为了能够准确的描述焊接接头材料力学性能不均匀分布规律，本文针对焊接接头材料力学性能不均匀性问题，利用ABAQUS有限元软件中的用户自定义材料(UMAT)模块对焊接接头力学性能不均匀分布规律进行表征。解决“三明治”焊接接头有限元仿真中存在的材料力学性能突变问题，为焊接接头力学性能分析与结构安全性评价提供新的有限元分析方法。以核电安全端焊接接头为例，应用建立的UMAT子程序对其进行安全性评价。完成的主要工作如下：

(1) 根据焊接接头材料力学性能分布规律，建立了表征焊接接头不均匀材料力学性能的UMAT子程序。

(2) 通过不均匀材料硬度压痕试验、硬度压痕试验ABAQUS有限元反演分析以及硬度压痕试验UMAT子程序数值模拟。获取三者硬度值分布规律，根据硬度值与屈服强度关系得到屈服强度分布规律，以验证UMAT子程序的准确性。

(3) 分别建立含缺陷的“三明治”焊接接头有限元模型与连续过渡焊接接头有限元模型。分析两种焊接接头有限元模型在不同屈服强度组配影响下，裂尖力学场变化规律。

(4) 以核电焊接接头为例，通过硬度压痕试验获取焊接接头硬度值分布规律，根据硬度值与屈服强度关系得到屈服强度分布规律。采用UMAT子程序表征焊接接头屈服强度分布规律并建立焊接接头有限元模型，分析核电安全端焊接接头不同位置裂纹裂尖力学场分布特点。

To describe the distribution of inhomogeneous mechanical properties of welded joint materials accurately, the User-Defined Material (UMAT) subroutine in ABAQUS software is used to describe the problem of distribution of inhomogeneous mechanical properties of welded joint materials. It solves the problem of sudden changes in the mechanical properties of materials in the finite element simulation of "sandwich" welded joints, and provides a new finite element analysis method for the mechanical performance analysis and structural safety evaluation of welded joints. Taking the welding end of the nuclear power safety terminal as an example, the safety evaluation is carried out by using the established UMAT subroutine. The main work performed is as follows:

(1) According to the distribution law of the mechanical properties of the welded joint materials, the UMAT subroutine is established to characterize the mechanical properties of the non-uniform materials of the welded joint.

(2) Through the indentation test of the uneven material hardness, the hardness indentation test ABAQUS finite element inversion analysis and the UMAT subroutine numerical simulation of the hardness indentation test. Obtain the distribution law of the three hardness values, and obtain the distribution law of the yield strength according to the relationship between the hardness value and the yield strength to verify the accuracy of the subroutine.

(3) Establish the finite element model of the "sandwich" welded joint with defects and the finite element model of the continuous transition welded joint respectively. Analyze the change law of crack tip mechanical field under the influence of different yield strength combinations of the two welded joint finite element models.

(4) Taking the nuclear power welding joint as an example, the hardness indentation test is used to obtain the distribution law of the hardness value of the welded joint, and the distribution law of the yield strength is obtained according to the relationship between the hardness value and the yield strength. The UMAT subprogram was used to characterize the distribution law of the yield strength of the welded joints and to establish a finite element model of the welded joints, and to analyze the characteristics of the mechanical field distribution of the crack tip at different positions of the welded joints of the nuclear power safety end.