1、NASA CONTRACTtM? -m-s- T - h d THE EFFECT OF ENVIRONMENT ON THE FATIGUE STRENGTHS OF FOUR SELECTED ALLOYS by T. R. Shives mzd J. A. Bennett Prepared under Contract No. R-14 by NATIONAL BUREAU OF STANDARDS Washington, D. C. f Or NATIONAL AERONAUTICS AND SPACE ADMINISTRATION . WASHINGTON, D. C. . AUGU
2、ST 1965 Provided by IHSNot for ResaleNo reproduction or networking permitted without license from IHS-,-,-I- - - TECH LIBRARY KAFB, NM ll099b89 NASA CR-267 THE EFFECT OF ENVIRONMENT ON THE FATIGUE STRENGTHS OF FOUR SELECTED ALLOYS By T. R. Shives and J. A. Bennett Distribution of this report is prov
3、ided in the interest of information exchange. Responsibility for the contents resides in the author or organization that prepared it. Prepared under Contract No. R-14 by NATIONAL BUREAU OF STANDARDS Washington, D. C. for NATIONAL AERONAUTICS AND SPACE ADMINISTRATION For sale by the Clearinghouse for
4、 Federal Scientific ond Technical Information Springfield, Virginio 22151 - Price $1.00 I - Provided by IHSNot for ResaleNo reproduction or networking permitted without license from IHS-,-,-III - THE EFFECT OF ENVIRONMENT ON THE FATIGUE STRENGTHS OF FOUR SELECTED ALLOYS By T. R. Shives and J. A. Ben
5、nett National Bureau of Standards SUMMARY Rotating beam fatigue tests of unnotched specimens of AlSl 4340 steel, free-cutting brass, titanium alloy Ti-4Al-4Mn, and magnesium alloy AZ6lA showed that the fatigue strength was lower in a moist environment than in a dry one. A coating of dodecyl alcohol
6、was found to have a beneficial effect on the fatigue strength of the steel and magnesium alloy specimens, but did not completely eliminate the effect of humidity. The effect of environment on crack propagation was investigated with notched specimens of the magnesium alloy. An interesting tarnishing
7、effect was observed on the fatigue portion of magnesium fracture surfaces. INTRODUCTION More than 30 years ago Gough and Sopwith (1) found that the fatigue strengths of many structural metals were lower in a normal atmosphere than they were in vacuum. In recent years there has been a renewed interes
8、t in many aspects of atmospheric corrosion fatigue, and the National Bureau of Standards has been conducting investigations in this field under National Aeronautics and Space Administration sponsorship. An earlier paper (2) reviewed the pertinent information in the literature up to early 1961; the p
9、resent paper is a report of further work on this project. Broom and Nicholson (3) established that water vapor is the only con- stituent of the normal atmosphere that affects the fatigue behavior of age- hardened aluminum alloys. The situation is not so clear for other structural metals and there is
10、 a need for additional work to evaluate the effects of particular atmospheric constituents on the fatigue strengths of a variety of metals. Knowledge of the effects of surface reactions will help in under- standing the mechanism of fatigue crack initiation, as it has in the case of aluminum alloys (
11、4). Since it is relatively easy to control humidity, and water vapor is thought to have an important effect on the fatigue properties of many metals, the first group of experiments was planned to evaluate the effect of varying humidity. The materials selected included four of the most common classes
12、 of structural metals except aluminum alloys; these were omitted because they had been studied previously (4). Provided by IHSNot for ResaleNo reproduction or networking permitted without license from IHS-,-,-Another purpose of this investigation was to determine if the beneficial effect of certain
13、polar organic liquids (5) is due only to the ability of the liquid film to protect the surface of the metal. Accordingly some of the materials were tested with a film of dodecyl alcohol as well as in the clean condition. Previous work had shown that this liquid was convenient to use and resulted in
14、nearly the same improvement in fatigue behavior as the most effective compounds invest1 gated (5). SYMBOLS The symbols used in this paper are defined below. SA Stress amplitude NF Cycles to fai lure NP Cycles for crack propagation * Runout - 20 x 106 cycles without failure. Provided by IHSNot for Re
15、saleNo reproduction or networking permitted without license from IHS-,-,-MATER I ALS AND TEST I NG PROCEDURES The four materials selected for this investlgatlon are listed In Table I. TABLE I. MATER I ALS Material Condition 1 . AISI 4340 Steel 2. Titanium Alloy Ti-4Al-4Mn 3. Brass, Federal Specifica
16、tion Composi tlon 22 4. M;e;ium Alloy Ser. 1-Q - 1475F T - 1100 F 33 Rc Ser. 11. Q - 1475 F T - 1150 F 29 R, Hot rol led, annealed One-half hard 78 RB Extruded Analysis (a) C 0.40 $ Cr 0.77 Mn 0.67 MO 0 -20 Nl 1.74 Sl 0.27 Al 4.2 $ C co.1 Mn 3.8 H2 0.006 N2 0.02 cu 60-63 $ Fe 0.15 max Pb 2.5-3.7 Zn
