THIRD PROGRESS REPORT on EFFECT OF LONG-TIME CREEP ON STRUCTURAL SHEET MATERIALS H. R. Yoorhees J. W.'Freeman The University of Michigan Contract AF 33(616)-8334 Project 1(8-7381) Task 73812 Aeronattiical Systems Division Air Force. Systems Command United States Air Force Wright-Patterson Ai-r, Force Base, Ohio

INTRODUCTION This program seeks to obtain information on changes in mechanical properties as a result of prolonged exposure under stress at 5500F. The experimental program was developed to furnish types of data now deemed most useful in evaluating materials for the construction of a Mach 3 transport airplane. The exposure stress of 67, 000 psi was selected as representative of the most probable design stress for that type of airplane. The program provides for exposure time periods of 2000, 5000, and 12, 000 hours at 550~F, as well as 30, 000 hours, to show the effect of time of exposure. It also includes a minimum number of tests designed to study the possibility of using results from shorter-duration exposures to predict changes in properties to be expected during a service life of 30, 000 hours. The following sections are repeated from the Second Progress Report to make the present report a self-contained survey of the research. EXPERIMENTAL PROGRAM Table 1 gives the exposure conditions and planned subsequent tensile tests, along with the present status of the tests. Testing is being done at 550'F as well as room temperature because data now available for alloys being considered for the Mach 3 airplane suggest that the properties at 550~F are the more critical. For this reason, as much testing at 550~F as possible has been included in the program. Duplicate tests are planned at room temperature after exposure of unnotched and Kt = 3 specimens for 30, 000 hours; but only single tests at 550~F. A fourth specimen is being left uncommitted, with the testing 1

conditions to be determined after the other three specimens have been tested. For all other exposures, only single tensile tests after exposure are now planned. The omission of replicate specimens is recognized not to be desirable but seemed acceptable to obtain data for more different conditions. Deviations from general trends should be no more troublesome to interpret than deviations from the average of replicate tests. The following sections indicate reasons for the choices of the actual exposure tests detailed in Table 1. Effect of Exposure at 5500F for 30, 000 Hours Under 67, 000 psi This will be measured by: (a) the change in short time tensile properties at room temperature and 550~F for unnotched strip specimens. (b) the change in short time tensile strength of edge notched (Kt = 3) specimens at room temperature and 550~F after exposure with the notch present. This notch is intended to simulate the effect of a stress concentration present from design considerations. (c) the change in ability to withstand a very sharp notch introduced during exposure. Unnotched specimens are being exposed. After exposure, ASTM sharp notches are machined into the specimens, and tensile tests conducted at room temperature and at 550~F. This procedure was selected on the basis that sharp notches and cracks should not be present originally in the airplane. It should be a severe test of changes during exposure in the important ability of the material to withstand sharp notches or cracks. A sharp notch present during exposure should undergo creep relaxation and thus provide a less severe test of changes in notch 2

sensitivity than a notch introduced after exposure. The procedure adopted does not, however, test the possibility that a sharp notch might cause cracking during exposure. Effect of Time at Exposure In addition to the exposure for 30, 000 hours, exposure of specimens for shorter times before tensile testing is providing data for study of possible methods of extrapolating from short time exposures, and also provides interim factual data without need to wait 30, 000 hours for an indication of the effect 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 and are then to be tensile tested at room temperature and 550PF. The 5000 and 2000 hour exposures included similar tests, plus two specimens in which sharp notches were machined after exposure, prior to tensile testing at room temperature and 550~F. Effect of Stress During Exposure A very limited study of the effect of stress during exposure was made possible by including a few specimens in the exposure furnaces with no stress applied. Exposure times of 30, 000, 12, 000, and 5000 hours are being covered, with subsequent tensile tests at room temperature and 550~F. "Accelerated" Exposures Table 1 lists some tensile tests to be carried out after prior creep under 67, 000 psi stress, but at 600-700~F. Results are to be analyzed to determine if effects of prolonged exposure can be predicted from shorter exposures at higher temperatures. Emphasis is on changes in properties of unnotched specimens, but a few specimens are included in 3

