Long term creep, 6 months - 1 year

[spears]

The basic tests to characterize an elastomer are the pure shear, simple tension, and equal biaxial tension. If I were interested in long term creep response would I simply gather data from these tests in the long term time frame I am interested in and fit to an appropriate model?

Also, I read that all elastomers are viscoelastic and in general all plastics. Does this mean that over very long load terms these materials will stretch or flow indefinitely.

Thanks

Karl

[sq]

Dear Karl,

To answer the second question first, almost all materials (including metals!) could be considered viscoelastic. This does not mean they will flow indefinitely. What it does mean is that their stress-strain response will be time and temperature dependent.

It sounds like you need a good introduction to viscoelastic properties of materials. A great introductory work is Ward, Hadley: "An Introduction to the Mechanical Properties of Solid Polymers". For more focus on polymer viscoelasticity, there is Aklonis, MacKnight: "Introduction to Polymer Viscoelasticity". If you want more references, respond here, and I'll give you the fuller list.

In more direct response to your first question, yes you can simply gather data from the long time frame from uniaxial or other loading. However, if you want to characterize the response over a very long time, you would then have to perform the experiment over an equally long time. But, there is an alternative!

For rheologically simple materials, one can use the time-temperature superposition principle. This idea, discussed very thoroughly in the above references, means that you can test a material for creep or stress relaxation at higher temperatures as a substitute for testing them at very long times. There are more than a few steps to this, especially developing a "master curve", which is something I suggest you read about.

Good luck!