Fracture mechanisms Ductile fracture –Occurs with plastic deformation Brittle fracture –Occurs...

35
Fracture mechanisms Ductile fracture Occurs with plastic deformation Brittle fracture Occurs with Little or no plastic deformation Thus they are Catastrophic meaning they occur without warning!

Transcript of Fracture mechanisms Ductile fracture –Occurs with plastic deformation Brittle fracture –Occurs...

Page 1: Fracture mechanisms Ductile fracture –Occurs with plastic deformation Brittle fracture –Occurs with Little or no plastic deformation –Thus they are Catastrophic.

Fracture mechanisms

• Ductile fracture– Occurs with plastic deformation

• Brittle fracture– Occurs with Little or no plastic deformation– Thus they are Catastrophic meaning they occur

without warning!

Page 2: Fracture mechanisms Ductile fracture –Occurs with plastic deformation Brittle fracture –Occurs with Little or no plastic deformation –Thus they are Catastrophic.

Ductile vs Brittle Failure

Very Ductile

ModeratelyDuctile BrittleFracture

behavior:

Large Moderate%Ra or %El Small

• Ductile fracture is nearly always

desirable!

Ductile: warning before

fracture

Brittle: No

warning

Page 3: Fracture mechanisms Ductile fracture –Occurs with plastic deformation Brittle fracture –Occurs with Little or no plastic deformation –Thus they are Catastrophic.

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

Example: Failure of a Pipe

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

Page 4: Fracture mechanisms Ductile fracture –Occurs with plastic deformation Brittle fracture –Occurs with Little or no plastic deformation –Thus they are Catastrophic.

• Evolution to failure:

• Resulting fracture surfaces (steel)

50 mm

Inclusion particlesserve as voidnucleationsites.

50 mm

From V.J. Colangelo and F.A. Heiser, Analysis of Metallurgical Failures (2nd ed.), Fig. 11.28, p. 294, John Wiley and Sons, Inc., 1987. (Orig. source: P. Thornton, J. Mater. Sci., Vol. 6, 1971, pp. 347-56.)

100 mmFracture surface of tire cord wire loaded in tension. Courtesy of F. Roehrig, CC Technologies, Dublin, OH. Used with permission.

Moderately Ductile Failure

necking

void nucleation

void growth and linkage

shearing at surface fracture

Page 5: Fracture mechanisms Ductile fracture –Occurs with plastic deformation Brittle fracture –Occurs with Little or no plastic deformation –Thus they are Catastrophic.
Page 6: Fracture mechanisms Ductile fracture –Occurs with plastic deformation Brittle fracture –Occurs with Little or no plastic deformation –Thus they are Catastrophic.

6

(c)2

003

Bro

oks/

Col

e, a

div

isio

n of

Tho

mso

n L

earn

ing,

Inc

. T

hom

son

Lea

rnin

g ™ is

a tr

adem

ark

used

her

ein

unde

r li

cens

e.

Figure 6.36 When a ductile material is pulled in a tensile test, necking begins and voids form – starting near the center of the bar – by nucleation at grain boundaries or inclusions. As deformation continues a 45° shear lip may form, producing a final cup and cone fracture

Page 7: Fracture mechanisms Ductile fracture –Occurs with plastic deformation Brittle fracture –Occurs with Little or no plastic deformation –Thus they are Catastrophic.

Ductile vs. Brittle Failure

Adapted from Fig. 8.3, Callister 7e.

cup-and-cone fracture brittle fracture

Page 8: Fracture mechanisms Ductile fracture –Occurs with plastic deformation Brittle fracture –Occurs with Little or no plastic deformation –Thus they are Catastrophic.

Brittle Failure

Arrows indicate point at which failure originated

Adapted from Fig. 8.5(a), Callister 7e.

Page 9: Fracture mechanisms Ductile fracture –Occurs with plastic deformation Brittle fracture –Occurs with Little or no plastic deformation –Thus they are Catastrophic.

9

(c)2003 Brooks/Cole, a division of Thomson Learning, Inc. Thomson Learning™ is a trademark used herein under license.

Figure 6.40 The Chevron pattern in a 0.5-in.-diameter quenched 4340 steel. The steel failed in a brittle manner by an impact blow

Page 10: Fracture mechanisms Ductile fracture –Occurs with plastic deformation Brittle fracture –Occurs with Little or no plastic deformation –Thus they are Catastrophic.

• Intergranular(between grains)

• Intragranular (within grains)

Al Oxide(ceramic)

Reprinted w/ permission from "Failure Analysis of Brittle Materials", p. 78.

Copyright 1990, The American Ceramic

Society, Westerville, OH. (Micrograph by R.M.

Gruver and H. Kirchner.)

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, Argonne

National Lab.)

