Embed Size (px)
Citation preview
DNV GL © 2016
Ungraded
SAFER, SMARTER, GREENERDNV GL © 2016
Ungraded
OIL & GAS
Subsea Materials Testing and Assessment for HPHT Applications
1
DNV GL Technology Week
October 31st, 2016
9am – 12pm
Colum Holtam, Rajil Saraswat, Ramgo Thodla
DNV GL © 2016
Ungraded
Agenda
Welcome and introductions (9am)
Safety moment
Introduction and background to API 17TR8: HPHT Design Guidelines for Subsea Applications
Introduction to fracture mechanics assessment
– Standard/Handbook approach
– FEA-based approach
Fracture mechanics assessment case study
– Fully clad subsea HPHT component
Break
Materials characterization and testing
Open forum
Close (Midday)
Lunch!
2
DNV GL © 2016
Ungraded
API 17TR8: HPHT Design Guidelines for Subsea Applications
3
DNV GL © 2016
Ungraded
API 17TR8: HPHT Design for Subsea Applications
HPHT design is a significant new challenge facing the subsea sector, particularly in the Gulf of Mexico
API 17TR8 provides HPHT Design Guidelines, specifically for subsea applications
First Edition issued February 2015
Second Edition under preparation
– Due for release in 2016 (tentative)
Could be several future Editions
4
DNV GL © 2016
Ungraded
API 17TR8: HPHT Design for Subsea Applications
5
Subsea HPHT design challenges
– T > 350°F
– P > 15ksi
– Harsh environmental conditions
– Sour production
– High H2S/Elemental S
– High Cl-
– Seawater with CP
– Low T (40°F)
– Also elevated T?
– Design approach
– Stress based vs. Fracture mechanics
– Failure modes
– Fracture
– Fatigue
DNV GL © 2016
Ungraded
Materials Focus
6
Subsea materials typically high strength steels
But elevated T and P generally requires use of high strength nickel based alloys and/or clad material
Modification of design philosophy (fracture mechanics vs. stress based)
– Environmentally assisted fracture & fatigue become critical in design
Testing required to characterize environmentally assisted cracking behavior
– SSR
– Fracture toughness
– Fatigue (FCGR / S-N)
Operating/Production conditions
– HPHT
Shut in conditions
– Seawater + CP at low T
DNV GL © 2016
Ungraded
API 17TR8: HPHT Design for Subsea Applications
7
Annex D: Material Characterization Protocols
– New in Second Edition
– Guidelines for use of metallic materials (low alloy steels and CRAs) for HPHT applications
– Generating material properties suitable for the application of fracture mechanics based approaches to the design of subsea equipment
DNV GL © 2016
Ungraded
Introduction to Fracture Mechanics
8
DNV GL © 2016
Ungraded
Principals of Fracture Mechanics
Equilibrium evaluation between:
– The Crack Driving Force (CDF) (load)
– The fracture toughness (capacity)
9
TestingAnalyses/Calculations
Who is the strongest?
Crack Driving Force (CDF) Fracture toughness (materials resistance)
CDF < CTODmat : Crack is stableCDF ≥ CTODmat : Crack is unstable (fracture or crack growth)
DNV GL © 2016
Ungraded
Fracture Mechanics “Engineering Critical Assessment” (ECA)
• Tensile properties• Fracture toughness • Fatigue crack
growth data
• Installation & Operation
• Primary (Static and Fatigue)
• Secondary (residual stress)
• Geometry• Size• Location• Orientation
A complex interaction between many input parameters (3 main types)
• Design data
DNV GL © 2016
Ungraded
Fracture Mechanics “Engineering Critical Assessment” (ECA)
Failure Assessment Diagram used to model failure by fracture / plastic collapse
Distinguishing between what is safe and unsafe
DNV GL © 2016
Ungraded
Pipeline ECA
12
Crack growth modelled to calculate critical initial flaw sizes
DNV GL © 2016
Ungraded
Flaw Types
13
2cBa
rm
2c
a
rm rm
B B
2c
p 2a
W is the mean circumference (2rm)
External flaw Internal flaw Embedded flaw
DNV GL © 2016
Ungraded
Tensile Properties
Stress strain curve
– Yield
– UTS
Location of flaw
– Weld metal
– Parent metal
14
DNV GL © 2016
Ungraded
Fracture Toughness
Fracture toughness parameter
CTOD
K
J
Specimen type
Compact Tension (CT)
Single Edge Notched Bend (SENB)
Single Edge Notched Tension (SENT)
15
DNV GL © 2016
Ungraded
16
Fatigue Crack Growth Analysis
• Fracture mechanics used to model fatigue crack growth through life
• Fatigue crack growth rate law (C, m)
∆
∆ ∆
1∆
DNV GL © 2016
Ungraded
Loads/Stresses
17
DNV GL © 2016
Ungraded
Different Approaches to Fracture Mechanics Analysis
18
Kr=KI/KIC KI σ
σ, Stress (Analytical method, Finite element analysis)
KI, Crack driving force
Standard/Handbook (BS 7910, API/ASME 579)
FEA model with a crack, cracked model
Lr, Plastic collapse ratio
Standard/Handbook (BS 7910, API/ASME 579)
FEA model with a crack, model
Lσσ
appliedloadcollapseload
