SEVENTH PROGRESS REPORT EFFECT OF 12, 000-HOUR CREEP EXPOSURE ON STRUCTURAL SHEET MATERIALS T. Mo Cullen J. W. Fre-em nan The Univers-ity.o-f M 6.ic'h"g:.n C ont., 33.... -8334 Pr 7ojec'8?7 3' 11) Project f(8-7381) Task 73812 Aeronautical Systems Division Air Force Systems Command United States Air Fbrce Wright-Patterson Air Force Base, Ohio

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INTRODUCTION This research investigation was initiated in June, 1961, with the objective of determining the influence of exposure at 5500F on the mechanical properties of vacuum melted AM350 sheet material. The data obtained in this program are to be used in the evaluation of alloys for possible use in the supersonic transport. It is anticipated that the principles developed in this investigation regarding the influence of exposure on the strength of properties of this alloy can be extended to other alloys of a similar type. AM350 sheet steel in the SCT condition and in the CRT condition is being exposed at 550~F 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. Specimens are being and have been exposed for time periods of 2, 000, 5, 000, 12, 000 and 30, 000 hours to evaluate the effects of exposure time on mechanical properties. The exposures for time periods of 12, 000 hours and less have been completed and the mechanical properties of the specimens measured. In addition to these a limited number of tests have been included in the program to evaluate the possibility of using shorterduration higher temperature exposures to predict changes in mechanical properties to be expected during the service life of the aircraft. EXPERIMENTAL PROGRAM The test materials have been described in full in previous progress reports. The same is true for the experimental procedures followed in this program. For this reason these sections will not be repeated in the present report. The program will evaluate the influence of exposure at 5500F for 30, 000 hours under a stress of 67, 000 psi on the mechanical properties of the AM 350 sheet material. This will be accomplished by observing the change in the- short-time tensile strength of both smooth and edge-notched (Kt i3) specimens at room temperature and 550 F, The edge-notches are intended to 1

simulate an area of stress concentration in the material. In addition, an objective of the investigation is to measure the ability of the alloy to withstand very sharp notches (Kt > 20) introduced after exposure. This research was included in the program on the basis that changes in the alloy during exposure might severely limit its ability to withstand cracks or sharp notches which might be encountered during the service life of the aircraft. In order to provide factual interim data without the necessity of waiting 30, 000 hours for an indication of the influence of exposure, shorter duration exposures of 2,000, 5, 000 and 12, 000 hours have also been included in the program. In addition, these tests will provide data for a study of possible methods of predicting the influence of long-time exposure from short-time tests. These shorter duration exposures included both smooth and edgenotched specimens which were tensile tested at both room temperature and 550~F after completion of the exposure. A limited study of the influence- of stress during exposure on the properties of the alloy in both the SCT and the CRT condition is being made by the inclusion of a few specimens which have no applied stress. Exposure times of 30, 000, 12, 000 and 5, 000 hours are being evaluated, with the subsequent tensile tests conducted at room temperature. Based on the assumption that the mechanical property changes induced by exposure to elevated temperatures should involve reactions of the type obeying the Arrhenius rate equation, exposures at 600~, 650~ and 700~F were included in the program. These exposures should induce equivalent property changes to those occurring in 30, 000 hours at 550~F. PRESENT STATUS OF THE PROGRAM The 2, 000, 5, 000 and 12, 000 hour exposures of the AM 350 sheet material in both the SCT and the CRT conditions have been finished and the tensile tests on the exposed samples completed. In addition, the accelerated exposures at 6000, 6500 and 700'F have been run and the tensile tests on these specimens completed. The results of these tests are reported in Table I. The 30,000 hour exposures have now been in progress for time periods of between 16, 000 and 20, 000 hours. The status of these exposures is shown in Table II.

RESULTS TO DATE The results obtained from the completed tensile tests along with the hardness data are reported in Table I. In Figure 1 the ultimate tensile strengths at 5500F and at room temperature, together with the elongation of the unnotched specimens, are plotted as a function of exposure time at 550~F. The following trends have been revealed by study of these data: 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. At 550~F, however, it is considerably stronger. (2) The notches of the intermediate acuity (Kt = 3) raised the net-section 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. At room temperature there was very little influence of notches of intermediate acuity on the net-section strength of the CRT material. (3) Sharp edge-notches reduced the net-section strength of the AM350 alloy in the SCT condition 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 probably not been significantly changed as the result of stressed or unstressed exposure at 550~F for time periods up to 12, 000 hours. A slight increase in strength was noted after exposure for 12, 000 hours. This increase amounted to about two percent of the original strength of the alloy. (2) There does not appear to be any appreciable change in the elongation of the unnotched specimens of the alloy in either the SCT or the CRT condition as the result of exposure for times up to 12, 000 hours, although, as was the case with the tensile strength of the material, a very slight increase in elongation was noted after 12, 000 hours of exposure. 3

(3) The accelerated, equivalent time exposures carried out at 6000, 6500 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 550PF. Earlier in the Investigation it was thought that unstressed exposure at 5500F of the SCT material may have caused a drop in subsequent room temperature yield strength. This drop, however, was not evident after 12, 000 hours of exposure. This indicates that the observed decrease in yield strength after 5, 000 hours of exposure was due to either material variability or to difficulties encountered during the testing of that speci men. Creep measurements taken during the exposures indicate that little strain has occurred. These measurements indicate that most of the specimens have contracted very slightly during the first 18, 000 hours of exposure. The amounts of contraction range from 0%o to 0. 03%0. It is likely that no significant amount of "creep" has taken place in the alloy in either the SCT or the CIR T condition. 4

