Quantitative Evaluation for Defects in Welded Joints

(1)建立了超声脉冲回波检测的数值模拟方法，为钢结构裂纹缺陷的无损检测提供了一种强有力的手段。通过这一模拟，详细研究了孔洞和缝隙处的绕射和反射波。采用声发射方法对CCT试件和角焊缝试件进行了疲劳试验，观察了裂纹萌生阶段和扩展阶段的行为。结果表明，即使是焊接接头，材料也存在裂纹萌生阶段，并在此阶段产生了较强的声发射信号。(3)对含下钻的角焊接头试件进行了疲劳试验，并对试件的尺寸进行了详细测量。研究了下钻尺寸和形状对疲劳强度的影响。此外，还对含下钻的模型进行了有限元分析，计算出下钻底部的应力集中。(4)针对裂纹从焊接根部萌生的情况，采用试验和分析相结合的方法，对承载角焊接头的疲劳强度进行了研究。焊缝根部非穿透段长度对疲劳强度有较大影响。

(1) Numerical simulation method of Ultrasonic Pulse-Echo Inspection, which potentially Is a powerful means for Non Destructive Evaluation of cracks and flaws of steel structures, is built. Through this simulation, the diffracting and reflecting waves at the voids and the slit are investigated in detail. Furthermore, suitable observation points to distinguish the shape of flaws from reflected waves are investigated.(2) Fatigue tests are carried out on CCT specimens and fillet welded specimens with Acoustic Emission (AE) method to observe the behavior of crack initiate stage and propagate stage. It is observed that crack Initiate stage exists even for a welded joints In the same way for materials, and relatively intense AE signals occur in this stage.(3) Fatigue tests are carried out on fillet welded joint specimens containing undercuttings of which dimensions are measured in detail. The effects of size and shape of undercuttings on the fatigue strength are Investigated. Besides, FEM analysis Is carried out on the model containing the undercuttings to calculate stress concentration at the bottom of undercuttings.(4) For the case that crack initiates from welded root, the fatigue strength of load-carrying fillet welded joints Is investigated through experimental and analytical approaches. the length of non-penetrate section of weld root much influences on fatigue strength.