Assessment method of COD for a surface flaw at a structural discontinuity

我们首先研究了单边裂纹板（SC）的弹塑性COD（σ_m），它是表面裂纹的横截面，受拉（σ_m）是由中心裂纹板（CC）受拉的COD（σ_m）和单边裂纹板受偏心弯曲应力（m ~*）的COD（σ_m）的某种叠加给出的。结果表明，σ_m应力下的SC COD值是由σ_m +0.75m ~* 应力下的CC COD值和m ~* 应力下的SC COD值叠加而得。在上述处理中，m*/sigma_m是sigma_m/sigma_y（sigma_y：屈服应力）的函数，当总屈服发生时，m*/sigma_m随sigma_m增大而减小，然后由表面裂纹工作拉伸和弯曲最深点处的K值求出表面裂纹横截面的工作拉伸应力和弯曲应力。采用简化梁理论，引入了拉弯联合作用下SC弯曲的屈服应力，并将该方法应用于海洋结构物T形管节点冠部表面缺陷的COD估算。估计COD的表面的中心是在良好的协议与实验。结果表明，在高工作应力条件下，由缺口尖端R端位移定义的CTOD值往往小于由Dugdale模型和铰链模型定义的三点弯曲CTOD值。

We invested at first that elasto-plastic COD's for a single edge cracked sheet(SC), which is a cross section of a surface crack, subjected to tension(sigma_m) were given by some superposition of COD's for a center cracked sheet(CC) subjected to tension and COD's for a single edge cracked sheet subjected to bending stress(m*) by eccentricity. As a result, COD's for SC by sigma_m are obtained by superposition of COD's for CC subjected to tens ion which stress is sigma_m plus 0.75m* and COD's for SC subjected to m*. In above treatment,m*/sigma_m is a function of sigma_m/sigma_y(sigma_y:yield stress) and decreases as sigma_m increases to zero when gross yield occurs.Then working tension stress and bending stress for a cross section of a surface crack are obtained by K value at the deepest point of a surface crack working tension and bending. By adapting a simplified beam theory, aperient yield stress for SC due to bending in case of combined tension and bending is introduced.This estimating method of COD's is applied to a surface flaw in a crown part of T tubular joint of an offshore structure. Estimated COD's at a center of a surface are in good agreement with experimental ones. As a result, it becomes clear that CTOD defined by the displacement of R end of a notch tip is tend to give smaller value in case of high working stress than CTOD defined by Dugdale model and hinge model for 3 point bending CTOD testing .