m8.uf1.2 properties of materials 2

8/8/2019 M8.UF1.2 Properties of Materials 2

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1.- Properties of materials

Part II

Grau Superior de Disseny en Fabricaci

MecnicaM08 Materials (MATFA)UF1 Propietats dels materials

Professor: Llus Claps i Badia


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UF Index

1.1. Preliminary knowledgePreliminary knowledge2.2. Physical and chemical propertiesPhysical and chemical properties

3.3. Mechanical propertiesMechanical properties

1.1. Quick summaryQuick summary2.2. Understanding stress and strainUnderstanding stress and strain

3.3. The tensile testThe tensile test

4.4. PropertiesProperties

4.4. Manufacturing propertiesManufacturing properties

5.5. Other: cost, environmental, non tangibleOther: cost, environmental, non tangible

6.6. Material selection processMaterial selection process


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3.1 Quick summary

Mechanical properties:Mechanical properties: Strength

tensile1.Yield strength (Re)2.Ultimate strength (Rm)

compressive strength flexural strength torsional, shear

Stiffness - Elasticity (E) Elongation (ductility) (A) Hardness (H) Resilience & toughness (KV) DBTT Fatigue limit Creep

Fracture toughness (Kic)

...

Tests:Tests: Tensile test Compressive test Beaming or flexural test Shear test, bending test, torsional, .

Hardness test Rockwell Brinell Vickers Shore, knoop

Impact test Charpy (V-notch test) Izod

Fatigue test

Creep test ...


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3.2 Understanding

stress and strain

Types of mechanical load (when external forcesare applied to pieces):

TENSILE

(i.e. a cable)

TORSION(i.e. a screw driver)SHEAR(i.e. a pin or a key)

BENDING OR FLEXURAL

(i.e. a beam)

COMPRESSIVE

(i.e. a column)


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3.2 Understanding

stress and strain

Suppose a mechanicalcomponent

Load this componentapplying a 20.000 N (2tones) external force aspicture shows.

In this case, thecomponent is mainlysubjected to a tensile load(also bending and shearload exist).

2 tones

18 12


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3.2 Understanding

stress and strain

MPamm

N

mmmm

N

A

F

93216

000.20

1812

000.20

2===

=

WHAT WILL HAPPEN?:

Each particle of material issubjected to STRESS (tensi).

Engineering units are MPa

(N/mm). In the past Kg/mm (). 10 MPa ~ 1 Kg/mm

Knowing the loads, engineerscan calculate the stress:


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3.2 Understanding

stress and strain

WHAT WILL HAPPEN?:

The load deform the piece.Strain is the measure of thedeformation.

As much stress, as muchstrain.

The strain can be:

ELASTIC: if material returns toto the original form when theload disapears.

PLASTIC: if strain remainswhen the load disapears.

VISCOELASTIC,VISCOPLSTIC: when a lot oftime is needed for strainhappens.


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3.2 Understanding

stress and strain

Elasticdeformation

Plasticdeformation


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3.3 The tensile test

The most fundamental mechanicaltest you can perform on a material

A standard specimen is stretchedslowly and progressively until itbreaks.

Load (force) and strain ismeasured and recordedcontinuously during the test.

Its a destructive test

The specimen is only subjected totensile load.

Standards: UNE-EN 10.002-1(metals at ambient T), UNE-EN-ISO 527(plastics), ...

SPECIMEN


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Test will be done in a specialhydraulic machine (tensile oruniversal testing machine).

Force is measured with a load cell.

Strain is measured with aextensometer gauge. Usually will beenough measuring the strain whenspecimen is already broken.

Virtual test:http://www.engsc.ac.uk/an/mini_projects/tensile/tensile_laboratory.html
http://www.engsc.ac.uk/an/mini_projects/tensile/tensile_laboratory.htmlhttp://www.engsc.ac.uk/an/mini_projects/tensile/tensile_laboratory.html


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1. The machine has recorded, a great number ofdates:

FLOAD FORCE (N) lDEFORMATION (mm)

1. With calculation, we must change load to stressand deformation to strain.

1. And finally, draw a graphics thaty (vertical): STRESS (MPa)x (horiz.): ,A STRAIN (-, %)

100

0

0

=

=

=

Al

l

A

F

A0is the section of the specimen

l0 is the measuring lenght of the specimen.


