effects of flaws and corrosion on the structural integrity ... · effects of flaws and corrosion on...
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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, [email protected]
www.soetelaboratory.ugent.be
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2
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
3
Welds are far from flawless
Stijn Hertelé - Lecture Lasgroep Zuid-Limburg - 13 September 2016
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4
Welds are far from flawless
Stijn Hertelé - Lecture Lasgroep Zuid-Limburg - 13 September 2016
What made this weld survive 65 years of operation?!
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5
Weld defect economics: repair or tolerate a safety risk?
Stijn Hertelé - Lecture Lasgroep Zuid-Limburg - 13 September 2016
NDT
‘Imperfection’
Accept WMSSafe
Potentially
unsafe (‘defect’)
Perform
ECA? N
Y
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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6
Weld economics:
the potential benefit of ECA
Stijn Hertelé - Lecture Lasgroep Zuid-Limburg - 13 September 2016
Imperfection length 2c
Imperfection
height a
2c
a
WMSsafe
pot. unsafe (defect)
Level 3
Level 2
Level 1
ECA
More input
required
More critical cases
(w.r.t. ease of repair,
number of defects found)
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7
Where to search?
Standards for workmanship and fitness-for-service
Stijn Hertelé - Lecture Lasgroep Zuid-Limburg - 13 September 2016
“Cookbook recepies”
towards weld assessment
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Cooking can be difficult
Stijn Hertelé - Lecture Lasgroep Zuid-Limburg - 13 September 2016
Idea:
Result:
Diagnosis:
working with the wrong ingredients
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Cooking can be difficult
Stijn Hertelé - Lecture Lasgroep Zuid-Limburg - 13 September 2016
Idea:
Result:
Diagnosis:
working with the wrong ingredients
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10
The importance of analysis input (“ingredients”):
Variability of properties hampers a clear-cut analysis
Stijn Hertelé - Lecture Lasgroep Zuid-Limburg - 13 September 2016
Four macrographs originating from
the same circumferential pipe weld
(having operated for 65 years)
3
6
9
12
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11
Current ECA procedures are based on highly
simplified representations of welds (“idealized welds”)
Stijn Hertelé - Lecture Lasgroep Zuid-Limburg - 13 September 2016
Base
metal
Base
metalWeld
metal
HAZ
(sometimes)
Surface
breaking
defect
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12
Measuring is knowing: mechanical testing of welds
provides valuable insights…
Stijn Hertelé - Lecture Lasgroep Zuid-Limburg - 13 September 2016
… but is expensive!
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13
How about numerical modelling as an alternative?
Stijn Hertelé - Lecture Lasgroep Zuid-Limburg - 13 September 2016
Finite element model of pipe containing a
circumferential weld defect
crap in = crap out
weld with defect
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14
Outline of this talk
Stijn Hertelé - Lecture Lasgroep Zuid-Limburg - 13 September 2016
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
15
Who are we?
Stijn Hertelé - Lecture Lasgroep Zuid-Limburg - 13 September 2016
16
Soete Laboratory is part of Ghent University’s
“Tech Lane Ghent Science Park” site
Stijn Hertelé - Lecture Lasgroep Zuid-Limburg - 13 September 2016
Soete
Laboratory
Offices
Test hall
17
What’s in a name?
Stijn Hertelé - Lecture Lasgroep Zuid-Limburg - 13 September 2016
Soete Laboratory
Walter SoeteDirector of lab, 1946-1982
Fracture mechanics pioneer
Emphasis on large scale testing
63 m
1000 m²
Test hall
18
Strategic alliances into metals research
Stijn Hertelé - Lecture Lasgroep Zuid-Limburg - 13 September 2016
IOF
(Industrieel OnderzoeksFonds)
e.g.:
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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19
Outline of this talk
Stijn Hertelé - Lecture Lasgroep Zuid-Limburg - 13 September 2016
Experimental
testing
Numerical
modelling
Workmanship
ECA
20
Fracture mechanics:
adding a dimension to material resistance
Stijn Hertelé - Lecture Lasgroep Zuid-Limburg - 13 September 2016
Stress
0Yield
strength
Crack driving force
0
Fracture
toughness
e.g.
