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，为钢结构裂纹和缺陷的无损检测提供了一种有力的手段。通过数值模拟，详细研究了孔隙和狭缝处的衍射波和反射波。此外，合适的观察点，以区分缺陷的形状从反射波进行了研究。(2)采用声发射（AE）方法对CCT试样和角焊缝试样进行疲劳试验，观察裂纹萌生阶段和扩展阶段的行为。研究发现，焊接接头也存在裂纹萌生阶段，材料也存在裂纹萌生阶段，且在此阶段声发射信号相对较强。(3)疲劳试验是在角焊缝接头试样上进行的，该试样含有详细测量尺寸的底切。研究了底切尺寸和形状对疲劳强度的影响。此外，还对含掏槽的模型进行了有限元分析，计算了掏槽底部的应力集中。(4)针对裂纹起源于焊接根部的情况，通过试验和理论分析，研究了角焊缝承载接头的疲劳强度。焊缝根部未熔透区的长度对疲劳强度影响较大。

(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.