1、Designation: G139 05 (Reapproved 2011)Standard Test Method forDetermining Stress-Corrosion Cracking Resistance of Heat-Treatable Aluminum Alloy Products Using Breaking LoadMethod1This standard is issued under the fixed designation G139; the number immediately following the designation indicates the
2、year oforiginal adoption or, in the case of revision, the year of last revision. A number in parentheses indicates the year of last reapproval. Asuperscript epsilon () indicates an editorial change since the last revision or reapproval.1. Scope1.1 This test method covers procedures for evaluation of
3、stress corrosion cracking (SCC) resistance by the breaking loadtest method, a concept which uses residual strength as themeasure of damage evolution (in this case environmentallyassisted cracking).1.2 This test method covers specimen type and replication,test environment, stress levels, exposure per
4、iods, final strengthdetermination, and statistical analysis of the raw residualstrength data.1.3 The test method was developed for use with heat-treatable aluminum alloys, that is, 2XXX alloys and 7XXXwith 1.2 to 3.0 % Cu, and test specimens oriented in theshort-transverse direction relative to grai
5、n structure (1, 2).2However, the residual strength measurements and the statisticsused to analyze the data are not specific to heat-treatablealuminum alloys and can be used for other specimen orienta-tions and different types of materials.1.4 This standard does not purport to address all of thesafet
6、y concerns, if any, associated with its use. It is theresponsibility 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. Referenced Documents2.1 ASTM Standards:3E8 Test Methods for Tension Testi
7、ng of Metallic MaterialsE691 Practice for Conducting an Interlaboratory Study toDetermine the Precision of a Test MethodG44 Practice for Exposure of Metals and Alloys by Alter-nate Immersion in Neutral 3.5 % Sodium Chloride Solu-tionG47 Test Method for Determining Susceptibility to Stress-Corrosion
8、Cracking of 2XXX and 7XXXAluminumAlloyProductsG49 Practice for Preparation and Use of Direct TensionStress-Corrosion Test SpecimensG64 Classification of Resistance to Stress-Corrosion Crack-ing of Heat-Treatable Aluminum Alloys3. Terminology3.1 Definitions of Terms Specific to This Standard:3.1.1 ce
9、nsora statistical term indicating that the valuefrom an individual observation may fall outside of the rangethat can be measured because of test procedures or conditions.3.1.2 samplethe nominally uniform, bulk material fromwhich individual stress-corrosion cracking specimens are ob-tained.4. Summary
10、 of Test Method4.1 This test method describes a procedure for using re-sidual strength after exposure to a corrosive environment toevaluate stress corrosion cracking susceptibility in heat treat-able aluminum alloy product forms such as sheet, plate,extrusions, forgings, and bar. These products gene
11、rally aremost susceptible to SCC in the long transverse direction ofsheet, the short transverse direction of plate, extrusions andforgings, and the transverse direction of rod and bar stock. Inthis test, tensile bars or direct tension sheet specimens, pre-pared according to Practice G49, are exposed
12、 to 3.5 weight %aqueous sodium chloride solution (Practice G44), are removedbefore they fail and are tension tested to determine the amountof corrosion damage that has occurred. The average retainedstrength is then calculated and the Box-Cox Transformationcan be used for statistical analysis of the
13、results.1This test method is under the jurisdiction of ASTM Committee G01 onCorrosion of Metals and is the direct responsibility of Subcommittee G01.06 onEnvironmentally Assisted Cracking.Current edition approved Sept. 1, 2011. Published September 2011. Originallyapproved in 2005. Last previous edit
14、ion approved in 2005 as G13905. DOI:10.1520/G0139-05R11.2The boldface numbers in parentheses refer to the list of references at the end ofthe standard.3For referenced ASTM standards, visit the ASTM website, www.astm.org, orcontact ASTM Customer Service at serviceastm.org. For Annual Book of ASTMStan
15、dards volume information, refer to the standards Document Summary page onthe ASTM website.1Copyright ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959, United States.4.2 The procedure calls for exposure of unstressed speci-mens which are used to factor out the
16、effects of pitting,intergranular, and general corrosion. These phenomena de-grade residual strength but do not require applied stress fortheir occurrence.5. Significance and Use5.1 The test method was developed for use with highstrength aluminum alloys (2XXX and Cu containing 7XXX)that are normally
