Research On Fretting Fatigue Life Of Dovetail Joint Under HCF－LCF Load

Dovetail, bolt, flange and spline are widely adopted in aero-engine. These structures are easy to suffer from fretting fatigue damage under vibration environment and time-dependent loads of aero-engine. Fretting fatigue makes aero-engine owner loss a lot of money every year. Due to the complication of fretting fatigue, available methods of fretting analysis and fretting fatigue life prediction can not satisfy engineering requirements. Thus the systematical studies on the dovetail fretting fatigue experiments and fretting fatigue lifing methods have not only the important significance, but also a engineering application value.Theoretical methods combined with experimental methods are conducted in the thesis. Based on large amount of fretting fatigue test and the newest productions in the fretting fatigue, deep studies have been undertaken in the area of fretting fatigue life prediction method and the main factors influencing fretting fatigue. The hybrid fretting fatigue life prediction model for dovetail joint was conducted. The hybrid model was based on fretting synthesis parameter and fracture mechanics.Experiencing prediction model was improved, and some main factors were discussed. HCF (High cycle fatigue) / LCF (Low cycle fatigue) load, the simulated dovetail specimens, have been tested under four load cases respectively.Fractographic investigations of typical cracks have also been fulfilled. In the light of fracture mechanics, a strain test data processing method was proposed to ascertain crack initiation life of specimen. This method can solve the problem of determining crack initiation life.A experiencing fretting fatigue life prediction method based on friction energy and equivalent stress concept have been verified using test data gotten in this study. A refined experiencing fretting fatigue life prediction method based on the ratio of high cycle to low cycle friction energy and equivalent stress concept have been proposed, and it's applicability was proved by test data. A new fretting fatigue life prediction method combining Ruiz parameter and crack propagation mode, with clear physical meaning, was introduced. The life prediction error by the hybrid method is about 20%. The precision of this combined method was satisfied with engineering requirement.At last, some factors, including element's size,friction coefficient, the ratio of high cycles to low cycles, the strss ratio and geometry size of the dovetail joint and so on, are evaluated under different value by numerical study using 2D and 3d finite element method. Their effects on the fretting fatigue life prediction of dovetail joint were illustrated. It shows that friction coefficient and dovetail angle play fatal role in the procession of fretting fatigue. Efficient measure should be adopted in aero-engine design and the best angle of dovetail is near 45o.