which sharp edge notches will be added after the creep exposure. Times of 2000 hours at 600~, 200 hours at 650~, and 20 hours at 700'F were selected to produce changes in subsequent tensile properties roughly the same as would 30, 000 hours at 550'F under the 67, 000 psi stress. The mechanisms by which exposure to creep should cause any changes in mechanical properties were reasoned to involve reactions of the type obeying the Arrhenius rate equation. Then the rate of property change (p) at constant stress may be expressed in terms of the gas control R, and a constant A and an energy E which are independent of temperature: - (E/RT) p =Ae Defined, in terms of the inverse of p, the time t at absolute temperature T for a given degree of property change becomes: C C) e /RT = E/RT t —- )e -A' e P A Converting to common logarithms and rearranging, T ( -log A' + log t ) - E/2. 3R = Constant at constant stress. For creep rupture of many alloys, -log A' is about 20. If this value and the above reasoning apply, times at 600~, 650~, and 700~F corresponding to 30, 000 hours at 550~F are, respectively, about 2090, 190, and 20 hours. The 200 hour exposures at 700~F were included to allow for an actual value of -log A' of 15 or less. 4

Other Tests Besides these tests, the following data will be obtained: 1) Creep measurements during exposure will be made on unnotched specimens except for two cases. When two unnotched specimens are exposed in tandem, creep will be measured only on one of the two. Creep measurements will not be made on wide specimens to be notched after exposure. 2) Hardness will be measured on the shoulder section of specimens before exposure, and of specimens after unstressed exposure. Measurements after stressed exposure will be limited to the wide specimens that are to receive sharp edge notches after creep exposure. 3) Selected specimens will be examined microscopically after exposure. TEST MATERIAL AND PROCEDURES All test specimens were sampled with their length in the direction of rolling of AM350 sheet from consumable-electrode melted Heat No. 23327, which had the following reported chemical composition: C Si Mn Cr Ni Mo N P S.084.21.65 16.50 4.29 2.94. 10.009.007 Specially-designed long furnaces were made to uniformly heat two specimens in tandem. The specimens being used are shown in Figure 1. The SCT material panels are too small to make the double gage length 5

wide specimen shown at the bottom of Figure 1 and is, therefore, to be exposed as single specimens. During exposure, the specimens are loaded by pins inserted through the holes at the ends of the specimens. Creep readings are made on the unnotched part of the specimens shown at the top of Figure 1. No creep readings are being taken on the wide specimens shown at the bottom of Figure 1. 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 tests at either room temperature or 550~F as the need may be. Special adapters pull against the shoulder fillets to avoid stressing the second gage section of the double specimen during tensile testing of the first. Rockwell "45N" superficial hardness measurements were taken for most specimens prior to the start of exposures. Hardness values after exposure are to be reported only 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 at the location to be notched, and (b) unstressed exposure for which hardness readings may be taken in the specimen shoulders. PRESENT STATUS OF THE PROGRAM As Table 1 shows, all planned exposures at 550~F have been started, and tensile tests completed for most of the specimens with 2000- or 5000-hour exposures. Some of the "accelerated" exposures at 600~, 650~ and 700 F have been finished. Additional such exposures now in progress should permit a decision as to whether any resulting 6

changes in tensile properties are sufficiently large to warrant completion of all the listed tests of this type. RESULTS TO DATE Creep strains during exposures so far are not reported because they were all negligibly small. During the first 6000 hours at 550~F under 67, 000 psi stress, measured creep remains below 0. 01 percent. Table 2 lists all tensile-test and hardness data obtained to date. Ultimate tensile strengths at 550'F and at room temperature for the three types of specimen, together with elongations for unnotched specimens only, are shown graphically in Figure 2 as a function of exposure time at 550~F. Study of Figure 2 reveals the following trends: 1) The SCT material is slightly weaker than the CRT at room temperature, but considerably stronger at 550~F. 2) The Kt = 3 notches raised net-section strengths at both room temperature and 550~F. 3) Except for a single test of SCT material at 5500F, sharp edge notches had little effect on net-section strength. 4) A consistently lower elongation after 5000 than after 2000 hours of exposure may signify a continuing loss of ductility with prolongation of creep exposure. Yield strengths and hardness values showed little change as a result of exposures at 550~F, except for an apparent drop in yield strength at room temperature from 179 to 159 ksi for SCT material exposure for 5000 hours without stress. 7

