ASTM G168-00(2006)

NOTICE: This standard has either been superseded and replaced by a new version or withdrawn.
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Designation: G168 − 00(Reapproved 2006)
Standard Practice for
Making and Using Precracked Double Beam Stress
Corrosion Specimens
This standard is issued under the fixed designation G168; the number immediately following the designation indicates the year of
original adoption or, in the case of revision, the year of last revision. A number in parentheses indicates the year of last reapproval. A
superscript epsilon (´) indicates an editorial change since the last revision or reapproval.
1. Scope E399 Test Method for Linear-Elastic Plane-Strain Fracture
Toughness K of Metallic Materials
Ic
1.1 This practice covers procedures for fabricating,
E1823 TerminologyRelatingtoFatigueandFractureTesting
preparing, and using precracked double beam stress corrosion
G15 Terminology Relating to Corrosion and CorrosionTest-
test specimens. This specimen configuration was formerly
ing (Withdrawn 2010)
designated the double cantilever beam (DCB) specimen.
G35 Practice for Determining the Susceptibility of Stainless
Guidelines are given for methods of exposure and inspection.
Steels and Related Nickel-Chromium-Iron Alloys to
1.2 The precracked double beam specimen, as described in
Stress-Corrosion Cracking in Polythionic Acids
this practice, is applicable for evaluation of a wide variety of
G36 Practice for Evaluating Stress-Corrosion-Cracking Re-
metals exposed to corrosive environments. It is particularly
sistance of Metals and Alloys in a Boiling Magnesium
suited to evaluation of products having a highly directional
Chloride Solution
grain structure, such as rolled plate, forgings, and extrusions,
G37 Practice for Use of Mattsson’s Solution of pH 7.2 to
when stressed in the short transverse direction.
Evaluate the Stress-Corrosion Cracking Susceptibility of
Copper-Zinc Alloys
1.3 The precracked double beam specimen may be stressed
in constant displacement by bolt or wedge loading or in G41 Practice for Determining Cracking Susceptibility of
Metals Exposed Under Stress to a Hot Salt Environment
constantloadbyuseofproofringsordeadweightloading.The
precracked double beam specimen is amenable to exposure to G44 PracticeforExposureofMetalsandAlloysbyAlternate
Immersion in Neutral 3.5 % Sodium Chloride Solution
aqueous or other liquid solutions by specimen immersion or by
periodic dropwise addition of solution to the crack tip, or G49 Practice for Preparation and Use of Direct Tension
Stress-Corrosion Test Specimens
exposure to the atmosphere.
G50 Practice for Conducting Atmospheric Corrosion Tests
1.4 This practice is concerned only with precracked double
on Metals
beam specimen and not with the detailed environmental
aspects of stress corrosion testing, which are covered in
3. Terminology
Practices G35, G36, G37, G41, G44, and G50.
3.1 Definitions of Terms Specific to This Standard:
1.5 This standard does not purport to address all of the
3.1.1 stress corrosion cracking (SCC) threshold stress
safety concerns, if any, associated with its use. It is the
intensity, K —the stress intensity level below which stress
Iscc
responsibility of the user of this standard to establish appro-
corrosioncrackingdoesnotoccurforaspecificcombinationof
priate safety and health practices and determine the applica-
material and environment when plane strain conditions are
bility of regulatory limitations prior to use.
satisfied.
3.1.1.1 Discussion—Terms relative to this subject matter
2. Referenced Documents
can be found in Terminologies G15 and E1823.
2.1 ASTM Standards:
D1193 Specification for Reagent Water
4. Summary of Practice
4.1 This practice covers the preparation and testing of
This practice is under the jurisdiction of ASTM Committee G01 on Corrosion
precracked double beam specimens for investigating the resis-
of Metals and is the direct responsibility of Subcommittee G01.06 on Environmen-
tance to SCC (see Terminology G15) of metallic materials in
tally Assisted Cracking.
Current edition approved Nov. 1, 2006. Published January 2007. Originally
various product forms. Precracking by fatigue loading and by
approved in 2000. Last previous edition approved in 2000 as G168 – 00. DOI:
mechanical overload are described. Procedures for stressing
10.1520/G0168-00R06.
For referenced ASTM standards, visit the ASTM website, www.astm.org, or
contact ASTM Customer Service at service@astm.org. For Annual Book of ASTM
Standards volume information, refer to the standard’s Document Summary page on The last approved version of this historical standard is referenced on
the ASTM website. www.astm.org.
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959. United States
G168 − 00 (2006)
specimens in constant displacement with loading bolts are
described, and expressions are given for specimen stress
intensity and crack mouth opening displacement. Guidance is
given for methods of exposure and inspection of precracked
double beam specimens.
