THE UNIVERSITY OF MICHIGAN COLLEGE OF ENGINEERING Department of Chemical and Metallurgical Engineering SCREENING PROGRAM ON SUPERALLOYS FOR TRISONIC TRANSPORT Report No. 4 INFLUENCE OF NOTCH ACUITY ON THE NOTCH STRENGTH OF RENE' 41, WASPALOY, AND D979 J. W. Schultz T. M. Cullen J. W. Freeman ORA Project 04368 prepared for: NATIONAL AERONAUTICS AND SPACE ADMINISTRATION Grant NsG-124-61 Washington 25, D. C. administered through: OFFICE OF RESEARCH ADMINISTRATION ANN ARBOR March 29, 1963

SUMMARY Rene' 41, Waspaloy, and D979 materials were subjected to a testing program designed to evaluate the influence of notch acuity on notch strength. Longitudinal and transverse specimens of the alloys were tested at room temperature, 650~, 800~, and 1000~F in the following conditions: (1) Rene' 41 - Cold reduced 35 percent and aged two hours at 1500~F (2) Waspaloy - Cold reduced 40 percent and aged two hours at 1500~F (3) D979 - Cold reduced 50 percent and aged 16 hours at 1100~F. The theoretical elastic stress concentration factors, Kt's, used in the program were 1. 0 (smooth specimens), 1. 5, 2. 1, 3. 1, 8.6 or 9.4, and >20 (ASTM sharp edge-notch). Notch-tensile strength ratios increased slightly with increasing stress concentration factor to about Kt = 2. 1 for all the alloys when tested in the transverse direction. Longitudinal specimens from Rene' 41 and Waspaloy at all test temperatures and D979 at room temperature also exhibited a slight increase in notch-tensile strength ratio at low values of Kt. D979, however, tested in the longitudinal direction at elevated temperatures showed a rapidly decreasing notch-tensile strength ratio with increasing value of the stress concentration factor above 1. 0, Rene' 41 displayed at all test temperatures a steady drop in notch strength and, consequently, in notch-tensile strength ratio with increasing notch acuity beyond the slight peak in the ratio which occurred at low values of Kt. Waspaloy exhibited a similar behavior

at room temperature but at elevated temperatures showed an increasing tendency toward exhibiting a minimum in the curve of notchtensile strength ratio versus Kt. This minimum occurred in the vicinity of Kt = 9. 4. D979 also showed the presence of a minimum at all temperatures except 650~F. At this temperature, the notch-tensile strength ratio for the alloy dropped rapidly to low values as a function of notch acuity and then remained relatively constant with increasing Kt. Duplicate tests on D979 material at 650~F showed increased scatter of results at the lower Kt values for longitudinal specimens. Transverse specimens exhibited a relatively constant amount of data scatter as a function of Kt. 2

INTRODUCTION As part of a screening program designed to evaluate the usefulness of selected sheet materials in the construction of a supersonic transport plane, a study is being carried out at the University of Michigan to determine the utility of the heat-resistant superalloys. Six different superalloys in a total of seventeen conditions of prior history are in the process of being evaluated. Materials suitable for use in a supersonic transport must possess adequate strength properties and be sufficiently stable under conditions of aerodynamic heating to maintain these properties. It is especially important for the materials to possess and maintain adequate fracture toughness (resistance to unstable crack propagation). D979 alloy in the cold reduced 50 percent and aged 16 hours at 1100'F condition displayed generally good properties during initial testing. However, longitudinal specimens tested at 650~, 800 ~ and 1000~F repeatedly fractured at stresses below the 0.2 percent offset yield strength. These fractures occurred in a brittle manner at pin holes, fillets, and in the gage section of the specimens under the extensometer collars. In an attempt to understand this behavior in a promising material, an investigation was initiated to determine what influence notches of low and intermediate acuities had on the strength properties of D979 alloy. Two other alloys in addition to the D979 were included in this investigation, Renew 41 and Waspaloy. These alloys were chosen because they also had shown excellent properties in tests carried out as part of the screening program. 3

