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Aviation History
1926
1926 - 0720.PDF
SUPPLEMENT TO FLIGHT SEPTEMBER 30, 1926 THE AIRCRAFT ENGINEER slip. Similar effects have since been obtained with single crystal specimens. (iv) The course of the final crack was such that it inter- sected the crystal boundaries almost orthogonally. No indication of intercrystalline fracture was found. The work being of a preliminary nature only, further tests of the same kind, and also tests employing other forms of loading, will be made. It is also hoped to study the effect of varying grain size on fatigue phenomena. SOME MECHANICAL TESTS OF CAST BARS OF ALPAX. By H. J. TAPSELL, A.C.G.I. R. & M. No. 1011 (M. 34) (9 pages and 2 diagrams). December, 1925. Price 9d. net. The work has been carried out in accordance with the approved programme on the mechanical properties of alloys of silicon and aluminium. The paper deals with the results obtained of some mechanical properties, at air temperature and at elevated temperatures, of cast silicon-aluminium bars (1£, 2 and 3 ins. diameter) called " Alpax," supplied by Messrs. Lightalloys, Ltd. Results are given of the following tests : Tensile tests, notched bar impact tests, Brinell hardness tests, and fatigue tests under direct reversed stresses. This material is a valuable casting alloy. The ultimate tensile strength compares favourably with that of chill-cast Y alloy, though inferior to that material in the heat-treated condition. The elongation is unusually high for a cast- aluminium alloy. The tensile strength falls steadily with rising temperature. At 250° C. the value—7 tons/sq. in.— is much lower than that of Y alloy as cast. At 350° C. the value—4 tons/sq. in.—does not compare well with that of Y alloy—7 tons/sq. in. for material as cast and nearly 12 tons for cast heat-treated material. In the sand-cast state this material is only slightly inferior and gives still a high elonga- tion and tensile strength. Castings up to 3 in. diameter are only slightly inferior in mechanical properties to castings of 1 in. diameter. The material is not free from casting defects. In this respect the 1 in. diameter chill-cast bars were least satis- factory, a number of them containing large cavities. The sand-cast bars were quite satisfactory. The bars of larger diameter, 2 and 3 in., were also, on the whole, satisfactory. The cavities detected in them were small for castings of such dimensions. The fatigue range is fair for a cast-aluminium alloy, but does not approach the results given by some heat-treated alloys. FURTHER EXPERIMENTS ON THE RELATION BETWEEN SKIN FRICTION AND HEAT TRANSMISSION. By Miss DOROTHY MARSHALL, B.SC. Work performed for The Engineering Research Board of the Department of Scientific and Industrial Research. R. & M. No. 1004 (Ae. 211). (19pages and 11 diagrams.) June, 1925. Price Is. net. Several previous workers, commencing with Osbome Rey- nolds in 1874, have made experiments to find the relation between the heat lost by a hot surface when cooled by a current of air passing over it, and its frictional resistance. Later work has been described in the reports of the Aeronautical Research Committee, R. & M. 94,* 1913, and R. & M. 243, f 1916. The experiments here described consist of two distinct series of tests :— (1) Tests made on a short heated section of pipe 5 in. in diameter through which a current of air was forced, the heat transmitted being estimated from the rise of temperature of the air. (2) Tests made on thin short cylinders or rings of nickel supported in the 3 ft. wind channel and heated electrically, the heat transmitted being measured from the energy supplied to the rings. In the second series of tests the surfaces of one cylinder were artificially roughened. • B,. & M. 94. Surface cooling and skin friction.—Lanchester. With a nAppendix by T. E. Stanton, D.Sc, M.Inst.t'.E. t B. & M. 243. Heat transmission over surfaces.—Is.P.L. As regards quantitative agreement between the observed heat transmission and the theoretical heat transmission calculated from the measured surface friction, the results of the pipe experiments showed as before a considerable dis- crepancy between the two quantities. In the experiments with smooth rings, the calculated heat was some 20 per cent, less than the observed heat; but with the roughened rings the deficiency was less, the second roughened ring giving fairly close agreement between the two values over a considerable range of speed. The investigation has proved of considerable interest in its bearing on the surface friction of thin plates in the neigh- bourhood of the leading edge, and the results are in fairly good agreement with the law of surface friction deduced by Blasius from the Boundary Layer Theory of Prandtl, both as regards the law of variation of frictional resistance with speed and the actual values of the forces. As regards the effect of surface roughness, the experiments show that this depends to a greater extent than was antici- pated on the dimensions and forms of the irregularities which constitute the roughness. SOME COMPARATIVE FATIGUE TESTS IN SPECIAL RELATION TO THE IMPRESSED CONDITIONS OF TEST. By H. J. GotTGH, M.B.E., B.Sc., and H. J. TAPSELL, A.C.G.I. Work performed for the Engineering Research Board of the Department of Scientific and Industrial Research. R. & M. No. 1012 (M. 35). 21 pages. April, 1926. Price Is. net. The report contains the results of a lengthy series of endu- rance tests on the two steels adopted by the Aeronautical Research Committee as standard material for comparative tests by various investigators. Three types of testing machines —the Haigh, Wohler and Stromeyer—have been used and three types of specimen have been employed. It is shown that the varying results obtained cannot be reconciled on any simple basis, such as, for example, the criterion of limiting shear stress. When, howei'er, the results are discussed with reference to the impressed conditions of test, i.e., the modifications in the conditions of stress and strain in the test piece, caused by super-elastic straining within the fatigue range and brought out by the characteristics of the testing machine used or of the shape of test piece employed, it is shown that the variations in calculated values of the fatigue range are of a similar order to those which would be expected from theore- tical considerations. Hence, the main result of the investigation is to furnish direct experimental evidence of the importance of the im- pressed conditions of test. From the practical point of view, this is equivalent to emphasizing the importance of ductility in a material which is subjected to fatigue action. The following is an account of some experiments on the fatigue strength of various metals which have been made from time to time, in the Engineering Department of the National Physical Laboratory. Some of the general con- clusions that can be drawn from the results of these tests may be of interest as they throw some light on various fundamental aspects of fatigue testing which have been rather neglected in the past. It is proposed to discuss the present results chiefly from two aspects :— (1) The influence of the type of loading employed on the fatigue strength of the metal under test and ; (2) The influence of the shape of specimen employed on the fatigue strength when specimens of several types are used in the same machine. A general conclusion that we have reached is that the fatigue range of a metal, as determined using a Wohler rotating bar machine, is-vej^r. largely unaffected by the type of specimen used (solid or hollow^.". These Reports are published by His Majesty's Stationery Office, London, and may be purchased directly from H.M. Stationery Office, at the following addresses : Adastral House, Kingsway, W.C. 2 ; 28, Abingdon Street, London, S.W. 1 ; York Street, Manchester ; 1, St. Andrew's Crescent, Cardiff ; and 120, George Street, Edinburgh ; or through any book- seller. 636/?
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