ccopps webinar slides
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World Centre for Materials Joining Technology
Fatigue of Welded Pressure VesselsCCOPPS WebinarCCOPPS WebinarCCOPPS WebinarCCOPPS Webinar
Wednesday 21Wednesday 21stst May 2008May 200815:00 15:00 –– 16:00 BST16:00 BST
Jim Wood University of Strathclyde
Steve Maddox The Welding Institute (TWI) y yg ( )
World Centre for Materials Joining Technology
Agenda
• Introduction and Relevant CCOPPS ActivityJim Wood
• Fatigue of Welded Pressure VesselsSteve Maddox
• Q&A SessionSteve Maddox & Jim Wood
• Closing RemarksJim Wood
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Industry Needs Survey
• Findings relevant to this webinar:– FEA use is increasing as is complexity of modelsFEA use is increasing as is complexity of models– Interfacing with codes of practice seen as issue– Happy with facilities in commercial codes in general
(exception is weld modelling and assessment + automation(exception is weld modelling and assessment + automation of the analysis process)
– Non-European Codes are used often by most respondents.P li i t il bl f d l d f• Preliminary report available for download from http://www.ccopps.eu/
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World Centre for Materials Joining Technology
World Centre for Materials Joining Technology
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World Centre for Materials Joining Technology
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World Centre for Materials Joining Technology
World Centre for Materials Joining Technology
World Centre for Materials Joining Technology
World Centre for Materials Joining Technology
World Centre for Materials Joining Technology
World Centre for Materials Joining Technology
Agenda
• Introduction and Relevant CCOPPS ActivityJim Wood
• Fatigue of Welded Pressure VesselsSteve Maddox
• Q&A SessionSteve Maddox & Jim Wood
• Closing RemarksJim Wood
World Centre for Materials Joining Technology
Fatigue design of welded pressure vessels• BS PD 5500 Annex C,
EN 13445-3 and ASME VIII, Division 2
• Detailed fatigue assessment - use of design curves - classification of weld detailsASME VIII, Division 2
(new structural stress approach)
- classification of weld details - stress concentrations - fatigue life improvement
• Fatigue failure in pressure vessels
• Fatigue design data
• Simplified assessment methods• Fatigue assessment of welding
imperfectionsFatigue design data• Stresses used in
fatigue design
imperfections• Future needs
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Effect of welding on fatigue resistance400
200
300
400
Stressrange,
50
100
R=0
MPa
50 R=0Grade 50 structural steel
Cycles105 106 107 108
10
Cycles
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Fatigue cracking in a welded joint
F ti kiFatigue crackingfrom weld root
XFatigue failure in plate at toe(from pre-existing sharp flaw at X)
X
( p g p )
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Key features of welds and welded joints• Sharp section changes - High SCFp g g• Local discontinuities - Crack initiation sites• High tensile residual stresses - Maximum mean stress
effect, compressive stresses damagingConsequences:
Relatively low fatigue strength• Relatively low fatigue strength,• dominated by fatigue crack growth• and controlled by full applied stress rangeand controlled by full applied stress range.• Fatigue life not increased by use of higher strength
material
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Fatigue failure in pressure vesselsSources of stress
Fatigue loadingSources of stress
concentration and hence fatigue cracking
• Pressure fluctuations• Temperature changes
• Joints (welds, bolts)• Geometric discontinuities
( i l d• Temperature differentials
• External mechanical
(openings, nozzles, ends, supports)
• Temporary attachments• External mechanical loading
• Vibration
Temporary attachments
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Fatigue failure from weld detailsFatigue cracking from inside ofFatigue cracking from inside ofFatigue cracking from inside of Fatigue cracking from inside of butt weld between a cylindrical butt weld between a cylindrical
shell and a flat endshell and a flat end
Fatigue cracking from nozzle Fatigue cracking from nozzle weld toeweld toe
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Fatigue design processCompare number of repetitions (n ) of stress range SrCompare number of repetitions (ni ) of stress range Sri
which vessel or part of vessel must withstand in its service life with the number (Ni ) withstood by representative
