1、Designation: G 139 05Standard Test Method forDetermining Stress-Corrosion Cracking Resistance of Heat-Treatable Aluminum Alloy Products Using Breaking LoadMethod1This standard is issued under the fixed designation G 139; the number immediately following the designation indicates the year oforiginal
2、adoption or, in the case of revision, the year of last revision. A number in parentheses indicates the year of last reapproval. Asuperscript epsilon (e) indicates an editorial change since the last revision or reapproval.1. Scope1.1 This test method covers procedures for evaluation ofstress corrosio
3、n 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 periods, final str
4、engthdetermination, 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 grain structure (1,
5、 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 thesafety concerns, if
6、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 Testing of Metallic
7、MaterialsE 691 Practice for Conducting an Interlaboratory Study toDetermine the Precision of a Test MethodG44 Practice for Evaluating Stress Corrosion CrackingResistance of Metals andAlloys byAlternate Immersion in3.5 % Sodium Chloride SolutionG47 Test Method for Determining Susceptibility to Stress
8、-Corrosion Cracking of High-Strength Aluminum AlloyProductsG49 Practice for Preparation and Use of Direct TensionStress-Corrosion Test SpecimensG64 Classification of Resistance to Stress-CorrosionCracking of Heat-Treatable Aluminum Alloys3. Terminology3.1 Definitions of Terms Specific to This Standa
9、rd:3.1.1 censora 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
10、.4. Summary 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 pr
11、oducts generally 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,
12、are exposed 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 analy
13、sis of the results.1This test method is under the jurisdiction of ASTM Committee G01 onCorrosion of Metals and is the direct responsibility of Subcommittee G01.06 onStress Corrosion Cracking and Corrosion Fatigue.Current edition approved Oct. 1, 2005. Published October 2005. Originallypublished as G
14、 139 96. Discontinued April 2003 and reinstated as G 139 05.Last previous edition G 139 96.2The 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 serviceas
15、tm.org. For Annual Book of ASTMStandards 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-men
16、s which are used to factor out the 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
17、 containing 7XXX)that are normally 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 met
18、hod has been developed for researchstudies 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
19、comparing materials with similar levels 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 spec
20、ifically to high strength aluminumalloys, 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 st
21、ressing directions, particularlythe 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 mea
22、sures such as coatings and inhibitorsand 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 environment
23、s or with respect to twoidentical structures 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. Des
24、igners must therefore make judg-ments 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 inten
25、tional overestimations of corro-sion 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 appropr
26、iateexposure stress. If the exposure 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 numbe
27、rs offailed specimens but not large 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 biasi
28、ng results. For example, environmen-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 whe
29、n comparison samples havedifferent 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 in
30、corporated in this testmethod are based 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 cond
31、ucted usingany specimen that can be 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 absolu
32、te numbers can bequite different.7.2 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 specime
33、ns may be used. The test sensitivityincreases 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
34、 and Exposure ConditionsThespecimens shall be stressed by axially loading in constantdeflection-type fixtures as in Figure 1 of Practice G49andexposed to the 3.5 % NaCl alternate immersion test perPractice G44. The number of specimens for each stresslevel/exposure time combination should be a minimu
35、m ofthree; five or more are preferable.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 s
36、pecimens. The unstressed specimens 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 ofG139052the s
37、ample. The more SCC resistant 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 st
38、ress level can be used but 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 mus
39、t be adjusted forthe sample 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-transve
40、rse specimens and moreresistant 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 envir
41、onment, it is advisable to 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 t
42、hat do not provide information 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 becomplete
43、d on the day the specimens 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
44、be calculated and recorded 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
45、real or merely data scatter.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 str
46、ength means that theability 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 Breaki
47、ng load data can be statistically 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 stresscorro
48、sion cracking data, as residual 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 willf
49、ail before they complete their 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 ex
