FEA Loads and Constraints: What Designers Should Know

Nov. 18, 2004
Loads for most FEA problems are just good guesses. Here's how to stop relying on guessing.

FEA loads and constraints: What designers should know

Bob Williams
Product Manager
Algor Inc.
Pittsburgh, Pa.

Engineers at Entergy Operations Inc., an operator of nuclear-energy facilities, used MES software to simulate heavy impacts to equipment. In this case, the question was: How does water level affect a tank's structural integrity as it is struck by a heavy object that might be hurled by a tornado?

MES let biomedical engineers at Wright Medical Technology Inc. evaluate contact stresses for this knee implant. Analysis helped predict wear patterns so designers could improve the implant.

This multiphysics simulation of a common singlepole residential circuit breaker combines electrostatic, transient heat transfer, and motion to determine performance characteristics such as trip time. The software simulates a voltage overload which heats and deforms a bimetallic strip that trips the mechanism to break the circuit.

Finite-element analysis (FEA) simulates how models react to a range of environments. You define the environments with a combination of loads and constraints which are vitally important to the overall simulation.

Although setting up a problem seems straightforward, a few of these factors often complicate the task. For example:

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. Engineers often rely on experience and judgment in determining loads and constraints, and how to apply them. Even experienced engineers have difficulty determining accurate values for these inputs. For example, suppose an impact analysis calls for simulating a structural response when several objects hit. A point load could be used to approximate the impact force. But what you think the load is and what it actually is often differs significantly.

Artificial loads and constraints complicate result evaluations by introducing "hot spots"—— localized zones of high stress in the model. For example, constrain a point and the area around it may show an artificial stress spike. There's no way around this modaccordingeling effect because it's a peculiarity of FEA mathematics. False spikes complicate results and prompt questions such as: Is the hot spot something to be concerned about, or can it be explained away? The answer also depends on engineering judgment.

For simple environments, you may be willing to tolerate small inaccuracies from modeling assumptions such as point loads or constraints. But what about more complex situations such as those involving motion, impact, timedependent changes, and combinations? These call for a better solution that comes from software that combines FEA with motion, such as Mechanical Event Simulation (MES) from our company.

This solution combines large-scale motion and stress analysis. It uses nonlinear time-dependent FEA to account for changes in the model's inertia, shape, and material behavior as it moves or is struck. There is no need to calculate or approximate loads because the forces and moments are balanced according to Newton's laws of motion.

For instance, it can be difficult to know the area of possible contact in assemblies, or the contact area may change over time. In such a scenario, it's nearly impossible to determine the loads or constraints that accurately represent the effect of touching parts. But when motion is combined with FEA, designers can simply model actual parts and let the software calculate contact loads.

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, it can apply point, surface, edge, and body loads. But you only define what you know. It's no longer necessary to guess when defining FEA inputs.

Many engineering scenarios also involve several physical phenomena, such as structural effects, fluid characteristics, thermal behavior, voltage effects, and more. Because of this, MES handles temperature and voltage data as inputs, letting users better simulate the real world.

Flexible simulation tools such as MES let users "know" less than ever about loads and constraints while accurately simulating complex, realistic scenarios.

Make Contact:
Algor Inc., (412) 967-2700,

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