Transcript Spring 2005 Properties of Materials

ÜYD
Properties of Material
ANADOLU
UNIVERSITY
Industrial Engineering Department
2 – Properties of Materials
Spring 2005
Saleh AMAITIK
Manufacturing Processes
Properties of Materials
STRUCTURE
PERFORMANCE
PROCESSING
PROPERTIES
Spring 2005
Manufacturing Processes
Properties of Materials
Mechanical Properties.
Mechanical properties are useful to estimate how parts will
behave when they are subjected to mechanical loads (stresses)
Properties include: Strength, ductility, hardness, elasticity, toughness,
creep, fatigue …etc.
Physical Properties.
Physical properties define the behavior of materials in
response to physical forces other than mechanical
Properties include: density, specific heat, melting point, thermal
expansion, conductivity, magnetic properties.
The manufacturing engineer should appreciate the design viewpoint
and the designer should be aware of the manufacturing viewpoint
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Manufacturing Processes
Mechanical Properties
Stress - Strain Relationships.
Strength
Ductility
Elasticity
Fatigue.
Creep.
Toughness
Hardness
These mechanical properties are usually determined by subjecting
prepared specimens to standard laboratory tests
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Manufacturing Processes
Stress – Strain Relationships
Three types of static stresses to which materials can be
subjected:
•
Tensile Stress - tend to stretch the material
•
Compressive Stress - tend to squeeze it
•
Shear Stress - tend to cause adjacent portions of
material to slide against each other
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Manufacturing Processes
Tensile Test
Most common test for
studying stress-strain
relationship, especially
metals
In the test, a force pulls the
material, elongating it and
reducing its diameter
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Manufacturing Processes
Tensile Test
ASTM (American Society for Testing and Materials)
specifies preparation of test specimen
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Manufacturing Processes
Tensile Test Machine
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Tensile Test
Typical tensile test progress
(1)
(2)
(3)
(4)
(5)
(6)
beginning of test, no load;
uniform elongation and reduction of cross-sectional area;
continued elongation, maximum load reached;
necking begins, load begins to decrease; and
fracture.
If pieces are put back together as in (6), final length can be measured
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Manufacturing Processes
Tensile Test Video Clip
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Tensile Test Video Clip
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Manufacturing Processes
Engineering Stress
Defined as force divided by original area:
F
e 
Ao
where
e = engineering stress;
F = applied force; and
Ao = original area of test specimen.
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Manufacturing Processes
Engineering Strain
Defined at any point in the test as
L  Lo
e
Lo
where
e = engineering strain;
L = length at any point during elongation
(instantaneous length); and
Lo = original gage length.
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Manufacturing Processes
Engineering Stress – Strain Curve in Tensile Test
It shows the basic relationship between stress and strain
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Manufacturing Processes
Engineering Stress – Strain Curve
Two Regions of Stress-Strain Curve
• The two regions indicate two distinct forms
of behavior:
1. Elastic region – prior to yielding of the material
2. Plastic region – after yielding of the material
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Manufacturing Processes
Elastic Region in Stress – Strain Curve
• Relationship between stress and strain is linear
• Material returns to its original length when stress is removed
• The material obeys Hooke's Law:
e = E e
where
E = modulus of elasticity (Young's modulus )
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Manufacturing Processes
Young’s Modulus
Young's modulus measures the resistance of a material
to elastic (recoverable) deformation under load .
Its value differs for different materials
A stiff material has a high Young's modulus and changes its shape
only slightly under elastic loads (e.g. diamond). A stiff material
requires high loads to elastically deform
A flexible material has a low Young's modulus and changes its
shape considerably (e.g. rubbers).
The stiffness of a component means how much it
deflects under a given load .
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Yield Strength in Stress – Strain Curve
As stress increases, a point in the linear
relationship is finally reached when the
material begins to yield.
– Yield Strength Y can be identified by the change
in slope at the upper end of the linear region
– Y = a strength property
– Other names for yield strength = yield point,
yield stress, and elastic limit
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Plastic Region in Stress – Strain Curve
Yield point marks the beginning of plastic
deformation.
The stress-strain relationship is no longer guided by
Hooke's Law .
As load is increased beyond Y, elongation proceeds
at a much faster rate than before, causing the slope
of the curve to change dramatically.
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Manufacturing Processes
Tensile Strength in Stress – Strain Curve
As the load is further increased, the engineering stress
reaches a maximum and then begins to decrease.
The maximum engineering stress is called the Tensile
Strength - TS (or Ultimate tensile Strength – UTS) of the
material.
TS =
Fmax
Ao
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Ductility in Tensile Test
Ability of a material to plastically deform without fracture
There are two common measures of Ductility