DNV GL © 2016
Ungraded
Fracture Mechanics Software
CRACKWISE (TWI commercial software)– BS 7910
Signal (Quest Integrity commercial software)– API/ASME 579– BS 7910
FlawSizer (DNV GL internal software)– BS 7910– DNV-OS-F101 App. A– DNV-RP-F108
FEA– ABAQUS
Spreadsheets/MathCAD
DNV GL © 2016
Ungraded
Fracture mechanics Assessment Case Study
20
DNV GL © 2016
Ungraded
Case Study Background
The latest HPHT design guidelines in API 17TR8 require that a fatigue and fracture assessment be conducted assuming there is a flaw located at the highest stress location
– The initial size of the assumed flaw is conventionally based on the reliability of the NDT methods used during manufacture of the component
Test programs required to quantify the behavior of high strength nickel based alloys exposed to HPHT production conditions including sour service as well as low alloy steels exposed to seawater with cathodic protection
Material properties data, including fracture toughness and fatigue crack growth rates, are used as inputs to fatigue and fracture assessments
FEA required to determine static and cyclic stresses based on design information
21
DNV GL © 2016
Ungraded
Objectives
Conduct a fracture mechanics assessment case study for a fully clad subsea HPHT component using standard FAD-based assessment methods, such as those found in BS 7910 and API/ASME 579, and FE-based methods
– Consider internal circumferential flaw
– Evaluate the differences between the approaches
22
DNV GL © 2016
Ungraded
Assessment Approach
Design Basis
Finite Element Analysis
Fracture mechanics analysis
23
Geometry, Material, Loads
Stress, StrainsFatigue Life
DNV GL © 2016
Ungraded
Component Details
Material: F22 internally clad with alloy 625 (weld overlay)
– 6mm thick clad layer
Simplified representative geometry:
Low cycle fatigue dominated by pressure loads
– Cycle between low temperature/low pressure and high temperature/high pressure
Operating pressure = 20ksi (137.8MPa)
Operating temperature = 400°F (204°C)
Hydrotest pressure 25ksi (172.25MPa)
– 1.25 x working pressure (ASME VIII Div. 3)
Sour environment
24
DNV GL © 2016
Ungraded
Steps in a Finite Element Analysis
Create structure geometry and develop mesh
Loadings and boundary conditions
– Pressure, supplemental loads, temperature distributions, residual stresses
– Initial boundary conditions
Material properties
Perform analysis
– Extract displacement/strains/stress, stress intensity factor, collapse loads
25
320
45
20
20
45
450
1520
298.
5
152.5
150
Units: mm
DNV GL © 2016
Ungraded
Simplified Component Geometry
26
F22 steel
Inconel 625 clad (6mm)
2D axi-symmetric model
DNV GL © 2016
Ungraded
FE Model – Thermal Analysis
Sequential coupling of thermal and stress analysis
Steady state thermal analysis
27
Fixed temperature
204°C
Sea water temperature 4°C
Heat transfer coefficient, 50 W/m2K Loc1
Loc2
DNV GL © 2016
Ungraded
FE Model – Stress Analysis, Boundary Conditions
Second order 8 noded axi-symmetric elements
28
Fixed
Internal pressure
Axial load and pressure end load
Refined mesh
DNV GL © 2016
Ungraded
Material Properties
High temperature stress-strain, CTE, specific heat, thermal conductivity
F22 material properties were obtained from ASME VIII Div. 2
Inconel 625 material properties were obtained from Special Metals
29
F22 steel Inconel 625
DNV GL © 2016
Ungraded
Loading steps
Hydro-static test pressure of 172.25MPa (25 ksi)
Remove hydro-static pressure
Apply axial load of 1.5 million lbf
Apply operating pressure of 137.8MPa (20ksi) and corresponding pressure end load
Import thermal strains from the thermal analysis model
30
DNV GL © 2016
Ungraded
Results – Axial Stress
31
Hydrotest Axial load Axial load
+
Internal pressure
Axial load
+
Internal pressure
+
Temperature
DNV GL © 2016
Ungraded
Axial Stress
32
Location 1
Location 2
Location 1
Location 2
DNV GL © 2016
Ungraded
Uncracked Model vs. Cracked Model
33
Uncracked FEA model
• Simpler as crack is not modelled
• Assessment is easier as commercial software is available
Cracked mesh FEA model
• Complicated as crack is explicitly modelled
• Accurate as the actual geometry is modelled instead of using simplified handbook solutions
DNV GL © 2016
Ungraded
Development of Finite Element Mesh in Cracked Model
– Flaw location, orientation and size
– Focused mesh around crack tip along crack front
– 3D FE model: surface cracks and other cracks that are not well reflected by 2D models
– 2D FE model: significantly reduces time and resources needed, provides same or good approximate results when problems can be described in 2D.