EXPERIMENTAL MATERIALS The alloys used in this investigation were in the form of 0. 025 -inch sheet material. Their reported chemical compositions in weight percent are listed in Table I. The conditions in which each of the three alloys were tested were as follows: (1) Rene' 41 - Cold reduced 35 percent and aged two hours at 1500~F (2) Waspaloy - Cold reduced 40 percent and aged two hours at 1500~F (3) D979 - Cold reduced 50 percent and aged 16 hours at 1100~F. Specimen blanks were cut from the cold worked sheet material and then aged in an electric furnace prior to being machined into finished specimens. 4

EXPERIMENTAL PROCEDURES The influence of notch acuity on the notch strength of the three alloys was evaluated at room temperature, 650~, 800~, and 1000~F. The theoretical elastic stress concentration factors used in the program were 1.5, 2o 1, 3. 1, and either 8. 6 or 9. 4. In addition, smooth specimens (Kt = 1, 0) and specimens with ASTM sharp edge-notches (Kt >20) were tested, Properties in both the longitudinal and transverse directions were measured to avoid misleading results from anisotropy effects which might be present. Test Specimens Unnotched Specimens The configuration of the smooth specimens used to measure unnotched properties (Kt 1, 0) is shown in Figure la. Specimens were prepared from rectangular blanks by millingo Ten specimens were machined at a time, using a fixture to clamp the blanks together and assure accurate alignment throughout the machining operations. Notched Specimens The geometry of the specimens with Ktis of 1, 5, 2o 1, 3. 1 and 8. 6 or 9.4 is shown in Figure lbo Only the notch root radius changed with the stress concentration factor. The values of the notch root radius for the different Kt's are as follows: 5

Stress Concentration Notch Root Radius, Factor, Kt inch 1.5 0.250 2.1 0.100 3.1 0.040 8.6 0.0044 9.4 0.0036 >20 <0. 0007 In the screening program, the resistance of the materials to catastrophic crack growth was evaluated using sharp edge-notched specimens of a design similar to that recommended by the ASTM (Ref. 1). The configuration of this specimen is shown in Figure lc. As was the case with the unnotched specimens, ten blanks were machined at one time, using a second fixture to maintain alignment. The reduced section was first milled to size. The notches were then ground almost to size with an alundum wheel on a 60-degree included angle. Notch roots for Ktfs of 1. 5 to 9 4 were lapped to final dimensions. Notch roots for the sharp (Kt >20) notches were finished by manually drawing a sharp carbide tool through the notches, using a shaper, until the required dimensions were obtained. Root radii and net section width were then measured using 50X optical comparatoro Tensile Tests All tensile tests were conducted with a 60, 000-pound capacity hydraulic tensile machine. Unnotched samples were strained at an approximate strain rate of 0. 01 inch per inch per minute up to about 2 percent deformation. The strain rate was then increased to about 0. 05 inch per inch per minute until failure. Notched specimens were loaded at a rate of 1000 psi net section stress per second. The test procedures followed those of References 1 and 2. 6

The specimens for tests at elevated temperatures were heated with an electric resistance furnace. Temperature variation along the gage length of the specimens was held to within ~ 5~F. Indicated test temperature was within ~ 3~F of the nominal temperature for all tests. 7

RESULTS AND DISCUSSION The notch strengths and the notch-tensile strength ratios for the three materials were evaluated as a function of notch acuity (stress concentration factor) at room temperature, 650~, 800~, and 1000~F. The results indicated that under certain conditions minimums in the notch-tensile strength ratio can occur at a lower notch acuity than obtained with the ASTM sharp edge-notch. Rene' 41 Table II lists the data obtained for Rene' 41 in the study of the influence of notch acuity on notch strength and on notch-tensile strength ratio. Rene' 41 exhibited a slight increase in notch-tensile strength ratio with initial increase in Kt (to about 3. 1), then decreased with further increases in Kt (Fig. 2). The literature indicates that most heat-resistant alloys show this type of behavior (Ref. 3). The ASTM sharp edge-notch (Kt >20) gave the lowest values of notch-tensile strength ratio at all temperatures and for both directions of testing. The longitudinal and transverse specimens behave very much alike except in two instances. First, replicate room temperature tests on specimens with Kt's of 8. 6 showed that specimens taken in the direction parallel to the direction of rolling (longitudinal) had significantly higher values of notch-tensile strength ratio than did specimens taken transverse to the direction of rolling (0. 95 and 0. 97 as opposed to 0. 79 and 0. 79). The second exception occurred at 10000F where the notch-tensile strength ratio fell off much more rapidly for the longitudinal specimens than it did for the transverse specimens. In this case, the transverse specimens maintained a high notch-tensile 8