% Elongation
lf = specimen length at fracture; and
lo = original specimen length
l f  l0
l0
x100
lf is measured as the distance between gage marks after two pieces of specimen are put back together
% Reduction of Area

A0  Af
A0
x100
lf = final (fracture) cross-sectional area of the specimen; and
lo = original cross-sectional area of the specimen
Brittleness is simply the lack of significant ductility
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Mechanical Properties of Various Materials
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True Stress
True Stress is defined as the ratio of the applied load F
to the actual (instantaneous) cross-sectional area A of
the specimen.
F

A
where
 = true stress;
F = applied force; and
A = actual (instantaneous) area resisting the load
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Manufacturing Processes
True Stress
True Strain (natural or logarithmic strain) is calculated as
l
dl
l
    ln
l
l0
l0
where
 = engineering strain;
L = length at any point during elongation (instantaneous length); and
Lo = original gage length.
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Manufacturing Processes
True Stress – Strain Curve
If previous engineering stress-strain curve were
plotted using true stress and strain values
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Compression Test
Applies a load that squeezes the ends of a cylindrical
specimen between two platens
(1) compression force
applied to test piece; and
(2) resulting change in
height
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Manufacturing Processes
Compression Test Machine
Video
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Manufacturing Processes
Engineering Stress in Compression
As the specimen is compressed, its height is reduced
and cross-sectional area is increased
F
e 
Ao
where
F = applied force
Ao = original area of the specimen
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Manufacturing Processes
Engineering Stress in Compression
As the specimen is compressed, its height is reduced
and cross-sectional area is increased
F
e 
Ao
where
F = applied force
Ao = original area of the specimen
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Manufacturing Processes
Engineering Strain in Compression
Engineering strain is defined
h  ho
e
ho
where
h = height at any point during elongation (instantaneous); and
ho = original height of the specimen
Since height is reduced during compression, value of e is negative (the
negative sign is usually ignored when expressing compression strain)
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Manufacturing Processes
Engineering Stress – Strain Curve in Compression Test
Shape of plastic region is different from tensile test
because cross-section increases
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Shear Stress and Shear Strain
Application of stresses in opposite directions on either
side of a thin element
Shear Stress
Shear stress is defined as
Shear Strain
F

A
Shear Strain is defined as
where
F = applied force; and
A = area over which deflection occurs.
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 

