ASTM G30-97(2015)
(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
5.1 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.
5.2 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 covers 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 G39 or direct tension (Practice G49)) 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)2 and in Practices G35, G36, G37, G41, G44, G103 and Test Method G123.
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.
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Designation: G30 − 97(Reapproved 2015)
Standard Practice for
Making and Using U-Bend Stress-Corrosion Test
Specimens
ThisstandardisissuedunderthefixeddesignationG30;thenumberimmediatelyfollowingthedesignationindicatestheyearoforiginal
adoptionor,inthecaseofrevision,theyearoflastrevision.Anumberinparenthesesindicatestheyearoflastreapproval.Asuperscript
epsilon (´) indicates an editorial change since the last revision or reapproval.
1. Scope 2. Referenced Documents
1.1 This practice covers procedures for making and using
2.1 ASTM Standards:
U-bendspecimensfortheevaluationofstress-corrosioncrack-
E3Guide for Preparation of Metallographic Specimens
ing in metals. The U-bend specimen is generally a rectangular
G1Practice for Preparing, Cleaning, and Evaluating Corro-
strip which is bent 180° around a predetermined radius and
sion Test Specimens
maintained in this constant strain condition during the stress-
G15TerminologyRelatingtoCorrosionandCorrosionTest-
corrosiontest.Bendsslightlylessthanorgreaterthan180°are
ing (Withdrawn 2010)
sometimes used. Typical U-bend configurations showing sev-
G35Practice for Determining the Susceptibility of Stainless
eral different methods of maintaining the applied stress are
Steels and Related Nickel-Chromium-Iron Alloys to
shown in Fig. 1.
Stress-Corrosion Cracking in Polythionic Acids
1.2 U-bend specimens usually contain both elastic and
G36Practice for Evaluating Stress-Corrosion-Cracking Re-
plastic strain. In some cases (for example, very thin sheet or
sistance of Metals and Alloys in a Boiling Magnesium
small diameter wire) it is possible to form a U-bend and
Chloride Solution
produceonlyelasticstrain.However,bent-beam(PracticeG39
G37Practice for Use of Mattsson’s Solution of pH 7.2 to
or direct tension (Practice G49)) specimens are normally used
Evaluate the Stress-Corrosion Cracking Susceptibility of
tostudystress-corrosioncrackingofstriporsheetunderelastic
Copper-Zinc Alloys
strain only.
G39Practice for Preparation and Use of Bent-Beam Stress-
Corrosion Test Specimens
1.3 This practice is concerned only with the test specimen
and not the environmental aspects of stress-corrosion testing G41Practice for Determining Cracking Susceptibility of
which are discussed elsewhere (1) and in Practices G35, G36, Metals Exposed Under Stress to a Hot Salt Environment
G37, G41, G44, G103 and Test Method G123. G44PracticeforExposureofMetalsandAlloysbyAlternate
Immersion in Neutral 3.5 % Sodium Chloride Solution
1.4 The values stated in SI units are to be regarded as
G49Practice for Preparation and Use of Direct Tension
standard.The inch-pound units in parentheses are provided for
Stress-Corrosion Test Specimens
information.
G103PracticeforEvaluatingStress-CorrosionCrackingRe-
1.5 This standard does not purport to address all of the
sistance of Low Copper 7XXX Series Al-Zn-Mg-Cu
safety concerns, if any, associated with its use. It is the
Alloys in Boiling 6 % Sodium Chloride Solution
responsibility of the user of this standard to establish appro-
G123TestMethodforEvaluatingStress-CorrosionCracking
priate safety and health practices and determine the applica-
of Stainless Alloys with Different Nickel Content in
bility of regulatory limitations prior to use.
Boiling Acidified Sodium Chloride Solution
This practice is under the jurisdiction ofASTM Committee G01 on Corrosion
of Metals and is the direct responsibility of Subcommittee G01.06 on Environmen-
tally Assisted Cracking. For referenced ASTM standards, visit the ASTM website, www.astm.org, or
Current edition approved Nov. 1, 2015. Published November 2015. Originally contact ASTM Customer Service at service@astm.org. For Annual Book of ASTM
approved in 1972. Last previous edition approved in 2009 as G30–97(2009). DOI: Standards volume information, refer to the standard’s Document Summary page on
10.1520/G0030-97R15. the ASTM website.
