Machine Design

New Ideas in FEA Solve Large Models Faster

Combining motion studies with FEA software and new kinematic elements solves large models quickly without sacrificing accuracy.

Michael Bussler
President Algor Inc.
Pittsburgh, Pa.

It wasn’t long ago that limitations in hardware and finite-element software allowed only linear, static stress analyses on CAD models of solid assemblies. Analysis capabilities took a big jump with the introduction of software that shows motion while calculating stresses for a full assembly of interconnected parts, and in acceptable time periods.

The latest event-simulation software simultaneously replicates motion (kinematics), dynamic loading and flexing (stresses) of an assembly of interconnected components (mechanisms) during a virtual “event.” This what-you-see-is-what-you-get environment examines the behavior of an entire mechanical system — bending, twisting, stretching, squashing, and buckling — without physically building a prototype. In addition, faster computer processors, bigger hard drives, and improved finite-element formations let almost any engineer run and study comprehensive scenarios in reasonable periods on a desktop computer.

The element formation that makes studying large models possible is called a kinematic element. It can significantly reduce analysis run times for assemblies with rigid components because kinematic elements behave like regular, flexible elements, except they produce no stress. Kinematic elements can be inserted in areas of the assembly where dynamic effects are essential, but stresses are of lesser importance. This saves time because it lets users focus the analysis on the mechanism being optimized. The first image, for example, shows a drivetrain- coupler model that includes stress-calculating finite elements along with kinematic elements to dramatically reduce the processing time.

Kinematic elements can be constrained or loaded with force, traction, pressure, and gravity. They also possess mass and transmit forces, which lets them produce motion and stress in flexible finite elements. Kinematic elements also have full contact and impact capabilities, so they interact with walls and objects made of other elements. Impact between objects is achieved using contact elements to simulate dynamic interaction between objects.

It takes only three steps to simulate a mechanical event on a CAD assembly using kinematic elements: Generate an FEA mesh on a CAD solid assembly, complete with realistic detail. Assign kinematic elements to assembly areas where stresses are not a concern. Then analyze the assembly to create a “virtual experiment,” which simultaneously incorporates motion, dynamic loading, and flexing over a user-defined period.

To generate an FEA solid mesh, transfer the CAD solid assembly into the FEA software. In this case, it’s Algor from Algor Inc. The analysis software’s capabilities can be launched from within solid modelers such as SolidWorks, Pro/Engineer, and Mechanical Desktop, when any one of them and the FEA program are installed on the same computer. When they are not, universal files such as Parasolid’s X_T or IGES lets users import models into the FEA program. Analyzing an automotive fastener assembly shows how kinematic elements work with event simulation software.

An engineer designed a fastener with Pro/Engineer. A menu pick launched the solidmodel- to-FEA interface which lets engineers view their entire model prior to FEA surface meshing to determine whether or not the design contains model defects, such as surface holes or self-intersecting surfaces. The engineer created an FEA surface mesh on the fastener model with an easy-to-use sliding mesh control, which quickly adjusts the density of the initial FEA surface mesh.

After creating an initial surface mesh, engineers can activate a finite-element model-building tool. In this case, it’s Algor’s Superdraw III. The tool provides access to automatic or manual surface mesh-refinement options, automatic mesh engines for FEA solid meshes using bricks, tetrahedron, or a combination of both.

In the fastener assembly example, a combination of advanced meshing techniques allow working directly on the FEA model surface. An advanced surface matching feature ensures that adjacent surfaces align correctly. Then the engineer used an automatic solid mesh engine to create a hybrid solid mesh based on the surface mesh. In this case, Algor’s solid meshing technology works inward from the optimized surface mesh to ensure that the best-shaped elements are on and near the surface where high stresses occur most often.

A next step defines the model parameters, including element types, material properties, boundary conditions, and the event’s duration. At this point, the engineer determines which elements to define as kinematic elements.

When working with a mechanism, rigid kinematic elements should be used to represent the stiffer members of the mechanism while flexible elements should be placed in model areas where stress information is important. Because stresses due to motion at the fastener hinges are important, the engineer specifies flexible elements for each of the nine hinges, and kinematic elements for the top and bottom components as well as the fastener arms.

Experience usually indicates what part of an assembly is apt to have high stresses and flexing. However, when a location is unknown, a trial run may be helpful using a simplified version of the model defined entirely with flexible elements. This trial event simulation can show where stresses are likely and help decide where to place flexible and kinematic elements in a more detailed model. Another approach constructs a model made entirely of kinematic elements to assess the kinematic functions of the event and then selectively insert regular finite elements in areas of stress.

The user begins the simulation as a final step using, in this case, Mechanical Event Simulation. The event can be viewed while it processes through two visualization utilities. One program graphs results such as displacements or velocity versus time as the analysis progresses. This makes it unnecessary to wait for an analysis to finish before reviewing results.

Another visualization program provides information about analysis accuracy. For example, this mode lets users view an event on screen also as it processes. Any model motion, such as flexing, buckling, material yielding, deformation, or deflections are shown for each time step as it happens. In addition, engineers can create analysis replay files in .avi format and bitmaps for presentations or reports to highlight areas of engineering concern. Furthermore, results can be extracted to provide critical step-by-step data. An accompanying image shows results at three times during an analysis.

Running an analysis with kinematic elements shows how the fastener assembly would function in the real world. Engineers gain accelerated design experience by building a virtual prototype using a computer, which previously could only have been learned through more expensive and time-consuming physical prototype testing.

© 2010 Penton Media, Inc.

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