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New Method Shows How Seals Really Age

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New Method Shows How Seals Really Age
New Method Shows How Seals Really Age

Video: New Method Shows How Seals Really Age

Video: New Method Shows How Seals Really Age
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Mechanical engineering seals must always have a long service life. Once installed, for example to protect the anchoring of the tower of a wind turbine in the sea against the ingress of salt water, the seal should function without problems for more than twenty years. The life of a seal is limited by setting or stretching. On the other hand, the material loses its elasticity over time due to chemical changes.

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Under the influence of oxygen or ozone, two effects can be observed that influence the aging of seals: On the one hand, the polymer chains and networks can break open under mechanical stress, on the other hand, additional oxygen bridges can arise in the network due to oxidation processes. Both effects influence seal-relevant properties such as rigidity, contact pressures or the ability to regain the original contour after deformation.

Extrapolation using the Arrhenius method

As a rule, engineers determine whether a material meets the requirements for a specific application by means of so-called "storage tests", in which the test object is exposed to temperatures of well over 100 ° C for a long time - usually 1000 hours. To predict temperature-dependent aging, engineers have so far extrapolated the measured values using a method that is named after the Swedish chemist and Nobel laureate Svante August Arrhenius.

The following applies: An increase in temperature by 10 ° C leads to a doubling of the reaction rate. This enables accelerated aging tests to be carried out at elevated temperatures. This method can work reliably if the correct test parameters are selected. Otherwise, the lifespan forecast can be very wrong. The prognosis can only be checked by measurements. It is understandable that this is not a satisfactory procedure, especially when talking about very long test times. It was therefore imperative to improve the methodology.

Seal manufacturer Freudenberg is pursuing two main approaches: Firstly , they significantly improved the service life model by coupling chemical oxidation equations, i.e. the oxygen attack on the elastomer, with the structural mechanical behavior of the material. In order to be able to calculate any geometry with this model, it was implemented numerically and implemented in a finite element program. This is able to calculate the local oxidation processes and the effect on the material behavior.


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Development with research

At the same time, however, it was also necessary to further develop the measurement methods with which the parameters for the material model are determined. The method developed with the central research, Freudenberg Technology Innovation, has been verified on material samples and is also used in the construction of offshore wind turbines. A library with the models is under construction. At the same time, the simulation is expanded so that application-specific cycles with changing temperatures and mechanical loads can be expected.

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