摘要
为减轻结构重量并保证结构的承载能力，高强钢以其高的强度和良好的塑韧性广泛应用于桥梁、建筑、航空航天等领域。焊接作为一种高效的连接方法，广泛应用于工程结构领域。由于焊接过程的特殊性，使得焊接缺陷不可避免，导致焊接接头容易产生低应力脆断破坏，显著降低了接头的承载能力，给焊接接头设计带来一定的局限性。为提高易产生缺陷的焊接接头的承载能力，本文从断裂力学角度出发，提出了基于应力强度因子(K因子)的焊接接头等承载设计思想，并给出了接头含不同缺陷时的等承载设计准则，使用该准则可以设计出承载能力与母材相当的焊接接头。
对于焊缝和母材区都含缺陷的接头而言，提出了焊缝和母材区的K因子比值与焊缝金属和母材金属的断裂韧度比值相等时即可满足等承载要求的设计思想；对于焊缝区存在缺陷而母材区无缺陷的接头而言，提出了焊缝区裂纹失稳扩展的临界应力与母材的抗拉强度相等时即可满足等承载要求的设计思想；对于焊缝区无缺陷而母材区存在缺陷的接头而言，提出了母材区裂纹失稳扩展的临界应力与焊缝区峰值应力相等且不高于焊缝金属的抗拉强度时即可满足等承载要求的设计思想；本文主要针对前两种接头的等承载设计进行讨论和研究。
本文研究了接头形状参数对含中心裂纹和单边裂纹对接接头形状因子的影响规律，研究表明，使用不考虑余高影响的传统的形状因子表达式计算含缺陷接头的形状因子是不合理的。提出了一种获得含裂纹复杂结构形状因子表达式的新方法，即首先采用有限元法研究结构各形状参数对其形状因子的影响规律，再引入一个能够统一各形状参数对形状因子影响规律的几何不连续形状协调参数，最后对具有不同形状参数结构的有限元结果以形状协调参数为自变量进行拟合，获得在传统形状因子表达式基础上的修正系数方程，进而获得较传统表达式更准确的形状因子表达式。本文利用该方法获得了考虑余高形状影响时的含中心裂纹和单边裂纹对接接头受单向拉伸载荷作用时的形状因子表达式。本文给出表达式能够更有效的指导含中心裂纹和单边裂纹对接接头的形状设计，并可获得相应的应力强度因子、临界应力、临界裂纹尺寸以及剩余寿命，以便对其进行安全性评估。
运用有限元法获得含缺陷复杂结构形状因子和K因子，需进行有限元建模、边界条件和材料属性的定义、加载计算以及数据处理等一系列步骤，过程繁琐。为提高含缺陷复杂形状接头K因子的计算效率和准确度，本文开发了含缺陷焊接接头K因子计算系统，对数据进行回归分析，可获得具有一定接头形状的含缺陷接头的形状因子和K因子表达式。本文开发了基于断裂参量K因子的焊接接头等承载设计系统，可确定等承载接头的形状参数，并能计算服役的含缺陷等承载接头的K因子、裂纹尖端张开位移、J积分以及接头剩余寿命，有利于对服役的等承载接头进行安全评估，进一步推广了等承载设计思想及应用。
对焊缝和母材都含缺陷等承载接头进行了实验研究，拉伸和疲劳试验结果表明，对于按照等承载接头形状参数设计的焊缝和母材都含中心裂纹或者单边裂纹的对接接头而言，即使焊缝区裂纹尺寸大于母材区裂纹尺寸，所有试件均断于含裂纹母材处，满足焊缝不先于母材破坏的等承载要求；对焊缝含缺陷母材无缺陷等承载接头进行了实验研究，对于按照等承载接头形状参数设计的焊缝含中心裂纹或者单边裂纹母材无缺陷的对接接头而言，所有拉伸试件均断于焊趾附近的母材处，且抗断应力达到母材的抗拉强度，满足等承载要求。所有疲劳试件均断于含裂纹焊缝处，其疲劳寿命远高于含相同缺陷承受相同疲劳载荷的普通接头的疲劳寿命。
基于断裂参量K因子的含缺陷等承载接头的危险截面从焊缝区域向母材区域转移，保证焊缝区不先于母材区破坏，接头承载能力能够达到母材的承载能力。焊接接头K因子计算系统能够计算含缺陷接头的K因子，对所得数据进行处理并回归分析后，可获得含缺陷接头K因子表达式。等承载接头设计专家系统能够获得等承载接头所需的形状参数。
In order to reduce weight and ensure the load carrying capacity of structure,high-strength steel is commonly used in bridges, building, aerospace fields andso on due to its high strength and good ductility. As an efficient joining process,welding has been widely used in engineering field. Weld defects can not beenavoided because of the welding particularity. It has been found that weld defectscan significantly deteriorate the load carrying capacity of joint. Typical brittlefracture is often observed in the defect-existent joint. In order to increase loadcarrying capacity of defect-existent joint, equal load carrying capacity (ELCC)method based on stress intensity factor is proposed by using fracture mechanicstheory. ELCC design criteria of joint with different defects are given.
