Thermoplastic polyurethanes (TPU) are often subject to highly dynamic loading conditions in engineering applications. Due to their robust mechanical properties, TPU materials form an excellent fit for dynamically loaded system components in many cases. However, for dynamically loaded TPU the long-term material behavior is of special interest, since TPU shows distinct creep, as generally observed in polymers. This article illustrates a rather simple but efficient and consistent method for predicting the long-term material behavior of a selected TPU grade under uniaxial dynamic loading conditions. The research arises from practical challenges of design engineers. These are often confronted with lifetime quantification issues of critical components, e.g. in a mechanical damping element under cyclic loading conditions, for which a permissible deformation may not be exceeded. In those cases the transient stress-strain behavior of the material is of special interest. As will be shown an important prerequisite for the derivation of a reliable material model is the acquisition of relevant creep data for the respective TPU material. In a second step, the creep data is extrapolated in time by employing a suitable method resulting in a time-dependent stress relaxation modulus function. Parallel Maxwell models expressed by Prony parameters yield the rheological properties of this function. Due to their derivation, these Prony parameters represent quasi-static material response. Nevertheless, by employing a novel dynamic-static loading analogy the Prony parameters form the basis for TPU lifetime prediction under uniaxial dynamic loading conditions. By comparing numerical FE results for a damper with experimental results from an endurance test, the proposed modeling concept demonstrates its validity. 

Graphical Abstract

Prediction of Long-Term Behavior for Dynamically Loaded TPU