Crack Tip Opening Displacement
(CTOD)
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21
The concept of CTOD
Stijn Hertelé - Lecture Lasgroep Zuid-Limburg - 13 September 2016
Original crack tip
CTOD
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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22
In addition to crack opening, there may be
stable crack extension
Stijn Hertelé - Lecture Lasgroep Zuid-Limburg - 13 September 2016
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Original
notch
CMOD
Crack extension
crack mouth opening displacement
CTOD “ductile tearing”
23
Advanced fracture mechanics concepts take
into account plasticity and stable crack extension
Stijn Hertelé - Lecture Lasgroep Zuid-Limburg - 13 September 2016
Stress
0Yield
strength
Crack driving force
0
Fracture
toughness
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becomes…
strain
stress
crack extension
crack
driving
force
Stress-strain
curve
“R-curve”
becomes…
24
Example R-curve
Stijn Hertelé - Lecture Lasgroep Zuid-Limburg - 13 September 2016
0
1
2
3
0 1 2
CT
OD
(m
m)
Da (mm)
W2G4
25
ECA is a fracture mechanics based
assessment of structural integrity
Stijn Hertelé - Lecture Lasgroep Zuid-Limburg - 13 September 2016
Defect
Applied
conditions
Strength
Toughness
ECA
NDT
Laboratory
testing
Compendia
of analytical
solutions © U
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26
Essential to ECA:
knowledge of weld properties and validity limits
Stijn Hertelé - Lecture Lasgroep Zuid-Limburg - 13 September 2016
property 1 (e.g., strength related)
property 2
(e.g., toughness related)
ECA valid
no ECA predictions possible
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27
Example ECA procedure:
the Failure Assessment Diagram
Stijn Hertelé - Lecture Lasgroep Zuid-Limburg - 13 September 2016
Source: Zerbst et al., Int J Press Vess Pip 77 (2000).
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28
Workmanship should be associated with little or no
validity criteria
Stijn Hertelé - Lecture Lasgroep Zuid-Limburg - 13 September 2016
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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29
Example WMS rule
Stijn Hertelé - Lecture Lasgroep Zuid-Limburg - 13 September 2016
Source: API 1104 - 2013
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30
Ghent University and WMS:The EPRG Tier 2 guidelines for weld defect assessment
Stijn Hertelé - Lecture Lasgroep Zuid-Limburg - 13 September 2016
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Flaw length
(mm)
Flaw
depth
(mm)
Weld should be overmatching in
strength
CVN > 30 J / 40 J
(min. / average of three tests)
Y/T < 0.90
and some other restrictions
t
3
4
5
3t 5t 7t
SAFE
POTENTIALLY
UNSAFE
31
Outline of this talk
Stijn Hertelé - Lecture Lasgroep Zuid-Limburg - 13 September 2016
Experimental
testing
Numerical
modelling
Workmanship
ECA
32
Two purposes of experimental testing
Stijn Hertelé - Lecture Lasgroep Zuid-Limburg - 13 September 2016
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
twi-global.com
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33
The result of an experiment is nothing more than the
collection of recorded signals
Stijn Hertelé - Lecture Lasgroep Zuid-Limburg - 13 September 2016
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Electrical signals(e.g. voltages)
Visual information(e.g. fracture surface)
0
0.5
0
1400000
0 3500
Voltage(V)
Time (s)
Load cellClip gauge
34
Mechanical experiments are expensive:
try to maximize their output!
