Published: Jan 1988
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Filament-wound composites are fabricated using an initial stress in the filament strand called the “winding tension.” This initial stress will interact structurally with the winding form and each of the wound layers incrementally as the fabrication process proceeds. The interactions will result in a variable residual stress distribution or possibly in total tension loss in some of the wound layers. Total tension loss before cure of the resin matrix produces strength loss in the affected layers and the whole structure.
An incremental construction procedure has been implemented into a finite element computer program for analysis of the winding process. The method involves switching “on” element stiffnesses in the finite element model as construction proceeds. The initial stress in the element being switched on due to winding tension is converted to nodal loads and applied to the currently existing structure. If any layer loses all its initial stress, then its material stiffness is reduced (assuming the uncured composite cannot carry fiber direction compressive loads). The incremental addition of element stiffnesses and modification of material properties make the analysis incrementally nonlinear.
An analytical and experimental study was conducted on a series of cylinders. The parameters investigated include winding mandrels of aluminum or plastic, two uniform winding tension levels, and two final composite thicknesses all using a Kevlar/epoxy composite. Two aluminum mandrel thicknesses were used to vary the mandrel stiffness, with the plastic mandrel serving as a very low stiffness mandrel. Residual stress distributions were predicted for all the wound parts, and these were used to calculate the mandrel strain during construction which was compared with measured response of strain gages on the mandrel. Correlations were very good. Tension loss in the inner layers was seen in two cases; these were for the highest winding tension and largest final composite thickness wound on the thin aluminum mandrel and wound on the plastic mandrel.
The study shows results of modeling the fabrication process and the consequence of fabrication parameter choices. Situations were produced which show strength loss in Kevlar/epoxy composites. It is therefore necessary to include fabrication engineering as part of the design of filament-wound structures.
composite materials, filament winding, residual stress, strength, finite-element model, fabrication model
Professor, Virginia Polytechnic Institute and State University, Blacksburg, VA