01–5 Hz.
Timeline of early fatigue research history
 1837: Wilhelm Albert publishes the first article on fatigue. He devised a test machine for
conveyor chains used in the Clausthal mines.[9]
 1839: Jean-Victor Poncelet describes metals as being tired in his lectures at the military
school at Metz.
 1842: William John Macquorn Rankine recognises the importance of stress
concentrations in his investigation of railroad axle failures. The Versailles train crash was
caused by axle fatigue.[10]
 1843: Joseph Glynn reports on fatigue of axle on a locomotive tender. He identifies the
keyway as the crack origin.
 1848: The Railway Inspectorate reports one of the first tyre failures, probably from a
rivet hole in tread of railway carriage wheel. It was likely a fatigue failure.
 1849: Eaton Hodgkinson is granted a small sum of money to report to the UK Parliament
on his work in ascertaining by direct experiment, the effects of continued changes of load
upon iron structures and to what extent they could be loaded without danger to their
ultimate security.
 1854: Braithwaite reports on common service fatigue failures and coins the term
fatigue.[11]
 1860: Systematic fatigue testing undertaken by Sir William Fairbairn and August Wöhler.
 1870: Wöhler summarises his work on railroad axles. He concludes that cyclic stress
range is more important than peak stress and introduces the concept of endurance limit.[9]

22. Micrographs showing how surface fatigue cracks grow as material is further cycled. From Ewing
& Humfrey, 1903
 1903: Sir James Alfred Ewing demonstrates the origin of fatigue failure in microscopic
cracks.
 1910: O. H. Basquin proposes a log-log relationship for S-N curves, using Wöhler's test
data.
 1945: A. M. Miner popularises Palmgren's (1924) linear damage hypothesis as a practical
design tool.
 1954: The world's first commercial jetliner, the de Havilland Comet, suffers disaster as
three planes break up in mid-air, causing de Havilland and all other manufacturers to
redesign high altitude aircraft and in particular replace square apertures like windows
with oval ones.
 1954: L. F. Coffin and S. S. Manson explain fatigue crack-growth in terms of plastic
strain in the tip of cracks.
 1961: P. C. Paris proposes methods for predicting the rate of growth of individual fatigue
cracks in the face of initial scepticism and popular defence of Miner's phenomenological
approach.
 1968: Tatsuo Endo and M. Matsuishi devise the rainflow-counting algorithm and enable
the reliable application of Miner's rule to random loadings.[12]
 1970: W. Elber elucidates the mechanisms and importance of crack closure in slowing
the growth of a fatigue crack due to the wedging effect of plastic deformation left behind
the tip of the crack.
High-cycle fatigue
Historically, most attention has focused on situations that require more than 104
cycles to failure
where stress is low and deformation is primarily elastic.
S-N curve

23. In high-cycle fatigue situations, materials performance is commonly characterized by an S-N
curve, also known as a Wöhler curve . This is a graph of the magnitude of a cyclic stress (S)
against the logarithmic scale of cycles to failure (N).
S-N curve for a brittle aluminium with an ultimate tensile strength of 320 MPa.
S-N curves are derived from tests on samples of the material to be characterized (often called
coupons) where a regular sinusoidal stress is applied by a testing machine which also counts the
number of cycles to failure. This process is sometimes known as coupon testing. Each coupon
test generates a point on the plot though in some cases there is a runout where the time to failure
exceeds that available for the test (see censoring). Analysis of fatigue data requires techniques
from statistics, especially survival analysis and linear regression.
The progression of the S-N curve can be influenced by many factors such as corrosion,
temperature, residual stresses, and the presence of notches. The Goodman-Line is a method to
estimate the influence of the mean stress on the fatigue strength.
Probabilistic nature of fatigue
As coupons sampled from a homogeneous frame will display a variation in their number of
cycles to failure, the S-N curve should more properly be an S-N-P curve capturing the probability
of failure after a given number of cycles of a certain stress. Probability distributions that are
common in data analysis and in design against fatigue include the log-normal distribution,
extreme value distribution, Birnbaum–Saunders distribution, and Weibull distribution.
Complex loadings
Spectrum loading
In practice, a mechanical part is exposed to a complex, often random, sequence of loads, large
and small. In order to assess the safe life of such a part:

24. 1. Reduce the complex loading to a series of simple cyclic loadings using a technique such
as rainflow analysis;
2. Create a histogram of cyclic stress from the rainflow analysis to form a fatigue damage
spectrum;
3. For each stress level, calculate the degree of cumulative damage incurred from the S-N
curve; and
4. Combine the individual contributions using an algorithm
5. Paris' law (also known as the Paris-Erdogan law) relates the stress intensity factor
range to sub-critical crack growth under a fatigue stress regime. As such, it is the most
popular fatigue crack growth model used in materials science and fracture mechanics.
The basic formula reads[1]
6. ,
7. where a is the crack length and N is the number of load cycles. Thus, the term on the left
side, known as the crack growth rate,[2]
denotes the infinitesimal crack length growth per
increasing number of load cycles. On the right hand side, C and m are material constants,
and is the range of the stress intensity factor, i.e., the difference between the stress
intensity factor at maximum and minimum loading
8. ,
9. where is the maximum stress intensity factor and is the minimum stress
intensity factor.[3]
10. History and use
11. The formula was introduced by P.C. Paris in 1961.[4]
Being a power law relationship
between the crack growth rate during cyclic loading and the range of the stress intensity
factor, the Paris law can be visualized as a linear graph on a log-log plot, where the x-axis
is denoted by the range of the stress intensity factor and the y-axis is denoted by the crack
growth rate.
12. Paris' law can be used to quantify the residual life (in terms of load cycles) of a specimen
given a particular crack size. Defining the stress intensity factor as
13. ,
14. where is a uniform tensile stress perpendicular to the crack plane and Y is a
dimensionless parameter that depends on the geometry, the range of the stress intensity
factor follows as
15. ,
16. where is the range of cyclic stress amplitude. Y takes the value 1 for a center crack in
an infinite sheet. The remaining cycles can be found by substituting this equation in the
Paris law
17. .
18. For relatively short cracks, Y can be assumed as independent of a and the differential
equation can be solved via separation of variables
19.
20. and subsequent integration

25. 21. ,
22. where is the remaining number of cycles to fracture, is the critical crack length at
which instantaneous fracture will occur, and is the initial crack length at which fatigue
crack growth starts for the given stress range . If Y strongly depends on a, numerical
methods might be required to find reasonable solutions.
23. For the application to adhesive joints in composites, it is more useful to express the Paris
Law in terms of fracture energy rather than stress intensity factors.[5]
Failure analysis
is the process of collecting and analyzing data to determine the cause of a failure, often with the
goal of determining corrective actions or liability. It is an important discipline in many branches of
manufacturing industry, such as the electronics industry, where it is a vital tool used in the
development of new products and for the improvement of existing products. The failure analysis
process relies on collecting failed components for subsequent examination of the cause or causes of
failure using a wide array of methods, especially microscopy and spectroscopy. The NDT or
nondestructive testing methods (such as Industrial computed tomography scanning) are valuable
because the failed products are unaffected by analysis, so inspection sometimes starts using these
methods.

