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Test Protocol for Measurement of Crack-Tip Opening Displacement (CTOD) and J-Fracture Resistance Curves Using Single-Edge Notched Tension (SENT) Specimens API TECHNICAL REPORT WELDTR1 FIRST EDITION, XXXX 2020

THIS DOCUMENT IS NOT AN API STANDARD; IT IS UNDER CONSIDERATION WITHIN AN API TECHNICAL SUBCOMMITTEE BUT HAS NOT RECEIVED ALL APPROVAL REQUIRED TO BECOME AN API PUBLICATION. IT SHALL NOT BE REPRODUCED OR CIRCULATED OR QUOTED, IN WHOLE OR IN PART, OUTSIDE OF API SUBCOMMITTEE ACTIVITIES EXCEPT WITH THE APPROVAL OF THE CHAIRMAN OF THE SUBCOMMITTEE HAVE JURISDICTION AND API STANDARDS DEPARTMENT STAFF. COPYRIGHT ® API. ALL RIGHTS RESERVED

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Copyright © 2020 American Petroleum Institute


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Foreword Nothing contained in any API publication is to be construed as granting any right, by implication or otherwise, for the manufacture, sale, or use of any method, apparatus, or product covered by letters patent. Neither should anything contained in the publication be construed as insuring anyone against liability for infringement of letters patent. Shall: As used in a standard, “shall” denotes a minimum requirement in order to conform to the specification. Should: As used in a standard, “should” denotes a recommendation or that which is advised but not required in order to conform to the specification. This document was produced under API standardization procedures that ensure appropriate notification and participation in the developmental process and is designated as an API standard. Questions concerning the interpretation of the content of this publication or comments and questions concerning the procedures under which this publication was developed should be directed in writing to the Director of Standards, American Petroleum Institute, 200 Massachusetts Ave, Washington, DC 20001. Requests for permission to reproduce or translate all or any part of the material published herein should also be addressed to the director. Generally, API standards are reviewed and revised, reaffirmed, or withdrawn at least every five years. A one-time extension of up to two years may be added to this review cycle. Status of the publication can be ascertained from the API Standards Department, telephone (202) 682-8000. A catalog of API publications and materials is published annually by API, 200 Massachusetts Ave, NW, Washington, DC 20001. Suggested revisions are invited and should be submitted to the Standards Department, API, 200 Massachusetts Ave, NW, Washington, DC 20001, [email protected].

mailto:[email protected]


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Introduction In this document, the values stated in SI units are to be regarded as the standard. Imperial values given in parentheses are for information only.


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Contents

Special Notes .................................................................................................................................... ii Foreword .......................................................................................................................................... iii Contents ............................................................................................................................................ v 1 Scope ........................................................................................................................ 1 2 Normative References .............................................................................................................. 1 3 Terms, Definitions, Acronyms, Abbreviations, Symbols, and Units ..................................... 1 3.1 Terms and Definitions ............................................................................................................... 1 3.2 Acronyms and Abbreviations ................................................................................................... 2 3.3 Symbols and Units .................................................................................................................... 2 4 Summary Test Method ............................................................................................................. 4 4.1 General ........................................................................................................................ 4 4.2 Specimen ........................................................................................................................ 4 4.3 Method ........................................................................................................................ 4 4.4 Results ........................................................................................................................ 5 5 Significance and Use ................................................................................................................ 5 6 SENT Specimen Details........................................................................................................... 5 6.1 Specimen Geometry ................................................................................................................. 5 6.2 Specimen Preparation .............................................................................................................. 8 6.3 Fatigue Pre-Cracking (When this Option is Used for Crack Preparation) .......................... 9 7 Instrumentation ...................................................................................................................... 10 7.1 Test Machine Instrumentation ............................................................................................... 10 7.2 Specimen Instrumentation ..................................................................................................... 10 7.3 Specimen Instrumentation with Double Clip Gauge ........................................................... 10 7.4 Electrical Potential Drop Instrumentation ............................................................................. 13 8 Test Procedure ...................................................................................................................... 13 8.1 Objectives and Overview ....................................................................................................... 13 8.2 Specimen Measurements ...................................................................................................... 14 8.3 Specimen Test Temperature ................................................................................................. 14 8.4 Knife Edge and Clip Gauge Attachment .............................................................................. 14 8.5 Electrical Potential Drop Procedure ...................................................................................... 15 8.6 Specimen Gripping and Alignment ....................................................................................... 16 8.7 Testing ...................................................................................................................... 16 8.8 Crack Measurement after Testing ......................................................................................... 17 8.9 Post-Test Sectioning of HAZ Specimens ............................................................................. 18 9 Analysis of Test Data .............................................................................................................. 19 9.1 Calculation of CMOD and CTOD Using Double-Clip-Gauge Procedure ......................... 19 9.2 Calculation of Crack Size when Unloading Compliance Method Is Used ........................ 20 9.3 Calculation of Crack Size when Potential Drop Method Is Used ...................................... 21 9.4 Calculation of J ...................................................................................................................... 21 9.5 Calculation of CTOD from J ................................................................................................... 23 9.6 Adjustment of Original Crack Size when Unloading Compliance Method Is Used ......... 24 9.7 CTOD R-Curve 25 9.8 J R-Curve ...................................................................................................................... 25


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10 Validity Requirements ............................................................................................................. 26 11 Test Report ...................................................................................................................... 27 Annex A ........................................................................................................................................... 29 Annex B ........................................................................................................................................... 31 Bibliography..................................................................................................................................... 33

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Test Protocol for Measurement of Crack-Tip Opening Displacement (CTOD) and J-Fracture Resistance Curves Using Single-Edge

Notched Tension (SENT) Specimens

1 Scope

This test protocol presents procedures and guidelines for the determination of SENT CTOD and J R-curves for the application of the results in strain-based design. The test protocol covers the testing of base materials and welds. The test results are presented as CTOD and J R-curves, i.e., plots of CTOD and J versus stable ductile crack growth. The specimen geometry and loading mode is designed to produce a level of crack-tip constraint in the test that is similar to the constraint experienced in service for a surface circumferential flaw in a pipe under tension or bending load. The test is intended to be used for pipe steels. Only ductile behavior is acceptable because the application of the results, i.e., the CTOD and J R-curves, is in strain-based design. The test is invalid for application in strain-based design if a brittle instability occurs during the test.

