Designation: G 139 – 05
Standard Test Method for
Determining Stress-Corrosion Cracking Resistance of HeatTreatable Aluminum Alloy Products Using Breaking Load Method1 This standard is issued under the fixed designation G 139; 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 supers cript epsilon (e) indicates an editorial change since the last revision or reapproval.
1. Sco Scope pe 1.1 This test method covers proc procedure eduress for evaluation evaluation of stress corrosion cracking (SCC) resistance by the breaking load testt met tes method hod,, a con concep ceptt whi which ch use usess res residu idual al str streng ength th as the measure meas ure of damag damagee evol evolution ution (in this case envir environme onmental ntally ly assisted cracking). 1.2 This test method covers covers specimen type and replication, replication, test environment, stress levels, exposure periods, final strength determ det ermina inatio tion, n, and sta statis tisti tical cal ana analys lysis is of the raw res residu idual al strength data. 1.3 1. 3 Th Thee te test st me meth thod od wa wass de deve velo lope ped d fo forr us usee wi with th he heat at-treatable aluminum alloys, that is, 2XXX alloys and 7XXX with wi th 1. 1.2 2 to 3. 3.0 0 % Cu Cu,, an and d te test st sp spec ecim imen enss or orie ient nted ed in th thee short-tran short -transvers sversee dire directio ction n rela relative tive to grai grain n struc structure ture (1, 2).2 However, the residual strength measurements and the statistics used use d to ana analyz lyzee the dat dataa are not spe specifi cificc to hea heat-t t-trea reatab table le aluminum alloys and can be used for other specimen orientations and different types of materials. 1.4 This sta standa ndard rd does not purport purport to add addre ress ss all of the safe sa fety ty co conc ncer erns ns,, if an anyy, as asso soci ciat ated ed wi with th it itss us use. e. It is th thee responsibility of the user of this standard to establish appro priate safety and health practices and determine the applicability of regulatory limitations prior to use. 2. Referenced Documents 2.1 ASTM Standards: 3 E 8 Test Methods for Tension Testing of Metallic Materials 1
Thiss test method Thi method is und under er the jurisdicti jurisdiction on of ASTM Committee Committee G01 on Corrosion of Metals and is the direct responsibility of Subcommittee G01.06 on Stress Corrosion Cracking and Corros Corrosion ion Fatigue. Currentt editio Curren edition n appro approved ved Oct. 1, 2005 2005.. Publi Published shed October 2005 2005.. Origin Originally ally published publis hed as G 139 – 96. Discontinued Discontinued April 2003 and reinstate reinstated d as G 139 – 05. Last previous edition G 139 – 96. 2 The boldface boldface numbers in parentheses refer to the list of reference referencess at the end of the standar standard. d. 3 For referenced ASTM standards, visit the ASTM website, www.astm.org, or contact ASTM Customer Service at service@
[email protected] astm.org. g. For For Annual Book of ASTM Standards volume Standards volume information, refer to the standard’s Document Summary page on the ASTM website website..
E 691 Practice for Conducting an Interlaboratory Study to Determine the Precision of a Test Method G 44 Pract Practice ice for Eval Evaluatin uating g Stre Stress ss Corro Corrosion sion Crack Cracking ing Resistance of Metals Metals and Alloys by Alternate Immersion Immersion in 3.5 % Sodium Chloride Solution G 47 Test Method for Determining Susceptibility to StressCorrosion Corro sion Crack Cracking ing of HighHigh-Stren Strength gth Aluminum Alloy Products G 49 Practice Practice for Prepa Preparati ration on and Use of Direc Directt Tensi ension on Stress-Corrosion Test Specimens G 64 Class Classific ificati ation on of Res Resist istance ance to Str Stress ess-Cor -Corros rosion ion Cracking of Heat-Treatable Aluminum Alloys 3. Terminology 3.1 Definitions of Terms Specific to This Standard: 3.1.1 censor —a — a sta statis tistic tical al ter term m ind indica icatin ting g tha thatt the val value ue from an individual observation may fall outside of the range that can be measured because of test procedures or conditions. 3.1.2 sample—the nominally nominally unifo uniform, rm, bulk mate material rial from which individual stress-corrosion cracking specimens are obtained. 4. Summ Summary ary of Test Test Method 4.1 Thi Thiss tes testt met method hod des descri cribes bes a pro proced cedure ure for using residual strength after exposure to a corrosive environment to evaluate stress corrosion cracking susceptibility in heat treatablee alu abl alumin minum um all alloy oy pro produc ductt for forms ms suc such h as she sheet, et, pla plate, te, extrusion extr usions, s, forg forgings, ings, and bar bar.. These products generally generally are most susceptibl susceptiblee to SCC in the long transverse transverse direction direction of sheet, shee t, the short trans transverse verse direction direction of plat plate, e, extru extrusions sions and forgings, and the transverse direction of rod and bar stock. In this test, tensile bars or direct tension sheet specimens, prepared according to Practice G Practice G 49 49,, are exposed to 3.5 weight % aqueous sodium chloride solution (Practice G (Practice G 44 44)), are removed before they fail and are tension tested to determine the amount of corrosion damage that has occurred. The average retained strength is then calculated and the Box-Cox Transformation can be used for statistical analysis of the results.