5. Significance and Use
5.1 Precracked specimens offer the opportunity to use the
principles of linear elastic fracture mechanics (1) to evaluate
resistance to stress corrosion cracking in the presence of a
pre-existing crack. This type of evaluation is not included in
conventional bent beam, C-ring, U-bend, and tension speci-
mens. The precracked double beam specimen is particularly
useful for evaluation of materials that display a strong depen-
dence on grain orientation. Since the specimen dimension in
the direction of applied stress is small for the precracked
double beam specimen, it can be successfully used to evaluate
short transverse stress corrosion cracking of wrought products,
such as rolled plate or extrusions. The research applications
NOTE 1—Schematic of the influence of corrosion product wedging on
SCC growth versus time curves in a decreasing K (constant displacement)
and analysis of precracked specimens in general, and the
test. Solid lines: actually measured curve for case of corrosion product
precracked double beam specimen in particular, are discussed
wedging that results in increase in crack growth with time; asterisks
in Appendix X1.
indicate temporary crack arrest. Dashed lines: true crack growth curve
excluding the effect of corrosion product wedging (9).
5.2 The precracked double beam specimen may be stressed
FIG. 1 Effect of Corrosion Product Wedging on Growth Crack
in either constant displacement or constant load. Constant
Versus Time Curve
displacement specimens stressed by loading bolts or wedges
are compact and self-contained. By comparison, constant load
NOTE 1—Do not allow corrodent in the crack to dry during periodic
specimens stressed with springs (for example, proof rings,
measurements to avoid repassivation at the crack tip and the resulting
discussed in Test Method G49, 7.2.1.2) or by deadweight change in corrosion conditions. Remove one specimen at a time from
corrodent. For tests conducted in deaerated test environments or in
loading require additional fixtures that remain with the speci-
environments that contain readily oxidizable species or corrosion
men during exposure.
products, interim crack length examinations may produce changes in the
5.3 The recommendations of this practice are based on the conditionsatthecracktipthatcan,inturn,affectcrackingbehaviorduring
the subsequent exposure period.
results of interlaboratory programs to evaluate precracked
specimen test procedures (2,3) as well as considerable indus-
6.2 Interferences in Visual Crack Length Measurements:
trialexperiencewiththeprecrackeddoublebeamspecimenand
6.2.1 Corrosion products on the side surfaces of the speci-
other precracked specimen geometries (4-8).
men can interfere with accurate crack length measurements.
Corrosion products on these surfaces may be removed by
6. Interferences
careful scrubbing with a nonmetallic abrasive pad. However,
6.1 Interferences in Testing:
for interim measurements, a minimum area of surface should
6.1.1 The accumulation of solid corrosion products or oxide
be cleaned to allow for visual crack length measurements if
films on the faces of an advancing stress corrosion crack can
reexposure is planned.
generate wedge forces that add to the applied load, thereby
6.2.2 Measurement on side grooved specimens may be
increasing the effective stress intensity at the crack tip (6-9).
difficultiftheadvancingcracktravelsupthesideofthegroove.
This self-loading condition caused by corrosion product wedg-
This is especially difficult with V-shaped grooves. Adjustment
ing can accelerate crack growth and can prevent crack arrest
of the direction and intensity of the lighting may highlight the
from being achieved. The effect of corrosion product wedging
location of the crack tip.
on crack growth versus time curve is shown schematically in
6.2.3 Often the crack length measured at the specimen
Fig. 1 (9). When wedging forces occur, they can invalidate
surface is less than in the interior, due to decreased stress
further results and the test should be ended.
triaxiality at the specimen surface. Alternatively, some condi-
6.1.2 Crack-tip blunting or branching out, or both, of the
tions produce an increase in crack length at the surface due to
plane of the precrack can invalidate the test. For valid tests, the
availabilityofthecorrodent.Ultrasonicmethodscanbeusedto
crack must remain within 610° of the centerline of the
obtain interim crack length measurements at the interior of the
specimen.
specimen but not near the specimen surface.
6.1.3 Drying or contamination of the corrodent in the crack
6.2.4 Transport of species in solution in the through-
duringinterimmeasurementsofthecracklengthmayaffectthe
thickness direction can be important for precracked double
cracking behavior during subsequent exposure.
beam specimens. This may affect measurement of crack length
since it can produce curvature of the crack front (that is,
variation in crack length from the edge to the center of the
The boldface numbers in parentheses refer to the list of references at the end of
this standard. specimen).
