At square one with FEA - part 1

At square one with FEA

A few solid elements

Four-node tetrahedron

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Six-node wedge

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Eight-node brick

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20-node brick

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Most automatic mesh generators create tetrahedral or tet elements, and hexahedron or brick elements. Tets come in four and 10-node varieties and bricks come in eight and 20-node versions. Some mesh generators also generate six-node wedges and five-node pyramids. Lower-order elements are good for quickly locating high-stress areas. High-order elements, the 10-node tets and 20-node bricks, provide a better picture of actual stresses at the cost of a larger model and more computing time.

The real challenges are finding skilled FEA users and managing the engineering department to take advantage of the software. In this first of several articles on getting FEA up and running, Ted Fryberger, a consulting engineer and instructor with DeepSoft Inc. in Columbia, Md. (www.deepsoftinc.com), makes a few suggestions for selecting qualified FEA engineers and establishing an FEA program.

"First off," says Fryberger, "FEA users need three things: academic training at the university level, practical experience doing manual engineering analyses, and the ability to use the FEA software correctly." Academic training for structural problems should be in statics, dynamics, strength of materials, and theory of elasticity. Tackling thermal, fluidic, electrostatic, and electromagnetic problems needs additional training. "Engineers and dedicated analysts should also have experience with manual stress-analysis calculations on calculators, spreadsheets, and computer programs they developed," he says.

"The person performing the analysis must be familiar with interchanging data between the FEA software and the CAD modeler used to create geometry," he adds. It is also important that analysts know their limitations as well as the program's. Engineering judgment, based on experience, is a crucial component of any engineering analysis. A few other points to consider include:

The range of analyses. It is quite broad, spanning linear, nonlinear, static, dynamic, steady state, transient state, deflection, stress, thermal, fluidic, electromagnetic, and more. This introduction deals only with linear-elastic and static structures, the most frequently encountered. "Linear-static analysis suffices for many products. Even when users want to do dynamic or nonlinear simulations, they always start with a static, linear-elastic analysis," says Fryberger. "Because if the product, or the FEA model of the product, can't survive a linear-static analysis then performing more complex dynamic or nonlinear simulation is a waste of time until these initial problems are fixed."

FEA in a nutshell applies a force to a stiffness matrix and then determines the resultant displacement. Stresses are then computed from the displacement vector. The relationship is expressed in the equation:

F = K x U

where F = force matrix, K = geometry stiffness matrix, and U = displacement matrix. Most FEA packages support many unit systems. Use one consistently and document the choice.

Preliminary FEA steps have a big affect on the quality of results. Even before preprocessing, users must define a strategy to model the product. "Surprisingly, most people don't even talk about this initial analysis strategy, and it's a critical step," says Fryberger.

For example, look at the part, assembly, or drawings and answer the questions:

• What failure modes are we looking for?
• Will it require a single analysis or a sequence of analyses, and how many types of analyses?
• Will the required tools pass information between each other?
• How do we determine satisfactory and unsatisfactory criteria for each analysis?
• How will the mesh be built -- manually, semiautomatically, or fully automatically, and why?
• Can the computer handle the anticipated model size?
• Can symmetry be used so only part of a model is needed?
• Will classical stress equations deliver useful information?
• Will 2D or 3D geometry be needed?

Future articles will cover specifying loads, selecting material models, checking input data, and working through an example problem.