Janett, Leslie G. (ed.) / The Wisconsin engineer
Volume 39, Number 3 (December, 1934)
Kommers, J. B.
Elastic strength, pp. 41-42
A New Standard Elastic Strength for Ductile Metals By J. B. KOMMERS DESIGN engineers are always interested in knowing D what allowable stresses may be used for a given ma- terial of construction. For a material like structural steel it is common practice to make a tensile test to help in answering this question. The tensile test determines, among IJLIwtJJ.Lilili^s, tflC UbL1C [l111 gS, Me ultimate tensile strength and the yield point of the material. An al- lowable or safe unit stress should be well below the ultimate, but it should also be be- low the yield point, because when the yield point is reached the ma- terial suffers per- manent deforma- tion. In other J. B. KOMMERS words, at the yield point and beyond, the material does not behave in an elastic manner, and will not return to its original dimensions when the load is removed. The above remarks apply particularly to structures subjected to static or quiescent loads. For machine parts subjected to millions of repeated or reversed stresses, the allowable unit stresses used in design must be still lower, in order to prevent the formation of a crack due to fatigue action. Also for cases involving impact, the allowable unit stresses must be lower than for static loads. Under the heading "Methods of Tension Testing of Metallic Materials," in the 1933 book of Standards of the American Society for Testing Materials, the following defini- tions are of interest in the present discussion. The four values defined are all intended to be approximations of the elastic strength. Elastic Limit-The greatest stress which a material is capable of developing without a permanent deformation re- maining upon complete release of the stress. Proportional Limit-The greatest stress which a material is capable of developing without a deviation from the law of proportionality of stress to strain (Hooke's Law). Yield Point-The stress in a material at which there oc- curs a marked increase in strain without an increase in stress. Yield Strength -The stress at which a material exhibits a specified limiting permanent set. The definitions of elastic limit and proportional limit re- mind one of some of the political and economic plans which are common topics of discussion today. They are examples of things which seem very simple on paper, but which are not simple when an attempt is made to get actual results. In determining the elastic limit it is necessary to apply loads to the specimen, reduce the loads to zero or nearly zero, and then determine from the extensometer measure- ments whether any permanent deformation or set has oc- curred. This process is continued with increasing loads until a permanent set is registered. A practical question which arises is the amount of permanent deformation which shall be used as a criterion. If the extensometer which is used can be read to 0.000001 of an inch, there will be an indication of permanent set at a much lower unit stress than if the extensometer can be read to only 0.0001 of an inch. Some experimenters have even reported that when delicate apparatus is used there is evidence of inelastic ac- tion at all stresses. The writer has also shown* that, at least for certain brasses, the repeated loads used in determin- ing the elastic limit may have the effect of improving the elasticity, so that the value obtained for elastic limit does not represent the material in its original condition. Besides this, the test is tedious and expensive and therefore is not a practical commercial test. In determining the proportional limit of a material it is necessary to plot the unit stress against the unit deforma- tion and then determine the lowest unit stress at which the curve first deviates from a straight line. There are many factors which may influence the result obtained from such a test. In a paper published in the 1929 proceedings of the American Society for Testing Materials, R. L. Templin, of the Aluminum Company of America, discusses some of these factors. He classifies them under five major headings: (1) the material tested, (2) the test specimen used, (3) the testing apparatus used, (4) the testing procedure followed, and (5) the interpretation of the results obtained. Under (1), Templin mentions definite grain due to method of manufacture, variations in structure due to voids, seams, inclusions, grain size, segregation of constituents, and uneven temper, any one of which may cause local yielding and affect the proportional limit. Under (2), he mentions variation in dimensions, straight- ness, and internal strain, resulting from the procedure used in preparing specimens. He cites an example of an alumi- num alloy, one specimen of which was slightly bent before testing, with the result that its proportional limit was much lower than that of a normal specimen. Machining of speci- *Engineering, London, June 22 and 29, 1934. December, 1934 Page 4 1
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