MCAD Modeling Methods-Trends in Design Analysis

1 Jun, 2005 By: IDSA ,Mike Hudspeth

Virtual testing finds problems up front.

"She canna take the strain, Capt'n!"
"You've got to do something, Engineer!"
"She wasn't built for this kind of thing!"
"Just a little while longer."
"She's gonna buckle!"
"Science Officer, what can we do?"
"Nothing, Captain. Analysis indicates we have 30 seconds to total failure."

Dramatic music blares as the tension builds to an impossible level and we. . . cut to a commercial.

Why is it that the parts we design fail? When they fail, is it in a consistent manner? Can the failure be predicted? Prevented? Everyone who designs mechanical parts has asked these questions. Finding answers is a different story. Fortunately, design analysis software can shed some light on these issues.


There are different types of design analysis. One of the most common is FEA (finite-element analysis). Computer-based FEA gives detailed information about the stresses and deflections inherent in a design (figure 1). FEA involves a mathematical model, which is an idealized and simple version of a physical situation. The computer model is created using assumptions about geometry, materials, loads and displacements. A faceted representation, or mesh, of a part or parts is used. Generally, the mesh is as simple as possible, representing only a small portion of the overall design. Once the model is boiled down to its lowest common denominator, the designer tells the FEA program what the model is made of, how it's fixed and what forces will act upon it. The program works for a while and then presents graphics showing where the weak spots are likely to be.
 Figure 1. The U.S. Navy uses FEA to estimate the response of shipboard equipment to underwater explosions.

FEA can save untold dollars by identifying problems well in advance of tooling. Most FEA programs are static, working with a model that is fixed in space. Dynamic FEA can handle multiple parts in a mechanism and put the assembly through its functional paces. Essentially, such a program analyzes every part in the assembly at every step in motion. Computer resources are occupied for a while, even on high-powered computers. To get FEA results in a reasonable timeframe, it's necessary to limit the input geometry, and thus limit the relevance of the results—this becomes a trade-off. Another thing to remember is that people who are not trained in FEA can very easily end up with erroneous results

Integrated CAD and Analysis

The ability to perform analysis from inside design software is extremely helpful (figure 2). Users no longer have to mess with iffy translations—now they get something useful from the start. Whatever is modeled can go directly to the analysis software for full testing. The obvious exception here is if users create a really bad model—no amount of cajoling can help with a corrupt model.
Figure 2. Analysis software is increasingly being integrated into modeling packages—this is a good way to put the tools on the engineering desk.

To every good thing, there is a downside. Generally, most professional modeling packages worth their salt now come with some kind of analysis capability. It's usually a pared-down version of a stand-alone product, such as COSMOSXpress found inside SolidWorks. This is a fairly easy-to-use package for entry-level analysis, but to go beyond basic analysis, a higher-level software must be used.

What happens if what comes with a modeling package doesn't do what users need it to? They need to buy a stand-alone package at full price. Though it costs more, users get full functionality, which could end up being a blessing in disguise. Investigate what the modeling package is capable of and compare it against what you need before making a final decision.

Simulation Analysis

Simulation is another trend that shows no sign of slowing down. In fact, its use is accelerating at a phenomenal pace. To test a new design in the old days, users had to build physical mockups and prototypes to put through their paces. This was expensive and time consuming. Often multiple copies had to be built because the models broke easily. When testing and marketing groups suggested changes, a whole new set of prototypes had to be made and tested.

Then there are tooling issues. If a model is given to five tooling vendors, each one will propose a different way to build it. I've often had to change a design just so it could be tooled correctly. Changes that occur at tooling can be outrageously expensive. It's better to find the problems before the design is that far along. This is where simulation comes in.

By building a model of a design and putting it through digital testing, users can discover what can be changed or replaced without incurring the costs in time and money that older methods did. Problems such as interferences require great pains to find and fix. By building a smart assembly of, say, an extending lamp arm, a designer can move it through its complete range of motion and tell if all the parts are the right size. Digital simulation can help identify problem areas as well as allow what-if questions that help put a product on the cutting edge.

Thermal Analysis

When objects that handle heat are designed, it's important to use some kind of thermal analysis software (figure 3). Whether it's a stove, an electronic device or the space shuttle, heat is an important consideration. Thermal analysis shows where the hot spots are in the parts.
Figure 3. Thermal analysis software can help identify heat transfer characteristics.

Structural Analysis

Structural analysis can indicate what parts need to be strengthened for a design not to collapse under its own weight. This is a make-or-break detail whether designing high-rise office buildings or vehicles. An interesting fact about analysis software is that the bigger the scenario and the more the parts that make up a system, the more general the analysis and the more realistic the analysis figures get.

Mold-Flow Analysis

With mold-flow analysis, a user imports a component, identifies the material it's to be made from and the type and location of the injection gate, and the software provides a detailed report that shows how the part will fill, what areas need draft for the part to come out of the mold easily and where the weld and flow lines will be. Again, many modeling software packages now offer some of this capability.

The Benefits

What are the benefits of design analysis? There are plenty—starting with higher-quality parts. Reliability is the key to manufacturing a trouble-free and profitable product. Consistently building parts that can be made easily and will attain a predictable service life is vital to a company's bottom line, and analysis can help. Safety is also an advantage—whether it's safety on the manufacturing floor or end-user safety. Functional testing through virtual prototyping reduces costs and time to market. With it, designers have a much better understanding of how a product performs under any and all conditions it's designed for.

To beat the competition to market, build a better product and increase return on investment, design analysis should be an important part of a designer's work.


About the Author: IDSA

About the Author: Mike Hudspeth

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