2 Normative References The following referenced documents are indispensable for the application of this document. For dated references, only the edition cited applies. For undated references, the latest edition of the referenced document (including any amendments) applies, except that new editions may be used on issue and shall become mandatory upon the effective date specified by the publisher or 6 months from the date of the revision (where no effective date is specified).

ASTM E4, Standard Practices for Force Verification of Testing Machines ASTM E1820-11, Standard Test Method for Measurement of Fracture Toughness

3 Terms, Definitions, Acronyms, Abbreviations, Symbols, and Units 3.1 Terms and Definitions For the purposes of this document, the following definitions apply 3.1.1 blunting Crack growth by plastic deformation at the crack tip 3.1.2 crack mouth open displacement CMOD Opening displacement at the mouth of the machined notch (specimen back surface)

3.1.3 compliance Reciprocal of the slope of the applied load vs. CMOD response during elastic unloading/reloading


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3.1.4 ductile tearing Crack growth by ductile mechanism (microvoid coalescence) 3.1.5 ligament Part of specimen between crack and back face (remaining cross section of specimen ahead of crack)

3.1.6 net thickness Distance between the roots of the side grooves 3.1.7 physical crack size Distance from specimen back surface to the crack front measured on the fracture surface 3.1.8 R-curve Plot of CTOD or J versus stable ductile crack growth 3.1.9 specimen width Side-to-side dimension of the specimen in the direction of crack growth 3.1.10 specimen thickness Side-to-side dimension of the specimen in the direction parallel to the crack front

3.2 Acronyms and Abbreviations

For the purposes of this document, the following acronyms and abbreviations apply.

CMOD crack mouth opening displacement

CTOD crack tip opening displacement

HAZ heat affected zone of weld visible after etching

3.3 Symbols and Units

For the purposes of this document, the following symbols and units apply.

a crack size

0a average original crack size from optical measurement

qa0 adjusted original crack size predicted from compliance or alternative method

fa final crack size from compliance

fpa physical final crack size from optical measurement

a∆ crack growth qaaa 0−=∆


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Apl area under the load vs. plastic crack mouth opening displacement curve

b remaining ligament (=W-a)

B specimen thickness in the direction parallel to the crack front

Beff effective thickness (for side-grooved specimens (Beff=B-(B-BN)2/B)

BN net thickness (distance between the roots of the side grooves in side-grooved specimens)

Ci crack mouth opening displacement (CMOD) compliance corresponding to the ith unload/load sequence

d diameter of screws in knife edge fixture

D distance between center of drilled hole and notch centerline

E elastic Young's modulus

E ′ ( )21 ν−=′ EE (plane strain) or E (plane stress)

h machined notch depth

h1 height of lower clip gauge

h2 height of upper clip gauge

hd depth of drilled hole for knife edge fixture

G geometry factor of SE(T) sample in K formula

H specimen length between grips

J J-integral

K stress intensity factor

LM estimated maximum load: ( ) TSNM BaWL σ0−=

n strain-hardening exponent

P load (force)

ΔP load increment

Pi load at point i

PM peak force for fatigue pre-cracking

PMAX maximum force applied for pre-cracking

PMIN minimum force applied for pre-cracking

PU maximum load during the test

PY yield limit load in tension of the remaining ligament: YSNY bBP σ=

R stress ratio: MAXMIN PPR =

S span between outer loading points in a three-point bend setup

s distance between centers of threaded holes in the thickness direction

ui normalized compliance corresponding to the ith load/unload sequence

V CMOD


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ΔV CMOD increment

Vpl plastic part of the CMOD

V1 crack opening displacement measured from the lower clip gauge

V2 crack opening displacement measured from the upper clip gauge

W specimen width in the direction of crack propagation η, γ, ψ parameters used in evaluation of the J-integral

YSσ 0.2% offset yield strength

TSσ ultimate tensile strength

Yσ effective yield strength (or flow stress)=(YS +TS)/2

δ crack tip opening displacement (CTOD) measured by triangulation from double clip gauge

δi value of CTOD immediately prior to the ith load/unload cycle

δJ CTOD calculated from J-integral

Mδ CMOD

ν Poisson's ratio 4 Summary Test Method

4.1 General

The test involves loading an SENT specimen to induce stable crack growth referred to as ductile tearing.

The preferred SENT specimen geometry is a BxB specimen. The preferred initial ratio is in the range of 0.25 – 0.35.

4.2 Specimen

The specimen is loaded in displacement control and is rigidly clamped during the test, i.e., “fixed grip loading”.

The specimen is instrumented with double clip gauges on a mounting fixture and/or a single clip gauge on integral knife edges machined in the test specimen.

4.3 Method

The preferred test method is to determine the R-curve by testing a single specimen using the unloading compliance (UC) method. Other methods to monitor crack size are allowed if they have been demonstrated to give accuracy equivalent to the UC method; procedures for the electrical potential drop method are included in this recommended practice.

The multiple-specimen method can be used as an alternative to the single-specimen method. The multiple-specimen method requires a minimum of six specimens to generate an R-curve. However, the multiple-specimen method is not recommended for use with J-integral testing because the method in Annex A16 in ASTM E1820-11 requires significant effort. The single-specimen method is preferred


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because it reduces the number of tests required and provides a more detailed description of the shape of the R-curve.

4.4 Results

The test results are presented as CTOD-R and J-R curves.

5 Significance and Use This test method measures CTOD and J R-curves suitable for use in strain-based engineering critical assessment (ECA) analysis of pipelines to predict the tensile strain capacity. The protocol allows for CTOD calculation using two different methods, i.e. direct CTOD measurement at original crack tip and CTOD from J at the current crack tip. Depending upon the methodology that is to be used for engineering analysis, selection should be made of the appropriate method of measurement for CTOD. The CTOD values obtained from these two methods will be different.

The measured R-curves may be dependent on the normalized initial crack depth a0/W. In general, the measured fracture resistance increases as the a0/W ratio decreases. It is important to test SENT specimens with an a0/W ratio that ensures that the test results are representative and useable in an ECA analysis. The preferred initial a0/W ratio is in the range of 0.25 – 0.35.

