The elasticity and resistance of the materials of engineering . the test piece, within cer-tain limits, affects very perceptibly the ultimate resistance. If a specimen of the shape shown in Fig. 2 be broken by atensile stress, it will, of course, fail in the reduced section MN,But before failure takes place, the reduced portion will be con- ^ siderably elongated and thenormal section correspond-ingly reduced, in conse-quence of the shearinge strains in the oblique planes^S^ shown by the dotted lines. (See Arts. 3 and 4.) When the reduced portion in the vicinityof MN is very short in comparison
The elasticity and resistance of the materials of engineering . the test piece, within cer-tain limits, affects very perceptibly the ultimate resistance. If a specimen of the shape shown in Fig. 2 be broken by atensile stress, it will, of course, fail in the reduced section MN,But before failure takes place, the reduced portion will be con- ^ siderably elongated and thenormal section correspond-ingly reduced, in conse-quence of the shearinge strains in the oblique planes^S^ shown by the dotted lines. (See Arts. 3 and 4.) When the reduced portion in the vicinityof MN is very short in comparison with its lateral dimensions,it includes the whole of very few of these oblique planes, if anyat all, consequently very little movement of these obliquelayers over each other can take place; in other words littleor no reduction of section can take place before rupture. Inthis latter case, then, a greater area of metal section will offerits resistance to the external tensile force, at the instant offailure, than in the former, and a correspondingly greater in-. Art. 32.] ULTIMATE RESISTANCE. 225 tensity of ultimate resistance will be found. Thus the shapeand dimensions of the test piece will considerably influence theultimate resistance and strains, as will soon be shown by ex-perimental results. All the preceding conclusions, though given in connectionwith wrought iron, are independent of the nature of the ma-terial, and apply equally to steel and cast iron. Since the reduction of area of the fractured section and theelongation of the bar are true measures of the ductility of theiron, these are or should be always measured with care. Table V. exhibits in a very plain manner the decrease ofultimate tensile resistance with the increase of sectional areaof round bars ; it is taken from the Report of the Committeesof the U. S. Board appointed to test Iron, Steel and otherMetals, etc., by Commander L. A. Beardslee, This decrease is probably partly due to the effect producedup
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