Stijn Hertelé - Lecture Lasgroep Zuid-Limburg - 13 September 2016
Two strategies:
Advanced test settings
Increasing the number of sensors
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Crack extension characterization by means of stiffness
(“unloading compliance”)
35
Crack depth is related to stiffness
Stiffness is measured by unloading compliance
Stijn Hertelé - Lecture Lasgroep Zuid-Limburg - 13 September 2016
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2WP2
0
500
1000
1500
0.00 1.00 2.00
CMOD (mm)
Fo
rce
(kN
) 1/UC
CMOD: crack mouth opening displacement
36
Unloading compliance testing:
Specimen is “breathing”
Stijn Hertelé - Lecture Lasgroep Zuid-Limburg - 13 September 2016
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37
Compliance increases as a crack extends
Stijn Hertelé - Lecture Lasgroep Zuid-Limburg - 13 September 2016
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0.05
0.10
0.15
0% 2% 4%
Applied strain (%)
Unlo
ad
ing c
om
plia
nce (
µm
/kN
)
Crack
extends
1/UC
38
Mechanical experiments are expensive:
try to maximize their output!
Stijn Hertelé - Lecture Lasgroep Zuid-Limburg - 13 September 2016
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
39
3D Digital image correlation (DIC):
optical profilometry and full field strain analysis
Stijn Hertelé - Lecture Lasgroep Zuid-Limburg - 13 September 2016
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2 cameras
(3D view)
Digital image correlation
software
Undeformed
specimen
Reference
Deformed
specimen
In-plane strains
40
Size doesn’t matter
Stijn Hertelé - Lecture Lasgroep Zuid-Limburg - 13 September 2016
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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
15 mm
15 mm
150 mm
200 mm
2 mm
41
Case study where DIC aided the interpretation of an
(expensive) test
Stijn Hertelé - Lecture Lasgroep Zuid-Limburg - 13 September 2016
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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
42
Widened pipe connection – old practice (1920s)
Stijn Hertelé - Lecture Lasgroep Zuid-Limburg - 13 September 2016
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OD 265 mm
WT 7 mm
where would you think it may fail under tension load?
43
DIC reveals the weakest spots of this connection
Stijn Hertelé - Lecture Lasgroep Zuid-Limburg - 13 September 2016
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44
Outline of this talk
Stijn Hertelé - Lecture Lasgroep Zuid-Limburg - 13 September 2016
Experimental
testing
Workmanship
ECA
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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46Stijn Hertelé - Lecture Lasgroep Zuid-Limburg - 13 September 2016
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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47Stijn Hertelé - Lecture Lasgroep Zuid-Limburg - 13 September 2016
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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48Stijn Hertelé - Lecture Lasgroep Zuid-Limburg - 13 September 2016
Unacceptable weld, according to the Fluxys criterion
3D profilometry (DIC)
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49Stijn Hertelé - Lecture Lasgroep Zuid-Limburg - 13 September 2016
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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50Stijn Hertelé - Lecture Lasgroep Zuid-Limburg - 13 September 2016
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
ECA
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51
Experimental
programme
Trends &
influences
Workmanship &
ECA
A look into the
future
Workmanship and ECA of corroded girth welds
52
Experimental
programme
Trends &
influences
Workmanship &
ECA
A look into the
future
Workmanship and ECA of corroded girth welds
53Stijn Hertelé - Lecture Lasgroep Zuid-Limburg - 13 September 2016
Ten girth welds were extracted from the
Belgian gas grid
Weld
number
API 5L
gradePipe seam
OD
(”)
WT
(mm)
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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54Stijn Hertelé - Lecture Lasgroep Zuid-Limburg - 13 September 2016
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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55Stijn Hertelé - Lecture Lasgroep Zuid-Limburg - 13 September 2016
Vickers hardness mapping reveals significant
heterogeneity in some tested welds
HV5
150 216 5 mm
How to uniquely quantify weld strength mismatch?
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56Stijn Hertelé - Lecture Lasgroep Zuid-Limburg - 13 September 2016
Simulation of weld and HAZ corrosion attack by
milling away material
base metal /
HAZ weld metal
5 mm
base metal /
HAZ
Reduction between 5% and 36% of wall thickness
Not representative for SCC or other sharp corrosion forms!
