effects of flaws and corrosion on the structural integrity ... · effects of flaws and corrosion on...

Stijn Hertelé - Lecture Lasgroep Zuid-Limburg - 13 September 2016

Effects of flaws and corrosion

on the structural integrity of welds

Are fitness-for-service standards unambiguous?

What is the role of material properties?

How can experiments help us?

… and how about numerical models?

Prof. dr. ir. Stijn Hertelé

Ghent University, Belgiumstijn.hertele@ugent.be

www.soetelaboratory.ugent.be

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Welds are a potential weakest structural link

Stijn Hertelé - Lecture Lasgroep Zuid-Limburg - 13 September 2016Laboratory Soete – Stijn Hertelé – Public PhD defence – May 22nd, 2012 Image source: lincolnelectric.com

Welder skills?

Welding

parameters?Alignment

of pipes?

Material?

Environment

Welds are far from flawless

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Welds are far from flawless

What made this weld survive 65 years of operation?!

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Weld defect economics: repair or tolerate a safety risk?

‘Imperfection’

Accept WMSSafe

Potentially

unsafe (‘defect’)

Perform

ECA? N

Repair

Economical

considerations

ECAAccept Safe

Potentially

unsafe

Repair

Cheap but very conservative

€€€

WMS: workmanship

ECA: engineering critical assessment

(= fitness-for-service or FFS)

Source: applusrtd.com

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Weld economics:

the potential benefit of ECA

Imperfection length 2c

Imperfection

height a

WMSsafe

pot. unsafe (defect)

Level 3

Level 2

Level 1

More input

required

More critical cases

(w.r.t. ease of repair,

number of defects found)

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Where to search?

Standards for workmanship and fitness-for-service

“Cookbook recepies”

towards weld assessment

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Cooking can be difficult

Result:

Diagnosis:

working with the wrong ingredients

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Cooking can be difficult

Result:

Diagnosis:

working with the wrong ingredients

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The importance of analysis input (“ingredients”):

Variability of properties hampers a clear-cut analysis

Four macrographs originating from

the same circumferential pipe weld

(having operated for 65 years)

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Current ECA procedures are based on highly

simplified representations of welds (“idealized welds”)

metalWeld

(sometimes)

Surface

breaking

defect

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Measuring is knowing: mechanical testing of welds

provides valuable insights…

… but is expensive!

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How about numerical modelling as an alternative?

Finite element model of pipe containing a

circumferential weld defect

crap in = crap out

weld with defect

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Outline of this talk

Experimental

testing

Numerical

modelling

Workmanship

ECAHow to improve?

The role of materials

How to support?

How to validate?How to optimize

result/cost balance?

Two case studies: Weld corrosion assessment

Assessment of heterogeneous welds

Who are we?

Soete Laboratory is part of Ghent University’s

“Tech Lane Ghent Science Park” site

Laboratory

Offices

Test hall

What’s in a name?

Soete Laboratory

Walter SoeteDirector of lab, 1946-1982

Fracture mechanics pioneer

Emphasis on large scale testing

1000 m²

Test hall

Strategic alliances into metals research

(Industrieel OnderzoeksFonds)

1st Tacon/MSC seminar

"Industrial examples of structural

integrity of metal structures"

(19/5/2016)

Devoted to

- establishing industrial relations

- screening research opportunities

- contract negotiations

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Experimental

testing

Numerical

modelling

Workmanship

Fracture mechanics:

adding a dimension to material resistance

Stress

0Yield

strength

Crack driving force

Fracture

toughness

Crack Tip Opening Displacement

(CTOD)

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The concept of CTOD

Original crack tip

Applied CTOD (as a function of load,

geometry, defect dimensions)

Analytical solutions

Finite element analysis

CTOD toughness

“CTOD testing”

twi-global.com

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In addition to crack opening, there may be

stable crack extension

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Original

Crack extension

crack mouth opening displacement

CTOD “ductile tearing”

Advanced fracture mechanics concepts take

into account plasticity and stable crack extension

Stress

0Yield

strength

Crack driving force

Fracture

toughness

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becomes…

strain

stress

crack extension

driving

Stress-strain

“R-curve”

becomes…

Example R-curve

Da (mm)

ECA is a fracture mechanics based

assessment of structural integrity

Defect

Applied

conditions

Strength

Toughness

Laboratory

testing

Compendia

of analytical

solutions © U

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Essential to ECA:

knowledge of weld properties and validity limits

property 1 (e.g., strength related)

property 2

(e.g., toughness related)

ECA valid

no ECA predictions possible

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Example ECA procedure:

the Failure Assessment Diagram

Source: Zerbst et al., Int J Press Vess Pip 77 (2000).

