Manufacturing of Active Composites with Integrated Piezoceramics

Kurzfassung

The rapid development of industrial products in the fields of traffic technology, mechanical engineering, etc. requires higher efficiency of materials and structures. Thus, lightweight design has become very important, mainly for reducing the effects of accelerated mass. New materials like high-performance fiber composites not only find application in aerospace technology, but are also gaining importance in terrestrial areas. Considering the excellent properties of weight, Carbon fiber reinforced polymers (CFRP) offer a variety of advantages in comparison with usual materials like steel, titan, or aluminum. On the other hand there are problems concerning the vibrational sensitivity due to low mass, tendency to buckling, and susceptibility to damage. A promising way to solve these problems is the integration of multifunctional materials such as piezoelectrics, Shape Memory Alloys (SMA), etc. Arising deformations, accelerations, or other physical measurements can be detected by sensors, technically processed, and eliminated with suitable real-time controlled actuators. To handle multifunctional material in different applications statements regarding the compatibility, the degree of efficiency and the overall electromechanical behavior of the material are important. Composite materials offer the unique chance to integrate multifunctional elements during the manufacturing process. The multifunctional elements become a part of the material itself. Besides the load-carrying function the integration of sensors and actuators within the composites forms a material which is able to detect and specifically influence its own loads, vibrations, or deformations. Because of some excellent properties (low energy consumption in quasi static applications, high efficiency, fast response, etc.) piezoelectric materials are in the center of interest. Usually thin monolithic piezoceramic wafers are used as structural actuators for active composites. To improve the mechanical and electrical performance it is often advantageous to pre-encapsulate the piezoceramic (e.g. in a polymer matrix). With this additional step the piezoceramic can be provided with a mechanical pre-compression, electrical insulation and mechanical stabilization. More recently active composites with piezoceramic fibers came into focus. These fibers (Æ 10-30 µm) are processed in form of patches with uniaxially aligned piezoceramic fibers embedded in a polymer matrix excited by interdigitated surface electrodes. With the use of the longitudinal piezoelectric effect and their excellent suitability for integration in curved structures piezoceramic fibers offer a number of advantages. Unfortunately these fibers are not yet available on a large scale. As there has been extensive work in the field of integration of thin lead zirconate-titanate piezo-ceramic (PZT) wafers (50x30x0.2mm) into CFRP at the Institute of Structural Mechanics, DLR-Braunschweig, this will be focused on in the following. Provided with uniformly electroded surfaces these wafers operating in the lateral, d31, mode. Fig. 1 shows the principle design of some active composites. In Fig. 1a the ceramic wafer is embedded between thin non-conductive fiber material (e.g. glass- or polyester fiber) which serve asinsulation. The load carrying CFRP-layers above and below the ceramic are used to electrically contact the piezoceramic, therefore the insulation layers are required to avoid short circuits. As the usual disturbing wires are not needed, the manufacture of the active composite has become easier and there is no additional weakening of the structure. In case of a break the piezoceramic will still work, as the broken pieces stay in the electrical field where they can be controlled. The tube in fig 1.b has the same lay-up with piezoceramic elements in each side of the tube. This structure is able to compensate deformations under thermal loads. In structures with arbitrary distributed ceramics carbon fiber bundles are used as a structural conformable way to contact the piezoceramic. Fig 1.c shows a beam structure with two piezo-ceramics. Each electrode in the active beam is contacted separately. In this way it can be used as strain actuator as well as a bimorph. Fig. 2 shows the operating diagram of the beam structure for both cases. Significant differences between mechanical and thermal properties of the ceramic plates as well as the extreme brittleness of the piezoceramic material demand sophisticated manufacturing techniques. Several manufacturing techniques have been used at the DLR including hand-lay-up and filament winding technique. Very good experience has been made with a sophisticated RTM technology, the so-called DP-RTM (Differential Pressure – Resin Transfer Moulding). This guarantees an extreme high quality and reproducibility of the components. The fiber material is laid out in dry state which facilitates the positioning of the electric cables and ceramics. The DP-RTM procedure becomes especially interesting as it is not necessary to provide massive moulds since the clamping forces in the autoclave are created by the differential pressure. Thus, a simple sheet plate can serve as sub mould while a vacuum foil is applied for the upper mould. Fiber volume content and fluid rate can directly be controlled by adjusting differential pressure during the stages of injection. In order to minimize the weight of the active fiber composite a high fiber volume content is required. This can be achieved by increasing the differential pressure. Simultaneously, the increasing mechanical load on the ceramics has to be considered since it might result in mechanical damage of the brittle actuators. The different process parameters like autoclave pressure, autoclave temperature and differential pressure have been optimized during several test procedures. The new materials and material systems that can be used as multifunctional actuators and sensors with new real-time control concepts and adaptive signal processing, have helped to establish a new class of structures called adaptive structures. The results of recent investigations indicate that these concepts contain very promising potentials for future structural and acoustic purposes. With the research progress and the realization of functional demonstrators as described above by some examples, the intention is now to make this technology available for common industrial applications. To achieve this aim an inter-disciplinary development process (fig. 3) has been established. “LEITPROJEKT ADAPTRONIK” is the title of a German national model project that will be conducted under the auspices of the Braunschweig-based Institute of Structural Mechanics. A consortium of seven major industry partners comprising five branches and seven medium and small-sized companies as well as further research partners was established as a center for the development of technologies within the model project. Besides many other research topics, the integration and development of piezoceramic plates and new piezoceramic fibers are of major importance within this project.