Introduction to finite element analysis
Introduction to finite element analysis

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Introduction to finite element analysis

Step 7 – Post testing and verification

The model can be adapted for each new car and any tests go towards verifying the computer model on a continuing basis.

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Transcript: Video 14

Lewis Butler
The tests that we actually carry out in the FE is representative of what we try and do on the car each season to verify its overall stiffness. And whilst this component isn’t ever tested in isolation in this manner, we know by measuring at different sections along its length how accurate the model is, and if this kind of model, basically with the assumptions that we’ve made doesn’t come out within about 5% of the tested value, then we’d probably flag it up as some kind of problem, and then re-investigate it after that.
Dr. Keith Martin, The Open University
It’s interesting that Red Bull have carried out detailed measurements of real test chassis tubs at various positions along the length-- the best form of verification. Interesting, also, that they’re disappointed if the measured values and computed results are not within 5% of each other. That’s a very satisfactory result, particularly with such a complicated part and with the non-isotropic material properties. Clearly, FEA is a very powerful simulation tool.
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Video 14
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The beauty of Red Bull’s approach to this model is that it is quite easy to match up with a real test and compare results.

The model itself has been refined over a few seasons and developed, based on subsequent testing of real tubs. This means the model can be used with confidence. Any improvements in torsional stiffness that it predicts are likely to be real.

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Dr. Keith Martin, The Open University
It’s important to consider the results of a finite element analysis with as much rigour as went into the modelling stage. The basic result is the deflection of the structure stored as displacements, ux, uy, and uz, at all the nodes. This is what the solver produces.
Other data are computed directly from these displacements. The displacements are differentiated to produce strains. And then stresses are found using the material properties.
Reaction forces at restraints are computed from the displacements and structural stiffness. Then we have to apply our engineering judgments on these predicted results. We would check for factors of safety and material yield, using perhaps the von Mises equivalent effective stress plots. For potential fatigue life predictions, we may be more interested in principal tensile stress plots. Remember that Red Bull had their own criteria for lifing the components, which would be logged and the components replaced on a regular basis.
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Video 15
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The National Agency for Finite Element Methods and Standards (NAFEMS) says that it is a common mistake in computer analysis to assume that the output, or results, of a processing job are as valid as the processing accuracy of the computer.

Instead NAFEMS recommends that it is safest to consider a set of results to be wrong until you are sure that they are at least of the expected orders of magnitude. For example computed reaction forces agree closely with hand calculated values and so on.

Remember, the computer won’t tell you that you’ve modelled the restraints properly, or that the material properties are correct.

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Transcript: Video 16

Dr. Keith Martin, The Open University
And don’t forget, in real life the engineers are responsible for making sure that variations in manufacturing, handling and transport, fitting on assembly, and use and abuse in service have all reasonably being covered in the worst case analysis. In Red Bull’s Formula One team, they have built up experience and expertise in the practical performance of the hub and the chassis tub and relating these to the simulation models.
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