Fatigue behaviour of Polarit 725 stainless steel

Fatigue tests on a stainless steel, Polarit 725, manufactured by Outokumpu Polarit have been performed. These tests are intended to provide some needed constant amplitude fatigue data and give a starting point for ongoing research work using variable amplitude loading. The Polarit 725 (AISI 304) was tested in two conditions. The first was hot rolled and the second was cold rolled to achieve 21.1% cold work. Fatigue tests included strain life, crack growth rate measurement in the Paris regime and crack growth rate measurement in the near threshold regime. Other tests included through the thickness hardness measurements and static tensile tests. High frequency stress controlled endurance limit testing was not possible for this material due to the significant amount of plastic deformation induced heating. The hot rolled material was very resistant to crack initiation during high cycle strain life testing. The material also showed progressive cyclic softening during the first 20% of life followed by a significant amount of hardening. The crack growth rate parameters and crack growth threshold parameters were comparable to more conventional steels. Crack growth rate specimens manufactured from the hot rolled material developed very straight crack fronts during testing while the specimens manufactured from the cold rolled material had very severe crack front angles that did not meet the requirements given in the testing standards. The crack front angle was possibly a result of internal residual stresses but this hypothesis has not been confirmed.