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Workmanship should be associated with little or no

validity criteria

property 1 (e.g., strength related)

property 2

(e.g., toughness related)

Workmanship valid

This comes

at the cost of

high conservatism

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Example WMS rule

Source: API 1104 - 2013

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Ghent University and WMS:The EPRG Tier 2 guidelines for weld defect assessment

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Flaw length

Weld should be overmatching in

strength

CVN > 30 J / 40 J

(min. / average of three tests)

Y/T < 0.90

and some other restrictions

3t 5t 7t

POTENTIALLY

UNSAFE

Experimental

testing

Numerical

modelling

Workmanship

Two purposes of experimental testing

Supporting an analytical ECA or workmanship analysis

(establishing validity)

Small scale, standardized testing

Eliminating the need for analytics

(Experimental level 3 ECA)

Large (preferably full) scale,

non-standardized testing

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The result of an experiment is nothing more than the

collection of recorded signals

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Electrical signals(e.g. voltages)

Visual information(e.g. fracture surface)

1400000

0 3500

Voltage(V)

Time (s)

Load cellClip gauge

Mechanical experiments are expensive:

try to maximize their output!

Two strategies:

Advanced test settings

Increasing the number of sensors

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Crack extension characterization by means of stiffness

(“unloading compliance”)

Crack depth is related to stiffness

Stiffness is measured by unloading compliance

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0.00 1.00 2.00

CMOD (mm)

) 1/UC

CMOD: crack mouth opening displacement

Unloading compliance testing:

Specimen is “breathing”

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Compliance increases as a crack extends

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0% 2% 4%

Applied strain (%)

extends

Mechanical experiments are expensive:

try to maximize their output!

Two strategies:

Advanced test settings

Increasing the number of sensors

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Crack growth characterization by means of stiffness

(“unloading compliance”)

Multiphysics approach: e.g. optical measurements

3D Digital image correlation (DIC):

optical profilometry and full field strain analysis

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2 cameras

(3D view)

Digital image correlation

software

Undeformed

specimen

Reference

Deformed

specimen

In-plane strains

Size doesn’t matter

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Miniature tension test

Cross weld tension test

Single-edge notched tension

(SENT) test

Medium curved wide plate

(MWP) test

Full scale pipe tension test

150 mm

200 mm

Case study where DIC aided the interpretation of an

(expensive) test

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Miniature tension test

Cross weld tension test

Single-edge notched tension

(SENT) test

Medium curved wide plate

(MWP) test

Full scale pipe tension test 200 mm

Widened pipe connection – old practice (1920s)

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OD 265 mm

WT 7 mm

where would you think it may fail under tension load?

DIC reveals the weakest spots of this connection

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Experimental

testing

Workmanship

Case study 1: Weld corrosion assessment

45Stijn Hertelé - Lecture Lasgroep Zuid-Limburg - 13 September 2016

Pipe corrosion management

Underground pipelines are protected against corrosion by two barriers:

‣ a protective coating.

‣ a cathodic protection system (CP).

Coating on a pipe is usually applied in the pipe mill while coating on the

girth weld is applied on site usually in lesser controlled conditions

corrosion concentrates in the girth weld area.

In some cases this can lead to shielding

of the cathodic protection system and result in corrosion.

Example of a defective

girth weld coating

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Standards cookbook?

Freedom in choosing ingredients

In practice…

Extract from ASME B31G-2012

(Manual for determining the

remaining strength of corroded pipelines)

Specification for weld material in 1966

was 28J Charpy U (1/1, -20°C)

All very nice and well

but is this weld tough

enough?

constructed in 1966

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The effect on maintenance economics is significant

From the specifications: ASME B31 G code can be applied on pipelines

constructed in 1983 and later.

For pipelines constructed before this date: unclear if toughness criterion

could be met decided to accept only metal loss in the girth weld cap.

Corrosion limit for

pre-1983 pipe

About 6 – 9 cut outs per year, representing 120 – 180 k€ maintenance cost.

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Unacceptable weld, according to the Fluxys criterion

3D profilometry (DIC)

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Launch of experimental research program (2012-2015)

Indications from literature:

“20% - 30% metal loss is acceptable independent of toughness”

Question to Soete Laboratory to investigate:

• how conservative is current Fluxys practice?

