异种金属焊接接头（DMWJ）广泛应用于核电、航空航天等重大装备领域，其界面区域因应力腐蚀导致的环境致裂问题，严重威胁着焊接结构的安全性和可靠性。本研究针对DMWJ断裂失效这一关键问题，以核电安全端DMWJ为研究对象，根据相场理论和数值模拟方式开发相场法数值模拟框架，从损伤力学的角度研究了连续过渡材料区域的单裂纹和双裂纹扩展行为。实现了对DMWJ界面裂纹扩展路径的预测，为焊接结构完整性评估提供了新的分析方法。完成的主要工作如下：

(1) 根据弹塑性相场理论推导裂纹扩展的演化方程，基于FORTRAN语言开发相场法的UMAT子程序，采用各向同向硬化本构方程建立弹塑性本构模型。开发UEL子程序框架，以弹性应变能和耗散能作为裂纹扩展驱动力建立相场法的扩展模型。通过二次开发技术，在INP文件进行参数化建模，耦合位移场和相场两层结构模型。通过对网格密度、裂纹长度尺度参数l与位移载荷的分析，研究不同参数对裂纹扩展路径的影响，确定裂纹扩展的尺寸参数。

(2) 通过显微压痕硬度实验获取DMWJ局部区域的硬度分布，利用硬度和力学性能参数的转化关系建立材料的力学性能参数梯度场。采用插值函数法将实验数据编写进UMAT子程序，表征DMWJ界面热影响区和熔合区的非均匀力学性能。结合UEL子程序与UMAT子程序，基于能量最小化原理来模拟单裂纹的扩展路径。研究在不同的裂纹位置时，力学性能不均匀性对DMWJ单裂纹扩展偏转路径的影响与变化规律，通过与参考文献的物理实验对比，验证相场法的准确性，分析弹性模量、屈服强度和硬化模量失配对DMWJ裂纹扩展路径产生的影响。

(3) 以材料分界面处的裂纹为基准裂纹，分别在DMWJ的热影响区和熔合区创建第二条裂纹，建立DMWJ双裂纹扩展相场模型。结合相场法的子程序来模拟双裂纹的萌生和扩展，描述两条裂纹之间的相互作用以及材料力学性能对裂纹扩展路径的影响。分析双裂纹融合的规律，以及不同的裂纹间距和裂纹长度对双裂纹扩展路径的影响。

Dissimilar Metal Welded Joint (DMWJ) are widely employed in critical equipment sectors such as nuclear power and aerospace. However, the interfacial regions of these joints are prone to severe environmental assisted cracking caused by stress corrosion, which significantly threatens the safety and reliability of welded structures. Aiming at the critical issue of fracture failure in DMWJ, this study takes the safe-end DMWJ in nuclear power plants as the research object. Based on the phase-field theory and numerical simulation methods, a phase-field numerical simulation framework was developed to systematically investigate the propagation behavior of single and double cracks in the continuously graded material regions from the perspective of damage mechanics. This framework enables the prediction of crack propagation paths in the DMWJ interface, providing a novel analytical method for the integrity assessment of welded structures. The main achievements are as follows:

(1) Derived the evolution equation for crack propagation based on elasto-plastic phase-field theory. Developed a UMAT (User Material) subroutine for the phase-field method using FORTRAN, and established an elasto-plastic constitutive model using an isotropic hardening constitutive equation. Developed a UEL (User Element) subroutine framework, establishing a phase-field crack propagation model with elastic strain energy and dissipated energy as the driving forces for crack extension. Implemented parameterized modeling in INP files through secondary development techniques, coupling a two-layer structural model of the displacement field and phase-field. The influence of different parameters (mesh density, crack length scale parameter l, and displacement loading) on crack propagation paths was investigated to determine the dimensional parameters for crack propagation.

(2) Obtained the hardness distribution in local regions of DMWJ through micro-indentation hardness experiments. Established a gradient field of material mechanical properties using the conversion relationship between hardness and mechanical performance parameters. Incorporated experimental data into the UMAT subroutine using interpolation functions to characterize the non-uniform mechanical properties of the heat-affected zone (HAZ) and fusion zone (FZ) in DMWJ interfaces. Simulated single crack propagation paths by combining UEL and UMAT subroutines based on the principle of energy minimization. Investigated the influence and variation patterns of mechanical property heterogeneity on the deflection paths of single cracks in DMWJ at different crack locations. The accuracy of the phase-field method was validated by comparing with physical experiments from literature, and the impact of mismatches in elastic modulus, yield strength, and hardening modulus on DMWJ crack propagation paths was analyzed.

(3) Taking the crack at the material interface as the reference, a second crack was introduced into the HAZ and FZ of DMWJ, respectively, to establish a phase-field model for double-crack propagation. The subroutines of the phase-field method were used to simulate the initiation and propagation of double cracks, describing the interaction between the two cracks and the influence of material mechanical properties on crack paths. The laws of double-crack coalescence and the effects of different crack spacing and crack length on double-crack propagation paths were analyzed.

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