304 S. Steel (metal)Reprinted w/permission from "Metals Handbook", 9th ed, Fig. 633, p. 650. Copyright 1985, ASM International, Materials Park, OH. (Micrograph by J.R. Keiser and A.R. Olsen, Oak Ridge National Lab.)

Polypropylene(polymer)Reprinted w/ permission from R.W. Hertzberg, "Deformation and Fracture Mechanics of Engineering Materials", (4th ed.) Fig. 7.35(d), p. 303, John Wiley and Sons, Inc., 1996.

3 mm

4 mm160 mm

1 mm(Orig. source: K. Friedrick, Fracture 1977, Vol. 3, ICF4, Waterloo, CA, 1977, p. 1119.)

Brittle Fracture Surfaces: Useful to examine to determine causes of failure

Page 11: Fracture mechanisms Ductile fracture –Occurs with plastic deformation Brittle fracture –Occurs with Little or no plastic deformation –Thus they are Catastrophic.

Failure Analysis – Failure Avoidance

• Most failure occur due to the presence of defects in materials– Cracks or Flaws (stress concentrators)– Voids or inclusions

• Presence of defects is best found before hand and they should be determined non-destructively– X-Ray analysis– Ultra-Sonic Inspection– Surface inspection

• Magna-flux• Dye Penetrant

Page 12: Fracture mechanisms Ductile fracture –Occurs with plastic deformation Brittle fracture –Occurs with Little or no plastic deformation –Thus they are Catastrophic.

• Stress-strain behavior (Room Temp):

Ideal vs Real Materials

TS << TSengineeringmaterials

perfectmaterials

E/10

E/100

0.1

perfect mat’l-no flaws

carefully produced glass fiber

typical ceramic typical strengthened metaltypical polymer

• DaVinci (500 yrs ago!) observed... -- the longer the wire, the smaller the load for failure.• Reasons: -- flaws cause premature failure. -- Larger samples contain more flaws!

Reprinted w/ permission from R.W. Hertzberg, "Deformation and Fracture Mechanics of Engineering Materials", (4th ed.) Fig. 7.4. John Wiley and Sons, Inc., 1996.

Page 13: Fracture mechanisms Ductile fracture –Occurs with plastic deformation Brittle fracture –Occurs with Little or no plastic deformation –Thus they are Catastrophic.

• Increasing temperature... --increases %EL and Kc

• Ductile-to-Brittle Transition Temperature (DBTT)...

Considering Temperature Effects

BCC metals (e.g., iron at T < 914°C)

Imp

act

Ene

rgy

Temperature

High strength materials (y > E/150)

polymers

More Ductile Brittle

Ductile-to-brittle transition temperature

FCC metals (e.g., Cu, Ni)

Adapted from Fig. 8.15, Callister 7e.

Page 14: Fracture mechanisms Ductile fracture –Occurs with plastic deformation Brittle fracture –Occurs with Little or no plastic deformation –Thus they are Catastrophic.
Page 15: Fracture mechanisms Ductile fracture –Occurs with plastic deformation Brittle fracture –Occurs with Little or no plastic deformation –Thus they are Catastrophic.
Page 16: Fracture mechanisms Ductile fracture –Occurs with plastic deformation Brittle fracture –Occurs with Little or no plastic deformation –Thus they are Catastrophic.

Flaws are Stress Concentrators!

Results from crack propagation• Griffith Crack Model:

where t = radius of curvature

of crack tip

o = applied stress

m = stress at crack tip

ot

/

tom K

a

21

2

t

Adapted from Fig. 8.8(a), Callister 7e.

Page 17: Fracture mechanisms Ductile fracture –Occurs with plastic deformation Brittle fracture –Occurs with Little or no plastic deformation –Thus they are Catastrophic.

Concentration of Stress at Crack Tip

Adapted from Fig. 8.8(b), Callister 7e.

Page 18: Fracture mechanisms Ductile fracture –Occurs with plastic deformation Brittle fracture –Occurs with Little or no plastic deformation –Thus they are Catastrophic.

Engineering Fracture Design

r/h

sharper fillet radius

increasing w/h

0 0.5 1.01.0

1.5

2.0

2.5

Stress Conc. Factor, K t

max

o=

• Avoid sharp corners!

Adapted from G.H. Neugebauer, Prod. Eng. (NY), Vol. 14, pp. 82-87 1943.)

r , fillet

radius

w

h

o

max max is the concentrated stress in the narrowed region

Page 19: Fracture mechanisms Ductile fracture –Occurs with plastic deformation Brittle fracture –Occurs with Little or no plastic deformation –Thus they are Catastrophic.