17、33-36 Others 0.50 Al 5.9 Q cu co.01 Fe 0.005 Mn 0.38 NI 0.001 St co.05 Zn 0.82 Tens1 le Strength (b) PSI MN/m2 156,000 1076 149,000 1027 62,100 428 . 47,800 330 (a) Analysis by producer for materials 1, 2, and 4; specification limits only given for brass, no analysis made. (b) Data for materials 1,
18、3, and 4 obtained by authors; data for Tl- c 30 113 102 70 82 ;: 47 40 32 61 59 42 5: 28 Serle Clean Low High Humidity Humidity ii Coated with Dodecyl Alcohol Low High Humidity Humidity Provided by IHSNot for ResaleNo reproduction or networking permitted without license from IHS-,-,-In figure 4 no d
19、istinction is made between low and hlgh humidity tests, the S-N curves being plotted to show the effect of the dodecyl alcohol coating only. In all cases, the coating was beneflclal, but the improvement was not as great as that found In the earlier experiments with the same alloy reported in referen
20、ce 5. Titanium Ti-4Al-4Mn Alloy There was considerable dispersion in the results for the titanium alloy specimens, as shown In Table III. in order to be able to observe the trends more easily, the values plotted In figure 5 were obtained by averaging the results at two stress amplitudes one ksi (6.9
21、 MN/,2) apart. it is clear that high humidity has a deleterious effect on the fatigue limit, the difference at 50$ runouts being equivalent to about 5% in stress amplitude. No attempt was made to obtain data in the finite life part of the S-N curve. There was more difficulty with fretting in the col
22、lets with these specimens than with the other materials tested. The flrst titanium specimens tested had a diameter of 0.24 in. (0.608 cm.) In the reduced section. During testing, fatigue caused by fretting between the specimen shank and the collet used to hold the specimen in the spindle produced pr
23、emature failure in the shank in several specimens. The diameter of the test section was reduced to 0.20 in. (0.508 cm.) in later specimens. Failure in the shank due to fret- ting was not entirely eliminated, but it was greatly lessened. All of the data presented in Table III and figure 5 were obtain
24、ed from 0.20 in. (0.508 cm.) diameter specimens. The titanium alloy exhibited a true fatigue limit behavior at a stress amplitude of approximately 100 ksl (690 MN/m2). This Is remarkably high in comparison with most steels and aluminum alloys on the basis of either fatigue strength: density ratio or
25、 fatigue strength: tensile strength ratio, the latter being nearly 2/3. Panseri and Mori (7) found an almost equally high fatigue limit: tensile strength ratio for a weaker Ti-6Al-4V alloy. Composition 22 Brass The results for clean brass specimens tested in both low and high relative humidity are s
26、hown in Table IV and in figure 6. The fatigue strength !n the high humidity environment was about 3% lower than that In a dry atmosphere throughout the range investigated. Magnesium AZ6lA alloy Smooth specimens of AZ61A magnesium alloy were first made with a diameter of 0.30 in. (0.762 cm.) in the r
27、educed section. After several specimens failed in the shank, the specimen diameter was reduced to 0.24 in. (0.608 cm.), which almost eliminated the difficulty. Provided by IHSNot for ResaleNo reproduction or networking permitted without license from IHS-,-,-I TABLE I I I. FAT I GUE TEST RESULTS FOR
28、TITAN I UM T I -4AL-4MN sA Ksf MN /m2 Low Humidity High Humidity 92 634 95 655 98 676 99 100 101 102 103 105 107 683 689 696 703 710 738 m 84 x io 3 78 x lo3 78 164: 100 x 103 56 x 103 85 3953 -j2 x 103 115 139 59 x 103 81 187 NF 38 x io3 112 51 x 103 b 41 x 103 65358 b 33 x 103 49 71 34 x 10 3 2: 2
29、05 Provided by IHSNot for ResaleNo reproduction or networking permitted without license from IHS-,-,-TABLE IV. FATIGUE TEST RESULTS FOR FREE-CUTTING BRASS S A Ksi MN/,2 30 207 31 214 32 221 34 234 36 248 38 262 Low Hum1 di ty I No. of NF* Specimens I 3 4005 x 103 I 2 2473 2 2809 3 1350 1 784 3 627 H
30、igh Humidity No. of Specimens NF” 3 2712 x io3 1 191 1 1883 3 956 1 69 3 529 * Antilog log mean values Provided by IHSNot for ResaleNo reproduction or networking permitted without license from IHS-,-,-The results for the smooth magnesium alloy specimens are shown in Table V, and the S-N curves in fi
31、gure 7. There was a good deal of dispersion In the data, but the effect of moisture on the fatigue behavior was pronounced. The deleterious effect of high humidity was greater at high stress levels than at the low end of the range investigated. Coating with dodecyl alcohol had a beneficial effect in
32、 the moist environment and to a lesser extent in the dry atmosphere. The difference in fatigue strength between the best ( 1 ow huml cl! tyr coated) condition and the poorest (high humidity, clean) ranged from 13% at NF = 350 x 103 to only 3% at NF = 2000 x 103. In terms of fatigue life this is a sm
33、aller improvement than that reported in reference 5 for the much weaker cast material. No consistent difference could be noted between the results with speci- mens that had been polished wet and those that had been polished dry. A series of tests was conducted in order to determine the effect of hum
