Research On Multi-axial Fatigue Life Prediction Method For Titanium Alloy

Many engineering components and structures in service such as aircraft engine blades, casings and main shafts are often under multiaxial stress states. Those structures always contain keyholes, lug bosses, grooves and welds etc. Multiaxial stresses often exist at notches even under uniaxial loads for the geometrical complexity. Some structures could be subjected to proportional, nonproportional loads or even random multiaxial loads. Therefore developing multiaxial theories and testing methods has very important significance in solving practical problems. Typical material of aircraft engine structure titanium alloy TC4 is chosen for this paper. Multiaxial tests and theories research of this material are conducted. The main contents of this paper include the following four aspects:Both uniaxial and multiaxial fatigue tests were conducted on titanium alloy TC4. The tests included quasi-static tensile、uniaxial tension/compression fatigue、pure torsion fatigue、constant amplitude multiaxial fatigue under proportional/nonproportional loadings, variable amplitude multiaxial fatigue, notch fatigue under costant/variable amplitude multiaxial fatigue. Test results show that cyclic softening can be observed under cyclic loading for titanium alloy TC4. Constant amplitude multiaxial fatigue tests were conducted on smooth thin-wall tubular specimens by displacement control mode. The results show that additional hardening is observed for titanium alloy TC4 under 45 o and 90 o out-of-phase loading conditions. The nonproportional loading paths cause more fatigue damage and decrease fatigue life in the region of short life. Multiaxial notch fatigue tests were conducted on two types of notch specimens by load control mode. The results show that fatigue damage under 45 o and 90 o out-of-phase loading conditions is smaller than 0o in-phase loading conditions and fatigue lives under nonproportional loading conditions are longer than proportional loading conditions.Constant amplitude multiaxial fatigue life prediction abilities for titanium alloy TC4 of Several commonly used multiaxial fatigue models were verified and evaluated. On this basis, a new mulatiaxial fatigue model was proposed based on the critical plane criteria. The model takes the maximum shear strain amplitude plain as the critical plane. The contribution of normal stress and strain on the critical plane to fatigue damage was taken into account in this model. The proposed model can be applied to multiaxial fatigue problems under proportional and nonproportional loadings and it can consider the effects of mean stress. The model was verified and evaluated by the constant amplitude multiaxial fatigue test data of titanium alloy TC4 and other five metal materials. The results show that the new multiaxial fatigue life model has high prediction accuracy.Aiming at the problems of fatigue life prediction under variable amplitude multiaxial loading, a new method for fatigue life prediction under variable amplitude multiaxial loading was proposed based on the proposed multiaxial fatigue model. The method was verified and evaluated by the variable amplitude multiaxial fatigue test data of titanium alloy TC4 and other three metal materials. The variable amplitude multiaxial fatigue life prediction results of these materials are all within a factor of two scatter band of the test results.The method to determine dangerous point for notched specimen under multiaxial loadings was proposed. The critical distance theory was used in multiaxial fatigue life prediction for notched specimen. The critical distance calculation methods for the point and line criteria used to correct fatigue damage gradient at notch root were proposed. The abilities of the point and line methods to correct fatigue damage gradient were verified by the multiaxial fatigue test data of TC4 notch spencimens. The results show that both methods have good and similar abilities to correct damage gradient.