ASTM G49-85(2000)
(Practice)Standard Practice for Preparation and Use of Direct Tension Stress-Corrosion Test Specimens
Standard Practice for Preparation and Use of Direct Tension Stress-Corrosion Test Specimens
SCOPE
1.1 This practice covers procedures for designing, preparing, and using ASTM standard tension test specimens for investigating susceptibility to stress-corrosion cracking. Axially loaded specimens may be stressed quantitatively with equipment for application of either a constant load, constant strain, or with a continuously increasing strain.
1.2 Tension test specimens are adaptable for testing a wide variety of product forms as well as parts joined by welding, riveting, or various other methods.
1.3 The exposure of specimens in a corrosive environment is treated only briefly because other standards are being prepared to deal with this aspect. Meanwhile, the investigator is referred to Practices G35, G36, and G37, and G44, and to ASTM Special Technical Publication 425 (1).
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Designation:G49–85 (Reapproved2000)
Standard Practice for
Preparation and Use of Direct Tension Stress-Corrosion
Test Specimens
ThisstandardisissuedunderthefixeddesignationG 49;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 3. Summary of Practice
1.1 This practice covers procedures for designing, prepar- 3.1 This practice covers the use of axially loaded, quantita-
ing, and using ASTM standard tension test specimens for tively stressed ASTM standard tension test specimens for
investigating susceptibility to stress-corrosion cracking. Axi- investigating the resistance to stress-corrosion cracking of
ally loaded specimens may be stressed quantitatively with metallic materials in all types of product forms. Consideration
equipment for application of either a constant load, constant is given to important factors in the selection of appropriate
strain, or with a continuously increasing strain. specimens, the design of loading equipment, and the effects of
1.2 Tension test specimens are adaptable for testing a wide these factors on the state of stress in the specimen as corrosion
variety of product forms as well as parts joined by welding, occurs.
riveting, or various other methods.
4. Significance and Use
1.3 The exposure of specimens in a corrosive environment
is treated only briefly because other standards are being 4.1 Axially loaded tension specimens provide one of the
most versatile methods of performing a stress-corrosion test
prepared to deal with this aspect. Meanwhile, the investigator
is referred to Practices G 35, G 36, and G 37, and G 44, and to because of the flexibility permitted in the choice of type and
size of test specimen, stressing procedures, and range of stress
ASTM Special Technical Publication 425 (1).
levels.
2. Referenced Documents
4.2 The uniaxial stress system is simple; hence, this test
2.1 ASTM Standards: method is often used for studies of stress-corrosion mecha-
E 8 TestMethodsforTensionTestingofMetallicMaterials nisms. This type of test is amenable to the simultaneous
G 35 Practice for Determining the Susceptibility of Stain- exposure of unstressed specimens (no applied load) with
less Steels and Related Nickel-Chromium-Iron Alloys to stressed specimens and subsequent tension testing to distin-
Stress-Corrosion Cracking in Polythionic Acids guish between the effects of true stress corrosion and mechani-
G 36 Practice for Evaluating Stress-Corrosion Cracking cal overload (2). Additional considerations in regard to the
Resistance of Metals and Alloys in a Boiling Magnesium significance of the test results and their interpretation are given
Chloride Solution in Sections 6 and 10.
G 37 Practice for Use of Mattsson’s Solution of pH 7.2 to 4.3 Wide variations in test results may be obtained for a
Evaluate the Stress-Corrosion Cracking Susceptibility of given material and specimen orientation with different speci-
Copper-Zinc Alloys men sizes and stressing procedures. This consideration is
G 44 Practice for Evaluating Stress-Corrosion Cracking significant especially in the standardization of a test procedure
ResistanceofMetalsandAlloysbyAlternateImmersionin for interlaboratory comparisons or quality control.
3.5 % Sodium Chloride Solution
5. Test Specimens
5.1 Whenever possible, tension test specimens used in
evaluating susceptibility to stress-corrosion cracking should
This practice is under the jurisdiction of ASTM Committee G-1 on Corrosion
conform to the dimensions of standard tension test specimens
of Metals and is the direct responsibility of Subcommittee G01.06 on Stress-
specified in Test Methods E 8, which contain details for
Corrosion Cracking and Corrosion Fatigue.
specimens machined from various product forms.
Current edition approved June 28, 1985. Published September 1985. Originally
published as G 49 – 76. Last previous edition G 49 – 76.
5.2 A wide range of sizes for tension test specimens is
The boldface numbers in parentheses refer to the list of references at the end of
possible, depending primarily upon the dimensions of the
this practice.
3 product to be tested. Because the stress-corrosion test results
Annual Book of ASTM Standards, Vol 03.01.
Annual Book of ASTM Standards, Vol 03.02. can be markedly influenced by the cross section of the test
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959, United States.
