Research On Mulitiaxial Fatigue Properties And Life Prediction For Notched Spcimen Of Titanium Alloy

Engineering components and complex structures usually contain stressconcentration or notches. The stresses and strains near the notch are three-dimensionalstate when parts are subjected to the alternating loading. The stresses and strains nearthe notch are usually lager than those of the smooth, and this will easily result in crackinitiation. Therefore, how to get local stress-strain of the notch and the accuratelocation of crack, it is significant for preventing fatigue failure at notch.In this paper, multi-axial fatigue tests were conducted on smooth thin-walledspecimens and notched thin-walled specimens made of TC21alloy. Fatigue charactersof TC21Titanium alloy, about cyclic softening/hardening and multi-axial fatigue,were analyzed and studied. Fatigue life of smooth thin-walled specimens waspredicted by the shear damage model and Fatemi-Socie method. Stresses and strainsat notch root were calculated using the elastic-plastic finite element analyses. In thisstudy, a method was proposed to identify dangerous point location of this type ofnotched specimen, and stresses and strains of the identified dangerous point locationwere used to estimate fatigue life of notched specimen. The present paper mainlyconcentrated on the following aspects:Firstly, uniaxial and multi-axial fatigue tests were implemented on TC21smooththin-walled specimens and the stress state of uniaxial/multi-axial load were analyzed.Under uniaxial tension and pure torsion loading, characters of cyclic softening wereshown for smooth thin-walled specimens, which was more evident with equivalentstrain amplitude increase. Under proportional and nonproportional multiaxial loading,cyclic softening appeared in both components of tension and torsion; degree of cyclicsoftening dependent on equivalent strain amplitude. Under variable amplitude loadingand block loading, cyclic softening appeared in torsion component, while was not thesame in the tension component. Additional hardening was evident undernon-proportional variable amplitude loading. Under proportional and nonproportionalblock loading, the effect of different load sequences on fatigue life was not identical.When non-proportional load was before the proportional loading, material was cyclicsoftening in early stage, and then cyclic hardening. When proportional loading wasbefore non-proportional loading, material was cyclic hardening under uniaxial loadingand45°non-proportional loading, cyclic softening under90°proportional loading. Secondly, fatigue life of smooth thin-walled specimen was predicted usingvon-Mises equivalent strain method, maximum shear strain method, shear damagemodel and Fatemi-Socie model. The results showed that von-Mises equivalent strainmethod and maximum shear strain were not satisfied. And results of the shear damagemodel and Fatemi-Socie method showed a good agreement with expetiment life.Therefore, the shear damage model and Fatemi-Socie method could be used to predictfatigue life of notched specimens.Thirdly, stress-strain analysis of notch root had been done by plastic-elastic finiteelement method. Node data were extracted from the1/2circumference of inner andouter surface at notch root to calculate the damage under proportional andnon-proportional loading. From the calculation results, it was found that the locationwith the most damage was always on the inner face of notch. The von Mises stressand strain of the most dangerous point were larger than other points.At last, in this paper, fatigue life of notched thin-walled specimen was estimatedwith the shear damage model and Fatemi-Socie method. The results showed the sheardamage model had a good prediction and the prediction of Fatemi-Socie method wasconservative.