162 AIRCRAFT ENGINEERING July, 1931 A Practical Article on the Meaning and Use of the New Method of Testing Material By E. Skerry* gauge length the extension would be 0·004 in., HIL E it is not expected in this short thu s :— pape r on the Proof Stress (from the point of view of its application to steel) to W I n testing materials the elastic limit reveal anything new, it is hoped, in view of its an d yield point were the criteria used increasing importance, that it may be of interest. for comparison and for specification Th e proof stress is of the utmost importance to purposes . But the fact that after the th e designer, and its appearance in recent specifi elasti c limit had been passed there was cations is noteworthy, showing as i t does a recogni a permanent set in the material made tion of the need for a stress value that can be th e yield point of little practical im accuratel y determined as against the rather crude portanc e to the designer , while in many method s of obtaining the yield stress, and the moder n materials it is , in any case, not impossibility of determining the elastic limit with readil y recognisable. The proof stress Fo r a 2-in. gauge length X = 0·0018 × 2 = any degree of confidence in the result. i s now becoming the almost universal 0·0036 in. (approximately 0·004 in.). criterio n and is being incorporated into Th e Proof Stress Defined specifications . It is felt, therefore, that Metho d of Reading Proof Stress thi s article on it will be of interest, to I t is defined as the greatest load per sq. in. which, Th e method of reading the proof stress from the practica l engineers. when applied for 15 seconds, an d removed, produces stress-strain diagram is illustrated in the diagram a permanent extension of not more than 0·1 per shown. The extensions measured in thousandths cent of the gauge length. (In the case of some of an in., representing the strain, are plotted as material s 0·5 per cent of the gauge length is used.) abscissa?, and small increments of intensity of Th e proof stress then, may be considered as the stress, in tons per sq. in., are plotted as ordinates. proof stress, and as a minimum figure in the case elastic limit plus a constant, and since the elastic Th e line of proportionality is continued hypothetic- of the higher limit of proof stress, an d the extensions limit is at the end of the elastic line, or line of ally and is represented by A B. If a second line obtaine d comply with these conditions, it is obvious proportionality, it follows that the proof stress is CD is drawn parallel, the two lines being a tha t the proof stress is a t some point between 40 th e stress a t which the stress-strain curve departs distanc e equal to 0·1 per cent of the gauge length an d 50 tons per sq. in. from tha t line b y an amount equal to the constant; apart , the proof stress is the point where the line or, alternatively, it may be defined as the stress at C D bisects the curve at P, and the load may Calculating the Actual Stress which an extension is obtained of an amount equal be read at 5. to th e constant after Hooke' s La w ceases to apply. This method is simply a means of checking the I t is interesting to note the relationship of stress I t will be seen tha t while th e elastic limi t and the rang e and does not indicate the actual stress. For an d strain in the diagram. (It must be remem yield stress arc th e definite result of strain induced thi s purpose i t is advisabl e t o calculate the extension bered tha t a 2-in. gauge length is again used.) The b y load, the proof stress is an artificial value whose for a small increment of load, say, 10 tons per stress at S is 42·5 tons per sq. in., and it will be relative position on the stress-strain curve may be sq. in., the proof stress occurring when this exten seen that the actual extension obtained at this altered by a variation of the constant. sion is reached, thus :— stress is 0·00875 in. The calculated extension is as I t follows tha t if stress is proportional to strain follows :— in a truly elastic body, then where A' = the extension. f = the load (in tons per sq. in.) . Finally, it will also be noted tha t the extensions l = the gauge length. obtaine d at 40 and 50 tons per sq. in. comply with E = direct modulus of elasticity. B y applying this load at a somewhat advanced th e conditions for a proof stress within that range. Th e figure of 12,500 tons per sq. in. is taken as stage of the test, but, of course, before the clastic representing E, though it is possible that the true limit is reached, errors which often occur in the proof stress can only bo determined by ascertaining early stages of the test due to the initial loading of British Association th e modulus for the particular class of steel under th e test piece are avoided. This is well illustrated Th e British Association for the Advancement of test . This, however, is neither necessary nor prac in one of the older specifications which calls for a Science is celebrating its centenary this year in ticable for production testing, and would lead to a minimu m proof stress of 40 tons per sq. in., and London from September 23-30. good deal of confusion and controversy, and in defines the proof stress as "th e load tha t produces Th e meetings of Section G (Engineering), of an y case would not make material difference to a total extension 0·002 in. greater per in. of gauge which Sir J. A. Alfred Ewing is President, will be th e result. lengt h than the extension produced by a load of held at the City and Guilds College, Exhibition Fro m the preceding formula it will be apparent 30 tons per sq. in." That is to say, for a 2-in. Road , S.W.7. The papers to be read include :— tha t the proof stress occurs at a load where an On September 24, "Metal Alloys in Relation to extension is obtained of Engineering Progress " by Dr. W. Rosenthain, and "Th e Effect of Temperature on Some of the Physica l Properties of Metals " by Prof. F . C. Lea ; Wher e a range of proof stress is specified, for on September 25, "Motorless Flight" by the example 40 to 50 tons per sq. in., the extension is Master of Sempill, "The Compressed-Air Wind calculated substituting the specified load for f, Tunne l at th e N.P.L. " b y Mr. E . F . Relf, an d "Th e thus: — New Wind Tunnels of the R.A.E." by Mr. R. McKinnon Wood ; on September 29, "Forc e Fits an d Shrinkage Fits " by Prof. E . G. Coker. British Standard Specifications Th e British Engineering Standards Association announc e the issue of the following new specifica tions . B.S.S. 87. Part I, 1931. Airscrew Hubs. B.S.S. 87. Part II, 1931. Engine Flange Fixings. 3S. 24 (cancelling 2S. 24), Bright Steel Bars for Keys . 4T. 8 (cancelling 3 T. 8). Annealed Seamless Brass Tubes. 2 T. 18 (cancelling T. 18). (A gauge length of 2 in. is used in the calculations.) Har d Drawn Seamless Brass Tubes . C.C. (AC) 3312. If the extensions thus calculated are taken as a Sectional List of British Standard Specifications for maximu m figure in the case of the lower limit of Aircraft Materials and Components. Copies are obtainable from the offices of the Association, a t 28, Victoria Street, London, S.W.I, * Mr. Skerry is Superintendent of the Testing Department at price 6d. each, plus postage. Messrs. Kayser, Ellison & Co.'s Carlisle Works, Sheffield.
Aircraft Engineering and Aerospace Technology – Emerald Publishing
Published: Jul 1, 1931