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G 139 – 05 procedure call callss for exposure of unst unstress ressed ed speci speci-4.2 The procedure mens wh mens whic ich h ar aree us used ed to fa fact ctor or ou outt th thee ef effe fect ctss of pi pitt ttin ing, g, intergranu inter granular lar,, and gener general al corr corrosio osion. n. These pheno phenomena mena degrade residual strength but do not require applied stress for their occurrence. 5. Signi Significanc ficancee and Use 5.1 Th 5.1 Thee te test st me meth thod od wa wass de deve velo lope ped d fo forr us usee wi with th hi high gh strength aluminum alloys (2XXX and Cu containing 7XXX)
high many of the specimens will fail before the end of their scheduled sche duled expos exposure ure peri periods. ods. The stat statisti istical cal proc procedure eduress included in this test method can accommodate small numbers of failed specimens but not large numbers. 6.2 The breaking breaking load test is applicable applicable to spec specimen imenss that havee bee hav been n exp expose osed d in nat natura urall and ser servic vicee env enviro ironme nments nts.. However, conditions in these environments may not be constant so consideration must be given to the period and timing of exposure to avoid biasing results. For example, environmen-
that are normally tested in 3.5 weight % NaCl by alternate immersion. However, the concept which uses residual strength a s a m ea s ur e o f d am ag e e v o l u t i o n ( i n t hi s c as e environmentally-assisted environmentally -assisted cracking) can, in principle, be applied to any alloy and environmental system. 5.2 5. 2 Th This is te test st me meth thod od ha hass be been en de deve velo lope ped d fo forr re rese sear arch ch 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 lar ly use useful ful for com compar paring ing mat materi erials als wit with h sim simila ilarr lev levels els 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. 5.3 The exposure exposure peri periods ods and conditions conditions that are desc described ribed
tal conditions that vary seasonally such as temperature, moisture, and pollutant concentration may affect the corrosivity of outdoor exposure stations. Direct material comparisons should be made using identical environmental conditions. 6.3 Some care is requ required ired when comparison comparison samples samples have differe dif ferent nt origi original nal (unco (uncorrode rroded) d) tens tensile ile stre strength ngth and fract fracture ure toughness toug hness values. Lar Large ge vari variation ationss in init initial ial prop properti erties es can either eit her red reduce uce or inc increa rease se the app appare arent nt dif differ ferenc ences es in SCC perfor per forman mance ce of the sam sample ples. s. To avo avoid id bia biass due to ten tensil silee properties, the statistical procedures incorporated in this test method are based on percentages of original strength. However, to examine the effect of fracture toughness, which affects residual strength, a flaw size calculation must be done using fracture mechanics techniques techniques (3).
in this test apply specifically high strength alloys, butmethod the statistical techniquestoshould be validaluminum for other alloy systems with different exposure conditions. 5.4 Altho Although ugh this particular particular procedure was prim primaril arily y intended tende d for test testing ing prod products ucts in the short short-tra -transver nsverse se stres stressing sing direction, it is useful for other stressing directions, particularly the long-transverse direction in sheet and thin plate products. 5.5 Deter Determina mination tion of the actual serviceabil serviceability ity of a mate material rial requires requi res stre stress-c ss-corros orrosion ion test testing ing perf performed ormed in the inten intended ded 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. 5.5.1 There is no good way to compare test 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 be tested in the actual service environment for the expected life of the component, no conclusive determination can be made about the suitability of a particular material for a particular parti cular application. application. Desig Designers ners must ther therefore efore make judgments on the suitability of particular materials for applications base ba sed d on kn know owle ledg dgee of th thee ma mate teri rial al an and d of th thee se serv rvic icee environment. To avoid service failures, the environment used for preliminary evaluations is often chosen based on a worst case scenario leading to intentional overestimations of corrosion damage.
7.1 The breaking breaking load procedure procedure may be conducted conducted using any specimen that can be axially stressed in a fixture that will sustain sust ain an appl applied ied displ displacem acement. ent. Howeve However, r, resu results lts obta obtained ined using different specimen geometries or stressing methods can not be directly compared. While the relative susceptibilities of the samples will not be changed, the absolute numbers can be quite different. 7.2 Whenever the geometry geometry of the metal sample permits, permits, the test should be conducted using smooth, round tension specimens prepared in accordance with Practice G Practice G 49 49.. In the case of sheet and other products that may be too thin to yield tensile bars, sheet tensile specimens may be used. The test sensitivity incr in crea ease sess wi with th th thee ra rati tio o of su surf rfac acee ar area ea to vo volu lume me in th thee specimen gage section; however tests made using round tensile specimens have shown that the same relative rankings can be achieved with different size specimens specimens (1).
