SIXTH PROGRESS REPORT on EFFECT OF LONG- TIME CREEP ON STRUCTURAL SHEET MATERIALS T. Mo. Cullen J. W. Freeman The University of Michigan Contract AF' 33('66)-8334 Project 1(8-7381) Task 738'12 Aeronautical Systems Division Air Force Systems Command United States Air Force Wright-Patterson Air Force Base, Ohio

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INTRODUC TION This research program was initiated for the purpose of obtaining data on the influence of long-time exposure at 550'F on the me chanical properties of AM350 sheet materialo The data obtained in the present program are to be used in the evaluation of the alloy for possible use in the construction of a trisonic transport airplane. It is anticipated that the principles developed in this investigation regarding the influence of exposure on the strength properties of AM350 alloy can be extended to other alloys of a similar type. AM350 material in both the SCT and the CRT conditions is being exposed under a stress of 67, 000 psi. This stress was selected on the basis of being representative of the most probable design stress for the aircraft. Exposure times of 2000, 5000, 12, 000 and 30, 000 hours at 550'F have been incorporated into the research program to determine the influence of time of exposure on mechanical properties. In addition, a limited number of tests have been included to evaluate the possibility of using shorter-duration higher temperature exposures to predict changes in mechanical properties to be expected during a service life of 30, 000 hours. The nature of this program is such that rather long time periods exist between those times when new data can be reported. As a result of this scheduling, no tests have been completed since the last progress report was submitted. As a consequence of this and in an effort to make the present report complete in itself this report will repeat a number of the sections of the last progress report, The section dealing with the status of the exposure tests has been revised and brougltup to date. EXPERIMENTAL PROGRAM Test Materials and Procedures The AM350 sheet material tested in this program was obtained from a single consumable-electrode melted heat produced by the Allegheny-Ludlum

Steel Corporation. This material, Heat No. 23327, had the following reported chemical composition: C Si Mn Cr Ni Mo N P S.084 21. 65 16. 50 4. 29 2. 94 10.009 o007 Thirteen sheets, each 12 inches wide by 24 inches long, of 0. 025 inch thick material were received after having been worked 20 percent. Longitudinal specimen blanks from three of these sheets were tempered to the CRT condition by a three hour treatment at 8500F prior to specimen machining. A similar number of the cold worked sheets were returned to the producer where they were cut into panels 12 inches wide by 8 inches long. These panels were annealed at 1950~F, pickled, conditioned for 5 to 10 minutes at 1710F and air cooled. Specimen blanks from five of the panels were sampled in the longitudinal direction at the University of Michigano Conversion to the SCT condition was accomplished by cooling to -100'F for 3 hours followed by tempering at 850~F for 3 hours, all prior to specimen preparation. Special furnaces were designed and constructed to uniformly heat the wide double gage length specimen shown at the bottom of Figure 1. The SCT panels were too short to be machined into this type of specimen and are, therefore, being exposed as single specimens. At the start of the exposure the specimens were loaded by means of pins inserted through holes at the ends of the specimen, Creep readings are being taken from the smooth section of the specimen shown at the top of Figure 1, No creep readings are being taken on the specimens of the type shown at the bottom of this figure. After exposure, a tensile test is conducted at room temperature on one of the test sections of the double specimen exposed to creep. The remaining section can then be used for a test at either room temperature or 550~F as the need may arise. Special adapters pull against the shoulder fillets to avoid stressing at the second gage section of the double specimen during tensile testing of the first. Superficial hardness measurements were taken from most specimens

prior to exposure. Hardness values after exposure are to be reported for the two conditions where readings after the exposure can be taken in the region of uniform exposure stress without affecting subsequent tensile tests: (a) wide unnotched specimens, with hardness readings in the gage section, but not in the plane of the notch, and (b) unstressed exposure for which hardness readings are to be taken in the specimen shoulders. Influence of Stressed Exposure at 550~F The effect of exposure at 550'F for 30, 000 hours under a stress of 67, 000 psi will be measured by: (a) The change in the short-time tensile properties at room temperature and 550~F of unnotched strip specimens. (b) The change in the short-time tensile strength of edge notched (Kt 3) specimens at room temperature and at 550~F after exposure with the notch present. This notch is intended to simulate the presence of an area of concentration in the material. (c) The change in ability to withstand a very sharp notch introduced after exposure. Unnotched specimens are being exposed. After exposure, ASTM sharp edge notches will be machined into the specimens, and tensile tests conducted at room temperature and at 5500F. This procedure was selected on the basis that sharp notches and cracks should not be originally present in the airplane. This should constitute a severe test of changes during exposure in the ability of the material to withstand sharp notches or cracks. A sharp notch present during exposure should cause creep relaxation and thereby should provide a less severe test of changes in notch sensitivity than a notch introduced after exposure. Effect of Exposure Time In addition to the exposure for 30, 000 hours, exposures for shorter times prior to tensile testing will provide date for study of possible methods of extrapolating from short time exposures, and also will provide factual interim data without the necessity of waiting 30, 000 hours for an ndi

