A T-2 tanker was lying quietly at pier when, without warning, she suddenly broke in two with a report that was heard for at least a mile. FRACTURE
A T-2 tanker was lying quietly at pier when,
without warning, she suddenly broke in two
with a report that was heard for at least a
mile.
FRACTURE
Types of fracture in metals• The concept of material strength and fracture has long beenstudied to overcome failures.
• The introduction of malleable irons during the revolution ofmaterial construction led to the perception of brittle and ductile fractures as well as fatigue failure in metals.
Failure in metallic materials can be divided into twomain categories:
Ductile failure : Ductile fracture involves a large amount of plastic deformation and can be detected beforehand.
Brittle failure:Brittle fracture is more catastrophic and has been intensively studied.
Ductile vs Brittle FailureVery
Ductile
Moderately
DuctileBrittle
Fracture
behavior:
Large Moderate%AR or %EL Small
• Ductile
fracture is usually
desirable!
Adapted from Fig. 8.1,
Callister 7e.
• Classification:
Ductile:
warning before
fracture
Brittle:
No
warning
• 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
Factors affecting modes of fracture
Strain rate
Metallurgical aspect
Temperature
State of stresses
(notch effect)
Loading condition
Ductile vs. Brittle Failure
cup-and-cone fracture brittle fracture
Failure modes
High energy is absorbed bymicrovoid coalescence during ductile failure (high energy fracture mode)
Low energy is absorbed duringtransgranular cleavage fracture(low energy fracture mode)
Less catastrophic More catastrophic
• Evolution to failure:
• Resulting
fracture
surfaces
(steel)
50 mm
particles
serve as void
nucleation
sites.
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 mm
Fracture surface of tire cord wire
loaded in tension. Courtesy of F.
Roehrig, CC Technologies, Dublin,
OH. Used with permission.
Moderately Ductile Failurenecking
s
void nucleation
void growth and linkage
shearing at surface
fracture
Microvoid shapeMicrovoid shape is strongly influenced by the type of loading.
Uniaxial tensile loading
Equiaxed dimples.
Shear loading
Elongated and parabolic dimples
pointing in the opposite directions
on matching fracture surfaces.
Tensile tearing
Elongated dimples pointing in the
same direction on matching fracture
surface.
Theoretical cohesive strength of metals• In the most basic term, strength is due to the cohesive forces between atoms.• The attractive and repulsive force acting on the two atoms lead to cohesive force between two atoms which varies in relation to the separation between these atoms.
The theoretical cohesive strengthσmax can be obtained in relation tothe sine curve and become.
wheregs is the surface energy (J/m2)ao is the unstrained interatomic spacing
Note: Convenient estimates of σmax ~ E/10.
Cohesive force as a function ofthe separation between atoms.
o
s
ltheoreticaa
Egs
Example: Determine the cohesive strength of a silica fibre,if E = 95 GPa, gs = 1 J.m-2, and ao = 0.16 nm.
• This theoretical cohesive strength is exceptionally higher thanthe fracture strength of engineering materials.• This difference between cohesive and fracture strength is due to inherent flaws or defects in the materials which lower the fracture strength in engineering materials.• Griffith explained the discrepancy between the fracture strength and theoretical cohesive strength using the concept of energy balance.
Fractographic observationin brittle fracture
The process of cleavage fractureconsists of three steps:
1) Plastic deformation to producedislocation pile-ups.2) Crack initiation.3) Crack propagation to failure.
Distinct characteristics of brittlefracture surfaces:
1) The absence of gross plasticdeformation.2) Grainy or Faceted texture.3) River marking or stress lines(chevron nothces).
Brittle fracture indicating the origin of thecrack and crack propagation path
Brittle FailureArrows indicate points at which failure originated
• Stress-strain behavior (Room T):
Ideal vs Real Materials
TS << TSengineering
materials
perfect
materials
s
e
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.
Stress Concentration for A Circular Hole
s
s
y
aq
x
r
sy=0
sx=-s
sy=3s
sx=0
• Tensile stresses reach 3 times of the applied stress at stress concentration points.
Flaws are Stress Concentrators!
where t = radius of curvature
so = applied stress
sm = stress at crack tip
Kt = stress concentration factor
ot
/
t
om Ka
s
ss
21
2
t
Adapted from Fig. 8.8(a), Callister 7e.
Concentration of Stress at Crack Tip
Adapted from Fig. 8.8(b), Callister 7e.
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
smax
so
=
• Avoid sharp corners!s
Adapted from Fig.
8.2W(c), Callister 6e.
(Fig. 8.2W(c) is from G.H.
Neugebauer, Prod. Eng.
(NY), Vol. 14, pp. 82-87
1943.)
r , fillet
radius
w
h
o
smax
Stress concentrations for different geometrical shapes
Stress Concentration at A Discontinuity
Crack PropagationCracks 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 stored in material as it is elastically deformed
• this energy is released when the crack propagates
• creation of new surfaces requires energy
plastic
Theories of brittle fracture
Griffith theory of brittle fracture
The first analysis on cleavage fracture was initiated by Griffith using the concept of energy balance in order to explain discrepancy between the theoretical cohesive strength and observed fracture strength of ideally brittle material (glass).
Irwin and Orowan modified the Griffith theory to include plastic deformation at the crack tip.
2a
t
s
s
Elastic energy released by crack formation:
E
ta22s
Energy to create new surfaces
ss atat gg 422
satE
taUUU g
s4
22
0 Potansiyel Enerji (U)
Çatlak boyu
(a)
4atgs
U
acr
E
ta 22s
02
42
E
att
a
Us
sg
a
E scr
gs
2
Griffith’s Fracture Theory
When Does a Crack Propagate?
