基于宏/微观作用机制的缩松缺陷区间离散表征方法及其应用

The shrinkage porosity defects, which always existed in large castings, have some characteristics such as high inhomogeneous and localization. Due to difficulties in integrating the macro/micro scale effects, the single factor analysis method, represented as pore size, is insufficient in the aspect of evaluation's validity and model's applicability. By carrying out the fatigue experiments of specimens containing different extent of shrinkage porosity, the project intends to study the influence of shrinkage porosity on fatigue performances by the combination methods of macro and micro scale. Multi macro/micro scale factors, such as region sizes, void ratio, curvature, pore diameter, dendrite arm spacing et al., are applied to describe the porosity shapes and other inhomogeneous and localization properties. Critical control factors will be determined, and their relationships between stress field or fatigue parameters are obtained, which is helpful for revealing the integrated action mechanism of macro and micro factors on the fatigue failure of defected castings. The step interval discrete quantitative algorithm is performed to divide the tested data into numerical interval for each critical factor,an arbitrarily porosity then could be described by the form of several interval sets, the life distribution graph of each numerical interval then will be constructed. Combining with probability statistics analysis and grey correlation methodology, the mapping matrix between discreted interval multi-factors sets and life evaluation sets, and effective weights of each factor will be constructed respectively. Finally, a new multilevel comprehensive evaluation algorithm considering the macro and micro effects of shrinkage porosity will be established for the life prediction of defected castings, and the critical tolerance of defects and corresponding interval sets would be calculated. The criterion of acceptable tolerances and more reasonable lifetime prediction method for large defects castings will be provided.

缩松缺陷经常存在于大型铸件中，具有高度的非均匀性和局部化特点。目前以孔洞尺寸为代表的单一因素分析方法，难以有效融合宏/微观尺度效应，在评估的有效性和模型的适用性上存在不足。项目通过含不同程度缩松缺陷的疲劳试验，采用宏、微观尺度相结合的思想，基于离散方法量化缩松缺陷对疲劳性能的影响。对缩松区域引入宏/微观尺度的多因素表征手段，包括形状尺寸、孔洞率、曲率、微孔洞尺寸、枝晶间距等。找出关键控制因素并确定与应力应变和疲劳参数的依赖关系，揭示缩松缺陷对疲劳失效的宏/微作用机制。对关键因素采用阶梯式区间离散的量化表征方法，画出各表征区间的寿命分布图谱，结合概率统计和灰色关联分析，分析离散表征区间与寿命评价集合之间的映射关系和影响权重。建立任意缩松缺陷对寿命影响的多因素综合评价算法，确定含缺陷铸件的剩余寿命区间和可容缩松程度的多因素临界区间集合表征。为大型铸件中缩松缺陷的定容和定寿提供科学依据和方法。

铸件中缩松缺陷具有高度的非均匀性和局部随机性，在形貌上表现为一种类似随机“云态”分布的特性，其实际的面积、形状、位置、模量等均为随机量。项目基于宏/微观相结合的思想，采用区间离散表征方法，研究了球墨铸铁材料中的随机缩松缺陷对铸件力学性能的影响。.（1）缩松缺陷铸件裂纹扩展速率的概率模型和剩余寿命可靠性分析：通过裂纹扩展试验得到了QT400-18材料稳定扩展速率公式中相关材料参数均服从对数正态分布，建立了QT400-18的裂纹扩展寿命概率模型。反推得到了材料的启裂门槛值和断裂韧性的分布形式及其估计。建立了不同初始裂纹下铸件指定寿命要求下的概率-寿命关系图谱。.（2）缩松缺陷对铸件疲劳寿命的影响因素分析和寿命模型：研究了含有不同缩松程度下的QT400-18球墨铸铁试样的疲劳寿命，失效横截面上的缩松程度区间为[0.67%, 5.91%]，对应的疲劳寿命散布的区间数量级从103到106。SEM分析表明，晶间不连续、微裂纹和夹杂是影响疲劳滑移和阻碍过程的主要因素，缩松区域的滑移带变得不规则和不连续。以ASIF为等效中间变量，将缩松缺陷等效为当量初始裂纹，建立了含缩松缺陷的试样的疲劳寿命预测模型。.（3）缩松缺陷随机性的区间离散化方法及对铸件有效力学性能影响：提出了改进的格形模型方法实现了缩松缺陷随机面积、形状、位置的模拟。通过该方法构造出大量的数值样本，能够覆盖铸件中出现的各种可能的随机“云态”形貌。通过统计分析，获得了随机缩松对铸件整体有效弹性模量和局部应力强度因子的影响趋势。（1）当缩松面积达到1.5%时，铸件的有效模量出现明显的下降；（2）缩松缺陷造成的应力强度因子大约在2.0；（3）垂直于加载方向的缩松尺寸对有效模量的影响更加显著；（4）相比其他位置的缩松，表面缩松缺陷要削弱铸件的有效弹性模量约1.5%。.项目提出的基于宏/微观机制结合概率模型的研究方法，获得了缩松缺陷劣化铸件性能的定量影响，为大型铸件中缩松缺陷的定容和定寿提供了科学依据和方法。