cation of the influence of exposure. Exposure times of 2000, 5000, and 12, 000 hours are being used. Unnotched and notched (Kt = 3) specimens are being exposed for 12, 000 hours. At the completion of the exposure the specimens will be tensile tested at room temperature and at 550'F. The 5000 and 2000 hour exposures included similar tests, plus two specimens in which sharp notches were machined after exposure and prior to tensile testing at room temperature and 550 F. Effect of Stress during Exposure A very limited study of the influence of stress during exposure is being made by the inclusion of a few specimens in the exposure furnaces which have no applied stress. Exposure times of 30, 000, 12, 000, and 5000 hours are being evaluated, with subsequent tensile tests to be conducted at room temperature. Accelerated Exposure Mechanical property changes induced by exposure to elevated temperatures should involve reactions of the type obeying the Arrhenius rate equation. If this is true, then it should be possible to use shorterduration higher temperature exposures to predict changes in mechanical properties to be expected during a service life of 30, 000 hours at 550~F. In order to test this hypothesis the Arrhenius rate equation was used to select exposure times at 600~F, 650~F, and 700~F which should induce equivalent mechanical property changes to those expected in 30, 000 hours at 550~Fo The corresponding times selected were 2000 hours at 600 F, 200 hours at 650~F, and 20 hours at 700'F. In addition, tests were run for 200 hours at 7000F to allow for the possibility that the rate constant selected for the Arrhenius equation was different from that expected.

PRESENT STATUS OF THE PROGRAM The 2000 and 5000 hour exposures of the AM350 sheet material have been finished and the tensile tests of the exposed samples completed. In addition, the accelerated exposures at 600'F, 650'F, and 700'F have been run and the tensile tests of these samples completed. The results of these tests are reported in Table I. The 12, 000-hour and 30, 000-hour exposures are now in progress. The status of these exposures is shown in Table II. The 12, 000-hour exposures are very near completion and will be finished in the next few weeks. The next progress report will include the data obtained from the tests on these specimens. The 30, 000-hour exposures now have been running approximately 16, 000 hours and should be completed about a year and one-half from now. RESULTS TO DATE The AM350 sheet materials do not appear to be subject to creep at 550~F under a stress of 67, 000 psi. During the first 16, 000 hours of exposure, measurements indicate that the total amount of creep has been less than 0. 01 percent. It is doubtful that the creep which has occurred to date is of any significance. The results of the completed tensile tests along with the hardness data are reported in Table I. In Figure 2, the ultimate tensile strengths at 550'F and at room temperature of the three different types of specimens, together with the elongations for the unnotched specimens, are plotted as a function of exposure time at 550~F. The following trends have been revealed by study of the data obtained thus far in the investigation. These trends have been divided into two categories, (1) properties of the unexposed material, and (2) properties of the material as influenced by exposure. Properties of the Unexposed Material (1) The alloy in the SCT condition is slightly weaker than the alloy in

the CRT condition at room temperature, however, at 550~F it is considerably stronger. (2) The notches of the intermediate acuity (Kt = 3) raised the netsection strength at 550'F and at room temperature for the alloy in the SCT condition. The alloy in the CRT condition also exhibited an increase in notch strength at 550~F, however, at room temperature there was very little influence of notches of intermediate acuity on the net-section strength of the material. (3) Sharp edge-notches reduced the net-section strength of the SCT material at 550~F by approximately 35, 000 psi. The alloy in the CRT condition had approximately the same net-section strength as the unnotched material at both room temperature and at 550~F. Properties of the Alloy as Influenced by Exposure (1) In both the CRT and the SCT conditions the strength properties of the material have not been significantly changed as the result of stressed or unstressed exposure at 550'F for 3000 or 5000 hours. (2) A slightly but consistently lower elongation after 5000 hours of exposure than after 2000 hours suggests that a reaction could be occurring during exposure which causes a continuing loss of ductility with prolongation of exposure. (3) The accelerated, equivalent time exposures carried out at 600~F: 650~F, and 700~F caused no appreciable changes in the strength properties or in the elongation of the AM350 sheet material in either the SCT or the CRT condition. (4) The yield strength and hardness data reported in Table I show that little change occurred in either property as the result of exposure at 550~F. The only exception to this was an apparent drop in yield strength at room temperature from 179 to 159 ksi for the SCT material exposed for 5000 hours without stress. The results to be obtained from the 12, 000 hour exposures should give an indication of whether or not this apparent drop in yield strength was actually the result of exposure.