7. Test Specimens
8. Exposure Procedure Procedure
6.1 The breaking breaking load test factors factors out pitti pitting ng corrosion corrosion that occurs in environments such as the 3.5 % NaCl solution used
8.1 Str Stressin essing g Pro Procedur ceduree and Expos Exposure ure Condi Condition tionss—The specimens specim ens sha shall ll be str stress essed ed by axi axiall ally y loa loadin ding g in con consta stant nt deflectiondeflec tion-type type fixtures as in Figur Figuree 1 of Pract Practice ice G 49 and expo ex pose sed d to th thee 3. 3.5 5 % Na NaCl Cl al alte tern rnat atee im imme mers rsio ion n te test st pe perr Practice G 44. 44 . The nu numb mber er of sp spec ecim imen enss fo forr ea each ch st stre ress ss level/ lev el/exp exposu osure re ti time me com combin binati ation on sho should uld be a min minimu imum m of three; five or more are preferable. 8.2 Stress Level—The minimum number of stress levels is two, one of which is a complete set of specimens exposed with no applied stress. For samples with unknown SCC resistance it is preferable to start with two or three stress levels in addition to the unstressed specimens. The unstressed specimens allow
in alternate immersion testing per Practice G Practice G 44 44.. The primary concern in using the breaking load test is choice of appropriate exposure stress. If the exposure stress is too low no damage will accumulate. On the other hand, if the applied stress is too
the damage caused by general, pitting and intergranular corrosion to be calculated and separated from damage caused by the applied stress. The other stress level(s) must be chosen for each individual sample by considering the expected performance of
6. Interferences
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G 139 – 05 the sample. The more SCC resistant the sample, the higher the stresses should be. The ideal maximum stress would be one that leads to significant damage by way of cracking but does not cause more than a few specimens to actually break into two pieces before the end of the scheduled exposure period (2). One stress level can be used but the statistical calculations only 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 Exposure Time—This parameter must be adjusted for the sample to be tested and the size and orientation of the test specimens. In general, two to four time periods (plus zero days with no stress) should be used with the maximum time being approximately ten days for short transverse tests on 2XXX and 7XXX alloys. In general, long-transverse specimens and more resist res istant ant all alloy oy sys system temss (su (such ch as 6XX 6XXX X all alloys oys)) sho should uld be exposed expos ed for longe longerr peri periods. ods. Class Classifica ification tion G 64 gives gives time periods for these situations which can be used to estimate a reasonable maximum exposure time. NOTE 1—For material material variants with unknown SCC perfor performance mance in the test environm environment ent,, it is advi advisab sable le to test a lim limited ited number number of pas pass/fa s/fail il specimens according to the procedures in Test Method G 47. 47 . This will provide guidance for choosing appropriate stress levels and exposur exposuree times for the sample. This can prevent the expenditure of large amounts of time and mon money ey for specimens specimens that do not provide provide inf inform ormati ation on with significant value.
8.4 Determination of Residual Strength—Upon completion of each exposure period, a set of specimens should be removed from test, rinsed, unstressed, and tension tested in accordance with Test Method E Method E 8. It is recommended that tensile testing be comp co mple lete ted d on th thee da day y th thee sp spec ecim imen enss ar aree re remo move ved d fr from om exposure. If a time delay between completion of exposure and tensile tensi le testi testing ng is unav unavoidab oidable, le, the speci specimens mens must be thor thor-oughly oug hly rin rinsed sed wit with h dei deioni onized zed wat water er,, sto stored red in a des desicc iccate ated d environmen envir onment, t, and the delay period should be recor recorded. ded. The breaking strength must be calculated and recorded for each test specimen. 8.5 The residual residual strength strength data can be used to show trends between bet ween sam sample pless by sim simply ply cal calcul culati ating ng ave averag ragee res residu idual al strength for each stress/time combination as shown in Fig. 1. 1. Howeve How ever, r, sta stati tisti stical cal pro proced cedure uress mus mustt be use used d to eva evalua luate te whether the trends are real or merely data scatter. 8.5.1 8.5 .1 Dur During ing the dev develo elopme pment nt of the bre breaki aking ng loa load d tes testt method, the variance of data within individual cells (a single sample/stress/time combination) has been shown to increase as resistance resis tance to SCC decr decreases eases.. This tendency for varia variance nce to increa inc rease se wit with h dec decrea reasin sing g res residu idual al str streng ength th mea means ns tha thatt the ability of the breaking load test to resolve differences between cells can be much greater for the better performing cells than the poorer perf performi orming ng cell cells. s. There Therefore, fore, plots of avera average ge residual strength can be very misleading.
NOTE 1—Some specimens in this set did fail before the end of their scheduled exposure periods, but these failed specimens scheduled specimens have not been included include d in the avera averages. ges. The avera averages ges represent only specim specimens ens that survived to be tensile tested. The upturn in the nine-day data at 310 MPa is caused by not including failed specimens. FIG. 1 Plot of Average Residual Strength Values for a Representative Data Set (one laboratory)
corrosion cracking data, as residual strength decreases, variance gener generally ally incr increases eases.. The foll following owing procedure assumes that a fixed number of specimens have been tested for each material variant, exposure period, and exposure stress. Some of these values will be left-censored, that is, some specimens will fail before they complete their scheduled exposure period. For such specimens the breaking load value is known to be less than or equal to the exposure stress but this procedure includes a sta statis tistic tical al met method hod for est estima imatin ting g the val values ues of tho those se dat dataa points. NOTE 2— 2—Appendi Appendix x X1 contains contains a sample Box-Cox calculation calculation that follows the procedure described in this section of the test method.
9.2 Tr Trans ansfor form m the ori origin ginal al val values ues,, X , by mea eans ns of the preliminary transformatio transformation n X tr 5
S D X X O
100
(1)
9.1 Breakin Breaking g loa load d dat dataa can be sta statis tisti tical cally ly ana analyz lyzed ed by
where X O is the average breaking load for no exposure for the given material variant. This transformation expresses the percent retention of original strength for each specimen, and thereby normalizes the residual strength of different materials. 9.3 The Box-Cox parameter parameterss are dete determin rmined ed using all data that have been generated simultaneously for relatively similar samples. For example, when testing several samples from one alloy that have been produced using various fabricating routes or are in different tempers, all data should be considered in determining the following parameters. This would also apply
following the steps outlined here. There are undoubtedly other procedures that will work but the Box-Cox transformation has demonstrated its usefulness for situations in which variance is not constant throughout the data set ( set (4 4,5). In the case of stress
to alloys from the same system. On the other hand, alloys that react differently to the test environment should be considered separately. This would be the case for comparisons of 6XXX versus 2XXX alloys, for example.
