Using analysis to gain a vital edge

Tom Shelley reports on some the benefits to be obtained by using advanced analysis, but cautions about the pitfalls

CAD analysis packages have an enormous potential to produce better designs and get them to market quicker, but it is still desirable to understand what they are actually doing, and there still have to be practical tests to verify that what the software is predicting is reflected in the real world.

Take software to model impacts, for example. SSA, based in Leafield near Oxford, sells and uses a package called NEiX, which is based on Nastran Finite Element Analysis, but has been tailored by Noran Engineering, now NEi Software, for analysing impacts: collapsing struts inside crashing vehicles, and more violent impacts such as explosions inside containers and missiles striking armour plate.

The big problem with this and other kinds of computer modeling is that, while the mathematical modelling may be good, materials deformed at high strain rates do not behave like materials deformed on a tensile testing machine, and much is still being learned about the physics and metallurgy. In particular, metals deformed at high strain rates may become superplastic and deform easily, or strain hardening may increase making them harder to deform, or they become brittle because dislocations do not have time to move. Composites made up of different materials, similarly, can be stronger or weaker at high strain rates, depending on many factors.

That does not mean that computer modelling is useless – far from it. But, as Jon Storey of SSA explains: "You always need to verify finite element analysis in some way." In the hands of engineers with expert knowledge, this is not a problem. "Initially", Storey told us, "Finite element analysis was a very specialist world. Now you often only have to push a button. It will produce a solution, but it may not be meaningful. This is one of the dangers."

Designers and users of real world armour plate strongly endorse this view. The designers all use computer simulation, some of it very advanced, but they all insist there is no substitute for real world tests and there is often a strong divergence between predicted and real world results.

If the task is fairly static however, simplified FEA integrated into CAD software can work extremely well. CAD expert and trainer Elise Moss, for instance, used SolidWorks to greatly improve the design of the support system for solar photovoltaic modules for Solar Infra, working in conjunction with Sandia National Laboratories in Albuquerque. She says that many passive solar modules sold in California cannot be used within 5m to 20m of the coast, because of problems with salt water corrosion, while others cannot survive high winds or heavy snow fall.

She says that using SolidWorks, and with help from Sandia Labs, which was using SolidWorks Simulation, she was able to come up with an aluminium alloy support that would withstand winds of up to 800mph and 20 feet of snow. This module is now on sale in the USA, following the building of a pilot installation in South Africa. "In the course of the investigation," she says, "we also learned a lot about the differences between the types of aluminium alloy and the lamination process to mount the cells. These incorporate thin copper conductors and have 'Mylar' on one side and white plastic on their backs. The three layers are laminated together in a press and the Mylar melts. The end result is like a grilled cheese sandwich in a Panini press. Then the laminated cells go into the aluminium frame."

Thermal Analysis (ATH), to study problems similar to the design of the laminated structures of the photovoltaics is one of the capabilities now offered within the Simulia element of Release 20 of Dassault Système's V5 PLM platform. It allows the calculation of steady-state or transient temperature distributions in response to the direct heating of a surface, the flow of a liquid past a surface, or the specified temperature of the surface. Thermal material properties can be temperature dependent. When analysing assemblies, the thermal conductivity across the interface between contacting parts can be specified.

Another new capability is non-linear structural analysis (ANL). When ATH is used in conjunction with ANL, structural analyses can be undertaken that include the effects of temperature distributions calculated by ATH, which can cause parts to expand and contract and also affect their material properties.

ANL can model the effects of large displacements, allows the incorporation of yielding of metals and can accommodate the behaviour of inherently non-linear materials such as rubber. The software can also model plasticity using either isotropic hardening for general use or kinematic hardening for low-cycle fatigue studies.

The new software further enables the effect of multiple steps to be analysed, where parameters such as loading, restraints and contact conditions change from step to step. A typical example is a pressure vessel subject to bolt tightening, followed by internal pressurisation and thermal loading. A variety of connections can be modelled, including springs and welds as well as bolts. Other connections include: rigid connections, virtual parts, and nonlinear springs and dashpots.

A high-profile vindication of the benefits of modeling can be seen in the crushing victory of BMW Oracle Racing's yacht "USA", in this year's America's Cup in February, which successfully exploited computational fluid dynamics (CFD), even if it did use rather a lot of computing power.

The yacht had a wing instead of a main sail, which at 57m high was longer than that on an Airbus 380. BMW Oracle Racing's Mario Caponnetto, said in an interview before the crucial races that, "In a very short time, the optimisation work of the wing profile has been carried on with the Star-CCM+ CFD code by our partner CD-adapco and exploiting a remote supercomputing cluster.
"For us, it was very important that the CFD code was able to give indications on the wing behavior as far as stall is concerned. That behavior was later validated during sea trials. Furthermore, we created a database of optimal wing shape based on all the possibly encountered wind situations. The database is installed on board and allows optimising, at any moment, wing efficiency."
When asked to go into details, he said, "First of all, we exploited the "client-server" architecture of the CD-adapco software. We could use a remote supercomputing cluster facility located in Italy. While sitting in our offices in Valencia or San Diego, we could check in real time the progress of the simulations running on the cluster. This happened thanks to a lightweight client, or if you like the final user, based on a Java interface, and a C++ server, or if you like the supercomputing cluster".
CFD is routinely used by designers of products from cars to domestic taps. Supercomputing clusters are not usually considered necessary.

Author
Tom Shelley

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