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Predictive Analysis and Flex Testing

Recently, Liquid Access approached Polynova Composites with the task of engineering a stiffer version of their Rocket competition slalom ski. While the current version performed well for skiers up to 135lbs., heavier skiers were requesting an increase in the ski’s stiffness for improved turning responsiveness. Since stiffness of a panel is dependent not only on the material’s flexural modulus, but is also a function of the cube of the thickness of the panel, increasing the core thickness would be the simplest solution. However, the desire to utilize the current tooling and manufacturing method excluded this option. To solve the problem, Polynova selected predictive finite elemental analysis as an aid in developing a new laminate. The analysis was then verified with three point bend testing of skis built with both the original and the proposed laminates.

The original Rocket Sit-Ski is infused with a vinyl ester resin and is comprised of a gel coat on the ski bottom, unidirectional carbon, a plain sheet of ½” foam core enveloped with Polybeam® 703 – a spacer fabric based IFR™, biaxial fiberglass, and a gel coat on the ski top. Vinyl ester resin was chosen because of its increased mechanical properties, particularly elongation to failure.

In manufacturing the gel coat is first sprayed into a standard female mold with a two-inch flange. Unidirectional carbon fiber is laid into the mold, followed by the Polybeam®. A plain sheet of ½” foam core precut to fit the mold is then laid in, followed by another ply of the Polybeam®. The biaxial fiberglass lamina and a semi rigid gel coated floating counter tool (float tool) are placed over the ply stack. The term floating here refers to the counter tools free positioning within the vacuum envelope. The Polybeam® extends beyond the float tool to accommodate the resin feed and vacuum port lines. The resin feed and vacuum ports are then appropriately placed, and the infusion proceeds. The ski is released at the end of the cycle, and trimmed for delivery.

Liquid Access set a flexural modulus multiple of two to three over the original ski as the design parameter for the new ski. Theoretical analysis of the original and candidate laminate schedules was made possible by VectorLam™ and Strand7 software. VectorLam™ accurately predicts mechanical properties of laminates built to various processes

including vacuum infusion. VectorLam’s™ ability to rapidly build and compare multiple laminate scenarios significantly decreased the time required in the initial design phase. The individual ply data from VectorLam™ was then imported into Strand7 FEA where the laminates were reconstructed. Strand7 is a general-purpose finite element analysis system consisting of pre-processor, solvers and post-processor. The laminated composites module provides fully interactive analysis of both symmetric and unsymmetrical composite laminates. The laminate engineering properties and the characteristic matrices are calculated based on standard laminate theory.

A major challenge was to accurately simulate the core under loading in conjunction with the laminate skins. Plate elements representing the laminate skins and brick elements representing the core accurately portrayed displacement but there was a gray area in the first mode of failure. Building the entire laminate utilizing Quad8 plate elements, which contain 8 equally spaced nodes around the perimeter of the plate, allowed for accurate representation of the interaction between core and skins without compromising displacement prediction. The core was assigned on the mid-plane and the laminate skins were offset by half the thickness of the core to accurately replicate the relationship between core and skins.

Utilizing additional unidirectional carbon on the top and bottom of the ski proved to be the most effective solution to meeting the design goal (2X to 3X flexural modulus). The added carbon unidirectional along the length of the ski increased the modulus which in turn decreased overall displacement of the ski under load.

To verify the predictive model, flexural testing of skis built with the original and proposed laminate was performed at the National Composites Center.

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