The knowledge of the mechanical properties, and particularly of
the elastic modulus and dissipation factors of films, is quite important
from a practical point of view. Recently, some apparatus has been set up
for measuring the dynamic moduli by the method of constant- frequency
forced vibrations. Resonance methods are unfit by the usual apparatus for
direct measurements, the films always exhibiting a resonance
It consists of a small metal
sheet clamped to one end; the film to be examined is glued to the top of
the other end by an epoxy resin standing both high and low temperatures.
The film is bent to 90° in respect of the sheet and fixed in the clamp
below. The film clamp is mobile in the vertical direction after loosing
and blocking the side screw through a micrometric movement adjusted by a
screw placed below the supporting framework. Horizontal movements by
blockable sliding guides are allowed. The system is inserted in a cell
where vacuum may be obtained and where an electrode, put above the metal
sheet unclamped end, excites forced vibrations. By the clamp vertical
moment it is possible to adjust the initial static stress in the film.
This may be evaluated on the basis or the sheet end deflection (e. g. by a
cathetometer) prior to performing the dynamic measurements. Thermostatting
is accomplished by inserting the cel1 into a thermostat, where temperature
may be varied.
The experimental tests show the
condition of maximum vibration amplitude and allow the measurement of the
system resonance frequency.
Then the frequencies corresponding to
different vibration amplitudes have been measured; they were chosen as
convenient fractions of the maximum amplitude. The experimental values of
nine points have usually been determined. In this way, the experimental
resonance curves of the system are plotted.
Such plots indicate which
is the best range of values of the parameters, on which to operate in
order to detect the different contributions of the system
The described measurements lead to the
conclusion that the indirect measurement method proposed here supplies
reliable results both with regard to the values of moduli , and with
regard to the possibility of detecting transitions even with low modulus