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Folsom Dam Gate Failure, July 1995

Mechanical PropertiesMechanical Properties Why mechanical properties?Why mechanical properties?Why mechanical properties?Why mechanical properties?

Need to design materials that can withstand applied load…

e.g. materials used in building bridges that can hold up automobiles, pedestrians…

materials for skyscrapers

materials for and designing MEMs and NEMs…

Space elevators?

materials for space exploration…

NASA

Issues to address…Issues to address…Issues to address…Issues to address…

• Stress and strain

• Elastic behavior

• Plastic behavior

• Strength, ductility, resilience, toughness, hardness

• Mechanical behavior of different classes of materials

For quality control, the test should preferably be as

simple, rapid and inexpensive as possible

•For predicting product performance the more relevant

the test to service conditions the more satisfactory it is

likely to be.

• For producing design data, the need is for tests which

give material property data in such a form that they can

be applied with confidence to a variety of configurations.

•For investigating failures the first difficulty is to establish

what to look for and then the prime need is for a test

which discriminates well.

Why Mechanical Test ??

Simple tension

Simple shear

Common States of Stress

Simple compression:

Bi-axial tension:

Hydrostatic compression:

• Standard methods are vital in ensuring reliable data

• True in all fields – Thus ASTM & ISO

• If a property is to be claimed it must be backed by

1. a method and

2. statistical analysis ( means ,SD)

• For materials science and engineering, materials development must be supported by proper results.

2

Original cross sectional area

Engineering stress

• You need to be aware of the

relationship between stress and strain

• You need to understand what each of

the terms relating to the mechanical

properties of materials are

• Initial linear

• Non-linear

What sort of behaviour is this called?

And this?

• The slope of this linear part of the line is called

• the modulus of elasticity or Young’s Modulus and is given the symbol E

• In this region we say the

material behaves “elastically”

E

tensile

MetalsAlloys

GraphiteCeramicsSemicond

Polymers

Composites/fibers

E(GPa)

Based on data in Table B2,Callister 6e.Composite data based on reinforced epoxy with 60 vol% of aligned carbon (CFRE),aramid (AFRE), or glass (GFRE) fibers.

Young’s Modulus, E

0.2

8

0.6

1

Magnesium,Aluminum

Platinum

Silver, Gold

Tantalum

Zinc, Ti

Steel, Ni

Molybdenum

Graphite

Si crystal

Glass-soda

Concrete

Si nitrideAl oxide

PC

Wood( grain)

AFRE( fibers)*

CFRE*

GFRE*

Glass fibers only

Carbon fibers only

Aramid fibers only

Epoxy only

0.4

0.8

2

4

6

10

20

40

6080

100

200

600800

10001200

400

Tin

Cu alloys

Tungsten

<100>

<111>

Si carbide

Diamond

PTFE

HDPE

LDPE

PP

Polyester

PSPET

CFRE( fibers)*

GFRE( fibers)*

GFRE(|| fibers)*

AFRE(|| fibers)*

CFRE(|| fibers)*

Eceramics >Emetals

>>Epolymers

3

• This is the amount of

the force needed to

deform a material to

a point where it

cannot return to its

original shape.

• Once past the yield permanent or plastic

deformation occurs

• This is the stress used for design

• Half plane of atoms inserted into lattice

• distortion of lattice

14

• Slip without dislocations

requires high shear force

• high theoretical strength

• all bond in plane broken

at same time

15

• Slip with dislocations

• dislocation glides along slip

plane

• slip facilitated by stress in

lattice ∴ less force

• only one bond broken and

reformed during glide

• dislocation slips to crystal

(‘grain’) surface

Plastic deformation and the role of

dislocations

•dislocation free materials have much greater strength

•materials contain dislocations16

CuYield strength

800 MPaNo dislocations

80 MPaDislocations

Dislocation motion occurs most readily on

Close packed planes

•“smoothest surface for slipping

And close packed directions

•“smoothest surface for slipping

4

• Maximum possible engineering stress in tension.

• Metals: occurs when necking starts.• Ceramics: occurs when crack propagation starts.• Polymers: occurs when polymer backbones are

aligned and about to break. Adapted from Fig. 6.11, Callister 6e.

(Ultimate) Tensile Strength, σTS

Results and AnalysisResults and Analysis

6061-T651Aluminum

Cold Rolled1018 Steel

Copper

C2600 Brass,half hard

Annealed1018 Steel

Stre

ss (

Mpa

)

Strain (mm/mm)

Metals

Some snap

Some bend

5

• Ductile failure:--one piece--large deformation

• Brittle failure:--many pieces--small deformation

Figures from V.J. Colangelo and F.A. Heiser, Analysis of Metallurgical Failures (2nd ed.), Fig. 4.1(a) and (b), p. 66 John Wiley and Sons, Inc., 1987. Used with permission.

“No or little plastic deformation before fracture”

“A significant amount of plastic deformation before fracture”

• true stress at fracture

• elongation at fracture (ductility)

• an idea of the toughness

• an idea of the resilience

• Note: these are “engineering” stress & strain data, there is also “true” stress & strain

• Plastic tensile strain at failure:

Adapted from Fig. 6.13, Callister 6e.

Ductility or %Elongation

6

• From the previous graphs:

• Ductile fracture � necking

• Brittle fracture � no necking

0

100

200

300

400

500

600

0 0.05 0.1 0.15 0.2 0.25 0.3 0.35

Str

ess, M

Pa

Strain

UTS

E

% elongation

Yield

• Brittle

• Ductile

• Strong, not very

ductile

• One that

undergoes

considerably

plastic

deformation