C h a p t e r 8

C h a p t e r 8

Failure

11/15/2015 7:27 AM

Dr. Mohammad Abuhaiba, PE1

Home Work Assignment

13, 18, 26, 30Due Tuesday 17/11/2015

4th exam on 29/11/2015

11/15/2015 7:27 AM

Dr. Mohammad Abuhaiba, PE

2

8.6 Fracture Toughness Testing

Impact test = used to evaluate

brittleness of a material under

conditions of high strain rates (103

mm/mm.s).

Two common tests:

Izod test: for plastic materials

Charpy

Units of Charpy: J

Units of Izod : J/m

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From amount of swing of pendulum, energy

dissipated in breaking specimen can be

obtained.

This energy is the impact toughness of

material.

Impact Toughness: ability of a material to

withstand an impact blow.

Fracture Toughness: ability of a material

containing flaws to withstand applied load.

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8.6 Fracture Toughness TestingDuctile to Brittle Transition Temperature

DBTT: temperature at which a

material changes from ductile to

brittle fracture

BCC metals have transition

temperatures

Most FCC metals do not

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FCC metals have high absorbed

energies, with the energy

decreasing gradually as the

temperature decreases.

Notch sensitivity: absorbed energies

are much lower in notched

specimens if the material is notch

sensitive.

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Relationship to stress-strain diagram:

Energy required to break a material during

impact testing is not always related to tensile

toughness.

Materials that have high impact resistance

are generally those that have high strength

and high ductility, and hence, high

toughness.

Metals that show excellent tensile toughness

may show a brittle behavior under high strain

rates.


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Figure 8.16:

Influence of

carbon

content on

Charpy V-notch

energy vs.

temperature

behavior for

steel.

8.7 Cyclic Stresses

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8.8 The S–N CurveRotating Cantilever Beam Test

Test is carried out at various stress amplitudes (Sa).

Stress amplitude = max stress

Number of cycles (N) it takes to cause totalfailure of specimen or part is recorded.

Curves are based on complete reversal of stress.

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3 3 3

32 32 10.19

M FL FLMax Stress

d d d

8.8 THE S–N CurveRotating Cantilever Beam Test

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Endurance limit: Max stress to which material

can be subjected without fatigue failure,

regardless of # of cycles

8.8 THE S–N CurveRotating Cantilever Beam Test

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Fatigue life: how long a component survives

at a particular stress

8.8 THE S–N CURVERotating Cantilever Beam Test

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Fatigue

strength: max

stress for which

fatigue will not

occur within a

particular

number of

cycles

8.8 THE S–N CURVERotating Cantilever Beam Test

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For carbon

steels,

endurance limit

is usually 0.4-0.5

UTS

Most materials

are notch

sensitive

8.9 Crack Initiation and Propagation

Fatigue failures typically occur in 3 stages:

1.Crack initiation at surface

2.Crack propagation as load continues to

cycle

3.Sudden fracture

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In polymers, as the material is subjected to

repetitive stresses, considerable heating

can occur near crack tips and the inter-

relationships between fatigue and creep

affect the overall behavior.

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Figure 8.22

Transmission

electron fracto-

graph

showing fatigue

striations in

aluminum. 9000

Figure 8.23 Fatigue

failure surface.

A crack formed at

top edge

Smooth region also

near the top

corresponds to area

over which the

crack propagated

slowly

Rapid failure

occurred over the

area having a dull

and fibrous texture


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8.12 Generalized Creep Behavior

Creep: Permanent elongation of a

component under a static load

maintained for a period of time at a

high temperature.

Essentially, in creep the material begins

to flow slowly.

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Mechanisms of creep of metallic

materials:

Diffusion

DL glide or climb

Grain boundary sliding

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When a material does actually creep

and then ultimately break, the fracture

is defined as stress rupture.

Normally, ductile stress-rupture fractures

include necking & presence of many

cracks that did not have the

opportunity to produce final fracture.

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Grains near the fracture surface tend to

be elongated.

Ductile stress rupture failures generally

occur at high creep rates and relatively

low exposure temperatures and have

short rupture times.

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Brittle stress rupture failures show little

necking and occur more often at

smaller creep rates and high

temperatures. Equiaxed grains are

observed near the fracture surface.

Brittle fracture typically occurs by

formation of voids at intersection of 3

grain boundaries and precipitation of

additional voids along grain boundaries

by diffusion processes.

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Figure 8.28 Typical

creep curve of strain

vs. time at constant

load and elevated

temperature


Creep Rates and Rupture times

In 1st stage of creep of metals,

many DLs climb away from

obstacles, slip, and contribute to

deformation.

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Eventually, rate at which DLs climb

away from obstacles equals rate at

which DLs are blocked by other

imperfections.

This leads to 2nd stage. The slope of

the steady portion is the creep rate.

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Eventually, during 3rd stage, necking

begins, & stress increases, &

specimen deforms at an

accelerated rate until failure

occurs.

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Rupture time: time required for

failure to occur

Either a higher temperature or a

higher stress reduces rupture time

and increases the creep rate.

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8.13 Stress & Temperature Effects

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