Fractura y Falla. Introducción a la
mecánica de fractura
1
FRACTURE
• How do cracks that lead to failure form?
• How is fracture resistance quantified? How do the fracture
resistances of the different material classes compare?
• How do we estimate the stress to fracture?
• How do loading rate, loading history, and temperature
affect the failure behavior of materials?
Ship-cyclic loading
from waves.
Adapted from chapter-opening photograph,
Chapter 8, Callister & Rethwisch 8e. (by
Neil Boenzi, The New York Times.)
Computer chip-cyclic
thermal loading.
Adapted from Fig. 22.30(b), Callister 7e.
(Fig. 22.30(b) is courtesy of National
Semiconductor Corporation.)
Hip implant-cyclic
loading from walking.
Adapted from Fig. 22.26(b),
Callister 7e.
3
Failure: Fracture, Fatigue and Creep
Temperature, Stress, Cyclic and Loading Effect
Ship-cyclic loading - waves and cargo.
It is important to understand the
mechanisms for failure, especially to
prevent in-service failures via design.
This can be accomplished via
- Materials selection.
- Processing (e.g. strengthening).
- Design Safety.
From Callister, photo Neal Noenzi (NYTimes)
Objective: Understand how flaws in a material initiate failure.
• describe crack propagation for ductile and brittle materials.
• explain why brittle materials fail well below theoretical expectations.
• define and use Fracture Toughness.
• define fatigue and creep and specify conditions in which they are operative.
• what is steady-state creep and fatigue lifetime?
5
Casting Porosity: Influence
of Hot Isostatic Pressing
8
Fracture
•Fracture: separation of a body into pieces due to stress, at temperatures below
the melting point.
Steps in fracture:
crack formation
crack propagation
Depending on the ability of material to undergo plastic deformation before the
fracture two fracture modes can be defined - ductile or brittle
• Ductile fracture - most metals (not too cold):
Extensive plastic deformation ahead of crack
Crack is “stable”: resists further extension unless
applied stress is increased
• Brittle fracture - ceramics, ice, cold metals:
Relatively little plastic deformation
Crack is “unstable”: propagates rapidly without increase in applied
stress
Ductile fracture is preferred in most applications
9
Fracture mechanisms

Ductile fracture
• Accompanied by significant plastic deformation
• Brittle fracture
– Little or no plastic deformation
– Catastrophic
10
Ductile vs Brittle Failure
• Classification:
Fracture
behavior:
Very
Ductile
Moderately
Ductile
Brittle
Large
Moderate
Small
Adapted from Fig. 8.1,
Callister & Rethwisch 8e.
%AR or %EL
• Ductile fracture is
usually more desirable
than brittle fracture!
Ductile:
Warning before
fracture
Brittle:
No
warning
11
Example – Failure of a Pipe
Ductile failure
one piece
large deformation
Brittle failure
many pieces
small 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.
12
13
Brittle vs. Ductile Fracture
Brittle materials
little plastic deformation and low
energy absorption before fracture
Ductile materials
extensive plastic deformation and
energy absorption (“toughness”)
before fracture
14
Brittle vs. Ductile Fracture
A. Very ductile
Soft metals (e.g. Pb, Au) at
room
temperature,
other
metals, polymers, glasses at
high temperature.
B. Moderately
ductile fracture
typical for ductile
metals
C. Brittle fracture
cold metals, ceramics.
15
Ductile Fracture
Dislocation Mediated
Necking
Cavity Formation
Crack propagation
Cavity coalescence to form a crack
Fracture
16
Moderately Ductile Failure
Evolution to failure
necking