fspecimens at same stress in fatigue tests, such that:
01nΣtnnn i321 01NnΣetc
Nn
Nn
Nn
i
i
3
3
2
2
1
1 .≤=+++
N values obtained from relevant design S-N curves
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Basis of fatigue design data and assessment method for welded jointsassessment method for welded joints
• BS PD 5500 and EN 13445 adapted from nominal stress-based fatigue rules for welded structures (bridges, offshore structures, etc)rules for welded structures (bridges, offshore structures, etc)
• Thus, grid of S-N curves each applicable to one or more particular weld detail, chosen on the basis of a classification system
• In general, curves related to structural stress rangeg , g• For potential fatigue failure from weld toe, structural stress range at toe
(hot-spot structural stress range) may be used• New ASME VIII uses only hot-spot structural stress calculated in a
specific way and converted to a fracture mechanics–based parameter called the ‘Equivalent structural stress range parameter’
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Fatigue design curves for weld details in BS PD 5500 and other UK Standards
E = 2.09 x 105 N/mm2 • Applicable to any of metals covered b PD 5500Stress
rangeSr
N/mm2
ClassDEFF2G
Sr5N = constant
for N > 107 cycles
by PD 5500, on basis of relative E values
Sr3N = constant
for N < 107 cycles
GW • For thickness
e > eref, where eref
=22mm allowable
Endurance N, cycles107
=22mm, allowable stress 25.0
refr ee.S ⎟
⎠
⎞⎜⎝
⎛=y e ⎠⎝
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Weld detail classification in BS PD 5500
Classification depends on:• welded joint geometry
direction of loading• direction of loading• crack initiation site• methods of manufacture and inspectionmethods of manufacture and inspection
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Stresses used in BS PD 5500 for fatigue design
• Structural (primary + secondary) stress range• Principal stress (not stress intensity) used directlyPrincipal stress (not stress intensity) used directly • Nominal stress for simple details (e.g. attachments,
seam welds)seam welds)• Nominal stress x SCF for structural details, or• Hot-spot structural stress at any weld toe (HigherHot spot structural stress at any weld toe (Higher
design curve than those used with nominal stress)• Net section of load-carrying fillet weldsNet section of load carrying fillet welds
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Fatigue design S-N curves for weld details in EN 13445
100
ClassClass = fatiguestrength in N/mm2
at 2 x 106 cycles•Currently applicable only
Stressrange
Sr
N/mm2
90807163565045
Sr5N = constant
for N > 5x106
cycles
to steel•For thickness
e > 25mm454032
Sr3N = constant
6
e > 25mm, allowable stress
25.025 ⎞⎛
Endurance N cycles2x106 5x106
rfor N < 5 x 106 cycles
25.0
r emm25.S ⎟
⎠⎞
⎜⎝⎛=
Endurance N, cycles
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Stresses used in EN 13445 for fatigue designfatigue design
• Structural (primary + secondary) stress rangeO• Option to use principal or equivalent stress range
• Nominal stress for simple details (e.g. attachments, ld )seam welds)
• Structural stress at any weld toe (Hot-spot stress)• Net section of load-carrying fillet welds
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Weld detail classification in EN 13445EN 13445 offers choice of using equivalent stress or principal stress. Since equivalent stress
Ring stiffener
Since equivalent stress has no direction, a consequence is that the Class 80
lowest detail Class must be assumed.Thus, the joint shown
Vessel shellHoop stress
Longitudinal , jwould be designed as Class 71.Class 71
Longitudinalstress
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Fatigue design curves from ASME VIII 1500
• Applicable to any of
1000
1500
ΔSess
MPa/mm-0.222
ASME 'Master' fatigue design curves
pp ymetals covered by ASME on basis of relative E values
200
300
400500
MPa/mm Mean - 2 SD
M 3SD
• Stress parameter depends on plate thickness, membrane/bending
100
200 Mean - 3SD(normal design curve)
membrane/bending stress ratio and applied stress ratio (Not units of stress)
103 104 105 106 107 108
Endurance, cycles
50
(Not units of stress)• No other thickness correction required
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Basis of fatigue design curves• Regression analysis of S N data obtained from tests• Regression analysis of S-N data obtained from tests
on actual welded joints• ≈2 5% probability of failure (mean 2 standard• ≈2.5% probability of failure (mean – 2 standard
deviations of log N [SD]):-BS PD 5500 design curves; included in ASMEBS PD 5500 design curves; included in ASME.
