Toughening Method Of The Titanium Alloy Laminates Through Presetting Unbonded Areas In Laminate Interfaces

Titanium alloy is widely applied to aeronautic structures because of its high specific strength,high corrosion resistance and high temperature resistance.However,its ability to resist crack growth is relatively poor.When a fatigue crack initiates in the titanium alloy structure,the crack growth life of the structure is short.Therefore,the toughening method of titanium alloy structure has been the focused research.A common and useful method of titanium alloy structure toughening is to transform it into laminates through diffusion-bonding technology.Usually the flaws inside the structure may cause stress concentration,which may lead to worse bearing capacity.However,the researchers also show that the flaws can change the stress distribution,block the fatigue crack growth and toughen the structure.Based on this idea,a promising toughening method of presetting unbonded areas inside the titanium alloy laminates is proposed.This paper focused on this toughening method of presetting unbonded areas in the titanium alloy laminates.The effectiveness of this method is certificated by test and numerical simulation.In addition,the location and size of the unbonded areas are studied to find and summarize the discipline.The distribution of the stress intensity factor in the crack front is calculated through theoretical and numerical method,which further helps reveal the mechanism of the toughening method.The toughening design is realized by adjusting the location and size of the unbonded areas.The study content and achievements are as follows.(1)Toughening method of the titanium alloy laminates through presetting unbonded areas in laminate interfaces and the technology to confirm its validity.The effect of unbonded areas on titanium allay laminates is analyzed through experimental test and numerical simulation.Thus the effectiveness of the toughening method of presetting unbonded areas in laminate interfaces is analyzed.First,the failure process model of titanium allay laminates with unbonded areas is built by XFEM,and the simulation result is obtained based on this model.Furthermore the titanium alloy laminate specimen with hole is prepared by diffusion-bonding technology,and then the beach marking method is applied in the fatigue test.With the trace lines,the growth path of the fatigue crack is achieved,and the crack growth property can be analyzed.The simulation result and the test result are identical,which shows the validity of the numerical model,and the fatigue crack growth properties are well predicted.The simulation result and the test result both show that the unbonded areas have great influence on the fatigue crack growth procedure,and reasonable layout of unbonded areas is an effective way of toughening method.(2)Parameter study of the unbonded areas in the titanium alloy laminates.The failure process of titanium allay laminates with unbonded areas is analyzed numerically,and it can be concluded that the toughness is affected by the location and the size of unbonded areas.The crack growth in surface and thickness direction is discussed,and the respective fatigue crack growth life and rate varies with different parameters.The simulation results show that the surface crack growth rate declines when the crack grows to the boundary of unbonded areas,and the decline point is determined by the size of unbonded areas.Meanwhile,when the crack in thickness direction grows to the boundary of unbonded areas,the crack is blocked by the unbonded areas,and the location of the unbonded areas decides when the crack redirects.It can be see that a reasonable location parameter can extend the fatigue crack growth life of the structure.(3)Mechanism analysis of the toughening method that presetting unbonded areas in laminate interfaces.We revealed the toughening mechanics by the stress intensity factor in the crack front through both theoretical method and numerical simulation.The crack front of the fatigue crack can be obtained from the fracture section of the specimen.Based on the crack front,a refined finite element model is simulated to get the 3-D stress intensity factor to reveal the mechanism of toughening.Then,the fatigue crack growth of the titanium laminates with a single unbonded area is divided into 3 stages.Based on these 3 stages,we found the corrected prediction formula of stress intensity factor,and it fits well with the finite element result.In conclusion,two main mechanisms are as follows.One,the unbonded areas change the path of the crack growth.The fatigue crack bypasses the unbonded areas before continuing grow in the thickness direction,and this extends the fatigue crack growth life of the laminates.Two,because of the existence of the unbonded areas,the crack growth path mainly locates at lower stress region away from the hole.This restrains the fatigue crack growth rate and increases the toughness of the laminates.(4)The layout design of the unbonded areas in the titanium alloy laminates based on the idea of single layer failure.Based on the research,we proposed the design philosophy of single layer failure to accomplish the toughening design of the structure.We design the layout of the unbonded areas in the titanium alloy laminates,including the layout of the unbonded areas in the interlamination and the location in the thickness direction.And three design schemes are proposed thereby.Moreover,we manufactured corresponding specimens and conducted experiments.The fatigue test shows that the fatigue crack growth life of single failure specimen is longer and the toughness is higher.Further research of the test result shows that the crucial parameter is the interlamination distance of the unbonded areas.In the end,the way to determine the size and location of critical interface is provided.