Collapsed elements (Spider web)
34
DNV GL © 2016
Ungraded
Model with Cracks under Axial Load and Internal Pressure
35
DNV GL © 2016
Ungraded
Crack Driving Force (CDF) estimated using 7 models
36
KI 856.4MPa mm0.5 for 1mm crack
Location 1
Location 2
DNV GL © 2016
Ungraded
Collapse Load, 1mm Crack, Location 1
Lr = P/Pc
– P, Applied load
– Pc, Collapse load (Load at which ligament goes beyond yield)
Elastic fully plastic analysis
37
Mises Stress Plastic strain
Pc=245.284MPa
Lr=137.8/245.284=0.561
DNV GL © 2016
Ungraded
Fracture Mechanics Assessment Inputs for Internal Flaw
1mm deep x fully circumferential (360°) internal surface flaw
– K solution (varies by assessment case)
– Reference stress solution (varies by assessment case)
Section thickness, B = 6mm (i.e. thickness of the clad layer only)
Fracture toughness, J0.2mm = 45N/mm
– Inferred from tests on similar materials in sour environment
Fatigue crack growth law for steels in air from BS 7910
Tensile properties for 625 at 4°C
– Yield strength = 479MPa
– Tensile strength = 965MPa
Young’s Modulus for alloy 625 = 207500MPa
Poisson’s ratio = 0.3
Primary membrane stress (at 1mm depth)
– Based on uncracked FE model
– Axial load applied after hydrotest + pressure only (worst case from FE)
– Pm = 360.7MPa
– Assume pure membrane since taking stress at flaw depth
Cyclic stresses (linearized over clad section thickness)
– Based on uncracked FE model
– Pressure only (worst case)
– ∆Pm = 240.0MPa
– ∆Pb = 199.0MPa
Assume zero residual stresses
– Perform sensitivity analysis
38
DNV GL © 2016
Ungraded
Failure Criterion
Failure defined as breach of the clad layer
– For internal flaw (Location 1), reference stress solution based on clad layer only
39
DNV GL © 2016
Ungraded
Assessment Cases1. Fracture check
– Calculate crack driving force (and critical flaw height)
a) Standard K solution - BS 7910 (Crackwise software)
b) Standard K solution - API/ASME 579 (Signal FFS software)
c) K from FEA
2. Fracture check
– Calculate collapse load
a) Standard reference stress solution - BS 7910 (Crackwise software)
b) Standard reference stress solution - API/ASME 579 (Signal FFS software)
c) Reference stress from FEA
3. Fatigue and fracture assessment – Cyclic operating stresses + fracture check above
– Calculate fatigue life
a) BS 7910 (Crackwise software) – standard K and reference stress solutions
b) API/ASME 579 (Signal FFS software) – standard K and reference stress solutions
– Compare with FEA
– nb cyclic stresses linearized from initial crack depth in this case
40
DNV GL © 2016
Ungraded
Results – Assessment Case 1
1. Fracture check
a) Standard K solution - BS 7910 (Crackwise software)
– K = 786.9Nmm-3/2 (1mm flaw)
– Critical flaw height = 2.1mm
b) Standard K solution - API/ASME 579 (Signal FFS software)
– K = 780.6Nmm-3/2 (1mm flaw)
– Critical flaw height = 2.1mm
c) K from FEA
– K = 856.4Nmm-3/2 (1mm flaw)
41
DNV GL © 2016
Ungraded
Results – Assessment Case 2
1. Fracture check
a) Standard reference stress solution - BS 7910 (Crackwise software)
– Lr = 0.9036, σref = 432.8MPa (1mm flaw)
b) Standard reference stress solution - API/ASME 579 (Signal FFS software)
– Lr = 0.895, σref = 428.9MPa (1mm flaw)
c) Reference stress from FEA
– Lr = 0.5618 (1mm flaw)
42
DNV GL © 2016
Ungraded
BS 7910 - 1mm flaw
43
DNV GL © 2016
Ungraded
API/ASME 579 – 1mm flaw
44
DNV GL © 2016
Ungraded
Results – Assessment Case 3
3. Fracture and fatigue
a) BS 7910 (Crackwise software) – standard K and reference stress solutions
– Fatigue life = ~1,549 cycles (1mm initial flaw height)
– Add safety factor
b) API/ASME 579 (Signal FFS software) – standard K and reference stress solution
– Fatigue life = ~1,432 cycles (1mm initial flaw height)
– Add safety factor
45
DNV GL © 2016
Ungraded
BS 7910 - 1mm flaw
46
DNV GL © 2016
Ungraded
API/ASME 579 – 1mm flaw
47
DNV GL © 2016
Ungraded
Comparison: BS 7910 and API/ASME 579 vs. FEA
3. Fracture and fatigue
– Used reduced cyclic stresses to more closely match FEA fatigue calculation
