A COMPARISON OF THE TENSILE AND BENDING PROPERTIES OF DENTAL GOLD WIRES

¹ School of Dentistry, University of Michigan, Ann Arbor, Mich.

An investigation was made of the relative values of the mechanical properties of precious metal wires in tension and in bending. The properties determined were modulus of elasticity and proportional limit in bending, and modulus of elasticity, proportional limit, and ultimate strength in tension.

A series of alloys in the form of 0.040 inch diameter wires was tested by means of a standard tensile test, and by a cantilever loading bend test. Each alloy was tested, first in the solution treated (soft), and second in the solution treated and aged (hard) conditions. The final values recorded for each alloy condition were the arithmetic averages of the data from three tensile specimens and three bend specimens. To obtain these individual values, stress-strain curves were plotted for each specimen, and the apparent proportional limit and modulus were determined from each curve.

The values found for proportional limit ranged from 50,000 psi to 130,000 psi in tension and from 70,000 psi to 215,000 psi in bending. A definite correlation between the results of the two tests was found. When plotted on semilog paper it yields a straight line having an empirical equation of the form Log_1OP_T - aP_B + b (1) where a and b are constants, P_T is the proportional limit in tension and P_B is the proportional limit in bending. a was found to be 3.004 x 10^-6, and b was found to be 4.478.

Earlier data by another investigator for similar alloys on specimens of the same size fit this curve very well. The average probable error of conversion from bend values to tensile values for the data of this investigation is 4.76 per cent; including the independent points it is 4.65 per cent. The alloys tested (covering the normal range of composition for wrought dental alloys) fit this single correlation in spite of varying clamping conditions in the bend test. The use of standard clamping conditions can be expected to yield an improved correlation unaffected by normal differences in alloy composition.

The results for modulus were more widely scattered but certain trends can be noted. A wide range of values was found for modulus in both tests. Tensile values ranged from 11 x 10⁶ psi to 18 x 10⁶ psi, while bend values ranged from 11 x 10⁶ psi to 19 x 10⁶ psi. As far as is known, this is a greater range than has previously been reported.

For most alloys and heat treatments, the modulus in bending is greater than that in tension. The effect of aging was to increase the modulus, both in bending and in tension, as the proportional limit was raised. An aging heat treatment increased the modulus, as found in bending, more than that found in tension.

Work is being continued on this project with the intention of extending it to a greater scope. It is believed that this comparison of tensile and bend properties of wires may be valuable, not only to the dental profession, but the correlation between tension and bending properties may be useful to those in industry where wires are employed.