26. UNIT III
SELECTION OF MATERIALS
Motivation for selection, cost basis and service requirements
Basis (or cost basis), as used in United States tax law, is the original cost of property, adjusted
for factors such as depreciation. When property is sold, the taxpayer pays/(saves) taxes on a
capital gain/(loss) that equals the amount realized on the sale minus the sold property's basis.
Cost basis is needed because tax is due based on the gain in value of an asset. For example, if a
person buys a rock for $20, and sells the same rock for $20, there is no tax, since there is no
profit. If, however, that person buys a rock for $20 and then sells the same rock for $25, then
there is a capital gain on the rock of $5, which is thus taxable. The purchase price of $20 is
analogous to cost of sales.
Typically, capital gains tax is due only when an asset is sold. However the rules for this are very
complicated. If tax is paid because the value has increased, the new value will be the cost basis
for any future tax.
Internal Revenue Service (IRS) Publication 551 contains the IRS's definition of basis: "Basis is
the amount of your investment in property for tax purposes. Use the basis of property to figure
depreciation, amortization, depletion, and casualty losses. Also use it to figure gain or loss on the
sale or other disposition of property."
A material's property is an intensive, often quantitative, property of some material.
Quantitative properties may be used as a metric by which the benefits of one material versus
another can be assessed, thereby aiding in materials selection.
A property may be a constant or may be a function of one or more independent variables, such as
temperature. Materials properties often vary to some degree according to the direction in the
material in which they are measured, a condition referred to as anisotropy. Materials properties
that relate to different physical phenomena often behave linearly (or approximately so) in a given
operating range. Modeling them as linear can significantly simplify the differential constitutive
equations that the property describes.
Some materials properties are used in relevant equations to predict the attributes of a system a
priori. For example, if a material of a known specific heat gains or loses a known amount of heat,
the temperature change of that material can be determined. Materials properties are most reliably
measured by standardized test methods. Many such test methods have been documented by their
respective user communities and published through ASTM International.

27. Selection for mechanical properties
A material's property is an intensive, often quantitative, property of some material.
Quantitative properties may be used as a metric by which the benefits of one material versus
another can be assessed, thereby aiding in materials selection.
A property may be a constant or may be a function of one or more independent variables, such as
temperature. Materials properties often vary to some degree according to the direction in the
material in which they are measured, a condition referred to as anisotropy. Materials properties
that relate to different physical phenomena often behave linearly (or approximately so) in a given
operating range. Modeling them as linear can significantly simplify the differential constitutive
equations that the property describes.
Some materials properties are used in relevant equations to predict the attributes of a system a
priori. For example, if a material of a known specific heat gains or loses a known amount of heat,
the temperature change of that material can be determined. Materials properties are most reliably
measured by standardized test methods. Many such test methods have been documented by their
respective user communities and published through ASTM International.
 Brittleness: Ability of a material to break or shatter without significant deformation when
under stress; opposite of plasticity
 Bulk modulus: Ratio of pressure to volumetric compression (GPa)
 Coefficient of friction (also depends on surface finish)
 Coefficient of restitution
 Compressive strength: Maximum stress a material can withstand before compressive
failure (MPa)
 Creep: The slow and gradual deformation of an object with respect to time
 Elasticity: Ability of a body to resist a distorting influence or stress and to return to its
original size and shape when the stress is removed
 Fatigue limit: Maximum stress a material can withstand under repeated loading (MPa)
 Flexibility: Ability of an object to bend or deform in response to an applied force;
pliability; complementary to stiffness
 Flexural modulus
 Flexural strength
 Fracture toughness: Energy absorbed by unit area before the fracture of material (J/m^2)
 Hardness: Ability to withstand surface indentation and scratching (e.g. Brinnell hardness
number)
 Plasticity: Ability of a material to undergo irreversible or permanent deformations
without breaking or rupturing; opposite of brittleness
o Ductility: Ability of a material to deform under tensile load (% elongation)
o Malleability: Ability to deform under compressive stress without developing
defects
 Poisson's ratio: Ratio of lateral strain to axial strain (no units)
 Resilience: Ability of a material to absorb energy when it is deformed elastically (MPa);
combination of strength and elasticity
 Shear modulus: Ratio of shear stress to shear strain (MPa)

28.  Shear strain: in the angle between two perpendicular lines in a plane
 Shear strength: Maximum shear stress a material can withstand
 Specific modulus: Modulus per unit volume (MPa/ m^3)
 Specific strength: Strength per unit density (Nm/kg)
 Specific weight: Weight per unit volume (N/m^3)
 Stiffness: Ability of an object resists deformation in response to an applied force; rigidity;
complementary to flexibility
 Surface roughness
 Tensile strength: Maximum tensile stress a material can withstand before failure (MPa)
 Toughness: Ability of a material to absorb energy (or withstand shock) and plastically
deform without fracturing (or rupturing); a material's resistance to fracture when stressed;
combination of strength and plasticity
 Viscosity: A fluid's resistance to gradual deformation by shear stress or tensile stress;
thickness
 Yield strength: The stress at which a material starts to yield (MPa)
 Young's modulus: Ratio of linear stress to linear strain
For many years Wood was the most favourable choice for construction of
Vehicle-bodies in the transportation sector.
Let us look at the reasons behind this choice:
 Traditionally wood was used in the transportation sector for building
Chariots, Animal drawn Carts, Palanquins etc – hence it became the
natural choice for building bodies of the automobiles, omnibuses etc.
at the first phase of industrial revolution.
 Wood has impressive mechanical properties. The elastic modulus of
wood is in the range of 8-20 GPa which is as good as materials like
PMMA and GFRP. The density of wood is about 0.6-75 Mg/m3
–
lighter than most of the polymers except polymeric foams. The
strength of the wood is about 30 MPa which is again comparable to
high-performance polymers.
 Other advantages of wood are recyclability, ease of machining and
aesthetically pleasing quality.
With the advent of mass-scale production and automation in car-industry, it
became necessary to replace wood by metals and metallic alloys. Typical
metal shaping technologies like sheet forming which can handle large batch
size (105
to 106
units per batch) became very much suitable for the massscale
production of vehicles.
There were two-choices in terms of use of metals and metallic alloys: Steel
and Aluminium Alloys. Why these materials became so popular for Car-