Exposures carried out to date at 600~, 6500, and 700~F caused no appreciable change except that the CRT specimen; exposed for 200 hours at 700~F and subsequently tested at 550~F had an ultimate strength which could be significantly higher than for unexposed material. A repeat test is planned to check this possibility. 8

Table 1 OUTLINE AND STATUS OF THE TESTING PROGRAM Outline of Initial Exposures to Creep Proposed Subsequent Tensile Tests a)Spec. Stress Temp Duration Date Started Temperature Date Completed Type (ksi) (~F) (hrs) CRT SCT Room 550~F b)Either CRT SCT U 67 550 30, 000 Jan. -Feb. 1962 X (Est: July 1965) U 67 550 30, 000 Jan. -Feb. 1962 X (Est: July 1965) U 67 550 30, 000 Jan. -Feb. 1962 X (Est: July 1965) U 67 550 30,000 Jan. -Feb. 1962 X (Est: July 1965) N 67 550 30, 000 Jan. -Feb. 1962 X (Est: July 1965) N 67 550 30, 000 Jan. -Feb. 1962 X (Est: July 1965) N 67 550 30, 000 Jan. -Feb. 1962 X (Est: July 1965) N 67 550 30, 000 Jan. -Feb. 1962 X (Est: July 1965) W 67 550 30, 000 Jan. -Feb. 1962 X (Est: July 1965) W 67 550 30,000 Jan. -Feb. 1962 X (Est: July 1965) U None 550 30, 000 Jan. -Feb. 1962 X (Est: July 1965) U None 550 30, 000 Jan. -Feb. 1962 X (Est: July 1965) U None 550 30,000 Jan. -Feb. 1962 X (Est: July 1965) U 67 550 12,000 Aug. -Sept. 1962 X (Est: March 1964) U 67 550 12,000 Aug. -Sept. 1962 X (Est: March 1964) N 67 550 12,000 Aug. -Sept. 1962 X (Est: March 1964) N 67 550 12,000 Aug. -Sept. 1962 X (Est: March 1964) U None 550 12,000 Aug. -Sept. 1962 X (Est: March 1964) U None 550 12,000 Aug. -Sept. 1962 X (Est: March 1964) N None 550 12, 000 Aug. -Sept. 1962 X (Est: March 1964) U 67 550 5,000 Feb. 1962 Jan. 1962 X Oct. 1962 Oct. 1962 U 67 550 5,000 Feb. 1962 Jan. 1962 X Oct. 1962 Oct. 1962 N 67 550 5,000 Feb. 1962 Jan. 1962 X Oct. 1962 Oct. 1962 N 67 550 5,000 Feb. 1962 Jan. 1962 X Oct. 1962 Oct. 1962 W 67 550 5, 000 Feb. 1962 May 1962 X Oct. 1962 (Feb. 1963) W 67 550 5, 000 Feb. 1962 May 1962 X Oct. 1962 (Feb. 1963) U None 550 5,000 Feb. 1962 Jan. 1962 X Oct. 1962 Oct. 1962 U 67 550 2, 000 Feb. -June Feb. -Mar. X Oct. 1962 May 1962 U 67 550 2,000 1962 1962 X June 1962 June 1962 N 67 550 2,000 Feb. -June Feb. -Mar. X June 1962 June 1962 N 67 550 2,000 1962 1962 X Oct. 1962 May 1962 W 67 550 2,000 Feb. -June Sept.1962 X May 1962 (Jan. 1963) W 67 550 2,000 1962 June 1962 X May 1962 Oct. 1962 U 67 600 2,000 Sept.1962 -- X (Jan. 1963) -- U 67 600 2,000 Dec. 1961 Dec. 1961 X Mar. 1962 Mar. 1962 U 67 650 200 -- -- X. _ U 67 650 200 Dec. 1961 Dec. 1961 X Mar. 1962 Mar. 1962 U 67 700 200 Dec.1961 Dec.1961 X.. U 67 700 200 Jan.1962 Jan.1962 X Mar.1962 Mar. 1962 W 67 700 200 Feb.1962 Feb.1962 X Mar.1962 Mar. 1962 W 67 700 200 Feb. 1962 Feb. 1962 X May 1962 Mar. 1962 U 67 700 20 -- -- X -- _ U 67 700 20 Dec.1961 Dec.1961 X Mar.1962 Mar.1962 a) U = Unnotched, 0. 350-inch gage width; N = Notched, Kt = 3; W = Wide unnotched during exposure, sharp edge notches for tensile tests b) Temperature of these tensile tests is to be selected after other results become available.

