(5-3-1) NPTEL - Properties of Materials at Cryogenic Temperature

8/10/2019 (5-3-1) NPTEL - Properties of Materials at Cryogenic Temperature

1/50

1

5


2/50

Earlier Lecture

Introduction to Cryogenics

Cryogens, Properties, T s diagram

Argon

Air

Nitrogen

Oxygen

Hydrogen

Helium Superfluid and its effects

2Prof. M D Atrey, Department of Mechanical Engineering, IIT Bombay


3/50

Outline of the Lecture

Title : Material Properties at Low Temperature

Structure of matter

Stress strain relationship

Mechanical properties of Metals and Plastics at

low temperature



4/50


Introduction

Properties of materials change, when cooled tocryogenic temperatures.

For example:

Rubber when quenched in LN2, it turns hardand breaks like a brittle material.

Wires made of materials like Nb Ti, exhibit

zero resistance when subjected to LHetemperatures (Superconductivity).


5/50


Introduction

The above examples show that material becomeshard and brittle at low temperature.

The electrical resistance decreases astemperature decreases.

Hence, a knowledge of behavior and propertieslike strength, ductility, thermal and electricalconductivities etc. of materials is necessary for

the proper design.


6/50


Structure of Matter

Solids are composed of atoms, which are boundtogether and are arranged in regular arrays.

Solids are broadly classified into two types

Metals

Non metalsFrom Cryogenicsperspective

Plastics

Glasses


7/50


Metals

Metals have a highly ordered structure.

The atoms are arranged in symmetrical crystallattices.

Most common of these lattice structures are

Face-Centered Cubic (FCC)

Body-Centered Cubic (BCC)

Hexagonal Close-Packed (HCP)


8/50


Lattice Structure of Metals

Face-Centered Cubic (FCC) An atom at each of the eight corners

and an atom at the center of each of the six faces. (Total = 14 atoms).

Body-Centered Cubic (BCC)

An atom at each of the eight cornersand one atom at the center of cube.

(Total = 9 atoms).


9/50


Hexagonal Close-Packed (HCP) An atom at each of the twelve corners, an

atom at the center of each of two verticalhexagonal ends and three atoms in waybetween the ends of prism. (Total = 17atoms).

The above lattice structures decides thenumber of slip planes in the crystal.

Slip planes are the directions within thecrystal in which the planes can slip ormove easily one over the other.

Lattice Structure of Metals


10/50


Slip Planes

Real crystals do not have perfect latticearrangements. There exists always some dislocationsdue to some imperfections.

The number of slip planes governs the movement of

dislocations and this governs the ductility and theimpact strength of any material.

The FCC structure has maximum number of slip

planes, while the BCC has the least. The HCPstructure falls in between the above two lattices.

As a result, the FCC solids are more ductile than theBCC and the HCP.


11/50


Properties of Materials

Sr. No. Property

1 Mechanical

2 Thermal

3 Electrical

4 Magnetic

From Mech Engg.perspective

From SCperspective


12/50

Stress strain curve


Stress(s)

Strain ( )

PL

When a ductilespecimen issubjected to a tensiletest, the stress strain relationship is

developed as shown.

Proportional Limit(PL) is the limit in

which the elongationof the specimen isdirectly proportionalto stress applied.

Young's

Modulus


13/50

Stress strain curve


Stress(s)

Strain ( )

D

E

F

G

PL

C

If this elongatingforce is removed,specimen regains itsoriginal shape elastic behaviour.

C is called as YieldPoint, the stress isYield Stress. The

point F is theUltimate TensileStress and G is thebreakage point.


14/50

Stress strain curve


Stress(s)

Strain ( )

D

E

F

GC

Brittle materials alsohave a ProportionalLimit (PL).

The stress strainrelationship for abrittle material is asshown (Dotted).

Stresses whenexceeded above PL,the brittle materialbreaks.

G

PL


15/50


16/50


Yield & Ultimate Strengths

Property Description

UltimateStress

It is the maximum nominal stressattained by a test specimen during a

simple tensile test.

YieldStress

It is the stress at which the strain ofa material shows a rapid increase

with an increase in stress, whensubjected to a simple tensile test.


17/50


Yield Strength

The yield strengthof variouscommonly usedmaterials increases

with decrease intemperature.

These materialsare normally alloys

of iron (steel) andaluminum etc.

050 100 150 200 250 300 350

200

400

600

800

1000

1200

1400

1600

YieldStrength,

MPa

304 Stainless Steel

9% Ni Steel

C1020 Carbon Steel

2024-T4 Aluminum

Temperature, K


18/50


Ultimate Strength Similar to the yield

strength, theultimate strength ofthe materials alsoincreases with

decrease intemperature.

Stainless steel has

the high strengthand is mostlypreferred atcryogenicapplications.

050 100 150 200 250 300 350

200

400

600

800

1000

1200

1400

1600

UltimateStren

gth,

MPa

304 Stainless Steel

9% Ni Steel

C1020 Carbon Steel

2024-T4 Aluminum

Temperature, K


19/50


The Ultimate and Yield strengths of the materiallargely depend on the movement ofdislocations.

