高温蠕变条件下表面裂纹扩展行为分析

Analysis of Surface Creep Crack Growth Behavior under High Temperature Creep Condition

裂纹的存在及其扩展严重限制着高温承压部件的使用寿命,而目前对结构三维蠕变裂纹扩展行为的认识及其预测仍比较有限。以典型耐热钢P91为例,在650℃下对其开展了不同应力和不同初始裂纹尺寸的表面裂纹蠕变扩展试验,通过断面观察获得了蠕变裂纹扩展形貌演化,并采用基于多轴蠕变损伤模型的有限元法、基于蠕变裂纹尖端参量C^(*)的有限元法以及英国标准BS7910推荐的简化计算方法进行表面裂纹蠕变扩展分析。结果表明,基于多轴蠕变损伤模型的有限元法能够合理地描述裂纹最终轮廓并准确地预测裂纹扩展时间。相比之下,基于C^(*)的有限元法和BS7910简化计算方法计算的裂纹形貌预测有明显偏差,并且较依赖于紧凑拉伸试样的蠕变裂纹扩展试验数据及其拟合的参数C和q,易导致过于保守的裂纹扩展时间预测。评估了不同计算方法对于表面裂纹蠕变扩展行为的预测能力。

The service life of high temperature pressurized equipment is remarkably limited by the existence and propagation of a crack. Unfortunately, the understanding and prediction on the three-dimensional creep crack growth(CCG) behavior of a structure are still very limited. For a typical heat-resistant steel, P91, creep tests under loads of various magnitudes are carried out at 650 ℃ using a series of tensile specimens containing elaborately prepared surface cracks with various initial length. The surface crack shape evolution is obtained by observing the fracture surfaces. Thereafter, the finite element(FE) analyses of CCG are carried out using a multiaxial creep-damage model. For the purpose of comparison, an FE method on the basis of the C^(*) parameter and a CCG calculation procedure recommended by the UK standard, BS7910, are also employed. It is found that the finite element solution based on the multiaxial creep-damage model can reasonably describe the final crack profile and show excellent capability in predicting the crack propagation time. In contrast, the FE method using the C^(*) and the calculation method from BS7910 result in significant deviation in the prediction of the crack shape. In addition, they are relatively dependent on the CCG test data for the compact tension specimen and its fitting parameters, C and q, and may lead to overly conservative predictions of the crack propagation time. The predictive capabilities of different calculation methods on the behavior of surface crack growth under creep condition were examined.