Study on the Detection of Very Small Fatigue Crack and the Evaluatum of Fracture in the Welded Structures

焊接零件的完整性通常由工人的眼睛检查。对于小于几百毫米的疲劳小裂纹，很难用肉眼检测出来。疲劳裂纹的存在会引起局部腐蚀、异常振动、漏水等，最终导致焊接结构的灾难性失效。为了保证结构的完整性，疲劳裂纹必须在裂纹扩展的早期阶段识别出来，并在维修中进行修复，本文研究了一种利用疲劳裂纹的监测结果来检测和评估微裂纹的方法。研究了疲劳裂纹扩展过程中裂纹尖端拉、压塑性区的形成。通过等幅加载、块幅加载和尖峰幅加载下的疲劳裂纹扩展试验以及K-试验，研究了表征裂纹尖端附近载荷-应变关系的迟滞环各区对疲劳裂纹扩展的作用。验证了所提出的监测方法的有效性，阐明了裂纹尖端滞后环对疲劳裂纹扩展的影响。

Integrity of welding parts is conventionally inspected by worker's eyes. It is very difficult to detect small fatigue crack less than several hundred millimeters by visual inspection. Existence of fatigue crack induces local corrosion, abnormal vibration, water leakage, etc. and finally fatigue crack propagation leads to catastrophic failure in welded structures. To secure the structural integrity, fatigue crack must be identified in early stage of crack propagation and must be repaired in maintenance.The present report investigates a method to detection of the infinitesimal fatigue damage and evaluation using monitoring results in wrap-around welded joint and toe along weld bead.And this report researches on the generation of the tensile and compressive plastic zones near the crack tip during fatigue crack propagation. And the role of each zone of the hysteresis loop denoting the relationship between loads and strains near the crack tip on fatigue propagation are studied by the fatigue crack propagation tests in the cases of constant amplitude loading, block amplitude loading and spike amplitude loading and the K& test.As a result of this study, I verified the validity of the proposed monitoring method and clarified the effects of hysteresis loop near crack tip to fatigue crack propagation.