strength ratio out to values of Kt >20. Waspaloy Waspaloy at room temperature exhibited only a slight peak in notch-tensile strength ratio at low Kt values (Fig. 3). The notchtensile strength ratio decreased somewhat when the value of Kt exceeded about 3. 1. At elevated temperatures, the peak persisted in the curve of notch-tensile strength ratio versus Kt, however, it shifted to a lower value of Kt at 1000~F as is shown in Figure 3. Table III lists the data obtained for Waspaloy during this investigation. Figure 3 shows the variation in notch-tensile strength ratio with theoretical stress concentration factor for Waspaloy. The principle feature of this figure is the minimum which appears in the elevated temperature curves at a Kt of about 9.4. At 650~F, this minimum only occurs in specimens taken in the longitudinal direction, however, at 800~F and 1000~F the minimum occurs both in the longitudinal and transverse specimens. Restating this in other terms, at 800~F and 1000~F Waspaloy has lower notch strength when the value of Kt is about 9. 4 than it does when Kt is in excess of 20 (ASTM sharp edgenotch). Figure 3 also shows that the drop-off in notch-tensile strength ratio occurs at lower values of Kt as the temperature is raised. While Waspaloy did show evidence of lower notch strength at a stress concentration factor of 9o 4 than it did at a value of greater than 20, the notch-tensile strength ratio did not fall below 0. 7 and, consequently, the existence of the minimum should not cause undue concern. D979 Table IV lists the results obtained in the study of the influence of notch acity on the notch strength and on he notch strength-tensile strength ratio of D979 alloy. Figure 4 shows the curves of notch9

tensile strength ratio versus Kt at the different test temperatures. This alloy exhibited a marked variation in properties with specimen orientation. At room temperature, values of notch-tensile strength ratio for specimens taken transverse to the direction of rolling were, in every case, significantly higher than those for specimens taken parallel to the direction of rolling. At 650~F, there is a crossover at a Kt value of about 13, above which the longitudinal specimens show a higher notch-tensile strength ratio. At 8000F, this cross-over took place at a stress concentration factor of approximately 5 and at 1000~F it occurred at a value of about 4, Minimums appear in the curves of notch-tensile strength ratio versus notch acuity at about Kt = 9. 4 for both longitudinal and transverse specimens at all temperatures except 650~F. Longitudinal specimens at 650~F exhibit a rapid drop in the ratio with increasing notch severity up to Kt 3. 1. Beyond this value, the notch-tensile strength ratio remains relatively constant. Transverse D979 specimens at 650~F exhibit a continual decrease in notch-tensile strength ratio with increasing Kto The transverse specimens of this material showed a peak in the notch-tensile strength ratio at about Kt = 1. 5. This peak decreased in magnitude with increasing temperature and at 1000~F, little evidence of its existence remained. Longitudinal specimens showed no evidence of such a peak except possibly at room temperature. Duplicate tests were run on this alloy at 650 ~F The specimens taken in the transverse direction showed little variability between samples while the longitudinal specimens showed significant variability of results at Kt's of 1. 5, 2, 1, and 3. 1. This is illustrated in Figure 4. A study of Figure 4 shows that at elevated temperatures the notch-tensile strength ratio of the longitudinal specimens decreased rapidly with increasing Kt. The fillets and pin holes of the smooth 10

specimens have stress concentration factors of 1. 15 and 1.75, respectively. For a value of Kt equal to 1,75, the notch-tensile strength ratio of D979 at 650~F is 0. 77, at 800~F, 0. 86, and at 1000~F, 0, 74. This deterioration of notch-tensile strength ratio at a stress concentration factor of only 1. 75 strongly indicates that the notch sensitivity of this alloy was responsible for its many failures at pin holes during tests of longitudinal smooth specimens at 650~, 800~, and 1000~F. This same explanation could also apply to the failures which occurred at the fillets. In this case (Kt = 1o 15), while the values of the notchtensile strength ratio were not as low (0. 91 at 650~, 0,97 at 800~, to 0. 92 at 1000 F), they quite probably were sufficiently low to have caused the failures which occurred at the fillets. This theory is enforced when one takes into account the significant amount of data scatter which occurred in D979 alloy in the region of the lower values of Kt. 11

