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

INTRODUCTION The contract originally specified exposure of AM350 sheet in the CRT and SCT conditions for 30, 000 hours under the stress causing 0. 5 percent creep deformation at 5500F in that time period. As shown in the First Progress Report, preliminary creep tests indicated that the stress required to produce this amount of creep would well exceed the yield strength. Even then, most of the creep would occur during the first few hours of testing. With 0. 5 percent creep established as an unsatisfactory exposure criterion, the program was re-evaluated with technical representatives of ASD. The objectives were changed to obtain the most information possible regarding changes in mechanical properties as a result of prolonged exposure under stress at 5500F. With de-emphasis of high-sensitivity creep measurements on the specimens during exposure, a considerable increase was possible in the number of specimens which could be exposed. The experimental program detailed in the next section was developed to furnish types of data now deemed most useful in evaluating materials for the constructionof 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 now 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. REVISED EXPERIMENTAL PROGRAM The exposure conditions and subsequent tensile tests now planned are given in Table 1 along with the status of the tests. It will be noted that testing at 550~F as well as room temperature has been added. This was done because data now available for alloys being considered for the Mach 3 airplane suggest that the properties at 5500F are more critical than those at room temperature. For this reason, it seemed desirable to add as much testing at 5500F as possible in the revised program. 1

Duplicate tests are planned at room temperature after exposure of unnotched and Kt = 3 specimens for 30, 000 hours; but only single tests at 5500F. A fourth specimen is being left uncommitted, with the testing 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 5500F 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 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, the revised program allows exposure of specimens for shorter times before tensile testing. This provides data for study of possible methods of extrapolating from short time exposures, and also provides interim factual data without waiting the full 30, 000 hours for an indication of the effect of exposure. 2

Exposure times of 2000, 5000, and 12, 000 hours will be used. Unnotched and notched (Kt = 3) specimens are to be exposed for 12, 000 hours and then tensile tested at room temperature and 550~F. The 5000 and 2000 hour exposures include similar tests, plus two specimens in which sharp notches will be 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 will be covered. Tensile tests at room temperature and 550~F are planned. "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 which sharp edge notches will be added after the creep exposure. Times of 2000 hours at 6000, 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 ({ ) 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) ^=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 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 6000, 6500, and 700~F corresponding to 30, 000 hours at 5500F are, respectively, about 2090, 190, and 20 hours. The 200 hour exposures at 7000F were included to allow for an actual value of -log A' of 15 or less. 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 4

in tandem. The specimens being used are shown in Figure 1. The SCT material panels are too small to make the double gage length 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 Table 1 shows the status of the tests. All of the 30, 000 hour exposures have been started. Most of the 5000 hour and part of the 2000 hour exposures are in progress. Some of the "accelerated" exposures at 6000, 6500, and 700~F have been started, with part of them now completed. The 12, 000 hour exposures will be started in the units made available by the completion of the 5000 hour exposures. The remaining exposures for 2000, 200, and 20 hours must await completion of those now in progress. RESULTS TO DATE All tests completed to date are listed in Tables 2 and 3. The following tabulation has been prepared to summarize the ultimate strength data. Where more than one test was conducted, averaged values were tabulated. 5

Room Temperature 550~F CRT SCT CRT SCT Ultimate Ultimate Ultimate Ultimate Strength N/S Strength N/ S Strength N/S Strength N/ S (psi) Ratio (psi) Ratio (psi) Ratio (psi) Ratio Original Unexposed Condition Unnotched 219,900 214, 000 168,500 194,000 Kt = 3 225,500 1.02 241,500 1.13 185,000 1.1 210,000 1.08 Sharp Notch 214,500 0.98 212, 150 0.99 169,450 1.05 159, 150 0.82 Exposed 550~F - 2000 hours 40, 000 psi (Unnotched) 215,500 90, 000 psi (Unnotched) 221,400 150, 000 psi (Unnotched) 221, 100 Exposed 600~F - 67, 000 psi 2000 hours (Unnotched) 172, 000 198,500 Exposed 650~F - 67, 000 psi 200 hours (Unnotched) 170,800 195,800 Exposed 700~F - 67, 000 psi 20 hours (Unnotched) 170,700 200,300 200 hours (Unnotched) 188,000 201,200 200 hours (Sharp Notch 209,800 0.96 208,800 0.98 -- 163, 000 0.84 added after Exposure)

