ASTM G168-17

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Designation: G168 − 00 (Reapproved 2013) G168 − 17
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 Scope*
1.1 This practice covers procedures for fabricating, preparing, and using precracked double beam stress corrosion test
specimens. This specimen configuration was formerly designated the double cantilever beam (DCB) specimen. Guidelines are
given for methods of exposure and inspection.
1.2 The precracked double beam specimen, as described in this practice, is applicable for evaluation of a wide variety of metals
exposed to corrosive environments. It is particularly suited to evaluation of products having a highly directional grain structure,
such as rolled plate, forgings, and extrusions, when stressed in the short transverse direction.
1.3 The precracked double beam specimen may be stressed in constant displacement by bolt or wedge loading or in constant
load by use of proof rings or dead weight loading. The precracked double beam specimen is amenable to exposure to aqueous or
other liquid solutions by specimen immersion or by periodic dropwise addition of solution to the crack tip, or exposure to the
atmosphere.
1.4 This practice is concerned only with precracked double beam specimen and not with the detailed environmental aspects of
stress corrosion testing, which are covered in Practices G35, G36, G37, G41, G44, and G50.
1.5 This standard does not purport to address all of the safety concerns, if any, associated with its use. It is the responsibility
of the user of this standard to establish appropriate safety safety, health, and healthenvironmental practices and determine the
applicability of regulatory limitations prior to use.
1.6 This international standard was developed in accordance with internationally recognized principles on standardization
established in the Decision on Principles for the Development of International Standards, Guides and Recommendations issued
by the World Trade Organization Technical Barriers to Trade (TBT) Committee.
2. Referenced Documents
2.1 ASTM Standards:
D1193 Specification for Reagent Water
E8/E8M Test Methods for Tension Testing of Metallic Materials
E399 Test Method for Linear-Elastic Plane-Strain Fracture Toughness K of Metallic Materials
Ic
E1823 Terminology Relating to Fatigue and Fracture Testing
G15 Terminology Relating to Corrosion and Corrosion Testing (Withdrawn 2010)
G35 Practice for Determining the Susceptibility of Stainless Steels and Related Nickel-Chromium-Iron Alloys to Stress-
Corrosion Cracking in Polythionic Acids
G36 Practice for Evaluating Stress-Corrosion-Cracking Resistance of Metals and Alloys in a Boiling Magnesium Chloride
Solution
G37 Practice for Use of Mattsson’s Solution of pH 7.2 to Evaluate the Stress-Corrosion Cracking Susceptibility of Copper-Zinc
Alloys
G41 Practice for Determining Cracking Susceptibility of Metals Exposed Under Stress to a Hot Salt Environment
G44 Practice for Exposure of Metals and Alloys by Alternate Immersion in Neutral 3.5 % Sodium Chloride Solution
This practice is under the jurisdiction of ASTM Committee G01 on Corrosion of Metals and is the direct responsibility of Subcommittee G01.06 on Environmentally
Assisted Cracking.
Current edition approved May 1, 2013Nov. 1, 2017. Published July 2013December 2017. Originally approved in 2000. Last previous edition approved in 20062013 as
G168 – 00 (2006).(2013). DOI: 10.1520/G0168-00R13.10.1520/G0168-17.
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 ASTM website.
The last approved version of this historical standard is referenced on www.astm.org.
*A Summary of Changes section appears at the end of this standard
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959. United States
G168 − 17
G49 Practice for Preparation and Use of Direct Tension Stress-Corrosion Test Specimens
G50 Practice for Conducting Atmospheric Corrosion Tests on Metals
3. Terminology
3.1 Definitions of Terms Specific to This Standard:
3.1.1 stress corrosion cracking (SCC) threshold stress intensity, K —the stress intensity level below which stress corrosion
Iscc
cracking does not occur for a specific combination of material and environment when plane strain conditions are satisfied.
3.1.1.1 Discussion—
Terms relative to this subject matter can be found in Terminologies G15 and E1823.
4. Summary of Practice
4.1 This practice covers the preparation and testing of precracked double beam specimens for investigating the resistance to
SCC (see Terminology G15) of metallic materials in various product forms. Precracking by fatigue loading and by mechanical
overload are described. Procedures for stressing 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 specimens. The precracked double beam specimen is particularly useful for
evaluation of materials that display a strong dependence 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 are compact and self-contained. By comparison, constant load
specimens stressed with springs (for example, proof rings, discussed in Test Method G49, 7.2.1.2) or by deadweight loading require
additional fixtures that remain with the specimen during exposure.
5.3 The recommendations of this practice are based on the results of interlaboratory programs to evaluate precracked specimen
test procedures (2, 3) as well as considerable industrial experience with the precracked double beam specimen and other precracked
specimen geometries (4-8).
