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Aviation History
1928
1928 - 1012.PDF
soSUPPLEMENT TO FLIGHT OCTOBER 25, 1928 THE AIRCRAFT ENGINEER unfortunately, some cases where the positions of holes are not accessible for punching. The horizontal studs S are used for holding the drilling jig R. This is moved into an adjoining position after the drilling of each set of eight holes. It is not claimed that the above methods of assembly would be adequate for large-scale production, but such production would warrant the spending of a larger sum of money in tools than is indicated by the appliances described. The amount of money that may be spent in tools preparing for a job should bear some proportionate relation to the number of parts required; Fig. 6, for instance, merely shows a simple but effective jig suitable for the requirements of the moment. (To be continued.) DESIGN STRESSES By CECIL D. HOLLAND, A.M.I.Ae.E. (Concluded from p. 73) The design stress may now be re-defined as follows :— " The maximum stress the material will be subjected to when structure is loaded to an amount—the load factor times the normal load'"—this maximum stress being 1-25 times the yield or proof stress of the material, with the following exceptions :— (a) When the ultimate stress is less than 1 • 25 times the yield or proof stress. (b) Struts and built-up sections where the stability of the section cannot be calculated, when the proof stress should be used. Having enunciated a policy, what are the various design values required by the Design Office ? Tension.—The ultimate yield or proof stress can be ascertained by the standard test methods ; and the design stress can then be determined. Compression pure.—E.g., very short struts and"bending. Same remarks as for tension. Compression for long struts.—As there is no known method of calculating the stability of a structural section, it is usual to use the yield or proof stress as the design stress. (As the number of materials that are likely to be used for long struts is small, a separate table could be" prepared for these.) Shear.—The same remarks apply to this as those given or tension and pure compression. It is interesting to note that in the case of steels, the design shear stress appears to have some direct relationship with the ultimate tensile stress. Thus at 20 tons ultimate the design shear stress is 75 per cent, of the ultimate, while at approximately 50 tons ultimate, the shear stress is 60 per cent, of the ultimate. A rough-and- ready rule is " The design shear stress is equal to half the ultimate tensile plus 5" (all in tons per sq. in.). This agrees well with known tests. Bearing.—Unfortunately. " bearing " is not a stress of the fundamental order in the sense tensile, compression and shear are, and up to the present, no standard test has been instituted to determine this. The need for knowing such a stress occurs in the design of attachments made by bolting oi riveting. Bearing stress can be best described as :—" The stress the material is subjected to by the bolt or pin trying to crush into it, which is measured by dividing the load by the projected area of the pin." An examination of a simple joint—see sketch below—will show that the stress under question is really a ease of constrained compression. J The lateral strain is constrained by the'material each side of the pin at right angles to the direction of load in the plane of the material, and if the bolt or rivet tightly grips the material, the strain at right angles to the plane of the material is also restricted to some extent. Values used in general engineering are usually derived from isolated experiments that are restricted in both range of materials and design. Empirical formulae based on such tests are ft, = 1| //. (ultimate) (Pippard and Pritehard). fb = 2 U (Morley). fai . when applied to the range of materials and construction that exists in the aircraft industry. Without attempting to become confused with unsolvable mathematics, the material which crushes in line with the pin appears to fail by compression, and the shearing of the fibres in line with the outside edges of the pin. The following empirical formula appears to cover the material and constructions within the limited experience of the author. fc + /« all design stresses. 936/
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