Elastomers and FEM: Creating material-specific material cards

Objective

Elastomers are widely used in mechanical engineering, medical technology, sporting equipment and household appliances. They are used in seals, couplings, bellows and bearings for their hyperelastic properties, sealing ability, friction resistance, vibration and damping insulation.

Modern applications create particular challenges when it comes to selecting materials. Fitting finite element method (FEM) models using relevant material data can result in reliable designs for elastometric components.

Several FEM models are able to depict the material behavior of elastomers to various degrees of quality. Engineers face two main challenges in this process:

• Selecting a suitable model for the intended use
• Carrying out sufficient tests for the parameterization of the model.

The project will tackle these issues. The objectives are to:

• Study the state of the art by means of existing standards and scientific publications. Include an evaluation of the main recommendations for characterizing elastomers in the study and record the challenges that arise during their application.
• Supplement this with an overview of available material models implemented in commercial FEM codes.
• Plan the creation of a reliable method for efficient and cost-effective elastomer modeling, taking into account customer-specific requirements (loading velocity, temperature, environment, etc.).
• Carry out necessary tests using the proposed method and provide material cards to the project partner.
• In the best-case scenario, this will lead to a targeted reduction in costs associated with the product development process.

Particular attention will be paid to how possible multiaxial effects, for example in seals, membranes and tires, are characterized. These effects significantly influence material behavior. Material parameters derived from the material behavior are an essential factor in the quality of the FE model depiction. Both 1D and 2D testing on elastomers can provide key data for material-specific component design in this regard.

There are some known difficulties in carrying out these tests, such as the influence of clamping on material behavior in uniaxial tensile tests, biaxial tensile tests (inflation test) and pure shear (planar tension test). That is why recommendations for executing the tests and optimizing clamping are being developed, with clear and unambiguous regulations being put into practice.

The method that results from this process will be an important tool for defining an optimized design strategy for elastomer components.

Focus areas and method

Elastomeric components transmit multiaxial loads. Material models fitted exclusively from the 1D tensile test data will only provide a rough estimate of the component behavior when used for the design. Although it is not complicated to conduct parameterization using data from a tensile test, there is often a big difference between load case predictions for certain applications and actual component behavior.

That is why this project focuses on systematically compiling material models and specifications for tests, making it possible to sufficiently depict multiaxial behavior.

To achieve this, initial research will be conducted at the outset, and its results will be supplemented with our suggestions for improving how testing is carried out. The known difficulties involving clamping for certain tests will be constructively tackled and eased. Recommendations will be developed for carrying out tests, and clear and unambiguous regulations will be derived. Necessary devices will be set up. Specimen geometries will be adjusted based on existing material or the semi-finished product.

Three elastomers will be mechanically characterized in consultation with participants. For this purpose, the three most important tests will be carried out, namely:

• The uniaxial tensile test
• 2D tensile text (inflation test)
• The pure shear test (planar tension test)

Strain will be recorded using 3D DIC (digital image correlation).

The tests will be carried out in defined temperature and environmental conditions, with a possible temperature range of -40 to 140 °C. When it comes to application, specimens can also be preconditioned in aggressive liquids or oils.

The data from the tests will be refined and fitted to material models. These will then be integrated into the FEM simulation (Ansys). This will allow measurement results to be generalized to correspond to any state of stress. Deviations in the models will be discussed. Best-fit models for the planned application will be provided, along with the appropriate material cards. Diagrams will be used to illustrate the advantages that modeling offers when compared to material characterization based on uniaxial tensile tests.

In summary, the project can be divided into the following work packages:

1. Conducting an overview of existing test standards and regulations and selecting an appropriate configuration for the project
2. Researching the literature in depth and systematizing methods
3. Adjusting the specimen geometry
4. Optimizing the test setups constructively
5. Carrying out mechanical tests
6. Evaluating the tests
7. Fitting models and creating material cards for the recommended FEM models
8. Comparing evaluations and recommendations

The project should result in a direct recommendation for action for the design engineer. It will also demonstrate methods for modeling elastomers in commercial FE codes in a way that is economical, yet also effective and reliable. The recommendation that results from the project will then represent an important tool for optimizing design strategies for elastomer components. In the best-case scenario, it will be possible to use the material cards created through the proposed method directly in the design of critical components. This will allow for an optimal, targeted reduction in the costs associated with the product development process. It will also be possible to qualify materials for use in specific applications.

The findings from this project will enable the project partners to expand their range of expertise and gain a clear advantage in terms of knowledge, thus becoming sought-after development partners.