Fatigue strength of VDSiCr spring steel under cyclic torsion and cyclic axial loading at different load ratios in the VHCF regime

Abstract

Ultrasonic fatigue tests are performed with VDSiCr spring steel under cyclic torsion loading. Based on earlier investigations [1, 2] a test-setup was developed for applying static mean torque, to allow torsion testing at load ratios R = 0.1, 0.35 and 0.5, respectively. VDSiCr spring steel is used for coil springs, which are cycled at positive load ratios in the very high cycle fatigue (VHCF) regime during service. The material is patented and oil quenched. Specimens are machined from wires of 8 mm diameter with a circular reduction of cross section in the centre to a diameter of 4 mm. Shot peening of the surface produces conditions similar to the practical application with surface compression stresses of about 700 MPa. Ultrasonic cyclic torsion tests are compared to ultrasonic cyclic axial loading tests at load ratios R = −1, 0.1 and 0.5. Specimens must vibrate in resonance at ultrasonic frequency in ultrasonic fatigue testing. Due to the different wavelengths of axial and torsional waves, specimen dimensions must be different. The inner sections of both specimens, where fatigue cracks are initiated, however have the same shape. Due to the stress gradient and the shorter wavelength of shear waves compared with longitudinal waves, the stressed volumes (i.e. stress amplitude greater 95% of nominal stress) are 36.0 mm3 in cyclic axial tests and solely 0.83 mm3 in cyclic torsion tests. Ultrasonic equipment used to control both tests is similar. However, different vibration gauges and ultrasonic converters are used. Cyclic load is applied in a pulse-pause-sequence to avoid specimen heating. Superimposed static torques (in cyclic torsion tests) or forces (in cyclic axial tests) are introduced at vibration nodes in both test setups.