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Engineering MaterialsStrength & Fracture Analysis
Chapter 12
Fracture Control & Design Codes
Professor R. BellDepartment of Mechanical & Aerospace Engineering
Carleton University
2013
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Fati ue Desi n A roaches
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Infinite Life
Unlimited safety design stresses below fatigue limit
Finite life safety factor = 20 x design life
Cracks will exist inspection and repair
Damage Tolerant Refinement of fail-safe philosophy
Use of FM to predict crack growth
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Fati ue Life Im rovement O tions
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I Lower design Stress
II Smaller Initial flaw size
III Small improvement in
toughness
IV Large improvement intoughness
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Purpose:
the operating function. Efficiently
Economically
Predict Service Loads and Conditions
Calculate Stresses
pec y a er a s Size Components and Members
Consider Possible Failure Modes
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General Yielding or Excessive Plastic Deformation
Buckling or General Instability (elastic or plastic)
Sub-Critical Crack Growth
Fatigue
Stress-Corrosion
Corrosion Fatigue
Unstable Crack Extension
Ductile or Brittle
t er Corrosion
Creep
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To Design Against Brittle Fracture and Fatigue
Select materials with good fracture toughness
Engineers will:
Eliminate or minimize stress raisers
Control welding procedures
A Fracture Control Plan is ust a formalization of basic
requirements into a set of guidelines for a specific structure.
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Identification
of factors that ma contribute to failure. Descri tion of service loadin andconditions.
Establishment
of relative contribution of each of these factors to a possible failure.
e erm na on
of relative efficiency and trade-offs of various design methods to minimizepossibility of failure.
Recommendation
of specific design considerations to ensure the safety and reliability of thestructure.
material selection
es gn s ress eve s fabrication
inspection
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Notch Toughness
at particular service temperature loading rate plate
thickness Size of Crack
at possible fracture initiation sites
Tensile Stress Level
including residual stress
Fracture mechanics has shown that all three factors can beinterrelated to predict the susceptibility of a structure to brittle failure.
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Fracture Toughness
KIC slow loading
KIC(t) loading rate
KID impact loading
Loads measured
wind - waves calculation
-
Static or Fatigue loading
Fatigue
onstant mp tu e - rotat ng mac nery Variable Amplitude - bridges aircraft
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Chemical or Environment
corrosion
stress corrosion
corrosion fatigue
Establishment of minimum service tempua y o a r ca on - con ro n a e ec s ze
Notch Toughness
Judgement
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Establish Relative Contribution of Each Factor
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to Possible Fracture
1. Tensile Stress Level2. Material Toughness3. Crack Size
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Effect of Plane Stress Plane Strain Conditions
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Trade-Offs Decrease Desi n Stress
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Trade-Offs Im rove Fabrication
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The effect of the 3 Factors on Total Life
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TENSILE STRESS lar e effect on life
FCG is decrease when F is decreased. Design stressrange (max -min) primary factor to control.
rate of FCG is low for small flaws. Quality of inspection is
primary factor to control. MATERIAL TOUGHNESS
large effect on life when moving from plane-strainbehaviour to elastic lastic behaviour.
small effect on life when moving from elastic-plasticbehaviour to plastic behaviour
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Elements of A Fracture ontrol Plan
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Design
Stress Distribution Information
Estimates of stable cack growth for typical periods of service Recommendation for sae operating periods between inspections
Materials Strength and fracture properties
Recommended Heat and Material treatmentsw u
Fabrication
Control of Residual Stresses Protection of required Strength and Fracture properties Maintain fabrication and materials records
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Elements of A Fracture ontrol Plan
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Inspection Fabrication Inspections NDT Inspections Proof Testing Estimates of largest defect sizes
Operations Control of Stress levels and stress fluctuations in service Corrosion Protection Periodic In-service inspections
Decommissioning
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Fracture Control O tions
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PERIODIC INSPECTION: repair upon crack detection
FAIL SAFE DESIGN: repair upon occurrence of partial failure
DURABILITY DESIGN: replacement or retirement after time H.
PERIODIC PROOF TESTING: repair after failure in proof test
STRIPPING eriodic removal of crack
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INSPECTION METHODS
Visual Naked Eye Accessibility
ene ran o oure qu - ccess y
Magnetic Particles UV Light Magnetic
X-Ray Small surface flaws difficult
Ultrasonic
Eddy Current Cheap Acoustic Emission Interpretation difficult
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and repaired.
e structure s es gne or to erance olarge damage.
Crack arresters Multiple Load Paths
ea e ore rea .
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Durabilit Desi n
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CRACK ai
assumed to exist
me s eterm ne or a sma crac togrow from ai to ap
Structure retired or replaced at H/2.
This may be wasteful.
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If toughness low apermissible < adetectable at time t component subjected to proof Fracture occurs if crack aproofexists
If no fracture occurs, therefore a safe operational life from aproofto apermissible is ensured
pipelines and pressure vessels - hydro-tests
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StrippingDepartment of Mechanical &
Aerospace Engineering
This is an option with smallundetectable cracks if apermissibleExists.
la er surfaceofde th
ps aa
After stripping it would take H hours
for crack to grow from as to ap again- . pwould exist, therefore strippingwould be repeated every H hours.
