Investigation On The Damage Tolerance Property And The High Cycle Fatigue Strength Of TC18 Titanium Alloy With Basket-weave Microstructure

The damage tolerance design concept is an important criterion to achieve the lowcost and long-life flight of the modern planes.With the increasing request of the safety and economy of planes,superior damage tolerance property and high cycle fatigue strength of the airplane components made by titanium alloys are needed.Titanium alloys with basket-weave microstructure are widely used in manufacturing critical airplane components due to the high fracture resistance.Thus understanding the influencing factors and rules of the damage tolerance property and high cycle fatigue strength for a given titanium alloy with different basket-weave microstruture features are very meaningful to broaden its application in aerospace field.However,due to the complexity of the influencing factors of damage tolerance property and high cycle fatigue strength,also due to the diversity of the basket-weave features of titanium alloy,finding the correct basket-weave features-damage tolerance property and the basket-weave features-high cycle fatigue strength interrelations of titanium alloy is the key problem need to be solved.In this dissertation,taking TC18 titanium alloy for example,its damage tolerance property and the high cycle fatigue behavior have been systematically and deeply studied by using the finite element simulation method combined with theoretical analysis and experimental study.The main results are as follows:Based on the analyses of microstructure,fracture surface and crack propagation path,the influence of basket-weave features on the fracture toughness and fracture mechanism of TC18 alloy has been studied.The results show that long and thick a platelets(with the length of above 15?m and thickness of about 1.5-2?m)can effectively deflect the main crack,which lead to rough fracture surface and tortuous crack propagation path,and finally increase the fracture resistance.Therefore,long and thick a platelets in microstructure is the necessary condition to achieve high fracture toughness of TC18 alloy.By considering the contributions of intrinsic fracture resistance(crack tip plastic work)and the extrinsic fracture resistance(the crack front tortuosity)to fracture toughness,the fracture toughness prediction model of TC18 alloy with basket-weave microstructure has been proposed and established.The prediction error of this model is within 10%,which shows good performance.By analyzing the prediction model,one conclusion can be drawn that contribution of the intrinsic fracture resistance to the fracture toughness of TC18 alloy is the major part,which occupies about 80%-95%.The contribution of extrinsic fracture resistance to the fracture toughness of TC18 alloy,however,is the minor part,which occupies about 5%-20%.Based on the analyses of microstructure,fracture surface and crack propagation path,the influence of surface oxidation on the fracture toughness of TC18 alloy has been studied.The results show that the fracture toughness of TC18 alloy generally shows no change when oxidized below 600?.However,the fracture toughness of TC18 alloy sharply decreases when oxidized above 700?.The brittle surface oxide layer and oxygen diffusion zone are the exact reasons causing this phenomenon.They can decrease the total area of shear lips on the fracture surface,decrease the crack propagation tortuosity and limit the formation of secondary cracks on the side surface of fracture toughness specimen.This can result in a big decrease of fracture resistance.The influence of basket-weave features on the fatigue crack propagation rates and fatigue crack propagation mechanism has been investigated.The fatigue crack propagation rates for TC18 alloy with five basket-weave features vary obviously at low?K levels and then tend to be similar with increasing ?X level.It can be explained in this way:at low ?K level,the crack tip plastic zone is small,cleavage fracture mechanism dominates the fatigue crack propagation process.In this situation,the crack deflecting effect caused by microstructure can greatly affect the fatigue crack propagation rates of TC18 alloy.With increasing ?K level,the crack tip plastic zone continuously increases and the fatigue striation mechanism becomes the main fracture mechanism.In this situation,the fatigue crack propagation resistance caused by crack deflection doesn't vary a lot for the five basket-weave features.Therefore,the relatively small variance of their fatigue crack propagation rates can be explained.Finally,the general Paris model based on yield strength and real fracture strain has been established.This model can fairly describe the fatigue crack propagation rates of TC18 titanium alloy in Paris zone.The high cycle fatigue crack initiation mechanism of TC18 alloy with different basket-weave features has been studied.The influence of crack initiation mechanism on the fatigue strength of TC18 alloy has also been investigated.The results show that high yield strength and high subsurface-crack initiation ratio in high cycle fatigue regime can both exert positive influence on its fatigue strength.The subsurface-crack initiation ratio of TC18 alloy is determined by the microstructure features.Coarse a phase is the weak phase for TC18 alloy with basket-weave microstructure,which is the prior fatigue crack initiation place.If the weak phase locates at the surface of fatigue specimen,then fatigue crack can most likely initiate at the specimen surface.However,if the weak phase locates at the subsurface of fatigue specimen,there is a big chance of the subsurface-crack initiation behavior.Generally speaking,microstructures with high transformed ? strength and limited volume fraction of weak phase can generally get high subsurface-crack initiation ratio.Based on Abaqus,taking the critical principal stress ?max(Maxps)as the damage onset criterion and the critical energy release rate Gc as the failure criterion,the crack tip damage process of the fracture toughness specimen has been simulated using XFEM method.Meanwhile,based on the seam crack method,the crack tip stress intensity factor at the failure point can be outputted,and the fracture toughness of TC18 alloy can be predicted finally.