a) BS 7910 (Crackwise software) – standard K and reference stress solutions
– Fatigue life = 15,520 cycles
b) API/ASME 579 (Signal FFS software) – standard K and reference stress solution
– Fatigue life = 15,567 cycles
c) FEA
– Fatigue = 16,340 cycles (at 2.1mm flaw depth)
– Note this does not represent failure in FE-based assessment due to lower Lr
48
DNV GL © 2016
Ungraded
Key Findings from Assessment of Internal Surface Flaw in Fully Clad Component
BS 7910 and API/ASME 579 similar for fully circumferential internal surface flaw
K at initial flaw depth (1mm) lower than FEA (i.e. FEA is more conservative for initial flaw depth)
Lr much lower in FEA (i.e. FEA is less conservative than Standard solutions)
Linearizing cyclic stresses over section thickness is more conservative than FEA
FEA would give longer fatigue life for internal flaw located in clad layer, e.g. due to lower Lr
49
DNV GL © 2016
Ungraded
Sensitivity Analysis – Residual Stresses
BS 7910 a) Zero residual stresses
– K = 786.9Nmm-3/2 (1mm flaw)
– Critical flaw height = 2.1mm
– Fatigue life = 1,549 cycles
b) PWHT (20% of yield strength)
– K = 995.9Nmm-3/2 (1mm flaw)
– Critical flaw height = 1.8mm
– Fatigue life = 1,260 cycles
50
DNV GL © 2016
Ungraded
Material Issues
51
DNV GL © 2016
Ungraded
Material Issues Identify key materials for the industry
– 718?
– F22?
– Clad F22?
Identify primary failure modes
– Fracture
– Fatigue
Identify key parameters that control performance
Quantify the performance of the key materials against these failure modes
Potentially develop database of material properties in environments
52
•F22•718Material
•SW + CP (low T)•Sour (HPHT)Environment
•Fatigue (FCGR, S-N)•Fracture (FT)Failure Modes
DNV GL © 2016
Ungraded
Materials Challenges for Subsea HPHT Applications
53
Nickel Based Alloys/CRA’s
HPHT Sour Environments
• Low pH/High H2S/High Cl-
• High T
• SCC and Corrosion Fatigue
Low T Shut in Conditions
• High H2S/Low pH
• Lower T (~40F)
• Fracture Issues
Low T CP Issues
• Lower T (~40F)
• Cathodic Protection
• Fatigue & Fracture Issues
Fabrication Challenges
• Welds/Clad layers (625)
• Alloy Selection (718/945/625+)
• Cu Plating issues –leading to low T H embrittlement
DNV GL © 2016
Ungraded
Current Acceptance Limits in ISO 15156-3
54
SCC behavior of precipitation hardened alloys has been evaluated in various environments using C-ring tests
DNV GL © 2016
Ungraded
Recent Work on PH Nickel Alloys to Develop a Robust Test Method
55
Corrosion2015 - 5497
DNV GL © 2016
Ungraded
Typical J-R curve
There is substantial decrease in the fracture toughness of IN718 at high chloride concentration at 400F
56
0 1 2 3 40
100
200
300
400400F
K-rateAir
: 16Nmm-3/2/s
K-rateEnv
:0.016Nmm-3/2/s
100psia H2S
500psia CO2
J-R CMOD - Air J-R CMOD - Air J-R 25wt% NaCl J-R 2.5wt% NaCl J-R 0.25wt% NaCl J-R 25wt% NaCl w/0.5wt% Acetic Acid J-R 2.5wt% NaCl w/0.5wt% Acetic Acid
J (N
/mm
)
a (mm)
DNV GL © 2016
Ungraded
Fracture toughness of IN718 – Effect of Chloride and Temperature
KJint is determined from J0.2mm, a sharp drop in KJint occurs in the presence of high chlorides in 718 at 400F
The effect of temperature at low chloride concentration is not significant
57
0.1 1 10
40
60
80
100
120
IN718100psis H2S200/500psia CO2
K-rate: 0.016Nmm-3/2/s
300F350F400F400F w/0.5wt% Acetic Acid
KJi
nt (M
Pam
)
Nacl (wt%)
DNV GL © 2016
Ungraded
Connecting Micro Process to Macroscopic Measurements
58
Micro process that lead to stabilization of pits or cracks appear to be similar
• Macroscopic Measurements (Erp)
• Repassivation potential which is associated with the driving for stabilization/repassivation of localized process
• Macroscopic Measurements for SCC
• Kint and CGR (da/dt)
H 1 2
4
3
FPZ
Ni/Fe - Matrix
MClx
Micro Processes During Cracking1. Establishment of crack tip chemistry 2. Local acidification3. Development of crack tip strain rate.4. Crack Propagation
Can the macroscopic measurements be related?