Table I RESULTS OF TENSILE TESTS FOR AM350 SHEFT E. xposure Conditions Test Subsequent Tests on Unnotched Specimens Notched (Kt = 3) asharp Edge Notches Temp. Stress Time Temp P.L. Offset Yield Strengths (ksi) Tensile Elongation (%) Tensile Strength Rockwell "45N" Hardness Tensile (~F) (ksi) (hrs) (~F) (ksi) 0.02% 0. 1% 0.2% Strength(ksi) Per 2" Per 0.5" (ksi) Before Exp. After Exp. $tren[~th (ksi) None Room 93 122 168 185. 5 218.5 28 -- 225.5 51 -- 214 None Room 100 126 165 191 217 16.5..... 51.5 - - 215. 1 None Room 110 137 171.5 182 224.2 23.5............. 550 4O 2000 Room 89 107 155 212.5 21 34........... b 178 550 67 2000 Room......... (178) 27-1.8 22 -- 2?0 52 51.5 212 550 90 2000 Room 119 142.5 154.5 186 221.4 16.5............. 550 150 2000 Room (145) 174 198 (208) 221 1 19.5............. 550 0 5000 Room 120 140.5 170 178. 5 219 20............. 550 67 5000 Room 150 174 182 185.5 222.2 19 -- 223.8 53.5 51.5 213 600 67 2000 Room 114 139 168 181 217.81 17 28........... 700 67 200 Room (70) 111 158 183 215.8..... 52.5 52 211.4 700 67 200 Room...................... 52 51 209.8 None 550 95 120 144 153 169 4 -- 185 50.5 -- 172. 1 None 550 96 115 143 153 168.8 4.5..... 50.5 -- 166.8 550 0 2000 550 89 112.5 135.5 147.5 171.2 4 10........... 550 67 2000 550 85 105 132 141 163.5 2 6 178.2 52 -- 160 550 67 5000 550 110 126 144 151 168.6 2 -- 181.8 53.5 51.5 170.5 600 67 2000 550 92 113 141 153 172 5 -- 175. 7 650 67 200 550 85 101 139 154 170.8 4 8........... 700 67 20 550 80 97 130 145 170. 7 4 6........... 700 67 200 550 95 104 140 152.5 188 4.5..... 52 51 c(>151) 700 67 200 550 103 117 139 150.5 171.2 4 8 --- 52.5 52 172 None Room 113 143 170 185.3 214.9 17 32 241.5..... 216 None Room 119 139 165 178 213. 1 12 26........ 208. 3 None Room 105 129 162 176 214 16.5 32........... 550 67 2000 Room................... 238.8..... 196. 5 550 67 2236 Room 123 146.8 170 179 212 18 -- 237.5........ 550 0 5000 Room 90 103 136 159 214.5 14............. 550 67 5000 Room 120 137 163 176 216 13.3 -- 238.5..... 209.4 700 67 200 Room...................... 53.5 52 208.8 None 550 70 89 119 135 193.6 5 12 210..... 159.3 None 550 80 98 126.9 141 194.4 6.5 12........ 159 550 67 2000 550 95 113.5 138 150 195.9 8.5 16 --- 54.5 55 159.2 550 0 2236 550 70 92.3 125.5 142 199.5 7.5............. 550 67 2236 550 80 106 132 147.5 199 7.5 -- 206. 1........ 550 67 5000 550 110 114 134 145 193.5 4.5 -- 208..... 161.5 600 67 2000 550 81 101 132 145. 5 198. 5 5. 5............. 650 67 200 550 75 95 122 140 195.8 6 12........... 700 67 20 550 87 101 131 146 200.3 5.5..... 53.5 51 163 700 67 200 550 96 118 141 151.5 201.2 6.............

