Types of fracture in metals • The concept of material strength and fracture has long been studied to overcome failures. • The introduction of malleable irons during the revolution of material 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 two main 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.
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Types of fracture in metals
• The concept of material strength and fracture has long been studied to overcome failures.
• The introduction of malleable irons during the revolution of material 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 two main 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 Failure Very
Ductile
Moderately
Ductile Brittle
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 by microvoid coalescence during ductile failure (high energy fracture mode)
Low energy is absorbed during transgranular 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 Failure necking
s
void nucleation
void growth and linkage
shearing at surface
fracture
Microvoid shape Microvoid 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, see fig.
The theoretical cohesive strength σmax can be obtained in relation to the sine curve and become.
where gs is the surface energy (J/m2) ao is the unstrained interatomic spacing
Note: Convenient estimates of σmax ~ E/10.
Cohesive force as a function of the 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 than the 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.
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.
Fractographic observation in brittle fracture
The process of cleavage fracture consists of three steps:
1) Plastic deformation to produce dislocation pile-ups. 2) Crack initiation. 3) Crack propagation to failure.
Distinct characteristics of brittle fracture surfaces:
1) The absence of gross plastic deformation. 2) Grainy or Faceted texture. 3) River marking or stress lines (chevron nothces).
Brittle fracture indicating the origin of the crack and crack propagation path
Brittle Failure Arrows indicate points at which failure originated
• Stress-strain behavior (Room T):
Ideal vs Real Materials
TS << TS engineering
materials
perfect
materials
s
e
E/10
E/100
0.1
perfect mat’l-no flaws
carefully produced glass fiber
typical ceramic typical strengthened metal typical 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
a q
x
r
sy=0
sx=-s
sy=3s
sx=0
• Tensile stresses reach 3 times of the applied stress at stress concentration points.
a b
s
s
x
y
b
aaxy
21ss
b
a21max ss
a
b2
Radius of curvatour at the tip of the ellipse
ss
a21max
Stress Concentration for
An Elliptic Hole
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 1.0 1.0
1.5
2.0
2.5
Stress Conc. Factor, K t
s max
s o
=
• 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
s max
Stress concentrations for different geometrical shapes
Stress Concentration at A Discontinuity
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 stored in material as it is elastically deformed
• this energy is released when the crack propagates
• creation of new surfaces requires energy
plastic
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
When Does a Crack Propagate? Crack propagates if the applied stress is
above 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 theory of brittle fracture Observed fracture strength is always lower than theoretical cohesive strength
Griffith explained that the discrepancy is due to the inherent defects in brittle materials 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 released elastic strain energy is at least equal to the energy 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 be
applied to brittle materials undergone plastic deformation
before fracture by including the plastic work, gp, into the total
elastic surface energy required to extend the crack wall, giving
the 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