6. Interferences
6.1 Interferences in Testing:
6.1.1 The accumulation of solid corrosion products or oxide films on the faces of an advancing stress corrosion crack can
generate wedge forces that add to the applied load, thereby increasing the effective stress intensity at the crack tip (6-9). This
self-loading condition caused by corrosion product wedging can accelerate crack growth and can prevent crack arrest from being
achieved. The effect of corrosion product wedging on crack growth versus time curve is shown schematically in Fig. 1 (9). When
wedging forces occur, they can invalidate further results and the test should be ended.
6.1.2 Crack-tip blunting or branching out, or both, of the plane of the precrack can invalidate the test. For valid tests, the crack
must remain within 610° of the centerline of the specimen.
6.1.3 Drying or contamination of the corrodent in the crack during interim measurements of the crack length may affect the
cracking behavior during subsequent exposure.
NOTE 1—Do not allow corrodent in the crack to dry during periodic measurements to avoid repassivation at the crack tip and the resulting change in
corrosion conditions. Remove one specimen at a time from corrodent. For tests conducted in deaerated test environments or in environments that contain
readily oxidizable species or corrosion products, interim crack length examinations may produce changes in the conditions at the crack tip that can, in
turn, affect cracking behavior during the subsequent exposure period.
6.2 Interferences in Visual Crack Length Measurements:
6.2.1 Corrosion products on the side surfaces of the specimen can interfere with accurate crack length measurements. Corrosion
products on these surfaces may be removed by careful scrubbing with a nonmetallic abrasive pad. However, for interim
measurements, a minimum area of surface should be cleaned to allow for visual crack length measurements if reexposure is
planned.
The boldface numbers in parentheses refer to the list of references at the end of this standard.
G168 − 17
NOTE 1—Schematic of the influence of corrosion product wedging on SCC growth versus time curves in a decreasing K (constant displacement) test.
Solid lines: actually measured curve for case of corrosion product wedging that results in increase in crack growth with time; asterisks indicate temporary
crack arrest. Dashed lines: true crack growth curve excluding the effect of corrosion product wedging (9).
FIG. 1 Effect of Corrosion Product Wedging on Growth Crack Versus Time Curve
6.2.2 Measurement on side grooved specimens may be difficult if the advancing crack travels up the side of the groove. This
is especially difficult with V-shaped grooves. Adjustment of the direction and intensity of the lighting may highlight the location
of the crack tip.
6.2.3 Often the crack length measured at the specimen surface is less than in the interior, due to decreased stress triaxiality at
the specimen surface. Alternatively, some conditions produce an increase in crack length at the surface due to availability of the
corrodent. Ultrasonic methods can be used to obtain interim crack length measurements at the interior of the specimen but not near
the specimen surface.
6.2.4 Transport of species in solution in the through-thickness direction can be important for precracked double 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 specimen).
7. Specimen Size, Configuration, and Preparation
7.1 Specimen Dimensions and Fabrication:
7.1.1 Dimensions for the recommended specimen are given in Figs. 2 and 3. As a general guideline, specimen dimensions
should ensure that plane strain conditions are maintained at the crack tip (1, 10). While there are no established criteria for ensuring
adequate constraint for a plane strain SCC test, some guidelines are given herein regarding specimen dimensions (see 7.1.3).
7.1.2 Specimen machining shall be in accordance with the standards outlined in Test Method E399. The principal considerations
in machining are that the sides, top, and bottom of the specimen should be parallel; the machined notch should be centered; and
the bolt holes should be aligned and centered. A typical bolt loaded specimen is shown in Fig. 4.
7.1.3 Recommendations for determining the minimum specimen thickness, B, which will ensure that plane strain conditions are
maintained at the tip of an SCC crack, are discussed in Brown (1) and Dorward and Helfrich (8). A Based on a conservative
estimate for the plane strain conditions, the minimum specimen thickness shall be made by adopting the thickness criteriacalculated
as B ≥ 2.5 (K /σ ) for plane, where K strain fracture toughness testing, as describedis determined per Test Method E399 in
Ic YS Ic
Testand σ Method is the 0.2 % offset yield strength in tension per Test Method E399E8/E8M. For bolt loaded precracked double
YS
beam specimens, the thickness, B, may also be influenced by the size of the loading bolts and the minimum thickness needed to
support the bolt loading.
7.1.4 The specimen half-height, H, may be reduced for material under 25 mm (1 in.) thick. The minimum H that can be used
is constrained by the onset of plastic deformation upon precracking or stresses in the leg of the specimen since this influences the
calculation of K. Outer fiber stresses shall not exceed the yield strength of the test material during precracking 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 magnified
as H is reduced.