void
nucleation
void growth
and linkage
shearing
at surface
fracture
Resulting fracture surfaces (steel)
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.
17
Ductile Fracture
Cap-and-cone fracture
in Al
Scanning Electron Microscopy: Fractographic studies at high resolution. Spherical “dimples”
correspond to micro-cavities that initiate crack formation.
18
Brittle Fracture
(Limited Dislocation Mobility)
No appreciable plastic deformation
Crack propagation is very fast
Crack propagates nearly perpendicular to the direction of the applied stress
Crack often propagates by cleavage – breaking of atomic bonds along specific
crystallographic planes (cleavage planes).
Brittle fracture in a mild steel
19
Microstructural Features of
Fracture in Metallic Materials
Transgranular
 Meaning across the grains (e.g., a transgranular fracture would
be fracture in which cracks would go through the grains).
Intergranular
 In between grains or along the grain boundaries.
Microvoids
 Development of small holes in a material.
20
Brittle Fracture
A. Transgranular fracture:
Fracture cracks pass through grains.
Fracture surface have faceted texture
because of different orientation of
cleavage planes in grains.
B. Intergranular fracture:
Fracture crack propagation is along grain
boundaries (grain boundaries are
weakened or embrittled by impurities
segregation etc.)
21
Transgranular Fracture
22
Intergranular Fracture
23
Brittle Fracture Surfaces
(between grains)
304 S. Steel (metal)
Intragranular,
Transgranular
(within grains)
304 S. Steel (metal)
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.)
3mm
1 mm
160mm
Reprinted w/ permission from R.W.
Hertzberg, "Defor-mation and Fracture
Mechanics of Engineering Materials",
(4th ed.) Fig. 7.35(d), p. 303, John Wiley
and Sons, Inc., 1996.
Intergranular
Polypropylene (polymer)
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.)
4 mm
(Orig. source: K. Friedrick, Fracture 1977, Vol. 3, ICF4, Waterloo, CA, 1977, p. 1119.)
24
25
26
27
Fracture 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.
28
Ideal vs Real Materials
• Stress-strain behavior (Room T):
E/10
 perfect mat’l-no flaws
TSengineering << TS perfect
carefully produced glass fiber
E/100
typical ceramic
0.1
materials
materials
typical strengthened metal
typical polymer
e
• DaVinci (500 yrs ago!) observed...
-- the longer the wire, the
smaller the load for failure.
• Reasons:
-- flaws cause premature failure.
-- larger samples contain longer 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.
29
Stress Concentration
Figure by N. Bernstein & D. Hess, NRL
Fracture strength of a brittle solid is related
to the cohesive forces between atoms. One
can estimate that the theoretical cohesive
strength of a brittle material should be ~
E/10. But experimental fracture strength is
normally E/100 - E/10,000.
This much lower fracture strength is
explained by the effect of stress
concentration at microscopic flaws. The
applied stress is amplified at the tips of
micro-cracks, voids, notches, surface
scratches, corners, etc. that are called stress
raisers. The magnitude of this amplification
depends on micro-crack orientations,
geometry and dimensions.
30
Flaws are Stress Concentrators!

Griffith Crack
1/ 2
a
m  2o  
 t 
t
 Kt o
where
t = radius of curvature
o = applied stress
m = stress at crack tip
Adapted from Fig. 8.8(a), Callister & Rethwisch 8e.
31
Flaws and Fracture
Actual fracture strength in most materials are significantly lower than expected from bond strengths.
Flaw/cracks can amplify or concentrate stress!
Max stress at the crack tip:
For long microcracks:
Stress concentration factor:
Kt 
m
a
2
o
t
Large Kt promotes failure
Avoid sharp corners!
Minimize crack size (a) and maximize radius of curvature (rt) if crack is unavoidable
32
Engineering Fracture Design
• Avoid sharp corners!