17、tested in 3.5 weight % NaCl by alternateimmersion. However, the concept which uses residual strengthas a measure of damage evolution (in this caseenvironmentally-assisted cracking) can, in principle, be appliedto any alloy and environmental system.5.2 This test method has been developed for research
18、studies of alloys and tempers with improved resistance to SCC.The test results permit different material variants to be com-pared with a high degree of confidence and with much moreprecision than the results of pass/fail tests. Thus, it is particu-larly useful for comparing materials with similar le
19、vels ofresistance to stress-corrosion cracking. The procedure could bemodified for use as a quality assurance tool but this has notbeen a primary purpose during its development.5.3 The exposure periods and conditions that are describedin this test method apply specifically to high strength aluminuma
20、lloys, but the statistical techniques should be valid for otheralloy systems with different exposure conditions.5.4 Although this particular procedure was primarily in-tended for testing products in the short-transverse stressingdirection, it is useful for other stressing directions, particularlythe
21、 long-transverse direction in sheet and thin plate products.5.5 Determination of the actual serviceability of a materialrequires stress-corrosion testing performed in the intendedservice environment, under conditions relating to the end use,including protective measures such as coatings and inhibito
22、rsand is outside the scope of this test method.5.5.1 There is no good way to compare test environments toactual service because most service environments have largeinherent variability with respect to a single structure that mayexperience many different environments or with respect to twoidentical s
23、tructures that serve in different locations. Unless asample can be tested in the actual service environment for theexpected life of the component, no conclusive determinationcan be made about the suitability of a particular material for aparticular application. Designers must therefore make judg-men
24、ts on the suitability of particular materials for applicationsbased on knowledge of the material and of the serviceenvironment. To avoid service failures, the environment usedfor preliminary evaluations is often chosen based on a worstcase scenario leading to intentional overestimations of corro-sio
25、n damage.6. Interferences6.1 The breaking load test factors out pitting corrosion thatoccurs in environments such as the 3.5 % NaCl solution usedin alternate immersion testing per Practice G44. The primaryconcern in using the breaking load test is choice of appropriateexposure stress. If the exposur
26、e stress is too low no damagewill accumulate. On the other hand, if the applied stress is toohigh many of the specimens will fail before the end of theirscheduled exposure periods. The statistical procedures in-cluded in this test method can accommodate small numbers offailed specimens but not large
27、 numbers.6.2 The breaking load test is applicable to specimens thathave been exposed in natural and service environments.However, conditions in these environments may not be con-stant so consideration must be given to the period and timingof exposure to avoid biasing results. For example, environmen
28、-tal conditions that vary seasonally such as temperature, mois-ture, and pollutant concentration may affect the corrosivity ofoutdoor exposure stations. Direct material comparisons shouldbe made using identical environmental conditions.6.3 Some care is required when comparison samples havedifferent
29、original (uncorroded) tensile strength and fracturetoughness values. Large variations in initial properties caneither reduce or increase the apparent differences in SCCperformance of the samples. To avoid bias due to tensileproperties, the statistical procedures incorporated in this testmethod are b
30、ased on percentages of original strength. How-ever, to examine the effect of fracture toughness, which affectsresidual strength, a flaw size calculation must be done usingfracture mechanics techniques (3).7. Test Specimens7.1 The breaking load procedure may be conducted usingany specimen that can be
31、 axially stressed in a fixture that willsustain an applied displacement. However, results obtainedusing different specimen geometries or stressing methods cannot be directly compared. While the relative susceptibilities ofthe samples will not be changed, the absolute numbers can bequite different.7.