2 4
The boldface numbers in parentheses refer to a list of references at the end of The last approved version of this historical standard is referenced on
this standard. www.astm.org.
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959. United States
G30 − 97 (2015)
FIG. 1 Typical Stressed U-bends
3. Terminology It is most useful for detecting large differences between the
stress-corrosion cracking resistance of (a) different metals in
3.1 For definitions of corrosion-related terms used in this
the same environment, (b) one metal in different metallurgical
practice see Terminology G15.
conditionsinthesameenvironment,or(c)onemetalinseveral
environments.
4. Summary of Practice
4.1 This practice involves the stressing of a specimen bent
6. Hazards
to a U shape. The applied strain is estimated from the bend
6.1 U-bends made from high strength material may be
conditions.Thestressedspecimensarethenexposedtothetest
susceptible to high rates of crack propagation and a specimen
environment and the time required for cracks to develop is
containing more than one crack may splinter into two or more
determined. This cracking time is used as an estimate of the
pieces. Due to the highly stressed condition in a U-bend
stress corrosion resistance of the material in the test environ-
specimen,thesepiecesmayleavethespecimenathighvelocity
ment.
and can be dangerous.
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
G30 − 97 (2015)
Examples of Typical Dimensions (SI Units)
Example L, mm M, mm W, mm T, mm D, mm X, mm Y, mm R, mm α,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)
two directions is quite different so it is important to define the However,itdoesnotnecessarilyprovidetestsofequalseverity
orientation of the test specimen. if the mechanical properties of the materials being compared
are widely different.
8.2 Specimen Dimensions—Fig. 2 shows a typical test
8.2.5 When wire is to be evaluated, the specimen is simply
specimen and lists, by way of example, several dimension
a wire of a length suitable for the restraining jig. It may be
combinations that have been used successfully to test a wide
desirabletoloopthewireratherthanusejustasimpleU-shape
range of materials. Other dimensional characteristics may be
(4).
used as necessary. For example, some special types of U-bend
configuration have been used for simulating exposure condi- 8.3 Surface Finish:
tions encountered in high temperature water environments 8.3.1 Any necessary heat treatment should be performed
relative to the nuclear power industry. These include double before the final surface preparation.
U-bend (2) and split tube U-bend (or reverse U-bend) (3) 8.3.2 Surface preparation is generally a mechanical process
specimens. butinsomecasesitmaybemoreconvenientandacceptableto
8.2.1 Whether or not the specimen contains holes is depen- chemically finish (see 8.3.4).
dent upon the method of maintaining the applied stress (see 8.3.3 Grinding or machining should be done in stages so
Fig. 1). that the final cut leaves the surface with a finish of 0.76 µm
8.2.2 The length (L) and width (W) of the specimen are (30µin.) or better. Care must be taken to avoid excessive
determined by the amount and form of the material available, heating during preparation because this may induce undesir-
the stressing method used, and the size of the test environment able residual stresses and in some cases cause metallurgical or
container. chemical changes, or both, at the surface. The edges of the
8.2.3 The thickness (T) is usually dependent upon the form specimen should receive the same finish as the faces.
of the material, its strength and ductility, and the means 8.3.4 When the final surface preparation involves chemical
available to perform the bending. For example, it is difficult to dissolution, care must be taken to ensure that the solution used
manually form U-bends of thickness greater than approxi- does not induce hydrogen embrittlement, selectively attack
mately 3 mm (0.125 in.) if the yield strength exceeds about constituents in the metal, or leave undesirable residues on the
1400 MPa (200 ksi). surface.
8.2.4 For comparison purposes, it is desirable to keep the 8.3.5 Itmaybedesirabletotestasurface(forexample,cold
specimen dimensions, especially the ratio of thickness to bend rolled or cold rolled, annealed, and pickled) without surface
radius, constant. This produces approximately the same maxi- metal removal. In such cases the edges of the specimen should
mum strain in the materials being compared (see 9.3). be milled. Sheared edges should be avoided in all cases.
G30 − 97 (2015)
FIG. 3 True Stress-True Strain Relationships for Stressed U-Bends
8.3.6 The final stage of surface preparation is degreasing. 9.3 The total strain (ε) on the outside of the bend can be
Depending upon the method of stressing, this may be done closely approximated to the equation:
before or after stressing.