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(MPa)

(%)

Breaking point

PLASTIC ZONE

ELASTIC

ZONEA strain

Re yield strength

Rm tensile strength

Corba real

Striction zone

Stress-strain curve

E Young modulus

0,2%


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3.4 Mechanical propertiesTensile strength, yield strength

RRmm TENSILE STRENGTH TENSILE STRENGTH ororUTSUTS (MPa)(MPa) (Resistncia a tracci)The highest stress in the stress-strain curve

RRee YIELD STRENTH YIELD STRENTH (MPa)(MPa) (Lmit elstic)The stress applied to a material at which plastic deformation starts to occur

Sometime is called proof stress Rp0,2 when offset method is used The most important property in structural materials. In brittle materials Rm = Re

Steel for cables

Re=Rm= 1.800 MPaStructural steel

Re = 275 MPa

Rm = 420 MPa

Nanotubes of carbon

Re=Rm~ 50.000 MPaCarbon fibres

Re=Rm~ 3.500 MPa


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STEEL

3.4 Mechanical propertiesYoung modulus

E YOUNGS MODULUS or ELASTIC MODULUSE YOUNGS MODULUS or ELASTIC MODULUS (MPa)(MPa)

It shows if a material is elastic or stiff.

The value of E is the slope of the stress-strain curve in the elastic zone.Its also the stress necessary to double the length of the specimen

stretching in elastic zone.

1.100.000 MPaRUBBER

+

-

DIAMOND

10 MPa

210.000 MPa

PP

1.500 MPa


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3. Propietats mecniques

3.6 Allargament

When this happened, the willow said to itself, I wish Iwas as strong as the Oak, instead of bending overwith every breeze then one day a large windstormwhipped through the field.

When the storm passed, and the darkness lifted, the

willow looked across the field, and was shocked todiscover that the oak was laying on the ground,broken. When the Gardener came into the field, thewillow said, Oh sir, what happened to the Oak? Howis it that I survived the storm, weak as I am, and theOak fell?

The Gardener said, Oh little willow-tree, do you not

understand what happened? When the winds blow,you bend with them, while the oak remains still. Sowhen a really powerful wind comes along, you canbend with the wind, and survive it. But the Oak cannotbend, and so if the wind is strong enough, it will break.For the Oak had a secret, a weakness within that noone looking at the outside could see. And the

Gardener went on his way, leaving the willow toponder what he said.

Moral: Strength within and strength without are not thesame, and one should cultivate strength within first.Also, when the winds of life blow, bend, and you maysurvive the real storms when they come. Try andresist them, and when the real storms come, you may

break instead.

In a field, there was an oak at one end, and awillow-tree at the other.

Whenever a wind moved through the field, thewillow swayed in the wind, while the oak remainedunmoved.


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3.4 Mechanical propertiesStrain

A STRAINA STRAIN (%)(%) (allargament)The ratio of the elongation to the original length (usually in %).

It shows if a material is ductile or brittle. Ductile materials can be shaped bydeformation processes.


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3.4 Mechanical propertiesCompressive strength, bending s.

In addition to the tensile test /tensile strength, there are othertests. Each type of load we saw(compressive, bending, torsionand shear) has its own test.

COMPRESSIVE TEST: speciallyused in ceramic materials forconstruction.

FLEXURAL TEST: We uses it forbrittle materials (we cant test itsby tensile test). With the flexuraltest results, its possible tocalculate the tensile strength.


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3.4 Mechanical propertiesToughness, resilience

RESILIENCE & KV - TOUGHNESSRESILIENCE & KV - TOUGHNESS (J/m(J/m)) (tenacitat)They show the capacity of a material to withstand impacts.RESILIENCE is the maximum amount ofenergy per area unit with only elasticdeformation. TOUGHNESS is the amount of energy per area unit necessary to break thematerial.Theoretically they are the area under the stress-strain curve (only the elastic zone inresilience and all the graphic in toughness case).

In practice, toughness is tested mainly with Charpy impact test (UNE 7475) or with Izodimpact test (less usual).

E

RRU

e

ee

2

2

12

==


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3.4 Mechanical propertiesToughness and temperature

DBTT (Ductile BrittleDBTT (Ductile BrittleTransition Temperature)Transition Temperature)

Many of the ductile materials(metals and plastics) turns tobrittle at low temperatures.

We can know the transitiontemperature testing a set ofspecimens at differenttemperatures with a Charpyimpact test.


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3.4 Mechanical propertiesHardness

H - HARDNESSH - HARDNESS (duresa)The resistance of a material to be penetrated or scratched.