29. design? Let us find the material indices most relevant from car-body
construction point of view. It is observed that three most significant issues in
car-body design are:
i. Stiffness of the sheets which is expressed as an objective to minimise
mass against a specified deflection limit. Minimisation of mass directly
implies the use of less amount of material and hence less cost per unit.
Also, mass minimisation would increase fuel efficiency of the vehicle.
For a flat panel of size (LxB), thickness t, modulus of elasticity E and
density ρ, this would involve the search for a material having
maximum value of an index
(E1/3
/ρ). Later we will discuss about the origin of such indices.
ii. Another important consideration is dent resistance. A similar study
would indicate that this requires the maximisation of an index (σy
t4
/k), where σy is the yield strength and k is the stiffness of the panel.
For many years Wood was the most favourable choice for construction of
Vehicle-bodies in the transportation sector.
Let us look at the reasons behind this choice:
 Traditionally wood was used in the transportation sector for building
Chariots, Animal drawn Carts, Palanquins etc – hence it became the
natural choice for building bodies of the automobiles, omnibuses etc.
at the first phase of industrial revolution.
 Wood has impressive mechanical properties. The elastic modulus of
wood is in the range of 8-20 GPa which is as good as materials like
PMMA and GFRP. The density of wood is about 0.6-75 Mg/m3
–
lighter than most of the polymers except polymeric foams. The
strength of the wood is about 30 MPa which is again comparable to
high-performance polymers.
 Other advantages of wood are recyclability, ease of machining and
aesthetically pleasing quality.
With the advent of mass-scale production and automation in car-industry, it
became necessary to replace wood by metals and metallic alloys. Typical
metal shaping technologies like sheet forming which can handle large batch
size (105
to 106
units per batch) became very much suitable for the massscale
production of vehicles.
There were two-choices in terms of use of metals and metallic alloys: Steel
and Aluminium Alloys. Why these materials became so popular for Car-

30. design? Let us find the material indices most relevant from car-body
construction point of view. It is observed that three most significant issues in
car-body design are:
i. Stiffness of the sheets which is expressed as an objective to minimise
mass against a specified deflection limit. Minimisation of mass directly
implies the use of less amount of material and hence less cost per unit.
Also, mass minimisation would increase fuel efficiency of the vehicle.
For a flat panel of size (LxB), thickness t, modulus of elasticity E and
density ρ, this would involve the search for a material having
maximum value of an index
(E1/3
/ρ). Later we will discuss about the origin of such indices.
ii. Another important consideration is dent resistance. A similar study
would indicate that this requires the maximisation of an index (σy
t4
/k), where σy is the yield strength and k is the stiffness of the panel.

31. UNIT IV
MODERN METALLIC MATERIALS
Dual phase steels,
Dual-phase steel (DPS) is a high-strength steel that has a ferrite and martensitic microstructure.
DPS starts as a low or medium carbon steel and is quenched from a temperature above A1 but
below A3 on a continuous cooling transformation diagram. This results in a microstructure
consisting of a soft ferrite matrix containing islands of martensite as the secondary phase
(martensite increases the tensile strength). Therefore, the overall behavior of DPS is governed by
the volume fraction, morpology (size, aspect ratio, interconnectivity, etc.), the grain size and the
carbon content.[1]
For achieving these microstructures, DPS typically contain 0.06–0.15 wt.% C
and 1.5-3% Mn (the former strengthens the martensite, and the latter causes solid solution
strengthening in ferrite, while both stabilize the austenite), Cr & Mo (to retard pearlite or bainite
formation), Si (to promote ferrite transformation), V and Nb (for precipitation strengthening and
microstructure refinement).[2]
The desire to produce high strength steels with formability greater
than microalloyed steel led the development of DPS in the 1970s.[3][4]
DPS have high ultimate tensile strength (UTS, enabled by the martensite) combined with low
initial yielding stress (provided by the ferrite phase), high early-stage strain hardening and
macroscopically homogeneous plastic flow (enabled through the absence of Lüders effects).
These features render DPS ideal materials for automotive-related sheet forming operations.
The steel melt is produced in an oxygen top blowing process in the converter, and undergoes an
alloy treatment in the secondary metallurgy phase. The product is aluminum-killed steel, with
high tensile strength achieved by the composition with manganese, chromium and silicon.
High strength low alloy (HSLA) steel
High-strength low-alloy steel (HSLA) is a type of alloy steel that provides better mechanical
properties or greater resistance to corrosion than carbon steel. HSLA steels vary from other steels
in that they are not made to meet a specific chemical composition but rather to specific