These data show the following: 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) Sharp notches did not significantly change strengths except for a significant reduction of the SCT material at 550 F. 4) The three specimens exposed under three different stresses at 550~F for 2000 hours did not show significant change in strength, as was previously reported. 5) The exposures carried out to date at 6000, 650~, and 700~F did not appreciably change properties except possibly in one case. The CRT specimen exposed for 200 hours at 7000F and subsequently tensile tested at 5500F had an ultimate strength which could be significantly higher than unexposed material and, therefore, indicative of a change. The following tabulation summarizes the yield strengths and elongation values from the tests to date on unnotched specimens: 0. 2% Offset Y. S. (psi) Elongation (To in 2 in. ) CRT SCT CRT SCT Room Temperature Unexposed 186,200 179,800 16-28 12-17 Exposed 550~F - 2000 hours 40,000 psi 178,000 21 90, 000 psi 186,000 16.5 150, 000 psi (208, 000) 19.5 550~F Unexposed 153,250 138,000 4 5.5 Exposed 600 ~F - 67, 000 psi 2000 hours 153, 000 145,500 5 5. 5 Exposed 650 F - 67, 000 psi 200 hours 154, 000 140,000 4 6 Exposed 700 ~F - 67, 000 psi 20 hours 145,000 146,000 4 5.5 200 hours 152,500 151,500 4.5 6 7

The following comments are derived from these data: 1) The SCT condition had lower yield strength prior to exposure than the CRT condition, with the difference being quite large at 550~F and only slight at room temperature. There was little difference in elongation between the two conditions. 2) Yield strength and, particularly, elongation were lower at 550~F than at room temperature. 3) The yield strength of the CRT condition did not change as a result of exposure under the conditions tested to date. The possible exception was an increase as a result of exposure for 2000 hours at 550'F under 150, 000 psi (the yield strength). 4) The yield strength of the SCT condition at 550'F appeared to increase slightly with increasing temperature of prior exposure. 5) No significant changes in elongation resulted from the exposures. Tables 2 and 3 include some elongation values for a gage length of 0. 5 inch. This was done to show whether or not the materials exhibited marked local necking at the point of fracture. This practice was initiated by request after some of the tests had been completed. Values for the 0. 5-inch gage length will not be available for most of the early tests. Hardness values obtained to date show very little change, if any, from creep exposure. 8

Table 1 Outline and Status of the Revised Testing Program Outline of Initial Exposures to Creep Proposed Subsequent Tensile Tests a)Spec. Stress Temp. Duration Date Started Temperature Date Completed Type (psi) (~F) (hrs) CRT SCT Room 550~F b)Either CRT SCT U 67, 000 550 30,000 Jan. 62 Jan. 62 X (Est: July 65) U 67, 000 550 30, 000 Feb. 62 Feb. 62 X U 67, 000 550 30, 000 Jan. 62 Jan. 62 X U 67, 000 550 30, 000 Feb. 62 Feb. 62 X N 67, 000 550 30, 000 Jan. 62 Jan. 62 X N 67, 000 550 30, 000 Feb. 62 Feb. 62 X N 67, 000 550 30, 000 Jan. 62 Jan. 62 X N 67, 000 550 30, 000 Feb. 62 Feb. 62 X W 67, 000 550 30,000 Feb. 62 Fcb. 62 X W 67, 000 550 30, 000 Feb. 62 Feb. 62 X U None 550 30, 000 Jan. 62 Jan. 62 X U None 550 30,000 Feb. 62 Feb. 62 X U None 550 30, 000 Feb. 62 Feb. 62 X U 67, 000 550 12, 000 X U 67, 000 550 12, 000 X N 67, 000 550 12, 000 X N 67, 000 550 12, 000 X U None 550 12,000 X U None 550 12, 000 X U 67, 000 550 5, 000 Feb. 62 Jan. 62 X (Est: Sept. 62) U 67, 000 550 5,s000 Feb. 62 Jan. 62 X N 67, 000 550 5, 000 Feb. 62 Jan. 62 X N 67, 000 550 5,000 Feb. 62 Jan. 62 X W 67, 000 550 5,000 Feb. 62 X W 67, 000 550 5, 000 Feb. 62 X U None 550 5, 000 Feb. 62 Jan. 62 X 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 1 (Concluded) Outline and Status of the Revised Testing Program Outline of Initial Exposures to Creep Proposed Subsequent Tensile Tests aSpec. Stress Temp. Duration Date Started Temperature Date Completed Type (psi) (~F) (hrs) CRT SCT Room 550~F CRT SCT U 67,000 550 2,000 X U 67,000 550 2,000 Mar. 62 Mar. 62 X N 67,000 550 2,000 X N 67,000 550 2,000 X W 67,000 550 2,000 Feb. 62 X W 67,000 550 2,000 Feb. 62 X U 67,000 600 2,000 X U 67,000 600 2,000 Dec. 61 Dec. 61 X Mar. 62 Mar. 62 U 67,000 650 200 X U 67,000 650 200 Dec. 61 Dec. 61 X Mar. 62 Mar. 62 U 67,000 700 200 X U 67,000 700 200 Jan. 62 Jan. 62 X Mar. 62 Mar. 62 W 67, 000 700 200 Feb. 62 Feb. 62 X Mar. 62 Mar. 62 W 67,000 700 200 Feb. 62 X Mar. 62 U 67,000 700 20 X U 67,000 700 20 Dec. 61 Dec. 61 X Mar. 62 Mar. 62 a) U - Unnotched, 0.350-inch gage width; N = Notched, Kt = 3; W = Wide unnotched during exposure, sharp edge notches for tensile tests