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Damage Tolerance Analysis
20,000 - 50,000 man hours for an aircraft
Coupon Tests and Verification
Inspections
Repairs and Periodic replacement
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Loss of Human Lives
Impact on environment
Replacement of Structure
Damage to buildings and surrounding structures
Down time - loss of production
Loss of sales and contracts
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Regimes of LEFM, EPFM and Collapse
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Failure Analysis Diagram FAD
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Fractures from brittle to fully plastic can be represented on a FAD
Normalized FAD
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Fitness for Purpose Procedures
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R6 - Procedure
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)(;
yL
r
mat
rP
PL
K
KK
,are a number of options within R6 which
may be used to evaluate the functionf, which defines the shape of the failureassessment curve. The simplest choicefor f is the option 1 curve:
rrr LLLF ))65.0exp(7.03.0)(1.01()(
62
y
urr LL
1
2
1
max
e as c approac requ res on y mater a propert es tens e an racture
toughness) and simple calculations of K and PL. Stress intensity factors, K, andlimit loads, PL, are available for many components in compendia or maycalculated using FEM
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BS 7910 (PD 6493) Procedure
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BS 7910 is a revision of PD6493:1991
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BS 7910 (PD 6493) Procedure
Department of Mechanical &
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BS 7910 is a revision of PD6493:1991.
In PD 6493 there are 3 levels of assessment wereg ven.
Level 1, termed the `Preliminary Assessment'
method,
This level includes a limit of 0.7 on Krand 0.8 on Sr.These two limits result in a safety factor of 2 on flaw
size.so res r c ng r o . m s nom o e ess an e
yield stress i.e flow 1.2 yield
These safety factors allow Level 1 to be used as a
C
r
where
rap assessmen o sa e y o an ex s ng s ruc ure.The fracture behaviour in Level 1 can also beestablished in term of CTOD
forceresistancethe
KforcedrivingtheYS
2
I
C
IE
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BS 7910 (PD 6493) Procedure
p f
Aerospace Engineering
PD 6493 - Level 2, the `Normal Assessment' method wassimilar to the 1980 edition (Rev. 2) of the R6 method.
s s e pre erre assessmen eve or e ma or y ostructural application and it is based on the Dugdale stripyield model
21
2 2secln,
rrrr SSK
rr
FS
KK
Level 3, the `Advanced Assessment' method was based onrevision 3 of R6 and allowed the user to take account
Lmat
.
levels, the fracture toughness could be expressed in terms ofCTOD or Kmat.
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BS 7910 (PD 6493) Procedure
p f
Aerospace Engineering
Fracture - FAD Fatigue
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Engineering Treatment Model - CDFAerospace Engineering
Crack Driving Force CDFGKSS Germany
,which for the fully plastic range, utilises the
material's stress-strain behaviourrepresented by a piece-wise power law toestimate the quantities 5, J-integral, and the
deformation of a cracked body
End-of-life failure conditions can be assessed
in terms of , applied load expressed as force, F,
moment, M, internal pressure, p, stress, ; applied strain, a.
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Engineering Treatment Model - CDFAerospace Engineering
Crack Driving Force CDFGKSS Germany
Point Value of Fracture
Failure conditions are given by the
intersection of the ETM curve with the,represented by Jmat or 5Ymat,
R-Curve Analysis
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SINTAP ProcedureAerospace Engineering
The SINTAP procedure offers both a FAD and a CDF route.They are complementary and yield identical results.
The underlying principles of the SINTAP method are:
a hierarchical structure based on quality of available data inputs; decreasing conservatism with increasing data quality; detailed guidance on determination of characteristic
input values such as fracture toughness;
the choice of representation of results in terms of a FAD or CDF; specifc methods for allowing for the effect of weld strength mismatch; guidance on dealing with situations of low constraint and, for
components containing pressurised fluids, leak before break analysis; compendia of solutions for stress intensity factors, limit load solutions
and weld residual stress profiles.
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SINTAP ProcedureAerospace Engineering
FAD Approach
Failure Assessment Linerr LfK )(
yr
matr LKK
structuretheoflimitcollapseplasticthedefinemaxrr LL
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SINTAP ProcedureAerospace Engineering
CDF Approach
Failure Assessment Linerr
K
LfK
)(
y
r
mat
r
K
structuretheoflimitcollapseplasticthedefinemaxrr LL
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ReferencesAerospace Engineering
PD6493:1991: Guidance on methods for assessing the acceptability of Flaws inFusion welded structures. British Standards Institution, London, 1991.
BS 7910:1999: (incorporating Amendment No.1) Guide on methods for assessingthe acceptability of Flaws in metallic structures. British Standards Institution,
, .
U. Zerbsta,*, R.A. Ainsworthb, K.-H. SchwalbeaBasic principles of analytical Flaw assessment methods,International Journal of PressureVessels and Piping 77 (2000) 855-867
C.S. Wiesnera, S.J. Maddox, W. Xua, G.A. Webster, F.M. Burdekin,R.M. Andrews, J.D. HarrisonEngineering critical analyses to BS 7910 the UK guide on methods for assessing theacceptability of Flaws in metallic structuresInternational Journal of Pressure Vessels and Piping 77 (2000) 883-893
P.J. Budden J.K. Shar les A.R. Dowlin
The R6 procedure: recent developments and comparison with alternative approachesInternational Journal of Pressure Vessels and Piping 77 (2000) 895-903 Ted L. Anderson, David A. Osage
API 579: a comprehensive Fitness-for-service guide
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n erna ona ourna o ressure esse s an p ng -
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ReferencesAerospace Engineering
R.A. Ainsworth, A.C. Bannister, U. Zerbst
An overview of the European aw assessment procedure SINTAP and its validation
International Journal of Pressure Vessels and Piping 77 (2000) 869-876
Broek, D., The Practical Use of Fracture Mechanics , Kluwer Academic Publishers,1988
Barsom, J., and Rolfe, S.T., Fracture and Fatigue Control in Structures, 2nd Ed.,Prentice-Hall, 1987
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