Crack Propagation

Cracks propagate due to sharpness of crack tip • A plastic material deforms at the tip, “blunting” the

crack.

deformed region

brittle

Energy balance on the crack• Elastic strain energy-

• energy is stored in material as it is elastically deformed• this energy is released when the crack propagates

• creation of new surfaces requires (this) energy

plastic

Page 20: Fracture mechanisms Ductile fracture –Occurs with plastic deformation Brittle fracture –Occurs with Little or no plastic deformation –Thus they are Catastrophic.

When Does a Crack Propagate?

Crack propagates if applied stress is above critical stress

where– E = modulus of elasticity s = specific surface energy

– a = one half length of internal crack

– Kc = c/0

For ductile materials replace s by s + p

where p is plastic deformation energy

212

/

sc a

E

i.e., m > c

or Kt > Kc

Page 21: Fracture mechanisms Ductile fracture –Occurs with plastic deformation Brittle fracture –Occurs with Little or no plastic deformation –Thus they are Catastrophic.

21

Section 6.10Fracture Mechanics

Fracture mechanics - The study of a material’s ability to withstand stress in the presence of a flaw.

Fracture toughness - The resistance of a material to failure in the presence of a flaw.

Page 22: Fracture mechanisms Ductile fracture –Occurs with plastic deformation Brittle fracture –Occurs with Little or no plastic deformation –Thus they are Catastrophic.

22

(c)2

003

Bro

oks/

Col

e, a

div

isio

n of

Tho

mso

n L

earn

ing,

Inc

. T

hom

son

Lea

rnin

g ™ is

a tr

adem

ark

used

her

ein

unde

r li

cens

e.

Figure 6.30 Schematic drawing of fracture toughness specimens with (a) edge and (b) internal flaws

Page 23: Fracture mechanisms Ductile fracture –Occurs with plastic deformation Brittle fracture –Occurs with Little or no plastic deformation –Thus they are Catastrophic.

23

(c)2

003

Bro

oks/

Col

e, a

div

isio

n of

Tho

mso

n L

earn

ing,

Inc

. T

hom

son

Lea

rnin

g ™ is

a tr

adem

ark

used

her

ein

unde

r li

cens

e.

Figure 6.31 The fracture toughness Kc of a 3000,000psi yield strength steel decreases with increasing thickness, eventually leveling off at the plane strain fracture toughness Klc

Page 24: Fracture mechanisms Ductile fracture –Occurs with plastic deformation Brittle fracture –Occurs with Little or no plastic deformation –Thus they are Catastrophic.

Fracture Toughness

Composite reinforcement geometry is: f = fibers; sf = short fibers; w = whiskers; p = particles. Addition data as noted (vol. fraction of reinforcement):1. (55vol%) ASM Handbook, Vol. 21, ASM Int., Materials Park, OH (2001) p. 606.2. (55 vol%) Courtesy J. Cornie, MMC, Inc., Waltham, MA.3. (30 vol%) P.F. Becher et al., Fracture Mechanics of Ceramics, Vol. 7, Plenum Press (1986). pp. 61-73.4. Courtesy CoorsTek, Golden, CO.5. (30 vol%) S.T. Buljan et al., "Development of Ceramic Matrix Composites for Application in Technology for Advanced Engines Program", ORNL/Sub/85-22011/2, ORNL, 1992.6. (20vol%) F.D. Gace et al., Ceram. Eng. Sci. Proc., Vol. 7 (1986) pp. 978-82.

Graphite/ Ceramics/ Semicond

Metals/ Alloys

Composites/ fibers

Polymers

5

KIc

(MP

a ·

m0

.5)

1

Mg alloys

Al alloys

Ti alloys

Steels

Si crystalGlass -soda

Concrete

Si carbide

PC

Glass 6

0.5

0.7

2

4

3

10

20

30

<100>

<111>

Diamond

PVC

PP

Polyester

PS

PET

C-C(|| fibers) 1

0.6

67

40506070

100

Al oxideSi nitride

C/C( fibers) 1

Al/Al oxide(sf) 2

Al oxid/SiC(w) 3

Al oxid/ZrO 2(p)4Si nitr/SiC(w) 5

Glass/SiC(w) 6

Y2O3/ZrO 2(p)4

K1c – plane strain stress concentration factor – with edge crack; A Material Property we use for design, developed using ASTM Std: ASTM E399 - 09 Standard Test Method for Linear-Elastic Plane-Strain Fracture Toughness K Ic of Metallic Materials

Page 25: Fracture mechanisms Ductile fracture –Occurs with plastic deformation Brittle fracture –Occurs with Little or no plastic deformation –Thus they are Catastrophic.
Page 26: Fracture mechanisms Ductile fracture –Occurs with plastic deformation Brittle fracture –Occurs with Little or no plastic deformation –Thus they are Catastrophic.
Page 27: Fracture mechanisms Ductile fracture –Occurs with plastic deformation Brittle fracture –Occurs with Little or no plastic deformation –Thus they are Catastrophic.

