【作者】 杨杰；

【导师】 王国珍；

【作者基本信息】 华东理工大学 ， 动力工程及工程热物理， 2014， 博士

【摘要】 裂尖拘束对材料与结构的断裂行为及韧性具有显著的影响,在结构完整性评定中,需要准确地纳入这种影响。拘束分为面内拘束和面外拘束,对二者各自己有一些表征参数,但对于实际结构中的三维(3D)裂纹,面内与面外拘束同时存在,如何用一个参数统一地定量表征这种复合拘束,并建立其与材料及焊接接头断裂韧性的统一关联,是结构完整性领域需要研究的重要基础性问题之一。本文通过数值计算、实验和理论分析相结合的方法,主要研究面内与面外裂尖拘束的统一表征参数及其与材料和焊接接头断裂韧性的统一关联,并初步探索了统一拘束参数在弹塑性结构完整性评定中的应用。研究主要结论如下：(1)基于裂尖前等效塑性应变(εp)等值线所围区域的面积,定义了一个新的面内与面外统一的拘束表征参数Ap。研究发现不同面内/面外拘束及其复合条件下材料的无量纲断裂韧性Jc/Jref与统一拘束参数√Ap之间存在线性关系。Jc/Jref-√Ap关联线与所选择的εp等值线无关,只取决于材料,它统一地表征了不同面内/面外裂尖拘束状态对材料断裂韧性的影响。(2)进一步研究发现,拘束参数Ap也可以统一地表征面内/面外几何拘束和焊接接头中的材料拘束。基于不同面内/面外/材料拘束条件下的断裂实验和Ap参数的计算,建立了核电异种金属焊接接头薄弱界面区裂纹的Jc/Jre(?)Ap统一关联线,它统一地表征了不同面内/面外拘束和材料拘束的复合作用对接头断裂韧性的影响。(3)通过微观实验观察揭示了面内/面外拘束对核电异种金属焊接接头断裂韧性和裂纹扩展阻力影响的机理。发现随面内拘束和面外拘束的增加,异种金属焊接接头中裂纹的断裂模式从延性断裂经由延性/脆性混合断裂转变为脆性断裂,相应的断裂韧性和裂纹扩展阻力曲线(J-R曲线)降低,裂纹扩展路径一般偏向于强度较低的材料一侧。(4)研究表明,面内拘束与面外拘束存在交互作用,试样中高的面内拘束增强了面外拘束效应,低的面内拘束对面外拘束不敏感。基于GTN模型的有限元数值模拟法可以用来计算获得不同面内、面外拘束条件下材料的J-R曲线和延性断裂韧性,从而可通过少量的实验结合数值模拟的方法来获得材料延性断裂时的Jc/Jref-(?)Ap关联线。(5)初步探索了统一拘束参数Ap在结构完整性评定中的应用。发现J-Ap复合参数J/Jref*(?)Ap)可以较准确地预测结构中3D裂纹前沿的起裂位置。用数值计算方法初步分析了J-√Ap双参数与材料Jc/Jre(?)Ap统一关联线相结合预测不同拘束状态的管道半椭圆3D表面裂纹断裂韧性与起裂载荷的方法。更多还原

【Abstract】 As constraint significantly affect the fracture behavior and fracture toughness of materials and structures, it needs to accurately consider the constraint effect in the integrity assessment of the structures. Constraint can be divided into two types, i.e. in-plane and out-of-plane, and each of them has its own characterization parameter. However, there exists a compound of both in-plane and out-of-plane constraints for three-dimensional (3D) crack in actual engineering structures. How to quantify both types of constraints by using a unified constraint parameter and to establish its correlation with fracture toughness of materials and welded joints is one of the most important foundational problems in the structural integrity area.In this dissertation, by using the combined approaches of the numerical simulations, experiments and theoretical analyses, the unified characterization parameter of in-plane and out-of-plane constraint and its correlation with fracture toughness of materials and welded joints was investigated. The application of the unified constraint parameter in the elastic-plastic structural integrity assessment containing the constraint effect was preliminarily analyzed. The main work and results obtained are as following:(1) Based on the area surrounded by the ep isoline ahead of crack tips, a unified constraint parameter Ap which can characterize both in-plane and out-of-plane constraints is defined. There exists a sole linear relation between the normalized fracture toughness Jc/Jref and(?) regardless of in-plane or out-of-plane constraint or both. The J(?)line is independent on the selection of the εp isolines, and only dependent on material. It characterizes the effect of different in-plane and out-of-plane constraints on fracture toughness of a material.(2) Further studies show that, the Ap is also a unified constraint parameter which can characterize both geometry constraint (in-plane and out-of-constraint) and material constraint in the welded joints. Based on the fracture tests under different in-plane/out-of-plane and material constraints and the calculation of Ap, the unified J(?) lines of interface cracks located at the weakest region in the dissimilar metal welded joint are established. It characterizes the combining effect of in-plane/out-of-plane and material constraints on fracture toughness of welded joint. (3) The mechanism of both in-plane and out-of-plane constraints effects on fracture toughness and fracture rsistance of a dissimilar metal welded joint in nuclear power plant is investigated by fracture tests and microscopic observations. It is found that with increasing in-plane and out-of-plane constraints, the fracture mode of the dissimilar metal welded joint changes from ductile fracture through mixed ductile and brittle fracture to brittle fracture, and the corresponding crack growth resistance (J-R curve) decreases. The crack growth path generally towards the material sides with lower strength.(4) There exists interaction between the in-plane constraint and the out-of-plane constraint. The higher in-plane constraint strengthens the out-of-plane constraint effect, while the lower in-plane constraint is not sensitive to the out-of-plane constraint. The finite element simulations based on the GTN damage model can be used to obtain the J-R curve and ductile fracture toughness of a material under different in-plane and out-of-plane constraints. Thus the Jc/Jref-(?) line of a material can be obtained by a small number of tests combined with the numerical simulations based on GTN damage model.(5) A preliminary research in the application of the unified constraint parameter Ap in the structural integrity assessment is carried out, and it is found that the parameter (?) which is a compound of J and Ap can predict the fracture location along a3D crack front in the structures. A method combining the J-(?) two parameter with the J(?) line of a material for predicting the fracture toughness and failure load of3D circumferential surface cracks in pipes are preliminarily studied by finite element calculations.更多还原