基于裂尖临界距离处累积塑性应变的疲劳裂纹扩展准则的高分辨率原位试验研究

Mechanisms of fatigue crack propagation as well as crack growth criteria have been and continue as one of the key and difficult topics in the field of fatigue and fracture, and provide the theoretical basis for fatigue design of structural component under fatigue loading. A hypothesis that ratchetting strain could serve as a driving force for fatigue crack growth and a fatigue crack growth criterion based on a critical distance and its accumulated plastic strain will be investigated. Influences of such factors as material microstructure, loading conditions and crack closure on the distribution and evolution of strain fields in the plastic zone ahead of the crack tip will be comprehensively discussed, using digital image correlation technique and via high-resolution in situ experiments of fatigue crack growth in low carbon stainless steel specimens, in order to find a scientific explanation for the microscopic mechanisms of fatigue crack growth driven by strain ratchetting in front of the crack tip. Interactions between microstructure, strain fields, crack path and crack growth rate will be examined, to define the critical distance and the critical value of the accumulated plastic strain, i.e. the only two physical quantities with the crack growth criterion. The evolution of the ratchetting strain at the critical distance will be investigated, to assist in establishing a fatigue crack growth rate model. Finite element (FE) analysis will be thereby carried out based on the criterion, aiming to validate the criterion and prove its reliability for FE studies. This proposal reflects an interdisciplinary study of Mechanics and Material Science. It is expected to elucidate some characteristics of the evolution of strain ratchetting and the intrinsic relationship between strain ratchetting and crack growth, and to provide a physical-based criterion for numerical simulation of fatigue crack growth.

疲劳裂纹扩展机理和准则的研究是疲劳断裂力学界研究的重点和难点之一，是疲劳构件设计的理论依据。本项目针对以棘轮应变作为裂纹扩展驱动力和基于临界距离处累积塑性应变的裂纹扩展准则开展研究，通过室温低碳不锈钢试件的高分辨率原位疲劳裂纹扩展试验和数字图像相关技术，研究材料微观组织结构、载荷条件和裂纹闭合等因素对裂尖前端塑性区内应变场分布和演化规律的影响，实现裂尖棘轮应变驱动疲劳裂纹扩展微观机理的科学解释；研究材料微观组织结构、应变场、裂纹扩展路径、裂纹扩展速率之间的关系，确定扩展准则中临界距离和累积塑性应变临界值，基于临界距离处棘轮应变随裂纹扩展的演化规律，建立基于棘轮应变的裂纹扩展速率模型；开展疲劳裂纹扩展的有限元模拟，检验裂纹扩展准则的适用性。本项目内容体现了力学和材料学科领域的交叉，可望阐明疲劳裂尖棘轮应变的演化规律及其与裂纹扩展的内在关联，为疲劳裂纹扩展数值模拟提供相应的物理判据。

面对现代工业对承受周期载荷的材料和结构的设计要求，理解和掌握材料中疲劳裂纹的细微观尺度下的扩展机理，已成为迫切需要解决的问题。项目围绕疲劳裂纹的扩展、裂尖应变场和位移场的分布和演变规律，采用试验研究来尝试解决疲劳裂纹扩展的微观机理和准则。项目采用数字图像相关技术开展了塑性和脆性材料的疲劳裂纹扩展试验研究工作，即低碳不锈钢小尺寸试件的高分辨率原位疲劳试验，和含预制裂纹岩石试件的原位疲劳试验。对于低碳不锈钢高分辨率原位疲劳试验，首先对小尺寸试件的材料组织表征和强度进行了评价分析试验，为后续疲劳试验提供参考数据；进一步地，开展了电子扫描电镜下高分辨率原位疲劳试验，重点研究了荷载大小、过载、加载频率和试件尺寸等因素的影响。对于含预制裂纹的岩石试件的原位疲劳试验，首先开展了准静态岩石的强度评价分析试验，进一步地，开展了疲劳载荷下岩石试件的裂纹扩展原位试验，重点研究了裂纹的扩展形貌、应变场和位移场的分布，分析了载荷大小、试件尺寸、加载频率和不同岩石材质等因素的影响。通过这些试验，初步形成了对塑性低碳钢材料和脆性岩石材料中疲劳裂纹扩展的机理有了初步的科学认识。.本项目研究取得的成果有：已发表标注基金号的学术论文5篇，另有1篇再审，2篇论文待发表。已发表论文中，被SCI收录3篇，EI收录1篇。项目共培养2名研究生，其中已经于2019年毕业1名。