Fatigue behavior:• Fatigue = failure under cyclic stress

• Stress varies with time. -- key parameters are S (stress

amplitude), m, and frequency

max

min

time

mS

• Key points when designing in Fatigue inducing situations: -- fatigue can cause part failure, even though max < c. -- fatigue causes ~ 90% of mechanical engineering failures.• Because of its importance, ASTM and ISO have developed many special standards to assess Fatigue Strength of materials

(Fig. 8.18 is from Materials Science in Engineering, 4/E by Carl. A. Keyser, Pearson Education, Inc., Upper Saddle River, NJ.)

tension on bottom

compression on top

countermotor

flex coupling

specimen

bearing bearing

Page 28: Fracture mechanisms Ductile fracture –Occurs with plastic deformation Brittle fracture –Occurs with Little or no plastic deformation –Thus they are Catastrophic.

Figure 8.8 Fatigue corresponds to the brittle fracture of an alloy after a total of N cycles to a stress below the tensile strength.

Page 29: Fracture mechanisms Ductile fracture –Occurs with plastic deformation Brittle fracture –Occurs with Little or no plastic deformation –Thus they are Catastrophic.

• Fatigue limit, Sfat: --no fatigue failure if

S < Sfat

Fatigue Limit is defined in: ASTM D671

Adapted from Fig. 8.19(a), Callister 7e.

Fatigue Design Parameters

Sfat

case for steel (typ.)

N = Cycles to failure103 105 107 109

unsafe

safe

S = stress amplitude

• However, Sometimes, the fatigue limit is zero!

Adapted from Fig. 8.19(b), Callister 7e.

case for Al (typ.)

N = Cycles to failure103 105 107 109

unsafe

safe

S = stress amplitude

Page 30: Fracture mechanisms Ductile fracture –Occurs with plastic deformation Brittle fracture –Occurs with Little or no plastic deformation –Thus they are Catastrophic.

For metals other than Ferrous alloys, F.S. is taken as the stress that will cause failure

after 108 cycles

Page 31: Fracture mechanisms Ductile fracture –Occurs with plastic deformation Brittle fracture –Occurs with Little or no plastic deformation –Thus they are Catastrophic.

• Cracks in Material grows incrementallytyp. 1 to 6

a~

increase in crack length per loading cycle

• Failed rotating shaft --crack grew even though

Kmax < Kc

--crack grows faster as • increases • crack gets longer • loading freq. increases.

crack origin

Adapted fromfrom D.J. Wulpi, Understanding How Components Fail, American Society for Metals, Materials Park, OH, 1985.

Fatigue Mechanism

mKdN

da

Page 32: Fracture mechanisms Ductile fracture –Occurs with plastic deformation Brittle fracture –Occurs with Little or no plastic deformation –Thus they are Catastrophic.

Figure 8.11 An illustration of how repeated stress applications can generate localized plastic deformation at the alloy surface leading eventually to sharp discontinuities.

Page 33: Fracture mechanisms Ductile fracture –Occurs with plastic deformation Brittle fracture –Occurs with Little or no plastic deformation –Thus they are Catastrophic.

Improving Fatigue Life

1. Impose a compressive surface stresses (to suppress surface crack growth)

N = Cycles to failure

moderate tensile mLarger tensile m

S = stress amplitude

near zero or compressive mIncreasing

m

--Method 1: shot peening

put surface

into compression

shot--Method 2: carburizing

C-rich gas

2. Remove stress concentrators. Adapted from

Fig. 8.25, Callister 7e.

bad

bad

better

better

Adapted fromFig. 8.24, Callister 7e.

Page 34: Fracture mechanisms Ductile fracture –Occurs with plastic deformation Brittle fracture –Occurs with Little or no plastic deformation –Thus they are Catastrophic.

Figure 8.20 Comparison of (a) cyclic fatigue in metals and (b) static fatigue in ceramics.

Page 35: Fracture mechanisms Ductile fracture –Occurs with plastic deformation Brittle fracture –Occurs with Little or no plastic deformation –Thus they are Catastrophic.

• Engineering materials don't reach theoretical strength.

• Flaws produce stress concentrations that cause premature failure.

• Sharp corners produce large stress concentrations and premature failure.

• Failure type depends on T and stress:

- for noncyclic and T < 0.4Tm, failure stress decreases with:

- increased maximum flaw size, - decreased T, - increased rate of loading.- for cyclic : - cycles to fail decreases as increases.

- for higher T (T > 0.4Tm): - time to fail decreases as or T increases.

SUMMARY