• hypothesized workmanship criterion:“20% metal loss on a girth weld is acceptable independent of toughness”

• effects of geometry and material (ECA procedure)?

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Launch of experimental research program (2012-2015)

Focus on weld related properties

Geometry misalignment (hi-lo)

corrosion metal loss

Material toughness (Charpy V-notch)

weld strength mismatch

Load residual stresses

Experimental

testing

Workmanship

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Experimental

programme

Trends &

influences

Workmanship &

A look into the

future

Workmanship and ECA of corroded girth welds

Experimental

programme

Trends &

influences

Workmanship &

A look into the

future

Ten girth welds were extracted from the

Belgian gas grid

number

API 5L

gradePipe seam

1 X46 Seamless 14 6.4

2 X60 S-SAW 20 5.6

3 X60 S-SAW 20 5.6

4 X60 L-SAW 16 6.5

5 X60 L-SAW 20 7.2

6 X46 Seamless 14 6.4

7 X60 D-SAW 36 10.2

8 X60 D-SAW 36 12.2

9 X60 D-SAW 36 10.2

10 X60 D-SAW 36 12.2

Years of installation:

1960’s-70’s

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Mechanical properties in a nutshell:

what you can expect from vintage pipe

Low Y/Tbetween 0.67 and 0.83

High ductilityuniform elongation > 0.10

Wide range of weld strength mismatchbetween –4% (undermatch)

and 50% (overmatch)

Potentially ‘low’ CVN toughnessminimum @ 20°C: 32 J

(full size equivalent)

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Vickers hardness mapping reveals significant

heterogeneity in some tested welds

150 216 5 mm

How to uniquely quantify weld strength mismatch?

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Simulation of weld and HAZ corrosion attack by

milling away material

base metal /

HAZ weld metal

base metal /

Reduction between 5% and 36% of wall thickness

Not representative for SCC or other sharp corrosion forms!

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Depth of weld metal loss:

average of three paths

0 10 20 30 40 50

Needle traverse (mm)

Center

“left”

“right”

“center”

Traverse (mm)

pipe A

pipe B

(lo)weld

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Thirty-three “medium wide plate” tests

have been performed

Soete Laboratory’s

2.5 MN universal test rig

120 mm ( 20” OD pipe)

150 mm (36” OD pipe)

Tension load

blunt “corrosion”

damage (machined)

girth weld

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3D digital image correlation for profilometry and

full field strain analysis

Axial strain0% 13%

Flattening of misaligned girth weld,

associated with plastic strain

concentration

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Example test

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Experimental

programme

Trends &

influences

Workmanship &

A look into the

future

Assessment criterion: gross section yielding

Rp0.2,A Rp0.2,B

Pipe ‘A’ Pipe ‘B’

Weld acceptable if

Relative failure stress > 100%

Failure stress

min(Rp0.2,A, Rp0.2,B)

Displacement (mm)

Stress (MPa)

Rp0.2,A

Rp0.2,B

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Load bearing capacity decreases as

wall thickness reduction is increased

0 10 20 30 40

tive fail

ess (%

Relative wall thickness reduction (%)

Gross section yielding

(axial stress =

yield strength weakest plate)

Relative

failure

stress (%)

workmanship?

reason for scatter?

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Three weld related influence factors

were identified

High weld strength overmatchReduces strain in weld

Low pipe steel Y/T-ratioReduces strain in HAZ

Low weld misalignmentAvoids local bending strains in weld

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Toughness was of no role for the tests performed!

(important w.r.t. workmanship criterion)

30 40 50 60 70

Min. CVN energy (J)(20 C, WMC & HAZ, full size equiv.)

No trend visible

DIC contours:longitudinal strain

‘Virtual clip gauge’

Weld strainat failure (%)

Weld strain

at maximum load

‘Virtual extensometer’

All tests

collapse dominated

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Understanding a “poor” weld required

detailed knowledge of material behaviour

0 10 20 30 40

tive fail

ess (%

Little misalignment

Strain hardening reasonable

Strength overmatching weld

why at the lower bound

of the scatter band?

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The “poor” weld is strength overmatching,

but highly heterogeneous

150 216 5 mm

Undamaged: 5.4% overmatch average (based on HV) © U

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After application of weld metal loss, its

strength properties are suddenly weak

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150 216 5 mm

After metal loss: 2.5% undermatch average

principal

strain (-)

Undeformed

Deformed (close to failure)

Misalignment has

flattened out

Severe local bending,

which contributes to failure

Initial misalignment:

25% of wall thickness

Severe misalignment is detrimental to load bearing

capacity

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Experimental

programme

Trends &

influences

Workmanship &

A look into the

future

Workmanship criterion:

It’s all about easy validity limits!