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57Stijn Hertelé - Lecture Lasgroep Zuid-Limburg - 13 September 2016
Depth of weld metal loss:
average of three paths
0
1
2
3
4
0 10 20 30 40 50
Pro
file
(m
m)
Needle traverse (mm)
Left
Center
Right
“left”
“right”
“center”
Traverse (mm)
pipe A
(hi)
pipe B
(lo)weld
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58Stijn Hertelé - Lecture Lasgroep Zuid-Limburg - 13 September 2016
Thirty-three “medium wide plate” tests
have been performed
Soete Laboratory’s
2.5 MN universal test rig
500
mm
120 mm ( 20” OD pipe)
150 mm (36” OD pipe)
Tension load
blunt “corrosion”
damage (machined)
girth weld
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59Stijn Hertelé - Lecture Lasgroep Zuid-Limburg - 13 September 2016
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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60Stijn Hertelé - Lecture Lasgroep Zuid-Limburg - 13 September 2016
Example test
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61
Experimental
programme
Trends &
influences
Workmanship &
ECA
A look into the
future
Workmanship and ECA of corroded girth welds
62Stijn Hertelé - Lecture Lasgroep Zuid-Limburg - 13 September 2016
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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63Stijn Hertelé - Lecture Lasgroep Zuid-Limburg - 13 September 2016
Load bearing capacity decreases as
wall thickness reduction is increased
80
90
100
110
120
130
140
150
160
0 10 20 30 40
Rela
tive fail
ure
str
ess (%
)
Relative wall thickness reduction (%)
Gross section yielding
(axial stress =
yield strength weakest plate)
Relative
failure
stress (%)
workmanship?
reason for scatter?
(ECA)
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64Stijn Hertelé - Lecture Lasgroep Zuid-Limburg - 13 September 2016
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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65Stijn Hertelé - Lecture Lasgroep Zuid-Limburg - 13 September 2016
Toughness was of no role for the tests performed!
(important w.r.t. workmanship criterion)
0
5
10
15
30 40 50 60 70
Wel
d s
trai
n a
t fa
ilure
(%
)
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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66Stijn Hertelé - Lecture Lasgroep Zuid-Limburg - 13 September 2016
Understanding a “poor” weld required
detailed knowledge of material behaviour
80
90
100
110
120
130
140
150
160
0 10 20 30 40
Rela
tive fail
ure
str
ess (%
)
Relative wall thickness reduction (%)
Little misalignment
Strain hardening reasonable
Strength overmatching weld
why at the lower bound
of the scatter band?
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67Stijn Hertelé - Lecture Lasgroep Zuid-Limburg - 13 September 2016
The “poor” weld is strength overmatching,
but highly heterogeneous
HV5
150 216 5 mm
Undamaged: 5.4% overmatch average (based on HV) © U
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68Stijn Hertelé - Lecture Lasgroep Zuid-Limburg - 13 September 2016
After application of weld metal loss, its
strength properties are suddenly weak
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HV5
150 216 5 mm
After metal loss: 2.5% undermatch average
69Stijn Hertelé - Lecture Lasgroep Zuid-Limburg - 13 September 2016
First
principal
strain (-)
0.13
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
0.00
© U
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70
Experimental
programme
Trends &
influences
Workmanship &
ECA
A look into the
future
Workmanship and ECA of corroded girth welds
Stijn Hertelé - Lecture Lasgroep Zuid-Limburg - 13 September 2016
71Stijn Hertelé - Lecture Lasgroep Zuid-Limburg - 13 September 2016
Workmanship criterion:
It’s all about easy validity limits!