ε 5 T/2R when T,,R
8.4 Identification of the specimen is best achieved by
where:
stamping or scribing near one of the ends of the test specimen,
T = specimen thickness, and
well away from the area to be stressed. Alternatively, nonme-
R = radius of bend curvature.
tallic tags may be attached to the bolt or fixture used to
maintain the specimen in a stressed condition during the test.
10. Stressing the Specimen
9. Stress Considerations
10.1 Stressing is usually achieved by either a one- or a
9.1 The stress of principal interest in the U-bend specimen two-stage operation.
iscircumferential.Itisnonuniformbecause(a)thereisastress
10.2 Single-stage stressing is accomplished by bending the
gradient through the thickness varying from a maximum
specimen into shape and maintaining it in that shape without
tensionontheoutersurfacetoamaximumcompressiononthe
allowing relaxation of the tensile elastic strain. Typical stress-
inner surface, (b) the stress varies from zero at the ends of the
ing sequences are shown in Fig. 4. The method shown in Fig.
specimen to a maximum at the center of the bend, and (c) the
4(a) may be performed in a tension testing machine and is
stress may vary across the width of the bend. The stress
often the most suitable method for stressing U-bends that are
distribution has been studied (5).
difficult to form manually due to large thickness or high-
9.2 WhenaU-bendspecimenisstressed,thematerialinthe strengthmaterialorboth.ThetechniquesshowninFig.4(band
outerfibersofthebendisstrainedintotheplasticportionofthe c) may be suitable for thin or low-strength material, or both,
true stress-true strain curve; for example, into Section AB in butaregenerallyinferiortothemethodshowninFig.4(a).The
Fig. 3(a). Fig. 3(b–e) show several stress-strain relationships method shown in Fig. 4(b) results in a more complex strain
that can exist in the outer fibers of the U-bend test specimen; system in the outer surface and may cause scratching. The
the actual relationship obtained will depend upon the method technique shown in Fig. 4(c) suffers from greater lack of
of stressing (see Section 10). For the conditions shown in Fig. control of the bend radius. The two types of stress conditions
3(d), a quantitative measure of the maximum test stress can be that can be obtained by the single-stage stressing method are
made (6). defined by point X in Fig. 3(b and c). In the latter case, some
G30 − 97 (2015)
FIG. 4 Methods of Stressing U-Bend Specimens—Single-Stage Stressing
elasticstrainrelaxationhasoccurredasaresultofallowingthe have proven satisfactory for this purpose. It is advisible to use
U-bend legs to spring back slightly at the end of the stressing flat metal washers (not shown) between the insulators and the
sequence.
bolt and nut to extend the life of the insulators. In some cases
the use of insulators can be avoided by using a restraining jig
10.3 Two-stage stressing involves first forming the approxi-
made from a metal similar or the same as that being tested,
mate U-shape, then allowing the elastic strain to relax com-
provideditdoesnotfailbystress-corrosioncrackinginthetest
pletely before the second stage of applying the test stress. A
environment. The bolt, nut and flat washers must resist
typical sequence of operations is shown in Fig. 5. The type of
corrosion in the test environment. UNS N10276 has been
equipment shown in Fig. 4(a and b) can also be used to
preform the U-shape. The test strain applied may be a satisfactory in many environments, although other materials
percentage of the tensile elastic strain that occurred during may be superior in highly oxidizing environments.
preforming (Fig. 3(d)) or may involve additional plastic strain
10.6 Some tests require that the U-bend specimen fit
(Fig. 3(e)).
through a 45/50 ground glass joint for exposure in an Erlen-
10.4 Theslope, MN,ofthecurveshowninFig.3(d)issteep
meyer flas
...
This document is not an ASTM standard and is intended only to provide the user of an ASTM standard an indication of what changes have been made to the previous version. Because
it may not be technically possible to adequately depict all changes accurately, ASTM recommends that users consult prior editions as appropriate. In all cases only the current version
of the standard as published by ASTM is to be considered the official document.
Designation: G30 − 97 (Reapproved 2009) G30 − 97 (Reapproved 2015)
Standard Practice for
Making and Using U-Bend Stress-Corrosion Test
Specimens
This standard is issued under the fixed designation G30; 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
1.1 This practice covers 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 G39 or direct tension
(Practice G49)) specimens are normally used to study stress-corrosion cracking of strip or sheet under elastic strain only.