32. mechanical properties. They have a carbon content between 0.05–0.25% to retain formability
and weldability. Other alloying elements include up to 2.0% manganese and small quantities of
copper, nickel, niobium, nitrogen, vanadium, chromium, molybdenum, titanium, calcium, rare
earth elements, or zirconium.[1][2]
Copper, titanium, vanadium, and niobium are added for
strengthening purposes.[2]
These elements are intended to alter the microstructure of carbon
steels, which is usually a ferrite-pearlite aggregate, to produce a very fine dispersion of alloy
carbides in an almost pure ferrite matrix. This eliminates the toughness-reducing effect of a
pearlitic volume fraction yet maintains and increases the material's strength by refining the grain
size, which in the case of ferrite increases yield strength by 50% for every halving of the mean
grain diameter. Precipitation strengthening plays a minor role, too. Their yield strengths can be
anywhere between 250–590 megapascals (36,000–86,000 psi). Because of their higher strength
and toughness HSLA steels usually require 25 to 30% more power to form, as compared to
carbon steels.[2]
Copper, silicon, nickel, chromium, and phosphorus are added to increase corrosion resistance.
Zirconium, calcium, and rare earth elements are added for sulfide-inclusion shape control which
increases formability. These are needed because most HSLA steels have directionally sensitive
properties. Formability and impact strength can vary significantly when tested longitudinally and
transversely to the grain. Bends that are parallel to the longitudinal grain are more likely to crack
around the outer edge because it experiences tensile loads. This directional characteristic is
substantially reduced in HSLA steels that have been treated for sulfide shape control.[2]
They are used in cars, trucks, cranes, bridges, roller coasters and other structures that are
designed to handle large amounts of stress or need a good strength-to-weight ratio.[2]
HSLA steel
cross-sections and structures are usually 20 to 30% lighter than a carbon steel with the same
strength.[3][4]
HSLA steels are also more resistant to rust than most carbon steels because of their lack of
pearlite – the fine layers of ferrite (almost pure iron) and cementite in pearlite.[citation needed]
HSLA
steels usually have densities of around 7800 kg/m³.[5]
Classifications
 Weathering steels: steels which have better corrosion resistance. A common example is
COR-TEN.
 Control-rolled steels: hot rolled steels which have a highly deformed austenite structure
that will transform to a very fine equiaxed ferrite structure upon cooling.
 Pearlite-reduced steels: low carbon content steels which lead to little or no pearlite, but
rather a very fine grain ferrite matrix. It is strengthened by precipitation hardening.
 Acicular ferrite steels: These steels are characterized by a very fine high strength
acicular ferrite structure, a very low carbon content, and good hardenability.
 Dual-phase steels: These steels have a ferrite microstruture that contain small, uniformly
distributed sections of martensite. This microstructure gives the steels a low yield
strength, high rate of work hardening, and good formability.[1]
 Microalloyed steels: steels which contain very small additions of niobium, vanadium,
and/or titanium to obtain a refined grain size and/or precipitation hardening.

33. A common type of micro-alloyed steel is improved-formability HSLA. It has a yield strength up
to 80,000 psi (550 MPa) but only costs 24% more than A36 steel (36,000 psi (250 MPa)). One of
the disadvantages of this steel is that it is 30 to 40% less ductile. In the U.S., these steels are
dictated by the ASTM standards A1008/A1008M and A1011/A1011M for sheet metal and
A656/A656M for plates. These steels were developed for the automotive industry to reduce
weight without losing strength. Examples of uses include door-intrusion beams, chassis
members, reinforcing and mounting brackets, steering and suspension parts, bumpers, and
wheels.[2][6]
Transformation induced plasticity steel
TRIP steel is a high-strength steel typically used in the automotive industry.[1]
TRIP stands for
"Transformation induced plasticity." It is known for its outstanding combination of Strength and
Ductility.
Contents
 1 Microstructure
 2 Metallurgical Properties
 3 Effect of Alloying Elements
 4 Applications
 5 References
Microstructure
TRIP steel has a microstructure consisting of retained Austenite in a ferrite matrix. Apart from
Retained Austenite it also contains hard phases like Bainite and Martensite.[2]
The higher silicon
and carbon content of TRIP steels results in significant volume fractions of retained austenite in
the final microstructure.
TRIP steels use higher quantities of carbon than Dual Phase steels to obtain sufficient carbon
content for stabilizing the retained austenite phase to below ambient temperature. Higher
contents of silicon and/or aluminium accelerate the ferrite/bainite formation. They are also added
to avoid formation of carbide in the bainite region.
Metallurgical Properties
During plastic deformation and straining, the retained austenite phase is transformed into
martensite. Thus increasing the strength by the phenomenon of Strain Hardening. This
transformation allows for enhanced strength and ductility.[3]
High strain hardening capacity and
high mechanical strength lend these steels excellent energy absorption capacity. TRIP steels also
exhibit a strong bake hardening (BH) effect following deformation.[4]
Research to date has not
shown much experimental evidence of the TRIP-effect enhancing ductility, since most of the
austenite disappears in the first 5% of plastic strain, a regime where the steel has adequate

34. ductility already. Many experiments show that TRIP steels are in fact simply a more complex
dual-phase (DP) steel.
Effect of Alloying Elements
The amount of Carbon determines the strain level at which the retained austenite begins to
transform to martensite. At lower carbon levels, the retained austenite begins to transform almost
immediately upon deformation, increasing the work hardening rate and formability during the
stamping process. At higher carbon contents, the retained austenite is more stable and begins to
transform only at strain levels beyond those produced during forming.
Applications
As a result of their high energy absorption capacity and fatigue strength, TRIP steels are
particularly well suited for automotive structural and safety parts such as cross members,
longitudinal beams, B-pillar reinforcements, sills and bumper reinforcements
The most common TRIP range of steels comprises 2 cold rolled grades in both uncoated and
coated formats (TRIP 690 and TRIP 780) and one hot rolled grade (TRIP 780), identified by
their minimum tensile strength expressed in MPa.
Maraging steel,
Maraging steels (a portmanteau of "martensitic" and "aging") are steels (iron alloys) that are
known for possessing superior strength and toughness without losing malleability, although they
cannot hold a good cutting edge. Aging refers to the extended heat-treatment process. These
steels are a special class of low-carbon ultra-high-strength steels that derive their strength not
from carbon, but from precipitation of intermetallic compounds. The principal alloying element
is 15 to 25 wt.% nickel.[1]
Secondary alloying elements, which include cobalt, molybdenum, and
titanium, are added to produce intermetallic precipitates,.[1]
Original development (by Bieber of
Inco in the late 1950s) was carried out on 20 and 25 wt.% Ni steels to which small additions of
Al, Ti, and Nb were made; a rise in the price of cobalt in the late 1970s led to the development of
cobalt-free maraging steels [2]
The common, non-stainless grades contain 17–19 wt.% nickel, 8–12 wt.% cobalt, 3–5 wt.%
molybdenum, and 0.2–1.6 wt.% titanium. Addition of chromium produces stainless grades
resistant to corrosion. This also indirectly increases hardenability as they require less nickel:
high-chromium, high-nickel steels are generally austenitic and unable to transform to martensite
when heat treated, while lower-nickel steels can transform to martensite. Alternative variants of
Ni-reduced maraging steels are based on alloys of Fe and Mn plus minor additions of Al, Ni, and
Ti where compositions between Fe-9wt.% Mn to Fe-15wt.% Mn have been used.[3]
The Mn has a
similar effect as Ni, i.e. it stabilizes the austenite phase. Hence, depending on their Mn content,
Fe-Mn maraging steels can be fully martensitic after quenching them from the high temperature
austenite phase or they can contain retained austenite.[4]
The latter effect enables the design of
maraging-TRIP steels where TRIP stands for Transformation-Induced-Plasticity.[5]

