Üyd properties of material anadolu u n i v e r s i t y industrial engineering department 2 –...

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


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.


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


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


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


Spring 2005

Tensile Test

ASTM (American Society for Testing and Materials) specifies preparation of test specimen


Spring 2005

Tensile Test Machine


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


Spring 2005

Tensile Test Video Clip


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.


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.


Spring 2005

Engineering Stress – Strain Curve in Tensile Test

It shows the basic relationship between stress and strain


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


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 )


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.


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


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.


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.


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


Spring 2005

Mechanical Properties of Various Materials


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


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


Spring 2005

True Stress – Strain Curve

If previous engineering stress‑strain curve were plotted using true stress and strain values


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


Spring 2005

Compression Test MachineVideo


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


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)


Spring 2005

Engineering Stress – Strain Curve in Compression Test

Shape of plastic region is different from tensile test because cross‑section increases


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


Spring 2005

Shear Stress – Strain Curve

Typical shear stress‑strain curve from a torsion test


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


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


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.


Spring 2005

Hardness Tests


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


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


Spring 2005

Temperature Effect

General effect of temperature on strength and ductility


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.


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.


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.


Spring 2005

Machinability, Formability and Weldability


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


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


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


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


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


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


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

Üyd properties of material anadolu u n i v e r s i t y industrial engineering department 2 –...

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