In order to guarantee the load-carrying capacity of under-matched joint withdefect reach that of the base metal, four different conditions have beenconsidered. The influences of joint shape parameters on shape factor ofcenter-cracked or edge-cracked butt joint are studied. The results showed that itis unreasonable to calculate the shape factor of center-cracked or edge-crackedbutt joint without considering weld reinforcement. A new approach to obtainingshape factor expression for defect-existent complicated structures is introduced.Firstly, the influences of structural shape parameters on shape factor should beinvestigated by using finite element method. Secondly, a coordinated parameterwhich can unify all influences of shape parameters on shape factor must beintroduced. Then a modified coefficient function on the basis of the traditionalshape factor expression can be obtained with regression analysis of all finiteelement results by using the coordinated parameter as the independent variable.Finally, the accurate shape factor expression considering the effect of allstructural shape parameters can be obtained. The accurate shape factorexpressions for center-cracked and edge-cracked butt joint under tensile load aregiven in this paper. The shape factor expressions given in this paper canefficiently guide defect-existent butt joint design. To conduct safety assessment,the corresponding stress intensity factor, critical stress, critical crack length andfatigue remaining life can also be obtained by using the shape factor expressions proposed in this paper.
The procedures of obtaining shape factor and stress intensity factor fordefect-existent complicated structures by using finite element method are asfollowing, establishing finite element model, defining material parameters andboundary conditions, submitting job and processing data. The procedure is toocomplicated and error-prone. In order to improve the efficiency and accuracy ofcalculating the shape factor and stress intensity factor for defect-existentcomplicated structures, the expert system of calculating stress intensity factor fordefect-existent joint is developed. The expert system of ELCC joint design whichcan provide the ELCC joint shape parameters is also developed. This expertsystem can also give the stress intensity factor, crack tip opening displacement,J-integral and fatigue remaining life of existing ELCC joint.
Tensile experiment and fatigue experiment of joint with center crack or edgecrack both in weld and base metal designed according to ELCC joint shapeparameters were tested. Although the crack length of weld metal is larger thanthat of the base metal, all the samples were fractured at the cracks in base metalzone which can satisfy the ELCC condition. Tensile experiment and fatigueexperiment of joint with center crack or edge crack only in weld metal designedaccording to ELCC joint shape parameters were also tested. All the tensilesamples were fractured at the base metal zone which is near to the weld toe, andtheir breaking stress can reach the tensile strength of base metal. This can satisfythe static ELCC condition. All the fatigue samples were fractured at the cracks inweld metal zone, but their fatigue remaining life were much higher than those ofthe normal joint with same crack.
The dangerous section of joint can be transferred from weld zone to basemetal zone after ELCC joint design. ELCC joint with defect would fracture at thebase metal zone and its load carrying capacity can reach that of the base metal.The stress intensity factor calculating system for defect-existent joint can be usedto efficiently calculate the stress intensity factor. The ELCC joint design systemcan easily provide the corresponding ELCC joint shape parameters.
引文
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