b
Where
 = deflection element; and
b = distance over which deflection occurs
Manufacturing Processes
Shear Stress – Strain Curve
Typical shear stress-strain curve from a torsion test
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Manufacturing Processes
Shear Elastic Stress – Strain Relationship
In the elastic region, the relationship is defined as
  G
where
G = shear modulus, or shear modulus of elasticity
For most materials, G  0.4E
where
E = elastic modulus
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Shear Plastic Stress – Strain Relationship
shear strength S = Shear stress at fracture
– Shear strength can be estimated from tensile
strength: S  0.7(TS)
Since cross-sectional area of test specimen in torsion test does not
change as in tensile and compression, engineering stress-strain
curve for shear  true stress-strain curve
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Hardness
The ability of a material to resist scratching, wear and indentation.
• Good hardness generally means material is resistant
to scratching and wear
• Most tooling used in manufacturing must be hard for
scratch and wear resistance
• Commonly used for assessing material properties
because they are quick and convenient.
• Several methods have been developed to measure the
hardness of materials.
• Most well-known hardness tests are Brinell and Rockwell
and Vickers.
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Manufacturing Processes
Hardness Tests
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Brinell Hardness Test
Widely used for testing metals and nonmetals of
low to medium hardness
A hard ball is pressed into specimen surface with a load
of 500, 1500, or 3000 kg
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Brinell Hardness Test
Brinell Hardness Number (BHN) = Load divided into
indentation area
BHN 
2F
Db ( Db  D  D )
2
b
where
BHN = Brinell Hardness Number;
F = indentation load, kg;
Db = diameter of ball, mm; and
Di = diameter of indentation, mm
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Manufacturing Processes
Temperature Effect
General effect of temperature on strength and ductility
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Toughness
Toughness is an estimate of how much energy
is consumed before the material fractures.
Energy consumed = work done = force x distance
which you can easily see, is related to the stress and strain. So:
Toughness = the strain energy = area under the stress-strain curve
To compute toughness, True stress and True strain are
used, which measure the instantaneous stress.
The tensile test can provide a measure of this property.
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Manufacturing Processes
Creep
If a material is kept under a constant load over a long
period of time, it undergoes permanent deformation.
This phenomenon is seen in many metals and several
non-metals.
For most materials, creep rate increases with increase
in temperature.
The phenomenon does not have much direct implication
in manufacturing, but has significant use in design of
parts that, for example, carry a load permanently during
their use.
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Manufacturing Processes
Fatigue
Fatigue is the fracture/failure of a material that is subjected to
repeating cyclical loading, or cyclic stresses.
There are two factors: the magnitude of the loading and the number
of cycles before the material fails.
The behavior is different for different materials
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Manufacturing Processes
Machinability, Formability and Weldability
Machinability: depends not only on worked
material but on applied machining process (range
of meanings).
Formability (malleability, workability): materials
suitability for plastic deformation (depends on
process conditions).
Weldability: depends on particular welding (joining)
technique.
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Manufacturing Processes
Physical Properties Defined
• Properties that define the behavior of materials in
response to physical forces other than mechanical
• Includes: volumetric, thermal, electrical, and
electrochemical properties
• Components in a product must do more than simply
withstand mechanical stresses
• They must conduct electricity (or prevent
conduction), allow heat to transfer (or allow its
escape), transmit light (or block transmission), and
satisfy many other functions
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Manufacturing Processes
Physical Properties in Manufacturing
• Important in manufacturing because they
often influence process performance
• Example:
– In machining, thermal properties of the work
material determine the cutting temperature,
which affects how long tool can be used
before failure
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Manufacturing Processes
Thermal Expansion
Thermal expansion is the name for this effect of temperature
on manufactured part.
Measured by coefficient of thermal expansion 
Coefficient of thermal expansion is defined as change in length
per degree of temperature, such as mm/mm/C.
Change in length for a given temperature change is:
L2 - L1 = L1 (T2 - T1)
where
 = coefficient of thermal expansion;
L1 and L2 are lengths corresponding respectively to
temperatures T1 and T2
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Manufacturing Processes
Thermal Expansion in Manufacturing
• Thermal expansion is used in shrink fit
and expansion fit assemblies
– Part is heated to increase size or cooled to
decrease size to permit insertion into
another part
– When part returns to ambient temperature, a
tightly-fitted assembly is obtained
• Thermal expansion can be a problem in
heat treatment and welding due to
thermal stresses that develop in material
during these processes
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Manufacturing Processes
Melting Properties of Metals
• Melting point Tm of a pure element =
temperature at which it transforms from
solid to liquid state
• The reverse transformation occurs at the
same temperature and is called the
freezing point
• Heat of fusion = heat energy required at
Tm to accomplish transformation from
solid to liquid
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Manufacturing Processes
Importance of Melting in Manufacturing
• Metal casting - the metal is melted and then
poured into a mold cavity
– Metals with lower melting points are generally
easier to cast
• Plastic molding - melting characteristics of
polymers are important in nearly all polymer
shaping processes
• Sintering of powdered metals - sintering does
not melt the material, but temperatures must
approach the melting point in order to achieve
the required bonding of powders
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Manufacturing Processes
Specific Heat
• The quantity of heat energy required to
increase the temperature of a unit mass of
material by one degree
• To determine the energy to heat a certain weight of
metal to a given elevated temperature:
H = C W (T2 - T1)
Where
H = amount of heat energy;
C = specific heat of the material;
W = its weight; and
(T2 - T1) = change in temperature
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