The predicted compressive stiffness and buckling strength of filament- wound cylinders using classical lamination theory is significantly higher than those observed experimentally. This discrepancy is partially influenced by the variation of mechanical properties in the region of fiber undulations. These regions are localized geo metric defects intrinsic to the filament-winding, weaving, and braiding processes. In the present work, the average mechanical properties of the fiber undulation region are quan tified using modified models of woven-fabric composites to account for the 3-dimensional effects. The mechanical properties thus determined can be incorporated as local element properties into global finite-element models. Preliminary results from large-displacement analyses of filament-wound cylinders are relatively more accurate when fiber undulations are accounted for.

The thermal response of a specific piezoceramic induced-strain actuator (the Penn State SPICES "frame" actuator) was investigated under two conditions: 1) as a free device; and 2) embedded in a woven glass/epoxy composite panel. Actuators were driven at various combinations of electric field strengths and frequencies. Field strengths ranged from 75 kV/m to 1.5 MV/rn (10 to 200 Vrms over a 135 micron thickness), while frequencies ranged from 100 Hz to 2000 Hz. The 50-ply composite panel was instrumented with thermocouples at 4 locations through the panel thickness. Temperature measurements were recorded continuously from an initial ambient isothermal state until a steady state temperature distribution was reached. Temperatures increased with frequency and field level, with heat generation roughly proportional to the frequency and to the square of the field level, consistent with a dielectric loss mechanism. The temperature rise at the actuator-composite interface, when driven at 100 Vrms and 500 Hz, was 50 C. The data indicate that self-heating in applications involving a combination of high field levels, high frequencies, and thick composites can result in high internal temperatures, and possibly lead to reduced performance and reliability.

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This project demonstrated the implementation of embedded optical fibers as sensors for detecting the onset of shell buckling instabilities and compressive failure in composite cylinders. In the present work, five 6-inch diameter cylinders (four filament-wound and one prepreg tape) with integrated optical fiber strain sensors were fabricated and tested in compression. The cylinders were instrumented with an axial strain gage for local strain measurements and a helically-wound 633-nm optical fiber for integrated strain measurement. The integrated strains along the optical fiber path were obtained by using a modified Mach-Zehnder interferometer with feedback electronics controlling PZT's in the reference arm. This distributed sensing technique provides a more reliable and sensitive means of detecting critical strains and the onset of pre-buckling deformations than the conventional local strain gage.