Mechanical Event Simulation Provides the Best Method to Simulate Large-Scale Motion for Finite Element Models

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

One of the latest trends in FEA is the simulation of large-scale motion using finite element models. Simulating large-scale motion is critical to accurately replicate the real-world behavior of many mechanisms and to determine how components will perform under conditions of impact, contact or other forces of concern. Engineers benefit from simulating the large-scale motion of finite element models because these models can accurately determine the resultant deformation and stresses without the need to make assumptions about the forces at work in the mechanical event.

Unfortunately, little attention has been paid to how FEA packages solve large-scale motion for these models ¾ if they do at all. The three available methods can vary widely in terms of ease-of-use, accuracy and functionality:

• Mechanical Event Simulation (MES) uses an automatic timestepping scheme that incorporates an implicit timestep method to yield a highly efficient and accurate solution.
• The Explicit Timestep Method determines a solution by "marching" along in time, extrapolating from the solution of the prior timestep. This method is fast for a given timestep, but requires many small timesteps to complete large-scale motion problems, rendering the processing speed unacceptable. In addition, detailed knowledge of the finite element model may be required in order to achieve reliable results.
• Motion Load Transfer requires engineers to use a separate kinematics package in order to obtain loads for FEA. Therefore, it does not actually simulate the motion of the finite element model, but instead produces loading from rigid-body motion with assumed stiffnesses. This loading is then used to perform a static stress analysis.

Mechanical Event Simulation

MES is Algor’s solution for replicating the large-scale motion of complex solid models. The software simultaneously produces motion, deformation and stresses in a single "What-You-See-Is-What-You-Get" process that eliminates the need to estimate loads and reduces the need for physical prototype testing.

Resulting from a five-year research partnership with Carnegie Mellon University, Algor’s MES automatic timestepping scheme utilizes an implicit timestep method. This type of solver is very stable and only requires that analysts specify the duration of the event and a solution capture rate in order to obtain an accurate solution. In addition, the automatic timestepping scheme enables the solver to compute at larger timesteps for periods of relative inactivity, such as a constant acceleration or deceleration, and then decreases the timestep size to capture periods of critical activity, like surface-to-surface contact or local buckling.

MES can also incorporate the use of Algor’s proprietary kinematic elements to facilitate the simulation of large-scale motion for complex solid models. These elements can be used in areas of the model for which stress results are of less importance, but where they still need to transmit loads. This enables shorter processing times without the need to simplify the geometry or compromise accuracy.

Because MES is based on an implicit timestep method, the software provides accurate solutions for large-scale motion simulations using finite element models. In addition, an automatic timestepping scheme and kinematic element technology can reduce overall computation times, making large-scale motion simulation practical with MES.

Explicit Timestep Method

The explicit timestep method extrapolates the solution of an analysis based on the calculations of previous timesteps. At first appearance, the explicit timestep method may seem attractive for large-scale motion analysis because it can solve individual timesteps so quickly. However, several significant drawbacks exist:

• The explicit timestep method requires many small timesteps to solve complex problems like large-scale motion. Generally, the same timestep size must be used throughout the entire analysis, which unnecessarily increases overall processing times.
• The explicit timestep method may require engineers to calculate the timestep size based on detailed knowledge of their finite element model, such as the length of the smallest element and the highest element stiffness.
• If the timestep size is incorrect, the solution can become unstable and lead to convergence problems. A misstep that leads to this instability may occur early in the analysis, but may not manifest itself until later in the analysis. Therefore, determining the source of an error in the analysis process can be difficult.

Because of the small timestep size required to avoid instability, simulating large-scale motion for finite element models using the explicit timestep method is often impractical.

Motion Load Transfer

Motion load transfer does not enable the simulation of large-scale motion using a finite element model. Instead, the process transfers loads determined using a kinematics program to a finite element model for static analysis.

The kinematics software used for load determination considers rigid-body motion only. Furthermore, such software packages assume joint stiffnesses and use simplified "line" structures in place of solid models. In addition, engineers cannot select the areas for load application in the static analysis since this is determined intrinsically by the software. Because of these limitations, the force data, and therefore the stress results, yielded from the motion load transfer process are typically inaccurate.

In addition to MES, Algor provides Inertial Load Transfer software that eliminates the limitations inherent with the motion load transfer process. This software automatically transfers dynamic loading at each node from an MES to a properly oriented static finite element stress analysis. The loads determined with MES are produced from flexible-body motion, take into account the movement of mass and can be filtered according to their magnitude to eliminate inconsequential loads from the static analysis.

Conclusion

Algor’s combination of an automatic timestepping scheme with an implicit timestep method for MES enables the most accurate and time-effective means of simulating the large-scale motion of a finite element model in order to determine resulting stresses. By incorporating MES into the design cycle, engineers and their employers benefit from accelerated design times, reduced physical prototyping and shorter times-to-market.