B. Specimen preparation procedure
An easy and repeatable autocoiling manufacturing process is employed to produce the NC
specimens.14 Coiled actuators are obtained from the precursor fibre (i.e. initial untwisted nylon
wire) by connecting its top end to a rotating motor and the bottom end to a suspended mass that
keeps the wire tensioned at constant force. The bottom of the wire is prevented from rotation around
the vertical axis (see in Figure 3) thus every turn of the motor adds one turn of twist to the fibre.
In a prescribed range of tensions, after several turns, the wire starts to buckle and assumes a coiled
shape. With the same precursor fibre, different coil shapes (with different coil indexes) can be
obtained by varying the value of the tension during twisting. Higher production loads originate coils
with smaller coil index.
Right after the twisting and coiling process, the NC is fixed to a frame that prevents the fibre
from untwisting or losing its shape when the weight is removed. The coil (together with the frame)
is exposed to heat treatment in air and then removed from the frame (without losing twist).