Under the action of the wind, wind turbine gears experience complex loading histories in which the amplitude of the load is varying. The variable load conditions make the load sequence and the load amplitude inverse number as important factors to be considered for gear fatigue analysis. In the present work, a damage-coupled elastic-plastic constitutive model based on the continuous damage mechanics (CDM) is established to analyze the influence of load sequence on rolling contact fatigue (RCF) crack initiation of a wind turbine gear. The crack initiation is indicated by reaching the critical threshold of the defined damage variable related to the shear stress range and the plastic strain. During repeated loading cycles, the deterioration of material is reflected by gradually reducing mechanical properties. The numerical simulation is carried out in the frame of ABAQUS using the subroutine of user defined material subroutine (UMAT). The two-level load conditions are selected and results of which are compared with those of constant amplitude load conditions. Different inverse numbers are utilized to investigate the influence of load amplitude inverse number on crack initiation life. Simulation results are compared with existing damage accumulation criteria. The results indicate that when the first crack initiates, the sum of lifetime ratio is greater than unity for the low-high load sequence, while this value is less than unity for the high-low load sequence. Furthermore, the increase of the load amplitude inverse number will weaken the influence of the load sequence with regard to the crack initiation life. The highly nonlinear characteristic of damage accumulation during fatigue process under variable loading histories is captured by the proposed method. This method provides a new way to reveal the contact fatigue process of gears under variable loading histories and lay a foundation for predicting the accurate fatigue life under the real, complex loading condition of a gear.

In comparison to conventional EDM, micro EDM distinguishes itself through its brief discharge duration, narrow discharge channel radius, and concentrated energy density. However, there remains a paucity of comprehensive research on the surface formation characteristics in this domain. This paper delves into the formation mechanism of the recast layer in micro EDM workpieces, scrutinizing the primary factors that influence the formation process and the morphological attributes of the recast layer. We conducted a series of single-pulse experiments and micro EDM trials. Utilizing surface fitting tools, our experimental findings facilitated the derivation of a relational expression between the recast layer thickness of high-speed steel and the discharge parameters in micro EDM. Notably, when the energy is below 100 μJ, the recast layer thickness remains under 10 μm. Specifically, at an energy level of 16 μJ, opting for a smaller capacitance of 2200 pf and a higher voltage of 120 V in micro EDM results in a thinner recast layer. This study serves as a cornerstone for future efforts aimed at controlling and assessing the surface morphology of micro EDM.