An approach to the description of time-dependent materials

A model is proposed for non-linear time-dependent materials, often referred to as materials with memory or hereditary type materials. It is applied to various types of loading: monotonous loading, creep and relaxation. As an example, the polymer material POM (polyoximethylene) is chosen. This material possesses strongly pronounced non-linearity, which is clearly seen in the stress–strain diagrams. The material was tested at three different loading rates, at creep conditions with three levels of stress and at relaxation conditions with three levels of strain. It is shown that the constitutive equation representing the model describes these types of loading using only one set of parameters. Determination of parameters is based on short-time experiments with constant stress rate loading. Prediction of material behaviour at long times for creep and relaxation processes can be made. Unloading is not considered in the paper but may be treated as well.