The spacings between the turns of the coil can be obtained from photos, as here, or more simply by just rotating the support cylinder until it is horizontal (so that creep will stop) - spacings are then readily measured directly with a ruler (or on a piece of paper which has been held against the coil and imprints of the positions of the turns obtained by light use of a pencil - ie using a "brass rubbing" technique).
In the basic creep rate equation
substitution of previously-derived expressions for the strain rate within the wire
where C is a constant, and the average shear stress in the wire
and hence an expression for the velocity of the Nth turn of the coil
in which K is a constant. This equation can be used to obtain various parameters from experimental data. For example, by plotting ln(s/t) against ln(N), the value of the stress exponent, n, can be obtained as the gradient.
It can be seen that the calculated stress exponent is about 1, which is consistent with the fact that it is diffusion creep which is the main mechanism in these solder specimens over this temperature range. (Diffusion creep tends to predominate at relatively high homologous temperatures.)
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