06/12/2023

Stress-Strain Characteristics of Rubber & Elastomers

Introduction

For the sake of this article we will use the terms Rubber and Elastomer interchangeably although that is not correct.  We are typically referring to Elastomers, of which Rubber is a subset.  Technically speaking, most would argue that the only Rubber is Natural Rubber, all other ‘Rubbers’ should be called Elastomers.  However, most of our customers use these terms interchangeably.

My Degree in Mechanical Engineering and Materials Science Degree focussed on a standard set of commonly used materials including metals (primarily steel), composites, ceramics, plastics, and a little bit of concrete (Civil Engineers learnt lots about concrete). 

Our curriculum was packed, so I am not surprised that more materials were not included.  Rubber is pretty unusual in the way it reacts mechanically so there was more to learn, and its use less common, so I understand why it was left out, but it is a really interesting material.  Yes, it has the same glass transition characteristics as many other materials at low temperatures, and Yes, it has a useful upper operating temperature limit, but how it deforms between these two temperatures is different.

Some basic information is useful if you are going to design products using Elastomers, hence this article which focusses on Modulus.  Almost none of our customers appreciate the complexity within Elastomeric compounds, which is fine as we work together to fill the important gaps. 

This article is designed to help educate Product Design Engineers.  AVMR do not know everything about Elastomers, but we have been working with them for around 40 years and have a good knowledge base and a network of experts.  If you need any help, please feel free to drop us a line.

Stress-Strain characteristics of Steel

Steel is a material most Engineers understand to a reasonable level.  Like many other metals and materials it stretches in a linear fashion until failure modes are initiated at the Yield point.  You may well be familiar with the below type of stress-strain curve:

Yield Strength: in tension or compression, is the point (yield point) at which a material will withstand a load and return to its original shape.  Below this point, elastic deformation occurs.  Stressing a material above this yield strength causes plastic, or permanent deformation.

0.2% offset Yield Strength: This is the usual definition of yield strength.

A key point to note in the above graph is that steel reacts to stress linearly up to its yield point.

Stress-Strain characteristics of Rubber

By comparison to the above, in the same test rubber is not linear in the way it reacts.  The below is designed to explain how rubber stretches at a very generic level; there is significant room for this curve to change shape.

In the above typical stress-strain curve for Rubber, the material has a higher modulus initially and at higher strains, whilst the modulus is lower during the middle part of extension.

The effort required to inflate a typical rubber balloon matches this curve.  Increased effort is initially required to force air into the balloon, but once the balloon has started to inflate the effort required to continue inflating reduces.  Once the balloon body is mostly inflated the effort required increases again and, at some point, the balloon will burst.

The initial material structure in the balloon is amorphous; the polymer strands are in a low-energy relaxed state and have little or no alignment with each other.  Overcoming that initial low-energy state takes more effort but as polymer strands start to give in to the applied force, they stretch and air is allowed in.  From here, there are fewer amorphous strands in place to continue the resistance, so the modulus reduces and further inflation becomes easier.  There is also a little heat produced which is transferred to the surrounding strands, this also offers heat energy and helps overcome their low-energy state.

While the polymer strands are aligning, if the load on the rubber is released the materials will typically recover to its original shape – this can be seen in a basic rubber band returning to its original size.  A balloon is slightly different in that the inflated area has normally jumped quickly to plastic deformation and is unlikely to fully recover, which makes it easier to inflate the second time.

Once a certain strain is achieved, most of the strands are aligned.  Further extension becomes more challenging again as the modulus increases.  This is due to a different mechanism, which is related to the bond strength between each polymer strand.

Why am I giving you this information?  If you, the Product Design Engineer, are familiar with specifying steel, or other traditional materials with very linear moduli, it would tempting to take a single modulus value for an Elastomer, but this might not give you the result you are expecting.

Specifying the Modulus of a Rubber Compound

Two rubber compounds can have different moduli throughout their stress-strain curves, but those moduli also change with extension, as we have seen from the above. 

If we add a second rubber compound (elastomer B) to our stress-strain curve, it could look like this:

In the above graph, we can see that elastomer A starts with a considerably lower modulus than B, but then at 300% strain elastomer A has a higher modulus than B.  These could both be the same type of rubber compound with the same high-level specification (e.g. EPDM, 60° ShA), however there are a wide range of potential ingredients within the compound which impact its performance.

In an ideal world, we would therefore expect the modulus of a rubber or elastomer compound to be given at a range of extensions.

Closing

Very few of our customers specify moduli for rubber compounds, some might be interested in knowing a modulus value, but it rarely comes in as a requirement.  If a material was properly specified, we would expect to see a range of moduli for the material, at different strains e.g. at 20%, 100% and 300% elongation.

In reality, as Elastomers are complex materials; while standards are available, they are broad and only focus on a small number of properties.  Customers generally come to us with a design and ask our advice on the Elastomer to use, we then work together with our network to build the first version which sometimes needs iterating.

A recent product did have a 300% strain specification on the elastomer, but during product testing we found the material was not collapsing in compression as the customer was expecting it to.  As a result, the material was having to withstand up to 500% strain in extension and failing.  With a few simple product design tweaks, we were able to get the product with same material to pass the required tests by helping the compressed side of the product to collapse more effectively which reduced the tensile strain to within limits.

A great result for all, enabled by strong relationships with our supplier and a relationship based on trust.