32、2 Whenever the geometry of the metal sample permits, thetest should be conducted using smooth, round tension speci-mens prepared in accordance with Practice G49. In the case ofsheet and other products that may be too thin to yield tensilebars, sheet tensile specimens may be used. The test sensitivit
33、yincreases with the ratio of surface area to volume in thespecimen gage section; however tests made using round tensilespecimens have shown that the same relative rankings can beachieved with different size specimens (1).8. Exposure Procedure8.1 Stressing Procedure and Exposure ConditionsThespecimen
34、s shall be stressed by axially loading in constantdeflection-type fixtures as in Figure 1 of Practice G49 andexposed to the 3.5 % NaCl alternate immersion test perPractice G44. The number of specimens for each stresslevel/exposure time combination should be a minimum ofthree; five or more are prefer
35、able.8.2 Stress LevelThe minimum number of stress levels istwo, one of which is a complete set of specimens exposed withno applied stress. For samples with unknown SCC resistance itis preferable to start with two or three stress levels in additionto the unstressed specimens. The unstressed specimens
36、 allowthe damage caused by general, pitting and intergranular corro-sion to be calculated and separated from damage caused by theapplied stress. The other stress level(s) must be chosen for eachindividual sample by considering the expected performance ofG139 05 (2011)2the sample. The more SCC resist
37、ant the sample, the higher thestresses should be. The ideal maximum stress would be onethat leads to significant damage by way of cracking but doesnot cause more than a few specimens to actually break into twopieces before the end of the scheduled exposure period (2).One stress level can be used but
38、 the statistical calculations onlyevaluate the performance of the sample at that stress level. Inother words, there is no good way to extrapolate and estimateperformance at higher or lower stress levels without actuallyconducting the test.8.3 Exposure TimeThis parameter must be adjusted forthe sampl
39、e to be tested and the size and orientation of the testspecimens. In general, two to four time periods (plus zero dayswith no stress) should be used with the maximum time beingapproximately ten days for short transverse tests on 2XXX and7XXX alloys. In general, long-transverse specimens and moreresi
40、stant alloy systems (such as 6XXX alloys) should beexposed for longer periods. Classification G64 gives timeperiods for these situations which can be used to estimate areasonable maximum exposure time.NOTE 1For material variants with unknown SCC performance in thetest environment, it is advisable to
41、 test a limited number of pass/failspecimens according to the procedures in Test Method G47. This willprovide guidance for choosing appropriate stress levels and exposuretimes for the sample. This can prevent the expenditure of large amounts oftime and money for specimens that do not provide informa
42、tion withsignificant value.8.4 Determination of Residual StrengthUpon completionof each exposure period, a set of specimens should be removedfrom test, rinsed, unstressed, and tension tested in accordancewith Test Method E8. It is recommended that tensile testing becompleted on the day the specimens
43、 are removed fromexposure. If a time delay between completion of exposure andtensile testing is unavoidable, the specimens must be thor-oughly rinsed with deionized water, stored in a desiccatedenvironment, and the delay period should be recorded. Thebreaking strength must be calculated and recorded
44、 for each testspecimen.8.5 The residual strength data can be used to show trendsbetween samples by simply calculating average residualstrength for each stress/time combination as shown in Fig. 1.However, statistical procedures must be used to evaluatewhether the trends are real or merely data scatte
45、r.8.5.1 During the development of the breaking load testmethod, the variance of data within individual cells (a singlesample/stress/time combination) has been shown to increase asresistance to SCC decreases. This tendency for variance toincrease with decreasing residual strength means that theabilit
46、y of the breaking load test to resolve differences betweencells can be much greater for the better performing cells thanthe poorer performing cells. Therefore, plots of average re-sidual strength can be very misleading.9. Statistical AnalysisBox-Cox Transformation9.1 Breaking load data can be statis
47、tically analyzed byfollowing the steps outlined here. There are undoubtedly otherprocedures that will work but the Box-Cox transformation hasdemonstrated its usefulness for situations in which variance isnot constant throughout the data set (4,5). In the case of stresscorrosion cracking data, as res
48、idual strength decreases, vari-ance generally increases. The following procedure assumesthat a fixed number of specimens have been tested for eachmaterial variant, exposure period, and exposure stress. Some ofthese values will be left-censored, that is, some specimens willfail before they complete t
49、heir scheduled exposure period. Forsuch specimens the breaking load value is known to be lessthan or equal to the exposure stress but this procedure includesa statistical method for estimating the values of those datapoints.NOTE 2Appendix X1 contains a sample Box-Cox calculation thatfollows the procedure described in this section of the test method.9.2 Transform the original values, X, by means of thepreliminary transformationXtr5SXXOD100 (1)where XOis the average breaking load for no exposure forthe given material variant. This transformation expresses thepercent retention