9. Statistical Analysis—Box-Cox Analysis—Box-Cox Transformation Transformation
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G 139 – 05 observed value 9.3.1 For all data cells with more than one observed (that is, noncensored value), calculate the average, m , and the standard deviation, s . Plot ln(s) versus ln(m), and determine the slope, a, of the best fit straight line. The parameter l in the Box-Cox transformation: Y 5 5 C
l X tr 2 1
(2)
is 1 − a . 9.3.2 The constant constant C can be chosen in any way that gives
5 C 5
(3)
X ltr ,max
where X tr,max is the max maximu imum m val value ue for X tr among among the noncensored values in the data set. This gives numbers in the range from 0 to 100, which is the same range as the values of X tr . 9.4 Gener Generate ate statistical statistically ly plaus plausible ible values for the censored observati obser vations, ons, repr represent esenting ing the fail failed ed speci specimens, mens, by unif uniform orm random number generation over the interval ( O, Y c), where Y c is the tra transf nsform ormati ation on of the cen censor soring ing value (th (that at is, the exposure stress). 9.5 Analy Analyze ze the compl complete, ete, transforme transformed d data set using stan stan-dard statistical techniques. A simple way of analyzing a set of data transformed to the Box-Cox metric is to find the averages and standard deviations of all cells in the data table. Since each cell has the same number of observations, the pooled estimate of the standard deviation for r cells is s p 5
Œ
2 r ! ~s21 1 s 22 1 ... 1 s 2r !~ N 2 2 r 2 2 c ! r ~ N 2
(4)
In this equation, N is the total number of observations, r the number of cells, and c the number of censored values. 9.5.1 Then the smallest differe difference nce in the averages averages of two cells that is statistically significant, the so-called least significant difference or LSD, is
ΠS D
LSD 5 t *ns p
2 n
(5)
This value can be used to compare two cells statistically to determine whether or not the data in the cells really comes from two populations with different means. 9.5.1.1 In this expression expression n is the number of observations per cell; the t-test coefficient, t , depends on the significance level chosen, and the degrees of freedom, n , are given by n
n 5 N 2 2 r 2 2 c
s p 5
Œ
n1s21 1 n 2s22 n1 1 n 2
(6)
For 95 % significance and n ' 100, t ' 2. As n becomes small, the value of t t n increases; this increases the value of the smallest difference which will be considered significant. For exact values for t , tables of student’s t-distribution must be consulted; the correct value will represent a two-tailed t-test. n
n
NOTE 3—The transformed LSD value(s) which has just been calculated applies to the entire data set over which the Box-Cox Transformation parameters were determined.
9.5.1.2 When comparing comparing data sets which have been considered separately, one should first pool the estimated variances from the two sets. For example, if the data sets are called 1 and
(7)
If bot both h var varian iance ce est estima imates tes are ass associ ociate ated d wit with h the sam samee number of degrees of freedom, the equation becomes s p 5
numbers of convenient size. One convenient choice is: 100
2, with variance estimates s12 and s22 and degrees of freedom n1 and n2 respectiv respectively ely,, the poole pooled d stand standard ard devia deviation tion will will,, in general, be
s21 1 s 22 2
ΠTo compare two averages which are not associated with the
same number of observations, n, the above expression for LSD is used, with n = n 1 + n 2 and s p equal to the above expression for the pooled standard deviation. 9.5.1.3 9.5.1. 3 A more elaborate elaborate statistical statistical analysis analysis of the data in this study can be based on the analysis of variance procedure. 9.5.2 A lower confidence confidence limit limit for the mean value value for any data cell can be calculated from the expression LCL 5 m B2C 2
t ns p
=n
,
(9)
where m B−C is the average Box-Cox transformed value and the t value represents a single-tailed t-test and is not the same as the t valu valuee used for the LSD above. For example, example, when a n
n
99 % LC LCL L is re requ quir ired ed an and d n ' 100, 100, th thee va valu luee of t is approximately 2.36. 9.6 If desired, desired, transform transform the LCL values back to either the X tr or the original X metrics. metrics. 9.7 The results results of the Box-Cox Box-Cox calculations calculations can be used to present the data graphically as in Figs. 2 and 3. 3. n
10. Interpretatio Interpretation n of Results 10.1 Str 10.1 Stress ess cor corros rosion ion cra cracki cking ng tes testt res result ultss are gen genera erally lly quite qui te rep reprod roduci ucible ble whe when n the app applie lied d str stress ess is eit either her hig high h enough to cause rapid failures of all specimens or so low that no damage is induced in the specimen. However, at intermediatee stre diat stresses sses there is consi considera derable ble varia variabilit bility y in speci specimen men performa perf ormance. nce. This varia variabili bility ty becom becomes es evide evident nt in pass pass/fai /faill testing when some but not all specimens from a group fail. Using the breaking load procedure, the variance can manifest itse it self lf ei eith ther er as sp spec ecim imen en fa fail ilur ures es or as la larg rgee va vari rian ance ce in measured residual strength. A large portion of this variability results resu lts from inhomogeniti inhomogenities es in the micr microstru ostructure cture of heat heat-treatable aluminum alloys and is independent of test procedure. 10.2 Stat Statisti istical cal results, such as the lower confid confidence ence limit and least significant difference, are intended to rank the stress corrosion cracking performance of different material variants for given environments, exposure periods, and applied stresses. 10.2.1 Becau Because se the stat statisti istical cal results are rela relative tive indicators indicators of performance in a given environment, different laboratories may not obtain the same absolute values for similar samples. This is discussed in detail in the Statement on Precision starting in 12.1 in 12.1 of of this test method. 10.2.2 10. 2.2 The These se sta statis tistic tical al res result ultss can cannot not be use used d to pre predic dictt performance in other situations (especially other environments) unless a correlation has already been developed. For example, SCC performance of low-Cu and Cu free 7XXX aluminum alloys in natural environments cannot be predicted based on