DNV GL © 2016
Ungraded
Relation between Localized Corrosion & SCC – 718
59
There is a strong co-relation between KJint and da/dt with the difference of Erp and OCP
When the Erp is lower than the OCP, KJint is substantially lower than when Erp>OCP
Susceptibility to cracking appears to be co-relatedwith susceptibility to localized corrosion
-100 0 100 200 300
40
60
80
100
120
da/dt (mm
/s) at KJ = 133M
Pam
(Erp - OCP)w/o H2S (mV vs RE at Temperature)
Kin
t (MPa
m)
-100 0 100 200 3001E-6
1E-5
1E-4
IN718100psis N2
500psia CO2
400F
Susceptible to localized corrosion at OCP
Not susceptible to localized corrosion at OCP
DNV GL © 2016
Ungraded
Relationship between Localized Corrosion & SCC – 625+
60
-100 0 100 200 300100
120
140
160
180
da/dt (mm
/s) at KJ = 133M
Pam
Erp-OCP (mV vs RE)
KJ (M
Pam
)
-100 0 100 200 300
2x10-6
4x10-6
6x10-6
8x10-6
1x10-5IN625+100psis N2
500psia CO2
400FSusceptibleto localized corrosion at OCP
Not susceptible to localized corrosion at OCP
There is a strong co-relation between KJint and da/dt with the difference of Erp and OCP.
As Erp approaches OCP, KJint is begins to drop.
Susceptibility to cracking appears to be co-relatedwith susceptibility to localized corrosion.
DNV GL © 2016
Ungraded
CP Effects
Effect of applied potential and HT on the fracture toughness of UNS N07718 has been explored using slow rising displacement tests
Kth has been performed at low pH as opposed to higher pH’s associated with seawater
The role of various parameters in seawater + CP needs to be explored to develop guidelines for Ni-Based alloys
61
DNV GL © 2016
Ungraded
Fracture Toughness - Shut In Conditions
718
– Fracture toughness decreases with K-rate and IGC increases
– No data on fatigue behavior in shut in conditions
F22
– Limited / No data on FT and fatigue behavior
62
0.01 0.1 1 10 1000
50
100
150
200
250
53
5.5 J0.2mm - Air J0.2mm - Environment Jmaxload - Air Jmaxload - Environment
J (N
/mm
)
K-rate (Nmm-3/2/s)
IN7183.5wt% NaCl40F (4.4°C)-1050mV SCE
7
Numbers indicatecharging time
Increasing K-rate
DNV GL © 2016
Ungraded
Fracture Toughness – Shut In Conditions - Rising Displacement vs. Step-Load
Rising Displacement Jint < Small Step Load Jint < Large Step Load Jint
Step load: ASTM F1624
Slow rising displacement: ASTM E1820 (modified)
Nominally same loading rate in all tests
~10 times slower than recommended in API 17TR8
Which toughness value to use?
– What is representative?
– What is conservative
63
0 10 20 30 4025
30
35
40
45
50
55
60
65
K-rate0.005N
mm
-3/2/s
4h hold
2h hold
IN7183.5wt% NaCl40F (4.4°C)-1050mV SCE2 day soak
J (N
/mm
)
Step size (lb)
DNV GL © 2016
Ungraded
Summary of Key Issues for CRA’s
PH Nickel Based CRAs used in HPHT Applications are susceptible to Environmentally Assisted Cracking.
Fracture toughness appears to drop in environment
– Mechanism appears to be one of SCC and electrochemically driven
– Intergranular cracking is observed
High strength CRAs also appear to be susceptible to H embrittlement at low temperature
– Sharp decrease in toughness with applied CP
– Mechanism is likely associated with the crack tip hydrogen generation and diffusion
A number of issues to be addressed in this area
– HPHT SCC & fracture toughness behavior
– Effect of environments on wrought and clad 625?
– Behavior of other PH Nickel Based CRAs – 625+/945?
– Low T behavior of PH Nickel Based CRAs
– What controls the H embrittlement – impurity content of Alloys?
– What is the impact of low T sour environments/coupled to steel?
Others?
