Mechanical properties of major clinical relevance are modulus of elasticity, yield strength (or proportional limit), hardness, and creep or distortion at elevated temperatures. Ultimate tensile strength, ductility, and toughness should also be reviewed,
although these properties have less relevance for metal-ceramic restorations. Except for hardness (and elevated temperature creep or distortion), all these mechanical properties are determined by loading a cast specimen of the alloy to the point of failure in a tension test at room temperature. The physical property of thermal contraction is critically important when choosing an alloy that is compatible with the porcelain selected. From a practical standpoint, the density is important to both the economics of alloy selection and the dental laboratory procedure with the casting machine.
Modulus of Elasticity. Figure 19-17 illustrates schematically the tensile stress-strain plot for a ductile casting alloy that undergoes substantial permanent deformation before fracture. This plot consists of two portions: (1) a linear or elastic region that ends at the proportional limit, where the stress is proportional to strain, and (2) a subsequent curved region corresponding to plastic or permanent deformation (that terminates when the test specimen fractures). The modulus of elasticity (called Young's modulus) is the slope of the stress-strain plot in the elastic region. The elastic modulus has the same value for tensile and compressive strains, which occur during bending of a prosthesis, where regions on opposite sides of the neutral axis (centerline for a
symmetric cross section) undergo opposite senses of deformation. An alloy with a higher modulus of elasticity has greater stiffness or rigidity for elastic deformation. For the fabrication of a long-span FPD, an alloy with a relatively high modulus of elasticity to reduce the amount of bending deflection under loading is preferred, since excessive flexure can cause fracture of the brittle porcelain (Fig. 19-18). The modulus of elasticity has units of stress/strain and is reported in units of GPa (1 gigapascal [GPa] = 103 pascals [Pa] = 145,000 psi [pounds per square inch]). The unit of 1 Pa = 1 N/m2 is much too small to be useful for the elastic modulus of materials.
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