80
90
100
110
120
130
140
150
160
0 10 20 30 40
Rela
tive fail
ure
str
ess (%
)
Relative wall thickness reduction (%)
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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72
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
Stijn Hertelé - Lecture Lasgroep Zuid-Limburg - 13 September 2016
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73
Plastic collapse assessment according to
BS7910:2013 – reference stress approach
max,a
f
a
ref
Reference stress
Applied primary stress Failure stress
Flow stress
Stijn Hertelé - Lecture Lasgroep Zuid-Limburg - 13 September 2016
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74
max,a
f
a
ref
Reference stress
Applied primary stress Failure stress
Flow stress
= f(geometry)
(NOT: residual stress)
Plastic collapse assessment according to
BS7910:2013 – reference stress approach
Stijn Hertelé - Lecture Lasgroep Zuid-Limburg - 13 September 2016
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75
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
ecap
eB1
B2
a
0
1
2
3
4
0 10 20 30 40 50
Pro
file
(m
m)
Needle traverse (mm)
Path i
Path ii
Path iii
Variation
of weld
misalignment
Damage depth
a (mm)
Position in direction transverse to weld (mm)
Metal
loss
Three
paths
(i, ii, iii)
2c
W
Average
hi-lo
ecap (mm)
Stijn Hertelé - Lecture Lasgroep Zuid-Limburg - 13 September 2016
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76
Reference stress solution for tension (a) and
bending (b) loaded plates showing metal loss
2
5.0
2
2
"13
"19
a
b
a
b
a
ref
” function of defect dimensions
a/B, 2c/W, a/2c(Willoughby and Davey, 1989)
Local misalignment bending stress
(linear-elastic solution in BS7910:2013)
Stijn Hertelé - Lecture Lasgroep Zuid-Limburg - 13 September 2016
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77
Plastic collapse assessment according to
BS7910:2013 – reference stress approach
max,a
f
a
ref
Reference stress
Applied primary stress Failure stress
Flow stressDegree of
conservatism
Stijn Hertelé - Lecture Lasgroep Zuid-Limburg - 13 September 2016
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78
Definition of flow stress: safe or objective
f =
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
Stijn Hertelé - Lecture Lasgroep Zuid-Limburg - 13 September 2016
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79
Predicted versus observed failure stresses
200
300
400
500
600
200 300 400 500 600
Ax
ial fa
ilu
re s
tres
s,
pre
dic
ted
by
Eq
. (5
) (M
Pa
)
Axial failure stress,
experimental (MPa)
Unsafe
Safe1:1
f =
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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80
0.6
0.8
1.0
1.2
1.4
0 15 30
Actu
al fa
ilu
re s
tress
/
pre
dic
ted
va
lue (
-)
Relative misalignment e/B (%)
Lineair ( )
Conservatism is introduced by
overestimated effect of misalignment
Actual / predicted
failure stress (-)
f = TSweld
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81
Linear-elastic misalignment correction does not
account for flattening of misalignment upon plastic
deformation
Axial strain0% 13%
Stijn Hertelé - Lecture Lasgroep Zuid-Limburg - 13 September 2016
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82
0.6
0.8
1.0
1.2
1.4
0 5 10
Actu
al fa
ilu
re s
tress
/
pre
dic
ted
va
lue (
-)
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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83
200
300
400
500
600
200 300 400 500 600
Ax
ial fa
ilu
re s
tres
s,
pre
dic
ted
by
Eq
. (5
) (M
Pa
)
Axial failure stress,
experimental (MPa)
Unsafe
Safe1:1
The effective misalignment concept is sound
f =
SMYS + 69 MPa
TSweld
Axial failure stress, predicted (MPa)
around 1:1
acceptable degree
of conservatism
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84
Experimental
programme
Trends &
influences
Workmanship &
ECA
A look into the
future
Workmanship and ECA of corroded girth welds
85
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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86
Experimental
programme
Trends &
influences
Workmanship &
ECA
A look into the
future
Workmanship and ECA of corroded girth welds
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
87
Outline of this talk
Stijn Hertelé - Lecture Lasgroep Zuid-Limburg - 13 September 2016
Experimental
testing
Numerical
modelling
Workmanship
ECA
88Stijn Hertelé - Lecture Lasgroep Zuid-Limburg - 13 September 2016
Simulations generate more output than experiments
Stress-strain at all points
(also within the structure)
(e.g. CTOD)
Derived quantities
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CTOD
89Stijn Hertelé - Lecture Lasgroep Zuid-Limburg - 13 September 2016
Numerical modelling: creating a “virtual lab”
Parameter space
Geometry
Materials
Conditions
Finite element
analysis…
Parametric
study
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When running properly:
no “experimental scatter”
“cheap” experiments
90Stijn Hertelé - Lecture Lasgroep Zuid-Limburg - 13 September 2016
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
91Stijn Hertelé - Lecture Lasgroep Zuid-Limburg - 13 September 2016
The difficult stage: model validation
Parameter space
Geometry
Materials
Conditions
Finite element
analysis…
Experiments
Validation
Parametric
study
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92Stijn Hertelé - Lecture Lasgroep Zuid-Limburg - 13 September 2016
Example model validation
(w.r.t. strain distribution predictions)
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11 (-)
0.035
0.000
(a) DIC (b) FEA
93
Actual weld
(complex)
Stijn Hertelé - Lecture Lasgroep Zuid-Limburg - 13 September 2016
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???
