基于奇异强度理论的船体焊接接头应力场和疲劳强度评估

Fatigue strength of ship structure plays a fundamental role in ship safety and vitality. In ship and ocean engineering, the most common welded joint of ship structure consists of plates, stiffeners and brackets. Geometrical discontinuities in welded components and structures provoke, as is well known, more or less localized perturbations of stress field. Stress concentrations at these corners cause cracks initiate and propagate under cyclic loadings. Therefore, knowledge of such stress distributions in the neighborhood of stress raisers is frequently required for fatigue strength assessment of notch specimens. The traditional method, also termed nominal stress method, is based on the design nominal stress, without taking into account explicitly the stress discontinuity due to the presence of the joint. Hot spot stress method is suggested for evaluating complex welded joints. However, regarding the definition and the solution of HSS, many proposals given are based on the surface extrapolation methods, which are easy to be affected by element and mesh size. Therefore, based on the singularity strength theory, this subject proposes a simple method using the singularity strength value ‘as’ to estimate the stress field distribution at the corner. Meanwhile, on the basis of singularity strength ‘as’, K-N method based on the parameter N-SIF and W-N method based on the parameter strain energy density are proposed to assess fatigue strength of typical ship welded joints. The results can provide a more advanced theory and simple method to accurately estimate the fatigue strength of ship welded joints.

船舶结构的疲劳强度对船舶的安全性和生命力有着十分重要的意义。在船舶与海洋工程领域，板、加强筋和肘板组成了船体最为常见的焊接接头。焊接构件和结构在几何上的不连续性会造成局部应力场的激增，拐角处的应力集中，形成了应力奇异效应，在交变应力下，会引起裂纹的萌生和扩展。了解焊接件切口处应力场分布情况，是分析切口件疲劳强度的前提。由于传统的名义应力方法没有考虑节点位置应力的不连续性，对于复杂焊接接头需要采用热点应力方法评估。然而，对于热点应力的定义和求解，多基于表面外插值的方法，结果受到单元类型和网格的影响。因此，本项目基于切口应力强度理论，引入了奇异强度值的概念，给出了预估船体焊接接头应力场的简易公式；同时，基于N-SIF和应变能密度概念提出评估切口件疲劳强度的K-N方法和W-N方法，将切口强度理论运用到船体典型节点的疲劳强度评估之中，为准确预估船体节点的疲劳强度提供更为先进的理论依据及简便方法。

对于复杂焊接接头，特别是含切口的焊接件，其应力场分析是十分复杂的过程，通常需要借助有限元方法进行拟合，但是有限元方法同样受到单元类型和单元尺寸的影响，不利于工程实际的运用。本课题在申请者原有研究成果和借鉴已有研究成果的基础上，引入了奇异强度值的概念，同时从理论上、数值上及试验上开展了对90°和135°复杂焊接接头的应力场和疲劳强度的研究，提出了一种评估船体复杂焊接接头应力场和疲劳强度的简易公式，在此基础上提出了相应的疲劳评估方法。最后，将2D平面应力场分布公式拓展到3D模型的应力场评估中，结合海洋工程中的管盘模型，得到了弯扭载荷下的切口应力场评估公式，并将切口应力法运用到复杂的多轴疲劳评估之中。本研究的研究方案为复杂焊接接头的应力场和疲劳强度评估提供了新的思路。