At lower temperatures, the internal energy ofatoms is low.

As a result, the atoms of the material vibrate

less vigorously with less thermal agitation.



20/50


When these agitations are low, the movementof dislocations is hampered.

It requires a very large stress to tear the

dislocations from their equilibrium positions.

Therefore, materials exhibit high yield andultimate strengths at low temperatures.



21/50


Materials exhibit fatigue failure when they aresubjected to fluctuating loads.

These failures can happen even if the stress

applied is much lower than the ultimate stressvalues.

Fatigue strength of a material is the stress at

which the specimen fails after a certain numberof cycles.

Fatigue Strength


22/50


Fatigue Strength

The fatiguestrengthincreases as thetemperaturedecreases.

The fatiguestrength ofstainless steel is

higher as shownin figure.

50 100 150 200 250 300 350Fatigue

Strengtha

tE6Cycles,

MPa

0

200

400

600

800

1000

1200

1400

1600

304 Stainless Steel

Beryllium - Copper

C1020 Carbon Steel

Temperature, K


23/50


Any fatigue failure begins with a microcrackinitiation.

At low temperatures, a large stress is required

to stretch the crack due to increase in ultimatestrength.

Therefore, like the ultimate strength, the

fatigue strength increases as the temperaturedecreases.

Fatigue Strength


24/50


In order to avoid fatigue failure, when aspecimen is subjected to fluctuating loads, theworking stress is maintained below a certainvalue called as Endurance Limit.

Beryllium Copper alloy is used inmanufacturing of flexure bearings. The workingstress is kept below the endurance limit to

avoid fatigue failure.

Fatigue Strength


25/50


Charpy and Izod tests are used to measure theresistance of a material to impact loading.

The energy absorbed when the material isfractured suddenly by a force is the measure ofimpact strength.

In both these tests, the difference in the heightattained by the hammer pendulum after the

impact (loss in potential energy), determinesthe impact strength of specimen.

Impact Strength


26/50


Impact Strength

050 100 150 200 250 300 350

20

40

60

80

100

120

140

160

Charpy

ImpactStrength,

MP

a304 Stainless Steel9% Ni Steel

2024-T4 Aluminum

C1020 Carbon Steel

Temperature, K

In general, theimpact strength ofthe materialsdecreases withdecrease in

temperature.


27/50


Impact Strength

050 100 150 200 250 300 350

20

40

60

80

100

120

140

160

Charpy

ImpactStrength,

MP


2024-T4 Aluminum

C1020 Carbon Steel

Temperature, K

Few of the materialsexhibit Ductile toBrittle Transition(DBT) at lowtemperatures.

The temperature atwhich this occurs iscalled as Ductile to

Brittle TransitionTemperature (DBTT).


28/50


Impact Strength

050 100 150 200 250 300 350

20

40

60

80

100

120

140

160

Charpy

ImpactStrength,

MP


2024-T4 Aluminum

C1020 Carbon Steel

Temperature, K

Carbon steelsundergo DBT at thetemperatures around80 to 100 K.

This causes a suddendecrease in theimpact strength ofthe material at that

temperature.

This decrease is asshown by the Scurve in the figure.


29/50


Impact Strength

050 100 150 200 250 300 350

20

40

60

80

100

120

140

160

Charpy

ImpactSt

rength,

MP


2024-T4 Aluminum

C1020 Carbon Steel

Temperature, K

Hence, thesematerials cannot beused for cryogenicapplications.

Stainless steel ismost prefferedmaterial from theimpact strength point

of view.


30/50


The impact strength of a material is largelygoverned by its lattice structure.

At low temperatures, the materials with BodyCentered Cubic (BCC) lattice, break easily.

This is due to reasons mentioned earlier on theslip planes and movement of dislocations.

As a result, the materials with BCC lattice arenot preferred for low temperature applications.

Impact Strength


31/50


The materials with Face Centered Cubic (FCC)or Hexagonal lattice have more slip planes.

These slip planes assist in plastic deformation

(rather than breaking) and hence increase theimpact strength of material even at lowtemperatures.

The materials with FCC and HCP lattices arepreferred for cryogenic applications.

Impact Strength


32/50


A material which elongates more than 5% ofthe original length before failure is called asductile material.

When a specimen is subjected to simple tensiletest, ductility is given as the measure of

Percentage elongation in the length ofspecimen at the failure (or)

Percentage reduction in cross sectional areaof the specimen at the failure.

Ductility


33/50


In general, theductility of thematerials decreaseswith decrease in thetemperature.

The materials whichundergo DBT, are notpreferred due to the

decrease in theductility.

Ductility

%E

longationbeforefailure

050 100 150 200 250 300 350

20

10

30

40

Temperature, K

304 Stainless Steel

9% Ni Steel

2024-T4 Aluminum

C1020 Carbon Steel

50

60

70

80


34/50


For stainless steel, thepercentage elongationis around 30% at 0 K meaning fairlyductile for cryogenic

applications.