Table 2 RESULTS OF TENSILE TESTS FOR AM350 SHEET 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 (VF) (ksi) (hrs) (F) (ksi) 0.02% 0. 1% 0.2% Strength(ksi) Per 2" Per 0.5" (ksi) Before Exp. After Exp. Strength (ksi) CRT Condition 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 40 2000 Room 89 107 155 178 212.5 21 34 -—..... —550 67 2000 Room......... — (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) 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 51.5 213 700 67 200 Room.................. — - 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 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 -- 650 67 200 550 85 101 139 154 170.8 4 8 -- 700 67 20 550 80 97 130 145 170.7 4 6 -—.... —c 700 67 200 550 95 104 140 152.5 188 4.5 -- -- 52 51 (>151) 700 67 200 550 -—.. —.......... — -- 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 --- 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 -—. —-- 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 5 50 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 --- 550 667 5000 550 110 114 134 145 193.5 4.5 -- 208 --- 600 667 2000 550 81 101 132 145.5 198.5 5.5 -- -—.... 650 667 200 550 75 95 122 140 195.8 6 12 -—........ 700 67 20 550 87 101 131 146 200.3 5.5 -- 53.5 51 163 700 667 200 550 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.

I" R. 1/2"D. EATENSOMETER ROOT RADIUS PIN HOLES 0.350" _' 0.5" 0.350 1.2 I/22' 0.9"- 3.35" 2.75" 8"8 I" R. OUTLINE FOR SINGLE SPECIMEN /SHARP NOTCH ADDED AFTER 5/8" CREEP PERIOD. 5/8"D. { + } I" 0.7" I * *' 1.6" 3/4" - I *n " -I -— I ~6.75" $ 12" Figure 1 - Specimen for Long-Time Creep and Subsequent Tensile Testing.

saCT Condition CRT Condition 240 240 —~~~~~~~~~~~~~~~~~~~ — 1 _ _- _ _- -'"~ — U 220 220 -~.- -- Cs I I as H 9 XX I < C jc ~200 -- -200 I~~~~~~~~~~~~~~~~~~~~~~~~~-, lac - l 180 1.~ 160 160 2,000 5,000 12,000 30,000 2,000 5,000 12,000 30,000:2,000 5,000 12,000 3030 I e,~ I, j w~~~~~~~ Fion 2 C Efect of CExposure, Hour Tyse Stecitm-en 0 Room 0-0 ~ ~ ~ - C~~~~~~~~~~~~~ ~~~~~C~ C 0 Sharp edge notches ~, Circled points indicate unstressed exposures. I0~~~~~~~~~~~~~~~~All other exposures at 67,000 psi stre uJ 00 A - -I I__I -- i _.1 _ _ - i 2,000 5,000 12,000 30,000 2,000 5,000 12,000 30,000 Test Temperature Duration of Creep Exposure, Hours Duration of Creep Exposure, Hours Type Specimen 550"F Room Unnotched o e Fig. 2 - Effect of Exposure at 5500F on Tensile Strength and Ductility of Vacuum-Melted AM350 Sheet. Notched, Kr.3 a a Sharp edge notches a Circled points indicate unstressed exposures. All other exposures at 67,000 psi stress