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Mann+Hummel and BASF: computer design for air intake manifolds |
Working with the customers – benefits for the customers
Mann+Hummel and BASF: computer design for air intake manifolds
Presented by Dr. Kay Brockmüller, Engineering Plastics Application Development
Trade Press Conference K 2004, June 23, 2004, Ludwigshafen, Germany
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Air induction systems are an important part of modern engine technology. By optimizing the design of the intake manifold it is possible to improve airflow into the combustion chambers and so maximize engine performance. Manifold design must also take into account the possibility — however unlikely — of the engine backfiring, in which case the manifold must cope with a sudden pressure without bursting.
In the past, designing a strong enough manifold was an iterative pro-cess in which prototypes were tested and modified until the required bursting strength was achieved. An alternative approach — first optimizing the design by computer simulation — was a definite improvement but not fully sufficient due to the limited accuracy in predicting when the component was likely to fail.
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Goal: 90% accuracy
In a joint engineering project, international development partner of the automotive industry MANN+HUMMEL and BASF improved the stress analysis to make it possible to predict burst pressures with 90% accuracy and so achieve big savings in costly prototype development and testing (figure 1). BASF contributed its expertise in plastics technology, while MANN+HUMMEL provided manufacturing know-how. Both partners employed finite element analysis and carried out performance tests on real parts.
Focussing on the weld seam
Today's plastic intake manifolds made from glass-fibre reinforced nylon are geometrically so complex that they are first moulded as individual pieces, which are then joined by vibration welding. Efforts to improve the various analysis methods were focussed on the weld seam, since it is here that mechanical failure is most likely to occur.
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Numerous single effects make the prediction more difficult
Local variation in weld strength: 5%
It is known that the strength of a weld at a particular location depends on the angle between the assembly direction and mating surfaces. The analysis showed, however, that taking this relationship into account improves the prediction accuracy by only 5%.
Taking internal stresses into account: 15%
Glass-filled mouldings are prone to warpage due to the orientation and non-uniform distribution of the reinforcing fibres. If the mating surfaces of two warped parts are forced together and welded, considerable stress will be introduced into the weld joint. In the case of the intake manifold, such stresses will supplement those due to internal pressure. A suitable algorithm enables these welding stresses to be taken into account, reducing the predictive error by about 15%.
Effect of anisotropy due to fibre orientation: 15%
If the results from a mould-fill simulation are used in a subsequent structural analysis computation, it is possible to predict how the material's anisotropy will affect the stresses within the part. By knowing precisely where critical deformation and stresses will occur, the error in the computed burst pressure can be reduced by a further 15% (figure 2).
Transformation errors with multiaxial and uniaxial stresses: 35%
The failure mechanism in the weld seam was also investigated. While a material's tensile characteristics are usually determined on standard test specimens along a single axis, the material in a real part is usually subjected to multiaxial loads. It is standard practice to transform the multiaxial stress condition into a uniaxial reference value. Trials on weld joints under a multiaxial load showed that the assumptions usually made in the transformation process lead to an error of about 35%.
Influence of the modelling method: 35%
The type of finite element model has an astonishingly high influence on the prediction accuracy. The shell-element models usually employed lead to an error of around 35% (figure 3).
It should be pointed out that the benefits gained in each of the cases discussed are not cumulative because one effect may be partially compensated by another.
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Significantly better than the state of the art
The new modelling and evaluation method takes into account the problems discussed. For example, it replaces the conventional shell model with a volume model and uses a multi-dimensional failure criterion.
In order to validate the new method in practice, four test specimen of various complexity, each having a weld seam, were studied. These were (figure 4):- a sphere comprising two hemispheres,
- an oblong specimen with rounded ends and two flattened sides,
- the same elongated specimen as 2., but with a wider and more offset weld seam,
- a simple intake manifold.
Results of pressure tests on the four specimen showed that the burst pressures predicted by the new modelling method are 90% accurate (figure 5). By contrast, predictions from conventional methods are only 55% accurate (averaged).
Armed with this new knowledge, BASF and MANN+HUMMEL are able to eliminate a lot of engineering uncertainty early in the development stage when predicting the bursting strength of the intake manifold. This not only saves much time and expense in the development of prototypes, but also allows better optimization of the design. And because optimized designs tend to require less material and are lighter, additional savings can be achieved in terms of material costs and shorter cycle times thanks to faster cooling.
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