The R-curves determined by this test method characterize the ductile fracture (tearing) resistance of the pipe and/or girth weld material. The R curve may be influenced by the weld geometry (cross sectional geometry, mostly related to weld bevel geometry) and by the relative mechanical properties of the girth weld and parent pipe material. In the presence of high weld overmatch, the weld width (dimension parallel to the pipe surface and perpendicular to the weld centerline) may significantly affect the measured R-curve. It is important that R-curve tests be performed on girth welds and parent pipe that are representative of production materials and conditions.

Owing to material heterogeneity in a girth weld, it is recommended that at least three weld metal and three HAZ specimens be tested at each location. The number of locations required to fully characterize the R-curve behaviour of a girth weld is dependent on the weld procedure (i.e., for example, if the weld was produced by mechanized or manual welding or if the pipe was welded in the fixed position (5G) or rotated during welding (1G).

6 SENT Specimen Details

6.1 Specimen Geometry

The recommended specimen geometry is of square (BxB) cross-section, (i.e., B = W) with a length between the grips (H) of 10W as shown in Figure 1. The length of the specimen at each end for gripping shall be at least 4W giving an overall minimum specimen length of 18W. The specimen width (W) shall be as close as possible to the pipe wall thickness, noting that the specimen has a surface notch.


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Figure 1—Recommended SENT Specimen Geometry

The specimen orientation relative to the pipe material or girth weld is illustrated in Figure 2; the notch shall be cut from the outside diameter OD or inside diameter ID according to agreement between stakeholders. The specimen axis corresponds to the longitudinal axis of the pipe.

Specimen centerline

“Daylight” between grips

H=10W Surface notch


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Figure 2—SENT Specimen Orientation with Respect to the Pipe

The specimens shall be prepared using either of the following options:

Notched using conventional machining followed by fatigue pre-cracking.

Notched using electro discharge machining (EDM).

Thin-wire EDM notching offers the following advantages over fatigue pre-cracking:

More accurate notch placement for HAZ tests.

Uniform notch front.

Target a0/W ratio can be achieved accurately.

1.1.1 For specimens that will be notched and fatigue pre-cracked, the notch shall be machined to a depth that ensures allowance for a minimum fatigue crack length of at least the larger of 0.05W or 1.3 mm from the machined notch. The notch profile shall be contained within the envelope shown in NOTE Surface notch can be OD or ID, by agreement

Figure 3. In order to facilitate fatigue pre-cracking at low stress intensity levels, the root radius of the machined notch should be 0.08 mm or less. The plane of the notch shall be perpendicular to the specimen surface within ±2 degrees.

For specimens that will be EDM notched the final notch (to a depth a0) must be made with a fine wire of diameter no larger than 0.15 mm (0.006 in.). However, the initial notching, to a depth not to exceed 2/3 of


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a0, may be done with a larger wire of diameter up to 0.25 mm (0.01 in.). The final EDM notch shall be no shorter than the larger of 0.05W or 1.3 mm.

NOTE Surface notch can be OD or ID, by agreement

Figure 3—Crack Starter Notch and Envelope of Fatigue Pre-crack

The preferred ratio of total size of the notch (machined notch plus final notch) to specimen width is in the range of 35.025.0 0 ≤≤ Wa .

Weld metal samples shall be notched at the weld centerline as defined in Section Error! Reference source not found.. HAZ samples shall be notched so that the fatigue pre-crack or EDM notch tip is located in the HAZ and within 0.5 mm of the weld fusion line.

Following fatigue pre-cracking or EDM notching, the specimens shall be side-grooved to promote plane-strain conditions and uniform stable crack growth across the specimen thickness during testing. The roots of the side grooves shall be located at the center of the specimen length and shall be perpendicular to the long dimension of the specimen. The recommended side-groove depth is 5% of the specimen thickness on each side giving a net specimen thickness (BN) equal to 0.90B. The root radius of the side-groove shall be in the range 0.5 to 0.7 mm. The included angle of the side-groove shall be less than 90 degrees. The recommended side-groove configuration is shown in Figure 4.

6.2 Specimen Preparation

Owing to weld residual stresses, the specimen blanks removed from a welded pipe may exhibit angular distortion. In such cases, the specimen blanks may be machined flat or tested in the misaligned condition and the treatment must be noted in the report. Bending of the specimen to reduce angular distortion is prohibited.

0.063 W max.

60o max.

1.3 mm min.

h

Fatigue crack

Back surface


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Figure 4—Recommended Side-Groove Configuration

6.3 Fatigue Pre-Cracking (When this Option is Used for Crack Preparation)

NOTE See 0 for details.

Specimens are fatigue pre-cracked in three-point bending.

The bend load fixture used for fatigue pre-cracking should be such that the stress distribution is uniform through the specimen thickness to ensure even fatigue crack growth.

The specimen should be accurately positioned within the load fixture to ensure that the plane of the notch is aligned with the central load point.

The specimen shall be fatigue pre-cracked at the highest practical frequency.

The recommended initial maximum fatigue force is given in equation (1)

MMAX PP 8.0= (1)

Where PM is defined by equation (2)

( )

ShWB

P YSM

σ25.0 −= (2)

The R-ratio (PMIN/PMAX) during fatigue pre-cracking shall be in the range 0 to 0.1.

5% B

Included angle <90o

Side groove Top view of a SENT

specimen

Detailed view of side groove

mm 7.0-5.0

Pre-crack or notch


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After fatigue pre-cracking to the desired Wa0 ratio, the specimen shall be side-grooved as described in 0.

7 Instrumentation

7.1 Test Machine Instrumentation

The test machine shall be instrumented so that the applied force can be measured continuously during the test.

The load cell shall be calibrated in accordance with ASTM E1820 and ASTM E4.

7.2 Specimen Instrumentation

The specimen shall be instrumented either with (a) a double clip gauge arrangement to permit the crack mouth opening displacement to be measured at two different heights above the specimen surface during the test or (b) with integral knife edges to accommodate one clip gauge allowing the CMOD to be measured directly; however, in this case only the J-integral and CTOD calculated from J (see Sections 9.5 and 9.6) can be determined.

7.3 Specimen Instrumentation with Double Clip Gauge

When a double clip gauge arrangement is used, the specimen shall be instrumented to permit the crack mouth opening displacement to be measured at two different heights above the specimen surface during the test.

