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Two FEs, one result: Low-cost, beam-based FE rises to challenge

April 26, 2013
FE software packages for modeling mechanical structures formulate model elements and nodes differently, but often arrive at very similar results

FEA software has converged on three types of elements. Truss-and-beam FEA quantifies generalized stresses in frame and weldments. Shell FEA predicts generalized and localized stresses in skins, plates, and sheet metal, and solid-element FEA accurately profiles stresses in all solid parts.

Red River researchers investigate designs for private-industry partners using myriad tools including FE software. In one recent project, a stand of 2 × 2 × ¼-in. steel tubing was modeled in low-cost and commercial FEA.

Simplified FEA based on truss-and-beam modeling dominates shareware options. It uses two-noded linear elements formulated by cubic-displacement numerics for accurate results with the fewest elements possible. One caveat: Elements are straight-line approximations, so accurately modeling curved frames requires more elements. Such software arose to give tinkerers — think amateur astronomers designing their own telescope frames — a low-cost way to verify mechanical structures. Companies also use such software chiefly to model long and thin frames.

In contrast, unabridged commercial FE software uses all mesh types to best model a given design. Here, higher-order node elements based on isoparametric formulations model complex, curved frames and solids, plus beam elements are generated from solid models in which cross-sectional properties are determined. Most test for fatigue, linear and nonlinear static, vibration, thermal, and dynamic phenomena.

Two programs, one result

This Grape beam-based model shows nodes, elements, and deflections. The latter are amplified by a factor of 1,600 to clearly show maximum displacement. Batch and auto-analysis reruns let engineers plug different materials, cross sections, and loading scenarios into one frame model to compare results and choose the best iteration.

On simple designs, the two software types often arrive at similar results. That’s what researchers at Red River College in Winnipeg, Canada, recently confirmed while designing a generator power-takeoff test stand. A model of the stand was analyzed in Simulation from SolidWorks, Waltham, Mass., and GBW32 V5.0 from Grape Software Inc., Beausejour, Canada. SolidWorks solid-model results were compared to Grape’s beam-element-based results. In both models, uniform beam loads simulated plate-to-frame load transfers. Maximum stresses differed by only 2.4%, and both FE packages confirmed that the stand’s deflections adhered to design criteria.

Like most free or low-cost options, Grape requires more input than commercial programs, so users must fully understand what and how they are modeling when manually defining nodes, elements, and properties. The benefit here is that students and even nonengineers can use it for both learning and design analysis. Grape also supports release nodes to model frames with hinged or pinned joints, or members that slide or rotate — and let frame-structure modifications better represent structures.

For this stand, calculated deflections in SolidWorks FE give results similar to those from Grape.

Red River College continues to use both Grape and SolidWorks, as the former effectively analyzes generalized stress on trusses and frames, while the latter models with solid elements to calculate localized stresses. Solidworks also excels in analyzing intricate frames with tight radii or varied cross sections — possible in Grape but requiring far more user input.

For more information, visit Grape Software Inc. • Edited by Elisabeth Eitel

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