35. Contents
 1 Properties
 2 Grades of maraging steel
 3 Heat treatment cycle
 4 Uses
 5 Physical properties
 6 References
 7 External links
Properties
Due to the low carbon content maraging steels have good machinability. Prior to aging, they may
also be cold rolled to as much as 90% without cracking. Maraging steels offer good weldability,
but must be aged afterward to restore the original properties to the heat affected zone.[1]
When heat-treated the alloy has very little dimensional change, so it is often machined to its final
dimensions. Due to the high alloy content maraging steels have a high hardenability. Since
ductile FeNi martensites are formed upon cooling, cracks are non-existent or negligible. The
steels can be nitrided to increase case hardness, and polished to a fine surface finish.
Non-stainless varieties of maraging steel are moderately corrosion-resistant, and resist stress
corrosion and hydrogen embrittlement. Corrosion-resistance can be increased by cadmium
plating or phosphating.
Grades of maraging steel
Maraging steels tend to be described by a number (200, 250, 300 or 350), which indicates the
approximate nominal tensile strength in thousands of pounds per square inch; the compositions
and required properties are defined in MIL-S-46850D.[6]
The higher grades have more cobalt and
titanium in the alloy; the compositions below are taken from table 1 of MIL-S-46850D:
Maraging steel compositions
Element Grade 200 Grade 250 Grade 300
Grade
350
Iron balance balance balance balance
Nickel 17.0-19.0 17.0-19.0 18.0-19.0 18.0-19.0
Cobalt 8.0-9.0 7.0-8.5 8.5-9.5 11.5-12.5
Molybdenum 3.0-3.5 4.6-5.2 4.6-5.2 4.6-5.2
Titanium 0.15-0.25 0.3-0.5 0.5-0.8 1.3-1.6
Aluminium 0.05-0.15 0.05-0.15 0.05-0.15 0.05-0.15

36. That family is known as the 18Ni maraging steels, from its nickel percentage. There is also a
family of cobalt-free maraging steels which are cheaper but not quite as strong; one exemplar is
Fe-18.9Ni-4.1Mo-1.9Ti. There has been Russian and Japanese research in Fe-Ni-Mn maraging
alloys.[2]
Heat treatment cycle
The steel is first annealed at approximately 820 °C (1,510 °F) for 15–30 minutes for thin sections
and for 1 hour per 25 mm thickness for heavy sections, to ensure formation of a fully
austenitized structure. This is followed by air cooling to room temperature to form a soft,
heavily-dislocated iron-nickel lath (untwinned) martensite. Subsequent aging (precipitation
hardening) of the more common alloys for approximately 3 hours at a temperature of 480 to
500 °C produces a fine dispersion of Ni3(X,Y) intermetallic phases along dislocations left by
martensitic transformation, where X and Y are solute elements added for such precipitation.
Overaging leads to a reduction in stability of the primary, metastable, coherent precipitates,
leading to their dissolution and replacement with semi-coherent Laves phases such as
Fe2Ni/Fe2Mo. Further excessive heat-treatment brings about the decomposition of the martensite
and reversion to austenite.
Newer compositions of maraging steels have revealed other intermetallic stoichiometries and
crystallographic relationships with the parent martensite, including rhombohedral and massive
complex Ni50(X,Y,Z)50 (Ni50M50 in simplified notation).
Uses
Maraging steel's strength and malleability in the pre-aged stage allows it to be formed into
thinner rocket and missile skins than other steels, reducing weight for a given strength.[7]
Maraging steels have very stable properties, and, even after overaging due to excessive
temperature, only soften slightly. These alloys retain their properties at mildly elevated operating
temperatures and have maximum service temperatures of over 400 °C (752 °F).[citation needed]
They
are suitable for engine components, such as crankshafts and gears, and the firing pins of
automatic weapons that cycle from hot to cool repeatedly while under substantial load. Their
uniform expansion and easy machinability before aging make maraging steel useful in high-wear
components of assembly lines and dies. Other ultra-high-strength steels, such as AerMet alloys,
are not as machinable because of their carbide content.
In the sport of fencing, blades used in competitions run under the auspices of the Fédération
Internationale d'Escrime are usually made with maraging steel. Maraging blades are superior for
foil and épée because crack propagation in maraging steel is 10 times slower than in carbon steel,
resulting in less blade breakage and fewer injuries.[8][9]
Stainless maraging steel is used in bicycle
frames and golf club heads. It is also used in surgical components and hypodermic syringes, but
is not suitable for scalpel blades because the lack of carbon prevents it from holding a good
cutting edge.

