X60管线钢疲劳特性的研究

Investigation of the fatigue characteristic of X60 pipeline steel

利用 2、4和 14mm的X6 0钢紧凑拉伸试样进行同样应力水平、固定应力比 (R =0 .1)的疲劳裂纹扩展试验 ,揭示出高韧性管道钢的疲劳裂纹扩展存在显著的厚度效应。由于应力水平低 ,相应的离面应力也低 ,不足以导致分层开裂 ,因此 ,裂纹端部的三维应力约束随厚度增加而增大。 4mm试样的疲劳裂纹速率远高于 2mm试样 ,但厚度再增大 ,裂纹扩展速率不再有显著变化。进一步对表面裂纹试样进行疲劳裂纹扩展试验 ,并用勾线法纪录裂纹扩展情况。利用穿透裂纹的da/dN -ΔK曲线预测表面裂纹扩展 ,用 2mm厚试样的数据控制裂纹表面长度的扩展 ,用 14mm厚试样的数据控制裂纹底部沿厚度方向的扩展 。

The effects of thickness on the fatigue crack propagation of X60 pipeline steel were investigated experimentally by using of compact tension specimens with 2,4 and 14mm , and the fatigue tests are identical in stress level and stress ratio .The results indicate that the effects of thickness on the fatigue crack propagation exist evidently . Since there is no delamination occurring due to low out-plane stress related to low stress level, the three-dimensional stress constraint near the crack-tip become serious as the thickness increasing .The fatigue crack propagation velocity of 4mm thickness specimen is remarkably higher than that of 2mm thickness, but if the thickness increase continually , the fatigue crack propagation velocity does not increase evidently any more. The fatigue propagation of surface-crack specimen was tested too. The fatigue propagation of surface-crack specimen was predicted by using da/dN-ΔK of through-crack specimens, i.e., using the data of 2mm thickness through-crack specimen to predict surface-crack propagation in length direction ,and using the data of 14mm thickness through-crack specimen to predict surface-crack propagation in depth direction .The results of prediction are essentially coincide with that of the experiment.