ASTM G30-97(2003)
(Practice)Standard Practice for Making and Using U-Bend Stress-Corrosion Test Specimens
Standard Practice for Making and Using U-Bend Stress-Corrosion Test Specimens
SIGNIFICANCE AND USE
The U-bend specimen may be used for any metal alloy sufficiently ductile to be formed into the U-shape without mechanically cracking. The specimen is most easily made from strip or sheet but can be machined from plate, bar, castings, or weldments; wire specimens may be used also.
Since the U-bend usually contains large amounts of elastic and plastic strain, it provides one of the most severe tests available for smooth (as opposed to notched or precracked) stress-corrosion test specimens. The stress conditions are not usually known and a wide range of stresses exist in a single stressed specimen. The specimen is therefore unsuitable for studying the effects of different applied stresses on stress-corrosion cracking or for studying variables which have only a minor effect on cracking. The advantage of the U-bend specimen is that it is simple and economical to make and use. It is most useful for detecting large differences between the stress-corrosion cracking resistance of (a) different metals in the same environment, (b) one metal in different metallurgical conditions in the same environment, or (c) one metal in several environments.
SCOPE
1.1 This practice describes procedures for making and using U-bend specimens for the evaluation of stress-corrosion cracking in metals. The U-bend specimen is generally a rectangular strip which is bent 180° around a predetermined radius and maintained in this constant strain condition during the stress-corrosion test. Bends slightly less than or greater than 180° are sometimes used. Typical U-bend configurations showing several different methods of maintaining the applied stress are shown in Fig. 1.
1.2 U-bend specimens usually contain both elastic and plastic strain. In some cases (for example, very thin sheet or small diameter wire) it is possible to form a U-bend and produce only elastic strain. However, bent-beam (Practice G 39 or direct tension (Practice G 49)) specimens are normally used to study stress-corrosion cracking of strip or sheet under elastic strain only.
1.3 This practice is concerned only with the test specimen and not the environmental aspects of stress-corrosion testing which are discussed elsewhere (1), in Practices G 35, G 36, G 37, G 41, G 44, G 103 and Test Method G 123.
1.4 The values stated in SI units are to be regarded as standard. The inch-pound units in parentheses are provided for information.
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 and health practices and determine the applicability of regulatory limitations prior to use. (For more specific safety hazard information see Section 10.)
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Standards Content (Sample)
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Designation: G 30 – 97 (Reapproved 2003)
Standard Practice for
Making and Using U-Bend Stress-Corrosion Test
Specimens
ThisstandardisissuedunderthefixeddesignationG 30;thenumberimmediatelyfollowingthedesignationindicatestheyearoforiginal
adoption or, in the case of revision, the year of last revision.Anumber in parentheses indicates the year of last reapproval.Asuperscript
epsilon (e) indicates an editorial change since the last revision or reapproval.
1. Scope G 1 Practice for Preparing, Cleaning, and Evaluating Cor-
rosion Test Specimens
1.1 This practice describes procedures for making and using
G 15 Terminology Relating to Corrosion and Corrosion
U-bend specimens for the evaluation of stress-corrosion crack-
Testing
ing in metals. The U-bend specimen is generally a rectangular
G 35 Practice for Determining the Susceptibility of Stain-
strip which is bent 180° around a predetermined radius and
less Steels and Related Nickel-Chromium-Iron Alloys to
maintained in this constant strain condition during the stress-
Stress Corrosion Cracking in Polythionic Acids
corrosion test. Bends slightly less than or greater than 180° are
G 36 Practice for Evaluating Stress-Corrosion Cracking
sometimes used. Typical U-bend configurations showing sev-
Resistance of Metals and Alloys in a Boiling Magnesium
eral different methods of maintaining the applied stress are
Chloride Solution
shown in Fig. 1.