The vertical distance between the two sets of knife edges on the double clip gauge fixture shall be at least 0.25W.

The knife-edge fixture for the clip gauges shall be attached to the specimen surface using either screws or welding. If the former is used, it is recommended that each knife edge should be attached using two screws as shown in Figure 5 and Figure 6. The screw holes shall be drilled and tapped so that the holes are perpendicular to the specimen surface. Hand drilling is prohibited. The distance between the centers of the two holes, s, shall be equal to the smaller of 5 mm or 0.5 B. The distance from the centers of the holes to the notch center-line, D, shall be equal to 1.5 to 2 times the screw diameter, d (see Figure 5). D shall not be larger than the crack size, a0. The screw hole depth, hd, shall be equal to the smaller of 5 mm or one half of the crack size, a0. The knife edges shall be parallel within 0.01 mm over the length of the mounted double clip gage fixtures. (Precision parallel bar with shims has been found to be effective [Weeks et al] and/or other methods using “template” may be helpful for ensuring the above). The screws shall all be fastened with equal and sufficient torque to prevent loosening during testing, including effects of temperature reduction and deformation.

The knife edges should be as small as possible but capable of supporting the clip gauges without shifting or moving during the test.

If the knife edges are attached by welding, the welds shall be small enough such that the welding heat does not affect the metallurgy of the notch tip region. The weld heat-affected zone (HAZ) shall be at least 3 mm from the notch (or pre-crack) tip. The welds shall be free from cracks or other defects that can extend during testing and cause relative movement between the knife edges and the specimen. The number and location of the welds shall be sufficient to prevent relative movement between the knife edges and the specimen throughout the duration of the test.


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Side view

Top view S±0.5%

hd ± 0.5%

D ± 0.5%

Notch surface

Figure 5—Recommended Threaded Hole Locations with Respect to the Specimen Notch

The spacing between holes (“s” in Figure 5) shall be equal to the smaller of 5 mm or 0.5 B. Screw size of 2 mm with 0.4 mm pitch

Figure 6—SENT Specimen with a Double Clip Gauge Fixture (Button Head Round Cap Screws)

D ± 0.5% hd ± 0.5%

s ± 0.5%


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A recommended design for a double clip gauge fixture with knife edges is shown in Figure 7. The double clip gauge fixture may not be suitable for specimen size less than 14 mm.

Figure 7—Recommended Double Clip Gauge Fixture


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A triangulation rule is used to evaluate both the crack-mouth and crack-tip opening displacements. The recommended clip gauges are relatively narrow to allow for both gauges to be mounted on a small knife-edge fixture as shown in Figure 6. Relatively compact clip gauges, as shown in Figure 8, are commercially available.

Figure 8—Clip-Gauge Geometry for 6-mm Working Range

7.4 Electrical Potential Drop Instrumentation

If the electrical potential drop method is used to monitor crack growth during the test, the specimen shall be suitably instrumented.

Typical electrical potential drop instrumentation includes the following:

A high quality constant current power source which is connected to the specimen through electrical leads. It is recommended that at least one end of the specimen, and preferably both, be insulated from the test machine (grips or load cell).

A nano-voltmeter which is connected to the specimen through electrical leads on either side of the notch.

8 Test Procedure

8.1 Objectives and Overview

The overall objective of the test method is to load a specimen in displacement control while monitoring the applied force and clip gauge displacements. The test is stopped after a certain amount of crack growth is achieved.

The double-clip-gauge method is used to obtain a direct measurement of CMOD and CTOD. The force and CMOD are used to calculate the J integral.

14 mm

27 mm

46 mm Notch as specified in ASTM E399

Gauge Width: 5mm

Cover

13 mm


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If the unloading compliance technique is used to measure crack growth, multiple load/unload cycles are conducted to measure the change in compliance as the applied force is increased. The load/unload cycles are not required if other methods are used to measure crack growth.

If the electrical potential drop method is used, the voltage across the specimen notch is measured continuously during the test and a plot of potential drop versus displacement (CMOD or CTOD) is generated. This plot is used to determine the point of crack growth initiation and to predict crack growth as a function of CTOD.

At the completion of each test, the specimen is removed from the load frame, heat tinted, and then cooled in liquid nitrogen prior to being broken into two pieces to expose the fracture surfaces. The crack growth at the end of the test is measured from optical measurements on the specimen fracture surfaces and compared to the crack growth measurements from unloading compliance, potential drop or other validated technique.

8.2 Specimen Measurements

The following specimen dimensions shall be measured before side-grooving. Specimen Thickness (B): The specimen thickness shall be measured at three equally spaced

positions along the specimen centerline. The maximum variation shall be less than the larger of 0.025 mm or 0.5%. The specimen thickness is taken to be the average of the three measurements.

Specimen Width (W): The specimen width shall be measured at three equally spaced positions along the specimen centerline. The maximum variation shall be less than the larger of 0.025 mm or 0.5%. The specimen width is taken to be the average of the three measurements.

Before testing, the specimen shall be marked with scribe lines to note the edges of the areas that will be gripped by drawing two lines, each at a distance of 5W from the specimen centerline. These lines shall be perpendicular to the long dimension of the specimen.

8.3 Specimen Test Temperature

The temperature control requirements of ASTM E1820 shall be met. The temperature of the specimen during the test shall be controlled and recorded to an accuracy of ±2°C. Special attention should be taken in controlling the temperature of the test environment when the potential drop method is used. The temperature of the specimen shall be stable and uniform during the test and shall not vary by more than ±2°C.

8.4 Knife Edge and Clip Gauge Attachment

The clip gauges shall be attached to the knife edges after the specimen is clamped in the test machine. A specimen instrumented with a double-clip-gauge setup is shown in Figure 9.

The clip gauges shall be firmly seated in the knife edges by lightly moving (wiggling) the gauges back and forth.


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Figure 9—Setup of SENT Specimen Instrumented with Double Clip Gauges

8.5 Electrical Potential Drop Procedure

If the potential drop method is used for crack growth measurement, the current and voltage leads shall be attached to the specimen prior to placing the specimen in the test machine. The positioning of the current and voltage leads is important and needs to be consistent for all specimens to ensure a constant calibration. The voltage leads shall be located in the center of the specimen as close to the notch as possible. It is recommended that at least one end (and preferably both ends) of the specimen be electrically isolated from the test machine, at either the grips or the load cell.

