ASTM G139-05(2011)
(Test Method)Standard Test Method for Determining Stress-Corrosion Cracking Resistance of Heat-Treatable Aluminum Alloy Products Using Breaking Load Method
Standard Test Method for Determining Stress-Corrosion Cracking Resistance of Heat-Treatable Aluminum Alloy Products Using Breaking Load Method
SIGNIFICANCE AND USE
The test method was developed for use with high strength aluminum alloys (2XXX and Cu containing 7XXX) that are normally tested in 3.5 weight % NaCl by alternate immersion. However, the concept which uses residual strength as a measure of damage evolution (in this case environmentally-assisted cracking) can, in principle, be applied to any alloy and environmental system.
This test method has been developed for research studies of alloys and tempers with improved resistance to SCC. The test results permit different material variants to be compared with a high degree of confidence and with much more precision than the results of pass/fail tests. Thus, it is particularly useful for comparing materials with similar levels of resistance to stress-corrosion cracking. The procedure could be modified for use as a quality assurance tool but this has not been a primary purpose during its development.
The exposure periods and conditions that are described in this test method apply specifically to high strength aluminum alloys, but the statistical techniques should be valid for other alloy systems with different exposure conditions.
Although this particular procedure was primarily intended for testing products in the short-transverse stressing direction, it is useful for other stressing directions, particularly the long-transverse direction in sheet and thin plate products.
Determination of the actual serviceability of a material requires stress-corrosion testing performed in the intended service environment, under conditions relating to the end use, including protective measures such as coatings and inhibitors and is outside the scope of this test method.
There is no good way to compare test environments to actual service because most service environments have large inherent variability with respect to a single structure that may experience many different environments or with respect to two identical structures that serve in different locations. Unless a sample can b...
SCOPE
1.1 This test method covers procedures for evaluation of stress corrosion cracking (SCC) resistance by the breaking load test method, a concept which uses residual strength as the measure of damage evolution (in this case environmentally assisted cracking).
1.2 This test method covers specimen type and replication, test environment, stress levels, exposure periods, final strength determination, and statistical analysis of the raw residual strength data.
1.3 The test method was developed for use with heat-treatable aluminum alloys, that is, 2XXX alloys and 7XXX with 1.2 to 3.0 % Cu, and test specimens oriented in the short-transverse direction relative to grain structure (1, 2). However, the residual strength measurements and the statistics used to analyze the data are not specific to heat-treatable aluminum alloys and can be used for other specimen orientations and different types of materials.
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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Standards Content (Sample)
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Designation: G139 − 05(Reapproved 2011)
Standard Test Method for
Determining Stress-Corrosion Cracking Resistance of Heat-
Treatable Aluminum Alloy Products Using Breaking Load
Method
This standard is issued under the fixed designation G139; the number immediately following the designation indicates the year of
original adoption or, in the case of revision, the year of last revision.Anumber in parentheses indicates the year of last reapproval.A
superscript epsilon (´) indicates an editorial change since the last revision or reapproval.
1. Scope G44PracticeforExposureofMetalsandAlloysbyAlternate
Immersion in Neutral 3.5 % Sodium Chloride Solution
1.1 This test method covers procedures for evaluation of
G47Test Method for Determining Susceptibility to Stress-
stresscorrosioncracking(SCC)resistancebythebreakingload
Corrosion Cracking of 2XXX and 7XXX Aluminum
test method, a concept which uses residual strength as the
Alloy Products
measure of damage evolution (in this case environmentally
G49Practice for Preparation and Use of Direct Tension
assisted cracking).
Stress-Corrosion Test Specimens
1.2 This test method covers specimen type and replication,
G64Classification of Resistance to Stress-Corrosion Crack-
test environment, stress levels, exposure periods, final strength
ing of Heat-Treatable Aluminum Alloys
determination, and statistical analysis of the raw residual
strength data.
3. Terminology
1.3 The test method was developed for use with heat-
3.1 Definitions of Terms Specific to This Standard:
treatable aluminum alloys, that is, 2XXX alloys and 7XXX
3.1.1 censor—a statistical term indicating that the value
with 1.2 to 3.0% Cu, and test specimens oriented in the
from an individual observation may fall outside of the range
short-transverse direction relative to grain structure (1, 2).
that can be measured because of test procedures or conditions.
However, the residual strength measurements and the statistics
3.1.2 sample—the nominally uniform, bulk material from
used to analyze the data are not specific to heat-treatable
which individual stress-corrosion cracking specimens are ob-
aluminum alloys and can be used for other specimen orienta-
tained.
tions and different types of materials.