0 10 20 30 40

tive fail

ess (%

Relative

failure

stress (%)

workmanship?

20% wall thickness

reduction is acceptable,

provided…

- weld is evenmatching

- weld is free of defects

- toughness criterion?

Preferably not!

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Can effects of key influence

parameters be

objectively

conservatively

predicted?

Soundness of

collapse based approach

Safe practical applicability

ECA: calculations should be safe and sound

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Plastic collapse assessment according to

BS7910:2013 – reference stress approach

Reference stress

Applied primary stress Failure stress

Flow stress

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Reference stress

Flow stress

= f(geometry)

(NOT: residual stress)

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Time to introduce some symbols

average thickness B = (B1 + B2)/2

relative wall thickness reduction a/B

misalignment e/B

metal loss width 2c/W

0 10 20 30 40 50

Needle traverse (mm)

Path i

Path ii

Path iii

Variation

of weld

misalignment

Damage depth

a (mm)

Position in direction transverse to weld (mm)

(i, ii, iii)

Average

ecap (mm)

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Reference stress solution for tension (a) and

bending (b) loaded plates showing metal loss

” function of defect dimensions

a/B, 2c/W, a/2c(Willoughby and Davey, 1989)

Local misalignment bending stress

(linear-elastic solution in BS7910:2013)

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Reference stress

Flow stressDegree of

conservatism

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Definition of flow stress: safe or objective

SMYS(base metal) + 69 MPa

actual TS(weld metal)

Modified ASME B31GDeveloped for vintage steel (low Y/T)

Criterion for weld strength mismatch

Should yield safe predictions

Test database studies Validated for pipe body corrosion

Should yield objective predictions

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Predicted versus observed failure stresses

200 300 400 500 600

ial fa

Axial failure stress,

experimental (MPa)

Unsafe

Safe1:1

SMYS + 69 MPa

TSweld

Axial failure stress, predicted (MPa)

should be around 1:1

(not below)

undesirably

excessive degree

of conservatism

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0 15 30

Relative misalignment e/B (%)

Lineair ( )

Conservatism is introduced by

overestimated effect of misalignment

Actual / predicted

failure stress (-)

f = TSweld

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Linear-elastic misalignment correction does not

account for flattening of misalignment upon plastic

deformation

Axial strain0% 13%

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0 5 10

Relative effective misalignment e/B (%)

Lineair ( )

Better predictions are obtained by the

introduction of “effective” weld misalignment

Actual / predicted

failure stress (-)

Replacing weld misalignment

by one third of its actual value

Theoretically motivated by

elastic-plastic analysis (%)

f = TSweld

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200 300 400 500 600

ial fa

Axial failure stress,

experimental (MPa)

Unsafe

Safe1:1

The effective misalignment concept is sound

SMYS + 69 MPa

TSweld

Axial failure stress, predicted (MPa)

around 1:1

acceptable degree

of conservatism

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Experimental

programme

Trends &

influences

Workmanship &

A look into the

future

A major question remains unanswered

Toughness criterion for the validity of the developed procedures?

Additional tests planned at low temperature (-60°C)

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Experimental

programme

Trends &

influences

Workmanship &

A look into the

future

Detailed weld characterization

Medium wide plate testing with DIC

Weld strength and misalignment

No influence of toughness

20% metal loss criterion

Plastic collapse assessment

Aiming towards standardization

Experimental

testing

Numerical

modelling

Workmanship

Simulations generate more output than experiments

Stress-strain at all points

(also within the structure)

(e.g. CTOD)

Derived quantities

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Numerical modelling: creating a “virtual lab”

Parameter space

Geometry

Materials

Conditions

Finite element

analysis…

Parametric

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When running properly:

no “experimental scatter”

“cheap” experiments

Who t(h)rusts zeros and ones?

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The Mars Climate Orbiter disintegrates in space

(1998):

NASA's $655-million robotic space probe plowed into

Mars's upper atmosphere at the wrong angle, burning

up in the process. The problem? In the software that

ran the ground computers the thrusters' output was

calculated in the wrong units (pound–seconds instead

of newton–seconds, as the NASA–Lockheed contract

had specified).

http://www.scientificamerican.com/article/pogue-5-most-embarrassing-software-bugs-in-history/

image: Wikipedia

The difficult stage: model validation

Parameter space

Geometry

Materials

Conditions

Finite element

analysis…

Experiments

Validation

Parametric

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Example model validation

(w.r.t. strain distribution predictions)

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11 (-)

(a) DIC (b) FEA

Actual weld

(complex)

Potential for accurate weld modelling

is generally not fully exploited

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General approach (e.g. used for standardized ECA procedures)

“Real world”

Analytical ECA

Idealized weld

(simple)

Idealized weld

(simple)

FE analysis???