G168 − 00 (2006)
7. Specimen Size, Configuration, and Preparation 7.1.3 Recommendations for determining the minimum
specimen thickness, B, which will ensure that plane strain
7.1 Specimen Dimensions and Fabrication:
conditions are maintained at the tip of an SCC crack, are
7.1.1 Dimensions for the recommended specimen are given
discussed in Brown (1) and Dorward and Helfrich (8).A
in Figs. 2 and 3. As a general guideline, specimen dimensions
conservative estimate for the specimen thickness shall be made
should ensure that plane strain conditions are maintained at the
by adopting the thickness criteria for plane strain fracture
crack tip (1,10). While there are no established criteria for
toughness testing, as described in Test Method E399. For bolt
ensuring adequate constraint for a plane strain SCC test, some
loaded precracked double beam specimens, the thickness, B,
guidelines are given herein regarding specimen dimensions
(see 7.1.3). may also be influenced by the size of the loading bolts and the
7.1.2 Specimen machining shall be in accordance with the minimum thickness needed to support the bolt loading.
standards outlined in Test Method E399. The principal consid-
7.1.4 The specimen half-height, H, may be reduced for
erations in machining are that the sides, top, and bottom of the
material under 25 mm (1 in.) thick. The minimum H that can
specimen should be parallel; the machined notch should be
be used is constrained by the onset of plastic deformation upon
centered; and the bolt holes should be aligned and centered.A
precracking or stresses in the leg of the specimen since this
typical bolt loaded specimen is shown in Fig. 4.
NOTE 1—All dimensions in mm (in.). Top and front views are shown for smooth specimen only; side view is shown for both smooth and side grooved
configuration.
NOTE 2—For Chevron notch crack starter, cutter tip angle 90° max.
NOTE 3—Radius at notch bottom to be 0.25 mm (0.01 in.) or less.
NOTE 4—Crack starter to be perpendicular to specimen length and thickness to within 62°.
NOTE 5—Initial COD (∆) may be increased to 12.7 mm (0.5 in.) to accomodate COD gage.
NOTE 6—All surfaces 32 µin. or better, tolerances not specified 60.127 (0.005).
NOTE 7—Continue with Chevron cutter on surface to machine grooves.
NOTE 8—Loading bolt holes shall be perpendicular to specimen center lines within 65°.
NOTE 9—Center line of holes shall be parallel and perpendicular to specimen surfaces within 62°.
NOTE 10—Center line of holes shall be coincident within 60.127 mm (0.005 in.).
FIG. 2 Detailed Machine Drawing for Smooth and Face Grooved DCB Specimen
G168 − 00 (2006)
NOTE 1—An optional bolt is shown which has a recessed hexagonal
socket to accept an Allen wrench.
FIG. 4 Bolt Loaded Precracked Double Beam Specimen
influences the calculation of K. Outer fiber stresses shall not
exceed the yield strength of the test material during precrack-
ing or stressing.
NOTE 2—The effect of notch geometry on specimen compliance and
stress intensity solutions, noted in 7.3.4.4, Note 4, 8.1.3, and Note 5,is
NOTE 1—All dimensions in mm (in). Tolerances not specified 60.127
magnified as H is reduced.
(60.005).
7.1.5 The overall length of the specimen, L, can be in-
NOTE 2—Suggested material: Strong enough not to fail in tension
creased to allow for more crack growth. Specimens of SCC
during loading or mechanical precracking.
susceptible material that are loaded in constant deflection to
NOTE 3—Bolt head design optional. Commercial stainless steel socket
head cap screws or hex head bolts are satisfactory.
high starting stress intensities may require additional crack
NOTE 4—Use one rounded end and one flat end bolt for loading each
growth to achieve crack arrest as defined in 10.1.
specimen. Commercial bolts or screws should be modified accordingly.
7.2 Specimen Configuration:
NOTE 5—To avoid galvanic corrosion between dissimilar bolt and
7.2.1 The recommended specimen configuration includes a
specimen metals, see 8.2.
FIG. 3 Machine Drawing for DCB Loading Bolts sharp starter notch, which may be either a straight through or
G168 − 00 (2006)
chevron configuration. The chevron configuration is recom- where:
1/2 1/2
mended for both the fatigue and the mechanical overload
K = stress intensity factor, MPa-m (ksi-in. ),
I
precracking operations (see Fig. 2).
P = applied load, MN (klbf),
a = crack length, m (in.),
7.2.2 The use of side grooves is optional. They may be
B = specimen thickness, m (in.),
helpful if any difficulty is experienced in keeping the crack in
B = specimen thickness at the machined notch for face
t
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