Engineering and Polymeric Materials

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Mechanical analysis

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DYNAMIC MECHANICAL

ANALYSER DMAis a measuring instrument which is used to

determine the dynamic characteristics of materials.

It applies a dynamic oscillating force to a sample andanalyzed the materials response to that cyclic force.

Basically, DMAdetermines changes in sample

properties resulting from changing in five

experimental variables: temperature, time, frequency,

force, strain.

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CONSTRUCTION OF DMA

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DYNAMIC MECHANICAL

ANALYSER

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DYNAMIC MECHANICAL

ANALYSER

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UNIVERSAL TESTING MACHINE

Mechanics Of Materials:

It is a branch of mechanics that develops therelation between external loads applied to adeformable body and intensity of internal forcesacting with in the body as well as the deformations of

the bodyStress

Strain

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Test FUNCTIONS

Tensile test

Compression test Shear

Bending

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CREEP

Time dependent deformation due to constant

stress at high temperature. It is the tendency of the material to move slowly or

deform permanently under the influence ofmechanical stresses.

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TYPES OF CREEP Instantaneous Creep: Mainly Elastic

Primary / Transient state Creep: Slope of strain vs.time decreases with time: Work hardening

Secondary / Steady state Creep: Rate of strainingconstant : work hardening and recovery

Tertiary Creep:Rapidly accelerating strain rate up to

fracture :Formation of internal cracks , grain boundaries

,separation and necking.

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CREEP: TEMPERATURE EFFECTS

AND STRESS

Instantaneous strain increases

Steady state creep rate increases

Tertiary state decreases time to rupture

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TERTIARY CREEP

The creep rate begins to accelerate as the cross

sectional area of the specimen decreases due tonecking or internal voiding allowed to proceed,the fracture will occur.

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MECHANISM OF CREEP

It includes certain mechanism:

o Dislocation Glideo Dislocation Creep

o Diffusion Creep

o Grain boundary sliding

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DISLOCATION GLIDE Dislocation Glide:

It involves dislocation moving along slip planesand overcoming batteries by thermal activation. Thismechanism occurs at high stress levels.

Dislocation Creep:

It involves the movement of dislocation which

overcome barriers by thermally assisted mechanismsinvolving the diffusion of vacancies or interstitials.

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Diffusion Creep:

It involves the flow of vacancies and interstitialsthrough crystal under the influence of applied stress .It occurs at high temperatures and low stress levels.

Grain boundary sliding:

It involves the sliding of grains past each other.

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CREEP TEST It is usually employed to determine the minimum

creep rate in secondary state. Like the creep test , stress

rupturing test , involves a tensile specimen under aconstant load at a constant temperature. The

difference being , stress rupture testing uses high

stresses and is always continued until failure of the

material occurs. The stress rupture test is used to

determine the time to failure and elongation.

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HARDNESS TESTING Resistance of metal to plastic deformation, usually by

indentation.

Refers to stiffness or temper, or to resistance toscratching, abrasion, or cutting.

The greater HARDNESSof the material, the greaterresistance is required to deform it.

Measurement can be defined as micro, macro or nanoscale according to force applied.

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MEASUREMENTS METHODS Four types of tests used by metal industry,

o Brinell hardness test: 10mm diameter ball with a loadof 500, 1000 or 3000kg.

o Rockwell hardness test:A core shape indenter, thedepth of penetration is measured.

oVickers hardness test: Pyramid shape indenter

o Knoop hardness test: Determine the micro hardness.

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BRINELL HARDNESS TEST

It is defined by forcing a hard sphere of a specifieddiameter under a specified load into the surface of amaterial and measuring the diameter of the

indentation left after the test.

The Brinell hardness number, or simply the Brinell

number, is obtained by dividing the load used, inkilograms, by the actual surface area of theindentation, in square millimeters.

Load is applied for 10 to 15 seconds.

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BRINELL HARDNESS TEST It consist of:

oWire basket

o

Handleo Throat

o Standard Brinell ball

o Test head

o Pressure lever

o Pressure gauge

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ROCKWELL HARDNESS TESTING It is a HARDNESS TEST, measurements based on the

net increase in the depth of impression as a load isapplied.

Two types of Rockwell hardness testing:

o Rockwell testing: Minor load is 10kg and major load(60, 100, or 150kg) is used regardless of the type of

indenter.o Rockwell superficial testing: Minor load is 3kg and

major load (15, 30,or 45kg) are used.

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VICKERS HARDNESS TESTINGA standard method for measuring HARDNESSof hard

surfaces.

It is a measure of the hardness of a material, calculatedfrom the size of an impression produced under load bya pyramid-shaped diamond indenter.