37. Maraging steel production, import, and export by certain states, such as the United States,[10]
is
closely monitored by international authorities because it is particularly suited for use in gas
centrifuges for uranium enrichment; lack of maraging steel significantly hampers this process.
Older centrifuges used aluminum tubes; modern ones, carbon fiber composite.[citation needed]
Physical properties
 Density: 8.1 g/cm³ (0.29 lb/in³)
 Specific heat, mean for 0–100 °C (32–212 °F): 813 J/kg·K (0.108 Btu/lb·°F)
 Melting point: 2,575 °F, 1,413 °C
 Thermal conductivity: 25.5 W/m·K
 Mean coefficient of thermal expansion: 11.3×10−6
 Yield tensile strength: typically 1,400–2,400 MPa (200,000–350,000 psi)[11]
 Ultimate tensile strength: typically 1.6–2.5 GPa (230,000–360,000 psi). Grades exist up
to 3.5 GPa (510,000 psi)
 Elongation at break: up to 15%
 KIC fracture toughness: up to 175 MPa·m1⁄
2
 Young's modulus: 210 GPa (30,000,000 psi)[12]
 Shear modulus: 77 GPa (11,200,000 psi)
 Bulk modulus: 140 GPa (20,000,000 psi)
 Hardness (aged): 50 HRC (grade 250); 54 HRC (grade 300); 58 HRC (grade 350)[citation
needed]
Nitrogen steel
Nitriding is a heat treating process that diffuses nitrogen into the surface of a metal to create a
case-hardened surface. These processes are most commonly used on low-carbon, low-alloy
steels. However, they are also used on medium and high-carbon steels, titanium, aluminum and
molybdenum. Recently, nitriding was used to generate unique duplex microstructure
(Martensite-Austenite, Austenite-ferrite), known to be associated with strongly enhanced
mechanical properties [1]
Typical applications include gears, crankshafts, camshafts, cam followers, valve parts, extruder
screws, die-casting tools, forging dies, extrusion dies, firearm components, injectors and plastic-
mold tools.
Contents
 1 Processes
o 1.1 Gas nitriding
o 1.2 Salt bath nitriding
o 1.3 Plasma nitriding
 2 Materials for nitriding
 3 History
 4 See also

38.  5 References
 6 Further reading
 7 External links
Processes
The processes are named after the medium used to donate. The three main methods used are: gas
nitriding, salt bath nitriding, and plasma nitriding.
Gas nitriding
In gas nitriding the donor is a nitrogen rich gas, usually ammonia (NH3), which is why it is
sometimes known as ammonia nitriding.[2]
When ammonia comes into contact with the heated
work piece it dissociates into nitrogen and hydrogen. The nitrogen then diffuses onto the surface
of the material creating a nitride layer. This process has existed for nearly a century, though only
in the last few decades has there been a concentrated effort to investigate the thermodynamics
and kinetics involved. Recent developments have led to a process that can be accurately
controlled. The thickness and phase constitution of the resulting nitriding layers can be selected
and the process optimized for the particular properties required.
The advantages of gas nitriding over the other variants are:
 Precise control of chemical potential of nitrogen in the nitriding atmosphere by
controlling gas flow rate of nitrogen and oxygen.
 All round nitriding effect (can be a disadvantage in some cases, compared with plasma
nitriding)
 Large batch sizes possible - the limiting factor being furnace size and gas flow
 With modern computer control of the atmosphere the nitriding results can be closely
controlled
 Relatively low equipment cost - especially compared with plasma
The disadvantages of gas nitriding are:
 Reaction kinetics heavily influenced by surface condition - an oily surface or one
contaminated with cutting fluids will deliver poor results
 Surface activation is sometimes required to treat steels with a high chromium content -
compare sputtering during plasma nitriding
 Ammonia as nitriding medium - though not especially toxic it can be harmful when
inhaled in large quantities. Also, care must be taken when heating in the presence of
oxygen to reduce the risk of explosion
Salt bath nitriding
In salt bath nitriding the nitrogen donating medium is a nitrogen-containing salt such as cyanide
salt. The salts used also donate carbon to the workpiece surface making salt bath a
nitrocarburizing process. The temperature used is typical of all nitrocarburizing processes: 550–

39. 570 °C. The advantages of salt nitriding is that it achieves higher diffusion in the same period
time compared to any other method. The advantages of salt nitriding are:
 Quick processing time - usually in the order of 4 hours or so to achieve
 Simple operation - heat the salt and workpieces to temperature and submerge until the
duration has transpired.
The disadvantages are:
 The salts used are highly toxic - Disposal of salts are controlled by stringent
environmental laws in western countries and has increased the costs involved in using
salt baths. This is one of the most significant reasons the process has fallen out of favor in
recent decades.
 Only one process possible with a particular salt type - since the nitrogen potential is set
by the salt, only one type of process is possible
Plasma nitriding
Plasma nitriding, also known as ion nitriding, plasma ion nitriding or glow-discharge nitriding,
is an industrial surface hardening treatment for metallic materials.
In plasma nitriding, the reactivity of the nitriding media is not due to the temperature but to the
gas ionized state. In this technique intense electric fields are used to generate ionized molecules
of the gas around the surface to be nitrided. Such highly active gas with ionized molecules is
called plasma, naming the technique. The gas used for plasma nitriding is usually pure nitrogen,
since no spontaneous decomposition is needed (as is the case of gas nitriding with ammonia).
There are hot plasmas typified by plasma jets used for metal cutting, welding, cladding or
spraying. There are also cold plasmas, usually generated inside vacuum chambers, at low
pressure regimes.
Usually steels are beneficially treated with plasma nitriding. This process permits the close
control of the nitrided microstructure, allowing nitriding with or without compound layer
formation. Not only the performance of metal parts is enhanced, but working lifespans also
increase, and so do the strain limit and the fatigue strength of the metals being treated. For
instance, mechanical properties of austenitic stainless steel like resistance to wear can be
significantly augmented and the surface hardness of tool steels can be doubled.[3][4]
A plasma nitrided part is usually ready for use. It calls for no machining, or polishing or any
other post-nitriding operations. Thus the process is user-friendly, saves energy since it works
fastest, and causes little or no distortion.
This process was invented by Dr. Bernhardt Berghaus of Germany who later settled in Zurich to
escape Nazi persecution. After his death in late 1960s the process was acquired by Klockner
group and popularized world over.

