Chapter 4 Year 3

8/13/2019 Chapter 4 Year 3

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CHAPTER 4 :

MECHANICALPROPERTIES OF

METALS


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TOPIC

1) TYPES OF MECHANICAL PROPERTIES

2) MECHANICAL TESTS OF METALS

3) HARDNESS TESTS 4) IMPACT TEST


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TYPES OF MECHANICAL

PROPERTIES

Defined as those properties which completely

define its behavior under action of external

load or forces. Those properties which associated with :

Its ability to resist failure

Its behavior under action of external forces

Knowledge of mechanical properties :

Essential for engineers in selecting suitable

materials for various applications.


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Mechanical

properties

Strength Ability to withstand or support external forces or load without rupture

Hardness Ability to resist deformation by abrasion, indentation or penetration and

scratching by harder bodies.

Ductility Ability to undergo appreciable plastic deformation before rupture.

Brittleness Ability to fracture when deformed . Opposite to ductility

Toughness Ability to absorb maximum energy up to fracture. Must be strong & ductile to be

tough. Shows ability to withstand impact. Value increase when temperature

increases.

Elasticity Ability to retain its original shape & size after removal of load

Plasticity Ability to experience permanent deformation without fracture when subjected to

external forces

Resilience Ability to absorb energy when it is elastically deformed

Malleability Ability to be deformed into thin sheets by rolling or hammering without fracture

Machinability Ability to be cut or removed by cutting tools in various ,machining operations.

Weldability Ability of 2 similar or dissimilar metals to be joined by fusion& with or without filler

Castability Ability to be formed into different shapes & sizes from its liquid state


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MECHANICAL TESTS OF

METALS

All mechanical properties of metal- established

by conducting tests on various testing

machines.

Types of mechanical test : Tensile test

Hardness test

Impact testCompressive test

Fatigue test

Creep test


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Tensile test

understand mechanical behavior by stress-strain test.

3 ways of load applicationtension, shear, compression

Most common mechanical stress-strain performed intension

Performed to determined ;

Elastic limit

Yield point

Ultimate tensile strength

% of elongation & reduction of area


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Standard tensile specimen

Circular cross section

Reduced section diameter = 12.8 mm

Reduce section length = 60mm

Gauge length = 50mm

Procedure :

Specimen held in holding grips of apparatus

Load applied gradually at a constant rate Specimen will be elongated until fracture

Elongationmeasured by extensometer


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o Data recordedload vs. elongation

o Normalized to engineering stress & engineering

strain

o Engineering stress :where : A0= original cross sectional area

before any load applied (m2)

F = instantaneous load (N)

o Engineering strain :where : li= original length

l0= instantaneous length


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Plot a graphstress vs. strain

OA is a straight line

Stress & strain proportional (Hookes Law)

where E = modulus of elasticitySlope corresponds to E

Called elastic deformation

When applied load is released, specimen returnsto its original shape.


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AB is a small curve

A is a point where elastic deformation end and

plastic deformation begin

Phenomenon of yielding occurs Called proportional limit

Initial departure from linearity

To determine yielding point precisely

Construct straight line parallel to elastic portion ofcurve at a specified strain offset (0.002)

B is the intersection of parallel line with curve

The stress defined as a yield strength,


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behavior for some steels

Elastic-plastic transition

Well defined

Occurs abruptly

Yield point phenomenon

Upper yield pointplastic deformation initiated.

decrease

Lower yield pointdeformation fluctuated at constant Yield strength , average of lower yield point


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BC is an upward curve

Stress increase to maximum limit (point C)

Called tensile strength , TS

Corresponds to maximum that can be

sustained by a structure in tension .

Necking begin to form- decrease of cross-

sectional area


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CD is a downward curve

Specimen continues to elongate

Requires lesser load to continue deformation.

decrease.D is a point of fracture or rupture

True stresswhere : A

i= instantaneous cross sectional area

True Strainwhere : li = instantaneous length


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Ductility

Measure degree of plastic deformation that has

been sustained at fracture

Can be expressed as :% elongation

% area reduction

% elongation (% E)where : lf= length after fracture

lo= original length


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% area reduction (%AR)

where : Af= cross sectional area at

fracture

Brittle material

Has little or no plastic deformation upon fracture

Has less than 5% elongation


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Hardness test

Measure hardness by forcing an indenter into

materials surface.

Indentermade of harder material

- usually in form of ball. pyramid or

cone

Early hardness test

Done by comparing with 10 standard mineral

Increasing hardness on Mohsscale

1) talc 6) orthoclase2) Gypsum 7) quartz

3) Calcite 8) topaz

4) Fluorite 9) corundum

5) Apatite 10) diamond


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Important test

Brinell Hardness Test

Rockwell Hardness Test

Viekers Hardness Test

Knoop Hardness Test


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Brinell Hardness Test

By Dr Johan August Brinell in 1900

Performed by pressing steel ball into surface of test

pieces using appropriate force.

Formula : Brinell Hardness Number , HB (or BHN)

Where D = diameter of steel ball (mm)

d = diameter of indentation (mm

P = applied load (kg)


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Rockwell Hardness Test

Devised in the USA

The most common method:

Simple to perform

Require no special skills Quick & direct reading

Performed when hardness is beyond range of Brinells

load is smaller than Brinells


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Viekers Hardness Test

The most accurate test

Has continuous scale of hardness (10 to 1000)

Indenter

Diamond square based pyramid with 136 angle

between opposite faces.

Load

Smaller than Rockwell & Brinell Between 1 and 1000 g

Suitable for:

Small, thin selected specimen region


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Where P = applied load (kg)

d = length of diagonal (mm)


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Knoop Hardness Test

Very much similar to Vickers

Indenter :

Diamond pyramid with short depth and diagonal

in ratio in ratio of 7:1

Measure diagonal length under microscope.

Knoop hardness number designated by HK

Where = longest diagonal

length


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Hardness scale for Knoop & Vickers

Approximately equivalent

Both are referred as micro hardness testing

Basis of reload & indenter size

Suitable for testing

Brittle materials (ceramics)

Extremely thin metalExceptionally hard, very shallow carburized or

nitride surface


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IMPACT TEST

Many machine parts are subjected to suddenapplied loadsimpact loads

Important engineering wise to have :

Material that can withstand impact load withoutfracturing

A hard, strong may not be suitable when

subjected to sharp sudden load.

Capacity of metals to withstands impact

without fracture

Impact resistance or impact strength

Indication of toughness


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Method of measuring toughness

Impact- testing apparatus

Types of impact-testing apparatus

Charpy

Izod


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Question

A cylindrical specimen of metal having a diameter of 12.88 mm and a gauge length

of 63.50 mm is tested using a tensile testing machine. The elongation measurement

are recorded in Table 3.

i) Plot the stress-strain curve on the graph paper provided based on data in

Table 3

ii) Based on the stress-strain curved plotted in (i) :

(1) Compute the modulus of elasticity

(2) Determine the yield strength at a strain offset of 0.002.(3) Determine the tensile strength

Determine the ductility in percent elongation and percent area reduction


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Chapter 4 Year 3

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