In the current study Active Fiber Composites (AFC) utilizing Lead-Zirconate-Titanate (PZT) fibers with Kapton screen printed interdigitated electrodes (IDE) were integrated into orthotropic glass fiber reinforced plastic (GFRP) laminates to investigate integration issues associated with smart structures and host laminate integrity. To aid in this goal surrogate or "Dummy" AFC (DAFC) were designed using a GFRP core and Kapton outer layers to match the longitudinal mechanical and interface properties of the AFC. These DAFC were used in place of real AFC to expedite test specimen manufacture and evaluation. This allowed efficient investigation of the impact of an integrated AFC-like inclusion on laminate mechanical integrity. Two integration techniques, cutout and simple insertion were investigated using DAFC, with little difference seen between the integrity of laminates prepared using these two methods. Using this testing scheme the influence of device placement in relation to position extending away from the laminate symmetric axis was found to have an effect on laminate integrity in tensile loading. As the DAFC were placed far from the laminate symmetry axis, the ultimate tensile strength and strain of the laminates decreased in a linear manner while the Young's modulus of the laminates remained constant. Similar trends were observed with integrated AFC specimens. The performance of integrated AFC was characterized using monotonic cyclic tensile loading with increasing strain levels. A transition region was observed between strains of 0.05%-0.50%, with a dramatic decrease in AFC sensitivity from a maximum to minimum value.
The scientific community has put significant efforts into the manufacturing and optimization of sensors and actuators made of piezoelectric fibres with interdigitated electrodes, well known as Active Fibre Composites (AFC). A great advantage of such AFC is their flexibility and the possibility to integrate them into composite structures.
In the current study an approach of optimizing the manufacturing process as well as the polarization of AFCs utilizing piezoelectric Lead-Zirconate-Titanate (PZT) fibres embedded in an epoxy matrix between interdigital Electrodes (IDE) screenprinted on Kapton will be discussed. During the poling process, an electric field is applied over the interdigitated electrodes of the AFC to its piezoelectric fibres along the fibre axis. One of the most important parameters of this polarization is, beside temperature and time, the applied voltage. An increase of the electric field results in an increase of the AFCs performance as shown by free-strain measurements.
The manufacturing process developed and used at Empa consists of laminating the piezoelectric fibres in an epoxy matrix between the electrodes. An essential goal of this lamination, carried out in a hot press, is to get a proper contact between piezo fibres and the electrode. By adding soft layers between the Kapton foil and the mould, the interdigitated electrodes are deformed by each single fibre and therefore build up a contact area which in its cross section can be described by a contact angle. This optimization of the manufacturing process is also shown by free strain measurements of the AFC.
The scientific community has put significant efforts in the
manufacturing of sensors and actuators made of piezoceramic fibers
with interdigitated electrodes. These allow for increased
conformability and actuation capability at high field regime. The
prediction of their coupled field behavior, however, is so far
limited to low field applications, where the piezoelectric
coupling coefficient is assumed to be constant. An approach, which
takes into account the strain driven nonlinearity of a
representative work cycle at high field regime is still lacking.
This study presents a nonlinear Finite Element Model to simulate
the free strain properties of Active Fiber Composites (AFCs) under
high electric field conditions. Input data for the fully
parametric model are the Representative Volume Element (RVE)
geometry and the material properties of its piezoceramic and epoxy
resin components. The high field properties of single PZT fibers
under free strain conditions were determined using a novel
characterization procedure. Free strain properties of the
actuators were measured experimentally, and important geometrical
parameters (contact angle between the fiber and the electrode,
average spacing between the fibers) were measured using
micrographical imaging. The results of the simulation show good
agreement with the free strain measurements, allowing for
prediction of a representative work cycle hysteresis. The
influence of important geometrical parameters on the actuator
properties such as electrode spacing and electrode-fiber contact
angle was investigated both numerically and experimentally.