|When simple calculations fail to describe stress and deflections in complex models, such as the proposed diaphragm, you'll know it's time to bring FEA on board. The big question then is, which package? Our experts have been there and done that, and offer several suggestions.|
According to expert users of finite-element software, there are several telltale signs that a company needs to get on the FEA bandwagon. One is when design cycles grow too short to comfortably finish projects, or when they turn away business because they cannot meet deadlines. "Or, if your company builds special machines in low quantities," says James Perrault, an MSC.Nastran user and engineering manager with McElroy Mfg. (www.mcelroymfg.com) in Tulsa. "There is not a lot of time to go through several designs to get it right. Another indicator is when stress studies based on simple equations or Roark's formulas for stress and strain are an oversimplification. Then you need to turn to FEA."
Once a department accepts that design schedules are tolerable only if they can do more in less time, they must decide which FEA program is right for them.
In a nutshell, our experts pinpointed ease-of-use, accuracy, cost, speed, and support as key issues with FEA software.
More specifically, they suggest first deciding what analysis makes sense. For example, if a department does a lot of straightforward stress studies, then a program with linear-static capability is a candidate. Most basic FEA programs provide this along with modal or vibration analysis, and buckling. Thermal analysis may be included as well. Of course, if products involve fluid flow, then contact software developers that specialize in CFD or computational fluid dynamics.
"Ask the salesperson if they sell something tailored to what you do," suggests Rob Calvet, an Algor user and senior optomechanical engineer with SiWave Inc., Arcadia, Calif. (www.siwaveinc.com). Software companies are starting to formulate products modularly. This way, users buy only what they need. When faced with new tasks, they merely add another module. "And ask whether or not the modules work together," he suggests.
To select candidate programs, look first at those compatible with your CAD system. "When first considering FEA, we looked for software that worked well with our 3D modeling system," says Perrault. "We eventually bought an FEA program that practically plugged into the CAD system."
When CAD and FEA programs work well together, loads and constraints need not be reassigned with each iteration of model geometry. "When an analysis turns up bad results, you'll want to quickly iterate to a solution that gives optimal material use, acceptable stress levels, and in a minimum time period," says Perrault.
Ease-of-use has wider implications than just software simplicity. "Uncomplicated software gets used more often," says Don Bartlett, a Cosmos user and senior staff engineer, Fanuc Robots Corp., Rochester Hills, Mich. (www.fanucrobotics.com). "All of our engineering staff uses a version integrated with our modeling software. The simplicity pays. "For example, the auxiliary axes on a recent weld positioner uses a heavy steel casting. After analysis, the designer got rid of many pounds where it was not needed. The arm weighs less, costs less, and has a lower dynamic load. All this because the FEA program was easy to use."
In addition to ease of use, consider what others in the company might need. "If there are divisions that deal with mechanical, structural,or thermal issues, ask about their needs," adds Calvet. "Talking to processing people about how they make things could also be helpful in pointing to needed capability," he says.
Let the people who will use the software select it. "I assigned a couple engineers to research available products and they returned with a proposal that I signed," says Perrault. "They've been satisfied with their decision and so have I."
Expect to find a wide variation in costs. "Software seems to come in low-cost packages with little support, moderate-cost software with reasonable support, and high-cost packages with large annual maintenance fees," says Calvet. "Some maintenance fees alone are difficult for small companies, so be sure to know what they will be. We've found that a moderately priced program with good support to be sufficient."
Support comes in different ways. "For instance, I wanted to take advantage of the press-fit capability in the software," says Bartlett. "But I wanted to model the part so constraints don't interfere with the answer. An example in the manual suggested using functions called soft springs and inertial release. What we wanted to know is how should we do this problem? We got good answers online. The FEA developer answered our questions in the user group. One offered to conduct an online demonstration to show how a feature works. There is no charge for this type of support," adds Bartlett.
Calculate an ROI for the software. Higher ups will probably want to see an ROI on such a purchase, and it's a good way to financially justify the investment. "Look for a payback in two years because the software will be outdated by then," says Perrault. Calculating ROI is not a precise science, but the task involves putting a dollar value on the time trimmed from a design cycle, the number of lawsuits it might avoid, and the number of prototypes and engineering change orders eliminated. Admittedly, those are hard to put a number on. "But make a best-guess," suggests Perrault.
