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Fatigue tests of a high carbon chromium steel were performed using rotating bending and ultrasonic axial cycling. Fatigue crack initiated at specimen interior for very-high-cycle fatigue (VHCF) with fish-eye pattern embracing fine-granular-area (FGA) originated from inclusion. The fatigue life from FGA to fish-eye and from fish-eye to the critical crack size was respectively calculated, so as to estimate the fatigue life contributed by FGA. The crack extension rate within FGA was also estimated. Our results demonstrated that the formation of FGA is responsible for a majority part of total fatigue life.

The problem of impact-entrainment relationship is one of the central issues in understanding saltation, a primary aeolian transport mode. By using particle dynamic analyser measurement technology the movement of saltating particles at the very near-surface level (1 mm above the bed) was detected. The impacting and entrained particles in the same impact-entrainment process were identified and the speeds, angle with respect to the horizontal, and energy of the impacting and entrained sand cloud were analysed. It was revealed that both the speed and angle of impacting and entrained particles vary widely. The probability distribution of the speed of impacting and entrained particles in the saltating cloud is best described by a Weibull distribution function. The mean impact speed is generally greater than the mean lift-off speed except for the 0Ð1-0Ð2 mm sand whose entrainment is significantly influenced by air drag. Both the impact and lift-off angles range from 0°to 180°. The mean lift-off angles range from 39°to 94°while the mean impact angles range from 40°to 78°, much greater than those previously reported. The greater mean lift-off and especially the mean impact angles are attributed to mid-air collisions at the very low height, which are difficult to detect by conventional high-speed photography and are generally ignored in the existing theoretical simulation models. The proportion of backward-impacting particles also evidences the mid-air collisions. The impact energy is generally greater than the entrainment energy except for the 0Ð1-0Ð2 mm sand. There exists a reasonably good correlation of the mean speed, angle and energy between the impacting and entrained cloud in the impact-entrainment process. The results presented in this paper deserve to be considered in modelling saltation.

Rotating bending (52.5 Hz) and ultrasonic (20 kHz) fatigue tests were performed on the specimens of a bearing steel, which were quenched and tempered at 150°C, 300°C, 450°C and 600°C, respectively, to investigate the influence of strength level and loading frequency on the fatigue behavior in veryhigh-cycle regime. Influences on fatigue resistance of materials, characteristics of S-N curves and transition of crack initiation site were discussed. The specimens with higher strength showed interior fracture mode in very-high-cycle regime and with slight frequency effect, otherwise cracks all initiate from the surface and the fatigue strength was much higher under ultrasonic cycling.

The velocity of a wind-blown sand cloud is important for studying its kinetic energy, related erosion, and control measures. PDA (particle dynamics analyser) measurement technology is used in a wind tunnel to study the probability distribution of particle velocity, variations with height of the mean velocity and particle turbulence in a sand cloud blowing over a sandy surface. The results suggest that the probability distribution of the particle velocity in a blowing sand cloud is stochastic. The probability distribution of the downwind velocity complies with a Gaussian function, while that of the vertical velocity is greatly complicated by grain impact with the bed and particle-particle collisions in the air. The probability distribution of the vertical velocity of fine particles (0·1-0·3 mm sands) can be expressed as a Lorentzian function while that of coarse particles (0·3-0·6 mm sands) cannot be expressed by a simple distribution function. The mean downwind velocity is generally one or two orders greater than the mean vertical velocity, but the particle turbulence in the vertical direction is at least two orders greater than that in the downwind direction. In general, the mean downwind velocity increases with height and free-stream wind velocity, but decreases with grain size. The variation with height of the mean downwind velocity can be expressed by a power function. The particle turbulence of a blowing sand cloud in the downwind direction decreases with height. The variations with height of the mean velocity and particle turbulence in the vertical direction are very complex. It can be concluded that the velocity of a sand cloud blowing over a sandy surface is mainly influenced by wind velocity, grain impact with the bed and particle-particle collisions in the air. Wind velocity is the primary factor influencing the downwind velocity of a blowing sand cloud, while the grain impact with the bed and particle-particle collisions in the air are the primary factors responsible for the vertical velocity.

This paper studies the formation mechanism of fine granular area (FGA) in high-strength steels and predicts the threshold value of its formation. Fatigue experiments are carried out by using rotating bending and ultrasonic fatigue testing machines on a high carbon chromium steel (GCr15) with three different heat treatments. The results show that the range of stress intensity factor at the periphery of FGA (K FGA) keeps constant with an average value of 5.2 MPa m 1/2 , which is close to the traditional crack growth threshold (K th,0), 5.0 MPa m 1/2. A theoretical model based on the plastic zone at crack tip is proposed to predict the value of K FGA and the predictions are in good agreement with experimental data.

The fatigue strength and crack initiation mechanisms of very-high-cycle fatigue (VHCF) for two low alloy steels were investigated. Rotary bending tests at 52.5 Hz 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 10 8 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 (K I ) at different regions of fracture surface were calculated, indicating that the K I value of fish-eye crack is close to the value of relevant fatigue threshold (DK th ). 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.

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