ASTM G38-73(1995)E1
(Practice)Standard Practice for Making and Using C-Ring Stress-Corrosion Test Specimens
Standard Practice for Making and Using C-Ring Stress-Corrosion Test Specimens
SCOPE
1.1 This practice describes the essential features of the design and machining, and procedures for stressing, exposing, and inspecting C-ring type of stress-corrosion test speci- mens. An analysis is given of the state and distribution of stress in the C-ring.
1.2 Specific considerations relating to the sampling process and to the selection of appropriate test environments are outside the scope of this practice.
1.3 The values stated in SI units are to be regarded as standard. The inch-pound units are provided for information.
1.4 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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e1
Designation: G 38 – 73 (Reapproved 1995)
AMERICAN SOCIETY FOR TESTING AND MATERIALS
100 Barr Harbor Dr., West Conshohocken, PA 19428
Reprinted from the Annual Book of ASTM Standards. Copyright ASTM
Standard Practice for
Making and Using C-Ring Stress-Corrosion Test Specimens
This standard is issued under the fixed designation G 38; 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.
This standard has been approved for use by agencies of the Department of Defense.
e NOTE—Section 14 was added editorially in October 1995.
1. Scope
1.1 This practice describes the essential features of the
design and machining, and procedures for stressing, exposing,
and inspecting C-ring type of stress-corrosion test specimens.
An analysis is given of the state and distribution of stress in the
C-ring.
1.2 Specific considerations relating to the sampling process
and to the selection of appropriate test environments are
outside the scope of this practice.
1.3 The values stated in SI units are to be regarded as
standard. The inch-pound units are provided for information.
1.4 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 appro-
priate safety and health practices and determine the applica-
bility of regulatory limitations prior to use.
2. Summary of Practice
2.1 This practice involves the preparation of and the quan-
titative stressing of a C-ring stress-corrosion test specimen by
application of a bending load. Characteristics of the stress
system and the distribution of stresses are discussed. Guidance
is given for methods of exposure and inspection.
3. Significance and Use
3.1 The C-ring is a versatile, economical specimen for
quantitatively determining the susceptibility to stress-corrosion
cracking of all types of alloys in a wide variety of product
forms. It is particularly suitable for making transverse tests of
FIG. 1 Sampling Procedure for Testing Various Products
tubing and rod and for making short-transverse tests of various
products as illustrated for plate in Fig. 1.
section so that the direction of principal stress (parallel to the
stressing bolt) is in the direction of minimum resistance to
4. Sampling
stress-corrosion cracking. For example, in the case of alumi-
4.1 Test specimens shall be taken from a location and with
num alloys (1), this is the short-transverse direction relative to
an orientation so that they adequately represent the material to
the grain structure. If the ring is not so oriented it will tend to
be tested.
crack off-center at a location where the stress is unknown.
4.2 In testing thick sections that have a directional grain
structure, it is essential that the C-ring be oriented in the
5. Specimen Design
5.1 Sizes for C-rings may be varied over a wide range, but
This practice is under the jurisdiction of ASTM Committee G-1 on Corrosion
of Metals and is the direct responsibility of Subcommittee G01.06 on Stress-
Corrosion Cracking and Corrosion Fatigue. The boldface numbers in parentheses refer to the list of references at the end of
Current edition approved Sept. 27, 1973. Published November 1973. this practice.
G38
C-rings with an outside diameter less than about 16 mm ( ⁄8 in.) were not made at the extreme edge.
are not recommended because of increased difficulties in 6.3 In the case of the notched C-ring (Fig. 3(d)) a triaxial
machining and decreased precision in stressing. The dimen- stress state is present adjacent to the root of the notch (5).In
sions of the ring can affect the stress state, and these consid- addition, the circumferential stress at the root of the notch will
erations are discussed in Section 6. A typical shop drawing for be greater than the nominal stress and generally may be
the manufacture of a C-ring is shown in Fig. 2. expected to be in the plastic range.