"I got a strong sense of company direction based on how they served me when learning the software, and the documentation's quality," says Carl Howarth, NE/Nastran user and owner of Howarth Development, Bloomfield, Mich (www.hdev.org). "It told me these people were serious. In contrast, one solver I had used was not upgraded in five years."
The question of how long a developer has been in business also comes up, and it might be a concern when buying a piece of capital equipment, suggests McElroy's Perrault. "You tend to be that way with software as well. But in this day and age, software changes so fast, it's less a concern." Rather than longevity, ask about the features the developer has planned for the next few releases.
Try it before you buy it. "Do not do business with anyone who will not let you test a fully functional program," adds Howarth. There are two ways to treat software before a purchase and they depend on its complexity. One way is to ask for a trial period to try it in your office. "The drawback to a 30 or 60-day trial period is that a fair test is difficult without training. Getting up to speed may take most of the trial period," says Perrault.
"The other option is to observe and time the vendor in a benchmark," he adds. "Find a part you've had difficulty analyzing. Or if you're upgrading the FEA package, use a model that has been difficult to mesh, apply constraints, or took too long to solve. Time the vendor and see if there is a measurable improvement. It's also useful if they benchmark on the same hardware you'll be using."
Be leery of canned demos. "They can look marvelous but might not be representative of what you'll be doing," says Perrault. And don't give the part to a vendor ahead of time, he cautions. That lets them work out the bugs and then come back with a smooth demo. Have them do it in front of you. So be willing to dedicate several hours to the effort.
And during a benchmark, listen to what the operator is muttering as he works. "If he says things such as 'Wow. It worked' be a bit leery of that particular program," suggests Perrault.
Get ready for a hardware upgrade. If anything in the model moves, needs a kinematic solution, or has multiple parts, it will eat computing power. Significant models may take days to compute.
The good news is that top-of-the-line Windows-based computers come with processors running at up to 3 MHz, and many can be fitted with over a gigabyte of RAM and 50-Gbyte hard drives. "You'll find FEA software takes a tremendous amount of RAM because of the huge stiffness matrix it builds for each part," says Perrault. "But memory is cheap, so load up on it," he recommends. "Large disks provide lots of scratch space to write temporary files."
A computer upgrade has other benefits as well. "We moved from a three-year purchasing cycle to half that," says Bartlett. "We're moving from 500-MHz to 2-GHz computers -- four times faster," says Bartlett.
Graphic displays may be less critical. "You probably won't spend much time rotating solid models and doing dynamic shading, so you won't need a top-of-the-line graphics card, even though it helps. Hardware configured to run solid-modeling systems should run any analysis package."
Those on a tight budget can get started with the computers on hand, but anticipate upgrading them as your skills improve. "The simulations we wanted were taking a long time to solve, so we upgraded to a dual-processor computer. The equipment-purchase decision comes down to anticipated usage and required turnaround time," says SiWave's Calvet.
And it's a mistake to skimp on office space and equipment. "Committing to FEA means someone will be spending a lot of tube time," says Howarth. "My computer is on a 4 X 6-ft table where I can comfortably spread out manuals and papers. I've seen people on small, cheap desks with the keyboard at the edge, resting their wrists on the mouse pad. That's an invitation to carpal-tunnel syndrome. Get a large desk, a comfortable chair, and at least a 19-in. monitor."
A last bit of advice
Plan for training. Have someone on the engineering team who knows FEA basics, theory, and limitations, so it's not treated as a black box.
Who gets trained first boils down to who has the aptitude and interest. "One trend in FEA today is ease of use," says Howarth. "But it's deceptive. The software is simpler to use, works faster, and does not require extensive education. On the downside, users may never understand the built-in assumptions. That leaves a margin for significant error. The real challenge is finding people good enough to understand when the answer is right and when it is not," he adds.
Learn an independent pre and postprocessor, such as Femap or HyperMesh. "It lets you pick up additional solvers as they are needed without learning new pre and postprocessors," suggests Howarth. "I'm on my third solver but I have not switched from the pre and post I started with."
Don't expect to apply FEA to every part in a design. "Engineering experience and simple calculations will tell you some parts are good enough," says Perrault. "Focus on parts that are high-stress or critical components. If you take the time to analyze everything, you won't get anything done."