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ME 494 Failure Modes of Engineering Materials

Sec. 6-1 Failure Theory Pt. 1 Page 1

CASE STUDY #6-1 Let’s Compare

Aluminum and

DETAILS:

Components (Beams)

How does the shape of the beams differ?

Why use rivets instead of welding aluminum?

What would explain these differences?

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FAILURE THEORY FOR STATIC LOADING

Failure

Theory

State of

σ and ε

for any case

UniaxialTest

Results

MacroscopicMaterial

Behavior

MicroscopicMaterial

Behavior

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TENSILE

BEHAVIOR of

METALS

Region 1-Elastic

Behavior

Interest in Design:

Recoverable deformation

Design considerations: Physical property, E, G and ν (i.e.

all steels have same stiffness )

Primary control from geometry

Region 2-Limited Plasticity

Interest in Design:

Limit for elastic behavior, SP or SY

Localized yield

Design considerations:

Processing (cold work, alloying, precipitation hardening )

Trade off between strength and ductility

Region 3-Large Scale Plasticity (or NOT)

Interest in Design:

Forming operations

Energy absorption (toughness, collapse load )

Design considerations:

Toughness is a compromise between ductility and strength

Avoid loss of toughness (brittle behavior )

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MICRO (& MACRO ) SCOPIC BEHAVIOR

DUCTILE vs. BRITTLE

BEHAVIOR

Material

under stress

can either

YIELD or

CLEAVE

CLEAVAGE

Fracture across atomic interface

Driven by NORMAL STRESS (σ)

Usually related to BRITTLE behavior

Limited necking

YIELD

Slip – movement of dislocations

Driven by SHEAR STRESS (τ)

Usually related to DUCTILE behavior

Necking before fracture

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DUCTILE BEHAVIOR

Most metals have crystal lattice structure (BCC,

FCC, HCP )

Ductile metals will SLIP

before CLEAVING

The Metallurgists were confused:

The theoretical slip strength of IRON is 1,500 ksi

The measured slip strength of IRON is 3 ksi

How can this be explained????? DISLOCATIONS!!!!

Proposed in 1934, Orowan, Polanyi and Taylor

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On microstructural level

plastic deformation is due to

movement of dislocations

Dislocations are

imperfections in the lattice

structure (edge or screw type)

Dislocation movement is

driven by shear stress (τ)

Dislocation continuesto move until it

reaches grain

boundary or material

surface

On macroscopic scale

dislocation movement is seen as yielding or plastic

deformation

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Most pure metals are very ductile

Metal is strengthened by Pinning Dislocations

Pinning of dislocations increases:

Hardness

Yield strength

Ultimate tensile strength

Fatigue strength (Up to a point!! )

But decreases:

Ductility

Absorption of

mechanical energy

BRITTLE BEHAVIOR

Reasons for BRITTLE BEHAVIOR:

Inherent characteristics (ceramics, cast iron )

Excessive cold work or alloying elements (carbon

in iron )

High strain rates or Low temperatures

(dislocation movement is restricted ) Environment (Stress Corrosion Cracking,

Hydrogen Embrittlement, Intergranular corrosion )

Presence of crack-like defect-Fracture mechanics

(Last stage of fatigue failure )

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WHAT IS FAILURE?

BRITTLE MATERIAL

NOTE: Fracture surface shows CLEAVAGE fracture▲:

Flat smooth surfaces

Sharp edges of fracture planes

DUCTILE MATERIAL

NOTE: Fracture surface shows DUCTILE failure▲:

Dimples from coalescence of micro-voids

Sharp edges from final plastic failure of ligaments

http://oregonstate.edu/instruct/engr322/Homework/Previous/S09/ENGR322HW7.html

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Untested SpecimenGray Cast iron

7075-T651 Aluminum

Compression

Tension

CASE STUDY #6-1 Aluminum and

MACROSCOPIC BEHAVIOR

What does this tell you?

Gray Cast Iron

2024 T351 Aluminum

Torsion Test

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X500

X90

X1.5x104

MICROSCOPIC BEHAVIOR

Microstructure

(Aluminum & )

Response to loading: Ductile or Brittle behavior?

Failure by cleavage or slip?

A. Pearlite: Laminations

of carbide & iron

B. Ferrite: Relatively pure

iron

C. Graphite Flakes: high

level of carbon

X500

X100

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1060-O UTS = 12.5 ksi

YS = 3 ksi RA = 91 %

Gray Iron Tens. Strength Comp. Strength

ASTM Class # (ksi) (ksi)

20 22 83

30 31 109

40 42.5 140

50 52.5 164

60 62.5 187

TENSION TEST: Stress-Strain Behavior

Aluminum

What does this tell you?

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Behavior: Gray

QUESTION: What are DARK GRAY areas on

fracture surface?

Fracture

Surface

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Behavior: Aluminum

High magnification

micrograph of central

region of cup-and-cone

failure

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1060-O UTS = 12.5 ksi

YS = 3 ksi RA = 91 %

Shotgun Quiz

How do you:

Make Gray less brittle?

Make Aluminum less

ductile?

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To increase ductility change

shape of graphite zones

vs. DUCTILE IRON

DUCTILE IRON

Fracture Surface Failed Torsion Specimen Tension Test Results

A. Pearlite: Laminations

of carbide & iron

B. Ferrite: Relatively pure

iron

C. Graphite Flakes: high

level of carbon

Elongation: 0.06%

X500

D. Pearlite: Laminations

of carbide & iron

E. Ferrite: Relatively pure

iron

F. Graphite Spheres: high

level of carbon

Elongation: 18%

X500

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To increase strength pin dislocations

Tension Test 2024 Aluminum

Tension Test Pure Aluminum

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Shotgun Quiz SOME QUESTIONS:

Is a BRITTLE material bad?

Are there advantages to

BRITTLE behavior?

Is a DUCTILE materialgood?

Are there disadvantages

to DUCTILE behavior?

Can we have it both ways?

Failure of case

hardened shaft

http://www.engr.sjsu.edu/wofmate/failshaft.htm

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494 Lec 21 FailThry1