Deformation Behavior of Solids

Usually, materials are divided into hard, medium-hard and soft or brittle and plastic or ductile materials. However, it makes more sense to consider elastic, plastic and viscous material properties. Then the three borderline cases elastic, elastic-plastic and elastic-viscous deformation behavior can be distinguished.

Elastic deformation behavior

Elastic deformation behavior means that no change in shape remains after the load is removed, i.e., with ideal elastic deformation behavior, the energy supplied is completely recovered. The deformation process is therefore reversible.

Theoretically, the elasticity is independent of the rate of expansion. The stress rate and temperature have no or negligible influence on the deformation behavior of elastic particles.

A material is described as brittle if its deformation is mainly elastic until it breaks.

Elastic-plastic deformation behavior

Essentially, the initial deformation of elastic-plastic materials is linear elastic. After reaching a yield criterion, they deform only plastically without further increase of stresses. The yield criterion is a function of temperature but is independent of the stress rate. 

As the temperature increases, yielding starts earlier. Below a certain temperature, a further reduction in temperature leads to embrittlement of the material, i.e., the yield criterion is suppressed. Elastic-plastic materials can also become brittle after extended flow processes (e.g., multiple stresses as can be realized in agitator bead mills). This effect is referred to as cold embrittlement.

Typical representatives of this group of materials are metals. Their comminution can be improved by lowering the temperature. More practical, however, is multiple stressing, which also leads to cold embrittlement; this can be realized, for example, in bead mills or, in the very fine particle range, in agitator bead mills.

Elastic-viscous deformation behavior

In the case of elastic-viscous material behavior, the viscous deformation component is largely dependent on the rate of expansion and temperature. Viscous solids can be recognized by the fact that the tension decreases with a fixed deformation or that the material deforms plastically even under constant load. Typical representatives of this class of materials are plastics.

The objective of a grinding processes can be very different. In some cases, even particle breakage should be completely avoided so as not to negatively affect the product properties. The focus of this consideration is on the particle shape or sphericity.

The target of the grinding or dispersing process can be defibrillation, i.e. detangling and separating fibers without shortening the original length of these fibers. Typical examples are e.g. CNTs (Carbon Nano Tubes) or cellulose fibers.

Delamination is when platelet-shaped particles are separated without changing their aspect ratio. The largest possible aspect ratio, i.e. the largest possible surface expansion with a very low thickness of the particles, can be of great importance, among other things, for fillers that are intended to achieve a barrier effect or for conductive coatings.

In special cases, however, the objective of a comminution process is also a targeted rounding, i.e. increasing the sphericity of the particles by breaking off or rubbing off superficial unevenness or roughnesses.

If, for example, such a process is then combined with a classification process, it is not only possible to change the particle shape in a targeted manner, but also to limit the upper particle size and thus to change the particle size distribution towards a monomodal product system. Furthermore, it is possible to dedust the product by means of a downstream classification and to limit the particle size distribution in its width.