Viscoelasticity in the Materials Industry

Many industrial materials have viscoelastic properties depending on their constituents, processing conditions and the conditions during use.

They can be very obviously viscoelastic when held in the hands, or this behaviour may not be apparent until heated up to 1000°C and put under a large stress. The relaxation time also varies a large amount. Whether this viscoelasticity is beneficial or not depends on the material’s type and intended use.

Memory foam mattresses are becoming very popular due to their ability to mould to the shape of the sleeper’s body, relieving pressure on painful joints. They have a layer of temperature-dependent viscoelastic material - when a load is applied (a person), the material relaxes away to take on the load’s shape. They then show slow springback when the load is removed.

The response of these materials to varying loads and varying loading rates is widely-investigated by memory foam manufacturers. As they are easily cut to regular cuboid shapes, they are suitable for compression testing. Full stress-strain analysis can be carried out at the press of a button using a Texture Analyser along with Exponent software, made simpler when the sample has a constant cross-section.

Polymers are long chained materials that creep due to stress induced molecular rearrangements. Some chains in the polymer may change partners causing the release of stress, some linear chains may eventually slip past each other (viscous flow) and some crosslinks or entanglements can relax in a reversible way. Polymers behave in a more elastic manner below their glass transition temperature, but as they are warmed up, these creep mechanisms become thermally activated and the polymer is able to flow at a lower stress.

Measuring Viscoelasticity

An example is silly putty. When a child stretches a piece of silly putty, they are interested by the flowing action between their hands. If it were more elastic, it would be more like stretching a rubber band. Too viscous, it would not be possible to hold and stretch the bulk of it; it would instantly flow away from the hands. The faster the putty is stretched, the more elastic it behaves (it is possible to snap a piece of putty at room temperature if pulled very quickly). Additionally, if silly putty is cooled down well below its glass transition temperature, it behaves as a hard, glassy plastic. The simplest way to test silly putty using a Texture Analyser is to compress a contained volume of the sample under varying deformation rates and to observe the force response when this deformation is held constant.

Concrete may seem completely elastic in its behaviour, but in fact it can shrink under stress over a period of several years. While compressed, chemically unbound water is pressed out of pores and evaporates, leading to shrinkage, which can be troublesome for pre-stressed concrete components used in construction. If concrete is fully dried before being built with, it does not creep, but it is almost impossible to dry concrete fully without severe cracking. This effect occurs at all stress levels but is most significant when designing pre‐stressed concrete parts like slender columns or floors. Accelerated testing must be used for such a slowly deforming material, and the TA.HDplus is ideal for this type of testing, with a capacity of 750kg force and a wide range of test attachments.

Although used as the definition of elastic behaviour, even a metal spring is viscoelastic at a high enough temperature and stress. This temperature depends on the type of metal. All materials show creep to different degrees at elevated temperature, and this may be caused by several different thermally activated mechanisms. The temperatures involved and the vacuum environment often necessary to prevent oxidation mean specialist equipment is necessary to study creep behaviour of metals.

There have been countless publications on experimental procedures using Texture Analysers to give insight into the rheological modelling of viscoelastic materials. This helps to characterise them and predict their behaviour under specific physical conditions. These viscoelastic models contain different combinations of springs and dashpots, showing complex behaviour that can represent different types of materials. Dynamic modulus curves are plotted along with creep and relaxation data, giving a full representation of the material’s time-dependent behaviour. The Exponent software allows for a large amount of flexibility both during the test itself and in the post-test analysis.

There is a Texture Analysis test for virtually any physical property. Contact Stable Micro Systems today to learn more about our full range of solutions.

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For more information on how to measure texture, please visit the Texture Analysis Properties section on our website.

The TA.XTplus texture analyser is part of a family of texture analysis instruments and equipment from Stable Micro Systems. An extensive portfolio of specialist attachments is available to measure and analyse the textural properties of a huge range of food products. Our technical experts can also custom design instrument fixtures according to individual specifications.

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