Aiguo Zhao

Fatigue strength and crack initiation mechanism of very-high-cycle fatigue for low alloy steels

The fatigue strength and crack initiation mechanisms of very-high-cycle fatigue (VHCF) for two low alloy steels were investigated. Rotary bending tests at 52.5xa0Hz with hour-glass type specimens were carried out to obtain the fatigue propensity of the test steels, for which the failure occurred up to the VHCF regime of 108 cycles with the S-N curves of stepwise tendency. Fractography observations show that the crack initiation of VHCF is at subsurface inclusion with “fish-eye” pattern. The fish-eye is of equiaxed shape and tends to tangent the specimen surface. The size of the fish-eye becomes large with the increasing depth of related inclusion from the surface. The fish-eye crack grows faster outward to the specimen surface than inward. The values of the stress intensity factor (KI) at different regions of fracture surface were calculated, indicating that the KI value of fish-eye crack is close to the value of relevant fatigue threshold (ΔKth). A new parameter was proposed to interpret the competition mechanism of fatigue crack initiation at the specimen surface or at the subsurface. The simulation results indicate that large inclusion size, small grain size, and high strength of material will promote fatigue crack initiation at the specimen subsurface, which are in agreement with experimental observations.

Very high cycle fatigue for GCr15 steel with smooth and hole-defect specimens

Very high cycle fatigue (VHCF) properties of a low temperature tempering bearing steel GCr15 with smooth and hole-defect specimens are studied by employing a rotary bending test machine with frequency of 52.5 Hz. Both smooth and hole-defect specimens break in VHCF regime with some difference in fatigue crack initiation. For smooth specimens, a fine granular area (FGA) is observed near the grain boundary in the fracture surface of the specimens broken after 10^7 cycles. But no FGA is observed in the hole-defect specimens broken in VHCF regime, and the VHCF crack does not initiate from the small hole at the surface as it does at low or high cycle fatigue regime. Internal stress is employed to explain the VHCF behavior of these two types of specimens. At last, an advanced dislocation model based on Tanaka and Mura model is proposed to illustrate the internal stress process and to predict fatigue crack initiation life with FGA observed in the fracture region.

An energy-equilibrium model for complex stress effect on fatigue crack initiation

Based on Tanaka and Mura’s fatigue model and Griffith theory for fracture, an energy-equilibrium model was proposed to explain the complex stress effect on fatigue behavior. When the summation of the elastic strain energy release and the stored strain energy of accumulated dislocations reach the surface energy of a crack, the fatigue crack will initiate in materials. According to this model, for multiaxial stress condition, the orientation of the crack initiation and the initiation life can be deduced from the energy equilibrium equation. For the uniaxial fatigue loading with mean stress, the relation between the maximum stress or the minimum stress and the stress amplitude is in agreement with an ellipse equation on the constant life diagram. If the ratio of the mean stress to stress amplitude is less than a critical value −0.17, and the stress amplitude keeps constant, the fatigue crack initiation life will decrease with the increase of the compress mean stress. In this model, the mean stress does not cause damage accumulation with the fatigue cycles in crack initiation. For this reason, the loading sequence of different load levels would induce the cumulative damage to deviate from the Palmgren-Miner cumulative damage rule. The procedure of estimating the damage under random loading is also discussed.