Time-dependent tension softening

A methodology that combines a rheological model for viscoelastic behaviour with a micromechanical model for tension softening through the effective spring concept is developed for materials exhibiting both tension softening and time-dependent behaviour. Spring elements with softening characteristics and dashpot with varying viscosity are used for this purpose. The characteristics of the spring elements and dashpot are obtained from a micromechanical model which relates the microstructure of the material to its tension softening response. The softening-type springs ensure gradual reduction in stiffness, whereas the dashpot with varying viscosity ensures that the retardation and relaxation times equal prescribed values. In this way, both the stress and crack opening are allowed to vary with time to reflect the real behaviour of a time-dependent tension softening material. The methodology is illustrated on the simple Poynting Thompson rheological model, without restricting its application to more sophisticated models.