Dissemination of IT for the Promotion of Materials Science (DoITPoMS)

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Spacings after coil has been crept under its own weight for 1 minute at 65 <sup>o</sup>C.

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. We say that the spacing of the Nth coil is the distance between the Nth and (N-1)th coils, i.e the 1st coil’s spacing is the distance between between the 0th and 1st coils – there is no 0th coil spacing. Hence the velocity of the Nth coil is it’s spacing divided by time as we assume steady state creep throughout.

In our analysis we use equation derived earlier and plot ln(s/t) against ln(N) so we can deduce the stress exponent from the gradient:

\[\ln \left( {\frac{s}{t}} \right) = K + n\ln (N) - \frac{Q}{{RT}}\]

where \(K = n\ln (B) - \ln (C)\)

Plot of ln(s/t) against ln(N)

Activation Energy 

Also, by plotting ln(s/t) against 1/T, for a given value of N, the activation energy Q can easily be determined from the gradient. The photos below are taken after creeping for one minute. The spacing between the 11th and 12th coils (in mm), i.e. the 12th coil spacing, are marked. We only use one coil spacing, but in practice we could use many more. Hence we could repeat the analysis many times whilst only having carried out 3 tests.