Since the voltage drop across the specimen notch is small, it is important that an accurate nano-voltmeter be used to record the change in voltage during the test.

Iron constantan thermocouple wire shall be used for the voltage leads to minimize thermal electromagnetic fields (EMFs) since the voltage leads will act as thermocouples.

To minimize radio frequency (RF) induced noise, the voltage leads shall be made of several wires twisted together.

The potential drop method is sensitive to temperature changes (specimen temperature and test laboratory temperature). If the specimen is tested below room temperature, the specimen shall be immersed in a liquid to minimize temperature fluctuations, and the laboratory temperature shall be controlled to ±2 ºC.

The use of reference probes is recommended to compensate for temperature fluctuations and to take into account changes in specimen or instrumentation resulting in changes in the measured voltage. Guidelines


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and further information on the potential drop technique may be found in ASTM E647 Appendix A6, “Guidelines for Electric Potential Difference Determination of Crack Size”.

8.6 Specimen Gripping and Alignment

Hydraulic grips are required to clamp the specimens. A high clamping force is important to ensure that the specimen does not slip during the test.

The maximum force to be applied to the specimen shall be estimated by Equation (3):

( ) NTSM BaWL 0−= σ (3)

The minimum gripping pressure shall be calculated based on LM and the appropriate equation described in the user’s manual for the hydraulic grips. The applied gripping pressure for clamping the specimen shall not be less than the calculated gripping pressure.

The specimen shall be aligned along the load axis of the test machine to minimize shear and bending moment forces. This alignment requires careful attention by the test operator.

The specimen length between the two gripped areas shall be equal to 10W±0.1W.

8.7 Testing

Before applying force to the specimen, the readings from both clip gauges shall be zeroed.

The specimen shall be loaded in displacement control at a constant rate not exceeding 0.025 mm/s. The loading rate should be such that the specimen reaches 0.5Py in a time between 0.3 and 3 minutes; a typical rate of 0.002 mm/s has been found satisfactory.

If the unloading compliance technique is used, the specimen shall be unloaded and loaded repeatedly at least five times within the range of 0.1 to 0.5Py (i.e., in the elastic load range) to pre-check the clip-gauge seating (where Py is the limit load based on yield stress Py = (W-ao) BNσys).

1.1.2 If the unloading compliance technique is used, the specimen is subjected to periodic unload/load cycles that are spaced approximately equally apart as shown in NOTE Only a limited number of unload/load sequences are shown; rather more may be needed to meet the requirements of 9.6.1.

Figure 10. The cycles shall be spaced at a maximum interval such that V1 (measured from the lower clip gauge) does not exceed 0.01W. There should be a minimum of eight unload/load cycles before maximum load, and occasionally substantially more if there is significant “apparent negative crack growth”, in order to find an adjusted value of initial crack size (see Section Error! Reference source not found.).

Prior to initiating each unloading, the specimen shall be held at a constant displacement for at least 5 seconds to allow the load to stabilize. The load drop during each elastic unloading shall be within the range of 0.35 to 0.5PY.

Plastic necking of the specimen ligament can significantly influence the estimation of crack size when using the compliance method. The maximum crack extension shall not exceed 20% of the initial ligament. This can be achieved approximately by stopping the test when the force drops 20% below the peak value.


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NOTE Only a limited number of unload/load sequences are shown; rather more may be needed to meet the requirements of 9.6.1.

Figure 10—Record of Force vs. Crack Opening Displacement Showing Equally-Spaced Unload/Load Sequences*

8.8 Crack Measurement after Testing

On completion of the test, the specimen shall be heat tinted to mark the tearing zone. Heat tinting for 30 minutes at about 300°C has been found satisfactory, but other suitable procedures are acceptable. The heat tint denotes the extent of ductile crack growth. The heat-tinted specimens shall be cooled in liquid nitrogen and broken into two pieces to expose the fracture surfaces. The fracture faces shall be protected to inhibit rusting. If light oil is used, the oil should be transparent to allow subsequent optical measurements of the fracture surface. The fracture surfaces shall be photographed for the permanent record soon enough after testing such that negligible surface rust has formed on the fracture faces.

The fracture surface of the specimen shall be examined and measurements shall be made to determine the initial crack size and the final crack size. The measured crack growth shall be compared with the crack extension predicted by unloading compliance or whatever other technique is used.

The initial and final crack lengths shall be measured at nine equally spaced points centered about the specimen centerline and extending to 0.005W from the root of the side grooves (see Figure 11). The original crack depth, 0a , and the final crack depth, pa (=afp), shall be calculated by averaging the near-

PU


18

surface measurements and combining the result with the remaining seven crack depth measurements to determine the average as described in the relationships shown in Equation (4)

( )

( )

81 9

0 0 0 02

81 9

2

2 8

2 8

i

i

ip p p p

i

a a a a

a a a a

=

=

= + +

= + +

∑

∑ (4)

Figure 11—Measurement Locations of Final Crack Growth Size (with Fatigue Precrack)

8.9 Post-Test Sectioning of HAZ Specimens

Each specimen notched in the HAZ shall be metallurgically examined, post-test, to check that the pre-crack tip (or notch tip) was adequately positioned.

Both halves of the specimen shall be sectioned at the point of maximum crack size. If the crack size is relatively constant along a large portion of the crack front, thus making the deepest point difficult to distinguish, then the location of sectioning shall be taken as close as is practical to the center of the specimen thickness.

Fatigue Precrack Front

Tearing Front

Machined Notch Front

Side-Groove Root


19

Two mating sections corresponding to the weld side and the base metal side shall be metallurgically prepared using standard polishing and etching techniques. It is recommended that the two sections be placed into a single metallurgical mount for polishing/etching and that these sections be placed as close together as possible to re-mate the fracture surfaces. The metallurgical microstructure at the tip of the pre-crack (or EDM notch) shall be identified and the distance from the crack tip to the fusion line recorded. A photographic record of the mating sections shall be made.

9 Analysis of Test Data

9.1 Calculation of CMOD and CTOD Using Double-Clip-Gauge Procedure

CMOD and CTOD values are calculated by extrapolating the upper and lower clip gauge displacements back to the specimen surface and original crack tip using a triangulation rule according to the profile described in Figure 12.

