Con-rod Fretting Fatigue Life Study Based On Fracture Mechanics Theory

Connecting rods used to be repeatedly repaired or scrapped in advance due to fretting fatigue on toothed mating surfaces,so two life methods including one that based on the accumulated maximum principal stress(AMPS)criterion and another that based on fracture mechanics are proposed in this paper.Finally,connecting rod is taken as an example,damage analysis and statistics are performed.Combined with the three-dimensional quasi-static finite element analysis,the maximum principal stress(MPS)criterion is used to predict the damage state on the con-rod toothed mating surfaces under every load case and overall load cases.Then the calculation of the con-rod fretting life using the above two life calculation methods are performed.Some practical measures related to con-rod design and process,are discussed and suggested to mitigate fretting fatigue damage.Followings are the main research contents and results:1.Based on the Residual Strength theory and the MPS criterion,the AMPS criterion is proposed to consider the material damage accumulation under cyclic alternating loads.The time concept of the load block cycle is introduced so that it can be used to predict the surface damage degree,crack initiation position and angle and to calculate the life expectancy.2.Based on the Paris formula,a full-life calculation model for fatigue crack initiation to propagation under multi-axial variable-amplitude stress state is proposed.The method for deriving the important material parameters involved in the life-span model is described.Based on Matlab,a GUI-based computing interface is programmed,which is easy to operate and highly computationally efficient.3.Taking connecting rod as an example,the damage analysis and statistics among some failure connecting rods are carried out.The three-dimensional quasi-static finite element analysis is performed then.Based on the numerical analysis results,the AMPS criterion is used to predict the crack initiation behavior and the most dangerous node is judged.Life of the most dangerous node is calculated by using two types of life calculation methods respectively,the results of which are in good agreement with the damage statistics.4.Effective suggestions on changing process parameters,improving surface quality,applying surface engineering technology and improving connecting rod structure design including the teeth's as well as the bolts',are put forward to slow down the fretting fatigue damage on toothed mating surfaces and offer powerful theoretical directions to improve the safety and reliability of connecting rod while used in engineering practice.