The technique of birefringence imaging was exploited to observe the evolution of creeping strain fields in transparent
PLZT 8/65/35 samples. PLZT samples with features that produce non-uniform fields were loaded with
constant voltage boundary conditions. The resulting birefringence contours evolve with time and can be related
to strain measurements. Three experimental arrangements are reported: partial surface electrodes producing
intense fields near an electrode tip, a round insulating hole producing local concentration of electric field, and
a thin, sharp crack producing crack tip fields. In each case, material was initially in the as-sintered (unpoled)
state, and was loaded with nominal electric field strengths that were well below the coercive field. However, the
birefringence imaging indicates significant remanent strain evolving over a time period of order 103s. The resulting
mean electric displacements are greatly enhanced relative to uniform field conditions at the same mean field
strength. The measurements show only weak interaction between thin cracks and the applied electric field, suggesting
that the thin cracks are effectively permeable. The results are of potential use in calibrating multi-axial
and time dependent material models.
The time-dependent remanent strain and polarization were measured in initially unpoled PZT-855 under electromechanical
loads. Various levels of constant compressive uniaxial stress combined with constant electric field
parallel to the stress axis were used to produce the creeping remanent strain and polarization. The remanent
quantities were deduced from measurements of total strain and electric displacement by subtracting the linear
(reversible) parts of strain and electric displacement, making use of previous measurements of the linear moduli
and accounting for their variation with material state. Mechanical compressive stresses alone produce remanent
strains that rapidly reach a saturated state. Under combined loading, increasing the compressive stress reduces
the observed creep rates caused by an additional electrical loading. The creep behaviour is most significant at
loads close to the coercive stress or electric field. At loads well above or below the coercive field levels, the
behaviour is nearly rate-independent.
In this study, measurements of the evolving linear elastic, dielectric and piezoelectric moduli of a soft ferroelectric
PZT are made during loadings of uniaxial compressive stress combined with an electric field. Using short pulses of
electric field and stress, the incremental remanent strain and polarization state of the material and the unloading
moduli were determined. The remanent quantities are treated as state variables, with a view to expressing the
moduli as functions of the material state. The piezoelectric moduli vary approximately linearly with polarization,
whilst the dielectric moduli and elastic compliances show more complex behaviour.
The dielectric and piezoelectric responses to a constant electric field have been measured on initially unpoled PZT-5H and PLZT 8/65/35. In particular, we are interested in the time dependent remanent strain and polarization due to unipolar electric field because they are associated with the domain switching behaviour. An experimental method has been developed to obtain the remanent strains and polarization by accounting for dielectric and piezoelectric effects. A linear dependence of dielectric and piezoelectric modulus on remanent polarization was found in PZT-5H, but not in PLZT 8/65/35. In PLZT 8/65/35, a transition was observed at a value of remanent polarization of 0.07 Cm-2 gradually evolving polarization to rapid switching. A change in the dominant mechanism of switching was also observed.