Figure 12—Illustration of a Growing-Crack Profile Used for Calculating both CMOD and CTOD using Double Clip-Gauge Method

The equation (5) shall be used to calculate CMOD:

( )1

1 2 12 1

MhV V V

h hδ = − −

− (5)

where V1 and V2 are the crack opening displacements measured at different heights above the specimen surface, h1 and h2, respectively.


20

The equation (6) shall be used to calculate CTOD:

( )1 0

1 2 12 1

h aV V Vh h

δ += − −

− (6)

The initial crack depth (ao) shall be determined according to Section Error! Reference source not found..

9.2 Calculation of Crack Size when Unloading Compliance Method Is Used

The elastic unloading compliance determined from the load-CMOD response curve as shown in NOTE Only a limited number of unload/load sequences are shown; rather more may be needed to meet the requirements of 9.6.1.

Figure 10 and described in Annex A is used to calculate the crack size corresponding to each unload/load sequence.

The effect of rotation on the unloading compliance can be significant, especially for deeply-cracked samples. Plastic deformation in the ligament changes the geometry, causing the crack mouth to move away from the load line thus reducing the CMOD compliance, and reducing the ligament size by necking. The following load-dependent equation takes the effect of both rotation and necking into consideration. The correction factor, F, is small for shallow cracks but can be appreciable for deep cracks. The value of the compliance [Equation (7)] corrected for rotation is as follows:

FCC icorri /= (7)

where PCMODCi ∆∆= / (8)

and y

i

PP

Wa-F 0165.01= (9)

Both a0 and limit load, PY, are evaluated at the initial values of crack length a=a0 and ligament size b=b0. (Note that, strictly speaking, current values of a and b should be used. However, by inserting the initial values the error is small and tedious iteration is avoided.)

The crack size is calculated using equation (10):

ai/W= 1.6485-9.1005ui+33.025ui2-78.467ui3+97.344ui4-47.227ui5 (10)

where ui is the normalized CMOD compliance corresponding to the ith unload/load sequence defined by equation (11):

corrieff

i CEBu

+=

11

(11)

where:


21

E = plane-stress value of E', i.e. E' = E.

( )PCMODC corri ∆∆= is the CMOD compliance for the ith unload/load sequence, corrected for rotation as specified in Section 9.2.2, and

BB-B-BB Neff /)( 2= (12)

9.3 Calculation of Crack Size when Potential Drop Method Is Used

The procedure to be followed to estimate crack size using the potential drop method is specified in Error! Reference source not found..

9.4 Calculation of J

The J evaluation equations below have been developed from finite element analysis (FEA) assuming plane-strain crack-tip constraint. Ji is calculated at point i at current values of crack length, ai; CMOD and load, Pi, using equation (13).

( ) ( )pli

ii J

EK

J +−

=22 1 ν

(13)

The value of the stress intensity factor, Ki, is calculated using equation (14).

( )

=

WaG

WBB

aPK i

N

iii

21

π

(14)

Where

112

1

−

=∑

=

j

j

ij

i

Wat

WaG (15)

Parameters tj (j=1 to 12) are given in Error! Reference source not found..

Table 1—Coefficients tj in Equation 15 for H/W=10 and 0.1≤ai/W≤0.7

j 1 2 3 4 5 6 7 8 9 10 11 12

tj 1.197 -2.133 23.886 -69.051 100.462 -41.397 -36.137 51.215 -6.607 -52.322 18.574 19.465 NOTE Values that can be used to check for correct transcription of Equation 15 to a spreadsheet are as follows: for ai/W=0.2 and 0.5, G(ai/W)=1.319 and 2.191 respectively

The value of the plastic part of the J integral Jpl(i) is calculated by equation (16).

( )

−−

−

+=

−

−−−

−

−−

)1(

)1()()1()1()(

)1(

)1()1()( 1

i

iii

N

iplipl

i

iCMODiplipl b

aaB

AAb

JJ LLDγη

(16)


22

where η CMOD(i-1) and γLLD(i-1) are evaluated from equation (17):

∑=

=10

0)/(

)(j

jij Wa

iCMODφη

(17)

Parameters, φj (j=1 to 10), are given in Error! Reference source not found.:

Table 2—Coefficients φj in Equation 17 for H/W=10 and 0.1≤ai/W≤0.7

j 0 1 2 3 4 5 6 7 8 9 10 φj 1.000 -1.089 9.519 -48.572 109.225 -73.116 -77.984 38.487 101.401 43.306 -110.770 NOTE Values that can be used to check for correct transcription of Equation 17 to a spreadsheet are as follows: for ai/W=0.2 and 0.5, ηCMOD(i)=0.9215 and 0.7601 respectively.

and

)(

)(

)()(

'

)1(1iLLD

iLLD

iLLDiLLD Wai

η

ηηγ −−−=

(18)

where

∑=

=10

0)/(

)(j

jij Wa

iLLDψη (19)

and

∑=

−=10

1

1' )/()(j

jij WajiLLD ψη

(20)

Parameters in Equations 19 and 20 are given in Table 3.

Table 3—Coefficients ψj in Equation 19 and 20 for H/W=10 and 0.1≤ai/W≤0.7

j 0 1 2 3 4 5 6 7 8 9 10 ψj -0.880 15.190 -35.440 18.644 18.399 -1.273 -12.756 -12.202 -4.447 5.397 14.187 NOTE Values that can be used to check for correct transcription of Equation 18 to a spreadsheet are as follows: for ai/W=0.2 and 0.5, γLLD(i)= -3.395 and 0.505 respectively

The quantity Apl(i)-Apl(i-1 is the increment of plastic area under the load versus plastic crack mouth opening displacement between points i-1 and i. The quantity Jpl(i) represents the total crack-growth-corrected plastic J at point i and is obtained by incrementing the existing Jpl(i-1) (using ηCMOD and APl) and then modifying the result to account for the crack growth increment (using γLLD). Accurate evaluation of Jpl(i) from Equation 16 requires small crack growth increments; the unloadings shall be spaced at a maximum


23

interval of either V1 (measured from the lower clip gauge) not to exceed 0.01W or not to exceed 0.01b0, whichever is the lesser as specified in 8.7. The quantity Apl(i) can be calculated as shown in Equation 21:

[ ]( ) 2/VVPPAA )1i(plpl(i))1i()i()1i(pl)i(pl −−− −++=

(21)

where Vpl(i) = (plastic part of the crack mouth opening displacement) = (Vi -PiCi).