G 37 Practice for Use of Mattsson’s Solution of pH 7.2 to
1.2 U-bend specimens usually contain both elastic and
Evaluate the Stress-Corrosion Cracking Susceptibility of
plastic strain. In some cases (for example, very thin sheet or
Copper-Zinc Alloys
small diameter wire) it is possible to form a U-bend and
G 39 PracticeforPreparationandUseofBent-BeamStress-
produceonlyelasticstrain.However,bent-beam(PracticeG 39
Corrosion Specimens
or direct tension (Practice G 49)) specimens are normally used
G 41 Practice for Determining Cracking Susceptibility of
to study stress-corrosion cracking of strip or sheet under elastic
Metals Exposed Under Stress to a Hot Salt Environment
strain only.
G 44 Practice for Evaluating Stress Corrosion Cracking
1.3 This practice is concerned only with the test specimen
ResistanceofMetalsandAlloysbyAlternateImmersionin
and not the environmental aspects of stress-corrosion testing
3.5 % Sodium Chloride Solution
which are discussed elsewhere (1), in Practices G 35, G 36,
G 49 Practice for Preparation and Use of Direct Tension
G 37, G 41, G 44, G 103 and Test Method G 123.
Stress-Corrosion Test Specimens
1.4 The values stated in SI units are to be regarded as
G 103 Practice for Performing a Stress-Corrosion Cracking
standard. The inch-pound units in parentheses are provided for
Test of Low Copper Containing Al-Zn-Mg Alloys in
information.
Boiling 6 % Sodium Chloride Solution
1.5 This standard does not purport to address all of the
G 123 Test Method for Evaluating Stress-Corrosion Crack-
safety concerns, if any, associated with its use. It is the
ing of Stainless Alloys with Different Nickel Content in a
responsibility of the user of this standard to establish appro-
Boiling Acidified Sodium Chloride Solution
priate safety and health practices and determine the applica-
bility of regulatory limitations prior to use. (For more specific
3. Terminology
safety hazard information see Section 10.)
3.1 For definitions of corrosion-related terms used in this
2. Referenced Documents practice see Terminology G 15.
2.1 ASTM Standards:
4. Summary of Practice
E 3 Methods of Preparation of Metallographic Specimens
4.1 This practice involves the stressing of a specimen bent
to a U shape. The applied strain is estimated from the bend
conditions. The stressed specimens are then exposed to the test
This practice is under the jurisdiction of ASTM Committee G01 on Corrosion
environment and the time required for cracks to develop is
of Metals and is the direct responsibility of Subcommittee G01.06 on Stress-
determined. This cracking time is used as an estimate of the
Corrosion Cracking and Corrosion Fatigue.
stress corrosion resistance of the material in the test environ-
Current edition approved April 10, 1997. Published February 1998. Originally
approved in 1972. Last previous edition approved in 1994 as G 30 – 94.
ment.
The boldface numbers in parentheses refer to the list of references at the end of
this practice.
3 4
Annual Book of ASTM Standards, Vol 03.01. Annual Book of ASTM Standards, Vol 03.02.
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959, United States.
G 30 – 97 (2003)
FIG. 1 Typical Stressed U-bends
5. Significance and Use 7. Sampling
5.1 The U-bend specimen may be used for any metal alloy
7.1 Specimens shall be taken from a location in the bulk
sufficiently ductile to be formed into the U-shape without
sample so that they are representative of the material to be
mechanicallycracking.Thespecimenismosteasilymadefrom
tested; however, the bulk sampling of mill products is outside
strip or sheet but can be machined from plate, bar, castings, or
the scope of this standard.
weldments; wire specimens may be used also.
7.2 In performing tests to simulate a service condition it is
5.2 Since the U-bend usually contains large amounts of
essential that the thickness of the test specimen, its orientation
elastic and plastic strain, it provides one of the most severe
with respect to the direction of metal working and the surface
tests available for smooth (as opposed to notched or pre-
finish, etc., be relevant to the anticipated application.
cracked) stress-corrosion test specimens. The stress conditions
are not usually known and a wide range of stresses exist in a
8. Test Specimen
single stressed specimen. The specimen is therefore unsuitable
for studying the effects of different applied stresses on stress-
8.1 Specimen Orientation—When specimens are cut from
corrosion cracking or for studying variables which have only a
sheetorplateandinsomecasesstriporbar,itispossibletocut
minor effect on cracking. The advantage of the U-bend
them transverse or longitudinal to the direction of rolling. In
specimen is that it is simple and economical to make and use.
