Elastomer materials and how they behave in the high-frequency spectrum

Objective

Providing mobility that is fit for the future is presenting major challenges for automobile manufacturers and their suppliers. One hurdle is determining the correct requirements for battery electric vehicles and their components so that the right materials can be chosen. Selecting the correct material is crucial for the functionality of the components used. Elastomers are ubiquitous in vehicle construction as vibration dampening components and isolation components. This includes bearings for the chassis area and powertrain, bearings for the battery packs and ancillary units, and electrical insulation and absorber elements to ensure comfort while driving.

The process of electrifying vehicles includes reassessing the property profiles of elastomer materials. On top of the requirements regarding temperature resistance, media resistance, service life and mechanical loads, there are also requirements for additional physical properties such as high-frequency behavior. Behavior under high-frequency excitation can be characterized by dynamic stiffening, damping and hysteresis of the elastomers. The excitation frequency varies between 50 and 3000 Hz. There has previously been very little reliable information about material behavior in this frequency spectrum.

Both the geometry of the elastomer components and the composition of the elastomer mixture play a crucial role at the high-frequency range. The selection of elastomer mixture components such as rubbers, fillers, plasticizers, crosslinking agents and processing aids contributes to creating, shifting and reducing resonances.

This joint research project will provide a fundamental understanding of the relationships between formulation composition and high-frequency behavior for purpose-oriented digital component design. This will allow earlier and more precise prediction of the properties of materials and components during the design phase. The aim is to study the physical properties of elastomer compounds in particular, looking at filler-filler interactions and the associated stiffening in the low amplitude range (Payne effect), and to provide model-based optimization approaches for suppressing or shifting resonance peaks in a targeted manner. Another factor that influences frequency behavior is the mixing process and the associated distribution of fillers in the elastomer matrix. This particular influencing factor will also be specifically addressed in the project. Finally, the influence of vulcanization parameters on the elastomer mixture and the accompanying high-frequency behavior will be studied. This will enable the project participants to identify suitable elastomer mixtures for designing components in a high-frequency field of application through the skillful selection of formulation components, an optimal mixing process and processing conditions suitable for the formulation. Additionally, the project is intended to inspire raw material suppliers in the elastomer industry to bring new raw materials to the market in order to meet a constantly growing demand from the future mobility sector, as well as to initiate new developments for the sector as required.

Focus areas and method

This joint research project is focused on identifying various factors that influence the high-frequency behavior of elastomers and how they are represented in virtual material models. It is intended to show the relationships between individual factors, such as formulation structure, the mixing process and vulcanization, and certain properties, particularly frequency response. Together with the project partners, these factors and their influences will also be measured and evaluated by means of a targeted series of tests. The main focus here is on filler-filler interaction. Fraunhofer LBF has a highly dynamic testing machine that can characterize elastomer specimens and components. Parameters such as dynamic stiffness and loss angle can be determined in the frequency range from 50 to 3000 Hz. This process also allows for various preloads and temperatures.

The following work packages will be addressed:

Determining the profile of requirements in close coordination with the project participants

  • Researching the state of the art in order to determine the behavior of elastomers; material modeling for simulations; assessing draft designs of components under high-frequency mechanical stress
  • Defining project participants’ profiles of requirements; grouping into clusters based on typical requirements
  • Defining the specimen geometry in order to measure the high-frequency behavior of the manufactured elastomer mixtures

Material preparation and characterization

  • Determining dynamic properties such as kd/ks and resonance peaks as a function of mechanical stress amplitude and temperature
  • Determining the factors that influence high-frequency behavior using selected model formulations (rubber base, fillers, coupling agents, plasticizers, cross-linking systems) with a focus on filler-filler interactions
  • Determining the influence of mixing parameters in the internal mixer on the high-frequency behavior of selected compounds (number of stages, temperature during the mixing process, degree of filling in the internal mixer)
  • Determining the influence of vulcanization parameters on the high-frequency behavior of selected compounds in the compression molding process (temperature and time)

Simulation and modeling in the high-frequency range

  • Implementing parametric material models and regression methods for determining the frequency-dependent material parameters from measurement data
  • Virtually modeling suitable geometries for specimens and manufacturing the selected specimen for measuring high-frequency behavior in the range from 50 to 3000 Hz
  • Verifying virtual component models based on frequency-dependent material parameters

A summary of structure-property relationships is derived on the basis of the results with the aim of a better mechanistic understanding of the influence of the formulation composition on high-frequency behavior. This understanding is then put into practice through an optimizable parametric material model that can be used in conventional finite element simulations. This will result in specific recommendations for creating the best possible formulation composition to use in the respective frequency spectrum.