--`,,```,,,,````-`-`,,`,,`,`,,`---
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(8)
G 139 – 05 ` , , ` ` ` , , , , ` ` ` ` ` ` , , ` , , ` , ` , , ` -
NOTE 1—In this case random values have been imputed for the failed specimens. Note the non-linear nature of the Box-Cox Metric (left Y-axis) as compared to the original metric (right Y-axis). NOTE 2—The Box-Cox transformation makes variance approximately constant throughout the entire plot. The least significant difference (LSD) can be used to compare any two values to determine whether or not they are differe different nt wit with h a give given n deg degree ree of confi confidenc dence. e. Exa Exampl mples es of this are shown on the graph; the four and six day 138 MPa values are indeed different while the four, six, and nine day 310 MPa values are all similar. Contrast this with Fig. with Fig. 1 where 1 where the differences appear to be larger at the higher stress level. FIG. 2 Plot of Averages in Box-Cox Transformed Metric (same data set as Fig. as Fig. 1) 1)
br brea eaki king ng lo load ad te test stss co cond nduc ucte ted d in 3. 3.5 5 we weig ight ht % Na NaCl Cl by alternate immersion (Practice G 44 44)) with any more accuracy than with traditional pass/fail approaches (Test Method G Method G 47 47). ). Thee re Th reas ason on is th that at th thee br brea eaki king ng lo load ad pr proc oced edur uree do does es no nott compensate when the test environment correlates poorly with service environments. 11.. Repo 11 Report rt
NOTE 1—This representation shows the stress/time combinations that cause significant SCC damage. From the LCLs the sample can be seen to perform very well at all stress levels during the two day exposure and at 138 MPa for the entire nine day period. However, stresses above 138 MPa and times longer than two days cause the residual strength of the material to decline more rapidly under an applied stress than under no applied stress. str ess. Det Determ ermina ination tionss of the stat statisti istical cal sign signific ificanc ancee of the these se res results ults requires analysis of the LSD as shown in Fig. 2. 2. FIG. 3 Plot Showing Lower Confidence Limit (LCL) Values for Each Cell (from data plott plotted ed in Figs. 1 and 2) 2)
12. Prec Precisio ision n and Bias 12.1 Statement on Precision: 12.1.1 The precision precision of the data from this test method was evaluated by way of an interlaboratory test program using three tempers of Alloy 7075 with different levels of stress corrosion cracking crac king susce susceptib ptibilit ility y. All eight labor laborator atories ies dist distingui inguished shed among amo ng the thr three ee tem temper perss con consis sisten tently tly.. The res result ultss of the interlaboratory test program agreed closely with long service
11.1 The following 11.1 following infor informati mation on shal shalll be repor reported: ted: 11.1.1 Identification of all samples, including alloy, alloy, temper, product produ ct form form,, thic thickness kness,, and speci specimen men loca location tion and orie orientantation. 11.1.2 11 .1.2 All raw dat dataa inc includ luding ing ori origin ginal al ten tensil silee str streng ength, th, exposure time, stress level, and raw breaking strength of each corroded corro ded specimen. specimen. This is best done in tabul tabular ar form using cells for each stress/time combination. The table shall note any specimens that failed before removal from test along with the day tha thatt the fai failur luree was det detect ected. ed. Whe Whenev never er pos possib sible, le, it is advisable to report fracture toughness in the same orientation as the SCC cracks would propagate. For example, for rolled plate that has been tested using short transverse SCC specimens the most appropriate appropriate value would be S-L plane-stra plane-strain in fracture toughness (K IC ).
and natural environment experience forof thethe three 7075 12.1.2 The research report research lists all lists raw datatempers. for the 4 eight laboratories. Numerical comparisons based on the BoxCox transformation are extremely difficult to interpret because each laboratory obtained a different transformation coefficient. Therefore, the individual data points were plotted to provide examples of the variability that should be anticipated by users of this procedure and were statistically analyzed in accordance with Practice E Practice E 691. 691.
11.1.3 11. 1.3 All calc calculat ulated ed stat statisti istical cal quant quantitie ities. s. The mini minimum mum would be average breaking strength and standard deviation for each data cell. 11.1.4 11. 1.4 All deviations deviations from the above procedure. procedure.
laboratory variation and, within each laboratory, exhibit scatter
NOTE 4—Owing to a testing error error for one of the stress levels, one of the eight test locations has been excluded from Fig. 4 and the some of the remaining remai ning numeri numerical cal and graphi graphical cal compar comparisons. isons.
