Post on 04-Apr-2018
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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