VHCF variable Amplitude II

Component design under consideration of stresses with
variable amplitudes and very high numbers of cycles
FKM 2018
Issue number 331
Project no. 309
136 pages

Member price visible after registration
Final report

Abstract:
In the experimental part of this research project, a 50CrMo4 heat-treatable steel was investigated at three different strengths (Rm = 919 MPa; 1174 MPa and 1500 MPa). For all strengths, unnotched samples were used which were loaded with constant amplitudes. For the strength Rm = 1500 MPa, notched specimens were also tested, which
were loaded with constant and variable amplitudes. The testing machines used were a servo-hydraulic (approx. 200 Hz), an electromechanical (approx. 700 Hz) and an ultrasonic testing machine (approx. 19,000 Hz). All specimens were fatigued until fracture or N = 108 to 109 (depending on the test frequency).
The maximum likelihood method, the modified arcsin?p method and the staircase method were investigated to assess the reliability of the statistical evaluation of Wöhler tests. For design practice, the evaluation of Wöhler tests using the maximum likelihood method is recommended. Furthermore, non-linear approach functions were examined for their suitability to describe Gaßner lines and different variants
of the Palmgren-Miner rule were compared with each other on the basis of the fatigue strength tests carried out here. The formulation according to Haibach [Spi81] is recommended as a non-linear approach function for approximating Gaßner test points.
Based on the high-speed tensile tests and the cyclically stabilized stress-strain curves, a mathematical method was developed that takes into account the type of control and the strain rate of the testing machine used and corrects the stresses in the Wöhler diagram
accordingly. By applying the method, the scatter band can be significantly reduced and the findings of the fractographic fracture surface tests can thus be explained. No shift of the failure location from the surface to the interior of the material could be observed with increasing service life, which is consistent with the results of the developed method.