PHYSICAL PROPERTIESDOE-HDBK-1017/1-93Properties of MetalTherefore, the criterion of fracture (that is, the limit of plastic deformation) for a plastic materialis likely to depend on tensile rather than compressive stress. Temperature change may modifyboth the plastic flow mode and the fracture mode.Figure 8 Charpy Test EquipmentThe quality known as toughness describes the way amaterial reacts under sudden impacts. It is defined asthe work required to deform one cubic inch of metaluntil it fractures. Toughness is measured by theCharpy test or the Izod test.Both of these tests use a notched sample. Thelocation and shape of the notch are standard. Thepoints of support of the sample, as well as the impactof the hammer, must bear a constant relationship tothe location of the notch.The tests are conducted by mounting the samples asshown in Figure 8 and allowing a pendulum of aknown weight to fall from a set height. Themaximum energy developed by the hammer is 120 ft-lb in the Izod test and 240 ft-lb in theCharpy test. By properly calibrating the machine, the energy absorbed by the specimen may bemeasured from the upward swing of the pendulum after it has fractured the material specimenas shown in Figure 9. The greater the amount of energy absorbed by the specimen, the smallerthe upward swing of the pendulum will be and the tougher the material is.Figure 9 Material Toughness TestIndication of toughness is relativeand applicable only to casesinvolving exactly this type ofsample and method of loading. Asample of a different shape willyield an entirely different result.Notches confine the deformation toa small volume of metal thatreduces toughness. In effect, it isthe shape of the metal in additionto the material composition thatdetermines the toughness of thematerial.MS-02Page 26Rev. 0
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