【作者】 高明；

【导师】 刘刚； 黄一；

【作者基本信息】 大连理工大学 ， 船舶与海洋结构物设计制造， 2011， 硕士

【摘要】 船舶与海洋平台属于大型复杂焊接钢结构。由于焊接结构的焊趾位置存在很高的非线性分布应力,焊趾处表面裂纹尖端附近的应力场必然受到影响。同时,由于非线性应力峰值的存在使得裂纹表面端点处的应力增大,加速表面裂纹沿着裂纹长度方向的扩展,改变裂纹扩展形状。裂纹尖端附近应力场的变化和裂纹形状的改变将影响表面裂纹的应力强度因子的大小。应力强度因子和裂纹形状的改变又将影响裂纹的扩展寿命。因此,讨论非线性分布应力对表面裂纹应力强度因子、裂纹扩展形状以及裂纹扩展寿命的影响对于准确计算焊接结构疲劳寿命具有重要意义。本文对对接接头焊趾处表面裂纹的裂纹扩展行为进行了大量有限元分析。分析中假设不存在焊接残余应力、气孔、未焊透、位错、咬边等焊接缺陷,而着重考察非线性分布应力对焊趾处表面裂纹扩展的影响。本文主要完成了以下三个方面的工作：1.分析非线性分布应力对表面裂纹应力强度因子的影响,以修正系数MK的形式予以体现,并考察了焊缝几何尺寸、表面裂纹形状和载荷形式对修正系数的影响。对修正系数的有限元分析结果进行了公式拟合。2.基于Newman-Raju提出的表面裂纹半椭圆扩展假设,研究了非线性分布应力对焊趾处表面裂纹扩展形状的影响。3.研究了非线性分布应力对焊趾处表面裂纹扩展寿命的影响,并从裂纹尖端附近应力场的变化和裂纹扩展形状的改变两个方面分析了非线性分布应力影响焊趾处表面裂纹扩展寿命的原因。分析表明：非线性分布应力对较小表面裂纹应力强度因子的影响显著,对深裂纹应力强度因子影响相对较小；本文中应力强度因子非线性分布应力修正系数拟合公式相对于有限元计算具有很高的精度；非线性分布应力的存在将影响表面裂纹在扩展过程中的形状改变,使得表面裂纹向浅长型发展；非线性分布应力的存在将大幅降低对接焊结构的表面裂纹扩展寿命；非线性分布应力对疲劳裂纹扩展寿命的影响可以分为裂尖附近应力场改变影响和裂纹形状改变影响两个方面,裂纹较浅时以应力场改变影响为主,裂纹较深时以形状改变影响为主。更多还原

【Abstract】 Ships and offshore platforms are large and complex welded steel structures. The weld toe leads to high nonlinear stress distributed across plate thickness, which affects the stress field around surface crack tip. Meanwhile, the stress becomes larger at intersection points of crack surface and free surface of the plate because of nonlinear stress peak, which accelerates the growth of the crack along length direction. Furthermore, the changes of the stress field and crack shape during propagation influence the magnitude of crack-tip stress intensity factors. Variations of both the crack shape and the stress intensity factor result in alteration of crack propagation life. So it is important to study the effects of the nonlinear distribution stress on the stress intensity factor, crack shape, and crack propagation life for more accurate fatigue life estimation of welded structures.Finite element method is used to analyze the surface crack growth at butt welded joints. The effects of residual stresses and welded defects such as slag inclusions, gas pores, misalignment, incomplete penetration at weld root are not considered. The thesis emphasizes on the effects of the nonlinear distribution stress on surface crack and the main works included are as follows:1. A correction factor of the stress intensity factor Mk is introduced to describe the effect of the nonlinear distribution stress. And the geometry parameters of weld toe, aspect ratio a/c and combined load conditions are considered. Numerical fitting is adopted on the formulas.2. The effect of the nonlinear distribution stress on the surface crack shape is discussed based on assumption of semi-ellipse proposed by Newman and Raju.3. The effect of the nonlinear distribution stress on crack propagation life is studied considering the variations of stress field around the surface crack and crack shape.It is found that the influence of the nonlinear distribution stress is significant on the stress intensity factor of small surface crack and, to a less degree, on that of relatively lager crack. The fitting formulas of stress intensity factors of nonlinear distribution stress are proved precise according to finite element analysis. The surface crack shape change is influenced by the nonlinear distribution stress and tends to be slim. The crack propagation life of the surface crack at weld toe is much lower than that in plate. The effect of the nonlinear stress on crack propagation life is considered by the variation of both the stress field around the crack and the crack shape. The results show that the change of crack-tip stress is the main reason for the decrease of the fatigue life for the small crack, but the alteration of crack shape is the main account for the big crack.更多还原