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Aviation History
1964
1964 - 1157.PDF
75 2-0 MACH NUMBER 30 Fig I (above) Variation of flight efficiency with Mach number Fig 2 (right) Variation of hot tensile strength after soaking at elevated temperature for IO,OOOhr, expressed as percentage of ultimate tensile strength at room temperature \solution treated and artificially tged; 50 25 50 100 130 TEMPERATURE ("C) DESIGN FOR MACH 2.2... of the most favourable alloy, soaking times were limited to lO.OOOhr. These were sufficient for demonstrating the superior properties of A-U2GN, which therefore became the choice of the Concord con- structors, and on which experimental efforts then became concen- trated, with tests of very long duration, including soaking tests of 30,000hr. This alloy has been known for many years, and its con- ditions for use were fairly rapidly adapted to the requirements of the Concord. Other alloys continue to be investigated. Static Properties Measurements of static properties were carried out at 20,130, and 150°C, and following tens of thousands of hours of heating at 130 and 150°C. Some 10,000 specimens tested in Great Britain and France show that strength properties of A-U2GN are only very slightly affected at temperatures up to 120°C, and show no significant change up to about 150°C. Other tests have shown that the effects of heating times are cumulative, i.e., that 15,000 flight cycles—rising to the selected temperature, followed by holding for 2hr at that temperature before cooling—produce the same result as continuous heating for 30,000hr. In fact, the reduction in strength does not exceed 2kg/mm2 (2,8451b/sq in) at 130°C for 30,000hr. This very slight variation, which is nevertheless taken into consideration for stress calculations, has practically no effect on any modification of the permissible stress ratio, which is chosen to prevent creep deformation and fatigue damage. Creep When subjected to elevated temperature and a constant load, materials become elongated as a function of time. This occurs in three stages, according to which the creep rate, having slowed considerably during the secondary stage, increases again during the tertiary stage. Tests were carried out at 130 and 150°C, for lO.OOOhr on all tested materials and extended to 30,000hr for A-U2GN. These tests eliminated the 2219 alloy, as well as A-U4GI, sol- ution-treated and artificially aged. The American alloy is very susceptible to creep, and its elongation reaches 0.1 % after lO.OOOhr at 130°C under a load of 8kg/mms. Elongation is less on A-U4GI (Fig 3); it remains below 0.1 % at lO.OOOhr at 130°C and at rela- tively high stresses, but the beginning of the tertiary stage occurs abruptly at low elongation values (between 0.05% and 0.20%). With A-U2GN elongation is very uniform, and the transition from secondary to tertiary creep occurs only at high stresses, after the elongation has already reached 0.5 %. Normally, at a stress of 8kg/mm2—i.e., of the order of magnitude which is not to be ex- ceeded if good fatigue strength is to be maintained—the creep elongation does not exceed 0.02% after 30,000hr at 130°C, which is negligible. Fatigue In spite of its superior properties and strength at elevated temperatures, X2020 could not be selected for the Concord, owing to its notch fatigue sensitivity. A-U2GN, on the other hand, fulfils all expectations based on its earlier applications, in the form of forgings, at temperatures which could approach 250°C with engine components. Its fatigue strength is, in fact, comparable—or, rather, superior—to that obtained for A-U4GI under conventional aircraft conditions, i.e., better than that of the alloy at present con- sidered one of the best from this point of view. The effect of heating on the fatigue strength of A-U2GN has been investigated by a very large number of tests on plain or notched specimens, and on assemblies at 20,150 and 175°C, with and without previous soaking. One result of these tests is that the fatigue strength of A-U2GN is the same as for A-U4GI, and is not affected either by the test temperature, or by preliminary heating for thousands of hours. Another result is that A-U2GN is a little less notch-sensitive after heating at 130°C for tens of thousands of hours than A-U4GI without heating. Crack propagation rate was also investigated, and it was found that at 20°C this is the same for A-U2GN and A-U4GI. With A-U2GN, however, this rate is reduced when the temperature increases, so that at 150°C it is half the 20°C value. But it should be noted that these phenomena may vary when the frequency of load application becomes very low. Thus the crack propagation rate at 150°C doubles when the frequency is reduced from 2,000 cyc/min to 20 cyc/hr. For these reasons studies of the fatigue behaviour of A-U2GN had to cover a very extensive range of frequencies of load appli- cations. A wide variety of testing machines had to be used, from vibrophores and conventional pulsators, to creep machines which were specially equipped to perform load modulation at very low frequency. i% AU4G1 T6 AU2GN T6 Fig 3 Creep elongation of two minium alloys at I3O"C Hours
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