max
Stress Conc. Factor, K t =
0
w
max
r,
fillet
radius
2.5
h
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.)
2.0
increasing w/h
1.5
1.0
0
0.5
1.0
sharper fillet radius
r/h
33
34
Crack Propagation
Cracks having sharp tips propagate easier than cracks
having blunt tips
• A plastic material deforms at a crack tip, which
“blunts” the crack.
deformed
region
brittle
ductile
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
35
Impact Testing
• Impact loading:
(Charpy)
-- severe testing case
-- makes material more brittle
-- decreases toughness
Adapted from Fig. 8.12(b),
Callister & Rethwisch 8e. (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.)
final height
initial height
36
Influence of Temperature on
Impact Energy
• Ductile-to-Brittle Transition Temperature (DBTT)...
Impact Energy
FCC metals (e.g., Cu, Ni)
BCC metals (e.g., iron at T < 914ºC)
polymers
Brittle
More Ductile
High strength materials (  y > E/150)
Temperature
Ductile-to-brittle
transition temperature
Adapted from Fig. 8.15,
Callister & Rethwisch 8e.
37
Criterion for Crack Propagation
Crack propagates if crack-tip stress (m)
exceeds a critical stress (c)
1/ 2
i.e., m > c
 2E s 
c  

 a 
where
– E = modulus of elasticity
– s = specific surface energy
– a = one half length of internal crack
For ductile materials => replace s with s + p
where p is plastic deformation energy
38
Critical Stress for Crack Propagation
Stress at which crack propagates
E = elastic modulus
gs = specific surface energy
gp = plastic deformation energy
i.e. For crack to propagate, enough stress must be applied to overcome energy needed
to create surface and cause plastic deformation.
Highly ductile
Brittle
39
40
41
Fracture Toughness
Measure of material’s resistance to brittle fracture.
In the presence of a crack, it’s related to critical stress for crack propagation and depends on
1) material size & geometry, 2) crack dimension & orientation, 3) manner in which the load is applied.
Gc = 2(gs+gp)
Fast-Fracture Condition:
K = c
a0  EGc = constant!!
Units of MPam
“stress intensity Hard to measure
factor”
Internal flaws
Measureable
(fixed) materials
properties
• fast fracture will occur when (in a material subjected to stress (s) a
crack reaches some critical size “a”; or, when a material contains
cracks of size “a” is subjected to some critical stress s.
• Point is that the critical combination of stress and crack length at
which fast fracture occurs is a MATERIAL CONSTANT!
Fracture Toughness:
Kc  Y c a
Relates to how the load is applied, crack orientation etc.
42
Geometry, Load, & Material
Condition for crack propagation:
K ≥ Kc
Stress Intensity Factor:
Depends on load & geometry.
Fracture Toughness:
Depends on the material, temperature,
environment, & rate of loading.
Values of K for some standard loads & geometries:

K  1.1 a
K   a
Units of K:
MPa(m)0.5
or
ksi(in)0.5
a
Adapted from Fig. 8.8, Callister 6e.
43
44
45
increasing
Fracture Toughness
Based on data in Table B5,
Callister 6e.
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.
46
Impact Behavior
Impact test

Measures the ability of a material to absorb the sudden application of a load
without breaking.
Impact energy

The energy required to fracture a standard specimen when the load is applied
suddenly.
Impact toughness

Energy absorbed by a material, usually notched, during fracture, under the
conditions of impact test.
Fracture toughness

The resistance of a material to failure in the presence of a flaw.
47
Properties Obtained from the
Impact Test
Ductile to brittle transition temperature (DBTT)
 The temperature below which a material behaves in a
brittle manner in an impact test.
Notch sensitivity
 Measures the effect of a notch, scratch, or other
imperfection on a material’s properties, such as toughness
or fatigue life.
48
(c)2003 Brooks/Cole, a division of Thomson Learning, Inc. Thomson Learning ™ is a trademark used herein under
license.
Ductile to Brittle Transition
Temperature
Results from a series of
Izod impact tests for a
super-tough nylon
thermoplastic polymer
49
Ductile to Brittle Transition

As temperature decreases a ductile material can become
brittle-ductile-to-brittle transition.

Alloying usually increases the ductile-to-brittle transition
temperature. FCC metals remain ductile down to very low
temperatures. For ceramics, this type of transition occurs at
much higher temperatures than for metals.