12.1.3 Fig. 4 shows plots of some raw data for two of the alloy 7075 tempers that were used in the interlaboratory test progra pro gram. m. The raw dat dataa sho show w con consid sidera erable ble lab labora orator tory-t y-too-
4
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` , , ` ` ` , , , , ` ` ` ` ` ` , , ` , , ` , ` , , ` -
NOTE 1—Plot shows that there is a correlation in the extent of damage between the T7X1 and T7X2 samples. FIG. 4 Seve Seven n Labor Laboratory atory Comparison Comparison of Raw Data
which increases as residual strength decreases. This nonuniform variance necessitates necessitates that stat statisti istical cal tech technique niquess such as the Box-Cox transformation be used. The scatter shown for the T7X1 temper is quite high because for many of the alternate immersion facilities this stress was close to a stress that would caus ca usee sp spec ecim imen enss to fa fail il be befo fore re th they ey we were re re remo move ved d fr from om exposure. 12.1.4 12. 1.4 Despite Despite the sca scatte tterr in Fig. Fig. 4, ther theree is cl clea earl rly y a consistency between the two sets of data for each laboratory. The laboratories that showed better performance for the T7X2 relati rel ative ve to oth other er lab labora orator tories ies als also o ten tended ded to sho show w bet better ter
12.1.5 The raw data from the brea breaking king load interlabora interlaboratory tory test program was statistically analyzed according to the proceduress in Prac dure Practice tice E 691. 691. The ana analys lysis is is bas based ed on sep separa arate te time/s tim e/stre tress ss com combin binati ations ons for the 707 7075-T 5-T7X1 7X1 sam sample ple.. The results are listed in Table in Table 1 with 1 with associated degree of freedom values and are plotted in Fig. 5. 5.
performance forshow the T7X1 relative other laboratories. T7X1 data also that the seventolaboratories tend to The fall into one of two groups. Three had relatively mild exposure condit con dition ionss whi while le the oth other er fou fourr had mor moree sev severe ere exp exposu osure re conditions. It is worth noting that no specimens from any of the labora lab orator tories ies fai failed led dur during ing exp exposu osure re pri prior or to the res residu idual al strength stren gth meas measurem urement. ent. There Therefore fore,, the dif differe ferences nces amon among g the facili fac ilitie tiess onl only y bec became ame evi eviden dentt whe when n the res residu idual al str streng ength th concept of the breaking load test was applied.
12.2.1 12. 2.1 the The proced pro cedure in Test Method Metho d this G 139 139 has biass bia because value of ure the breaking load in case is no defined only in terms of this test.
12.1.6 No over overall all estimates estimates of varia variance nce or corr correspon esponding ding confidence intervals have been calculated for the data because the variance is not constant throughout the data set. 12.2 Statement on Bias:
13. Keyw Keywords ords 13.1 alternate immersion; immersion; aluminum alloys; alloys; corrosion; heattreatable aluminum alloys; outdoor exposure; residual strength; SCC; stress-corrosion cracking; tension testing
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G 139 – 05
NOTE 1—Repeatability and reproducibility plotted versus overall average residual strength for the 7075-T7X1 material tested in the breaking load loa d inte interla rlabora boratory tory tes testt prog program ram.. Bot Both h stat statisti istics cs exhi exhibit bit the typi typical cal stress-c stre ss-corr orrosio osion n cra cracki cking ng beha behavior vior;; that is, the var varian iance ce incr increas eases es as residual strength is degraded. FIG. 5 Repeatability and Reproducibility Plotted Versus Overall Residual Strength TABLE TAB LE 1 Statistical Analysis Analysis of the Variance in the Interlaboratory Test Program (short transverse tests of 7075-T7X1 plate exposed to 3.5 % NaCl solution according to Practice G Practice G 44 44)) NOTE 1—For each 138 and 207 MPa exposure-stress, time-period, combination the repeatability repeatabil ity has eight degrees of freedom (DOF) and the laboratory and reproducibility have 32 DOF. The corresponding DOF values for 310 MPa are 7 and 28. NOTE 2—In addition to the repeatability and reproducibility values called for by Practice E 691, 691, the actual variability due to laboratory differences has been included here in the column, under “Laboratory.”
