The Study On Fretting Fatigue Damage Mechanism Of Fastening Surface Under Characteristic Loading Of IC Engine

The internal-combustion engine is a complex mechanical system which consists of thousands of parts. All these parts are assembled using bolted joints or interference fit. As the difference of the material and machining process of the contact interface, the contact behavior of the parts is different. The contact interface suffers pretension load and cyclic loading when the engine works. Fretting fatigue is a common failure around the contact interfaces under such loading conditions, making the survice life of the IC engine is much shorter. With the development of the IC engine, fretting fatigue becomes a major failure problem which influences the survice life of the IC engine. However, the fretting fatigue mechanism of the IC engine is not fully investigated at present. It due to the special character of this phenomenon: for the complicated connecting relationship of the parts as well as the effect of the cyclic loading, the contact interfaces of the IC engine suffer cyclic contact load and tangential load, while the published research of fretting fatigue just consider the effect of the cyclic tangential load.To inveatigete the fretting fatigue phenomenon of the IC engine, this study focus on this problem though mechanical analysis, numerical calculation and experimential inveatigation. Though this study, the fretting fatige mechanism of the IC engine was promulgated; a fretting fatigue damage prediction model was proposed; the main effect factors which influence the fretting fatigue of the IC engine was pointed out. The main works of this paper was:(1) A fretting related damage(FRD) parameter was proposed by analyzing the fretting fatigue test data and the mechanism of fretting fatigue. This parameter is succinct and has specific physical meaing. This parameter can reflect the relationship between fretting factors and the fretting fatigue damage.(2) A fretting fatigue prediction methodology was proposed besed on critical plane method. Fretting fatigue damage prediction model was proposed by modified plain fatigue model using FRD parameter. This methodology can fully use of the mature reseaech result of the plain fatigue and relect the relationship between the plain fatigue and fretting fatigue as well. Meanwhile, this methodology can predict the location and the propagating orientation of the fatigue crack exactly.(3) A experimential study was carried out to investigate the fretting fatigue behavior of the bearing cap bolted jonint on the V type IC engine. Meanwhile, an experimential process was proposed for structural model test. The test result shows that the fatigue crack initiate at the middle area of the contact interface and has a angle of 17 to 33 with the contact interface, then the creak orientated itself perpendicular to the contact interface. This phenomenon is the same as the failure of the V type engine.(4) Finite element method was used to analyze the stress distribution on the contact interface of the Vtype engine and structural model. The analysis result shows that the stress distribution changed a lot between pretension condition and exploding condition. The contact interface at the same side of the exploding cylinder has a tendency of out of contact. On the contrary, the contact interface at the opposite side of the exploding cylinder has a tendency of cut-over.(5) Over consideration of the fretting fatigue test and the finite element analysis result, the fretting fatigue damage mechanism of the V type engine was proposed. Meanwhile, it can be concluded that the main factors which directly influence the fretting fatigue behavior of the V type engine are the bolt pretension load and the additional rotate torque caused by the bearing load. This study suggested that appropriate increasing of the bolt pretension load and using extended skirt block with cross-bolted main bearings design are beneficial to the fretting fatigue lives.