Commercial Rotational Instruments

Abstract

Materials generally show a broad spectrum of behaviour, the rheological properties lying between the extremes of a truly elastic and an ideally viscous response. The rheological testing procedures involve measuring the response of a material to an applied deformation but, given that ideality in response is rarely encountered, a straightforward rheological test is not usually sufficient for a complete material characterisation. Also, materials often show considerable time dependent changes as well as markedly non—linear responses to an applied stress or strain, so that the rheological tests need to be chosen carefully. The deformation, or strain, can be applied in a variety of ways but the aim of any study, in establishing a relationship between material properties and the stress—strain—time conditions, means that only relatively simple and well characterisable shear conditions are used. Understandably, the most usual approach is to shear the material between two surfaces. Now, since it is only the relative movement within the material that is important, the approach used does not require that any one surface be stationary. However, from a practical point of view, a simple arrangement is to move the surfaces in contact with the material rather than move the material itself.