Crack propagates if the applied stress isabove critical stress
where– E = modulus of elasticity
– gs = specific surface energy
– a = one half length of internal crack
For ductile => replace gs by gs + gp
where gp is plastic deformation energy
212
/
sc
a
E
gs
i.e., sm > sc
Griffith’s theory of brittle fracture
Observed fracture strength isalways lower than theoreticalcohesive strength
Griffith explained that the discrepancy is due to the inherent defects in brittlematerials leading to stress concentration lower the fracture strength
Consider a through thickness crack of length 2a,subjected to a uniform tensile stress σ, at infinity.
Crack propagation occurs when the releasedelastic strain energy is at least equal to theenergy required to generate new crack surface.
The stress required to create the new crack surface is
In plane strain condition, it is given by:
Modified Griffith equation• The Griffith equation is strongly dependent on the crack size a,and satisfies only ideally brittle materials like glass.
• Irwin and Orowan suggested Griffith’s equation can beapplied to brittle materials undergone plastic deformationbefore fracture by including the plastic work, gp, into the totalelastic surface energy required to extend the crack wall, givingthe modified Griffith’s equation as follows
s
Criterion of Failure
gs and gp are material properties.
Gc is called critical energy release rate, and it is a material property.
a
EGccr
s For a given crack length, a, Failure occurs if s >
Also, if the s is given we can find the critical crack length for failure.
Failure occurs if G > Gc
Gc = 2(gs + gp ) (J / m2)
Applied energy release rate is G=s2a/E
In many cases we would like to know the design stress.
Linear Elastic Fracture Mechanics
It can be shown that the stress field, s, at the tip of a crack is a
function of the stress intensity factor, K.
Notice: s infinity as r 0
r
q
K is a function of the applied stress, the crack length, and the geometry.
K = aY s
K= f(s,a)
Usually
Critical K that a material can stand: Kc the fracture toughness.
)( mMPa
Failure occurs if K > Kc
1
Y
aYK s
12.1
Y
aYK s P
P
atPK /
(c)
GEK
E
KG
c
c
2
G or K, which approach is correct
From Griffith, a
GE
s
From LEFM, aK s /
If we write in terms of material properties
Fracture Toughness
Based on data in Table B5,
Callister 7e.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 crystal
Glass -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
40
506070
100
Al oxideSi nitride
C/C( fibers) 1
Al/Al oxide(sf) 2
Al oxid/SiC(w) 3
Al oxid/ZrO 2(p)4
Si nitr/SiC(w) 5
Glass/SiC(w) 6
Y2O3/ZrO 2(p)4
Toughness versus Strength
• Crack growth condition:
• Largest, most stressed cracks grow first!
Design Against Crack Growth
K ≥ Kc = aY s
--Result 1: Max. flaw size
dictates design stress.
max
cdesign
aY
K
s
s
amax
no fracture
fracture
--Result 2: Design stress
dictates max. flaw size.2
1
s
design
cmax
Y
Ka
amax
sno fracture
fracture
• Two designs to consider...
Design A--largest flaw is 9 mm
--failure stress = 112 MPa
Design B--use same material
--largest flaw is 4 mm
--failure stress = ?
• Key point: Y and Kc are the same in both designs.
Answer: MPa 168)( B sc• Reducing flaw size pays off!
• Material has Kc = 26 MPa-m0.5
Design Example: Aircraft Wing
• Use...max
cc
aY
K
s
sc amax A
sc amax B
9 mm112 MPa 4 mm--Result:
ac decreases dramatically with
decreasing toughness, espically if
the design stress is to be increased.
Design against fracture
Loading Rate
• Increased loading rate...-- increases sy and TS
-- decreases %EL
• Why?
s
e
sy
sy
TS
TS
largere
smallere
An increased rate gives less time for
dislocations to move past obstacles.
Impact Testing
• Impact loading:-- severe testing case
-- makes material more brittle
-- decreases toughness
(Charpy)
Impact Testing
final height initial height
Adapted from Fig. 8.12(b),
Callister 7e. (Fig. 8.12(b) is
adapted from H.W. Hayden,
W.G. Moffatt, and J. Wulff, The
Structure and Properties of
Materials, Vol. III, Mechanical
Behavior, John Wiley and Sons,
Inc. (1965) p. 13.)
• Increasing temperature...--increases %EL and Kc
• Ductile-to-Brittle Transition Temperature (DBTT)...
Temperature
BCC metals (e.g., iron at T < 914°C)
Imp
act E
ne
rgy
Temperature
High strength materials (s y > E/150)
polymers
More DuctileBrittle
Ductile-to-brittle transition temperature
FCC metals (e.g., Cu, Ni)
Adapted from Fig. 8.15,
Callister 7e.
Temperature vs. Charpy
• Pre-WWII: The Titanic
• WWII: Liberty ships
• Problem: Used a type of steel with a DBTT ~ Room temp.
Reprinted w/ permission from R.W. Hertzberg,
"Deformation and Fracture Mechanics of Engineering
Materials", (4th ed.) Fig. 7.1(a), p. 262, John Wiley and
Sons, Inc., 1996. (Orig. source: Dr. Robert D. Ballard,
The Discovery of the Titanic.)
Design Strategy:Stay Above The DBTT!
• An oil tanker that fractured in a
brittle manner by crack propagation
around its girth.