纳入面内与面外统一裂尖拘束的结构安全评价方法

At present, the domestic and international safety assessments of mechanical structures with defects are mainly based on the conventional single-parameter fracture mechanics. The crack-tip constraint effect has not been effectively incorporated in the assessments, thus the assessment results are over- conservative or non-conservative. The international assessment approaches developed by using the in-plane constraint parameters T and Q have not incorporated the out-of-plane constraint effect on the fracture behavior of structures, and the assessment results are not accurate. For the three-dimensional cracks in actual engineering structures, both in-plane and out-of-plane constraints exist simultaneously due to different structural geometries, crack sizes and loading modes, thus it is necessary to develop new safety assessment methods of structures incorporating both in-plane and out-of-plane constraints. In this project, using the combination methods of fracture experiments of specimens with different constraints, the numerical calculations of constraint and fracture parameters for three-dimensional cracks in the specimens and typical structures and theory analyses, the applicability of the unified in-plane and out-of-plane constraint parameter Ap based on crack-tip equivalent plastic strain for brittle and ductile fracture under conditions of a wide range of constraints and loading levels will be investigated. The research contents in this project also include the technical method establishing the correlation between the unified constraint parameter Ap and fracture properties of materials and the technology roadmap and approaches of the safety assessments of structures and welded joints based on the parameter Ap. These studies will lay the technical foundation for developing the structural integrity assessment approaches which can accurately incorporate both in-plane and out-of-plane crack-tip constraints.

目前国内外含缺陷机械结构的安全评价主要是基于传统断裂力学的单参数方法，未有效纳入裂尖拘束效应的影响，因而其评价结果过于保守或非保守。国际上发展的基于面内裂尖拘束参数T和Q的评价方法未有效纳入面外拘束对结构断裂行为的影响，而存在不准确性。对于实际结构中的三维裂纹，由于结构几何、裂纹尺寸和加载方式的不同，面内与面外拘束同时存在，因而需要发展能统一纳入面内与面外裂尖拘束的结构安全评价新方法。本项目拟采用不同拘束试样的断裂实验、试样与典型结构三维裂纹拘束和断裂参数的数值计算和理论分析相结合的方法，研究基于裂尖等效塑性应变的面内与面外统一裂尖拘束参数Ap在宽范围拘束条件和载荷水平下，对脆性断裂和延性断裂两种失效模式的适用性；研究统一拘束参数Ap与材料断裂性能建立关联的技术方法；研究基于参数Ap的结构及焊接接头安全评价的技术路线和方法。为建立准确纳入面内与面外统一拘束的结构完整性评价方法奠定技术基础。

材料的断裂韧性受裂尖拘束的影响，随拘束的增加韧性降低。在基于传统单参数断裂力学的结构完整性评价中，通常用实验室平面应变高拘束试样测得的下限断裂韧性评定实际结构低拘束浅短裂纹或高拘束深长裂纹的安全性，其评价结果过于保守或非保守。为提高评价的准确性，需要采用合适的拘束参数纳入拘束效应的影响。目前基于面内拘束参数T 、Q等的评价方法未有效纳入面外拘束对结构安全评价的影响，而存在不准确性。因而需要发展能统一纳入面内与面外拘束的结构安全评价新方法。本项目采用数值计算与模拟、断裂韧性试验和理论分析相结合的方法，主要围绕纳入面内与面外统一裂尖拘束的结构安全评价方法的相关理论和技术基础开展研究。研究内容和取得的主要成果包括：探明了面内与面外统一裂尖拘束参数Ap的宽范围适用性，建立了Ap与材料延性断裂韧性、脆性断裂韧性及焊接接头断裂韧性的统一关联。建立了基于Ap参数的含裂纹管道的脆性断裂评定方法。为便于工程计算和应用，提出了一个基于裂尖张开位移(CTOD)的面内与面外统一拘束参数Ad。建立了基于J-Ap和J-Ad两参数纳入面内与面外拘束的含裂纹管道的延性断裂评定的裂纹扩展力(CDF)法和失效评定图（FAD）法。探明了面内/面外几何拘束和材料拘束对焊接接头裂纹延性断裂阻力的影响及机理。发展了纳入几何和材料拘束的焊接接头结构的断裂预测和安全评价方法。探明了承压管道不同类型表面裂纹尺寸对蠕变统一拘束参数Ac和断裂参数C*的影响规律及材料蠕变性能对管道裂纹参数Ac的影响规律。建立了不同几何试样蠕变拘束与管道裂纹拘束的关联。初步建立了基于C*-Ac二参数纳入面内/面外统一拘束的承压管道蠕变裂纹扩展寿命评价的方法。这些成果对发展纳入面内/面外拘束效应的二参数断裂力学理论具有重要的学术意义；对发展纳入面内与面外统一拘束的结构安全评价的技术方法和标准规范奠定了坚实的技术基础，因而具有很重要的实际意义。