METHOD FOR PRODUCING A COMPOSITE
STRUCTURE INCLUDING A
This application is based on and claims the priority under 35 U.S.C. §119 of German Patent Application 197 32 513.0, filed on Jul. 29, 1997. The entire disclosure of German Patent Application 197 32 513.0 is incorporated herein by reference.
FIELD OF THE INVENTION
The invention relates to a method for producing a composite structure of actuators, including a planar or plateshaped piezo-electric element and fiber composite panels arranged on the two opposite major surfaces thereof.
It is generally known to manufacture composite structures of planar or plate-shaped panels with a planar or plateshaped piezoceramic element arranged or sandwiched therebetween. Such composite structures are increasingly being used as components for high velocity actuators, for example such actuators to be used for active noise damping. There is also a hope to develop such composite structures for actuating or controlling the rotors of helicopters.
The known plate-shaped piezoceramic elements that are typically used in such applications, i.e. in high velocity actuators, have a high tension stiffness and compression stiffness, as well as a high compression strength, but a relatively low tensile strength and relatively low achievable active strain. These disadvantages of the known piezoceramic elements make it impossible to successfully use such piezoceramic elements in composite structures for controlling or actuating the rotors of helicopters. Namely, the rotors are subject to substantial tensile strain due to the centrifugal force arising during operation of the rotor, as well as tensile strains resulting from the bending moments. The known piezoceramic elements used in known composite structures are not able to withstand such substantial tensile strains, and thus cannot be used for actuating or controlling the helicopter rotors.
SUMMARY OF THE INVENTION
In view of the above, it is an object of the invention to provide a method for producing or manufacturing a composite structure that is suitable for controlling or actuating the rotors of helicopters, or generally for forming a motive element of an actuator, in which on the one hand the useable active strain is not substantially reduced, while on the other hand a tensile loading of the composite structure will not result in a tensile loading of the piezoceramic element itself. More directly, it is an aim of the invention to provide a method of manufacturing a composite structure including a piezoceramic element in combination with fiber composite panel layers, wherein the piezoceramic element is under compression while the fiber composite panels are under tension in a neutral or resting state of the composite structure. The invention further aims to overcome or avoid the disadvantages of the prior art, and to achieve additional advantages, as apparent from the present description.
The above objects have been achieved in a method for producing a composite structure including a planar or plateshaped piezoelectric element sandwiched between two fiber composite panels, according to the invention. In the method,
the fiber composite panels are each first subjected to a pre-stressing force effective in the plane of the respective panel. In this manner, each fiber composite panel is prestressed and preferably pre-strained or elongated, and then
5 held in this condition. The pre-stressing forces can be applied by any known force-applying or straining means, for example by engaging the edges of the panels and applying mechanical loads thereto, or by other effects.
Next, the pre-stressed fiber composite panels are adhe
10 sively bonded onto the piezoelectric element over the entire contact surface therebetween, such that the piezoelectric element is securely and firmly sandwiched and embedded between the panels. Only after the adhesive bonding has been completed, e.g. the adhesive has cured, the pre
15 stressing forces are removed or released from the prestressed fiber composite panels. As a result, once the prestressing forces are removed, the fiber composite panels have a tendency to contract or shrink back to their respective un-stressed and un-strained resting condition, whereby the
20 fiber composite panels are placed under internal tension, while the piezoelectric element bonded thereto is placed under internal compression.
Particularly according to the invention, the pre-stressing forces applied to the fiber composite panels are specifically
25 selected and controlled in magnitude and direction, dependent on the specific parameters of the composite structure and the application at hand. Namely, the magnitude and direction of application of the pre-stressing forces is so selected, that the composite structure will withstand the
30 passive tensile strains, and the tensile loads acting thereon in its intended application, without overcoming the precompression of the piezoelectric element and thereby subjecting the piezoelectric element to tension loads. Moreover, the pre-stressing forces must be limited to avoid over
35 compressing the piezoelectric element beyond its limits, or unnecessarily limiting the range of useable active strain of the finished composite structure. The proper application of the pre-stressing forces can also take into account the isotropy or anisotropy of the respective fiber composite
By applying the pre-stressing with a targeted or customtailored magnitude and direction, the resulting composite structure can be optimized in such a manner that the passive tensile strains effective on the piezoceramic element are
45 selected to be of such a magnitude that the piezoceramic element just withstands these passive tensile strains. In this manner, the range of useable active strain is maximized. The present method is particularly suitable for producing composite structures for controlling or actuating the rotors of
50 helicopters. In this field of application, the exact selection of the magnitude and direction of the pre-stressing is especially advantageous, in view of the high tensile forces and corresponding high stresses that arise during operation.
BRIEF DESCRIPTION OF THE DRAWINGS
In order that the invention may be clearly understood, it will now be described in connection with an example embodiment, with reference to the drawings, wherein:
FIG. 1 is a schematic perspective view of a composite 60 structure produced according to a method of the invention;
FIG. 2 is a schematic plan view of one of the fiber composite panels of the composite structure, with respective arrows schematically representing the pre-stressing forces applied to the panel; 65 FIG. 3 is a schematic vertical section through a composite structure, during its manufacturing according to the method of the invention; and