• ≈ 0.1% probability curves (mean - 3SD):-BS PD 5500 simplified design methods; all EN 13445BS PD 5500 simplified design methods; all EN 13445 design curves for weld details; generally required for ASMES
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Application of design curves for weld detailsweld details
• No effect of applied mean stress in PD 5500 and EN 13445; mean stress correction in ASMEEN 13445; mean stress correction in ASME
• No effect of material tensile strength• No effect of welding process• No effect of welding process• May need to be reduced to allow for corrosive
environment; no specific guidance in BS PDenvironment; no specific guidance in BS PD 5500 or EN 13445 but penalty factors specified in ASME
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Stresses used in ASME VIII Div. 2 for fatigue design of welded joints
• Structural (primary + secondary) stress range based on through-thickness stress distribution obtained by numerical analysis y
• Structural equivalent stress range parameter (a function of material’s fatigue crack propagation properties (m=3.6),applied structural stress range (Δσ) material thickness (t)applied structural stress range (Δσ), material thickness (t),membrane to bending stress ratio and applied stress ratio):
S σΔ
Messess
f.I.tS
m1
m2m2−=
σΔΔ
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Generalized stress parameter (Maddox, 1974)
)(d)c2/ashapefrontcrack&t/a(fY,aYK,)K(CdN/da
a
m
fa
πσΔΔΔ ===
NtCI)Y(
)(d
m
)1(mm
tat
a
1
2mt
fa
tia
σΔπ
⎤⎡
==∫∴ −
( )
ttanconsC1N.
Ite.i
)1( m1
2m
σΔ ==⎥⎥⎥
⎦
⎤
⎢⎢⎢
⎣
⎡
⎟⎟
⎠
⎞
⎜⎜
⎝
⎛ −
( )t'parameterstressdGeneralize'where
,ttanconsaN.or
1*
m*
m1
2m
σΔσΔ
σΔ
⎟⎟⎞
⎜⎜⎛
=
=
−
)SASMEcf(I.t
*or
Ip
essm1
m2m2 ΔσΔσΔ M−=
⎟⎠
⎜⎝
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ASME Equivalent structural stress range parameter ΔSessg p ess
Based on:• Structural stress (presumably assumed to allow forStructural stress (presumably assumed to allow for
SCF factor Mk normally applied to stress intensity factor))
• Fatigue life mainly growth of a pre-existing crack• Implicit assumptions made about initial flaw size p p
and shape• Specific fatigue crack growth rate relationshipp g g p
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Hot-spot structural stress approach
Fatiguek “Hot spot”crack Hot-spot
Hot-spot stress = structural stress at weld toe.at weld toe.It includes all stress concentrating effects except the local notch effect of the weld toeof the weld toe.
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Hot-spot structural stress from through-thickness stress distribution
Structural stressActual stress
σhot-spot
τmτ(y) (y)t
σ σb
τ(y) σx(y)
σmσb
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Methods for calculating the hot-spot structural stressstructural stress
· Using surface stresses (e.g. measured):Surface stress extrapolation (SSE)
· Using through-thickness stress distribution (e.g. from numerical analysis):- Through-thickness integration (TTI)- Nodal forces (NF) method
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Hot-spot stress developments in British Standards• Current TWI joint-industry project aims to produce guidance on hot-spot
str ct ral stress approach for incl sion in British Standard fatig estructural stress approach for inclusion in British Standard fatigue design rules (initially BS 7608)
• Comparison of the three methods of calculating hot-spot stress (SSE, TTI and NF) from FEATTI and NF) from FEA
• Solid and shell elements, examination of sensitivity to mesh size• Case studies on range of structural components• All methods mesh sensitive but mesh sensitivity least for simple welded
joints in plates under unidirectional loading• Findings so far indicate that nodal force method most mesh sensitive of
the three when applied to structural component• Mesh insensitivity of the ASME method may be for a restricted set of
possible mesh types and weld meshing options.
