Üyd properties of material anadolu u n i v e r s i t y industrial engineering department 2 –...
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ÜYD
Properties of Material
ANADOLU U N I V E R S I T YIndustrial Engineering Department
2 – Properties of Materials
Spring 2005
Saleh AMAITIK
Manufacturing Processes
Spring 2005
Properties of Materials
PROPERTIES
PERFORMANCE
STRUCTURE
PROCESSING
Manufacturing Processes
Spring 2005
Properties of Materials
Mechanical Properties.
Physical Properties.
Properties include: Strength, ductility, hardness, elasticity, toughness, creep, fatigue …etc.
The manufacturing engineer should appreciate the design viewpoint
and the designer should be aware of the manufacturing viewpoint
Mechanical properties are useful to estimate how parts will
behave when they are subjected to mechanical loads (stresses)
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.
Manufacturing Processes
Spring 2005
Mechanical Properties
Stress - Strain Relationships.
Strength ElasticityDuctility
HardnessToughness
Fatigue.
Creep.
These mechanical properties are usually determined by subjecting
prepared specimens to standard laboratory tests
Manufacturing Processes
Spring 2005
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
Manufacturing Processes
Spring 2005
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
Manufacturing Processes
Spring 2005
Tensile Test
ASTM (American Society for Testing and Materials) specifies preparation of test specimen
Manufacturing Processes
Spring 2005
Tensile TestTypical tensile test progress
(1) beginning of test, no load;
(2) uniform elongation and reduction of cross‑sectional area;
(3) continued elongation, maximum load reached;
(4) necking begins, load begins to decrease; and
(5) fracture.
(6) If pieces are put back together as in (6), final length can be measured
Manufacturing Processes
Spring 2005
Engineering Stress
Defined as force divided by original area:
oe A
F
where
e = engineering stress;
F = applied force; and
Ao = original area of test specimen.
Manufacturing Processes
Spring 2005
Engineering Strain
Defined at any point in the test as
o
o
LLL
e
where
e = engineering strain;
L = length at any point during elongation (instantaneous length); and
Lo = original gage length.
Manufacturing Processes
Spring 2005
Engineering Stress – Strain Curve in Tensile Test
It shows the basic relationship between stress and strain
Manufacturing Processes
Spring 2005
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
Manufacturing Processes
Spring 2005
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 )
Manufacturing Processes
Spring 2005
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.
Manufacturing Processes
Spring 2005
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
Manufacturing Processes
Spring 2005
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.
Manufacturing Processes
Spring 2005
Tensile Strength in Stress – Strain Curve
TS = oA
Fmax
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.
Manufacturing Processes
Spring 2005
Ductility in Tensile Test
Ability of a material to plastically deform without fracture
1000
0 xl
ll f
There are two common measures of Ductility
% Elongation
% Reduction of Area 1000
0 xA
AA f
lf = specimen length at fracture; and lo = original specimen lengthlf is measured as the distance between gage marks after two pieces of specimen are put back together
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
Manufacturing Processes
Spring 2005
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.
A
F
where
= true stress;
F = applied force; and
A = actual (instantaneous) area resisting the load
Manufacturing Processes
Spring 2005
True Stress
True Strain (natural or logarithmic strain) is calculated as
where
= engineering strain;
L = length at any point during elongation (instantaneous length); and
Lo = original gage length.
0
ln0
l
l
l
dll
l
Manufacturing Processes
Spring 2005
True Stress – Strain Curve
If previous engineering stress‑strain curve were plotted using true stress and strain values
Manufacturing Processes
Spring 2005
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
Manufacturing Processes
Spring 2005
Engineering Stress in Compression
As the specimen is compressed, its height is reduced and cross‑sectional area is increased
oe A
F
where
F = applied force
Ao = original area of the specimen
Manufacturing Processes
Spring 2005
Engineering Stress in Compression
As the specimen is compressed, its height is reduced and cross‑sectional area is increased
oe A
F
where
F = applied force
Ao = original area of the specimen
Manufacturing Processes
Spring 2005
Engineering Strain in Compression
Engineering strain is defined
o
o
h
hhe
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)
Manufacturing Processes
Spring 2005
Engineering Stress – Strain Curve in Compression Test
Shape of plastic region is different from tensile test because cross‑section increases
Manufacturing Processes
Spring 2005
Shear Stress and Shear Strain
Application of stresses in opposite directions on either side of a thin element
Shear Stress Shear Strain
AF b
Shear stress is defined as Shear Strain is defined as
where F = applied force; and A = area over which deflection occurs.
Where
= deflection element; and
b = distance over which deflection occurs
Manufacturing Processes
Spring 2005
Shear Stress – Strain Curve
Typical shear stress‑strain curve from a torsion test
Manufacturing Processes
Spring 2005
Shear Elastic Stress – Strain Relationship
In the elastic region, the relationship is defined as
Gwhere
G = shear modulus, or shear modulus of elasticity
For most materials, G 0.4E
where E = elastic modulus
Manufacturing Processes
Spring 2005
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
Manufacturing Processes
Spring 2005
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.
Manufacturing Processes
Spring 2005
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
Manufacturing Processes
Spring 2005
Brinell Hardness Test
Brinell Hardness Number (BHN) = Load divided into indentation area
)(
222ibbb DDDD
FBHN
where
BHN = Brinell Hardness Number;
F = indentation load, kg;
Db = diameter of ball, mm; and
Di = diameter of indentation, mm
Manufacturing Processes
Spring 2005
Temperature Effect
General effect of temperature on strength and ductility
Manufacturing Processes
Spring 2005
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.
Manufacturing Processes
Spring 2005
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.
Manufacturing Processes
Spring 2005
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
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.
Manufacturing Processes
Spring 2005
Machinability, Formability and Weldability
Manufacturing Processes
Spring 2005
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
Manufacturing Processes
Spring 2005
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
Manufacturing Processes
Spring 2005
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
Manufacturing Processes
Spring 2005
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
Manufacturing Processes
Spring 2005
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
Manufacturing Processes
Spring 2005
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
Manufacturing Processes
Spring 2005
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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