G49
specimen, this factor should be given careful consideration
with regard to the object of the investigation. Although larger
specimens may be more representative of most actual struc-
tures, they often cannot be machined from product forms to be
evaluated; and they present more difficulties in stressing and
handling in the laboratory. Also, larger specimens of some
materials may require longer exposure periods than smaller
specimens.
5.3 Smaller cross-section specimens are widely used be-
cause they (a) have a greater sensitivity to the initiation of
stress-corrosion cracking, (b) usually give test results more
quickly, and (c) permit greater convenience in testing. On the
other hand, the smaller specimens are more difficult to ma-
chine, and their performance is more likely to be influenced by
extraneous stress concentrations resulting from non-axial load-
ing,corrosionpits,etc.Therefore,specimenslessthanabout10
mm(0.4in.)ingagelengthor3.0mm(0.12in.)indiameterare
not recommended for general use.
5.4 Tension specimens containing machined notches have
been used in studies of stress-corrosion cracking and hydrogen
embrittlement (3). The presence of a notch induces a triaxial
stressstateattherootofthenotchwhereintheactualstresswill
be greater by a concentration factor dependent on the notch
geometry. Advantages of such specimens include the probable
localization of cracking to the notch region and acceleration of
failure. However, unless directly related to practical conditions
of usage, spurious results may ensue.
NOTE 1—The behavior shown is generally representative, but the
5.5 Tension specimens containing a machined notch in
curves will vary with specific alloys and tempers.
which a mechanical precrack (for example, a fatigue or tension
FIG. 1 Effect of Loading Method and Extent of Cracking or
crack) has been started will be the subject of another ASTM
Corrosion Pattern on Average Net Section Stress
standard. Various types of precracked specimens are discussed
in other publications (2, 4).
occur: (a) fracture by mechanical overload of a material that is
6. Stress Considerations
not susceptible to stress-corrosion cracking, or (b) stress-
corrosion cracking of a material at an unknown stress higher
6.1 There are several factors that may introduce bending
moments on specimens, such as a longitudinal curvature, than the intended nominal test stress. The occurrence of either
of these phenomena would interfere with a valid evaluation of
misalignmentofthreadsonthreaded-endroundspecimens,and
the corners of sheet-type specimens. The significance of these materials with a relatively high resistance to stress corrosion.
These considerations must be taken into account in experi-
factors is greater for specimens with smaller cross sections.
Even though eccentricity in loading can be minimized to equal ments undertaken to determine “threshold” stresses. The sig-
nificance of these factors is discussed further in Section 10.
the same standards accepted for tension testing machines,
inevitably, there is some variation in the tensile stress around
7. Stressing Methods
the circumference of the test specimen which can be of such
magnitude that it will introduce considerable error in the 7.1 General Considerations:
7.1.1 Tension specimens may be subjected to a wide range
desired stress. Tests should be made on specimens with strain
gages affixed to the specimen surface (around the circumfer- of stress levels associated with either elastic or elastic and
ence in 90° or 120° intervals) to verify strain and stress plastic strain. Because the stress system is intended to be
uniformity and determine if machining practices and stressing essentially uniaxial (except in the case of notched specimens),
jigs are of adequate tolerance and quality. great care must be exercised in the construction of stressing
6.2 Another consideration is the possible increase in net frames so that bending stresses are avoided or minimized.
section stress that will occur when corrosion develops during 7.1.2 Although a number of different stressing frames have
the environmental exposure (1, 5). As shown schematically in been used with tension specimens, three basic types are
Fig. 1, there are two limiting curves: one for zero stiffness considered herein: constant (sustained) load, constant strain
(dead weight) and the other for infinite stiffness (ideal constant (deformation), and continuously increasing strain. A constant
strain). In actual testing with various types of stressing frames, loadcanbeobtainedwithdeadweight,buttrulyconstantstrain
such as those shown in Figs. 2-4, the increase in net section loading is seldom achieved because a stressing frame with
stress will be somewhere in between. When the net section infinitestiffnesswouldberequired.Stress-corrosiontestresults
stress becomes greater than the nominal gross section stress can be influenced by the type of loading in combination with
and increases to the point of fracture, either of two events can the design of the test specimen; therefore, the investigator
G49
FIG. 2 Spring-Loaded Stressing Frame (7)
Fig. 2 (7). The principle of the proving ring, as used in the
calibration of tension testing machines, has also been adapted
to stress-corrosion testing to provide a simple, compact, and
easily operated device to apply axial load (8); see Fig. 3(a).
The load is applied by tightening a nut on one of the bolts and
is determined by carefully measuring the change in ring
diameter.Anothersimilarbutlesssophisticatedringdevicecan
also be used, the difference being that the load is applied with
a hydraulic jig (8) as shown in Fig. 3(b). In either ring device,
the bolt contains a keyway to prevent a torsional stress from
being applied to the specimen while tightening the nut.
7.2.2 Constant Strain—Stress-corrosion tests performed in
low-compliance tension testin
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