ccopps webinar slides

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 ( )


Agenda

• Introduction and Relevant CCOPPS ActivityJim Wood

• Fatigue of Welded Pressure VesselsSteve Maddox

• Q&A SessionSteve Maddox & Jim Wood

• Closing RemarksJim Wood


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/


Agenda






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


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


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 )


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


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


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


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


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’


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 ⎠⎝


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


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


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


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


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


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


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


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


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


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−=

⎟⎠

⎜⎝


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


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.


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


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


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.


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


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


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


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


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.


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


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


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


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


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)


Thank you for your attentionThank you for your attention


Agenda





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