34、idity on crack propagation, using notched specimens of magnesium AZ6lA. All tests were run at SA = 10 Ksi (69 MN/m2). Prellminary tests were conducted to determlne the number of cycles requlred to form a small crack. Specimens wei- sectioned after various numbers of cycles and examined microscopical
35、ly, and it was found that a crack was usually present in clean specimens after 105 x 103 cycles in a low humidity atmosphere. On the basis of these results, all specimens were run clean under low humidity conditions for 105 x 103 cycles, then they were divided into four groups, each group being run
36、to failure under one of the following conditions: a Clean, low relative humidity b) Clean, high relative humidity 4 Coated with dodecyl alcohol, low relative humidity d) Coated with dodecyl alcohol, high relative humidity The results of these tests are shown in Table VI. It Is clear that both humidi
37、ty and the polar liquid had a significant effect, and the two effects seem to be nearly independent. The high humidity reduced the average crack propagation life by 64% from the life under condition “a” while the presence of the liquid film raised It by 35%. 9 Provided by IHSNot for ResaleNo reprodu
38、ction or networking permitted without license from IHS-,-,-TABLE V. FATIGUE TEST RESULTS FROM SMOOTH MAGNESIUM AZ6lA sA Ksi MN/m 22 152 7 6082 x 103 23.5 162 7 712 25 1-P 17 375 27 186 8 311 sA KS i MN/,2 22 152 23.5 162 25 172 27 186 I Low Humi di ty Clean No. of Specimens Coated wl th Dodecyl Alco
39、hol No. of Specimens NF* 3 5369 x 103 9 450 5 344 I Hiqh Humidity Clean No. of Specimens NF* 11 1922 x 103 4 8245 13 158 10 89 , Coated with Dodecyl Alcohol No. of NF* Specl met-is 4 3176 x io 3 9 240 5 152 * Antilog log mean values 10 Provided by IHSNot for ResaleNo reproduction or networking permi
40、tted without license from IHS-,-,-TABLE Vi . FATIGUE TEST RESULTS FOR NOTCHED MAGNESIUM AZ6lA (SA = 10 KSI, 69 MN/m2) (All specimens were stressed for 105,000 cycles under condition (a) to initiate a crack. Np does not include these cycles.) Condition No. of Soecimens NP * a Low relative humidity, c
41、lean 7 521 x 103 b High relative humidity, clean 3 201 C Low relative humi di ty, coated with dodecyl alcohol 3 735 d High relative humidity, coated with dodecyl alcohol 4 243 * Antilog log mean values TARNISHING OF FRACTURE SURFACES An interesting type of film formation was noted on fractures of th
42、e magnesium alloy specimens. When first broken the surfaces were bright, but several months after testing some of the surfaces had become discolored like that at the left in figure 8. Upon examining all of the fractured magnesium specimens, It was found that only those that had been tested in hlgh h
43、umidity had tarn1 shed in this way. The discoloration could be observed only on specimens that had been stored In closed containers, because other- wise the entire specimen acquired a grey coating. The tarnish film, which varied in color from greenish tan to dar.k grey, was exactly the same on both
44、sides of each fracture (fig. 9) and was 1 imi ted to that part of the surface which had failed in fatigue. By runnlng more specimens and periodically looking at the fractures, it was found that the tarnish became noticeable within two to three weeks after fracture. Efforts to determlne the nature of
45、 these films have thus far been unsuccessful. DISCUSSION All of the materials tested showed higher fatigue strengths in a dry atmosphere than in a moist one. However , theeffect was relatively small except for the magnesium alloy. The results of the present experiments indicate that the decreases in
46、 fatigue strength for the steel, brass and 11 Provided by IHSNot for ResaleNo reproduction or networking permitted without license from IHS-,-,-titanium alloy were less than 5$, whereas Bennett (4), for example, has reported a decrease of 14% for an aluminum alloy over approximately the same range o
47、f humidity. A part of this difference may be due to the higher speed of the present tests, as it was shown in earlier work on this project (6) that the influence of high humidity is more pronounced at 3000 cpm than at 9000 cpm. The tests reported in reference 4 were run at only 1800 cpm. Despite the
48、 possible mitigating effect of the high testing speed, it appears that the influence of humidity on fatigue strength of brass is much less than the effect of all atmospheric constituents, as indicated by the vacuum tests of Gough and Sopwith (1). They reported an improvement of 26% in the fatigue limit of annealed brass when tested in vacuum as compared with tests in air. Thus it is probable that constituents other than water vapor have an influence on the fatigue behavior of brass. There are few data in the literature with which to compare the results on the 4343 st