64
DNV GL © 2016
Ungraded
Rapid Material Characterization for HPHT
65
DNV GL © 2016
Ungraded
Typical Operating Conditions
Primary loading scenarios
– Fatigue loading from pressure transients which are relatively quick (~hours)
– Fatigue loading from thermal transients which are relatively long (~1-2days)
– Static loading associated with long steady operations (~20-30days)
66
Pre
ssu
re
Tem
per
atu
re
15ksi/20ksi
Ambient
350F/400F
Time
20 – 50 days
~a few days~10’s min to hours
DNV GL © 2016
Ungraded
Loading Scenarios
Low cycle fatigue and static crack growth behavior may not be discretephenomenon but part of a continuum of the same phenomenon
Development of a single specimen methodology to capture all of the critical design parameters
67
Time
Load
Ambient
20 – 50 days
Environmentally Assisted Crack PropagationFa
tigue
Con
trol
led
Static Crack Growth Controlled
~a few days~10’s min to hours
Loading scenario’s involve low cycle fatigue which in the presence of environments can lead to environmentally assisted fatigue crack growth
Constant load in environment can lead to static crack growth
Primary environments of interest
– Production environment
– Seawater + CP environment
Characterize the FCGR and static crack growth behavior in environment is essential for design
DNV GL © 2016
Ungraded
FCGR Screening Methodology
68
Kmax
Time
K
Kmin, R = 0.13
Kmin, R = 0.4
Kmin, R = 0.6
Decreasing Frequency Decreasing
Frequency
Decreasing Frequency
• Perform tests at constant K under decreasing frequency
• Decrease K by increasing R-ratio but keeping Kmaxconstant
• This allows for keeping the size of the static fracture process zone fixed
• Frequency changes can be made in a sequence so as not to disturb the crack chemistry
DNV GL © 2016
Ungraded
FCGR Response Type 1
– No environmental effects i.e. no frequency dependence
– Perform tests at any convenient frequency
Type 2
– Cycle dependent environmentally assisted crack growth rate, strong dependence on frequency followed by a plateau FCGR
– Perform tests at plateau frequency, if very low could perform at higher frequency and knock down the measured curve by an appropriate amount
– Knock up the measured curve can be estimated by ratio of FCGRplateau/FCGRin-ar
69
Type 3Crack growth rate dominated by static crack growth rate with no evidence of plateau upto to low frequencies (1mhz/0.3mHz). Can also be identified by a FCGR 1/f
Type 1
f (Hz)
da/
dN
(mm
/cy
cle)
Op
erating
Frequ
ency In-Air
DNV GL © 2016
Ungraded
Transition from Fatigue to Static Crack Growth
70
Kmax
Time
K
Kmin, R = 0.6
1h 3h 24h
Con
stan
t at
Km
ax
Increasing hold time Increasing
hold time
• FCGR is typically a function of the cyclic loading however static CGR is a function of the stress intensity (or perhaps more accurately crack tip strain rate)
• Transition from fatigue to static crack growth rate by introducing periods of static holds
DNV GL © 2016
Ungraded
Crack Growth Rate Measurements Under K-control
71
Time (s)
a (m
m)
Type 1
• Smooth transition from long hold periods to constant K behavior and a linear a vs Time response i.e. a steady CGR is maintained
Type 2
• Change in crack length varies with time under constant K and a non-linear a vs Time response is obtained – a decrease in CGR with time
Type 3
• No detectable crack growth is observed
da/
dt
(mm
/s)
Time (s)
Detection Limit
Note Y-axis is CGR
DNV GL © 2016
Ungraded
Potential Options Under K-control
72
da/
dt
(mm
/s)
Con
stan
t K
at
K1
Con
stan
t K
at
K2
Co
nst
ant
K a
t K
3
Ris
ing
K u
nd
er
dK
/d
a co
ntr
ol
Ris
ing
K u
nd
er
dK
/d
a co
ntr
ol
• The CGR at constant K will be used to determine the strategy for subsequent testing.
• The factors that will influence subsequent testing will be based on the measured CGR and the K-level
• If the CGR is “low” K will be increased under dK/da control to a higher value of K where the CGR will be measured at a constant K. This process will be repeated until the CGR reaches a “high” value.
• If the CGR when transitioning to constant K is “high” K will be decreased under dK/da control to lower value of K and the CGR measured at constant K. The process will be repeated until the CGR reaches a “low” value.
K3>K2>K1
“Low” CGR
“Intermediate” CGR
“High” CGR
K
Kth will be identified based on a threshold value of CGR.