7.1.5 The overall length of the specimen, L, can be increased to allow for more crack growth. Specimens of SCC susceptible
material that are loaded in constant deflection to high starting stress intensities may require additional crack growth to achieve
crack arrest as defined in 10.1.
7.2 Specimen Configuration:
G168 − 17
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 (Δ) may be increased to 12.7 mm (0.5 in.) to accomodateaccommodate COD gage.
NOTE 6—All surfaces 32 μin. or better, tolerances not specified 60.127 (0.005).
NOTE 7—Continue For V-shape side groove, continue with Chevron cutter on surface to machine grooves. For U-shape side groove, machine groove
with radius cutting tool such as a ball end mill, size equal to notch height.
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.).
NOTE 11—The crack length at the start of the exposure test (a ) is achieved by fatigue or mechanical precracking. Precracking length shall extend 2.5
to 3.8 mm (0.10 to 0.15 in.) from the tip of the machine notch at the specimen surface, see 7.3.4.3.
FIG. 2 Detailed Machine Drawing for Smooth Face and FaceSide Grooved DCB Specimen
7.2.1 The recommended specimen configuration includes a sharp starter notch, which may be either a straight through or
chevron configuration. The chevron configuration is recommended for both the fatigue and the mechanical overload precracking
operations (see Fig. 2).
7.2.2 The use of side grooves is optional. They may be helpful if any difficulty is experienced in keeping the crack in the center
of the specimen. The side groove configuration may be machined with the chevron V-shaped cutter or with a U-shaped cutter.
radius cutting tool. The depth of each side groove should not exceed 5 % of B, such that the net thickness, B , will be at least 90 %
n
of B.
7.2.3 Specimens machined from rectangular product can have six possible orientations (see Test Method E399) relative to the
direction of loading and the direction of crack propagation, 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 the most frequently used to avoid crack branching
7.2.4 More detailed discussions of the factors described in this section are given in Brown (1), Sprowls et al (6), and Sprowls
(9).
7.3 Specimen Preparation:
7.3.1 Specimen surfaces along the path of expected crack propagation may be polished to assist in crack measurement.
7.3.2 Specimens shall be cleaned and degreased prior to precracking and testing. Successive ultrasonic cleaning in 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 side surfaces of the specimen following precracking is necessary, then this
should be performed by lightly wiping these surfaces and not by immersion of the specimen into the cleaning or degreasing media.
NOTE 3—Only chemicals appropriate for the metal or alloy of interest shall be used. All chemicals shall be of reagent grade purity.
7.3.3 Specimens shall be fully machined, including surface grooves, prior to precracking. Precracked specimens shall be stored
in a dry atmosphere prior to environmental exposure.
7.3.4 Fatigue Precracking:
G168 − 17
NOTE 1—All dimensions in mm (in). Tolerances not specified 60.127 (60.005).
NOTE 2—Suggested material: Strong enough not to fail in tension during loading or mechanical precracking.
NOTE 3—Bolt head design optional. Commercial stainless steel socket head cap screws or hex head bolts are satisfactory.
NOTE 4—Use one rounded end and one flat end bolt for loading each specimen. Commercial bolts or screws should be modified accordingly.
NOTE 5—To avoid galvanic corrosion between dissimilar bolt and specimen metals, see 8.2.
FIG. 3 Machine Drawing for DCB Loading Bolts
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
7.3.4.1 Fatigue precracking shall be performed under sinusoidal cyclic loading with a stress ratio 0.05 < R < 0.2, where R =
P /P . Any convenient cyclic load frequency may be used for precracking.
min max
7.3.4.2 The maximum stress intensity factor (K ) to be applied during fatigue precracking shall not exceed two thirds of the
max
target starting stress intensity for the environmental exposure.
7.3.4.3 The fatigue precrack shall extend 2.5 to 3.8 mm (0.10 to 0.15 in.) from the tip of the machined notch at the specimen
surface. The plane of the crack shall be within 610° of the centerline of the specimen. The resulting crack length, a , shall be
o
measured on both specimen surfaces, and the two values averaged. The measuring instrument shall have an accuracy of 0.025 mm
(0.001 in.).
G168 − 17
7.3.4.4 The stress intensity factor during precracking shall be computed from the following equation (2, 11):
1/2 3/2
K 5 @3.464 P a ~110.673~H/a!!#/@~B ! H # (1)
I n
H
3.464·P·a· 1 1 0.673
S D
a
K 5 (1)
I 1⁄2 3⁄2
~B · B ! ·H
n
where:
1/2 1/2
K = stress intensity factor, MPa-m (ksi-in. ),
I
P = applied load, MN (klbf),
a = crack length, m (in.),
B = specimen thickness, m (in.),
B = specimen thickness at the machined notch for face grooved specimens, m (in.) (B = B for smooth face specimens), and
n n
H = specimen half height, m (in.).