The value depends of the type of test. Most usually tests and scalesare:

-HB, HBW (Brinell,

UNE-EN-ISO 6506-1)- HV (Vickers, UNE-EN-ISO 6507-1)

- HR_(Rockwell, UNE-EN-ISO 6508-1)

- HS_(Shore)

- HK (Knopp, microhardness)

Advantages of hardness test:1. Usually is a non-destructive test (although little traces rest in

material)

2. Testing machines are cheap and we can perform the tests veryquickly.

3. Tensile strength can be approached from hardness value.


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HARDNESS TESTS

Hardnesstester

BRINELL TEST(HB): Uses a carbide ball as penetrator.Shape is measured with a microscope..

VICKERS TEST(HV): Uses a diamond pyramidal penetrator.The shape is measured with a microscope. The most

universal hardness test (metals, ceramics, ).ROCKWELL TEST: Uses a steel or carbide ball (HRB) odiamond cone (HRC). The device measures the distancepenetrated. More quick and cheap than others.

SHORE TEST: It measures the bounce of a diamond-tipped

hammer. Applied mainly to rubber and plastic materials. Thedevice is known as scleroscope.

KNOOP TEST (HK): Microhardness, it can measure thinplates or coatings. Similar to Vickers.

FILE PENETRATION: Its a workshop quick test. If filepenetrates or not, material is more or less to 60 HRC


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CONVERSION

TABLESF

OR

STEELS

Quenche

d,tempered

Nottempered


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3.4 Mechanical propertiesFatigue resistance

FATIGUE LIFEFATIGUE LIFE (N)andand FATIGUE STRESS LIMITFATIGUE STRESS LIMIT (MPa)Fatigue is a failure of a material due to cyclic loading. The maximum stress of every cycle isless than the yield strength of the material.Fatigue is a stochastic process. Dates are given for a specified probability.Fatigue stress limit is the stress value necessary to fatigue damage occurs with a specifiednumber of cycles.Fatigue life is the number of stress cycles (of a specified character) that a specimen sustains

before failure occurs.Material performance against fatigue are commonly characterized by an S-N curve.


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Example of fatigue collapsed shaft:

2) Lamelar tearing(beaches)

3) Final

fracture

1) Start point ornucleation

DAMAGE

COLLAPSE


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The Norwegian rigAlexander Kiellandcollapse at the North Sea in1980.Of the 212 people aboard,123 died.A fatigue failure of a steelbracing was the cause of thedisaster. Fissure starts at apoor profile of a hydrophonefillet weld.


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3.4 Mechanical propertiesCreep

CREEPCREEP ((termoflunciatermofluncia)) Creep is a slow deformation of materials subjected to constant charges for long time,

even if stress is less than yield strength.

Creep depends on temperature. It occurs up to 30% of melting point in metals andup to 40-50% of melting point in ceramics.

Creep is evaluated with tests (1000 to 10.000 hours). The result of tests are drew ina graphic.

Larson-Miller graphic (stress-time-temperature)Strain versus time


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3.4 Mechanical propertiesCreep

Jet turbine and steam turbine blades caneasily have a creep failure. They arecontinuously subjected to centrifugal forceand high temperatures.

The movement of ice in a glacier flow is anexample of creep.

Creep occur in many plastics at room

temperature. Results of this are notorious indaily objects: The wardrobe shelves (wood + plastic glues)

always increase deformation with time

Plastic cable ties (Unex) always come loosewith time


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3.4 Mechanical propertiesToughness to fracture

KKITIT FRACTURE TOUGHNESSFRACTURE TOUGHNESS

Describes the ability of a material containing a crack, to resist the growing of thisfracture, avoiding a catastrophic failure in brittle fracture mode.

All the materials contain micro-fissures. There are a effect ofstress concentration inthis points.

Ceramics and very hardened metals, arent good structural materials in tensile loadbecause they present brittle fracture.

Ductile fracture:

Deformation + breakingBrittle fracture:

Only breaking


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3.4 Mechanical propertiesToughness to fracture

1944, Portland.

Suddenly, the USS T-2Schenectady , cracked in half.

When the accident happened,the ship were in port, in calmweather, and not charged. Thewater was cold.

It was de first accident of nearly800, involving the Liberty,Victory and Tanker T-2 shipclasses.

The cause of the failure was notunderstood at the time. Really,low-grade steel of hull became

brittle in low-temperature.Since then, naval engineershave to consider the brittlefracture in hull structuralmaterials and their welds.

m8.uf1.2 properties of materials 2

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