喷丸强化残余应力对AISI304不锈钢疲劳裂纹扩展行为的影响

目的 探究喷丸强化残余压应力对AISI 304不锈钢疲劳裂纹扩展行为的影响规律.方法 建立并联合紧凑拉伸(CT)试样三维有限元模型和对称胞元喷丸有限元模型,发展一套多步骤数值模拟方法.首先,建立AISI 304不锈钢CT试样的三维有限元模型,模拟不同外加交变载荷工况下的疲劳裂纹扩展过程.基于线弹性断裂力学理论,利用裂纹闭合技术,计算不同裂纹长度对应的应力强度因子范围,采用修正的Paris公式计算疲劳裂纹扩展速率,并通过试验数据对计算结果进行考核.其次,建立多弹丸分层逐次冲击靶面的对称胞元喷丸有限元模型,模拟100％和200％喷丸覆盖率下的残余应力场,并通过试验数据对该对称胞元喷丸有限元模型的有效性进行验证.最后,将喷丸强化诱导的残余应力场以读写外部文件的方式导入CT试样三维有限元模型,模拟在内部残余应力场和外部交变载荷共同作用下的疲劳裂纹扩展行为.结果 对于相同的喷丸工况,保持外加载荷比不变而减小最大外加载荷,或者保持最大外加载荷不变而减小外加载荷比,喷丸强化诱导的残余压应力对疲劳裂纹扩展的抑制作用愈加显著.对于相同的外加载荷工况,200％喷丸覆盖率工况比100％喷丸覆盖率工况更能有效降低AISI 304不锈钢的疲劳裂纹扩展速率.结论 喷丸强化诱导的残余压应力场能够有效抑制AISI 304不锈钢的疲劳裂纹扩展.

Effects of Shot Peening-induced Residual Stresses on Fatigue Crack Propagation Behavior of AISI 304 Stainless Steel

In order to study the effects of shot peening-induced residual stresses on fatigue crack propagation behavior of AISI 304 stainless steel, a multi-step simulation method was developed by combining the three-dimensional finite element model of compact tension (CT) specimen and the symmetric cell model of shot peening. Firstly, a three-dimensional finite element model of the CT specimen of AISI 304 stainless steel was established to simulate the fatigue crack expansion process under different applied alternating load conditions, the stress intensity factors with respect to different crack lengths were accordingly computed based on the principle of linear elastic fracture mechanics (LEFM) and the use of virtual crack closure technique (CCT), and the fatigue crack propagation rates were resultantly calculated by using the modified Paris law. The predictions of fatigue crack propagation rate were validated by the experiment data. Secondly, the symmetric cell model with the assumption that multiple shots sequentially impact on the target surface layer by layer was created to simulate the shot peening-induced residual stress fields associated with the 100% and 200% coverages, and the predicted residual stresses were validated by the experimental results. Lastly, the residual stress fields induced by shot peening were then imported into the CT model with the method of reading and writing files, and the fatigue crack propagation behaviors under the combined effects of the internal residual stresses and external applied loads were then investigated. The obtained results show that, in the case of the same shot peening conditions, with the decrease of the maximum applied loads when the applied load ratio remains a constant, or with the decrease of the applied load ratios when the maximum applied load remains a constant, the fatigue crack propagation rates would reduce more significantly. On the other hand, for the same applied loads, when compared with the shot peening case of 100% coverage, the residual stresses induced by shot peening with 200% coverage can more effectively reduce the fatigue crack propagation rate of AISI 304 stainless steel. It is therefore concluded that the compressive residual stresses induced by shot peening have the capability of reducing the fatigue crack propagation rate of AISI 304 stainless steel.