NOTE 4—The stress intensity solutions provided by Eq 1, Eq 2, and Eq X1.2 are based on theoretical compliance of specimens of the recommended
configuration in Fig. 2. They have been validated by the work of Fichter (11). However, significant deviation in starter notch geometry and specimen half
height may result in inaccurate K values (12, 13).
I
7.3.5 Mechanical Precracking:
7.3.5.1 Specimens that are precracked by mechanical overload shall be precracked immediately prior to, and as the initial step
of, the environmental exposure test initiation. It may be convenient to support the specimen in a vise during the mechanical
precracking procedure. Mechanical precracking may be difficult on higher toughness materials; for example, aluminum alloys with
1/2
K > 25 MPa-m . Regardless of the material toughness, mechanical precracking is also difficult for specimens that are machined
IC
with the crack propagation direction normal to predominant grain orientation; for example, L-T or S-T (see Test Method E399)
orientations in rolled plate.
7.3.5.2 Crack mouth opening displacement, V , shall be monitored with a clip-on crack mouth opening displacement (COD)
o
gage during precracking. A typical COD gage is described in Test Method E399, Annex A1.
7.3.5.3 The mechanical precrack shall be extended 2.5 to 3.8 mm (0.10 to 0.15 in.) from the tip of the machined notch at the
specimen surface. The resulting crack length, a , shall be measured on both specimen surfaces, and the two values averaged. The
o
measuring instrument shall have an accuracy of 0.025 mm (0.001 in.).
7.3.5.4 The resulting stress intensity after mechanical precracking will be K , the stress intensity for mechanical crack arrest.
Ia
If K is greater than the target starting stress intensity, then K shall be used as the starting stress intensity for the stress corrosion
Ia Ia
test (that is, K ). If a mechanically precracked specimen is inadvertently overloaded, no attempt shall be made to reduce the initial
io
stress by partially unloading the specimen. This will produce compressive stresses at the crack tip, which will retard or prevent
crack initiation. If K is less than the target starting stress intensity, then adjustment of crack mouth opening, V , should be made
Ia o
following procedures provided in 8.1 (Eq 3).
7.3.5.5 The resulting stress intensity factor, K , should be computed from the following equation (11):
Ia
1/2 2 2
K 5 ~V E!/$2.309 H ~a /H10.673! @111.5~C /a !2 1.15~C /a ! #% (2)
Ia o o o o o o
V ·E
o
K 5 (2)
2 2
Ia
a C c
0 0 0
1⁄2
2.309·H · 1 0.673 · 1 1 1.5 · 2 1.15·
S D F S D S DG
H a a
0 0
where:
1/2 1/2
K = stress intensity factor at crack arrest, Mpa-m (ksi-in. ),
Ia
V = crack mouth opening displacement, m (in.),
o
E = Young’s Modulus, MPa (ksi),
a = starting crack length at start of exposure test, m (in.),
o
C = distance from load line to COD gage attachment location, m (in.), and
o
H = specimen half height, m (in.).
7.4 Residual Stress Effects—Residual stresses can have an influence on SCC. The effect can be significant when test specimens
are removed from material in which complete stress relief is impractical, such as weldments, as-heat treated materials, complex
wrought parts, and parts with intentionally produced residual stresses. Residual stresses superimposed on the applied stress can
cause the local crack-tip stress intensity factor to be different from that calculated from externally applied forces or displacements.
Irregular crack growth during precracking, such as excessive crack front curvature or out-of-plane crack growth, often indicates
that residual stresses will affect subsequent SCC growth behavior. Changes in the zero-force value of crack mouth opening
displacement as a result of precrack growth is another indication that residual stresses will affect the subsequent SCC growth.
8. General Procedure
8.1 Stressing Procedure:
G168 − 17
8.1.1 Precracked double beam specimens may be stressed either in constant displacement or constant load. The constant
displacement condition may be achieved by a wedge inserted in the machined notch or by loading bolts. The constant load
condition may be achieved through the use of dead weight loading or approximated with the use of proof rings with adequate
compliance to minimize load reduction that will occur during the test due to crack growth in the specimen (3).
8.1.2 Suggested loading bolts are shown in Fig. 3. A precracked double beam specimen stressed in constant displacement with
two bolts is shown in Fig. 4. The loading bolts shall be tightened until the crack mouth opening displacement (V ) reaches a value
o
corresponding to the desired target starting stress intensity value for the measured precrack length. The bolts shall be tightened in
small increments, alternating between the two, such that the specimen is deflected symmetrically about the centerline. Another
approach is to mount the nonstressed end of the specimen in a vice and use two wrenches, turning both wr
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