The ductile-to-brittle transition can be measured by impact
testing: the impact energy needed for fracture drops suddenly
over a relatively narrow temperature range – temperature of
the ductile-to-brittle transition.
50
Effects of Temperature
Increasing temperature increases %EL and Kc
Adapted from C. Barrett, W. Nix, and A.Tetelman, The Principles of Engineering Materials, Fig. 6-21, p. 220, Prentice-Hall, 1973. Electronically reproduced by
permission of Pearson Education, Inc., Upper Saddle River, New Jersey.
1551
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license.
Charpy V-Notch Properties
The Charpy V-notch
properties for a BCC
carbon steel and a FCC
stainless steel. The FCC
crystal structure typically
leads top higher
absorbed energies and
no transition
temperature
52
53
Famous example failures: Liberty ships
USS Esso Manhattan, 3/29/43
Fracture at entrance to NY harbor.
John P. Gaines, 11/43
Vessel broke in two off
the Aleutians (10 killed).
http://www.uh.edu/liberty/photos/liberty_summary.html
USS Schenectady, 1/16/43
Liberty tanker split in two while
moored in calm water at the
outfitting dock at Swan Island, OR.
Coast Guard Report: USS Schenectady
Without warning and with a report which was heard for at least a mile, the deck and sides of the
vessel fractured just aft of the bridge superstructure. The fracture extended almost instantaneously to the
turn of the bilge port and starboard. The deck side shell, longitudinal bulkhead and bottom girders fractured. Only the
bottom plating held. The vessel jack-knifed and the center portion rose so that no water entered. The bow and stern
settled into the silt of the river bottom.
The ship was 24 hours old.
Official CG Report attributed fracture to welds in critical seams that “were found to be defective”.
Adapted from D. Johnson
54
Design Strategy:
Stay above the DBTT!
Pre-WWII: The Titanic
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.)
WWII: Liberty ships
Reprinted w/ permission from R.W. Hertzberg,
"Deformation and Fracture Mechanics of Engineering
Materials", (4th ed.) Fig. 7.1(b), p. 262, John Wiley and
Sons, Inc., 1996. (Orig. source: Earl R. Parker, "Behavior of
Engineering Structures", Nat. Acad. Sci., Nat. Res. Council,
John Wiley and Sons, Inc., NY, 1957.)
Problem: Used a type of steel with a DBTT ~ Room temp.
1655
The Importance of Fracture Mechanics
 Selection of a Material
 Design of a Component
 Design of a Manufacturing or Testing Method
 Griffith flaw - A crack or flaw in a material that concentrates and
magnifies the applied stress.
56
Homework
57
Fatigue
(Failure under fluctuating / cyclic stresses)
Under fluctuating / cyclic stresses, failure can occur at loads considerably lower
than tensile or yield strengths of material under a static load: Fatigue
Estimated to causes 90% of all failures of metallic structures (bridges, aircraft,
machine components, etc.)
Fatigue failure is brittle-like (relatively little plastic deformation) - even in
normally ductile materials. Thus sudden and catastrophic!
Applied stresses causing fatigue may be axial (tension or compression), flextural
(bending) or torsional (twisting).
Fatigue failure proceeds in three distinct stages: crack initiation in the areas of
stress concentration (near stress raisers), incremental crack propagation, final
catastrophic failure.
58
Fatigue: S-N curves
(stress-number of cycles to failure)