many cases the stress-corrosion cracking resistance in these
It is most useful for detecting large differences between the
two directions is quite different so it is important to define the
stress-corrosion cracking resistance of (a) different metals in
orientation of the test specimen.
the same environment, (b) one metal in different metallurgical
8.2 Specimen Dimensions—Fig. 2 shows a typical test
conditions in the same environment, or (c) one metal in several
specimen and lists, by way of example, several dimension
environments.
combinations that have been used successfully to test a wide
6. Hazards
range of materials. Other dimensional characteristics may be
used as necessary. For example, some special types of U-bend
6.1 U-bends made from high strength material may be
configuration have been used for simulating exposure condi-
susceptible to high rates of crack propagation and a specimen
tions encountered in high temperature water environments
containing more than one crack may splinter into two or more
relative to the nuclear power industry. These include double
pieces. Due to the highly stressed condition in a U-bend
specimen,thesepiecesmayleavethespecimenathighvelocity U-bend (2) and split tube U-bend (or reverse U-bend) (3)
specimens.
and can be dangerous.
G 30 – 97 (2003)
Examples of Typical Dimensions (SI Units)
Example L, mm M, mm W, mm T, mm D, mm X, mm Y, mm R, mm a,rad
a 80 50 20 2.5 10 32 14 5 1.57
b 100 90 9 3.0 7 25 38 16 1.57
c 120 90 20 1.5 8 35 35 16 1.57
d 130 100 15 3.0 6 45 32 13 1.57
e 150 140 15 0.8 3 61 20 9 1.57
f 310 250 25 13.0 13 105 90 32 1.57
g 510 460 25 6.5 13 136 165 76 1.57
h 102 83 19 3.2 9.6 40 16 4.8 1.57
FIG. 2 Typical U-Bend Specimen Dimensions (Examples only, not for specification.)
8.2.1 Whether or not the specimen contains holes is depen- 8.3.3 Grinding or machining should be done in stages so
dent upon the method of maintaining the applied stress (see that the final cut leaves the surface with a finish of 0.76 µm (30
Fig. 1). µin.) or better. Care must be taken to avoid excessive heating
8.2.2 The length (L) and width (W) of the specimen are during preparation because this may induce undesirable re-
determined by the amount and form of the material available, sidual stresses and in some cases cause metallurgical or
the stressing method used, and the size of the test environment chemical changes, or both, at the surface. The edges of the
container. specimen should receive the same finish as the faces.
8.2.3 The thickness (T) is usually dependent upon the form 8.3.4 When the final surface preparation involves chemical
of the material, its strength and ductility, and the means dissolution, care must be taken to ensure that the solution used
available to perform the bending. For example, it is difficult to does not induce hydrogen embrittlement, selectively attack
manually form U-bends of thickness greater than approxi- constituents in the metal, or leave undesirable residues on the
mately 3 mm (0.125 in.) if the yield strength exceeds about surface.
1400 MPa (200 ksi). 8.3.5 It may be desirable to test a surface (for example, cold
8.2.4 For comparison purposes, it is desirable to keep the rolled or cold rolled, annealed, and pickled) without surface
specimen dimensions, especially the ratio of thickness to bend metal removal. In such cases the edges of the specimen should
radius, constant. This produces approximately the same maxi- be milled. Sheared edges should be avoided in all cases.
mum strain in the materials being compared (see 9.3). How- 8.3.6 The final stage of surface preparation is degreasing.
ever, it does not necessarily provide tests of equal severity if Depending upon the method of stressing, this may be done
the mechanical properties of the materials being compared are before or after stressing.
widely different. 8.4 Identification of the specimen is best achieved by
8.2.5 When wire is to be evaluated, the specimen is simply stamping or scribing near one of the ends of the test specimen,
a wire of a length suitable for the restraining jig. It may be well away from the area to be stressed. Alternatively, nonme-
desirable to loop the wire rather than use just a simple U-shape tallic tags may be attached to the bolt or fixture used to
(4). maintain the specimen in a stressed condition during the test.
