The Application of a New Structural Index to Compare Titanium Alloys With Other Materials in Airframe Structures

Abstract

Abstract The evaluation of structural materials for airframes is based on the strength/density ratio or, more precisely, the ultimate strength/density ratio, the compressive yield strength/density ratio, and the structural indexes for plates and columns. The structural indexes developed for plate elements have, in the past, been based on the formulaSe=1K1/2[12(1−μ2)π2]1/2Fbu3/2Er1/2 The application of this relationship to complex sections depends on the weighted average of the various plate elements that constitute the section and on the proper selection of their end fixities. Past experience indicates that this method is often unreliable and does not predict the failing load of the section, since the formula predicts only the stress at which the first buckle occurs in each respective element. A new method of predicting the failing load of a section in compression was suggested in 1954 by R. A. Needham. His method has been used to develop a new structural index for the evaluation of the strength/density ratio of materials in compression. In this report, this new structural indexSe=2.56Fcc2.18(ηFcyE)0.59 is compared with the results of compression tests on aluminum sections at room temperature, and similar tests on titanium-alloy angles and channels at room and elevated temperatures. The aluminum test data are from Needham’s report; the titanium-alloy test data were supplied by Convair (San Diego) and Boeing (Seattle). Excellent agreement is obtained in all cases. It is believed that the new structural index will provide a reliable method of evaluating the elevated-temperature performance of materials such as titanium, in comparison with other materials, when only the compressive stress-strain curves are available at each temperature.