1.4 This standard does not purport to address all of the
4. Summary of Test Method
safety concerns, if any, associated with its use. It is the
4.1 This test method describes a procedure for using re-
responsibility of the user of this standard to establish appro-
sidual strength after exposure to a corrosive environment to
priate safety and health practices and determine the applica-
evaluate stress corrosion cracking susceptibility in heat treat-
bility of regulatory limitations prior to use.
able aluminum alloy product forms such as sheet, plate,
2. Referenced Documents
extrusions, forgings, and bar. These products generally are
most susceptible to SCC in the long transverse direction of
2.1 ASTM Standards:
sheet, the short transverse direction of plate, extrusions and
E8Test Methods for Tension Testing of Metallic Materials
forgings, and the transverse direction of rod and bar stock. In
E691Practice for Conducting an Interlaboratory Study to
this test, tensile bars or direct tension sheet specimens, pre-
Determine the Precision of a Test Method
pared according to Practice G49, are exposed to 3.5 weight %
aqueous sodium chloride solution (Practice G44), are removed
This test method is under the jurisdiction of ASTM Committee G01 on
before they fail and are tension tested to determine the amount
Corrosion of Metals and is the direct responsibility of Subcommittee G01.06 on
of corrosion damage that has occurred. The average retained
Environmentally Assisted Cracking.
strength is then calculated and the Box-Cox Transformation
Current edition approved Sept. 1, 2011. Published September 2011. Originally
approved in 2005. Last previous edition approved in 2005 as G139–05. DOI:
can be used for statistical analysis of the results.
10.1520/G0139-05R11.
4.2 The procedure calls for exposure of unstressed speci-
Theboldfacenumbersinparenthesesrefertothelistofreferencesattheendof
the standard.
mens which are used to factor out the effects of pitting,
For referenced ASTM standards, visit the ASTM website, www.astm.org, or
intergranular, and general corrosion. These phenomena de-
contact ASTM Customer Service at service@astm.org. For Annual Book of ASTM
grade residual strength but do not require applied stress for
Standards volume information, refer to the standard’s Document Summary page on
the ASTM website. their occurrence.
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959. United States
G139 − 05 (2011)
5. Significance and Use 6.2 The breaking load test is applicable to specimens that
have been exposed in natural and service environments.
5.1 The test method was developed for use with high
However, conditions in these environments may not be con-
strength aluminum alloys (2XXX and Cu containing 7XXX)
stant so consideration must be given to the period and timing
that are normally tested in 3.5 weight % NaCl by alternate
ofexposuretoavoidbiasingresults.Forexample,environmen-
immersion. However, the concept which uses residual strength
tal conditions that vary seasonally such as temperature,
as a measure of damage evolution (in this case
moisture, and pollutant concentration may affect the corrosiv-
environmentally-assistedcracking)can,inprinciple,beapplied
ity of outdoor exposure stations. Direct material comparisons
to any alloy and environmental system.
should be made using identical environmental conditions.
5.2 This test method has been developed for research
6.3 Some care is required when comparison samples have
studiesofalloysandtemperswithimprovedresistancetoSCC.
different original (uncorroded) tensile strength and fracture
The test results permit different material variants to be com-
toughness values. Large variations in initial properties can
pared with a high degree of confidence and with much more
either reduce or increase the apparent differences in SCC
precision than the results of pass/fail tests. Thus, it is particu-
performance of the samples. To avoid bias due to tensile
larly useful for comparing materials with similar levels of
properties, the statistical procedures incorporated in this test
resistancetostress-corrosioncracking.Theprocedurecouldbe
method are based on percentages of original strength.
modified for use as a quality assurance tool but this has not
However, to examine the effect of fracture toughness, which
been a primary purpose during its development.
affects residual strength, a flaw size calculation must be done
5.3 The exposure periods and conditions that are described
using fracture mechanics techniques (3).
inthistestmethodapplyspecificallytohighstrengthaluminum
7. Test Specimens
alloys, but the statistical techniques should be valid for other
alloy systems with different exposure conditions.
7.1 The breaking load procedure may be conducted using
any specimen that can be axially stressed in a fixture that will
5.4 Although this particular procedure was primarily in-
sustain an applied displacement. However, results obtained
tended for testing products in the short-transverse stressing
using different specimen geometries or stressing methods can
direction, it is useful for other stressing directions, particularly
not be directly compared. While the relative susceptibilities of
the long-transverse direction in sheet and thin plate products.
the samples will not be changed, the absolute numbers can be
5.5 Determination of the actual serviceability of a material
quite different.
requires stress-corrosion testing performed in the intended
7.2 Wheneverthegeometryofthemetalsamplepermits,the
service environment, under conditions relating to the end use,
test should be conducted using smooth, round tension speci-
including protective measures such as coatings and inhibitors
mens prepared in accordance with Practice G49. In the case of
and is outside the scope of this test method.
sheet and other products that may be too thin to yield tensile
5.5.1 Thereisnogoodwaytocomparetestenvironmentsto
bars, sheet tensile specimens may be used. The test sensitivity
actual service because most service environments have large
increases with the ratio of surface area to volume in the
inherent variability with respect to a single structure that may
specimengagesection;howevertestsmadeusingroundtensile
experience many different environments or with respect to two
specimens have shown that the same relative rankings can be
identical structures that serve in different locations. Unless a
achieved with different size specimens (1).
sample can be tested in the actual service environment for the
expected life of the component, no conclusive determination
8. Exposure Procedure
can be made about the suitability of a particular material for a
8.1 Stressing Procedure and Exposure Conditions—The
particular application. Designers must therefore make judg-
specimens shall be stressed by axially loading in constant
ments on the suitability of particular materials for applications
deflection-type fixtures as in Figure 1 of Practice G49 and
based on knowledge of the material and of the service
exposed to the 3.5% NaCl alternate immersion test per
environment. To avoid service failures, the environment used
Practice G44. The number of specimens for each stress
for preliminary evaluations is often chosen based on a worst
level/exposure time combination should be a minimum of
case scenario leading to intentional overestimations of corro-
three; five or more are preferable.
sion damage.