The diagonal of the resulting indention is measured

under a microscope.

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VICKERS HARDNESS TESTING This method indent the material in the form of

pyramid with an angle of 136 between opposite faces.

The load is applied for 10 to

15 seconds.

It can b used for broad

hardness range and suitable

for small particles.

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KNOOP HARDNESS TESTING

The relative MICRO HARDNESSof material canb determined by this method.

In this method, a pyramid shaped diamondindenter with apical angle of 130 is pressedagainst material.

The HARDNESSof the material is determined by

the depth to which the knoop indenterpenetrates.

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FATIGUE TESTING In materials sciences, FATIGUEis the weakening of a

material caused by repeatedly applied load.

Progressive and localized structural damage thatoccurs when a material is subjected to cyclic loading.

The maximum stress that cause such damage may lessthan the strength of the material as Ultimate Tensile

Stress Limit or Yield Stress Limit.

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FATIGUE TESTINGA method to predict the FATIGUElife of materials is

UNIFORM MATERIAL LAW(UML).

Thismethod(UML)was developed for FATIGUElifeprediction of Aluminum and Titanium alloys by theend of 20thcentury.

It is extended to high strength steel and cast iron.

It is also measured by DMA (Dynamic MechanicalAnalysis)

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FATIGUE TESTING It occurs when a material is subjected to repeated

loading and unloading.

If the loads are above a certain threshold, microscopiccracks will begin to form at the surface etc. Eventuallya crack will reach a critical size and suddenly the

structure will fracture.

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FACTOR CAUSING FATIGUE FAILURE

A maximum tensile stress of sufficiently high value.

A large amount of variation or fluctuation in applied

stress.A sufficiently large number of cycles of the applied

stress.

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FATIGUE LIMITA value below which the material will not fail for any

number of cycle.

FATIGUE is usually related with tensile stress butFATIGUEcracks have reported due to compressiveloads.

Its life is influenced by many factors such astemperature, surface finish, metallurgicalmicrostructure and oxidizing agents etc.

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FATIGUE TESTING

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STRESS RELAXATION It describes how polymers relieve stress under constant

strain. Because they are viscoelastic, polymers behavein a nonlinear fashion.

This fashion is described by both STRESSRELAXATIONand a CREEP, which describes howpolymers strain under constant stress.

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PARAMETERS AFFECT STRESS

RELAXATION Nonmaterial parameters that affects are:

o Magnitude of initial loading

o Speed of loading

o Temperature (isothermal/non-isothermal conditions)o Loading medium

o Friction and wear

o

Long-term storage

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STRESS RELAXATION Constant strain is applied

Stress releases as a

function of time.

STRESS RELAXATION

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STRESS RELAXATION Temperature also affect the STRESS RELAXATION of

polymer.

oWhen the temperature is greater, then chain mobilityis high and stress is relaxed rapidly.

oWhen the temperature is lower, then chains are frozen

and stress relaxation is slow and sometimes isnegligible.

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IZOD CHARPY TESTING The Izod test consists of a pendulum with a

determined weight at the end of its arm swingingdown and striking the specimen while it is held

securely in a vertical position. The impact strength isdetermined by the loss of energy of the pendulum asdetermined by precisely measuring the loss of heightin the pendulum's swing.

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IZOD CHARPY TESTINGAt WMT&R Izod and CHARPY TESTING are

performed at temperatures ranging from -320F toover 2000F.

The Izod test, like the CHARPY TEST,is also used totest materials at low temperature to try to simulateconditions that may occur in the actual use of thematerial.

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IZOD CHARPY TESTING The Ductile to Brittle Transition Temperature may be

obtained by testing a number of identical specimens atdifferent temperatures, and then plotting the impact

energy as a function of temperature, the ductile-brittletransition becomes apparent as the resulting curveshows a rapid decline in impact strength as thetemperature increases.

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IZOD CHARPY TESTING

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DART IMPACT TESTING Impact test is of enormous importance.

A collision between two objects can often result indamage to one or both of them

The damage might be a scratch crack or break.

It is an important property.

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IMPACT RESISTANCEWhen two objects collide damage is often done

How well something resists damage is called its impactresistance

An impact test measures how much energy isabsorbed when an object breaks under high speedcollision

The safety of many consumer products depends ontheir resistance to breaking

But impact resistance is difficult to quantify.

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TOUGHNESS NOT HARDNESS Impact test is about resisting impact

This is often called a materials toughness

It is the amount of energy a material can absorbbefore fracturing.

Units joules per meter cube.

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DART IMPACT TESTING

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