Note Y-axis is CGR
DNV GL © 2016
Ungraded
Static & Ripple Crack Growth Rate Measurements
73
da/
dt
(mm
/s)
K
Kth
Low CGRCGRKn
High CGR
da/
dt
(mm
/s)
K
Time
Kmax
K
Kth will be determined based on an increase above low values of CGR with increasing K
Static CGR measurements at different K-values to determine the K vs CGR relationship
Determine effect of small ripples to establish CGR at low K
Re-establish constant K behaviorbefore transitioning to next K level
DNV GL © 2016
Ungraded
Constant K tests - No K-rate effect Development of test methodology to
transition from a fatigue pre-crack to a statically loaded crack
– Decreasing K
– Decreasing frequency
– Increasing hold time
Measure CGR
46.550
R =
0.3
/0.3
3Hz
R =
0.5
/0.0
5Hz
R =
0.5
/0.0
1Hz
R =
0.7
/0.0
01H
z
R =
0.7
/0.0
01H
z
3600
s
R =
0.6
/0.0
01H
z
R =
0.6
/0.0
01H
z
3600
s
R =
0.6
/0.0
01H
z
9000
sR =
0.6
/0.0
01H
z
8640
00s
Con
stan
t K
thold
718
– 40F
– -1050mV SCE
– 3.5wt% NaCl, pH = 8.2
DNV GL © 2016
Ungraded
718 – Overall Record W/Reference
Overall record of crack length as a function of time
Transition to static crack growth by cycling at high K and high frequency initially followed by decreasing K and frequency
Low frequency low K followed by introduction of hold times at Kmax
0 200 400 600 800 1000 1200 1400 1600 180017.8
18
18.2
18.4
18.6
18.8
19
2.6156e-008 mm/s5.8514e-008 mm/s5.83e-008 mm/s
8.9433e-008 mm/s
1.8572e-007 mm/s5.7564e-009 mm/s9.9861e-007 mm/s
4.0929e-006 mm/s
4.0419e-005 mm/s
Time (h)
a (m
m)
6/15/2016 @ 0 hours
8/22/2016 @ 0 hours
718Kmax = 50ksiin3.5wt% NaCl-1050mV SCE40F
DNV GL © 2016
Ungraded
Effect of Hold Time on CGR
Addition of hold time at 1mHz leads to a slight decrease in the CGR, increasing hold times doesn’t to decrease the CGR
Suggests that the CGR at the long hold times is associated with static crack growth
800 900 1000 1100 1200 1300 1400 1500 1600 1700
18.65
18.7
18.75
18.8
18.85
18.9
2.5599e-008 mm/s
5.8514e-008 mm/s
5.83e-008 mm/s
8.9433e-008 mm/s
Time (h)
a (m
m)
1mHz + 86400s1mHz + 9000s1mHz + 3600s1mHz718
Kmax = 55MPam0.5R = 0.63.5wt% NaCl-1050mV SCE40F
DNV GL © 2016
Ungraded
Effect of Kmax on CGR
At higher Kmax, the CGR at the low frequencies is significantly higher
77
1900 2000 2100 2200 2300 240017.5
18
18.5
19
19.5
20
20.5
21
21.5
221.17e-007 mm/s
1.9613e-007 mm/s2.6156e-007 mm/s1.91e-006 mm/s
2.4469e-005 mm/s
1.602e-006 mm/s
Time (h)
a (m
m)
Raw500/500 + 86400s500/500 + 900s500/50050/501.5/1.550/50
7183.5wt% NaClpH = 8.2-1050mV SCE40FKmax = 60ksiin
DNV GL © 2016
Ungraded
Effect of Hold Time on CGR at Kmax of 72ksi√in
No transition to static CGR was possible at these levels of Kmax.
Further tests are needed to establish a K value at which stable static crack growth can be established.
2500 2550 2600 2650 2700 2750 2800 285022
23
24
25
26
27
28
2.2439e-007 mm/s5.6312e-007 mm/s2.6474e-006 mm/s9.048e-006 mm/s
5.1905e-005 mm/s
1.5329e-005 mm/s
Time (h)
a (m
m)
10/12/2016 @ 1 h
Raw500/500 + 9000s500/50050/5010/101.5/1.55/5
7183.5wt% NaClpH = 8.2-1050mV SCE40FKmax = 72ksiin
DNV GL © 2016
Ungraded
SEM of Fracture Surfaces
Fracture surface exhibits evidence of intergranular cracking.
79
DNV GL © 2016
Ungraded
Summary of the effect of “Frequency”
Significant effect of hold time on the CGR
The effect of frequency is more apparent at the lower frequencies.
– More so when hold times were introduced
Static CGR at constant K was difficult to establish. Low CGR was observed under 1day unload cycles.
1E-5 1E-4 1E-3 0.01 0.1 1
1E-4
1E-3
0.01
Kmax = 79.2MPam
Kmax = 66MPam
K = 34.6MPam, R = 0.2 K = 25.3MPam, R = 0.5 K = 22.2MPam, R = 0.6 K = 26.4MPam, R = 0.6 K = 31.4MPam, R = 0.6
da/d
N (m
m/c
ycle
)
f (Hz)
7183.5wt% NaCl-1050mV SCE40F
Kmax = 55MPam
DNV GL © 2016
Ungraded
FCGR and Static CGR – Kmax Effects on 625+
Crack growth rate as a function of “frequency” and hold time.
Increasing rise time leads to a decreasing crack growth rate.
No Steady State CGR under constant K conditions can be established at 50ksi√in.
200 300 400 500 600 700 800 900 1000
26.55
26.6
26.65
26.7
26.75
26.8
2.6512e-008 mm/s
4.3772e-008 mm/s
8.5496e-008 mm/s
2.1124e-007 mm/s
Time (h)
a (m
m)
Raw500/500 + 86400s500/500 + 9000s500/500 + 3600s500/500
625+3.5wt% NaClpH = 8.2-1050mV SCEKmax = 50ksiin0.5R = 0.6
DNV GL © 2016
Ungraded
FCGR and Static CGR – Kmax Effects on 625+
Crack growth rate as a function of “frequency” and hold time.
Increasing rise time leads to a decreasing crack growth rate.
No Steady State CGR under constant K conditions can be established at 60ksi√in.
