NNR IN RAPIDLY ROTATED METALS By - Nottingham eTheses ...

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Consequently any resulting inaccuracy in the Knight shift value
quoted here is much smaller than the experimental error.
(b) Pressure. For those specimens of aluminium powder encapsul-
ated in resin the magnitude of the rotationally induced stresses
can be estimated using the expressions given in Section 4.2.3.
The maximum stresses in a rotating solid cylinder occur at the rot-
ation axis where at =r= 1/8(3 + u)pb2w2. The density'of the
aluminium powder slugs was found to be 1.6 g cm 3.
The precise
value of u is unknown but can be assumed to be less than but approx-
imately equal to 1. Consequently at a rotation speed of 4 kHz
the maximum pressures generated within the specimen are approxi-
mately 0.8 x 107 N m;
-2 .
The same stress equations cannot automat-
ically be applied to those powder samples packed directly into the
rotors. However with the assumption that the metal powder can be
considered as an isotropic viscous fluid, it can be shown by element-
ary calculation that the radial stress generated in the specimen is
a roximatel
22 (59)
pp y Apr w The maximum stress acts therefore at
the periphery of the sample. At the rotation speed employed of 4 kHz
(and substituting the density of aluminium metal for p) this press-
ure is given by 1.4 x 107 Nm2
Kushida and Murphy (96)
found that for a hydrostatic pressure
of 109 Nm2 the fractional increase in the aluminium Knight shift
was 1.31%. Hence, with the assumption that rotation and hydrostat-
ic pressures are comparable in their effects, the expected variat-
ion in the Knight shift at the highest rotationally generated stress
is 0.3 ppm. The mean pressure shift will lie within this value.
It follows that such effects have a negligible influence
experimental Knight shift value obtained.
on the