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Fatigue data expressed in terms of hot-spot stress range (SSE method)
300
400
500
200
300Hot-spot
stress rangeat weld toe
N/mm2
Cl E
90100
Class E
Pressure vessels - nozzles Plate welded components
95% fid i t l
104 105 106 107 3x107
Endurance, cycles
60
95% confidence intervals enclosing data
Endurance, cycles
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Comparison with mean – 2SD S-N curves derived from ASME VIII
500
300
400
500
Hot-spotstressrange
at
Pressure vessel data Structural specimen
or component data
Mean ± 2SD
200
atweld toe
N/mm2
BS PD 5500Cl E
90100
ASME VIII, Div.2R ≤ 0, t ≤ 16mmlower: all membraneupper: all bending
Class E
104 105 106 107 3x107
Endurance cycles
60
upper: all bending
Endurance, cycles
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Comparison with mean – 3SD S-N curves derived from ASME VIII
500
300
400
500
Hot-spotstressrange
at
Pressure vessel data Structural specimen
or component data
Mean ± 3SD
200
atweld toe
N/mm2
EN 13445Class 71
90100
Class 71Class 63
ASME VIII, Div.2R ≤ 0, t ≤ 16mmlower: all membrane
104 105 106 107 3x107
Endurance cycles
60
upper: all bending
Endurance, cycles
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Misalignment as a source of stress concentration
Bending stresses due to misalignmentto misalignment
Same guidance on calculation of SCF is given in PD5500 and EN 13445PD5500 and EN 13445
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Treatment of weld details subject to combined or multi-axial loadingg
ΔσL = Maximum change in σL
Δ Maximum change inσ ΔσH = Maximum change in σH
Δσ45 = Maximum change in σH45σH
σ45
σL
Principal stresses then calculatedfrom ΔσL, ΔσH, and Δσ45
Same approach in PD5500 and EN 13445. Current research should provide better method for out-of-phase loading.
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ASME t t t f ld d t il bj tASME treatment of weld details subject to multi-axial loading
5.022
ess 1m21m2 3)(F
1S ⎥⎥⎤
⎢⎢⎡
⎟⎟⎞
⎜⎜⎛
+⎟⎟⎞
⎜⎜⎛
=λΔσΔΔ
essMessess
m1
m2m2
m1
m2m2
I.tf.I.t)(F⎥⎥⎦⎢
⎢⎣
⎟⎠
⎜⎝
⎟⎠
⎜⎝
−−δWhere:F i h l di ( i i l t di ti i t tFor in-phase loading (principal stress directions remain constant throughout loading cycle), F(δ) = 1while For out-of-phase loading (principal stress directions change during loading cycle), F(δ) = function of applied normal and shear stresses and out-of-phase angle, or conservative value of 1/√2
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Elastic-plastic (E-P) conditionsStressrange
BS PD 5500 & EN 13445:
Design S-N curveCorrectedforplasticity
For stress ranges > twice yield:E-P strain obtained directly from analysis and
converted to stress or plasticityorStresses from design S-N curves reduced by
factor that depends on load source (mechanical or thermal) and stress range
ASME VIII, Div. 2:
103 104 105 106 107 108
Endurance N, cycles
/ yield.
For every case:
E-P strain obtained directly from analysis (Neuber’s rule & material’s cyclic stress-strain properties) and converted to stress rangecyclic stress strain properties) and converted to stress range
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Fatigue life improvement methods
• BS PD 5500 and EN 13445 allow weld toe i di d di i i d igrinding and corresponding increase in design
classification• ASME VIII accepts weld toe grinding, TIGASME VIII accepts weld toe grinding, TIG
dressing or hammer peening. Equivalent structural stress parameter design curves increased accordingly with greatest benefit inincreased accordingly, with greatest benefit in high-cycle regime
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Assessment of welding flawsGuidance given in BS PD 5500 and ENGuidance given in BS PD 5500 and EN
13445 on flaw assessment based on fitness-for-purpose:
• Reference to BS 7910Reference to BS 7910• Specific recommendations on:1. Planar flaws not acceptable2. Buried flaws - inclusions, porosity;, p y;3. Deviations from design shape -
misalignment, peaking, ovality.ASME seems to accept planar flaws since
equivalent structural stress parameter can be calculated for cracks up to 10% of section thickness
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Fatigue design of pressure vessels -future needs
• Parametric hot-spot SCFs for pressure vessel details
• Elastic-plastic fatigue• Effect of environment (corrosive, elevated
temperature, hydrogen)• Closer link between design and fabrication quality• Experimental methods for design (draft for EN• Experimental methods for design (draft for EN
13445 now available)
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Thank you for your attentionThank you for your attention
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Agenda
• Introduction and Relevant CCOPPS ActivityJim Wood
• Fatigue of Welded Pressure VesselsSteve Maddox
• Q&A SessionSteve Maddox & Jim Wood
• Closing RemarksJim Wood
World Centre for Materials Joining Technology
Agenda
• Introduction and Relevant CCOPPS ActivityJim Wood
• Fatigue of Welded Pressure VesselsSteve Maddox
• Q&A SessionSteve Maddox & Jim Wood
• Closing RemarksJim Wood