【作者】 蔡峥嵘；

【导师】 花银群；

【作者基本信息】 江苏大学 ， 机械制造及其自动化， 2010， 硕士

【摘要】 激光冲击强化技术是一种利用高功率短脉冲激光与材料相互作用产生高压冲击应力波的力效应来改善金属机械性能的表面改性技术。它可使材料产生塑性变形、残余压应力、晶粒细化以及高密度位错等,能有效地提高材料疲劳强度,在航天、汽车等现代制造领域具有广阔前景。本文从激光冲击波诱导残余应力场的机理出发,对激光搭接冲击强化钛合金进行了实验研究和数值模拟,并研究了激光冲击对疲劳裂纹扩展的抑制机理以及残余应力场对紧凑拉伸试件(CT试件)疲劳裂纹扩展特性的影响,取得了如下研究成果：根据弹塑性动力学理论,探讨了激光冲击波的产生和传播机理,残余应力场的形成和估算方法。以ANSYS/LS-DYNA为平台,建立了激光搭接冲击强化钛合金有限元分析模型,阐述了激光搭接冲击强化数值模拟的有限元理论基础,讨论了模型建立、网格划分过程中的关键问题,获得了冲击后残余应力场的分布。针对搭接率、激光功率密度及脉宽对残余应力场的影响进行了数值模拟研究,模拟结果表明：增加搭接率、功率密度和激光脉宽,不但可以增加表面残余压应力,而且可以增加残余压应力层深度L_P；但当脉宽增加到一定值时,搭接区表面残余压应力达到最大值,继续增加会导致搭接区表面残余压应力的降低；搭接率增加到一定程度时,继续增加会导致残余压应力层深度的降低。阐述了激光冲击改善疲劳裂纹扩展特性的基本原理,进行了TC4钛合金的激光冲击与疲劳裂纹扩展实验,讨论了激光冲击对CT试件疲劳裂纹扩展的影响。实验数据表明：激光冲击对疲劳裂纹的扩展起到了很大的抑制作用,激光冲击后试件的疲劳寿命为冲击前的1.38倍,疲劳裂纹的扩展速率为冲击前的78%。建立CT件有限元模型,对激光冲击前后的钛合金CT件模型进行了疲劳裂纹扩展的模拟,分别模拟了峰值压力和搭接率对疲劳裂纹扩展的影响,结果显示冲击后的寿命是冲击前的1.46～3.03倍,疲劳裂纹扩展速率为冲击前的0.95～0.127倍。更多还原

【Abstract】 Laser shock processing is a new surface modification technology, which makes use of the mechanical effect of high pressure shockwave generating in the process of interaction between metal materials and high power density, short pulse. It causes metal materials to yield and plastically deform, thereby the surface layer develops dislocation of high density, twins and high level compressive residual stress. Those prolong the fatigue life of metal materials greatly. LSP has extensive applied foreground in some modern manufacturing such as aerospace industry, automotive engineering and so on.According to the theory of residual stress field formation by laser shock wave, Laser overlapping shock processing of TC4 alloy was researched by the methods of experiments and simulations. The mechanism of laser shock to restrain the growth fatigue crack and the effect of Residual stress field on the properties of CT specimen fatigue crack growth were researched. The achievements were:According to the theory of plastic-elasticity dynamics, the basic theory of the shock wave generation and propagates in material as well as the formation mechanism and evaluation method of residual stress field was described. Based on ANSYS/LS-DYNA software, the laser overlapping shock FEA analysis models were founded, considering the finite element theory. Some key problems in Model and meshing were discussed and the distribution of the residual stress field generated by LSP was obtained. The impact on the residual stress field by overlapping rate,the laser power density and the pulse width were researched by numerical simulation, the results show that:Increasing the overlapping rate, power density and laser pulse not only increased the surface residual stress and increased the depth of residual compressive stress, but when the pulse width has reached a certain value, the surface residual stress of overlapping area achieved maximum, continue to increase the overlapping rate will lead to the reduction of surface residual stress; when overlapping rate has got to a certain value, continue to increase it will lead to the reduction of the depth of residual compressive stress.The improved mechanism of LSP on the characteristics fatigue crack growth were expounded, the experiments of LSP on TC4 titanium alloy and fatigue crack propagation were did on, the impact of LSP on CT specimen fatigue crack growth were discussed. The experimental data show that:the LSP has a significant inhibitory effect on the fatigue crack growth, after LSP, the fatigue life of CT specimen was 1.38 times of that of before and the fatigue crack growth rate was 78% of that before.The CT FEA model was founded, the fatigue crack growth of TC4 CT specimen before and after LSP was simulated, the expect on fatigue crack growth of peak pressure and the overlapping rates were simulated, the results show that:the fatigue life of after LSP was 1.46～3.03 times of that of before, and the fatigue crack growth rate was 0.95～0.127 of that of before.更多还原