基于分形理论的复杂应力状态下高温低周疲劳表面短裂纹行为研究

Study on Behavior of Surface Short Cracks for Low Cycle at High Temperature and Complex Stress State Based on Fractal Theory

含有裂纹的固体从物理角度看实际上是一个非线性耗散系统,可以用分形理论加以描述。疲劳短裂纹的扩展路径可视为分形曲线,裂纹扩展路径的不规则性可用分形维数加以描述。裂纹扩展的分形维数中隐含着诸多的物理因素,如材料微观组织结构、加载条件等。以复杂应力状态下高温低周疲劳表面短裂纹为研究对象,对20钢进行高温低周疲劳试验,观测试样表面疲劳短裂纹的萌生扩展及合体的演化过程,对试验结果进行分形分析,得到裂纹分形维数随循环过程的演化特征:疲劳短裂纹的萌生扩展行为具有分形特征,分形维数随着疲劳进程的发展而稳定地成比例增加;分形维数不产生缺口依存性,可适用于各种不同应力比的复杂应力状态短裂纹的表征;分形维数可作为把握材料总体损伤状态的参数,为材料短裂纹阶段的寿命预测提供依据。

From the physical point of view,the solid containing cracks is actually a non-linear dissipation system,so it can be described by fractal theory.The propagation of short fatigue crack is a fractal curve,the irregular path can be described by fractal dimension.The fractal dimension of the crack implicates many physical factors,such as microstructure of material and loading conditions.The short crack for low cycle at high temperature is studied.The material 20 steel in various stress state is experimentally studied,the evolution process of initiation,propagation and coalescence of short cracks on the specimen surface are observed.By analyzing experiment results,characters of crack fractal dimensions evolving with the cycle process are as follows,the behavior of short fatigue cracks satisfies fractal feature,with the development of the fatigue process,the fractal dimension is increased.