Fatigue properties of a material (S-N curves) are tested in
rotating-bending tests in fatigue testing apparatus:
Result is commonly plotted as S (stress) vs. N (number of cycles to failure)
Low cycle fatigue: high loads, plastic and elastic deformation
High cycle fatigue: low loads, elastic deformation (N >105)
59
FATIGUE TEST METHODOLOGIES
Fatigue
Fatigue = failure under cyclic stress.
specimen
bearing
compression on top
bearing
motor
counter
flex coupling
tension on bottom
Adapted from Fig. 8.16,
Callister 6e. (Fig. 8.16 is
from Materials Science in
Engineering, 4/E by Carl. A.
Keyser, Pearson Education,
Inc., Upper Saddle River, NJ.)
Stress varies with time.
key parameters are S and sm
Key points: Fatigue...
can cause part failure, even though smax < sc.
causes ~ 90% of mechanical engineering failures.
1761
Cyclic Stress and Strain Fatigue
Results of the Fatigue Test
 Endurance limit - An older concept that defined a stress below
which a material will not fail in a fatigue test.
 Fatigue life - The number of cycles permitted at a particular
stress before a material fails by fatigue.
 Fatigue strength - The stress required to cause failure by fatigue
in a given number of cycles, such as 500 million cycles.
 Notch sensitivity - Measures the effect of a notch, scratch, or
other imperfection on a material’s properties, such as toughness
or fatigue life.
63
Fatigue: Cyclic Stresses
Periodic and
symmetrical
about zero
stress
Periodic and
asymmetrical
about zero
stress
Random
stress
fluctuations
64
Fatigue: Cyclic Stresses
Cyclic stresses are characterized by maximum, minimum and mean
stress, the range of stress, the stress amplitude, and the stress ratio
Remember the convention that tensile stresses are positive, compressive stresses are
negative
65
Fatigue: S—N curves
Fatigue limit (endurance limit) occurs for some materials (some Fe and Ti allows).
In this case, the S—N curve becomes horizontal at large N. The fatigue limit is a
maximum stress amplitude below which the material never fails, no matter how
large the number of cycles is.
66
Fatigue: S—N curves
In most alloys, S decreases continuously with N. In these cases the fatigue
properties are described by:
Fatigue strength:
stress at which fracture
occurs after specified
number of cycles (e.g.
107)
Fatigue life:
Number of cycles to fail
at specified stress level
67
Fatigue Design Parameters
Fatigue limit, Sfat:
 no fatigue if S < Sfat
Sometimes, the fatigue limit
is zero!
S = stress amplitude
unsafe
case for
Al (typ.)
safe
103
105
107
109
N = Cycles to failure
Adapted from Fig.
8.17(a), Callister 6e.
Adapted from Fig. 8.17(b),
Callister 6e.
68
S-N Graph
The stress-number of cycles to failure (S-N) curves for a tool steel
and an aluminum alloy
(c)2003 Brooks/Cole, a division of Thomson Learning, Inc. Thomson Learning ™ is a trademark used herein under license.
69
Fatigue Behavior of BCC versus FCC Metals
Fractographic Evidence of
Crack Growth
fatigue
striations
10 mm
71
Ni-base ALLOY TURBINE BLADE FATIGUE
FAILURE
Fatigue clam shell marker–band color changes due to
differences in service-related oxide layer thickness
Design and Safety Factors
Design uncertainties mean we do not push the limit.
Factor of safety, N
Often N is
between
y
 working 
1.2 and 4
N
Example: Calculate a diameter, d, to ensure that yield does not occur in
the 1045 carbon steel rod below. Use a factor of safety of 5.
 working 
220,000N


