【作者】 桑毅；

【导师】 陈瑞芳；

【作者基本信息】 江苏大学 ， 机械制造及其自动化， 2009， 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过程中产生的高压冲击应力波的力效应来改善金属机械性能的一种表面改性技术,可使材料产生塑性变形、残余压应力、以及高密度位错等,能有效地提高了材料疲劳强度。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,对激光冲击强化钛合金残余应力场进行了实验研究和数值模拟,并研究了残余应力场对小孔件疲劳性能的影响以及疲劳寿命预测方法,取得如下研究成果。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,建立了激光冲击强化钛合金的三维和准二维有限元分析模型,阐述了激光冲击强化数值模拟的有限元理论基础,讨论了建模过程中的几个关键问题,获得了冲击后残余应力场的分布。针对激光功率密度对残余应力场的影响进行了数值模拟研究,模拟结果表明:在激光功率密度超过一定阈值后,最大残余压应力出现在表层之下,且随着功率密度增加,表面残余压应力不断降低直至出现残余拉应力。阐述了局部应力应变法的原理,应力集中与疲劳强度因子k_f的概念。收集并分析2024铝合金,7050铝合金的激光冲击与疲劳实验,讨论了激光冲击对缺口疲劳的影响。结果表明:激光冲击后,冲击区表面质量对疲劳寿命改善效果影响很大,尤其是表面粗糙度的影响。粗糙度明显改善时,寿命可提高6～9倍,无明显改善时,只能提高2～3倍。对于同样冲击效果的试件,不同的疲劳载荷水平会影响到寿命增幅。当疲劳加载水平高时,寿命提高幅度会变小,这可能与残余应力松弛有关。激光冲击改变屈服强度和弹性模量并引起晶粒细化,均对疲劳裂纹的扩展有很好的抑制作用。建立缺口件有限元模型,对激光冲击前后的钛合金小孔件模型进行了疲劳加载模拟与寿命预测,分别使用平面应变下缺口根部应力结果和非平面应变下缺口根部中间位置应力结果预测疲劳寿命,前者得到的寿命预测值要高于后者,冲击后的寿命是冲击前的1.9～3.3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 shock processing of TC4 Ti alloy was researched by the methods of experiments and simulations.The effection of residual stress field to fatigue life of center-hole specimen as well as the prediction methods of which 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 3D FEA and semi-2D FEA analysis models were founded,considering the finite element theory. Some key problems were discussed and the distribution of the residual stress field generated by LSP was obtained.The effects of laser shock power density was simulated which indicated that the peak surface residual compressive stress increased as the power density increased when the laser power density was below a fixed value,and then decreased,even to tension residual stress.At the same time,the maximum compressive residual stress appeared below the surface was increasing all the time.The local stress-strain method was described by focusing the stress concentration and the fatigue notch factor k_f The laser shock processing experiments and fatigue experiments of 2024 Al alloy and 7050 Al alloy were collected and discussed.The results indicated that the qualification of the shocked region make a big effect to fatigue life improving, especially the roughness.Fatigue life can be increased by 6～9 times under good-improved roughness and 2～3 times under the normal roughness.The amplification of fatigue life will be increase when the fatigue loads level down.The improvements of yield stress and elastic modual induced by laser shock processing make good effords to prevent the crack propagation.Fatigue load simulation and life prediction was done for center-hole specimen of Ti alloy with and without LSP.The fatigue lives gained by the stress results at the middle position of the notch root in the case of no plane strain are shorter than which gained by the stress results in the case of plane strain.The fatigue lives are increased about 1.9～3.3 times.更多还原