Study On Fatigue Fracture Behavior Of Titanium Alloy Under Single Tensile Overload

Titanium alloy has the characteristics of high specific strength,wide operating temperature range and excellent corrosion resistance,and is widely used in aircraft engines.Such as the preferred materials for advanced engine compressor disks,compressor blades and fan blades.Titanium alloy parts mainly bear the fatigue load spectrum,and it is of great significance to grasp their fatigue cracking behavior under the action of variable amplitude load.At present,there are many researches on the crack growth characteristics of titanium alloys under constant amplitude load,but there are few researches on the fatigue crack growth characteristics under variable amplitude load.This thesis investigates the crack propagation characteristics of TC4-DT titanium alloy under a single amplitude overload condition during the constant amplitude process.In this thesis,the fatigue crack growth test of TC4-DT titanium alloy under constant amplitude load is carried out.The effect of the microstructure of the fractured specimen on the crack propagation life is investigated.The results show that the lamellar ? phase thickness and grain size are the key factors that affect the specimen's total fatigue life,In the microstructure,the thicker lamellar ? phase increases the strain absorption energy at the crack tip and can delay the crack propagation;Larger grain size makes the crack propagation rate lower.In order to further investigate the effect of applying a single tensile overload over the crack growth behavior in a constant amplitude cyclic load,Eight kinds of crack growth tests with different overload ratios are carried out,and the crack growth path,the fracture characteristics of the fracture before and after overload,the influence of the Vickers hardness near the fracture before and after overload,and the influence of a single tensile overload on the crack growth rate are observed and analyzed,.The size of the plastic zone is calculated based on the Irwin plastic zone model,and the obtained results show that:1)Crack propagation path:the crack originates from the cut,the crack is perpendicular to the loading direction,and the propagation is zigzag or jagged,During the expansion process,cracked branches and closures appeared.The propagation path of the main crack in the lamellar structure is through crystal growth,Some cracks propagate in the intragranular mode along the intragranular bundling direction,and some cracks propagate through the bundling direction.2)Effect on fracture characteristics:when the overload ratio is 1.5 and 2.0,the overload line cannot be observed on the fracture,When the overload ratio is 2.5,2.8,3.0,and 3.2,the overload line can be observed on the fracture,As the overload ratio increases,the overload line becomes more and more obvious;WhenR_OL=3.5,after1.45×10⁶cycles,no fatigue crack growth was observed.Observing the fracture characteristics before and after overload,it is found that the position near the overload line is mainly characterized by fatigue bands.After the overload line,the fast expansion feature is observed as a tough socket+cleavage,and then a dense fatigue band is observed.Then it was observed that the fracture is a mixed feature of fatigue band+cleavage+tough fossa.3)Effect on microhardness:Overload will cause the hardness to increase,making the Vickers hardness value at the location where the overload is applied higher than the Vickers hardness value on both sides.This is because a large tensile overload causes a large plastic deformation of the material,Hardens the material,resulting in an increase in hardness.4)Effect on crack growth rate:When the overload ratio R_OL=1.5,in the fitted curve,the crack length a increases with the cycle cycle N.Failure to clearly observe the effect of overload hysteresis;When the overload ratio R_OL=2.0,2.5,the crack length a after overload increases slowly with the cycle cycle N,After the number of hysteresis cycles N_D,the crack length increases rapidly with the cycle number,When the overload ratio R_OL=3.0,3.2,after the overload is applied,the crack length a keeps almost constant as the cycle cycle N increases,and it expands again after the crack stagnate.The crack length a increases rapidly with the cycle cycle N increase;After a certain number of cycles,the crack growth rate gradually increases from the minimum value,Until the horizontal position before overload is restored,and continue to follow the law of equal amplitude loading change.5)Effect on the plastic zone:A single tensile overload is applied to form a large monotonous plastic zone at the crack tip,the plastic zone is roughly Y-shaped,And as the overload ratio increases,the plastic zone increases.