Study On The Effects Of Extrusion Process On The Fretting Fatigue Life Of Rotor Torsion Arm Hole

As one of the key parts in helicopter rotor system,the torsion arm component is subjected to complex high-frequency vibration and alternating load during flight,and its connection holes connection are prone to fatigue damage.In order to improve the anti-fatigue performance of the torsion arm hole,engineers in enterprises usually use eold extrusion process to strengthen the hole.However,what kind of extrusion process parameters can obtain the optimization fatigue life of the torsion arm hole?which is always a problem to be solved for the enterprise.Therefore,this paper mainly includs the following aspects:(1)The mechanical characteristics of torsion arm hole are analyzed by means of a three-dimensional nonlinear contact algrithm in the finite element model.Based on the the Smith-Watson-Topper(SWT)and Fatemi-Sociie(FS)damage criteria in multiaxial fatigue theory and the fatigue test,the crack initiation position and fatigue life of the torsion arm hole under the alternating load are predicted.The accuracy of the prediction of these two damage criteria is compared.The results show that the SWT criterion is more accurate than the FS criterion.It is also found by microscope observation that the fatigue failure of the torsion arm hole belongs to Type Ⅰ damage,and that there are multiple crack sources on the surface of the torsion arm hole and they extend along the radial hole at the same time and expand.(2)Secondly,the hole extrusion reinforcement analysis of the torsion arm was carried out to study the distribution of the residual compressive stress of the hole surface and the changing trend of the residual compressive stress of the hole surface under different extrusion process.The results show that the residual pressure stress distribution of the hole surface is extremely uneven after extrusion,the residual compressive stress near the extruding exit is relatively large,followed by the position near the extruding entrance,and the middle of the hole is relatively small.The residual compressive stress increases first and then decreases with the distance from the hole edge getting farther and farther,and finally becomes stable.With the increase of extrusion amount,the maximum residual compressive stress of the hole surface gradually increased,and it basically stabilized when the extrusion amount exceeded 0.5 mm.At the lower level of friction coefficient,the change of friction coefficient has little effect on the residual compressive stress of torsion arm.However,the radial residual compressive stress at the extruding entrance and the middle of the hole is sensitive to the change of friction coefficient,while the residual compressive stress at the extruding exit is not sensitive.(3)Finally,the fatigue life of the torsion arm hole under the hole extrusion process is predicted to determine the optimal extrusion process.The relationship between residual compressive stress and fatigue life is also analyzed.The results show that the fatigue life of the torsion arm hole appears to increase with the increase of extrusion,and finally it tends to be stable.The position of the torsion arm hole breakpoint as the extrution exit,the life improvement effect is best.In this case,the fatigue life is best when the extrusion amount is 0.3 mm.Residual compressive stress and fatigue life are positively correlated.The reason why it ean increase fatigue life is that it ean not only reduce the eyclic strain,but also reduce the average cyclic stress.