94Stijn Hertelé - Lecture Lasgroep Zuid-Limburg - 13 September 2016
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
???
95Stijn Hertelé - Lecture Lasgroep Zuid-Limburg - 13 September 2016
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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96Stijn Hertelé - Lecture Lasgroep Zuid-Limburg - 13 September 2016
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
97Stijn Hertelé - Lecture Lasgroep Zuid-Limburg - 13 September 2016
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
98
Outline of this talk
Stijn Hertelé - Lecture Lasgroep Zuid-Limburg - 13 September 2016
Experimental
testing
Numerical
modelling
Workmanship
ECA
Case study 2: Assessment of heterogeneous welds
99Stijn Hertelé - Lecture Lasgroep Zuid-Limburg - 13 September 2016
Automatic hardness testing of weld macrographs
reveals weld heterogeneity
4 Vickers indentations / mm²
Vickers
hardness
(5 kg)
240
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100Stijn Hertelé - Lecture Lasgroep Zuid-Limburg - 13 September 2016
Weld heterogeneity is a fact of life
Hardness variations up to 50% have been observed at Soete Laboratory
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101Stijn Hertelé - Lecture Lasgroep Zuid-Limburg - 13 September 2016
Crack driving force in heterogeneous welds: What
properties matter?
Potential influence of…
Average properties
of entire weld
Surrounding
properties
Properties
at flaw tip
External
load
Flaw
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ON
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102Stijn Hertelé - Lecture Lasgroep Zuid-Limburg - 13 September 2016
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.”
Approach approved by the best © U
Gent
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ll rig
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or
all
countr
ies. C
annot
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isclo
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or
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ithout
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ritt
en
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orizatio
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f U
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ON
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TIA
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ent
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103Stijn Hertelé - Lecture Lasgroep Zuid-Limburg - 13 September 2016
Going back to Soete Laboratory’s philosophy…
© U
Gent
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ll rig
hts
reserv
ed f
or
all
countr
ies. C
annot
be d
isclo
sed, used,
or
repro
duced w
ithout
prio
r w
ritt
en
specific
auth
orizatio
n o
f U
Gent
–C
ON
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EN
TIA
L –
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atio
n –
UG
ent
pro
prie
tary
in
form
atio
n.
Actual weld
(complex)
“Real world”
Analytical ECA
Idealized weld
(simple)
Actual weld
(complex)
FE analysis
104Stijn Hertelé - Lecture Lasgroep Zuid-Limburg - 13 September 2016
Going back to Soete Laboratory’s philosophy…
© U
Gent
–A
ll rig
hts
reserv
ed f
or
all
countr
ies. C
annot
be d
isclo
sed, used,
or
repro
duced w
ithout
prio
r w
ritt
en
specific
auth
orizatio
n o
f U
Gent
–C
ON
FID
EN
TIA
L –
privile
ged in
form
atio
n –
UG
ent
pro
prie
tary
in
form
atio
n.
