基于曲线裂纹等效厚度的断裂准则研究

The most widely used fracture criterion is based on the two-dimensional fracture theory. Within the framework of two-dimensional fracture theory, the fracture toughness is dependent on the geometry of structures, rather than a material constant. For a straight-through cracked plate, the fracture toughness will change for 2 to 3 times just changing the thickness of plate. Though the fracture toughnesses of various thicknesses can be measured by a large number of tests, it is still difficult to apply the thickness-dependent fracture toughnesses to assess fracture of part-through curve cracked structures due to the lack of reasonable physical meanings of thicknesses in these structures. The three-dimensional fracture toughness developed in recent years is considered as a material constant, which provides new ideas for fracture assessment of part-through curve cracked structures. This project will firstly focus on the stress field and constraint state of border along the part-through curve crack line, and then found an equivalent relationship between part-through curve crack and straight-through crack.The whole line of the curve crack border is divided to several segments and each of them is treated as a straight-through cracked plate with a certain thickness. On that basis, the three-dimensional fracture criterion of part-through curve cracked structures will be established which will remove the theoretical defect and solve the technical problem in fracture assessments of complex engineering structures using the tested fracture toughnesses of standard specimens.

当前工程中广泛应用的断裂准则是基于二维断裂理论建立起来的。在二维理论框架内，断裂韧性并非是材料常数，而是与结构的几何形状密切相关。对于穿透直裂纹板，仅厚度的变化就能够引起断裂韧性2-3倍的变化。即使通过大量实验得到了各个厚度下的断裂韧性，也无法将其应用于非穿透曲线裂纹结构的评估。近年来发展起来的三维断裂韧性被认为是与试件厚度无关的材料常数，为实现非穿透曲线裂纹结构的断裂评估提供了新的思路。本项目将重点研究非穿透曲线裂纹尖端的应力场与约束状态，基于曲线裂纹与穿透直裂纹尖端应力场和约束状态的内在本质联系，建立非穿透曲线裂纹前沿与穿透直裂纹尖端的等效关系，将非穿透曲线裂结构的裂纹前沿线纹分解为不同厚度的一系列穿透直裂纹板。在此基础上进一步建立非穿透曲线裂纹结构的三维断裂准则，克服当前基于标准试件测定的断裂韧性无法合理推广至复杂工程结构断裂评估的理论缺陷与技术难题。

当前结构安全评估逐渐接受并采用了以断裂力学为基础的损伤容限设计理念，其核心概念建立在结构不可避免地存在裂纹类损伤的前提之上，其学科基础是断裂力学。含裂纹结构的剩余强度评估是损伤容限设计的核心任务。当前工程中广泛应用的断裂准则是基于二维断裂理论建立起来的。在二维理论框架内，断裂韧性并非是材料常数，而是与结构的几何形状密切相关。对于穿透直裂纹板，仅厚度的变化就能够引起断裂韧性数倍的变化。即使通过大量实验得到了各个厚度下的穿透直裂纹板断裂韧性，由于实际结构中的裂纹大多为非穿透曲线裂纹，因此也无法对其进行准确评估。近年来发展起来的三维断裂韧性被认为是与试件几何结构无关的材料常数，为实现非穿透曲线裂纹结构的断裂评估提供了新的思路。首先，本项目建立了非穿透曲线裂纹的弹塑性参数化通用有限元模型，基于该模型系统研究了不同尺寸非穿透曲线裂纹尖端的应力场，并基于曲线裂纹与穿透直裂纹尖端应力场和约束状态的内在本质联系，建立了非穿透曲线裂纹前沿与穿透直裂纹尖端的等效关系，将非穿透曲线裂纹前沿线纹分解为不同厚度的一系列穿透直裂纹板。在此基础上进一步建立了非穿透曲线裂纹结构的三维断裂准则，克服当前基于标准试件测定的断裂韧性无法合理推广至复杂工程结构断裂评估的理论缺陷与技术难题。其次，发展了基于原位SR-μCT对疲劳裂纹萌生及扩展的实时观察技术，基于此可以实现非穿透曲线裂纹的损伤演化检测及扩展寿命评估。另外，发展了利用电弧放电预制非穿透曲线裂纹的方法，基于该方法可以实现任意尺寸初始裂纹的预制和亚毫米级的开裂面，为后续各种非标准断裂试件的制备提供了技术手段。最后，本项目系统分析了动态裂纹尖端应力场，揭示了动态断裂韧性随速度变化的机理。