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, 2009Nov. 1, 2015. Published May 2009November 2015. Originally approved in 1972. Last previous edition approved in 20032009 as
G30–97(2003).G30–97(2009). DOI: 10.1520/G0030-97R09.10.1520/G0030-97R15.
FIG. 1 Typical Stressed U-bends
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959. United States
G30 − 97 (2015)
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) and in Practices G35, G36, G37, G41, G44, G103 and Test Method G123.
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.
2. Referenced Documents
2.1 ASTM Standards:
E3 Guide for Preparation of Metallographic Specimens
G1 Practice for Preparing, Cleaning, and Evaluating Corrosion Test Specimens
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
G39 Practice for Preparation and Use of Bent-Beam Stress-Corrosion Test Specimens
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
G49 Practice for Preparation and Use of Direct Tension Stress-Corrosion Test Specimens
G103 Practice for Evaluating Stress-Corrosion Cracking Resistance of Low Copper 7XXX Series Al-Zn-Mg-Cu Alloys in
Boiling 6 % Sodium Chloride Solution
G123 Test Method for Evaluating Stress-Corrosion Cracking of Stainless Alloys with Different Nickel Content in Boiling
Acidified Sodium Chloride Solution
3. Terminology
3.1 For definitions of corrosion-related terms used in this practice see Terminology G15.
4. Summary of Practice
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 environment and the time required for cracks to develop is
determined. This cracking time is used as an estimate of the stress corrosion resistance of the material in the test environment.
5. Significance and Use
5.1 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.
5.2 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.
6. Hazards
6.1 U-bends made from high strength material may be susceptible to high rates of crack propagation and a specimen containing
more than one crack may splinter into two or more pieces. Due to the highly stressed condition in a U-bend specimen, these pieces
may leave the specimen at high velocity and can be dangerous.
The boldface numbers in parentheses refer to a list of references at the end of this standard.
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.
G30 − 97 (2015)
Examples of Typical Dimensions (SI Units)
Example L, mm M, mm W, mm T, mm D, mm X, mm Y, mm R, mm α, 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)
7. Sampling
7.1 Specimens shall be taken from a location in the bulk sample so that they are representative of the material to be tested;
however, the bulk sampling of mill products is outside the scope of this standard.
7.2 In performing tests to simulate a service condition it is essential that the thickness of the test specimen, its orientation with
respect to the direction of metal working and the surface finish, etc., be relevant to the anticipated application.
8. Test Specimen
8.1 Specimen Orientation—When specimens are cut from sheet or plate and in some cases strip or bar, it is possible to cut them
transverse or longitudinal to the direction of rolling. In many cases the stress-corrosion cracking resistance in these two directions
is quite different so it is important to define the orientation of the test specimen.
8.2 Specimen Dimensions—Fig. 2 shows a typical test specimen and lists, by way of example, several dimension combinations
that have been used successfully to test a wide range of materials. Other dimensional characteristics may be used as necessary. For
example, some special types of U-bend configuration have been used for simulating exposure conditions encountered in high
temperature water environments relative to the nuclear power industry. These include double U-bend (2) and split tube U-bend (or
reverse U-bend) (3) specimens.
8.2.1 Whether or not the specimen contains holes is dependent upon the method of maintaining the applied stress (see Fig. 1).
8.2.2 The length (L) and width (W) of the specimen are determined by the amount and form of the material available, the
stressing method used, and the size of the test environment container.
8.2.3 The thickness (T) is usually dependent upon the form of the material, its strength and ductility, and the means available
to perform the bending. For example, it is difficult to manually form U-bends of thickness greater than approximately 3 mm (0.125
in.) if the yield strength exceeds about 1400 MPa (200 ksi).
8.2.4 For comparison purposes, it is desirable to keep the specimen dimensions, especially the ratio of thickness to bend radius,
constant. This produces approximately the same maximum strain in the materials being compared (see 9.3). However, it does not
necessarily provide tests of equal severity if the mechanical properties of the materials being compared are widely different.
8.2.5 When wire is to be evaluated, the specimen is simply a wire of a length suitable for the restraining jig. It may be desirable
to loop the wire rather than use just a simple U-shape (4).
