testing-machine

Century Dictionary and Cyclopedia

1. n. A machine or apparatus for ascertaining by experiment or test the resistance offered by a material to a stress which tends to deform or break it. The machine may be adapted to cause failure by tension or pulling apart; by compression or crushing; by bending or fiexure; or by torsion or twisting. One machine may be adapted for any of the first three methods by simple change. The torsion machine is usually specially designed, and any of the others may be so. The elements of such a machine are: the grips or holding apparatus; the straining apparatus for applying load; the apparatus for observing, measuring, or recording the intensity of the applied load; the apparatus for observing or recording the deformations under applied load. The grip or holder for tension-tests is usually a form of wedge which moves in slots with converging sides. These wedge-grips are borne in holders, one of which is part of the straining mechanism and moves with it, while the other is borne by the table or platform to which the levers of the weighing mechanism are attached. When the specimen is placed between these two sets of holders, a movement of the straining mechanism to separate the two holders transmits its effort through the test-piece to the weighing levers, and thus to the graduated weigh-beam, on which a poise of known weight can be moved out until the stress in the specimen is just balanced by the weight on the beam. The stress can be gradually increased up to the capacity of the weighing beam to record it, or until the test-piece parts; the reading of the poise on the graduated beam then gives the breaking-load, which is reduced to unit load by dividing the test-load by the measured area of the test-piece. The weighing or reducing levels may be simple or compound; the fulcrums may be knife-edges, or thin flexible plates may be used as fulcrums (A. H. Emery' s system). The plate-fulorums do not have a friction increasing with the load in an unknown or variable ratio, as is the case in knife-edge fulcrums. The straining effect may be produced by geared screws, or by hydraulic cylinder action for large machines; in small ones a simple lever action is possible. In some forms the travel of the weighing poise is made automatic by electric or mechanical detents, so that with the lift of the beam the poise is caused to travel, and on its drop the travel stops. For measuring deformations, either a micrometric or vernier pair of scales is used, one element being attached to the specimen near the grip at one end, and the other element being similarly attached near the other. As the two ends are separated by the straining effort, one scale passes by the other and the stretch is measured for any load. The change in rate of stretch with equal increments of load indicates the elastic limit; when the stretch begins to increase steadily the yield-point is passed. An easy extension of this principle enables the moving scale to trace on a paper attached to the fixed scale a diagram in which the abscissas are deformations and the ordinates the load applied. Valuable facts and priuciples as to ductility, modulus of elasticity, and other properties can be deduced from such records. The ordinary capacities of testing-machines vary from 100,000 to 300,000 pounds. Unusual capacities are 900,000 and 1,200,000 pounds. Early testing-machines were made by Fairbairn and Kirkaldy in Great Britain, and later by Wicksteed. Early American types were devised by Richards, Miller, Riehlé, Thurston, and officers of the United States Ordnance Board. Thurston' s torsional machine has been one of the best known of its class. Every important steel manufacturer uses the testing-machine, and most specifications demand its use before a consignment of product is accepted. Special forms are also in extensive use for cement, springs, wire, textile products, twine, paper, and all material in which strength is an importantfactor. Special types of testing-machines have also been devised to test the abrasive resistance of brick for paving, the lubricating quality of oils and greases, resistance to frequently applied loads, etc.
2. n. Any machine used to test an isolable quantity or property in a substance.