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Strength Problem in the Design of Smart Structures

German Aerospace center (DLR), Institute of Composite Structures and Adaptive Systems Lilienthalplatz 7, D 38108, Braunschweig, Germany, matthias.piening{at}dlr.de

It is a common practice to design an actuator system for maximum structural efficiency. Thus the design is driven by the ratio of output to input in terms of force and stroke. As the action of the actuator and the reaction of the elastic body are dependent on the material properties, it is mandatory to know the stress distributions within the elastic body system and the associated strength limits. In order to avoid local failure and degradation due to fatigue, it is necessary to consider the local stress conditions in the early design process. The integrated actuator is an elastic body like the surrounding body system, with individual structural behavior and strength properties. Both together are exposed to the stresses and strains caused by the action of the actuator and the operational loads which act on the whole system at the same time, i.e., mechanical, thermal, or moisture loads. The presently used 2D models are well suited to predict the strength behavior of homogeneous metallic structures. But as soon as the elastic body system becomes inhomogeneous, has a complex shape, or is anisotropic, the solutions are at least questionable. Transverse stresses cannot be calculated and the effect of changes in the layer stacking sequence can not be estimated. But the results of static and fatigue tests showed a strong dependence of the failure behavior on the stacking order. The inverse physical problem is the action of active layers in a laminate. The placement of active layers within the cross section has a decisive effect on the long-term efficiency of the active laminate. Therefore it is to be taken into account in the early design phase.

Key Words: actuators • stress distributions • structural efficiency • delaminations.

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