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Service Life Simulation on Plastics under the Influence of Aggressive Fluids

01.04.2008
Components in the vehicle engine compartment must withstand high temperatures, mechanical vibration and the effect of aggressive substances such as fuel, brake fluid or engine oil.

Until now, the durability of plastic components has been extremely difficult to assess, considering the variety of influencing factors. This is particularly important for critical safety components, whose failure involves a significant danger potential and subsequent liability risks.

At the "JEC" trade show held from 1 to 4 April in Paris, at its stand U74, hall 1.3, Fraunhofer LBF will present solutions for simulating the service life of components made from plastic and other composite materials.

The number and usage of components made from plastic and composite materials has been growing steadily throughout the last few years. Until now, reliable, material-specific testing methods and rating concepts for these plastic components, enabling an assessment of their expected service life, have not been available. Within the framework of a Wing Project of the German Federal Ministry for Education and Research lead by Robert Bosch GmbH, engineers are currently working on new material technologies. Research scientists at the Fraunhofer Institute for Durability and System Reliability LBF in Darmstadt are developing a simulation method which will enable the reliable assessment of fatigue and ageing of plastic components under various ambient influences. Using specimens made from thermoplastic materials and resembling the actual components, researchers have investigated the behaviour of the materials in laboratory tests, as a function of material, geometry, manufacturing process and various environmental factors (such as temperature, fuel, or oil). These parameters are critical to strength, load-carrying capability and service life of the materials.

The effect of these combined parameters on material fatigue can subsequently be shown in a numerical simulation of the components. "First of all we analyse the vibratory strength of plastic specimens immersed in an oil bath, by introducing cyclic loads", explains Andreas Büter, Head of Department at Fraunhofer LBF. "Depending on the load at which specimen fatigue or fracture occurs, the fatigue strength, i.e. the relationship between the cyclic load applied and the maximum permitted number of vibrations can be calculated." On the basis of the results obtained, a diagram called Wöhler curve is determined by the engineers, which enables a statistical assessment of the fatigue life of a component.

How a fuel rail, for example, will withstand engine vibrations under simultaneous contact with fuel, is simulated on the basis of the results obtained by means of a numerical model of the component. "We calculate the stress and strain occurring in the material at various loads", explains Büter. "We know from experiments the stresses the material can withstand without damage, and at which strains damage occurs. Hence we can assess the service life of plastic components with maximum reliability".

The purpose of the simulation models, which are adapted to the specific material in hand, is to enable designers to take into account ageing processes and the effects of various ambient factors on plastic components at an early stage in the development phase - similar to metal components. This will reduce the cost and time involved in design modifications and adaptation of shape.

Scientific contact at the show:
Dr. Andreas Büter, Julia Hartmann
mobile +49 172 6184202
julia.hartmann@lbf.fraunhofer.de

Anke Zeidler-Finsel | Fraunhofer Gesellschaft
Further information:
http://www.lbf.fhg.de/

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