 d 2 / 4 


y
N
5
73
Fatigue Crack Growth

Once a crack is present in a material, it will tend to grow
under the influence of cyclic loading.

The crack may be initiated by fatigue, or may be preexisting from manufacture, or may be caused by an
impact, or similar event (e.g., a thermal shock.)

The crack will grow to a critical length then fracture of the
component will occur.
74
Fatigue: Crack Initiation and
Propagation
Three stages of fatigue failure:
1. Crack initiation in the areas of stress concentration
(near stress raisers)
2. Incremental crack propagation
3. Final rapid crack propagation after crack reaches
critical size
75
Fatigue: Crack Initiation and
Propagation
Crack initiation
at the sites of stress concentration (microcracks, scratches, indents, interior corners,
dislocation slip steps, etc.). Quality of
surface is important.
Crack propagation



Stage I: initial slow propagation along
crystal planes with high resolved
shear stress. Involves just a few
grains, and has flat facture surface.
Stage II: faster propagation
perpendicular to the applied stress.
Crack grows by repetitive blunting
and sharpening process at crack tip.
Rough fracture surface.
Crack eventually reaches critical
dimension and propagates very
rapidly.
76
Crack Growth Mechanisms
Load reduced
No Load
Loaded
Slip
No Load
Max Load
Loaded again77
Fatigue Fracture Surface
Schematic representation
of a fatigue fracture surface
in a steel shaft, showing the
initiation region, the
propagation of fatigue crack
(with beam markings), and
catastrophic rupture when
the crack length exceeds a
critical value at the applied
stress
(c)2003 Brooks/Cole, a division of Thomson Learning, Inc. Thomson Learning ™ is a trademark used herein under
license.
78
Cyclic Stress-Controlled Fatigue
Total Fatigue Life (NT) is the Sum of the Number of
Cycles Require to Initiate a Crack (Ni) plus the
Number of Cycles to Propagate the Crack (NP)
Factors That Affect Fatigue Life

Magnitude of stress (mean, amplitude..)

Quality of surface (scratches, sharp transitions and edges)
Solutions:

Polishing (removes machining flaws etc.)

Introducing compressive stresses (compensate for applied tensile stresses) into thin
surface layer by “Shot Peening” – firing small shot surface to be treated. High-tech
solution- ion implantation, laser peening.

Case Hardening – create C or N rich outer layer in steels by atomic diffusion from the
surface. Makes harder outer layer and also introduces compressive stresses.