9.5 Calculation of CTOD from J

Values of the crack-tip opening displacement δJ shall be calculated from J using equations (22) to (30).:

Yi

iiJ m

Jσ

δ = (22)

where

Y

Y

iipici

Yici

PPforPP

mmm

PPformm

>

−−=

≤=

1)()(

)(

(23)

and

21)( A

Wa

Am iic +=

(24)

where

( ) ( ) ( )32

1 1000263.0100986.011152.01293.0 nnnA +−+−= (25)

( ) ( ) ( )32

2 1000427.010194.01297.00867.3 nnnA −+−= (26)

and

21)( B

Wa

Bm iip +=

(27)

where


24

( ) ( ) ( )32

11000200.0100567.010634.00169.1 nnnB −+−=

(28)

( ) ( ) ( )32

21000393.0101487.011216.06969.0 nnnB −+−=

(29)

and n is the strain-hardening exponent determined from the stress-strain curve:

YS

n

YS

YS

YS

forE

forE

σσσσσ

ε

σσσε

>

=

=

1

≤

(30)

9.6 Adjustment of Original Crack Size when Unloading Compliance Method Is Used

All crack size and CTOD pairs, ia and iδ or δJi, shall be identified corresponding to each unload/load cycle during the test.

As shown in Figure 13, the initial unloading compliance data may indicate that the crack growth is negative. This is nonphysical and requires correction. Any spurious “apparent negative crack growth” values shall be ignored, so that only values beginning with the minimum crack length and ending at the last pair measured before maximum load was attained shall be used in equation (31) to estimate aoq. There must be at least eight pairs remaining in the data set for this method for crack size adjustment to be valid.

The initial crack size, qa0 , shall be determined using the set of points ia and iδ or δJi, and the relationship of equation (31):

( ) 3

22

10 ++41+= δCδC./δaδa q (31)

The following procedure is used to determine an adjusted estimate of the original crack size. Equation 30 is fitted to the (ai,δi) data pairs using the method of least squares to determine qa0 ; a procedure to do this is given in ASTM E1820, Appendix X1. The adjusted initial crack size shall be compared with the average initial crack size to qualify the test (see 10.2).

When the single-clip-gauge method to determine the J-R curve is used, aoq is similarly determined. However, in this case, the (ai, Ji) data pairs are used ending at the last pair measured before maximum load was attained. To determine aoq, equation 32 is fitted following ASTM 1820-11, Annex A9, using the method of least squares. There must be at least eight pairs remaining in the data set for this method for crack size adjustment to be valid.

( ) 3

22

10 2/ JCJCJaJa Yq +++= σ (32)


25

For each predicted ia , the corresponding crack growth shall be calculated as determined by equation (33):

qii aaa 0−=∆ (33)

9.7 CTOD R-Curve

The CTOD R-curve shall be obtained by plotting δi or δJi versus ia∆ as shown in Figure 14.

∆a

CTOD

i-1 i i+1

NOTE Each point in the curves corresponds to a load/unload sequence described in NOTE Only a limited

number of unload/load sequences are shown; rather more may be needed to meet the requirements of 9.6.1. Figure 10.

Figure 13—Illustration of a CTOD R-Curve

The CTOD R-curve (Figure 14) shall be fitted to the power law equation (34) to determine αδ and ηδ.:

( ) δηδα aCTOD ∆= (34)

The data shall be fitted from Δa=0.5 mm to the last measured crack growth value using the method of least squares. Error! Reference source not found. illustrates the fitting procedure.

9.8 J R-Curve

The J R -curve shall be obtained in a similar fashion to the CTOD R-curve, except that the ordinate is Ji

rather than δi, and ∆a is determined using aoq from equation (32).


26

The J R curve determined above shall be fitted to the power law equation (35) to determine αJ and ηJ

( ) JaJ J

ηα ∆= (35)

The data shall be fitted from Δa=0.5 mm to the last measured crack growth value using the method of least squares.

0.5∆a (mm)

CTOD = 1.36(∆a)0.71

0

1

2

3

0 1 2 3

CTO

D (m

m)

SENT R-curve

δ η

Power law fit

0.5∆a (mm)

CTOD = 1.36(∆a)0.71

0

1

2

3

0 1 2 3

CTO

D (m

m)

SENT R-curve

δ η

Power law fit

Figure 14—Curve Fitting of CTOD R-Curve by a Power Law Equation to Determine αδ and ηδ

10 Validity Requirements The following validity checks shall be performed prior to commencing an R-curve test:

αδ ηδ


27

The fatigue crack on both sides of the specimen shall extend at least 0.05W or 1.3 mm, whichever is larger, from the machined notch.

For weld-metal-notched tests, the notch or pre-crack shall be located at the weld centerline. The position of the centerline shall be at a point halfway between the outermost weld toes on the OD surface of the weld. These weld toes are the locations where the weld metal touches the base metal on either side of the cap pass(es).

For HAZ-notched tests, the pre-crack tips (or notch tips in the case of EDM-only notching) on both sides of the specimen shall be on the weld fusion line or entirely within the HAZ and within 0.5 mm of the fusion line.

The following validity checks shall be performed after completing an R-curve test for base material:

None of the nine physical measurements of initial crack size shall differ by more than the larger of 0.05W or 0.1a0 from the average initial crack length, 0a .

None of the nine physical measurements of final crack size shall differ by more than 0.05W or 0.1a0 from the average final crack length, afp..

For specimens from the weld zone (weld metal or HAZ), the above two crack straightness requirements shall be relaxed to the larger of 0.1W or 0.2a0.

The calculated initial crack size, a0q,, in 9.6 shall not differ from a0 by more than 0.5 mm.

The calculated crack growth from elastic compliance or potential drop at the last unload/load sequence,

qff aaa 0−=∆ , shall not differ from the optically measured crack growth, Δaf = afp – a0, by more than

0.20Δafp.

For HAZ specimens, the location of the pre-crack tip (or notch tip in the case of EDM-only notching) as determined on the post-test cross sections shall be located on the weld fusion line or entirely within the etched HAZ and within 0.5 mm of the fusion line.