CONCLUSIONS A limited evaluation of the influence of notch acuity on the notch strength of Rene' 41, Waspaloy, and D979 sheet materials at room temperature, 650~, 800~, and 1000~F has been completed. The data obtained give a good indication of the behavior of the materials under the influence of notches less severe than the sharp notch used in the original screening program of possible supersonic transport materials. The conclusions reached are summarized as follows: (1) The notch-tensile strength ratio generally had maximum values at Kt's of 1. 5 to 3. 1 regardless of specimen orientation or test temperature with the exception of D979 tested parallel to the direction of rolling at 650~, 800~, and 1000~F where the ratios fell off sharply at Kt >1. 0. (2) In at least one case for each alloy, at intermediate KtVs of 8.6 to 9.4 the notch-tensile strength ratio fell to values as low as or lower than those obtained for Kt >20. This raises the question of whether the ASTM sharp edge-notch specimen is the ultimate test for fracture toughness in the screening program. (3) The cause of brittle fracture of longitudinal unnotched specimens of D979 below the 0. 2% offset yield strength at 650~, 8000, and 1000~F in the pin holes and at the fillets is its pronounced sensitivity to dull notches (Kt >1 to 2). (4) Waspaloy exhibited the least anisotropy and loss of notch strength with increasing notch severity at the temperatures investigated. In no case did notch-tensile strength ratio fall below 0.7. (5) Rene3 41 exhibited little anisotropy and the loss of notch strength appeared severe only for Kt >20 at 650~, 800~, and 1000~F. 12

REFERENCES 1. Special ASTM Committee: Fracture Testing of High-Strength Sheet Materials, Chap. I. ASTM Bull. Jan, 1960, pp 29-40; Chap. II, ASTM Bull. Feb. 1960, pp 18-28. 2. Manning, C. R., Jr., and Heimerl, G. J,: An Evaluation of Some Current Practices for Short-Time Elevated-Temperature Tensile Tests of Metals. Langley Research Center, Langley Field, Virginia, NASA TN-D-420, September, 1960. 3. Voorhees, H. R., and Freeman, J. W. Notch Sensitivity of High-Temperature Alloys. WADC Technical Report 59-470, March 1960, 13

Table I CHEMICAL COMPOSITION OF EXPERIMENTAL MATERIALS Alloy C Si Mn Cr Ni Mo Si Al W P S B Fe Co Zr D979 0.078 0. 15 0.18 15.02 43.97 4.06 3.04 1.02 3.57 0.007 0.006 0.12 Bal Waspaloy 0.08 0.07 0.04 19.63 Bal 4.26 2.99 1.40 --- --- 0.007 0.0048 2.30 13.49 0.03 Rene'41 0.10 0.06 0.06 18.48 Bal 9.37 3.19 1.42 --- -- 0.007 0.0047 2.20 10.43 --- D979 alloy (Heat W23211) was produced by the Allegheny Ludlum Steel Corporation 4P Waspaloy (Heat B119) and Rene' 41 (Heat R216) were produced by the Metallurgical Products Division of the General Electric Company

