Elastomer (hyperelastic) Characterization

Introduction

The analysis of elastomers in finite element analysis often require the use of hyperelastic material models. These models require that material specimens of the subject material be stretched and stress-strain data collected. Most hyperelastic models perform better if multiple states of strain are represented. Elastomers should be characterized at the temperature of the application.

Multiple Strain States

There are 3 strain states which are particularly useful in characterizing dense elastomers. They are simple tension, pure shear and equal biaxial extension. If the elastomer is confined in its application and the compressible nature of the elastomer is a concern, a volumetric compression experiment (bulk modulus) may also be advisable. This section is divided into experimental sections as follows:

Typical Dense Elastomer Experiments:


Simple Tension

Simple tension experiments are very popular for elastomers. There are several standards for the testing of elastomers in tension. However, the experimental requirements for analysis are somewhat different than most standardized test methods. The most significant requirement is that in order to achieve a state of pure tensile strain, the specimen must be much longer in the direction of stretching than in the width and thickness dimensions. The objective is to create an experiment where there is no lateral constraint to specimen thinning.

outline of rubber tensile test specimen D412

Simple Tension Test Specimen Outline

Viton rubber tensile test specimen in an environmental chamber in an Instron instrument

Simple Tension Test in an Environmental Chamber Fitted with Optical Glass for use with a Laser Extensometer


Planar Tension (Pure Shear)

The pure shear experiment used for analysis is not what most of us would expect. The experiment appears at first glance to be nothing more than a very wide tensile test. However, because the material is nearly incompressible, a state of pure shear exists in the specimen at a 45 degree angle to the stretching direction. The most significant aspect of the specimen is that it is much shorter in the direction of stretching than the width. The objective is to create an experiment where the specimen is perfectly constrained in the lateral direction such that all specimen thinning occurs in the thickness direction.

free body diagram of a pure shear or planar tension test specimen

Planar Tension Test Specimen Outline

planar tension or pure shear speciemn in a tensile test machine

Planar Tension (Pure Shear) with Laser Extensometer


Equal Biaxial Extension

For incompressible or nearly incompressible materials, equal biaxial extension of a specimen creates a state of strain equivalent to pure compression. Although the actual experiment is more complex than the simple compression experiment, a pure state of strain can be achieved which will result in a more accurate material model.

image of a rubber biaxial test specimen

Equal Biaxial Test Specimen

image of an equal biaxial rubber test specimen in a biaxial extension test instrument

Equal Biaxial Extension Experiment


Volumetric Compression (Bulk Modulus)

Volumetric compression is an experiment where the compressibility of the material is examined. In this experiment, a cylindrical specimen is constrained in a fixture and compressed. The actual displacement during compression is very small and great care must be taken to measure only the specimen compliance and not the stiffness of the instrument itself. The initial slope of the resulting stress-strain function is the bulk modulus. This value is typically 2-3 orders of magnitude greater than the shear modulus for dense elastomers.

image of a volumetric compression test fixture for nearly incompressible elastomers

Volumetric Compression Experiment


Simple Compression

The compression experiment is also a popular test for elastomers. When testing for analysis, pure states of strain are desired and this is especially difficult to achieve experimentally in compression for dense materials. However, for foam materials, the error due to friction between the specimen and platen is not as significant.

rubber compression test D575

Simple Compression Test


Specimen Preparation

Material testing experiments on elastomers typically require sheets of elastomer from which test specimens are die cut. If elastomer sheets aren't available, sometimes sheets can be skived from thicker slabs or actual parts.

typical specimen cutting layout for hyperelastic test speciemns cut from sheet

Specimen Shapes Stamped from Sheets

image of a slicing machine used to make thin slices of elastomer for cutting of test specimens

Bandknife Slicing System to Create Rubber Sheets From Bulk Pieces