Ductility

%E

longationbeforefailure

050 100 150 200 250 300 350

20

10

30

40

Temperature, K

304 Stainless Steel

9% Ni Steel

2024-T4 Aluminum

C1020 Carbon Steel

50

60

70

80


35/50


Embrittlement at Low Temp


Ductile

FCC HCP

Cu, Ni Titanium

Cu-Ni alloys

Al & alloys

Aust. SS

Brittle

HCP BCC

Zinc Iron, Carbon

Molybdenum

Niobium

Most Plastics

Embrittlement of Structural Materials at LowTemperature.


36/50


Hardness is the measure of depth of thestandard indentation made on the surface of thespecimen by a standard indenter.

Common hardness tests inlcude Brinell test Vickers test Rockwell test

Hardness is directly proportional to the ultimatestress of a material. Hence, it follows the sametrend, i.e. increases as the temperature isdecreased.

Hardness


37/50


The three commonly used elastic moduli are

Youngs Modulus Shear Modulus

Bulk Modulus

With the decrease in temperature, thedisturbing vibrations and thermal agitation ofmolecules decrease.

These will increase the inter-atomic forces andthereby, reducing the strain at lowtemperatures.

Elastic Moduli


38/50


Elastic Moduli To produce same

strain at lowtemperature, greaterstress is required.

In other words, toproduce the samestress at lowtemperature, less

strain is required.

As a result theYoungs modulusincreases.

Stress(s)

Strain ( )

300 K100 K


39/50


Elastic Moduli

050 100 150 200 250 300 350

40

80

120

160

200

240

280

320

Young'sModu

lus,

GPa

Temperature, K

C1020 Carbon Steel9% Ni Steel

304 Stainless Steel

2024-T4 Aluminum

The Youngs modulusof various commonlyused materials is asshown in the adjacentfigure.

The elastic moduliincreases (slightly)with the decrease in

temperature.

All the three elasticmoduli follow the sametrend.


40/50


Non Metals (Plastics)

Plastics or polymers are made of long chains ofmolecules.

Each molecule has thousands of atoms heldtogether and arranged in tangled arrays.

The intermolecular forces that unite the differentpolymer molecules are van der Waals force.


41/50


Tensile Strength (Plastics) The strength of

various commonlyused plastics is asshown. The strengthincreases with the

decrease in thetemperature.

Of all the plastics,

PTFE (Teflon) is theonly one which canbe deformedplastically to a smalldegree at 4K.

050 100 150 200 250 300 350

34.48

68.96

103.44

137.92

172.4

206.88

241.36

275.84

UltimateStress,

MPa

Polyethylene terephthalate (Mylar)

Polytetrafluoroethylene (Teflon)Polytrifluoromonochloroethylene (Kel-F)Polyvinyl chlorideNylon

Temperature, K


42/50


Properties of Plastics

The effect of stress on plastics or elastomers isvery less as compared to metals.

These solids yield partly by uncoiling the longchain of molecules and sliding over one another.

This motion is facilitated by the thermal energypossessed by the molecules.

At low temperatures, material deformation ismore difficult due to decrease in thermal energy.


43/50

Summary

Prof. M D Atrey, Department of Mechanical Engineering, IIT Bombay 43

Stainless steel is the best material for thecryogenic applications.

Carbon steel cannot be used at low temperature asit undergoes a Ductile to Brittle Transition (DBT).

Ultimate and Yield strength, fatigue strength ofany material increase at lower temperature whileimpact strength, ductility decrease at lower

temperature.

PTFE (Teflon) can be deformed plastically at 4 K ascompared to other materials.


44/50


A self assessment exercise is given afterthis slide.

Kindly asses yourself for this lecture.


45/50

Self Assessment1. Non metals are classified into ____ and ______.

2. _________ forces unite the different polymermolecules in plastics.

3. The atoms in a metal are arranged in___________.

4. The Ultimate Strength of materials _____ with

decrease in temperature.

5. ______ metal is mostly preferred in cryogenicapplications.



46/50

Self Assessment6. Thermal agitation in molecules is _______

proportional to temperature.

7. ______ failure occurs when materials aresubjected to fluctuating loads.

8. ________ is used in manufacturing of flexurebearings.

9. _________ governs the impact strength of amaterial.

10. ________ property of the material decreaseswith decrease in temperature.



47/50

Self Assessment11. ________ material cannot be used at low

temperatures.

12. ______ crystal structures are most preferred incryogenic applications.



48/50

Answers


1. Glasses, Plastics2. Van der Waals

3. Symmetrical crystal lattice

4. Increases5. Stainless steel

6. directly

7. Fatigue

8. Beryllium Copper alloy

9. Lattice structure


49/50

Answers


10. Percentage elongation

11. Carbon steel

12. FCC


50/50

Thank You!

(5-3-1) NPTEL - Properties of Materials at Cryogenic Temperature

Documents

Transcript of (5-3-1) NPTEL - Properties of Materials at Cryogenic Temperature