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MAAE 4012Strength & Fracture
Chapter 8
Professor R. BellDepartment of Mechanical & Aerospace Engineering
Carleton University
2013
Chapter 8 - EAC 1
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Environmentall Assisted Crackin
Department of Mechanical
& Aerospace Engineering
Under the action of a monotonic stress, corrosive environments can
Effect of Environment on Crack growth
levels that would not cause initiation or crack growth in inertenvironments
Occurs at a stress much less than the yield stress or at a stress
n ens y muc ess an e rac ure oug ness
Frequently no obvious indication of the effect of corrosion on sub-
critical crack growth Results in unex ected failure once the critical crack len th is
reached
Two Major Types of Environmentally Assisted Cracking
Stress Corrosion Cracking
Hydrogen Embritt lement
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Environmentally Assisted Cracking
Department of Mechanical
& Aerospace Engineering
No universal theory to explain these damage processes
Lack of theory is due to the complex nature of
Fracture Mechanics at the crack tip
The electrochemistry of the corrosion process
The physical metallurgy of the materials
Chapter 8 - EAC 3
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Examples of Stress Corrosion
Department of Mechanical
& Aerospace Engineering
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Examples of Stress Corrosion
Department of Mechanical
& Aerospace Engineering
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Examples of Stress Corrosion
Department of Mechanical
& Aerospace Engineering
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Material-Environment Corrosion Combinations
Department of Mechanical
& Aerospace Engineering
Material Susceptible Environments
Ordinary Steels NaOH solutions
,
Mixed acids (i.e. N2SO4HNO3)
HCN solutionAcidic H2S solutions
Seawater
Aluminum Alloys NaCl H2O2 solutions
NaCl solutions and seawater
Air and water
Stainless Steels Acidic chloride solutions (MgCl2, BaCl2)
NaCl-H2O2 solutions
Seawater
Copper Alloys Ammonia vapours & solutionsWater and water vapour
Not to be taken as complete
Chapter 8 - EAC 7
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Stress Corrosion Test Methods
Department of Mechanical
& Aerospace Engineering
Two main Categories of Corrosion Tests
Time to-Failure Tests
Crack growth rate tests
Tests based on Smooth specimens - crack initiation
Tests based on re-cracked FM s ecimens stress
corrosion crack growth
Chapter 1 - Introduction 8
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Stress Corrosion Test Methods
Department of Mechanical
& Aerospace Engineering
Ti-8Al-1Mo-1V alloy in 3.5% NaCl solution
Initial K plotted against time to failure
Threshold KISCC or KIEAC
No rigorous proof of an environmental
threshold, therefore the use of KIEAC or
treated with caution
< IEAC o a ure even a ter ong exposure
KIEAC < K < KIC Subcritical flaw growth with final fracture
K >K Immediate failure with initial loadin
Chapter 8 - EAC 10
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Cantilever Beam Test Specimen
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& Aerospace Engineering
Procedure to obtain K ISCCrequires multiple specimens
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Wedge-Opening-Loading Test Specimen
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& Aerospace Engineering
Basic principle of WOL specimen
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K ISCC - A Material Property
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& Aerospace Engineering
Canti lever beam specimen
Constant load
The KI value increases
as the crack lengthincreases
Increase in K leads
to fracture of specimen
Wedge Opening Loading specimen - WOL
The KI value decreases as the crack length increases Decreasing load
Constant displacement i.e. Constant crack opening displacement
Decrease in K leads to crack arrest at K
Chapter 8 - EAC 13
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K ISCC - A Material Property
Department of Mechanical
& Aerospace Engineering
Influence of specimen geometry and test duration
Elapsed Time (hrs) Apparent K ISCC(Ksi in)
100 170
1,000 115
,
Influence of Cut-off Time on KISCC measured
with Cantilever Bend Specimens
Influence of specimen geometry on time to
failure for 4340 steel Barsom & Rolfe p 288
Chapter 8 - EAC 14
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Crack Growth Rate Tests
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& Aerospace Engineering
Involves the measurement of the