Actual weld
(complex)
“Real world”
Analytical ECA
Idealized weld
(simple)
Actual weld
(complex)
FE analysis
105Stijn Hertelé - Lecture Lasgroep Zuid-Limburg - 13 September 2016
Modelling of heterogeneous welds,
by assigning element-specific material properties
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ithout
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Each element is linked to its
corresponding hardness value
obtained from a hardness map
106Stijn Hertelé - Lecture Lasgroep Zuid-Limburg - 13 September 2016
Relation between Vickers hardness and stress-strain
properties (“hardness transfer function”)?
© U
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0.
25
21
2
R1
9
R1.
5
4.5
5
0.5
EDM2WEIGHT:
A4
SHEET 1 OF 1SCALE:1:1
DWG NO.
TITLE:
REVISIONDO NOT SCALE DRAWING
MATERIAL:
DATESIGNATURENAME
DEBUR AND
BREAK SHARP
EDGES
FINISH:UNLESS OTHERWISE SPECIFIED:
DIMENSIONS ARE IN MILLIMETERS
SURFACE FINISH:
TOLERANCES:
LINEAR:
ANGULAR:
Q.A
MFG
APPV'D
CHK'D
DRAWN
Simple:
StandardsAccurate:
Miniature tensile testing
200
400
600
800
1000
100 200 300 400
Rp0.2,
Rm
(MPa)
Vickers hardness HV
Rm
Rp0.2
ISO15653:
base metal
weld metal
ISO18265
Comparison with local HV indentations
allows to calibrate the transfer function
Which one to choose?
107Stijn Hertelé - Lecture Lasgroep Zuid-Limburg - 13 September 2016
Miniature tensile testing at Soete Laboratory
2 mm
10 px
60 µm
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ithout
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ON
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TIA
L –
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atio
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ent
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atio
n.
108Stijn Hertelé - Lecture Lasgroep Zuid-Limburg - 13 September 2016
Miniature tensile testing at Soete Laboratory
© U
Gent
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ll rig
hts
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or
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countr
ies. C
annot
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isclo
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ithout
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orizatio
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ON
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109Stijn Hertelé - Lecture Lasgroep Zuid-Limburg - 13 September 2016
Experimental validation of the
heterogeneity modelling approach
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Gent
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ll rig
hts
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ies. C
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ithout
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n.
70
80
90
100
110
0.0 0.2 0.4 0.6
Ten
sile
fo
rce
(kN
)
Plastic CTOD (mm)
Experiment
Simulation
Based on SENT testing
110Stijn Hertelé - Lecture Lasgroep Zuid-Limburg - 13 September 2016
Going back to Soete Laboratory’s philosophy…
© U
Gent
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ll rig
hts
reserv
ed f
or
all
countr
ies. C
annot
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isclo
sed, used,
or
repro
duced w
ithout
prio
r w
ritt
en
specific
auth
orizatio
n o
f U
Gent
–C
ON
FID
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TIA
L –
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ged in
form
atio
n –
UG
ent
pro
prie
tary
in
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atio
n.
Actual weld
(complex)
“Real world”
Analytical ECA
Idealized weld
(simple)
Actual weld
(complex)
FE analysis
111Stijn Hertelé - Lecture Lasgroep Zuid-Limburg - 13 September 2016
Deformation around weld defect under tension:
45° “slip lines”
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cracked section
0.08
0.00
112Stijn Hertelé - Lecture Lasgroep Zuid-Limburg - 13 September 2016
“Weld homogenization”: consider average properties
along 45° slip lines originating from the defect
© U
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Actual
weld
Idealised
weld
HAZ base metal (HVb)base metal (HVb)
M(s) = HVw(s)/HVb
45 FLFR
O
Heq,L Heq,R
Meq HVb
a
Heq,L Heq,R
HAZ
HVb
Predictions of load bearing capacity mostly accurate within 5%
113Stijn Hertelé - Lecture Lasgroep Zuid-Limburg - 13 September 2016
www.soetelaboratory.ugent.be