Table 2 Results of Tensile Tests for AM350 Sheet in the CRT Condition Hardness Subsequent Tensile Properties Exposure Conditions (Rockwell "45N") Temp. Stress Duration Before After Temp. Proportional Offset Yield Strengths (psi) Tensile Elong. (o) (~F) (psi) (hrs) Exp. Exp. (~F) Limit (psi) 0.02% 0.1% 0. 2% Strength(psi) Per 2" Per 0.5" Unnotched Specimens (No Prior Exposure) 48.5 Room 93,000 122,000 168,000 185,500 218,500 28 (No Prior Exposure) 49 Room 100,000 126,000 165,000 191,000 217,000 16.5 (No Prior Exposure) 51 Room 110,000 137,000 171,500 182,000 224,200 23.5 550 40,000 2000 Room 89,000 107,000 155,000 178,000 212, 500 21 34 550 90,000 2000 Room 119,000 142,500 154,500 186,000 221,400 16.5'550 150,000 2000 Room (145,000) 174,000 198,000 (208,000) 221,100 19.5 (No Prior Exposure) 49.5 550 95,000 120,000 144,000 153,000 169,000 4 (No Prior Exposure) 50 550 96,000 115,000 143,000 153,500 168,800 4.5 600 67,000 2000 550 92,000 113,000 141,000 153,000 172,000 5 650 67,000 200 550 85,000 101,000 139,000 154,000 170,800 4 8 700 67,000 20 550 80,000 97,000 130,000 145,000 170,700 4 6 700 67,000 200 52 550 95,000 104,000 140,000 152,500 188,000 4.5 Notched Specimens (Kt= 3) (No Prior Exposure) 49 Room 225, 500 (No Prior Exposure) 49 550 185,000 Exposed Unnotched; Sharp Edge Notches Added Before Tensile Test (No Prior Exposure) 51 Room 214, 000 (No Prior Exposure) 51.5 Room 215, 100 700 67,000 200 52 51 Room 209,800 (No Prior Exposure) 50.5 550 172, 100 (No Prior Exposure) 50.5 550 166,800 700 67,000 200 52 51 550 a)(> 151, 000) a) Specimen shoulder tore; no fracture at the notch.

Table 3 Results of Tensile Tests for AM350 Sheet in the SCT Condition Hardness Subsequent Tensile Properties Exposure Conditions (Rockwell "45N") Temp. Stress Duration Before After Temp. Proportional Offset Yield Strengths (psi) Tensile Elong. (%) (OF) (psi) (hrs) Exp. Exp. (0F) Limit (psi) 0.02% 0. i1% 0.2% Strength (psi) Per 2" Per 0.5" Unnotched Specimens (No Prior Exposure) Room 113,000 143,000 170,000 185,300 214,900 17 32 (No Prior Exposure) 52.5 Room 119,000 139,000 165,000 178,000 213, 100 12 26 (No Prior Exposure) 52.5 Room 105,000 129,000 162,000 176,000 214,000 16.5 32 (No Prior Exposure) 550 70,000 89,000 119,000 135,000 193,600 5 12 (No Prior Exposure) 550 80,000 98,000 126,900 141,000 194,400 6.5 12 600 67,000 2000 550 81,000 101,000 132,000 145,500 198, 500 5.5 650 67,000 200 550 75,000 95,000 122,000 140,000 195,800 6 12 700 67,000 20 550 87,000 101,000 131,000 146,000 200,300 5.5 700 67,000 200 550 96,000 118,000 141,000 151,500 201,200 6 Notched Specimens (Kt 3) (No Prior Exposure) Room 241, 500 (No Prior Exposure) 550 210,000 Exposed Unnotched; Sharp Edge Notches Added Before Tensile Test (No Prior Exposure) Room 216,000 (No Prior Exposure) Room 208, 300 700 67,000 200 53.5 52 Room 208,800 (No Prior Exposure) 550 159, 300 (No Prior Exposure) 550 159,000 700 67,000 200 53.5 51 550 163,000

I" R. 0.025" ROOT RADIUS 1/2"D. EATENSOMETER 0.350" 0.5" 0.350" 1.2 1.4~ -1 LV2" 0.99" 3.355" 2.75" W~~~~_ ~8"1 I" R. OUTLINE FOR SINGLE SPECIMEN SHARP NOTCH ADDED AFTER i 5/8" D CREEP PERIOD. I T I" 0.7" 1.6" -3/4'" — 4.5" l 6.75"' 12" Figure 1 - Specimen for Long-Time Creep and Subsequent Tensile Testing.