Optimizing geometry – avoid internal corners, notches etc.
80
Improving Fatigue Life
Impose a compressive surface stress (to suppress surface cracks from growing)
Method 1: Shot Peening
Method 2: Carburizing
shot
put
surface
into
compression
Remove stress concentrators.
C-rich gas
Adapted from
Fig. 8.22, Callister 6e.
Adapted from
Fig. 8.23, Callister 6e.
bad
better
bad
better
81
Surface Treatment to Improve Fatigue
Resistance
SHOT–PEENED SURFACE in STEEL
Residual Stress Pattern after Shot Peening
Influence of Grinding & Shot–Peening
on Overall Fatigue Limetime
Laser Shot Peening Applied to Jet Engine Components
Factors That Affect Fatigue Life:
Environmental Effects
Thermal fatigue: Thermal cycling causes expansion and contraction, hence
thermal stress, if component is restrained.
Solutions:
 Eliminate restraint by design
 Use materials with low thermal expansion coefficients
Corrosion fatigue: Chemical reactions induce pits which act as stress raisers.
Corrosion also enhances crack propagations.
Solutions:
 Decrease corrosiveness of medium, if possible
 Add protective surface coating
 Add residual compressive stresses
86
Surprising Fatigue Failures
Creep
Creep is a time-dependent and permanent deformation of materials when subjected to a
constant load at a high temperature (> 0.4 Tm). Examples: turbine blades, steam generators.
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Elevated Temperature Effects on Materials
Standard Tensile Test
• Strain Rate Controlled Deformation
• Monitor Load versus Displacement to
Yield Stress versus Strain
Standard Creep Test
• Stress or Load Controlled Deformation
• Monitor Displacement versus Time to
Yield Strain versus Time Data
Stages of Creep
1. Instantaneous deformation,
mainly elastic.
2. Primary/transient creep. Slope
of strain vs. time decreases with
time: work-hardening
3. Secondary/steady-state creep.
Rate of straining is constant:
balance of work-hardening and
recovery.
4. Tertiary. Rapidly accelerating
strain rate up to failure: formation
of internal cracks, voids, grain
boundary separation, necking,
etc.
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Creep Test Data Analysis
Region I: Primary Creep
Region II: Secondary (Steady-State) Creep
Region III: Tertiary Creep
Deformation Mechanisms:
Region I: Strain Hardening mechanisms
associated with dislocation-obstacle
interactions. Strain rate decreasing.
Region II: Dynamic balance between
hardening and recovery mechanisms.
Dislocation-obstacle interactions versus
dislocation-dislocation annihilation.
Constant strain rate.
Region III: Prior to fracture, void growth,
cavitation, sample necking. Strain rate
increasing.
Mostly interested in Region II: Steady State
As this region represents the major portion
of component life.
Parameters of Creep Behavior
The stage secondary/steady-state creep is of longest duration and the steady-state creep
rate
is the most important parameter of the creep behavior in long-life
applications.
Another parameter, especially important in short-life creep situations, is time to rupture,
or the rupture lifetime, tr.
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Creep: Strain Time Plot
(c)2003 Brooks/Cole, a division of Thomson Learning, Inc. Thomson Learning ™ is a trademark
used herein under license.
A typical creep curve showing the strain produced as a function of
time for a constant stress and temperature
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Creep Behavior
Occurs at elevated temperature, T > 0.4 Tmelt
Deformation changes with time.
Adapted from Figs. 8.26 and 8.27, Callister 6e.
2195
Creep: Stress and Temperature Effects
With increasing stress or temperature
The instantaneous strain increases
The steady-state creep rate increases
The time to rupture decreases
96
Creep
Stress and Temperature Effects
The effect of temperature or applied stress on the creep curve
(c)2003 Brooks/Cole, a division of Thomson Learning, Inc. Thomson Learning ™ is a trademark used herein under license.
97
Creep
Stress and Temperature Effects
The stress/temperature dependence of the steady-state creep rate can be
described by
where Qc is the activation
energy for creep, K2 and n
are material constants.
(Remember the Arrhenius
dependence on
temperature for thermally
activated processes that we
discussed for diffusion?)
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Secondary Creep
stress exponent (material parameter)
 Q c 
.
n
es  K 2 exp 
 activation energy for creep
 RT  (material parameter)
Most of component
life spent here.
Strain rate is constant
at a given T, s
strain hardening is
balanced by recovery
Strain rate increases
for larger T, s
applied stress
200
100
40
20
10
Stress (MPa)
427C
538C
649C
Adapted from Fig. 8.29, Callister 6e.(Fig. 8.29 is from Metals
Handbook: Properties and Selection: Stainless Steels, Tool
Materials, and Special Purpose Metals, Vol. 3, 9th ed., D.
Benjamin (Senior Ed.), American Society for Metals, 1980, p.
131.)
strain rate
material const.
1
10-2
10-1
Steady state creep rate es (%/1000hr)
22
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Mechanisms of Creep
Different mechanisms are responsible for creep in different materials and under different
loading and temperature conditions. The mechanisms include
Stress-assisted vacancy diffusion
Grain boundary diffusion
Grain boundary sliding
Dislocation motion
Different mechanisms result in
different values of n, Qc.
100
Creep Mechanisms
101
Alloys for High-Temperature Use
(turbines in jet engine, hypersonic airplanes, nuclear reactors, etc.)
Creep is generally minimized in materials with:
 High melting temperature
 High elastic modulus
 Large grain sizes (inhibits grain boundary sliding)
Following materials are excellent resilient to creep:
 Stainless steel
 Refractory metals (containing elements of high melting
point, like Nb, Mo, W, Ta)
 Superalloys (Co, Ni based alloys)
102
High Temperature Structural Materials
103
Creep: Engineering Applications
104
Practical Implications for Engineering Applications
Negative or Limiting Implications:
Time dependent deformation means
component part shapes, sizes, properties vary
over time.
Critical Applications:
• Turbine blades in Jet Engines and Gas
Turbine Engines
• Nuclear Fuel Elements (Both in-service
and Long Term Storage)
Practical Implications for Engineering Applications
Negative or Limiting Implications:
Time dependent deformation means
component part shapes, sizes, properties vary
over time.
Critical Applications:
• Turbine blades in Jet Engines and Gas
Turbine Engines
• Nuclear Fuel Elements (Both in-service
and Long Term Storage)
107
Summary
Make sure you understand language and concepts:
Brittle fracture
Charpy test
Corrosion fatigue
Creep
Ductile fracture
Ductile-to-brittle transition
Fatigue
Fatigue life
Fatigue limit
Fatigue strength
Fracture toughness
Impact energy
Intergranular fracture
Izod test
Stress raiser
Thermal fatigue
Transgranular fracture
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