Investigation of the Damage Behavior of Polyurethane in Stress Relaxation Experiments and Estimation of the Stress-at-Break σ b with a Failure Envelope

In stress relaxation experiments the investigated polyurethane exhibits an unexpected, but repeatable failure during the relaxation period. Images, taken by a camera and a high speed camera, displayed crack initiation several minutes before rupture occurred. The crack growth rate then accelerates and leads to failure rapidly. The present investigation indicates different methods for analyzing the damage evolution with focus on the appearance of damage at low strains, accumulation of damage processes at higher strains, the influence of time on damage evolution and the identification of recovery phenomena during unloading and in the unloaded state. The results show that not only long times at high strains but also the loading history, especially loading and unloading processes, accelerate damage evolution. In a preliminary study the characterization of the ultimate tensile properties by a failure envelope appears to provide an adequate method for estimating the stress-at-break σb and the time-to-break tb in stress relaxation experiments.