On the creep, recovery, relaxation and elastic "memory" of some renneted milk gels

Abstract

Small pressures are applied to one end of a column of renneted milk gel (curd) in a U-tube and the displacements at the other end are magnified a hundredfold. As a first approximation, the curd behaves as a body, except that the creep curve shows a much greater rapid strain than does the recovery curve. Relaxation at constant strain follows a power relation between stress and time with an exponent of 0.40. The time taken for the stress to halve itself is proportional to the initial (rapid) rate of straining. Under selected conditions, creep and relaxation curves during ageing approximate to Boltzmann's Superposition Principle. Hysteresis curves, obtained by repeated loading and unloading, show marked stiffening, followed by a (Bauschinger) softening when the direction of loading is reversed. Following suitable stressing in alternate directions, curd shows elastic memory, i.e. the recovery curves change direction. Such behaviour necessitates additions to the Burgers model. A modification of Boltzmann's Superposition equation is discussed and Graham's equation, which includes an Andrade term, has been applied to some of the data. The relative significance of the analytical and integrative approaches to these problems is considered.