Fretting Running State under Complex Alternating Loads

The present common-used fretting fatigue models simplify the actual loading conditions of engineering cases and ignore the effect of variable normal load. In this paper, the contact behavior of the fretting fatigue model subjected to variable normal load and axial load was investigated. The running state of fretting under alternating load was studied. A Q-P curv analysis method was proposed. The contact state under typical loading conditions was discussed, and the influence of different loading parameters on the fretting running state was revealed. It shows that the proposed Q-P curves analysis method can provide guidance for the study of the fretting running state under actual engineering working conditions. A two-dimensional (plane strain) model was established by using the finite element software ABAQUS to simulate the contact state of the fretting pad and the specimen under cyclic normal load and axial load. The model was divided into 3 main parts: The fretting pad, the specimen and the transverse spring. The normal load was applied to the upper edge of the fretting pad and the axial load was applied to the right edge of the specimen. The main purpose of this simulation was to study the effect of variable normal load and phase difference on the contact behavior, so a bulk axial load with stress ratio R= ‒1, σB,a=100 MPa and cyclic normal load Pm=225.2 N/mm and Pa =77 N/mm (which corresponds to maximum contact stress P0 from 140 MPa to 200 MPa). Three phase difference φ=0°, 45°, 90° and two spring stiffness kf =2 084 N/mm, 3 770 N/mm (corresponding to the Young’s modulus of the spring Es = 1 990 MPa, 3 600 MPa, respectively) were used to evaluate the effect of multi-axial loadingon the micro-action fatigue operating condition, with the reaction force at the left edge of the output transverse spring equal tothe tangential force. A series of fretting fatigue test corresponding to the simulation was performed on a biaxial fretting fatigue testing system The biaxial fretting fatigue testing system has two hydraulic actuators and three load cells, through which the loading of variable normal loads P, bulk load FB and the measurement of tangential forces Q could be implemented. A series of tests were performed using loading conditions (FB,a=10 kN; FB,m =0 kN; Pa=1.54 kN; Pm= ‒4.5 kN; φ=0°, 45°, 90°) and the loading loops with different phase differences were recorded to evaluate the rationality of the Q-P curve analysis method. For proportional loading conditions, the Ft-D curve is a straight line for a small pad fixture stiffness, the curve changes to a regular parallelogram for a larger stiffness. In these conditions, the Q-P curves are linear functions, and the slope of the curve increases as the increase of spring stiffness. For non-proportional loading conditions, the shape of the Ft-D curve is no longer a parallelogram and becomes very peculiar. The Q-P curves obey the ellipse function. The shape of the ellipse is determined by pad fixture stiffness, normal and bulk loading parameters, the phase difference etc. The proposed Q-P curves analysis method can provide a guidance for the study of the fretting fatigue running state, and also provides a method for further discussion of fretting fatigue or fretting wear behavior.