40. Plasma nitriding is often coupled with physical vapor deposition (PVD) process and labeled
Duplex Treatment, with enhanced benefits. Many users prefer to have a plasma oxidation step
combined at the last phase of processing to produce a smooth jetblack layer of oxides which is
resistant to wear and corrosion.
Since nitrogen ions are made available by ionization, differently from gas or salt bath, plasma
nitriding efficiency does not depend on the temperature. Plasma nitriding can thus be performed
in a broad temperature range, from 260 °C to more than 600 °C.[4]
For instance, at moderate
temperatures (like 420 °C), stainless steels can be nitrided without the formation of chromium
nitride precipitates and hence maintaining their corrosion resistance properties.[5]
In the plasma nitriding processes, nitrogen gas (N2) is usually the nitrogen carrying gas. Other
gasses like hydrogen or Argon are also used. Indeed, Argon and H2 can be used before the
nitriding process during the heating of the parts to clean the surfaces to be nitrided. This cleaning
procedure effectively removes the oxide layer from surfaces and may remove fine layers of
solvents that could remain. This also helps the thermal stability of the plasma plant, since the
heat added by the plasma is already present during the warm up and hence once the process
temperature is reached the actual nitriding begins with minor heating changes. For the nitriding
process H2 gas is also added to keep the surface clear of oxides. This effect can be observed by
analysing the surface of the part under nitriding (see for instance [6]
).
Materials for nitriding
Examples of easily nitridable steels include the SAE 4100, 4300, 5100, 6100, 8600, 8700, 9300
and 9800 series, UK aircraft quality steel grades BS 4S 106, BS 3S 132, 905M39 (EN41B),
stainless steels, some tool steels (H13 and P20 for example) and certain cast irons. Ideally, steels
for nitriding should be in the hardened and tempered condition, requiring nitriding take place at a
lower temperature than the last tempering temperature. A fine-turned or ground surface finish is
best. Minimal amounts of material should be removed post nitriding to preserve the surface
hardness.
Nitriding alloys are alloy steels with nitride-forming elements such as aluminum, chromium,
molybdenum and titanium.
History
Systematic investigation into the effect of nitrogen on the surface properties of steel began in the
1920s. Investigation into gas nitriding began independently in both Germany and America. The
process was greeted with enthusiasm in Germany and several steel grades were developed with
nitriding in mind: the so-called nitriding steels. The reception in America was less impressive.
With so little demand the process was largely forgotten in the US. After WWII the process was
reintroduced from Europe. Much research has taken place in recent decades to understand the
thermodynamics and kinetics of the reactions involved.

41. Smart materials are designed materials that have one or more properties that can be
significantly changed in a controlled fashion by external stimuli, such as stress, temperature,
moisture, pH, electric or magnetic fields.
Other keywords related to smart material are such as shape memory material (SMM) and shape
memory technology (SMT).[1]
Contents
 1 Types
 2 See also
 3 References
 4 External links
Types
There are a number of types of smart material, some of which are already common. Some
examples are as following:
 Piezoelectric materials are materials that produce a voltage when stress is applied. Since
this effect also applies in the reverse manner, a voltage across the sample will produce
stress within the sample. Suitably designed structures made from these materials can
therefore be made that bend, expand or contract when a voltage is applied.
 Shape-memory alloys and shape-memory polymers are materials in which large
deformation can be induced and recovered through temperature changes or stress changes
(pseudoelasticity). The shape memory effect results due to respectively martensitic phase
change and induced elasticity at higher temperatures.
 Magnetostrictive materials exhibit change in shape under the influence of magnetic field
and also exhibit change in their magnetization under the influence of mechanical stress.
 Magnetic shape memory alloys are materials that change their shape in response to a
significant change in the magnetic field.
 pH-sensitive polymers are materials that change in volume when the pH of the
surrounding medium changes.
 Temperature-responsive polymers are materials which undergo changes upon
temperature.
 Halochromic materials are commonly used materials that change their colour as a result
of changing acidity. One suggested application is for paints that can change colour to
indicate corrosion in the metal underneath them.
 Chromogenic systems change colour in response to electrical, optical or thermal changes.
These include electrochromic materials, which change their colour or opacity on the
application of a voltage (e.g., liquid crystal displays), thermochromic materials change in
colour depending on their temperature, and photochromic materials, which change colour
in response to light—for example, light sensitive sunglasses that darken when exposed to
bright sunlight.
 Ferrofluid

42.  Photomechanical materials change shape under exposure to light.
 Polycaprolactone (polymorph) can be molded by immersion in hot water.
 Self-healing materials have the intrinsic ability to repair damage due to normal usage,
thus expanding the material's lifetime
 Dielectric elastomers (DEs) are smart material systems which produce large strains (up to
300%) under the influence of an external electric field.
 Magnetocaloric materials are compounds that undergo a reversible change in temperature
upon exposure to a changing magnetic field.
 Thermoelectric materials are used to build devices that convert temperature differences
into electricity and vice versa.
Smart materials have properties that react to changes in their environment. This means that one
of their properties can be changed by an external condition, such as temperature, light, pressure
or electricity. This change is reversible and can be repeated many times. There are a wide range
of different smart materials. Each offer different properties that can be changed. Some materials
are very good indeed and cover a huge range of the scales.
shape memory alloys
A shape-memory alloy (SMA, smart metal, memory metal, memory alloy, muscle wire,
smart alloy) is an alloy that "remembers" its original shape and that when deformed returns to
its pre-deformed shape when heated. This material is a lightweight, solid-state alternative to
conventional actuators such as hydraulic, pneumatic, and motor-based systems. Shape-memory
alloys have applications in industries including automotive, aerospace, biomedical and robotics.
Contents
 1 Overview
 2 One-way vs. two-way shape memory
o 2.1 One-way memory effect
o 2.2 Two-way memory effect
 3 Superelasticity
 4 History
 5 Crystal structures
 6 Manufacture
 7 Properties
 8 Practical limitations
o 8.1 Response time and response symmetry
o 8.2 Structural fatigue and functional fatigue
o 8.3 Unintended actuation
 9 Applications
o 9.1 Industrial
 9.1.1 Aircraft and spacecraft
 9.1.2 Automotive

