What Designers Should Know about FEA Loads and Constraints

Entergy Operations, Inc., a leading operator of nuclear energy facilities, used ALGOR's MES software to simulate a car impacting a critical water storage tank to determine if a lower water level could be maintained without affecting the tank's structural integrity during tornado conditions.

Biomedical engineers at Wright Medical Technology, Inc. used ALGOR's MES to evaluate the contact stresses for this artificial knee implant. The analysis results helped to predict wear patterns so that the performance of the implant could be optimized.

This ALGOR multiphysics simulation of a common single-pole residential circuit breaker combines electrostatic, transient heat transfer and MES analyses to determine the trip time and behavior. When subjected to an overload condition, the bimetallic strip in the breaker deforms due to Joule heating and trips the mechanism to break the circuit.

Bob Williams
Product Manager
ALGOR, Inc.
Pittsburgh, PA

This article was published in Machine Design, "FEA Loads and Constraints: What Designers Should Know", November 18, 2004.

When you perform finite element analysis (FEA), you are setting up a virtual model of a real-world situation to see how a product will react in its environment. You define that environment through a combination of loads and constraints and the decisions and/or assumptions that you make about those loads and constraints are vitally important to the overall accuracy of the simulation.

While it may seem like a straightforward task, there are often complicating factors related to defining loads and constraints such as:

• It's not always easy to know what the loads and constraints should be - particularly for scenarios involving motion, impact, time-dependent changes or multiphysics phenomena. Historically, engineering experience and judgment was heavily relied upon to determine loads and constraints and how to best apply them. However, even experienced engineers can have difficulty determining accurate values for these critical inputs. For example, an impact analysis involves simulating the response when multiple objects hit. A point load could be used to approximate the impact force. But, what you think the load is and what the load actually is often differs significantly.

• Artificial loads and constraints complicate results evaluation by introducing "hot spots" in the model. For example, if you constrain a point, the nearby results will be artificially spiked. There's no way around this modeling effect; it's part of the traditional FEA process. However, it complicates looking at results. Is the hot spot something to be concerned about or can it be explained away? Determining the answer depends on engineering judgment.

When simulating simple environments, you may be willing to accept inaccuracies from modeling assumptions such as point loads or constraints. But, for other, more complex situations – often involving motion, impact, time-dependent changes and/or multiphysics – ALGOR's Mechanical Event Simulation (MES) provides a better solution.

Removing the Need to Approximate Loads and Constraints

ALGOR's MES combines large-scale motion and stress analysis and uses nonlinear time-dependent FEA to properly account for the changing inertia, shape and material behavior of the model as it undergoes motion or experiences impact. With MES, there is no need to calculate or approximate loads because the forces and moments are automatically balanced according to Newton's laws of motion.

For example, with assemblies in contact, you may not even know the area of possible contact or the area of contact may be changing over time. In this scenario, it's nearly impossible to figure out approximate loads or constraints that accurately represent the effect of touching parts. With MES, you simply model the actual parts and let the software automatically calculate the contact loads. Returning to our previous impact example, in ALGOR's MES, you would model the separated bodies, turn on gravity and let physics do the rest. There is no need to guess and define any assumed impact loads because the forces and moments are automatically balanced by the software according to Newton's laws of motion.

Like basic FEA, MES provides as much flexibility as possible to help users apply known loads and constraints in ways that make sense. For example, you can apply point, surface, edge and body loads. But, with MES, you don't have to define anything more than what you're confident of and know. You simply no longer need to make guesses when defining FEA input.

Many engineering scenarios also involve multiphysics phenomena where the model must include not only structural effects but also fluid characteristics, thermal behavior, voltage effects and more. Because of this, MES supports taking temperature and voltage data as input for multiphysics analyses so users can better simulate the real-world environment.

Thus, MES is a flexible simulation tool that allows users to "know" less than ever about loads and constraints while accurately simulating more complex, realistic design scenarios.