激光冲击诱导应力波对残余应力孔洞形成的影响

Effect of Laser Shock Induced Stress Waves on Formation of Residual Stress Holes

目的揭示残余应力孔洞形成的影响机制。方法采用有限元动力学仿真,实现对激光冲击诱导应力波传播过程的捕捉和特性分析。结果在2000 ns以内的光斑作用区域,材料的初始应力仅受到激光冲击诱导Ⅱ类应力波的扰动作用,此扰动作用由光斑中心向外依次减弱,引起的应力波动幅值由2459 MPa降至224 MPa;在其他工艺参数相同的情况下,残余应力孔洞随着峰值压力和脉宽的增加而变得更加明显,应力幅值差σ_(k)分别由0、114 MPa增至454、568 MPa,孔洞直径d_(k)分别由0、0.8 mm增至约1.4 mm,光斑直径的影响相对较弱。在激光能量3 J的作用下,空间高斯分布对应结果存在应力孔洞,σ_(k)、d_(k)分别为190 MPa、1 mm。将空间平顶分布激光能量增至7 J,对应结果由3 J激光能量时的无应力孔洞转变为存在明显应力孔洞,σ_(k)、d_(k)分别为634 MPa、1.4 mm。另外,在激光能量3 J的作用下,材料1的应力孔洞最显著,σ_(k)、d_(k)分别为207 MPa、1.4 mm,均高于材料2和材料3的对应值。凸圆面、平面和凹圆面对应的σ_(k)分别为212、190、83 MPa,d_(k)均为1 mm。结论在圆形光斑激光冲击载荷作用下,加载和卸载过程中形成的应力波在光斑中心的汇聚作用均较强,导致光斑中心区域材料持续出现正反向屈服,进而形成残余应力孔洞;在相同参数条件下,激光能量越高,则材料的力学性能越低,应力波扰动作用越强,应力孔洞现象越明显。

In order to reveal the mechanism of the influence of laser shock induced stress waves on the formation of residual stress holes,the propagation process of laser shock induced stress wave were captured,and its characteristics were analyzed,by extracting node stresses at different times,based on the finite element dynamic simulation analysis.The results showed that within 2000 ns,the initial stress of the material in the area affected by the light spot was only disturbed by the ClassⅡstress wave induced by laser shock,and the disturbance gradually stabilized over time;The initial stress disturbance caused by ClassⅡstress wave was gradually weakened from the center of the light spot outward,the amplitude of stress fluctuation caused by it decreased from 2459 MPa to 224 MPa(the amplitude of stress fluctuation in the central region exceeded the yield strength of the material itself);In cases where other process parameters were the same,the residual stress hole phenomenon became more pronounced with the increase of peak pressure and pulse width(the stress amplitude differenceσ_(k)increased from 0 MPa and 114 MPa to 454 MPa and 568 MPa,respectively,and the stress hole diameter d_(k)increased from 0 mm and 0.8 mm to approximately 1.4 mm in the hole area).The influence of spot diameter on residual stress hole phenomenon was relatively weak.Under the action of 3 J laser energy,there was a stress hole in the residual stress result corresponding to the spatial Gaussian distribution,andσ_(k)and d_(k)were 190 MPa and 1 mm,respectively.By increasing the laser energy of the spatial flat top distribution to 7 J,the corresponding residual stress result changed from no stress holes at 3 J laser energy to an obvious stress hole,σ_(k)and d_(k)were 634 MPa and 1.4 mm respectively,and the characteristic values of stress holes were greater than the spatial Gaussian distribution under the same peak pressure condition;In addition,under the action of 3 J laser energy,the stress hole corresponding to material 1 was the most significant,σ_(k)and d_(k)were 207 MPa and 1.4 mm,respectively,which were higher than those of material 2 and material 3,and even exhibited secondary stress hole.The characteristic related to stress holeσ_(k)under the geometric conditions of convex circular surface,flat and concave circular surface were 212 MPa,190 MPa and 83 MPa,respectively,and d_(k)were all 1 mm.In conclusion,under the action of circular spot laser shock load,the stress waves formed during the loading and unloading processes have a strong convergence effect at the center of the spot,resulting in continuous positive and negative yielding of the material in the center area of the spot,and the formation of residual stress holes;Under the same parameter conditions,the higher the laser energy,the lower the mechanical properties of the material,the stronger the stress wave disturbance action,and the more obvious the residual stress hole phenomenon.In addition,different surface geometric features mainly affect the stress amplitude difference in the hole area.