Zhi Yong Huang

Relation between the Mechanical Behaviour of a High Strength Steel and the Microstructure in Gigacycle Fatigue

Many components can reach or exceed 109 cycles in their service time. When fatigue life is beyond 106, the Wöhler S-N curve was always considered to be asymptotic in horizontal axis, but the fatigue behaviour over 106 cycles can not be neglected. It is not usual to carry out a fatigue test beyond 109 cycles due to the conventional fatigue test’s constraints, time consuming and expensive. High strength steel is widely applied in automobile, railway industry after surface treatment in order to improve performance of material in practice. Carburizing process hardens surface to increase wear and fatigue resistance and shot peening has a beneficial effect on the material fatigue strength from the surface residual compressive stresses. A piezoelectric gigacycle fatigue machine is used to do the tests in gigacycle regime on specimens with different surface treatments. The effect of different surface treatments is investigated in gigacycle regime at a frequency of 20KHz with a fixed stress ratio R=0.1 at room temperature. Moreover, Scanning Electron Microscopy (SEM) observations of fracture surfaces are analyzed to evaluate the mechanism of damage related to surface treatments, microstructure scored inclusion size. The role of inclusions and microstructure is emphasized at 109 cycles.

Dislocations Gliding Study by IR Thermography in C-Mn Steels with Different Solute Atoms Content in the Gigacycle Fatigue Domain

Tests were performed on two Carbon-Manganese steels (A42 and A48 steels, French standard) in the gigacycle fatigue domain thanks to a piezoelectric fatigue machine working at 20000Hz. During the tests, temperature recordings were achieved by an infrared camera for various stress amplitudes. The main difference between the two steels compositions was the aluminum content (0.045% for the A42 steel and 0.004% for the A48 steel), and the carbon content (0.140% for the A 42 steel and 0.198% for the A48 steel). In the A48 steel, the few aluminum content induces a higher free content of solute nitrogen in the lattice. Mechanical spectroscopy tests were performed and gave qualitative results on the solute contents repartition in the lattice. The temperature increase recorded during the fatigue tests for the two steels are different at the beginning of the tests. The differences can be explained by the different repartition of the solute atoms which induces a different dislocation gliding between the two materials. At the end of the tests, the thermal recordings are similar and attributed to the evolution of the solute atoms repartition and the dislocation structure.

Effect of low cycle fatigue pre-damage on very high cycle fatigue

Carbon-manganese steel is often applied in components of pipes in nuclear plant. Ultrasonic fatigue tests following low cycle fatigue (LCF) cycles damaged are used to study the strength of very high cycle fatigure (VHCF). The comparison of test results of simple VHCF and cumulative fatigue (LCF plus VHCF) shows that LCF load influences the following VHCF strength. Continuum damage mechanics model is extended to VHCF region. The effect of LCF load on VHCF is studied by an improved cumulative damage model.

High-Cycle Fatigue and Thermal Dissipation Investigations for Low Carbon Steel Q345

Carbon steel is a kind of metallic material that widely used in construction, machinery, manufacturing and other domains. In the mechanical structure system, long-term cyclic stress may cause the mechanical components failure. In this work, the characteristic of fatigue crack propagate in low carbon steel Q345 and the effect of loading frequency to the fatigue property of Q345 steel were investigated. Meanwhile, the dispersion of high-cycle fatigue of life of the Q345 steel under high fatigue testing frequency was analyzed, and the P-S-N curve with the test data was given out. With the help of infrared camera, temperature rise curve during fatigue test was analyzed to study the thermal dissipation of Q345 steel.

Stress Ratio and Notch Effects on the Very High Cycle Fatigue Properties of a Near-Alpha Titanium Alloy

Ultrasonic fatigue tests up to 1010 cycles were performed on a turbine engine titanium alloy (Ti-8Al-1Mo-1V) at the stress ratio (R) of −1 with smooth specimens and at R = −1, 0.1 and 0.5 with notched specimens. As a result, with increase of fatigue life, the source of reduced fatigue life caused by multi-point surface crack initiation changes from crack propagation stage to crack initiation stage in the high cycle fatigue regime. Notch effect further promotes the degeneration of high cycle and very high cycle fatigue strength at R > −1. The bilinear model, extended from the Goodman method, can better estimate the mean stress sensitivity of this titanium alloy. The fatigue mean stress sensitivity and fatigue-creep mean stress sensitivity of this material increased with the increase of fatigue life. The new model, based on the Murakami model, can provide more appropriate predictions for notch fatigue strength.

Micromechanical Modeling of Elastic-Viscoplastic Behavior of Armco-Fe at High Strain Rate

Heterogeneous dynamical stress-strain response of Armco-Fe was investigated at high strain rates through the Split Hopkinson Pressure Bar (SHPB) testing. It was found that the viscoplastic deformation in BCC ferrite grains is affected by the strain rate. Thermal softening and variation in crystal orientations under high-strain-rate loading were used in the elastic-viscoplastic modeling. The micromechanical analysis with self-consistent transition and homogenization was used for estimation of the global impact response of the material. The results from modeling were found in good agreement with the experimental data.