Investigation On Residual Stress And Fatigue Performance Of The Rod With A Hole By Laser Shock Processing

Laser shock processing(LSP)of small holes is an emerging technology to strengthen small holes.When the material is irradiated by the laser with high power density and short pulse,plastic deformation happens,generating compressive residual stress around the hole and at certain depth.Because of these characteristics,it has a promising prospect in aircraft landing gears,automobile industry and other fields.The desired strengthening result can be obtained by controlling laser parameters.In this paper,the numerical simulation method and experimental method are combined jointly to investigate influences of different parameters on residual stress fields and fatigue lives of the 304 stainless steel rods with holes.The main contents and conclusions are as follows:(1)In the first place,the theoretical analyses were carried out.Comparing with conventional hole strengthening technologies,laser shock processing possesses outstanding advantages.The progress of laser shock processing at home and abroad is reviewed.Subsequently,the formation of laser shock wave,the formation mechanism of compressive residual stress,the estimation formula of compressive residual stress,and the mechanisms revealing that the compressive residual stress retards the fatigue crack initiation and delays the fatigue crack growth,were introduced respectively.(2)The software ABAQUS was utilized to establish the 3-D models and conduct the numerical simulations.Multiple laser shocking at the same spot and the sequences of drilling hole were firstly simulated to prepare for the subsequent investigation on the residual stress fields affected by advancing directions of laser shocking,peak pressures of shock waves,diameters of rods and the number of shocking layers.The results show that the residual stress hole appears at the center of the spot after the surface is impacted by single spot.The residual stress along the depth direction is symmetrically distributed after laser shock processing.The residual stress field is better when the specimen is treated with post-drilling after LSP,with residual stress fluctuating slightly and compressive residual stress distributing in mid-thickness region of the hole wall.The advancing directions of laser shocking barely affect the residual stress distribution.The peak pressure of the shock wave and the number of shocking layers have similar effects on the residual stress field.As the peak pressure and the number of shocking layers gradually increase,the compressive residual stress also increases gradually,after the compressive residual stress getting saturated,its increasing amplitude decreases;while tensile residual stress increases gradually,and its increasing amplitude increases.As for the rod with small diameter,the tensile residual stress is introduced in the mid-thickness of the hole wall after laser shock processing.With the diameter getting bigger,the tensile residual stress gradually decreases and transforms into compressive residual stress,and then compressive residual stress increases slightly.(3)Finally,corresponding experiments were carried out.It can be seen that micro-pits appear in the surface of the rod after laser shock processing,which are difficult to see clearly by eyes.According to the numbers of stress cycles,fatigue lives of rods subjected to laser shock processing increase to different degrees.Among three kinds of rods,shocked rods with ablative layers have the longest fatigue lives.Compared with untreated rods,the fatigue striation spacing decreases after laser shock processing.