Table II NOTCH STRENGTH AND NOTCH-TENSILE STRENGTH RATIO OF RENE' 41 AS A FUNCTION OF NOTCH ACUITY Notch Strength (1000 psi) Stress Concentration Factor Room Temp. 650~F 800~F 1000~F Kt La TO L T L T L T 1 244 237 229 222 221 217 221 218 1.5 252 Z48 235 234 231 230 226 222 264 256 2.1 26 228 234 230 228 210 221 266 261 263 254 3. 1 z68 254 232 226 233 238 202 224 268 260 241 186 8.6 237 188 169 171 179 174 161 181 196 >20 196 182 138 155 142 142 116 140 195 Notch-Tensile Strength Ratio Stress Concentration Factor Room Temp 650~F 800~F 1000~F Kt L T L T L T L T 1 1.00 1.00 1.00 1.00 1.00 1.00 1.00 1.00 1.01 1.05 1.5 1.01 1.05 1.03 1.05 1.05 1.06 1.02 1.01 1.06 1.08 1.07 1.09 2.1 1.07 1.09 1.00 1.05 1.04 1.05 0.95 1.02 1.07 1.10 1.06 1.07 3.1 1.06 1.07 1.01 1.02 1.06 1.10 0.91 1.03 *1.08 1.10 0.97 0.79 8.6 0o975 079 0.74 0.77 0.81 0.80 0.73 0.83 0. 79 >20 0.79 0.77 0.60 0.70 0.64 0.65 0.52 0.64 0.78 - Longitudinal specimen orientation - Transverse specimen orientation 15

Table III NOTCH STRENGTH AND NOTCH-TENSILE STRENGTH RATIO OF WASPALOY AS A FUNCTION OF NOTCH ACUITY Notch Strength (1000 psi) Stress Concentration Factor Room Temp. 650~F 800~F 1000~F Kt La Tb L T L T L T 1 237 232 211 204 204 199 204 199 1.5 246 235 220 218 212 208 206 202 218 212 221 211 2.1 246 236 2 214 210 207 209 221 221 224 210 3.1 244 242 2 210 214 174 173 222 220 9.4 221 216 155 185 144 152 160 151 161 182 >20 212 201 163 149 172 166 169 166 Notch-Tensile Strength Ratio Stress Concentration Factor Room Temp. 650~F 800~F 1000~F Kt L T L T L T L T 1 1.00 1.00 1.00 1.00 1.00 1.00 1.00 1.00 1,5 1.04 1.01 104 107 1.04 1.05 1.01 1.02 1.03 1.04 2.1 1.04 1.02 1.05 1.03 1.05 1.06 1.02 1.05 1. 05 1.08 1.06 1.03 3.1 1.03 1.04 1.0 103 1.08 0.85 0.87 1 05 1. 08 0.73 0.91 9.4 0.94 0.93 0 76 0.89 0.71 0.76 0.78 0.76 O. 76 0.89 >20 0.90 0.87 0.77 0.73 0.84 0.83 0.83 0.83 - Longitudinal specimen orientation - Transverse specimen orientation 16

Table IV NOTCH STRENGTH AND NOTCH-TENSILE STRENGTH RATIO OF D979 AS A FUNCTION OF NOTCH ACUITY Notch Strength (1000 psi) Stress Concentration Factor Room Temp. 650~F 800~F 1000~F Kt La Tb L T L T L T 1 273 262 244 237 235 233 214 234 218 251 1.5 276 283 1 214 240 170 236 175 240 206 232 2.1 261 280 6 188 209 146 212 163 220 151 174 3.1 240 262 151 174 166 189 135 169 144 151 9.4 171 173 14 19 143 123 122 104 >20 18 0 1 43 123 158 136 126 115 176 132 126 120 Notch- Tensile Strength Ratio Stress Concentration Factor Room Temp. 650~F 800~F 1000~F Kt L T L T L T L T 1 1.00 1.00 1.00 1.00 1.00 1.00 1.00 1.00 0.89 1. 06 1.5 1.01 1.09 0.8 1.0 0.91 1.03 0.80 1.01 0.72 1.01 0.84 0.98 2.1 0.96 1.07 0.67 0.93 0.80 0.90 0.68 0.91 3.1 0.88 1.00 0.62 0.73 0.71 0.81 0.63 0.72 0.77 0.78 9.4 0.60 0.66 0.59 0.64 0.61 0.53 0.59 0.44 0.61 0.59 0.59 0.52 0.49 >20 0.65 0.73 0.59 0.53 0.67 0.58 0.59 0.54 0.53 0.51 a - Longitudinal specimen orientation - Transverse specimen orientation 17