TABLE I RESULTS OF TENSILE TESTS FOR AM350SHEET Exposure 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 Ex__pp. After Exp, Strength (ksi) CRT Condition None Room 93 122 168 185.5 218.5 28 -- 225.5 5l -- 214 None Room 100 126 165 191 217 16.5..... 51.5 -- 215. l None Room 110 137 171.5 182 224.2 23.5............ 550 40 2000 Room 89 107 155 178 212,5 21 34.......... 550 67 2000 Room........ b(178) 221,8 22 -- 220 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 22 1.7 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 51.5 213 550 0 12000 Room 136 156 183 193 230 24.7..... 49 49.8 --- 550 67 12000 Room 9 l 120 165 187 227 22.7 -- 230.5 49..... 600 67 2000 Room 114 139 168 181 217.8 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 Z000 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 51.5 170.5 550 0 12000 550 95 109 139 150 173 4, 3..... 49 50.3 --- 550 67 12000 550 95 108 137 151 171. 5 4.3 -- 183 49 50.7 --- 600 67 2000 550 92 113 141 i 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 4.3..... 52 5l c(>l 51) 700 67 200 550 103 117 139 150.5 171.2 4 8 --- 52,5 52 172 SCT Condition 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 54 52.5 196.5 550 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 13.5 -- 238.5 53.5 53.5 209.4 550 0 12000 Room 117 158 177 189 ZZ3 Z0.0..... 53. 5 50.8 --- 550 67 12000 Room 116 130 159 176.5 224, 17.7 -- 247 54.0..... Y00 67 200 Room...................... 53.5 52 208.8 None 550 70 89 119 135 193.6 5 1Z 2 10 -- -- 159.3 None 550 80 98 126.9 141 194.4 6.5 12....... 159 550 67 2000 550 95 i 13.5 138 150 195.9 8.5 16 --- 54.5 55 159.2 550 0 Z236 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 53 52 161.5 550 0 12000 550 100 119 141 154 198.9 6.7..... 53.5 51.8 --- 550 67 12000 550 115 125 137 151 198.8 7.3 -- 213.5 53.5 52.6 --- 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 10l 131 146 200,3 5.5..... 53.5 5l 163 700 67 200 550 96 118 141 151.5 201.2 6............ a) Exposed unnotched. Sharp edge notches added before tensile test b) By "drop of needle"; extensometer erratic c) Specimen shoulder tore; no fracture at the notch

TABLE 2 STATUS OF 30,000 HOUR EXPOSURE TESTS AT 5500F Specimen Specimen Alloy Accumulated Estimated Stress Code Type (a) Condition (b) Time to Date (Hours) Completion (psi) E -6 W SCT 18648 7/17/65 67, 000 E-7 W SCT 18648 7/17/65 67, 000 E-4 U SCT 18384 7/28/65 67, 000 C-6 U SCT 18384 7/28/65 None E-1 U SCT 18744 7/13/65 67, 000 C-3 U SCT 18744 7/13/65 None C-i U SCT 16728 10/ 4/65 67, 000 E-2 U SCT 16728 10/ 4/65 67, 000 D-4 U SCT 16728 10/ 4/65 None 37 W CRT 18552 7/21/65 67, 000 55 U CRT 18648 7/17/65 None 36 U CRT 18384 7/28/65 67, 000 56 U CRT 18552 7/21/65 None 32 U CRT 19152 6/26/65 67,000 52 U CRT 19152 6/26/65 67, 000 53 U CRT 19152 6/26/65 None 41 U CRT 18744 7/13/65 67, 000 E-4 N SCT 18552 7/28/65 67, 000 E-1 N SCT 18744 7/13/65 67, 000 C-i N SCT 16728 10/ 4/65 67, 000 E-2 N SCT 16728 IO/ 4/65 67, 000 36 N CRT 18552 7/28/65 67, 000 32 N CRT 19152 6/26/65 67, 000 52 N CRT 19152 6/26/65 67, 000 41 N CRT 18774 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 8500F. SCT = Annealed at 1950'F, conditioned for 10 minutes at 1710'F, A. C., held three hours at -100'F and tempered three hours at 8500F.

SCT Condition CRT Condition 240 *Accelerated, 240- Accelurated, * Equivalent Time Eposures Equivalent Time Exposures 220 22I * _ _ _ _ _ _ _ _ 3 ~ ~ ~ ~ ~ ~ t A - A 01 ~~ ~~~~~~ ~~~~~~~~~~~~~~~~~~~~~~~_ ~ I 0 N 200 o2..0) -,.- [ -160 0~ ~ ~ ~ ~ ~ ~ ~~~~~~~~~~~~~~- (1 Exposure at 550 F AclrtdEpsr.E20 ___ _ Test Temperature Test Temperature~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~es Tmpraur C14 04~~~~~~~~~~ Specimen Type 550 IF Room 50F Ra 0p 0- Unnotched 0 * C~~.2~ ~~ ~~- ___ Notched, Ks 3 g * o, 10 _ _ _ _ _ _ _ _ _sAA o 0 0 -~~~~~~~~~~~~~~ Sharp Edge NathsoAA A L L o ~ ~ ~ ~ 0 ~~~ Circled points indicate unstressed exposures.~~~~~~~~~~~~~~~~~~Cicle pins idiat u UO ~~~~~~~~~~~~~~~~~~~~~~~~All other exposures at 6700pisrs 0 - 0 2,000 5,0 120030,00 0 12,000 30,000 Duration of Creep Exposure, Hours Duration of Creep Exposure, Hours Figure 1. Effect of Exposure at 5500 on Tensile Strength Land Ductility of Vacuum-Melted AM350 Sheet at 5500 F and at Room Temperature. Predicted Strengths and Ductilities Based on the Results of Accelerated Tests Conducted at 6000, 60 65C Sandp000dgAe PlottchtnEqiaentEpsur Tim of 3000Hus

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