ASTM G168-00

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Designation:G168–00
Standard Practice for
Making and Using Precracked Double Beam Stress
Corrosion Specimens
This standard is issued under the fixed designation G 168; 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 (e) indicates an editorial change since the last revision or reapproval.
1. Scope G 15 Terminology Relating to Corrosion and Corrosion
Testing
1.1 This practice covers procedures for fabricating, prepar-
G 35 Practice for Determining the Susceptibility of Stain-
ing, and using precracked double beam stress corrosion test
less Steels and Related Nickel-Chromium-Iron Alloys to
specimens. This specimen configuration was formerly desig-
Stress Corrosion Cracking in Polythionic Acids
nated the double cantilever beam (DCB) specimen. Guidelines
G 36 Practice for Preforming Stress-Corrosion Cracking
are given for methods of exposure and inspection.
Tests in a Boiling Magnesium Chloride Solution
1.2 The precracked double beam specimen, as described in
G 37 Practice for Use of Mattson’s Solution of pH 7.2 to
this practice, is applicable for evaluation of a wide variety of
Evaluate the Stress-Corrosion Cracking Susceptibility of
metals exposed to corrosive environments. It is particularly
Copper-Zinc Alloys
suited to evaluation of products having a highly directional
G 41 Practice for Determining Cracking Susceptibility of
grain structure, such as rolled plate, forgings, and extrusions,
Metals Exposed Under Stress to a Hot Salt Environment
when stressed in the short transverse direction.
G 44 Practice for Evaluating Stress Corrosion Cracking
1.3 The precracked double beam specimen may be stressed
ResistanceofMetalsandAlloysbyAlternateImmersionin
in constant displacement by bolt or wedge loading or in
3.5 % Sodium Chloride Solution
constantloadbyuseofproofringsordeadweightloading.The
G 49 Practice for Preparation and Use of Direct Tension
precracked double beam specimen is amenable to exposure to
Stress Corrosion Test Specimens
aqueous or other liquid solutions by specimen immersion or by
G 50 Practice for Conducting Atmospheric Corrosion Tests
periodic dropwise addition of solution to the crack tip, or
on Metals
exposure to the atmosphere.
1.4 This practice is concerned only with precracked double
3. Terminology
beam specimen and not with the detailed environmental
3.1 Definitions of Terms Specific to This Standard:
aspects of stress corrosion testing, which are covered in
3.1.1 stress corrosion cracking (SCC) threshold stress in-
Practices G 35, G 36, G 37, G 41, G 44, and G 50.
tensity, K —the stress intensity level below which stress
Iscc
1.5 This standard does not purport to address all of the
corrosion cracking does not occur for a specific combination of
safety concerns, if any, associated with its use. It is the
material and environment when plane strain conditions are
responsibility of the user of this standard to establish appro-
satisfied.
priate safety and health practices and determine the applica-
3.1.1.1 Discussion—Terms relative to this subject matter
bility of regulatory limitations prior to use.
can be found in Terminologies G 15 and E 1823.
2. Referenced Documents
4. Summary of Practice
2.1 ASTM Standards:
2 4.1 This practice covers the preparation and testing of
D 1193 Specification for Reagent Water
precracked double beam specimens for investigating the resis-
E 399 Test Method for Plane-Strain Fracture Toughness of
tance to SCC (see Terminology G 15) of metallic materials in
Metallic Materials
various product forms. Precracking by fatigue loading and by
E 1823 Terminology Relating to Fatigue and Fracture Test-
mechanical overload are described. Procedures for stressing
ing
specimens in constant displacement with loading bolts are
described, and expressions are given for specimen stress
intensity and crack mouth opening displacement. Guidance is
This guide is under the jurisdiction ofASTM Committee G01 on Corrosion of
given for methods of exposure and inspection of precracked
Metals and is the direct responsibility of Subcommittee G01.06 on Environmentally
double beam specimens.
Assisted Cracking.
Current edition approved May 10, 2000. Published August 2000.
Annual Book of ASTM Standards, Vol 11.01.
3 4
Annual Book of ASTM Standards, Vol 03.01. Annual Book of ASTM Standards, Vol 03.02.
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G168
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
and analysis of precracked specimens in general, and the
precracked double beam specimen in particular, are discussed
in Appendix X1.
5.2 The precracked double beam specimen may be stressed
in either constant displacement or constant load. Constant
displacement specimens stressed by loading bolts or wedges
NOTE 1—Schematic of the influence of corrosion product wedging on
are compact and self-contained. By comparison, constant load
SCC growth versus time curves in a decreasing K (constant displacement)
specimens stressed with springs (for example, proof rings,
test. Solid lines: actually measured curve for case of corrosion product
discussed in Test Method G 49, 7.2.1.2) or by deadweight wedging that results in increase in crack growth with time; asterisks
indicate temporary crack arrest. Dashed lines: true crack growth curve
loading require additional fixtures that remain with the speci-
excluding the effect of corrosion product wedging (9).
men during exposure.
FIG. 1 Effect of Corrosion Product Wedging on Growth Crack
5.3 The recommendations of this practice are based on the
Versus Time Curve
results of interlaboratory programs to evaluate precracked
specimen test procedures (2,3) as well as considerable indus-
trialexperiencewiththeprecrackeddoublebeamspecimenand
conditionsatthecracktipthatcan,inturn,affectcrackingbehaviorduring
other precracked specimen geometries (4-8).
the subsequent exposure period.
6.2 Interferences in Visual Crack Length Measurements:
6. Interferences
6.2.1 Corrosion products on the side surfaces of the speci-
6.1 Interferences in Testing:
men can interfere with accurate crack length measurements.
