FEA-Based Mechanical Event Simulation Moves Beyond the Limitations of Kinematic Motion-Only Simulation and FEA Stress Analysis

Michael L. Bussler
President
Algor, Inc.
Pittsburgh, PA

The computer-aided engineering software industry has recently jumped onto the "simulation" bandwagon in order to address the need to combine motion and stress analysis. By simulating motion, engineers hope to avoid inaccuracies due to assuming forces inherent in traditional linear static finite element analysis (FEA). However, engineers need to be aware of an essential difference between various "simulation" software packages: whether or not the software accurately accounts for flexing (stresses) as well as motion.

Processes that account for motion fall into two categories: motion load transfer and Mechanical Event Simulation (MES). While motion load transfer interfaces an FEA package to a kinematics package, MES replicates motion and flexing to produce stresses in a single software package without requiring load transfer, resulting in greater efficiency and accuracy.

What You Get with Motion Load Transfer.

Kinematics software represents rigid-body motion in fully coupled mechanisms and yields forces at each instance in time. The maximum force from a kinematic analysis can be used as input for a static finite element analysis, which yields a stress analysis at a single moment in time. Although this process of motion load transfer is an improvement over linear static FEA, users of kinematics packages must make assumptions about stiffnesses, thereby introducing inaccuracies in the generated force values. In addition, motion load transfer only produces stress results at one moment in time. The engineer must spend a good deal of effort in repeatedly feeding back results between the stress and kinematic packages for each instance. Using motion load transfer is therefore inefficient and not practical for real work.

What You See is What You Get with Mechanical Event Simulation.

MES is performed by Accupak/VE, which produces motion, flexing and stresses in a single What-You-See-Is-What-You-Get process, eliminating the need to input a force. MES produces accurate results by avoiding the need to make assumptions inherent in the motion load transfer process and is more efficient since it is a single process.

For instance, when engineers use a kinematics package, they assume that all motion is rigid-body motion. On the other hand, motion and flexing simulation models bend, twist, stretch and squash on the computer screen as a result of motion, and the stresses produced by the motion and flexing are produced for analysis at the same time.

Joints can be modeled with great accuracy in a motion and flexing simulation model. On the other hand, a kinematics package requires engineers to make assumptions about stiffness at all joints and links within the mechanism.

Motion and flexing simulation with MES takes into account phenomena such as resonance and load stiffening, while static FEA processors typically used in the motion load transfer process cannot deal with these factors.

Simultaneous motion and flexing simulation with MES is more efficient than motion load transfer because it does not require feedback between multiple software packages. The What-You-See-Is-What-You-Get process helps engineers to see the cause-and-effect relationship between motion, flexing and stress. The results are clear even to non-engineers.

Other Considerations

Accupak/VE handles contact and impact for simulating the interaction of independently moving parts of an assembly or separate objects, such as occurs when a container falls into a net. Motion-only simulations are limited to fully coupled mechanisms; therefore, they cannot produce forces or stresses from the interaction of two or more completely separate parts and/or objects.

Accupak/VE also interfaces with CAD systems and offers proprietary "kinematic" elements to make MES with CAD solid models and assemblies practical. This proprietary Algor technology enables engineers to perform simulations with highly detailed models that look more like real components and assemblies than the simplified models that were previously used for their processing speed. Using the new kinematic elements, an engineer can simulate an event using a complete CAD solid model or assembly in a practical amount of time, yielding a computer simulation that performs as an actual virtual prototype and provides motion, flexing and stress results at each point in time.

Because Accupak/VE enables engineers to create virtual prototypes, it can reduce or replace physical prototype testing. Reducing prototype testing decreases the time and expense of getting products to market, thus enabling companies to develop products faster without compromising safety. The more efficient the design cycle is, the faster companies can fulfill consumer needs and desires and the more competitive and profitable the company is to its investors.