43.  9.1.3 Robotics
 9.1.4 Civil Structures
 9.1.5 Piping
 9.1.6 Telecommunication
o 9.2 Medicine
 9.2.1 Optometry
 9.2.2 Orthopedic surgery
 9.2.3 Dentistry
o 9.3 Engines
o 9.4 Crafts
 10 Materials
 11 References
 12 External links
Overview
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The two main types of shape-memory alloys are copper-aluminium-nickel, and nickel-titanium
(NiTi) alloys but SMAs can also be created by alloying zinc, copper, gold and iron. Although
iron-based and copper-based SMAs, such as Fe-Mn-Si, Cu-Zn-Al and Cu-Al-Ni, are
commercially available and cheaper than NiTi, NiTi based SMAs are preferable for most
applications due to their stability, practicability[1][2][3]
and superior thermo-mechanic
performance.[4]
SMAs can exist in two different phases, with three different crystal structures
(i.e. twinned martensite, detwinned martensite and austenite) and six possible
transformations.[5][6]
NiTi alloys change from austenite to martensite upon cooling; Mf is the temperature at which the
transition to martensite completes upon cooling. Accordingly, during heating As and Af are the
temperatures at which the transformation from martensite to austenite starts and finishes.
Repeated use of the shape-memory effect may lead to a shift of the characteristic transformation
temperatures (this effect is known as functional fatigue, as it is closely related with a change of
microstructural and functional properties of the material).[7]
The maximum temperature at which
SMAs can no longer be stress induced is called Md, where the SMAs are permanently
deformed.[8]
The transition from the martensite phase to the austenite phase is only dependent on temperature
and stress, not time, as most phase changes are, as there is no diffusion involved. Similarly, the
austenite structure receives its name from steel alloys of a similar structure. It is the reversible
diffusionless transition between these two phases that results in special properties. While
martensite can be formed from austenite by rapidly cooling carbon-steel, this process is not
reversible, so steel does not have shape-memory properties.

44. In this figure, ξ(T) represents the martensite fraction. The difference between the heating
transition and the cooling transition gives rise to hysteresis where some of the mechanical energy
is lost in the process. The shape of the curve depends on the material properties of the shape-
memory alloy, such as the alloying.[9]
and work hardening.[10]
One-way vs. two-way shape memory
Shape-memory alloys have different shape-memory effects. Two common effects are one-way
and two-way shape memory. A schematic of the effects is shown below.

45. The procedures are very similar: starting from martensite (a), adding a reversible deformation for
the one-way effect or severe deformation with an irreversible amount for the two-way (b),
heating the sample (c) and cooling it again (d).
One-way memory effect
When a shape-memory alloy is in its cold state (below As), the metal can be bent or stretched and
will hold those shapes until heated above the transition temperature. Upon heating, the shape
changes to its original. When the metal cools again it will remain in the hot shape, until
deformed again.
With the one-way effect, cooling from high temperatures does not cause a macroscopic shape
change. A deformation is necessary to create the low-temperature shape. On heating,
transformation starts at As and is completed at Af (typically 2 to 20 °C or hotter, depending on the
alloy or the loading conditions). As is determined by the alloy type and composition and can vary
between −150 °C and 200 °C.
Two-way memory effect
The two-way shape-memory effect is the effect that the material remembers two different
shapes: one at low temperatures, and one at the high-temperature shape. A material that shows a
shape-memory effect during both heating and cooling is said to have two-way shape memory.
This can also be obtained without the application of an external force (intrinsic two-way effect).
The reason the material behaves so differently in these situations lies in training. Training
implies that a shape memory can "learn" to behave in a certain way. Under normal
circumstances, a shape-memory alloy "remembers" its low-temperature shape, but upon heating
to recover the high-temperature shape, immediately "forgets" the low-temperature shape.
However, it can be "trained" to "remember" to leave some reminders of the deformed low-
temperature condition in the high-temperature phases. There are several ways of doing this.[11]
A
shaped, trained object heated beyond a certain point will lose the two-way memory effect.
Superelasticity
SMAs also display superelasticity, which is characterized by recovery of unusually large strains.
Instead of transforming between the martensite and austenite phases in response to temperature,
this phase transformation can be induced in response to mechanical stress. When SMAs are
loaded in the austenite phase, the material will transform to the martensite phase above a critical
stress, proportional to the transformation temperatures. Upon continued loading, the twinned
martensite will begin to detwin, allowing the material to undergo large deformations. Once the
stress is released, the martensite transforms back to austenite, and the material recovers its
original shape. As a result, these materials can reversibly deform to very high strains – up to 8
percent. A more thorough discussion of the mechanisms of superelasticity and the shape-memory
effect is presented by Ma et al.[12]
History

46. The first reported steps towards the discovery of the shape-memory effect were taken in the
1930s. According to Otsuka and Wayman, A. Ölander discovered the pseudoelastic behavior of
the Au-Cd alloy in 1932. Greninger and Mooradian (1938) observed the formation and
disappearance of a martensitic phase by decreasing and increasing the temperature of a Cu-Zn
alloy. The basic phenomenon of the memory effect governed by the thermoelastic behavior of
the martensite phase was widely reported a decade later by Kurdjumov and Khandros (1949) and
also by Chang and Read (1951).[7]
The nickel-titanium alloys were first developed in 1962–1963 by the United States Naval
Ordnance Laboratory and commercialized under the trade name Nitinol (an acronym for Nickel
Titanium Naval Ordnance Laboratories). Their remarkable properties were discovered by
accident. A sample that was bent out of shape many times was presented at a laboratory
management meeting. One of the associate technical directors, Dr. David S. Muzzey, decided to
see what would happen if the sample was subjected to heat and held his pipe lighter underneath
it. To everyone's amazement the sample stretched back to its original shape.[13][14]
There is another type of SMA, called a ferromagnetic shape-memory alloy (FSMA), that changes
shape under strong magnetic fields. These materials are of particular interest as the magnetic
response tends to be faster and more efficient than temperature-induced responses.
Metal alloys are not the only thermally-responsive materials; shape-memory polymers have also
been developed, and became commercially available in the late 1990s.
Crystal structures
Many metals have several different crystal structures at the same composition, but most metals
do not show this shape-memory effect. The special property that allows shape-memory alloys to
revert to their original shape after heating is that their crystal transformation is fully reversible. In
most crystal transformations, the atoms in the structure will travel through the metal by diffusion,
changing the composition locally, even though the metal as a whole is made of the same atoms.
A reversible transformation does not involve this diffusion of atoms, instead all the atoms shift at
the same time to form a new structure, much in the way a parallelogram can be made out of a
square by pushing on two opposing sides. At different temperatures, different structures are
preferred and when the structure is cooled through the transition temperature, the martensitic
structure forms from the austenitic phase.
Manufacture
Shape-memory alloys are typically made by casting, using vacuum arc melting or induction
melting. These are specialist techniques used to keep impurities in the alloy to a minimum and
ensure the metals are well mixed. The ingot is then hot rolled into longer sections and then drawn
to turn it into wire.
The way in which the alloys are "trained" depends on the properties wanted. The "training"
dictates the shape that the alloy will remember when it is heated. This occurs by heating the alloy