Bending fatigue of high strength strip steel at 20 kHz and the influence of defects
Jens Bergström1, Mohamed Sadek1, Waqas Tofique2, Alexander Löf2, Chris Millward2
1Department of engineering and physics, Karlstad University
SE-651 88 Karlstad, Sweden
2voestalpine Precision Strip AB
684 92 Munkfors, Sweden
High strength strip steel is used in many applications as in engines and compressors. High precision in dimensional tolerances, good surface finish, high tensile strength and fatigue strength are required. One vital requirement is a high fatigue strength at long life lengths, and, hence, it is related to high cleanness of the steel microstructure with minimal internal or surface defects. One application is the use in flapper valves where high fatigue strength in combination with impact fatigue strength is demanded. A high static strength and adequate damping of valve material are also desired. One concern in characterization of the material is difficulties in obtaining the fatigue strength of the thin high strength strips, and in particular the bending fatigue strength. The present study has its focus on the development of a method to obtain the bending fatigue strength at 20 kHz fatigue load frequency, and to evaluate the influence of defect distribution on the fatigue strength. An ultrasonic 20 kHz test instrument working in resonance along the load train is used, and with a bending fixture and a specimen geometry particularly developed for this purpose. Reference testing in uniaxial fatigue of strip specimens is performed. Evaluation of initiating defect distributions and crack initiation mechanisms as fine granular area (FGA) assessments are made by SEM fractography. LOM and SEM-EBSD microstructure and mechanical characterization are performed. Final analysis of the fatigue strength and its dependence on defect distributions is performed using failure probability statistics.