Exposure Time
Days 2 2 2 2 4 4 4 4 6 6 6 6 9 9 9 9
` , , ` ` ` , , , , ` ` ` ` ` ` , , ` , , ` , ` , , ` -
Exposure Stress
Average Residual Strength
Repeatability (variance within one laboratory)
Laboratory (variance between different laboratories)
Reproducibility (total variance which combines repeatability and different laboratories)
s r
95 % Confidence Inter val
s L
95 % Confidence Inter val
s R
95 % Confidence Interval
MPa (ksi)
%of Original Strength
%
%
%
%
%
%
0 138 (20) 207 (30)
96.46 96.36 93.40
1.05 1.56 5.53
0.85–1.39 1.26–2.06 4.44–7.31
2.99 2.36 4.74
1.90–6.15 1.34–4.99 1.35–10.7
3.17 2.83 7.28
2.16–5.95 2.03–4.68 5.59–10.4
310 (45) 0 138 (20) 207 (30) 310 (45) 0 138 (20) 207 (30) 310 (45) 0 138 (20) 207 (30) 310 (45)
85.47 93.65 91.22 88.33 78.00 90.99 88.84 83.70 63.62 87.26 84.62 75.07 64.70
5.59 1.50 10.56 8.59 8.80 1.66 1.75 7.52 17.78 1.24 2.73 10.31 10.05
4.44–7.56 1.21–1.99 8.49–14.0 6.91–11.4 6.99–11.9 1.33–2.19 1.41–2.31 6.05–9.95 14.1–24.0 1.00–1.65 2.19–3.61 8.29–13.6 7.97–13.6
17.72 4.73 4.43 7.54 16.39 4.85 5.50 9.30 8.97 4.80 6.21 11.05 10.58
11.0–39.4 3.03–9.72 0.00–12.6 2.34–16.9 9.47–37.0 3.09–9.97 3.52–11.3 4.79–19.9 0.00–25.6 3.11–9.83 3.86–12.8 5.05–24.1 4.23–25.1
18.58 4.97 11.45 11.43 18.61 5.13 5.77 11.96 19.92 4.96 6.78 15.12 14.59
12.3–37.9 3.36–9.44 9.27–15.0 8.76–16.5 12.8–34.3 3.49–9.66 3.91–11.0 8.78–18.7 15.8–27.0 3.33–9.63 4.68–12.3 11.3–22.8 10.7–22.6
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G 139 – 05 APPENDIX (Nonmandatory Information) X1. SAMPLE CALCULATION CALCULATION FOR BOX-COX TRANSFORMATION TRANSFORMATION
X1.1 The following following calculat calculations ions use actual actual raw data from one of the laboratories that participated in the cooperative test program. To simplify the example one temper/time combination has been displayed but, of course, the calculated values are based on the whole data set for that laboratory. Starting with the raw data for each tensile specimen, the example follows the procedure from Section Section 9 of the test method: X1.1.1 X1. 1.1 Tr Trans ansfor form m dat dataa to per percen centt of max maximu imum m res residu idual al strength using the original strength value ( X 0) of 77.5 ksi, X tr 5 ~ X / X 0! 3 100
(X1.1)
TABLE X1.2 X1.2 Raw Data Transform to Percent of Maximum Residual Strength Exposure Applied
Applied
Raw Data X tr 1 (%) X tr 2 (%) X tr 3 (%) X tr 4 (%) X tr 5 (%)
Time (Days)
Stress (ksi)
Stress (%)
6 6 6 6 9 9 9 9
0 20 30 45 0 20 30 45
0 25.8 38.7 58.1 0 25.8 38.7 58.1
87.8 86.4 81.2 Failed 82.7 83.2 75.0 81.2
86.6 86.5 77.1 Failed 82.1 82.4 77.3 Failed
86.8 86.5 84.3 61.9 82.6 85.5 81.7 Failed
87.6 83.7 79.6 Failed 83.6 83.3 69.1 81.4
86.0 86.8 83.1 82.5 82.8 76.9 62.3 Failed
X1.1.2 Tra Transfo nsform rm data to Box-Cox metric, 5 100/ ~100l! 3 ~ X tr l 2 1 ! Y 5
(X1.2)
l 5 1 2 a
where: a = the slope of the best fit line in Fig. X1.1, X1.1, in this case l = 9.76. X1.1.3 Gener Generate ate random values for failed specimens specimens in the Box-Cox metric
Y ran 5 Y exp 3 ~ Rand~0,1!
(X1.3)
where: Y exp
= the expos exposure ure stre stress ss tran transfor sformed med to the BoxCox metric using the above procedure and, Rand(0 Ran d(0,1) ,1) = a rando random m number number bet betwee ween n 0 and and 1. X1.1.4 Calcu Calculate late average and stan standard dard deviation deviation for each cell in the Box-Cox metric. X1.1.5 Calculate Calculate Least Significant Significant Dif Differen ference ce (LSD) and Lower Confidence Limit (LCL). X1.1.5 X1. 1.5.1 .1 Use the sta standa ndard rd dev deviat iation ionss cal calcul culate ated d for the Box-Cox metric to determine s p for the overall data set. For this sample data the s p = 5.14 and t = = 1.98 based on 134 Degrees of Freedom Freed om (DOF) = (200 (200 obser observatio vations) ns) − (40 cells cells)) − (26 (26 censored values). ` , , ` ` ` , , , , ` ` ` ` ` ` , , ` , , ` , ` , , ` -
X1.1.5.2 Therefore, using the two-tailed two-tailed t-test to determine the LSD with 95 % confidence the equation is TABLE X1.1 X1.1 Raw Data for Breaking Load Calculations Exposure Applied Time Stress (Days) (ksi) 6 6 6 6 9 9 9 9
0 20 30 45 0 20 30 45
Raw Data
Applied Stress (%)
TB1 (ksi)
TB2 (ksi)
TB3 (ksi)
TB4 (ksi)
TB5 (ksi)
0 25.8 38.7 58.1 0 25.8 38.7 58.1
68.1 67.0 62.9 Failed 64.1 64.5 58.1 62.9
67.1 67.1 59.7 Failed 63.6 63.8 59.9 Failed
67.2 67.0 65.3 47.9 64.0 66.3 63.3 Failed
67.9 64.9 61.7 Failed 64.8 64.6 53.5 63.1
66.7 67.3 64.4 63.9 64.1 59.6 48.3 Failed
LSD 5 1.98 3 5.14 3 =~2/5! 5 6.43
(X1.4)
and the equation to determine LCL with 99 % confidence using one-tailed t-test is, LCL 5 m B2C 2 ~2.36 3 5.14! / =25 5 m B2C 2 5.42
(X1.5)
X1.1. X1 .1.6 6 Use th thee LS LSD D to de dete term rmin inee wh whet ethe herr or no nott th thee average Box-C average Box-Cox ox value valuess (and hence the original measured measured values) are different. In this case, two cells must have average Box-Cox transformed values, m B−C , that differ by at least 6.43 to be consi considered dered statistical statistically ly dif differen ferentt at a 95 % confid confidence ence level. The entire data set is shown in Fig. 2 which includes markers for the LSD values. X1.1.7 To dete determin rminee LCL for an indi individua viduall cell cell,, subt subtract ract 5.42 5.4 2 fro from m the Box Box-Co -Cox x met metri ricc val value. ue. The Then n tra transf nsform orm the differe dif ference nce back to perce percentage ntage of orig original inal maxi maximum mum resi residual dual strength or strength value using the following equation, l
X tr 5 Y / ~100/ ~100 ! 1 1 !~1/ l!