8.3 Surface Finish:
9. Stress Considerations
8.3.1 Any necessary heat treatment should be performed
before the final surface preparation. 9.1 The stress of principal interest in the U-bend specimen
8.3.2 Surface preparation is generally a mechanical process is circumferential. It is nonuniform because (a) there is a stress
but in some cases it may be more convenient and acceptable to gradient through the thickness varying from a maximum
chemically finish (see 8.3.4). tension on the outer surface to a maximum compression on the
G 30 – 97 (2003)
inner surface, (b) the stress varies from zero at the ends of the 4(a) may be performed in a tension testing machine and is
specimen to a maximum at the center of the bend, and (c) the often the most suitable method for stressing U-bends that are
stress may vary across the width of the bend. The stress
difficult to form manually due to large thickness or high-
distribution has been studied (5).
strengthmaterialorboth.ThetechniquesshowninFig.4(band
9.2 When a U-bend specimen is stressed, the material in the
c) may be suitable for thin or low-strength material, or both,
outerfibersofthebendisstrainedintotheplasticportionofthe
butaregenerallyinferiortothemethodshowninFig.4(a).The
true stress-true strain curve; for example, into Section AB in
method shown in Fig. 4(b) results in a more complex strain
Fig. 3(a). Fig. 3(b–e) show several stress-strain relationships
system in the outer surface and may cause scratching. The
that can exist in the outer fibers of the U-bend test specimen;
technique shown in Fig. 4(c) suffers from greater lack of
the actual relationship obtained will depend upon the method
control of the bend radius. The two types of stress conditions
of stressing (see Section 10). For the conditions shown in Fig.
that can be obtained by the single-stage stressing method are
3(d), a quantitative measure of the maximum test stress can be
defined by point X in Fig. 3(b and c). In the latter case, some
made (6).
elastic strain relaxation has occurred as a result of allowing the
9.3 The total strain (e) on the outside of the bend can be
U-bend legs to spring back slightly at the end of the stressing
closely approximated to the equation:
sequence.
e5 T/2R when T ,, R
10.3 Two-stage stressing involves first forming the approxi-
mate U-shape, then allowing the elastic strain to relax com-
where:
pletely before the second stage of applying the test stress. A
T = specimen thickness, and
typical sequence of operations is shown in Fig. 5. The type of
R = radius of bend curvature.
equipment shown in Fig. 4(a and b) can also be used to
preform the U-shape. The test strain applied may be a
10. Stressing the Specimen
percentage of the tensile elastic strain that occurred during
10.1 Stressing is usually achieved by either a one- or a
preforming (Fig. 3(d)) or may involve additional plastic strain
two-stage operation.
(Fig. 3(e)).
10.2 Single-stage stressing is accomplished by bending the
specimen into shape and maintaining it in that shape without 10.4 The slope, MN, of the curve shown in Fig. 3(d) is steep
allowing relaxation of the tensile elastic strain. Typical stress- (equal to Young’s modulus). Therefore, it is often difficult to
ing sequences are shown in Fig. 4. The method shown in Fig. reproducibly apply a constant percentage of the total elastic
FIG. 3 True Stress-True Strain Relationships for Stressed U-Bends
G 30 – 97 (2003)
FIG. 4 Methods of Stressing U-Bend Specimens—Single-Stage Stressing
FIG. 5 Method of Stressing U-Bend—Two-Stage Method
prestrain and there is a danger of leaving the specimen surface recommended that the stress conditions shown in Fig. 3(bore)
under compressive stress. For this reason and also because it be achieved. Hence, the final applied strain prior to testing
results in a more severe test (that is, higher applied stress), it is consists of plastic and elastic strain. To achieve the conditions
G 30 – 97 (2003)
shown in Fig. 3(b and e), it is necessar
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