Study On The Effect Of Laser Peening With Different Power Densities On Fatigue Life Of Titanium Alloy Fastener Hole

In the aircraft structure,the fastening hole is a very common feature structure of parts.Due to the working environment of the aircraft is very poor,and the small hole is a typical stress concentration structure,fatigue cracks often occur in this area.Laser shock processing(LSP)is a new type of strengthening method,which has many advantages over traditional strengthening methods.Many researches at home and abroad are limited to the measurement of surface residual stress and the analysis of the residual stress of the hole wall.The three-dimensional stress near the pore structure has a significant influence on the fatigue gain after LSP.Therefore,it is of great significance to study the different laser parameters on the three-dimensional stress distribution of the hole structure to predict the fatigue life of the hole structure.In this paper,based on the numerical simulation and experimental methods,the effects of laser power density on the three-dimensional stress distribution and fatigue life of the titanium alloy hole structure have been studied.The main research contents are as follows:(1)Based on ABAQUS finite element analysis software,a numerical simulation model of laser shock processing of titanium alloy hole structure was established.The three-dimensional stress distribution of hole structure under different laser shock parameters was analyzed.The results show that When the number of impact layers is one,the residual stress distributions of the A-plane(pre-impact surface)and B-plane(secondary impact surface)of the small-hole structural have large differences,and the differential value increases as the peak pressure increases.When the number of impact layers is two,with the increase of peak pressure,the residual tensile stress peak of pore wall first increases and then decreases.When the peak pressure reaches 5GPa,the residual tensile stress peak value reaches the maximum.From the surface residual stress distribution,the residual compressive stress is in the impact region and the residual tensile stress is outside the impact region.As the peak pressure gradually increases,the residual compressive stress in the impact region gradually increases,but there is a saturation value.Outside the impact region,the residual tensile stress increases as the peak pressure increases.(2)Fatigue life analysis of the titanium alloy hole structure with LSP was carried out by FE-SAFE finite element analysis software.The results show that,from the fatigue life clouddiagram,the lowest fatigue life of the small hole structure after the laser shock is found on the hole wall position,and with the increase of the peak pressure,the length of the fatigue risk weak zone on the hole wall decreases gradually.The fatigue risk of small hole structures increases gradually outside the impact area.Under the different peak pressure,the fatigue life of the hole structure is also different.With the increase of the peak pressure,the fatigue life increases first and then decreases,and then continues to increase.(3)According to the results of simulation analysis,reasonable laser shock parameters and fatigue test parameters are selected to perform LSP test on the 6mm titanium alloy double-hole sample,and residual stress measurement and fatigue test are performed on the processed sample.The results are as follows: there is a certain error between the residual stress measurement results and the numerical simulation results,but the trend of change is consistent.With the increase of laser power density,the fatigue life gain of the hole structure increases first and then decreases,and then continues to increase,which is in accordance with the simulation results.(4)Through the morphological analysis of typical fatigue fractures,it is concluded that the fatigue of the small-hole fracture at the non-impact end appears near the hole corner,and the fatigue crack initiation at the impact end appears on the hole wall.In addition,when the power density is small,the fatigue crack initiation generated by the hole wall is larger and more dispersed.When the power density is large,the fatigue crack initiation is dispersed more concentrated.By studying the effect of laser power density on the three-dimensional stress and fatigue life of the small hole structure,and analyzing the internal reasons,this has guiding significance for the parameter selection during laser shock processing.