1200 1300 1400 1500 1600 1700 1800 1900 2000
26.6
26.7
26.8
26.9
27
27.1-1.7456e-008 mm/s
3.9197e-008 mm/s
1.0481e-007 mm/s
2.0084e-007 mm/s
1.6524e-006 mm/s
Time (h)
a (m
m)
RawConstant K500/500 + 86400s500/500 + 9000s500/50050/50
625+3.5wt% NaClpH = 8.2-1050mV SCEKmax = 60ksiin0.5R = 0.6
DNV GL © 2016
Ungraded
Effect of Frequency
FCGR increases sharply with decreasing frequency, with FCGR increasing by about 10x as the frequency decreases from 1mHz to 0.01mHz.
The effect is apparent at two different Kmax values.
1E-5 1E-4 1E-3 0.011E-4
1E-3
Kmax = 60ksiinR = 0.63.5wt% NaClpH = 8.2-1050mV SCE40F
Hold Times No Hold Times
da/d
N (m
m/c
ycle
)
Frequency (Hz)
500/500
500/500+9000s
500/500+86400s
50/50
1E-5 1E-4 1E-31E-4
1E-3
500/500+3600s
Hold Times No Hold Times
da/d
N (m
m/c
ycle
)Frequency (Hz)
Kmax = 50ksiinR = 0.63.5wt% NaClpH = 8.2-1050mV SCE40F
500/500+9000s
500/500
500/500+86400s
DNV GL © 2016
Ungraded
Transition from FCGR to Static CGR
Crack growth rate as a function of “frequency” and hold time.
Increasing rise time leads to a decreasing crack growth rate.
Steady State CGR under constant K conditions appears to be established at 90ksi√in.
Steady State CGR at constant K is about 2.5710-7mm/s.
2100 2150 2200 2250 2300 2350
27
27.5
28
28.5
29
29.5
30
30.5 2.573e-007 mm/s3.2198e-007 mm/s8.2561e-007 mm/s
1.303e-006 mm/s
5.316e-006 mm/s
6.0753e-005 mm/s
2.1376e-005 mm/s
6.7077e-006 mm/s
Time (h)
a (m
m)
RawConstant K900/100 + 86400s900/100 + 9000s900/10090/103/19/190/10
625+3.5wt% NaClpH = 8.2-1050mV SCKmax = 90ksiin0.5R = 0.6
DNV GL © 2016
Ungraded
Summary of Frequency Effect
Decreasing frequency leads to an increasing CGR per cycle.
At a high Kmax the increase in FCGR is about 100x higher than the values at low frequency.
– This appears to be a result of the combination of Kmaxand K effect.
More work is needed to establish the K vs CGR behavior.1E-5 1E-4 1E-3 0.01 0.1
1E-4
1E-3
0.01 900/100+86400s
Kmax = 90ksiinR = 0.63.5wt% NaClpH = 8.2-1050mV SCE40F
da/d
N (m
m/c
ycle
)
Frequency (Hz)
900/100+9000s
900/10090/10
9/1 3/1
DNV GL © 2016
Ungraded
Open Questions Environmental Variables
– What is the effect of applied potential?
– What is the effect of temperature?
Effect of loading variables
– What is the K vs CGR behavior in cases where steady state CGR can be established?
– What is the effect of dK/da?
– Is there is an effect of sample size on the K vs CGR behavior i.e. is plasticity a critical factor in these results?
– Can lower CGR on the order of about 110-8mm/s be established.
Metallurgical Variables
– Is there an effect of grain boundary precipitation like sigma phase on the susceptibility of high strength nickel based alloys.
– What is the deformation mode in associated with crack propagation in these systems –Does it change based on the nature of the precipitation 718 vs 625+?
Can the overall behavior be modelled based on a crack tip strain rate formulation?
DNV GL © 2016
Ungraded
Summary
Hydrogen embrittlement of 718 in seawater + CP conditions appears to be sensitive to test methodology
Constant displacement tests do not exhibit any evidence of cracking in long term exposure tests in seawater + CP conditions.
Rising displacement tests on pre-cracked specimens appear to show susceptibility to environmentally assisted cracking.
– Decreasing K-rate lead to a decreasing Kth value at a
– CGR is sensitive to K-rate in the rising displacement tests
Static CGR tests were performed by transitioning from fatigue pre-cracking in environment to static crack growth with introduction of hold times
CGR is significantly lower than those observed in the rising displacement tests at the same K-values.
DNV GL © 2016
Ungraded
Static Crack Growth Analysis• Fracture mechanics used to model static crack growth through life
• Static crack growth rate law (C1, n(scc))
88
1
DNV GL © 2016
Ungraded
Open Forum
Questions?
89
DNV GL © 2016
Ungraded
SAFER, SMARTER, GREENER
www.dnvgl.com
Thank you
90
Dr. Colum [email protected]+1 281-396-1000
Dr. Rajil [email protected]+1 281-396-1000
Dr. Ramgo [email protected]+1 614-761-1214
