Quasi-steady wind varies both in time and space and has a great influence on a wind turbine gearbox's fatigue life. Quasi-steady wind fields are described by mean wind speed and turbulence intensity, and the influence of these two parameters on wind turbine gearbox's fatigue life is studied. Based on the two-step decoupled method (the first step is global analysis and the second is gearbox dynamic analysis), a model which can calculate the complex gearbox's fatigue damage from gear's dynamic force and SN parameters is established. The fatigue damage of every gear calculated from the established model is then analysed. The fatigue damage result under certain wind loads is first analysed, and the vulnerable gears among each stage are found. Then, how mean wind speed and turbulence intensity influences fatigue damage is studied, and the damage comparison factors are calculated, which finds the 3 rd stage pinion to be the gear most sensitive to wind loads, and it requires more attention when designing gearboxes. Finally, the gears' fatigue damage sensitivities to mean wind speed and turbulence intensity are analysed and the condition in which fatigue damage is more sensitive to wind loads is found, this is meaningful when designing pitch control systems.

The dissipative contact force model plays a key role in predicting the response of multibody mechanical systems. Contact-impact event can frequently take place in multibody systems and the impact pair is often affected by supporting forces which are treated as external spring forces. However, the external spring forces are ignored during the derivation process of existing dissipative contact force models. Considering the influences of external spring forces, the fact is discussed that the crucial issues, including relative velocity and energy loss, in modeling dissipative contact force are different compared to the same issues analyzed in existing literatures. These differences can result in obvious errors in describing the collision response in multibody systems. Thus, a comparative study is carried out for examining the performances of several popular dissipative contact force models in multibody dynamics. For this comparison, a method associated with Newton's method is proposed to calculate the contact force that meets the Strong's law of energy loss and this force is used as reference. The comparative results show that the models suitable for both hard and soft contact exhibit good accuracy when contact equivalent stiffness is far larger than external spring stiffness by two orders of magnitude. Conversely, these models can cause varying degree and obvious errors in contact force, number of collisions, etc., especially when the difference in stiffness is close to or less than one order of magnitude.