Soete Laboratory’s philosophy is different

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Actual weld

(complex)

“Real world”

Analytical ECA

Idealized weld

(simple)

Actual weld

(complex)

FE analysis

Example: modifying the weld to its desired shape:

Nodal coordinate transformations

Flat plate,

simplified weld geometry

Easy to model

Curved plate,

detailed weld geometry

Coordinate

transformations

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Understanding of weld behaviour is aided by

automated analysis pre- and postprocessing

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Analytical ECA

Idealized weld

(simple)

Actual weld

(complex)

FE analysis

Models constructed by

object-oriented scripting

(rather than CAD-based)

Automated construction

of parametric studies

In-house developed models of a wide range of

configurations

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CTOD test

SENT test

curved

wide plate

full pipe

Spider-web

crack tip mesh

Experimental

testing

Numerical

modelling

Workmanship

Case study 2: Assessment of heterogeneous welds

Automatic hardness testing of weld macrographs

reveals weld heterogeneity

4 Vickers indentations / mm²

Vickers

hardness

(5 kg)

170 © U

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Weld heterogeneity is a fact of life

Hardness variations up to 50% have been observed at Soete Laboratory

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Crack driving force in heterogeneous welds: What

properties matter?

Potential influence of…

Average properties

of entire weld

Surrounding

properties

Properties

at flaw tip

External

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The current ECA treatment of weld heterogeneity

is “fairly simplistic”

e.g. EU FITNET (fitness-for-service)

procedure, MK8 (2008)

§6.3.3.1

“It should be recognised that weld tensile

properties may vary through the thickness

of a component and may be dependent

on specimen orientation. The range of

weld metal microstructures sampled can

often lead to a high degree of scatter. The

use of the lowest tensile properties

irrespective of orientation and position is

necessary to provide a conservative

result.”

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Going back to Soete Laboratory’s philosophy…

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Actual weld

(complex)

“Real world”

Analytical ECA

Idealized weld

(simple)

Actual weld

(complex)

FE analysis

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Actual weld

(complex)

“Real world”

Analytical ECA

Idealized weld

(simple)

Actual weld

(complex)

FE analysis

Modelling of heterogeneous welds,

by assigning element-specific material properties

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Each element is linked to its

corresponding hardness value

obtained from a hardness map

Relation between Vickers hardness and stress-strain

properties (“hardness transfer function”)?

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EDM2WEIGHT:

SHEET 1 OF 1SCALE:1:1

DWG NO.

TITLE:

REVISIONDO NOT SCALE DRAWING

MATERIAL:

DATESIGNATURENAME

DEBUR AND

BREAK SHARP

FINISH:UNLESS OTHERWISE SPECIFIED:

DIMENSIONS ARE IN MILLIMETERS

SURFACE FINISH:

TOLERANCES:

LINEAR:

ANGULAR:

APPV'D

Simple:

StandardsAccurate:

Miniature tensile testing

100 200 300 400

Rp0.2,

Vickers hardness HV

ISO15653:

base metal

weld metal

ISO18265

Comparison with local HV indentations

allows to calibrate the transfer function

Which one to choose?

Miniature tensile testing at Soete Laboratory

60 µm

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Miniature tensile testing at Soete Laboratory

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Experimental validation of the

heterogeneity modelling approach

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0.0 0.2 0.4 0.6

Plastic CTOD (mm)

Experiment

Simulation

Based on SENT testing

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Actual weld

(complex)

“Real world”

Analytical ECA

Idealized weld

(simple)

Actual weld

(complex)

FE analysis

Deformation around weld defect under tension:

45° “slip lines”

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cracked section

“Weld homogenization”: consider average properties

along 45° slip lines originating from the defect

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Actual

Idealised

HAZ base metal (HVb)base metal (HVb)

M(s) = HVw(s)/HVb

45 FLFR

Heq,L Heq,R

Meq HVb

Heq,L Heq,R

Predictions of load bearing capacity mostly accurate within 5%

www.soetelaboratory.ugent.be

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