8.3 Surface Finish:
8.3.1 Any necessary heat treatment should be performed before the final surface preparation.
G30 − 97 (2015)
FIG. 3 True Stress-True Strain Relationships for Stressed U-Bends
8.3.2 Surface preparation is generally a mechanical process but in some cases it may be more convenient and acceptable to
chemically finish (see 8.3.4).
8.3.3 Grinding or machining should be done in stages so that the final cut leaves the surface with a finish of 0.76 μm (30 μin.)
or better. Care must be taken to avoid excessive heating during preparation because this may induce undesirable residual stresses
and in some cases cause metallurgical or chemical changes, or both, at the surface. The edges of the specimen should receive the
same finish as the faces.
8.3.4 When the final surface preparation involves chemical dissolution, care must be taken to ensure that the solution used does
not induce hydrogen embrittlement, selectively attack constituents in the metal, or leave undesirable residues on the surface.
8.3.5 It may be desirable to test a surface (for example, cold rolled or cold rolled, annealed, and pickled) without surface metal
removal. In such cases the edges of the specimen should be milled. Sheared edges should be avoided in all cases.
8.3.6 The final stage of surface preparation is degreasing. Depending upon the method of stressing, this may be done before or
after stressing.
8.4 Identification of the specimen is best achieved by stamping or scribing near one of the ends of the test specimen, well away
from the area to be stressed. Alternatively, nonmetallic tags may be attached to the bolt or fixture used to maintain the specimen
in a stressed condition during the test.
9. Stress Considerations
9.1 The stress of principal interest in the U-bend specimen is circumferential. It is nonuniform because (a) there is a stress
gradient through the thickness varying from a maximum tension on the outer surface to a maximum compression on the inner
surface, (b) the stress varies from zero at the ends of the specimen to a maximum at the center of the bend, and (c) the stress may
vary across the width of the bend. The stress distribution has been studied (5).
9.2 When a U-bend specimen is stressed, the material in the outer fibers of the bend is strained into the plastic portion of the
true stress-true strain curve; for example, into Section AB in Fig. 3(a). Fig. 3(b–e) show several stress-strain relationships that can
exist in the outer fibers of the U-bend test specimen; the actual relationship obtained will depend upon the method of stressing (see
Section 10). For the conditions shown in Fig. 3(d), a quantitative measure of the maximum test stress can be made (6).
9.3 The total strain (ε) on the outside of the bend can be closely approximated to the equation:
G30 − 97 (2015)
FIG. 4 Methods of Stressing U-Bend Specimens—Single-Stage Stressing
ε5 T/2R when T,,R
where:
T = specimen thickness, and
R = radius of bend curvature.
10. Stressing the Specimen
10.1 Stressing is usually achieved by either a one- or a two-stage operation.
10.2 Single-stage stressing is accomplished by bending the specimen into shape and maintaining it in that shape without
allowing relaxation of the tensile elastic strain. Typical stressing sequences are shown in Fig. 4. The method shown in Fig. 4(a)
may be performed in a tension testing machine and is often the most suitable method for stressing U-bends that are difficult to form
manually due to large thickness or high-strength material or both. The techniques shown in Fig. 4(b and c) may be suitable for
thin or low-strength material, or both, but are generally inferior to the method shown in Fig. 4(a). The method shown in Fig. 4(b)
results in a more complex strain system in the outer surface and may cause scratching. The technique shown in Fig. 4(c) suffers
from greater lack of control of the bend radius. The two types of stress conditions that can be obtained by the single-stage stressing
method are defined by point X in Fig. 3(b and c). In the latter case, some elastic strain relaxation has occurred as a result of allowing
the U-bend legs to spring back slightly at the end of the stressing sequence.
10.3 Two-stage stressing involves first forming the approximate U-shape, then allowing the elastic strain to relax completely
before the second stage of applying the test stress. A typical sequence of operations is shown in Fig. 5. The type of equipment
shown in Fig. 4(a and b) can also be used to preform the U-shape. The test strain applied may be a percentage of the tensile elastic
strain that occurred during preforming (Fig. 3(d)) or may involve additional plastic strain (Fig. 3(e)).
10.4 The slope, MN, of the curve shown in Fig. 3(d) is steep (equal to Young’s modulus). Therefore, it is often difficult to
reproducibly apply a constant percentage of
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