6.4 The possibility of residual stress should always be
6. Stress Considerations
considered, especially when C-rings are machined from prod-
6.1 The stress of principal interest in the C-ring specimen is
ucts that contain appreciable residual stress or when C-rings
the circumferential stress. It should be recognized that this
over about 6.35 mm ( ⁄4 in.) thick are heat treated after being
stress is not uniform (2, 3). First, there is a gradient through the
machined. It is generally not advisable to heat treat finish-
thickness, varying from a maximum tension on one surface to
machined C-rings because of the likelihood of developing
a maximum compression on the opposite surface. Secondly, the
residual stresses in the ring.
stress varies around the circumference of the C-ring from zero
NOTE 1—When specimens are exposed to corrosive media at elevated
at each bolt hole to a maximum at the middle of the arc
temperatures, the possibility of relaxation of stress during the exposure
opposite the stressing bolt; the nominal stress is present only
period should be investigated. Relaxation can be estimated from known
along a line across the ring at the middle of the arc. Thus, when
creep data for both the ring and the stressing bolt.
the specimen is stressed by measuring the strain on the tension
6.5 An advantage of the C-ring is that it can be stressed with
surface of the C-ring, the strain gage should be positioned at
high precision and bias by application of a measured deflec-
the middle of the arc in order to indicate the maximum strain.
tion. The sources of error in stressing are those that are inherent
Thirdly, the circumferential stress may vary across the width of
with the use of measuring instruments (micrometers, strain
the ring, the extent of the variation depending on the width-
gages, etc.) as discussed in 6.2-6.4 and Annex A1.
to-thickness and diameter-to-thickness ratios of the C-ring. In
6.6 The calculated stress applies only to the state of stress
general, when loaded as shown in Fig. 3 (a, b), the tensile stress
before initiation of cracks. Once cracking has initiated the
on the outer surface will be greater at the extreme edge than at
stress at the tip of the crack, as well as in uncracked areas, has
the center, while when loaded as shown in Fig. 3 (c), the tensile
changed.
stress on the inner surface will be less at the edge than at the
7. Stressing Methods
center (4).
6.2 Another characteristic of the stress system in the C-ring 7.1 The C-ring, as generally used, is a constant-strain
is the presence of biaxial stresses; that is, transverse as well as specimen with tensile stress produced on the exterior of the
circumferential stresses are developed on the critical test ring by tightening a bolt centered on the diameter of the ring.
section. The transverse stress will vary from a maximum at the However, a nearly constant load can be developed by the use of
mid-width to zero at the edges, and will be the same sign as the a calibrated spring placed on the loading bolt. C-rings also can
circumferential stress. In general, the transverse stress may be be stressed in the reverse direction by spreading the ring and
expected to decrease with decreasing width to thickness and creating a tensile stress on the inside surface. These methods of
increasing diameter to thickness ratios. An example is shown in stressing are illustrated in Fig. 3. Proper choice of a minimum
Fig. 4 where the transverse tensile stress at the mid-width of a bolt diameter or a spring constant is, of course, required to
19.00 mm (0.748 in.) outside diameter by 1.537 mm (0.0605 assure achieving true constant strain or constant load stressing.
in.) thick by 19.0 mm (0.75 in.) wide C-ring of aluminum alloy 7.2 The most accurate stressing procedure is to attach
7075-T6 was equal to about one third of the circumferential circumferential and transverse electrical strain gages to the
tensile stress. In this example the circumferential stress was surface stressed in tension and to tighten the bolt until the strain
uniform over most of the width of the C-ring; measurements measurements indicate the desired circumferential stress. The
NOTE 1—If stock is undersize or tube stock is used dimensions can be varied to suit size of section from which the specimen must be cut.
FIG. 2 C-Ring Type of Stress-Corrosion Specimen
G38
NOTE 1—For Fig 3 (d) a similar notch could be used on the tension side of (b) or (c).
FIG. 3 Methods of Stressing C-Rings
7.3 When several rings of the same alloy and dimensions
are to be loaded, it is convenient to determine a calibration
curve of circumferential stress versus ring deflection as in Fig.
4 to avoid the inconvenience of strain gaging each ring.
7.4 The amount of compression required on the C-ring to
produce elastic straining only, and the degree of elastic strains
can be predicted theoretically (2, 3). Therefore, C-rings may be
stressed by calculating the deflection required to develop a
desired elastic stress by using the individual ring dimensions in
a modified curved beam equation as shown in Table A1.1. The
accuracy of calculated stresses is shown in Fig. 4 by the
agreement of the calculated curve and the actual data points.
See Annex A1 for the equation for stressing C-ring specimens.
7.5 In the case of notched specimens a nominal stress is
assumed using the ring outside diameter measured at the root
of the notch. Consideration then should be given to the stress
concentration factor (K ) for the specific notch when calculat-
T
ing the D required to develop the intended stress.