8.2 Stress Level—The minimum number of stress levels is
6. Interferences
two,oneofwhichisacompletesetofspecimensexposedwith
6.1 The breaking load test factors out pitting corrosion that noappliedstress.ForsampleswithunknownSCCresistanceit
occurs in environments such as the 3.5% NaCl solution used is preferable to start with two or three stress levels in addition
in alternate immersion testing per Practice G44. The primary to the unstressed specimens. The unstressed specimens allow
concerninusingthebreakingloadtestischoiceofappropriate the damage caused by general, pitting and intergranular corro-
exposure stress. If the exposure stress is too low no damage siontobecalculatedandseparatedfromdamagecausedbythe
will accumulate. On the other hand, if the applied stress is too appliedstress.Theotherstresslevel(s)mustbechosenforeach
high many of the specimens will fail before the end of their individual sample by considering the expected performance of
scheduled exposure periods. The statistical procedures in- the sample.The more SCC resistant the sample, the higher the
cluded in this test method can accommodate small numbers of stresses should be. The ideal maximum stress would be one
failed specimens but not large numbers. that leads to significant damage by way of cracking but does
G139 − 05 (2011)
notcausemorethanafewspecimenstoactuallybreakintotwo
pieces before the end of the scheduled exposure period (2).
Onestresslevelcanbeusedbutthestatisticalcalculationsonly
evaluate the performance of the sample at that stress level. In
other words, there is no good way to extrapolate and estimate
performance at higher or lower stress levels without actually
conducting the test.
8.3 ExposureTime—Thisparametermustbeadjustedforthe
sample to be tested and the size and orientation of the test
specimens.Ingeneral,twotofourtimeperiods(pluszerodays
with no stress) should be used with the maximum time being
approximatelytendaysforshorttransversetestson2XXXand
7XXX alloys. In general, long-transverse specimens and more
resistant alloy systems (such as 6XXX alloys) should be
exposed for longer periods. Classification G64 gives time
periods for these situations which can be used to estimate a
reasonable maximum exposure time.
NOTE 1—For material variants with unknown SCC performance in the
test environment, it is advisable to test a limited number of pass/fail
specimens according to the procedures in Test Method G47. This will
NOTE 1—Some specimens in this set did fail before the end of their
provide guidance for choosing appropriate stress levels and exposure
scheduled exposure periods, but these failed specimens have not been
timesforthesample.Thiscanpreventtheexpenditureoflargeamountsof
included in the averages. The averages represent only specimens that
time and money for specimens that do not provide information with
survived to be tensile tested. The upturn in the nine-day data at 310 MPa
significant value.
is caused by not including failed specimens.
8.4 Determination of Residual Strength—Upon completion
FIG. 1 Plot of Average Residual Strength Values for a Represen-
ofeachexposureperiod,asetofspecimensshouldberemoved
tative Data Set (one laboratory)
from test, rinsed, unstressed, and tension tested in accordance
withTest Method E8. It is recommended that tensile testing be
completed on the day the specimens are removed from
exposure. If a time delay between completion of exposure and
that a fixed number of specimens have been tested for each
tensile testing is unavoidable, the specimens must be thor-
materialvariant,exposureperiod,andexposurestress.Someof
oughly rinsed with deionized water, stored in a desiccated
these values will be left-censored, that is, some specimens will
environment, and the delay period should be recorded. The
fail before they complete their scheduled exposure period. For
breakingstrengthmustbecalculatedandrecordedforeachtest
such specimens the breaking load value is known to be less
specimen.
thanorequaltotheexposurestressbutthisprocedureincludes
a statistical method for estimating the values of those data
8.5 The residual strength data can be used to show trends
points.
between samples by simply calculating average residual
strength for each stress/time combination as shown in Fig. 1.
NOTE 2—Appendix X1 contains a sample Box-Cox calculation that
However, statistical procedures must be used to evaluate follows the procedure described in this section of the test method.
whether the trends are real or merely data scatter.
9.2 Transform the original values, X, by means of the
8.5.1 During the development of the breaking load test
preliminary transformation
method, the variance of data within individual cells (a single
X
sample/stress/timecombination)hasbeenshowntoincreaseas
X 5 100 (1)
S D
tr
X
O
resistance to SCC decreases. This tendency for variance to
increase with decreasing residual strength means that the
where X is the average breaking load for no exposure for
O
ability of the breaking load test to resolve differences between
...
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