11 Test Report The test report shall include, at minimum, the following information:

A summary of the test procedure.

Type of test specimen (base material, weld metal or HAZ).

Material properties including yield and tensile strength at room temperature for base material and weld metals.

Specimen dimensions including side groove depth and root radius, and the heights of lower and upper clip gauges above the specimen surface.

Test temperature, and loading rate including time to reach 0.5Py.

Load and crack opening displacement records, including data for both lower and upper clip gauges for double-clip-gauge method.


28

Optically measured initial and final crack depths at nine equally spaced points and the average according to 8.8, and the average initial crack depth and final crack depth estimated by unloading compliance or alternative method used.

Photographic record of the fracture surfaces. Also, in the case of HAZ specimens, photographic record of the crack tip cross sections as described in 8.9.

Initial and final crack sizes, a0q and , calculated according to Section 9.6 and 9.2 respectively.

Qualified CTOD and J R-curves, including indication of the point of maximum load.

α and η as determined in Section 9.7 and Section 9.8 for each R-curve reported: CTOD-R, J-R, and CTODJ-R.


29

Annex A (Normative)

Calculation of Compliance In Figure A.1, a typical force versus CMOD curve is plotted. The value of CMOD, Mδ , either determined from Equation 5 (with double clip gauges) or directly measured (with single clip gauge), is used to calculate compliance. The slope of the unload cycle should be calculated by linear regression using the method of least squares fitting to obtain the slope of a straight line through the unload data points. The CMOD compliance for the ith unload cycle is defined as the inverse slope:

0102030405060708090

100

0 1 2 3 4 5COD (mm)

Forc

e (k

N)

Figure A.1—Load versus CMOD Record

Data filtering is required to reduce errors in the compliance measurement owing to noise originating from the clip gauge response to the initiation of the unload and reload cycle. Figure A.2(a) is an enlarged view of one unload/reload cycle obtained from the force versus CMOD data plotted in Figure A.1. Two transient zones are visible, one at the start of the unload cycle and one at the start of the reload cycle. The transient zone is characterized by a nonlinear response in the force versus CMOD data. The nonlinearity may be due to a lag in the response of the clip gauge as the load direction is changed or due to the continuation of plastic deformation near the crack tip as the load is decreased past the initiation point of the unload cycle. It is necessary to remove (truncate) the data points in the transient regions from the compliance calculation to ensure an accurate measurement of the elastic response of the crack mouth during the load cycle. Figure A.2(b) is an example of a truncated data set obtained from the data plotted in Figure A.1(a). A straight line is fitted through the unload data to obtain the slope of the data using the method of least squares fitting.

CMOD (mm)


30

(a)

(b)

Figure A.2—(a) Enlarged View of an Unload/Reload Sequence, (b) Example Results of Filtered Data for Calculating Compliance of ith Unload Cycle

CMOD (mm)


31

Annex B (Normative)

Calculation Of Crack Size Using Electrical Potential Drop Method The voltage change across the specimen notch shall be measured during the test and plotted against the calculated CTOD (using the double clip gauge method) as shown in Figure B.1 or against the measured CMOD (using a single clip gauge).

Schematic Voltage versus CTOD Record

0

5

10

15

20

25

30

35

0.0 0.2 0.4 0.6 0.8 1.0 1.2CTOD (mm)

Volta

ge (N

ano

Volts

)

Point of Stable Crack Growth

Initiation

δend - δinit δendδinit

Figure B.1—Plot of Voltage Drop versus CTOD

During the early stage of elastic loading, there should be no change in voltage across the notch. However, as plasticity develops at the crack tip and through the ligament, there is a linear change in voltage with displacement. This is due to the material resistivity increasing with plasticity. The initiation of slow stable crack growth coincides with the point at which the voltage versus displacement trace deviates from the linear region associated with plasticity (blunting).

A validated electrical potential drop calibration curve shall be used to predict the stable crack growth during the test as a function of the potential drop following the initiation of stable crack growth. The values of stable crack growth obtained using the calibration curve do not include blunting.

As an alternative to the use of a validated calibration curve, a linear relationship can be assumed between voltage change and crack growth (after the initiation of stable crack growth) provided the crack growth (Δa) is less than 0.1W. The calculated values of stable crack growth do not include blunting. For larger amounts of crack growth (Δa > 0.1W), a linear relationship between voltage and stable crack growth cannot be assumed.


32

The voltage versus calculated CTOD record shall be analyzed as follows to develop a CTOD R-curve:

The voltage (Vinit) and CTOD (δinit) values at the point of predicted stable crack initiation shall be determined from the voltage versus CTOD record.

The calculated value of CTOD (δend) at the end of the test shall be determined and the voltage and displacement corresponding to the point of initiation shall be recorded.

The calculated CTOD range (δend – δinit) shall be divided into at least 10 and preferably 20 equal increments.

A table shall be created that lists the calculated values of CTOD and stable crack growth (excluding blunting) for each increment.

The calculated values of stable crack growth shall be adjusted, to account for crack tip blunting, by adding a fraction of the CTOD value at the initiation of stable crack growth (δinit) equal to δinit /1.4 to the calculated values of stable crack growth.

When using the single-clip-gauge method, the same steps shall be followed but using J or CTOD-from-J rather than calculated CTOD. To adjust for blunting, values of δinit /1.4 or Jinit /2σY shall be added to the values of crack growth.


33

Bibliography [1] ASTM E647, Standard Test Method for Measurement of Fatigue Crack Growth Rates [2] ISO 12135, Metallic materials–Unified method of test for the determination of quasistatic fracture

toughness [3] CANMET Report 2011-xx (TR-R), Recommended Practice: Fracture Toughness Testing Using

SE(T) Samples with Fixed-Grip Loading, January 2012 [4] Measurement of Crack-Tip Opening Displacement (CTOD) Fracture Resistance Curves Using

Single-Edge Notched Tension (SENT) Specimens, ExxonMobil Upstream Research Company Report, 20 September 2010. Internal Document

[5] “Fracture toughness instrumentation techniques for single-specimen clamped SE(T) tests on X100

line pipe steel: experimental setup,” Paper No. S07-03, 6th International Pipeline Technology Conference 2013, Ostend, Belgium, 6-9 October, 2013. Weeks et.al

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