1.6 o.6 z5 _ o. 50 RAD. 1. 0.88 1.83 j 2.0. 1.83 7. 0 la. Smooth (unnotched) specimen (Kt = 1.0). NOTCH RADII 0. 250, 0. 100, 0. 040, 0.0044, & 0. 0036 r- "* \ —1.00 0625 + - -10.50-0 - Il I 0. 50 - I I08 |1.6 RAD. 0. 38 0. 884 1.13,. 1.0043 1.00 ~ 1.13 l 6.0. lb. Notched specimen for Kt = 1. 5, 2. 1, 3. 1, 8. 6, and 9. 4. NOTCH RADIUS <0. 0007 600 0.625 0. 70 1.6 0. ByL -1.00 RAD. 0. 38 0.188 1. 13 1L 1.00 _ 1.00 1.13 _ 6. o0 18

1.2 ' I I I I I I I I I!- 1. 2 II I I I o 1. 1 Room - 1. 1 - ~O 1.0 - 10 - '. 2 4-r Temperatures^~, — 0.8 - -0.8 - 0 0 0 0 ~~~~~ ~~~0. 5 9~~~~~ 0.95 10... 0.9 10.7 U o 0. - 0. o 0. - - 0.6 0. 1 o. 6 0.75 - - I ---- I-0. 5 1.0 2. 0 3. 0 4.0 5.0 10.0 20.0 1.0 2. 0 3.0 4.0 5.0 10.0 20.0 Stress Concentration Factor, Kt Stress Concentration Factor, Kt Fig. 2 Influence of Notch Acuity on the Tensile Properties of Rene' 41. rjIJ O 0.6 2: 0.9 609 si 0.8 5 0.8 Stress Concentration Factor, Kt Stress Concentration Factor, Kt Fig. 2 Influence of Notch Acuity on the Tensile Properties of Rene' 41.

1.2 1.2 o; < 1.1- Room 1 650_F o. 1 - 1o Temperature 1. 0 0 0. 9 0.9 2 0.6 -0.6: 0.8 II 0.85 1.0 2.0 3.0 4.0 5.0 10.0 20.0 1.0 2.0 3.0 4.0 5.0 10. 0 20.0 Stress Concentration Factor, Kt Stress Concentration Factor, Kt O- Longitudinal properties A- Transverse properties 0 1.22 I" o 1. 1 -0o 1.1 H.800~F 1000OF 1. - % 1. 0 o O 0.6 0.69 0.5 II-, - 0 I I I I I I —I I I__ _ _ _ _ __5_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ 1.0 2.0 3.0 4.0 5.0 10.0 20.0 1.0 2.0 3.0 4.0 5.0 10.0 20.0 Stress Concentration Factor, Kt Stress Concentration Factor, Kt Fig. 3 Influence of Notch Acuity on the Tensile roperties of Waspaloy. 0.6 0 0. 0.96 1.0 2.0 3.0 4.0 5.0 10.0 20.0 o. 0 2. 0 3.0 4.0 5.0 100 20.0 Stress Concentration Factor, Kt Stress Concentration Factor, Kt Fig. 3 Influence of Notch Acuity on the Tensile Properties of Waspaloy.

1.1 l 1.1, o 1.0 Room o 1.0 650~F Stress Concentratin F, Temperature. 0.9 \ 0'9 \ 0.8- 0 0.8 r) U) 0. 7 0.7 0o~r - I. ~ I~0.6 0i.4 Inl c o I I o t,TeniI I0.4 Properties o D979. ^ 0.6 - 00 9 - 0.5- 0.5 1.0 2.0 3.0 4.0 5.0 10.0 20.0 1.0 2.0 3.0 4.0 5.0 10.0 20.0 Stress Concentration Factor, Kt Stress Concentration Factor, Kt O- Longitudinal properties A- Transverse properties I-' I1.1 i. t,,, 1 1 I ], I [ [II 1.0 o 0o 10f 800F.0 10000F 0.9 A 0.9 0. 8 1 o0.8 '0 7 - 0.7 0o. 6~o- - 0.6 z Z O0. I I I I I I I I_ 0.4 II I I 1.0 2.0 3.0 4.0 5.0 10.0 20.0 1.0 2.0 3.0 4.0 5.0 10.0 20.0 Stress Concentration Factor, Kt Stress Concentration Factor, Kt Fig. 4 Influence of Notch Acuity on the Tensile Properties of D979.