crack growth rate per unit time da/dt as
a function of the instantaneous stress
intensity factor
3 Regions
Region I
rate of SCC growth is strongly
dependent on the magnitude of KI SIF below Kth cracks do not
propagate
Threshold SIF corresponds to
KISCC
Chapter 8 - EAC 15
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Crack Growth Rate Tests
Department of Mechanical
& Aerospace Engineering
Region II
SCC growth above KISCC
strength steels in gaseous H2 aswell as other environment- material
systems appear to be independent
Type B primary driving force
for crack growth is not mechanical
the crack tip such as chemical,
electrochemical, mass transport,
diffusion and absorption processes
Region III
rate of SCC growth increases
rapidly with KI as KI approaches
Chapter 8 - EAC 16
IC C
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Corrosion Fatigue
Department of Mechanical
& Aerospace Engineering
By definition Fatigue in a corrosive environment
Corrosion fatigue can occur in most materials
Corrosion fatigue is time dependent therefore
wave shape
Crack Initiation and crack growth both affected by
Different from SCCEffect of environment and frequency on
S-N curve of Mild Steel Schijve p45
Chapter 8 - EAC 17
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Corrosion Fatigue Crack Growth
Department of Mechanical
& Aerospace Engineering
Effect of load shape
Crack Growth rates below KISCC under
various load shapes Barsom &Rolfe, p 321
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Prevention of Corrosion Fatigue failures
Department of Mechanical
& Aerospace Engineering
Isolate the environment and material metallic coatings paint ceramic and rubber
liners clading
Alter the severity of the environment chemically remove aggressive constituents
increase pH decrease temp, flow rate and concentration
Alter the surface characteristics introduce surface compressive stresses (reduces crack
initiation and propagation)
Substitute a more corrosion resistant material
design the components to prevent the initiation and propagation of cracks to a critical size
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Prevention of Corrosion Fatigue failures
Department of Mechanical
& Aerospace Engineering
Cathodic Protection
Welded T Plate
Initiation Life
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Prevention of Corrosion Fatigue failures
Department of Mechanical
& Aerospace Engineering
Cathodic Protection
Welded T Plate
Total Life
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Prevention of Corrosion Fatigue failures
Department of Mechanical
& Aerospace Engineering
Cathodic Protection
Welded T Plate
Total Life
vs Air Data
vs design curve
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Corrosion Fatigue failures
Department of Mechanical
& Aerospace Engineering
1968, failure of pressure bulkhead
Loss of control of rudder and elevators
85 fatalities no survivors
Corrosion prevention by sealing paste
and drain hole
Corrosion cracks started at A and grew
along the edge of the bonded strip
At cruising altitude temp difference
across the bulkhead - condensation
small quantities of water is trapped at edgeof bonded strip local corrosion damage
and thus fatigue cracks
Inspection of this area difficult If bonded strip bonded outside bulkhead
Corrosion resistance of this Al alloy poor Bonded strip better than riveted strip
Sealer applied to prevent corrosion
If sealer applied to edge of bonded strip
corrosion fatigue avoided
Ref. Schi ve, 475
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Hydrogen Embrittlement
Department of Mechanical &
Aerospace Engineering
Hydrogen is present as atmospheric humidity
At high temperatures atmospheric molecular hydrogen can
This is capable of diffusing though metallic crystals Atomic hydrogen can also be present as a result of:
Electrolysis
Electroplating
Corrosion reactions
,zirconium, the dissolved hydrogen reacts to form brittle hydridecompounds
In zirconium this can lead to delayed hydride cracking
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Department of Mechanical &
Aerospace Engineering
In 1974 water leaked from the primary transport system into thegas annulus
The Zircaloy-2 tubes were removed and it was found that the
Chapter 8 - EAC 25
ea was n e area o e ro e o n
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Department of Mechanical &
Aerospace Engineering
In a Pickering reactor there are 139 pressure tubes, each about 6 m (240 in) long,
. . . .
They are made from a Zirconium alloy with 2.5% niobium Zr-Nb.
The pressure tubes are joined to end fittings which are made of stainless steel.
