The Phase-field model emerges as a promising method since it can automatically capture the initiation, propagation, coalescence, and branching of cracks without any tracking algorithms. Despite its success in simulating brittle and quasi-brittle cracks, tensile (mode I) and shear (mode II) fractures cannot be distinguished effectively. Moreover, the phase-field model is rarely applied to specimens at the structural level (i.e., reinforced concrete beams), due to the demanding computational cost. To address these shortfalls, two sets of phase-field variables are formulated to describe the tensile and shear fractures of concrete, respectively, through crack-direction-based decomposition of strain energy density into tensile, shear and compressive parts. Computational burdens are greatly alleviated by implementing the model with an explicit finite element solver (Abaqus/Explicit), allowing for massive parallel computing. The sensitivity analysis is performed to investigate the effects of critical parameters with a 1D numerical tensile bar. The double-phase-field model is applied to study the bending behavior of composite beams consisting of normal-strength concrete (NC) overlayer and ultra-high performance concrete (UHPC) substrate. To this end, five UHPC-NC beams, with various thickness ratios of UHPC and NC layers, were prepared and loaded to failure. District bending strengths, cracking patterns, deformation characteristics and failure modes of the four beams were revealed and discussed. It shows that the proposed model is necessary to accurately capture the bending behavior of UHPC-NC composite beams. This model is also applied to discuss the effects of critical parameters on the bending behaviors of UHPC-NC composite beams. To benefit potential users, detailed explanations regarding the data structures of input files and user subroutines (VUEL, VUMAT, VUSDFLD in Abaqus/Explicit) are given and released on GitHub repository.

相场模型成为一种很有前途的方法，因为它可以自动捕获裂纹的起始、扩展、合并和分支，而无需任何跟踪算法。尽管它在模拟脆性和准脆性裂纹方面取得了成功，但不能有效地区分拉伸（模式 I）和剪切（模式 II）断裂。此外，由于要求的计算成本，相场模型很少应用于结构级别的试件（即钢筋混凝土梁）。为了解决这些不足，通过基于裂缝方向的分解，分别制定了两组相场变量来描述混凝土的拉伸和剪切断裂。应变能密度分为拉伸、剪切和压缩部分。通过使用显式有限元求解器 ( Abaqus/Explicit )实现模型，大大减轻了计算负担，允许大规模并行计算。进行敏感性分析以研究关键参数对一维数值拉伸杆的影响。应用双相场模型研究由普通强度混凝土（NC）覆盖层和超高性能混凝土（UHPC）基材组成的组合梁的弯曲行为。为此，准备了五根具有不同厚度比的 UHPC 和 NC 层的 UHPC-NC 梁，并将其加载到失效状态。区域弯曲强度，开裂模式，揭示和讨论了四根梁的变形特征和破坏模式。它表明，所提出的模型对于准确捕捉 UHPC-NC 复合梁的弯曲行为是必要的。该模型还用于讨论关键参数对 UHPC-NC 复合梁弯曲行为的影响。为了使潜在用户受益，有关输入文件和用户子例程（ Abaqus/Explicit中的VUEL、VUMAT、VUSDFLD ）的数据结构的详细说明已在 GitHub 存储库上给出并发布。