A few solid elements

Four-node tetrahedron
Four-node tetrahedron
 
Six-node wedge
Six-node wedge
 
Eight-node brick
Eight-node brick
 
20-node brick
20-node brick
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.



Suggestions for hardware and software

A brief outline of the FEA process would be the same regardless of software vendor. What does change are particular command names and how they are used in the software. Most FEA programs provide accurate answers when properly used. But some have advanced functions and features or more-efficient user interfaces. So there may be a program that better suits one type of analysis or CAD interface than another.

Most FEA programs run on Windows or Unix workstations, although the techniques and concepts work the same on a mainframe or supercomputer. "Users should consider buying or using the most powerful system their budget allows and their product requires," says Fryberger. "Hardware horsepower is far more important for FEA than it is for 2D or 3D CAD. A Pentium IV or equivalent should be the CPU of choice." One-half to a full gigabyte of RAM is recommended for small to medium-sized models, and more for larger ones. Go with high-speed hard drives with 30 to 40 Gbytes at a minimum. Include a fast, powerful graphics board along with a 21-in. CRT or LCD. A 3D-pointing device, such as the Labtec Spaceball, is supported by many solid modelers and some FEA programs. These features describe a minimum system. More memory and speed are better. Another plus is a fast Ethernet LAN for local communications and broadband Internet access.