6.1.1 The accumulation of solid corrosion products or oxide
Corrosion products on these surfaces may be removed by
films on the faces of an advancing stress corrosion crack can
careful scrubbing with a nonmetallic abrasive pad. However,
generate wedge forces that add to the applied load, thereby
for interim measurements, a minimum area of surface should
increasing the effective stress intensity at the crack tip (6-9).
be cleaned to allow for visual crack length measurements if
This self-loading condition caused by corrosion product wedg-
reexposure is planned.
ing can accelerate crack growth and can prevent crack arrest
6.2.2 Measurement on side grooved specimens may be
from being achieved. The effect of corrosion product wedging
difficultiftheadvancingcracktravelsupthesideofthegroove.
on crack growth versus time curve is shown schematically in
This is especially difficult with V-shaped grooves. Adjustment
Fig. 1 (9). When wedging forces occur, they can invalidate
of the direction and intensity of the lighting may highlight the
further results and the test should be ended.
location of the crack tip.
6.1.2 Crack-tip blunting or branching out, or both, of the
6.2.3 Often the crack length measured at the specimen
plane of the precrack can invalidate the test. For valid tests, the
surface is less than in the interior, due to decreased stress
crack must remain within 610° of the centerline of the
triaxiality at the specimen surface. Alternatively, some condi-
specimen.
tions produce an increase in crack length at the surface due to
6.1.3 Drying or contamination of the corrodent in the crack
availabilityofthecorrodent.Ultrasonicmethodscanbeusedto
duringinterimmeasurementsofthecracklengthmayaffectthe
obtain interim crack length measurements at the interior of the
cracking behavior during subsequent exposure.
specimen but not near the specimen surface.
6.2.4 Transport of species in solution in the through-
NOTE 1—Do not allow corrodent in the crack to dry during periodic
measurements to avoid repassivation at the crack tip and the resulting
thickness direction can be important for precracked double
change in corrosion conditions. Remove one specimen at a time from
beam specimens. This may affect measurement of crack length
corrodent. For tests conducted in deaerated test environments or in
since it can produce curvature of the crack front (that is,
environments that contain readily oxidizable species or corrosion prod-
variation in crack length from the edge to the center of the
ucts, interim crack length examinations may produce changes in the
specimen).
7. Specimen Size, Configuration, and Preparation
The boldface numbers in parentheses refer to the list of references at the end of
this standard. 7.1 Specimen Dimensions and Fabrication:
G168
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 E 399. 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
may also be influenced by the size of the loading bolts and the
(see 7.1.3).
minimum thickness needed to support the bolt loading.
7.1.2 Specimen machining shall be in accordance with the
7.1.4 The specimen half-height, H, may be reduced for
standards outlined in Test Method E 399. The principal con-
material under 25 mm (1 in.) thick. The minimum H that can
siderations in machining are that the sides, top, and bottom of
be used is constrained by the onset of plastic deformation upon
the specimen should be parallel; the machined notch should be
precracking or stresses in the leg of the specimen since this
centered; and the bolt holes should be aligned and centered.A
influences the calculation of K. Outer fiber stresses shall not
typical bolt loaded specimen is shown in Fig. 4.
exceed the yield strength of the test material during precrack-
7.1.3 Recommendations for determining the minimum
ing or stressing.
specimen thickness, B, which will ensure that plane strain
conditions are maintained at the tip of an SCC crack, are
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 (D) 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 within6 5°.
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
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
7.2.3 Specimens machined from rectangular product can
have six possible orientations (see Test Method E 399) relative
to the direction of loading and the direction of crack propaga-
tion, namely, S-L, S-T, T-L, T-S, L-T, and L-S. In wrought
products, the S-L orientation is usually the most critical and is
NOTE 1—All dimensions in mm (in). Tolerances not specified 60.127
the most frequently used to avoid crack branching
(60.005).
7.2.4 More detailed discussions of the factors described in
NOTE 2—Suggested material: Strong enough not to fail in tension
this section are given in Brown (1), Sprowls et al (6), and
during loading or mechanical precracking.
Sprowls (9).
NOTE 3—Bolt head design optional. Commercial stainless steel socket
7.3 Specimen Preparation:
head cap screws or hex head bolts are satisfactory.
NOTE 4—Use one rounded end and one flat end bolt for loading each 7.3.1 Specimen surfaces along the path of expected crack
specimen. Commercial bolts or screws should be modified accordingly.
propagation may be polished to assist in crack measurement.
NOTE 5—To avoid galvanic corrosion between dissimilar bolt and
7.3.2 Specimens shall be cleaned and degreased prior to
specimen metals, see 8.2.
precracking and testing. Successive ultrasonic cleaning in
FIG. 3 Machine Drawing for DCB Loading Bolts
acetone and methyl alcohol is suggested. Specimens shall not
be recleaned after precracking to prevent contamination of the
crack with cleaning or degreasing chemicals. If cleaning of the
NOTE 2—The effect of notch geometry on specimen compliance and
side surfaces of the specimen following precracking is neces-
stress intensity solutions, noted in 7.3.4.4, Note 4, 8.1.3, and Note 5, is
sary, then this should be performed by lightly wiping these
magnified as H is reduced.
surfaces and not by immersion of the specimen into the
7.1.5 The overall length of the specimen, L, can be in-
cleaning or degreasing media.
creased to allow for more crack growth. Specimens of SCC
NOTE 3—Only chemicals appropriate for the metal or alloy of interest
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