Chapter 8 - EAC 26
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-
Department of Mechanical &
Aerospace Engineering
The stainless steel end fitting was
revealed in the tube
When rolling the tubes into the end
the parallel part of the end fittingsto produce an over-extended joint
stresses
The cause of the cracking was
Cracking (DHC)
Chapter 8 - EAC 27
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Department of Mechanical &
Aerospace Engineering
The stainless steel end fitting was
in the area of high residual stress
Hydrogen, normally in the pressuretubes had migrated to the area of
hi h residual stress and a smallcrack initiated
Propagation was by fracture of thehydrides which are brittle when
cold Once initiated the crack proceed to
grow through the wall thickness byrepeated formation and fracture ofthe hydrides when the system was
When the system was hot thehydrogen was in solution and crack
growth did no proceed
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Department of Mechanical &
Aerospace Engineering
The cause of the rolling problem was
into the tapered section of the end fitting
The residual stress reached levels of 700MPa
MPa
Fuel channel safety was not significantly
effected because of the leak-before-breakcriterion
Cracks were about 15-20 mm (0.6-0.8in.) in surface length and were confined to
a very small region
The crack len ths were si nificantl less
than the critical crack length of about 3 in. The leak-before-break criterion was well
demonstrated
Chapter 8 - EAC 29
f h l
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Department of Mechanical &
Aerospace Engineering
J.M. Barsom and S.T. Rolfe, Fatigue & Fracture Control in Structures,Prentice Hall, 1987.
P.A. Ross-Ross, The Investigation into the Cracking of Pressure Tubes inPickering Units 3 and4, From Steam to Space, CSME 1996.
Standard Method for Plane-Strain Fracture Toughness of MetallicMaterials. ASTM Specification E-399-83
Chapter 6 - Fatigue 30
D t t f M h i l &
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Department of Mechanical &
Aerospace Engineering
N.E. Dowling, Mechanical Behavior of Metals, Prentice Hall, Newersey, .
A. Baumel and T. Seeger, Materials Data for Cyclic Loading,Elsevier, Amsterdam, 1990.
. ,Prismatic Bodies with Arbitrary Non Linear Stress-Strain Law,
ASME Journal of Applied Mechanics, vol. 28, 1961, pp. 544-550.
K. Molski and G. Glinka A Method of ElasticPlastic Stress andStrain Calculation at a Notch Root, Material Science andEngineering, vol. 50, 1981, pp. 93-100.
A. Moftakhar, A. Buczynski and G. Glinka, Calculation of Elasto-
Plastic Strains and Stresses in Notches under MultiaxialLoading, International Journal of Fracture, vol. 70, 1995, pp.357-373.
Chapter 6 - Fatigue 31
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References
Department of Mechanical &
Aerospace Engineering
J.M. Barsom and S.T. Rolfe, Fatigue & Fracture Control inStructures, Prentice Hall, 1987.
Bannantine, J.A., Comer, J.J. and Handrock, J.L. "Fundamental ofMetal Fatigue Analysis (Prentice Hall, 1990)
mar- aess, . a gue an oo - s ore ee s ruc ures .(Tapir Publishers, Trondheim, Norway, 1985)
Chapter 6 - Fatigue 32
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ReferencesDepartment of Mechanical &
Aerospace Engineering
J.M. Barsom and S.T. Rolfe,
Fatigue & Fracture Control in Structures , Prentice Hall, 1987.
P.A. Ross-Ross,
e nves ga on n o e rac ng o ressure u es n c er ngUnits 3 and4 , From Steam to Space, CSME 1996.
Standard Method for Plane-Strain Fracture Toughness of Metallic
Materials .ASTM S ecification E-399-83
Brock, D.
"Elementary Engineering Fracture Mechanics"
2nd Edition, Martinus Ni hoff Publishers, 1982
Brock, D.
"The Practr ical Use of Fracture Mechanics
Kluwer Academic Publishers 1988
Anderson, T.L Fracture Mechanics - Fundamentals and Applications
2 nd Edition CRC Press Boca Raton 1995
Chapter 6 - Fatigue 33