8. Machining
8.1 When rings are machined from solid stock, precautions
should be taken to avoid practices that overheat, plastically
FIG. 4 Stresses in 7075-T6 Aluminum Alloy C-Ring Stress-
deform, or develop residual stress in the metal surface.
Corrosion Specimen (4)
Machining should be done in stages so that the final cut leaves
circumferential (s ) and transverse (s ), stresses are calcu-
C T the principal surface with a clean finish of 0.7 μm (30 μin.) rms
lated as follows:
or better. Necessary machining sequences, type of tool, feed
rate, etc., depend upon the alloy and temper of the test piece.
s 5 E/~1 2 μ !·~e 1 μe !, and
C C T
Lapping, mechanical polishing, and similar operations that
s 5 E/~1 2 μ !·~e 1 μe !
T T C
produce flow of the metal should be avoided.
where:
9. Surface Preparation
E 5 Young’s modulus of elasticity,
9.1 A high-quality machined surface is the most desirable
μ 5 Poisson’s ratio,
for corrosion test purposes unless one wants to test the
e 5 circumferential strain, and
C
as-fabricated surface of a tube or bar; it should, of course, be
e 5 transverse strain.
T
degreased before exposing the specimen. In order to remove
NOTE 2—When using electrical strain gages with thin-walled C-rings, a
heat treat films or thin layers of surface metal that may have
correction should be allowed for the displacement of the gage from the
become distorted during machining, chemical or electrochemi-
surface of the ring. All traces of the gage and the adhesive must be
cal etches may be used. The choice of such a treatment will
removed from the C-ring before it is exposed.
depend upon the alloy of the test piece. Care should be
NOTE 3—Stresses may be calculated from measured strains using the
exercised to choose an etchant that will not selectively attack
modulus of elasticity, provided the stresses and strains do not exceed the
proportional limit. constituents in the metal or will not deposit undesirable
G38
residues on the surface. Etching or pickling should not be used 11.3 Determination of cracking time is a subjective proce-
for alloys that may undergo hydrogen embrittlement. dure involving visual examination that under some conditions
9.2 It is generally the best procedure to complete the surface can be very difficult, as noted in Section 12, and depends on the
preparation before the C-ring is stressed except for a possible skill and experience of the inspector.
final degreasing of the critically stressed area.
12. Inspection
9.3 Every precaution should be taken to maintain the
12.1 Highly stressed C-rings of alloys that are appreciably
integrity of the surface after the final preparation; that is, avoid
susceptible to stress-corrosion cracking tend to fracture
finger printing and any rough handling that could mar the
through the entire thickness or to crack in a way that is
finish.
conspicuous. Frequently, however, with lower applied stresses,
10. Specimen Identification
or with more stress-corrosion-resistant alloys, cracking begins
10.1 Specimen numbers may be scribed on one of the tips slowly and is difficult to detect. Small cracks may initiate at
adjacent to the cut-away segment of the C-ring. No markings multiple sites and be obscured by corrosion products, and an
of any kind should be made on the critically stressed arc arbitrary decision must be made to declare a specimen “failed.”
between the bolt holes. Nonmetallic tags may be attached to Inasmuch as C-rings do not always fracture, it is preferable to
the stressing bolt by means of a second nut. report the first crack as the criterion of failure. It is common
practice to make this inspection with the naked eye or at a low
11. Exposure Methods
magnification. If there are indications noted that cannot be
11.1 The C-ring, because of its small size and the simple
established definitely as a crack by this type of examination,
methods of stressing, can be exposed to almost any kind of
the investigator should either (a) note the date of this first
corrosive environment (6). The specimens should be supported
suspicion of cracking and continue the exposure of the speci-
in such a way that nothing except the corrosive medium comes
men, watching for further growth that will confirm the first
in contact with the critically stressed area. No part of an
indication as the failure date, or (b) discontinue exposure of the
exposure rack should be allowed to touch the surface or the
specimen and perform a metallographic examination of a cross
edges of the critically stressed region.
section taken through the suspected crack to establish whether
11.2 Care must be exercised to avoid galvanic effects
there is cracking. Metallographic examination of fractured or
between the C-ring, the stressing bolt, and exposure racks. It is
cracked C-rings can also be helpful in determining whether the
essential also to prevent crevice corrosion that could develop
failure was caused by stress-corrosion cracking or
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