Study On Failure Mechanism Of Bending Fretting Fatigue

Fretting fatigue is the process of material damage due to micrometer-scaled movements at the interface between contact bodies caused by cyclic fatigue loads and fretting fatigue may finally result in lower fatigue strength and earlier failure than plain bending fatigue. Fretting fatigue occures widely in various structure components of aviation, transportaion and marine engineering, such as in turbine engine, railway axle, overhead electrical conductors, bolts, and so on. Fretting fatigue is the main failure mode of these structure components, so in order to assess the safety and reliability of these components, the damage characteristics and failure mechanism must be revealed. Fretting fatigue can be devided into three types:the tension and compression fretting fatigue, bending fretting fatigue and torsion fretting fatigue, accoding to different types of fatigue loads. In recent years, the fretting fatigue has been widely studied and some typical achievements have been obtained about the experimental techniques, numerical analysis and mechanism discussion. Howerer, most of the exsited studies focused on the tension and compression fretting fatigue, the bending and torsion fretting fatigue behaviors were rarely discussed. LZ50steel (cyclic stabilizing material),316L stainless steel (cyclic hardening material), tempered42CrMo steel (cyclic softening material) and6061-T6aluminium alloy (cyclic softening material) were widely used in the structure parts, such as train axle, where they would be subjected to bending fretting fatigue in the service. Therefore, the experimental study and finite element analysis were carried out to reveal the failure characteristics of bending fretting fatigue in this paper for these four kinds of metals. The obtained results can provide solid fundamentals for the preventation of bending fretting fatigue failure.In order to reveal the failure characteristics of bending fretting fatigue about the four kinds of materials, the following studies have been carried out in this thesis:1. The bending fretting fatigue and plain bending fatigue experiments of tempered42CrMo steel and6061-T6aluminium alloy were carried out to analysis the difference between the bending and plain bending fatigue behaviors. The relation of fatigue lives and cyclic bending loads was studied by systematic bending fretting fatigue experiments; the damage evolution characteristics during the bending fretting were revealed by observing the morphology of fracture surfaces under different bending loads and the fretting damage of specimens after different numbers of cycles.2. A cyclic elasto-plastic constitutive model which can describe the material characteristic (i.e. cyclic hardening, cyclic stabilizing, cyclic sofetening) reasonably was implemented into ABAQUS finite element software as a UMAT user subroutine.3. Based on the experiments of bending fretting fatigue, a simplified two dimensional plane strain finite element model with an equivalent normal force transformation from a three dimensional finite element model was established.4. The simplified two dimensional finite element model with the UMAT subroutine of cyclic constitutive model was used to simulate the bending fretting process, and then the stress-strain responses in the contact zone were analyzed. The effects of cyclic bending load, friction coefficient and normal force on the bending fretting were discussed.5. The damage evolution characteristics and failure mechanisms of bending fretting fatigue for four materials were analyzed by combining the experimental and numerical analyses, which is helpful to prevent the bending fretting fatigue failure. Finally, the crack iniation locations and lives of bending fretting fatigue were predicted by using a suitable critical plane model and volume average method. The predictions were in agreement with the experimental results.