X1.2 Summa Summary ry X1.2.1 The Aver X1.2.1 verage age Y col column umn ind indica icates tes tha thatt no str stress ess related damage occurred in the specimens that were tested at 20 ksi since the differences between the 0 ksi and 20 ksi Box-Cox transformed values (2.61 for six days and 0.17 for nine days) were less than for the LSD value (6.43) calculated above. On the other hand, stress related damage occurred in the 30 ksi specimens and to a greater extent in the 45 ksi specimens. X1.2.2 The LCL values in the last column correctly correctly show that the specimens are subject to failure when the applied test stress is 45 ksi. The 45 ksi/six day result of zero comes from the mb−c value value bei being ng les lesss tha than n the subtract subtractor or for the LCL calculation. The LCL values in the original residual strength or percent of original strength metrics can be plotted to show SCC performance as a function of time as shown in Fig. 3. 3. However, the LSD is still required to test differences between cells.
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(X1.6)
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G 139 – 05 TAB TABLE LE X1.3 Value Values s From the Box-Cox Transformation Transformation for Each Specimen and Cell NOTE 1—Boldface numbers were randomly generated from X1.1.3 from X1.1.3.. Box-Cox Values for Each Tensile Bar
Exposure Time
Applied Stress (ksi)
Applied Stress (%)
Y 1
6 6 6 6 9 9 9 9
0 20 30 45 0 20 30 45
0 25.8 38.7 58.1 0 25.8 38.7 58.1
28.2 24.1 13.0 0.035 15.6 16.6 6.0 13.0
Y 2
24.7 24.4 7.9 0.396 14.5 15.0 8.1 0.221
Y 3
25.0 24.2 18.8 0.9 15.4 21.7 14.0 0.431
Y 4 27.5 17.6 10.8 0.203 17.4 16.8 2.7 13.4
Average Y Average Y
Y 5 23.1 25.1 16.4 15.2 15.7 7.7 1.0 0.043
25.69 23.08 13.39 3.35 15.75 15.58 6.35 5.43 5.
Standard Deviation 2.1 3.1 4.4 6.6 1.0 5.1 5.1 7.1
TABLE X1.4 Final Value Values s of the LCL for Each Cell Exposure Time
Applied Stress (ksi)
Applied Stress (%)
Average Y
Average X tr (%)
LCL (%)
LCL (ksi)
6 6 6 6 9 9 9 9
0 20 30 45 0 20 30 45
0 25.8 38.7 58.1 0 25.8 38.7 58.1
25.69 23.08 13.39 3.35 15.75 15.58 6.35 5.43
87.0 86.1 81.4 70.6 82.8 82.7 75.4 74.2
84.9 83.7 77.2 0 79.3 79.1 61.9 40.3
65.8 64.9 59.8 0 61.4 61.3 48.0 31.2
NOTE 1—The log of the cell averages of residual strength plotted versus the log of the cell standard deviations. No imputed values are included in this plot. The linear best fit line has the equation:
5 17.18 2 8.796 X , R ˆ 2 2 5 0.683 Y 5
(7)
FIG. X1.1 Log of Cell Averages of Residual Strength
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G 139 – 05 REFERENCES (1) Sprowls, D. O., Bucci, R. J., Ponchel, B. M., Brazill, R. L., and Bretz, P. E., “A Study of Environ Environmental mental Characterizati Characterization on of Conven Conventional tional and Advanced Advanced Alu Aluminu minum m Allo Alloys ys for Sel Select ection ion and Des Design. ign. Pha Phase se II—The Breaking Load Method,” NASA CR-172387 , National Aeronautics and Space Administration Administration,, Washingto ashington, n, DC, 1984. (2) Colv Colvin, in, E. L., and Emp Emptag tage, e, M. R., “The Bre Breakin aking g Loa Load d Met Method: hod: Results and Statistical Modifications from the ASTM Interlaboratory Test Te st Progra Program,” m,” New Met Methods hods for Corr Corrosio osion n Testi esting ng of Alum Aluminum inum
(3) Luk Lukas asak ak,, D. A., Bu Bucc cci, i, R. J. J.,, Co Colv lvin, in, E. L. L.,, an and d Lif Lifka ka,, B. W., “Damage-Based Assessment of Stress Corrosion Performances Among Aluminum Alloys,” New Methods for Corrosion Testing of Aluminum Alloys, ASTM STP 1134, ASTM, 1992, p. 101. (4) Fung, C. A., “Statistical Topics in Off-Line Quality Control,” Ph.D. Thesis, University of Wisc Wisconsin-M onsin-Madison, adison, Madison, WI, 1986. (5) Emp Emptage tage,, M. R., and Hin Hinkle, kle, A. J., Proceedings, Join Jointt Sta Statist tistica icall Meetings, Meetin gs, American Statistical Association, Association, Anaheim, CA, August 7,
Alloys, ASTM STP 1134, ASTM, 1992, p. 82.
1990.
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