Figs. 13 and 14 show a form of single-lever testing machine designed by J. H. Wicksteed (Proc. Inst. Mech. Eng., 1882) ; the machine shown exerts a maximum force of 3o tons, but ioo tons or more are exerted by similar machines in common use. AA is the lever, on which there is a graduated scale. The stress on the Test-piece is measured by a weight W which can be moved through a measured distance along AA by a handwheel H connected with 4149-56 have been based, by permission, on the article written by Sir J. A. Ewing for the iith edition of this Encyclopaedia.
gearing SS. The Hooke shaft R is shown in a separate sketch in fig. 14. The upper end of the sample is gripped in a holder which hangs from a knife edge 3 inches from the fulcrum of the lever; the lower end is gripped in a similar holder which is jointed to a cross-head C, connected by adjustable screws (to provide for specimens of different lengths) to a lower cross-head B, on which the hydraulic plunger exerts its thrust. G is a counterpoise which pushes up the plunger when the water is allowed to escape. In this machine the hydraulic pressure is applied by means of an auxiliary plunger Q of small diameter. Q is driven by a belt on pulley D.
Pressure being admitted to the main plunger, a load is imposed upon the specimen, and the weight W is then run out along the lever until this just "floats" between the stops EE. The load (and hence the stress) can thus be determined, and it remains to measure the accompanying strain.
ments made on one side of the specimen only are liable to be in error on account of bending caused by slight eccentricity of loading: accordingly, it is best to measure the relative displacement of two pieces at tached in such a manner as to share equally in the strain on both sides.
The extensometer of J. A. Ewing is illus trated (diagrammatically) by fig. 15. Two clips B and C are attached to the specimen, each by means of a pair of opposed screws. Between B and C is a rod B', hinged to B and fitting into a spherical recess in C. A bar R hangs from C by a hinge Q, and carries a mark which is read by a microscope attached to B. Thus, as the specimen stretches (B' remaining of constant length), the bar R is pulled up rela tively to the microscope, and the amount of the movement is measured by a micrometer scale in the eye-piece. Calibration is effected by means of a micrometer screw attached as indicated at B.
Strain Beyond the Elastic Limit: Influence of Time. 51. Within the region of "plastic" distortion (i.e., where the stress and strain are not directly proportional) it is found that the behaviour of a metal is largely in fluenced by the rate at which the load is increased ; the full strain corresponding to a given load is reached only after a percep tible time. Fig. 16 gives some results obtained by Ewing (J. A.
Ewing, Strength of Materials, P 1914), from tensile tests of soft iron wire. The "fast" load ing caused rupture in 4 minutes: the "slow" loading took 5,000 times as long. When a tensile load, of amount exceeding the elastic limit, is applied and then kept constant, the specimen is ob served to extend at first rapidly and then more slowly. In general it appears that the slow extension comes ultimately to an end; but when the applied load is nearly equal to that which would imme diately break the specimen, the extension, after slowing down, quickens again and continues until rupture occurs. If on the other hand, the specimen is subjected to an extension which is main tained constant, it is found that the load required to maintain that extension gradually diminishes.