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采用数值模拟方法对核电站316L不锈钢弯头的应力腐蚀裂纹扩展行为进行了研究。首先针对不锈钢厚壁弯头(外径355.6 mm,内径275.6 mm)进行有限元建模,在弯头内壁上创建出与实际裂纹相符的半椭圆状3D缺陷作为裂纹形状,其裂纹张开位移(δ_i)由Dugdale模型计算确定;然后根据有限元计算结果,建立裂纹应力强度因子(K)随裂纹深度(a)及附加应力(P)变化的拟合公式,结合实验数据得到管材在2种冷变形量下的应力腐蚀裂纹扩展速率(da/dt)拟合公式,利用迭代方法计算了裂纹穿透管壁所需的时间,为核电站安全评估提供了有效依据。研究显示,当弯头部位的冷变形量较小(硬度为230~245 HV)且在理想情况下(无初始附加应力),弯头被应力腐蚀裂纹穿透耗时最长(约57 a);当初始附加应力增加至200 MPa,此失效时间约缩减至前者的1/5(无应力释放)、2/7(应力释放一半)以及3/7(应力完全释放);保持初始附加应力不变(200 MPa)并提高弯头部位冷加工变形量(由硬度为230~245 HV提高到275~300 HV),弯头的大变形部位被穿透时间约缩短至小变形部位失效时间的2/5(无应力释放)、3/8(应力释放一半)以及1/3(应力完全释放),由此可见应力释放程度的降低和冷加工变形量的增加均导致了核电站316L不锈钢弯头剩余寿命的缩短。
The numerical simulation method was used to study the stress corrosion crack propagation of 316L stainless steel elbow in nuclear power plant. Firstly, the finite element method is used to simulate the stainless steel thick-walled elbow (the outer diameter is 355.6 mm and the inner diameter is 275.6 mm), and a semi-elliptical 3D defect corresponding to the actual crack is created on the inner wall of the elbow as a crack shape. δ_i) was calculated and determined by Dugdale model. Based on the results of finite element analysis, a fitting formula of crack stress intensity factor (K) with crack depth (a) and additional stress (P) was established. Based on the experimental data, The fitting formula of da / dt under deformation is used to calculate the time required for the crack to penetrate the pipe wall, which provides an effective basis for the safety assessment of nuclear power plant. Studies have shown that elbows take the longest to penetrate through stress corrosion cracking when the amount of cold deformation is small (hardness 230-245 HV) and in the ideal case (without initial stress) (about 57 a ); When the initial additional stress increased to 200 MPa, this failure time reduced to about 1/5 of the former (no stress release), 2/7 (half stress relief) and 3/7 (full stress release); the initial additional stress (200 MPa) and increase the cold working deformation of the elbow part (from 230 to 245 HV to 275 to 300 HV), and the large deformation part of the elbow is shortened to the point of failure time of the small deformation part 2/5 (no stress release), 3/8 (stress release half), and 1/3 (stress full release), we can see that the reduction of the stress release and the increase of the deformation of cold work all lead to the surplus of 316L stainless steel elbow Shortened life expectancy