Hysteresis losses decrease with increasing temperature. This same behaviour has been detected
on other polymeric materials.15,16 Figure 5 reports the percentage of hysteresis losses at
different temperatures, calculated on the average response over the last two loading cycles and
expressed as the amount of dissipated energy over the elastic energy provided to the sample
during the stretching phase.
• As temperature increases, the curves show an increasing steep segment (here referred to as
preload knee) at small deformations (ΔL/L0 < 0.02), i.e. the specimen presents much higher
stiffness at low stretches. This is because the coils start touching each other.
• At higher temperatures, the first training cycle is closer to the stabilized response.
• When the RC is kept for 2 s at 50% deformation, slower hysteresis phenomena induce a slight
stress relaxation (see Figure 5).
• Due to hysteresis, during the unloading phase of isothermal tensile cycles at 25 ◦C, the sample
suffers from buckling, i.e, its stress falls to zero, as it approaches the reference configuration.
The responses of the RC averaged on loading and unloading cycles are shown in Figure 6 for
different temperatures. For deformations up to 40 %, the higher the temperature, the higher the
mean force exerted by the RC in correspondence of a given strain. The loading curves at the
different temperatures mutually intersect in a tight region around 45 % strain and 60 MPa nominal
stress. For larger strains, the behaviour of the force with respect to the temperature is the opposite,
i.e. for very large values of the stress (and, thus, of the strain), the NC tends to extend when
temperature is increased. Notice also that the slopes of the curves are different. This means that it
is possible to control the stiffness of the actuator by varying the temperature. For example, for a
deformation ΔL/L0 = 0.07 a stiffness of 101 MPa or 78 MPa can be obtained with a temperature of
25 ◦C or 122 ◦C, respectively. This feature can be useful in a number of applications, e.g. vibration
control.