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Chapter 9 : Failure

This presentation includes figures and images from Callister text and instructor materials.

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Brittle fracture surface

transgranular intergranular

160mm 4 mm

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Intergranular

(betweengrains)

Intragranular

(within grains)316 S. Steel

(metal)Reprinted w/

permission from

"Metals Handbook",9th ed, Fig. 650, p.

357. Copyright 1985,ASM International,

Materials Park, OH.(Micrograph by D.R.

Diercks, ArgonneNational Lab.)

304 S. Steel

(metal)Reprinted w/permission

from "Metals Handbook",9th ed, Fig. 633, p. 650.

Copyright 1985, ASMInternational, Materials

Park, OH. (Micrograph byJ.R. Keiser and A.R. Olsen,

Oak Ridge National Lab.)

4 mm 160mm

BRITTLE FRACTURE

Initiation point

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Evolution to failure:

neckingvoidnucleation

void growth

and linkageshearingat surface fracture

Resulting

fracture

surfaces(steel)

50 mm

particles

serve as void

nucleation

sites.

50 mm

100 mm

From V.J. Colangelo and F.A. Heiser,Analysis of Metallurgical Failures(2nd

ed.), Fig. 11.28, p. 294, John Wiley andSons, Inc., 1987. (Orig. source: P.

Thornton, J. Mater. Sci., Vol. 6, 1971,pp. 347-56.)

Fracture surface of tire cord wire

loaded in tension. Courtesy of F.Roehrig, CC Technologies, Dublin, OH.

Used with permission.

MODERATELY DUCTILE FAILURE

Cup and cone ductile fracture

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Ductile Fracture surface

From center of specimen From shear edge of specimen

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Fracture Modevariation in load

direction

Mode I

Tension

Mode II

sliding

Mode III

Tearing

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Stress conc. factor:

Large Ktpromotes failure:

FLAWS ARE STRESS CONCENTRATORS

Stress distributionin front of a hole:

Kt = 2(a/t)1/2

2/1

02

t

m

a

OR

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Stress intensity factor and Fracture toughness

-Stress intensity near crack isdistributed throughout the

volume, decreasing further away.

- The stress intensity depends on

stress applied and flaw location

andsize.

-For the given geometry

aYK

Compare to stress

concentration factor:

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Stress intensity factor - Fracture toughness

Note axes orientation: stress applied in y-direction

Plane-stress: Very thin (plate)sample, (stress developed in z

direction is small compared to

that in y and x)

Plane-strain: Thick plate, (strain

developed in z direction is small

compared to that in y and x)

Stress does develop in z direction.

- most constrained configuration.

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Fracture toughness

aYK aYK

cc

Critical applied stress

for fracture

aYK cIc

Plane strain conditionsmost constrained,so brittle fracture occurs at lower stress for

same crack length.

Material property, just like yield strength or

elastic modulus, for crack opening mode, plane

strain, for this geometry

FromMechanical Metallurgy, Dieter

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Crack growth condition:

Y a

Largest, moststressed cracks grow first!

--Result 1: Max flaw size

dictates design stress.

--Result 2: Design stress

dictates max. flaw size.

design Kc

Y amax

amax 1

Kc

Ydesign

2

K Kc

DESIGN AGAINST CRACK GROWTH

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Increasing temperature...

--increases %EL and Kc

Ductile-to-brittle transition temperature (DBTT)...

Impact test/temperature effect

DBTT

initial heightfinal height

sample

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Crazing in polystyrene

Crazeoccurs in polymers

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Slip bands withingrains formed by

fatigue

Surface notches

formed by fatigue

Images from Schaffer, 1999