Stress-Dependent Permeability of Porous Polyimide Under Stress Loading and Unloading.

Porous polyimide (PPI) is a key material for the oil supply parts in the bearing units of the momentum wheels and reaction wheels. The permeability of PPI determines the oil supply rate in the bearing contact area, thus affecting the performance of the bearing units and wheels. However, few studies have focused on the effect of alternating stress on the permeability of PPI in running bearing units. Here, a study on the stress-dependent permeability of PPI under stress loading and unloading is conducted. After experimental and theoretical analyses, permeability is reduced at most by 37% when the effective stress σeff is between 0.75 and 26.45 MPa. The elastic-structural deformation of capillary cross-section and tortuosity sensitivity lead to a negative correlation between σeff and permeability, and permeability hysteresis. When σeff is greater than 15 MPa and the pressure gradient is greater than 40 MPa/m, the permeability decreases and then increases during stress unloading because of the blocking and unblocking of detached particles. A fitting formula for the tortuosity variation caused by detached particles is proposed to describe this permeability behavior. This study contributes to the application and improvement of PPI in bearing units and similar working conditions.