TABLE II STATUS OF EXPOSURE TESTS 12,000 - Hour Exposures at 550~F Specimen Specimen Alloy Accumulated Estimated Stress Code Type(a) Condition(b) Time to Date (Hrs.) Completion (psi) C-7 U SCT 11736 1/11/64 None D-2 U SCT 11544 1/19/64 None D-3 U SCT 11544 1/19/64 67,000 D-5 U SCT 11544 1/19/64 67,000 34 U CRT 11736 1/11/64 None 45 U CRT 11736 1/11/64 67,000 46 U CRT 11736 1/11/64 67,000 43 U CRT 11544 1/19/64 None D-2 N SCT 11544 1/19/64 None D-3 N SCT 11544 1/19/64 67,000 D-5 N SCT 11544 1/19/64 67,000 45 N CRT 11736 1/11/64 67,000 46 N CRT 11736 1/11/64 67,000 43 N CRT 11544 1/19/64 None 30, 000 - Hour Exposures at 550~F E-6 W SCT 16464 7/17/65 67, 000 E-7 W SCT 16464 7/17/65 67,000 E-4 U SCT 16200 7/28/65 67,000 C-6 U SCT 16200 7/28/65 None E-1 U SCT 16560 7/13/65 67,000 C-3 U SCT 16560 7/13/63 None C-1 U SCT 14568 10/ 4/65 67,000 E-2 U SCT 14568 10/ 4/65 67,000 D-4 U SCT 14568 10/ 4 /65 None 37 W CRT 16368 7/21/65 67,000 55 U CRT 16464 7/17/65 None 36 U CRT 16200 7/28/65 67,000 56 U CRT 16368 7/21/65 None 32 U CRT 16968 6/26/65 67,000 52 U CRT 16968 6/26/65 67,000 53 U CRT 16968 6/26/65 None 41 U CRT 16560 7/13/65 67, 000 E-4 N SCT 16368 7/28/65 67,000 E-1 N SCT 16560 7/13/65 67,000 C-1 N SCT 14568 10/4 /65 67,000 E-2 N SCT 14568 10/ 4 /65 67, 000 36 N CRT 16368 7/28/65 67,000 32 N CRT 16968 6/26/65 67,000 52 N CRT 16968 6/26/65 67,000 41 N CRT 16560 7/13/65 67, 000 a) U = Unnotched, 0.350-inch gage width; N = Notched, Kt = 3; W = Wide unnotched during exposure, sharp edge notches for tensile tests. b) CRT = Cold rolled 20 percent plus three hours at 850~F SCT = Annealed at 19500F, conditioned for 10 minutes at 17100F, A. C., held three hours at -1000F and tempered three hours at 8500F

0.025" ROOT RADIUS I/ 2" D. EXT EN SOM ET ER Pi N HOLES 0.350" 0.5" 0.350" 1.2" 1/2" 0.9"1 3.35" 2.75" V2 8"1 I"R. OUTLINE FOR SINGLE SPECIMIN SHARP NOTCH ADDED ATR5/8'' D.CREEP PERIOD. 11-T~~~~~~ fit 0.7" 1.6.63/4"- 4.511 -- 6.75" 12" Figure 1 - Specimen for Long-Time Creep and Subsequent Tensile Testing.

SCT Condition CRT Condition 240- ALcele^ot d, 240 - - Acselerated, x osursEq ivlent Time Expss qial Tie posures.=20E -1 dEtrus0 2 am500FRo 220 _ ii 22=0 I II~ 1 ni 20O I$_ _1L I I,,- ~ ~ ~ ~ ~ ~ ~ ~ 0A 01 0 2,000 5,000 12,000 30,000 0 2,000 5,000 12,000 30,000 30. 30 Duration of Creep ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~Exposure, Hour 55DAclratio fCed Exposure, or X X~~~~~. 20 0 0 Figure 20. E2ect ofecimeS Exposure at 550~ on Tensile Strength and Ductility of Vacuum-MelType A 50 S at 550 F Rnom C14~~~~~~~~~~~~~~~~~~C1 Room0Temperature. Predided Strengths and Dudihities Based on the ResultsUnnatch o t T C d.2 Nothed, Kt 3 E T o a ]0 I4~~~~L_'~~.,,1 i I_ JU I I 0~~~~~~~~~~~~~~~~~~ 160 ~~~~~~ - 16C~~~~ - -0 0 2,000 5,000 12,000 302,000 5,000 12,000 30,000 Duration of Creep Exposure, Hours Duration of Creep Exposure, Hours Figure 2. Effect of Exposure at 550~ on Tensile Strength and Ductility of Vacuum-Melted AM350 Sheet at 550~ F and at Room Temperature. Predicted Strengths and Ductilities Based on the Results of Accelerated Tests Conducted at 600T, 65S50 and 700R F Are Plotted at an Equivalent Exposure Time of 30,000 Hours.

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