Numerical Investigation Of Strength Mismatch Constraint Effect On Fracture Toughness In Pipeline Steels

In recent years,due to the development and application of high pressure and large-sized pipeline,the higher strength and toughness of pipeline steel have been required.Because of the nature of welding,there is often a strength mismatch between the weld metal and the base metal in the pipeline welding process.And abundant researches have shown that strength mismatch has a great effect on the fracture behavior of welded joints.Therefore,the study has important scientific significance and engineering application value.In this paper,numerical simulation was performed to study the effect of mismatch constraint on the fracture toughness and crack growth resistance curve of pipeline steel.Based on the elastic-plastic fracture mechanics theory,the complete Gurson model(CGM)was used to study the crack growth resistance curves of the standard SENB(Single Edge Notched Bending),SENT(Single Edge Notched Tension),circumferential axisymmetric cracked pipe,WP(Wide Plane)and the MBL(Modified Boundary Layer)models with mode-I crack.The results show that the geometric constraint Q,caused by geometric structure of model,leads to the different ductile fracture behaviors;and with the increase of Q value,the fracture toughness decreases.Compared with the other specimens,the SENT specimen has a similar fracture resistance curve behavior with the cracked pipe.In addition,the fracture toughness(Crack Tip Opening Displacement,CTOD)of SENB and SENT samples is tested.The test results further prove the accuracy of finite element analysis.The SENB and SENT specimens were selected to find out the effect of strength mismatch constraint on the ductile tearing process.Following results can be obtained.Firstly,varying from 0.8 to 1.3,weld strength mismatch My,the ratio between the yield stress of weld metal and base metal,shows a strong effect on the fracture toughness and ductile crack growth resistance curves for welding pipes.With the increase of mismatch ratio,the ductile fracture resistances significantly increase for both the standard SENB and the shallow and deep-cracked SENT specimens and pipes.Secondly,the mismatch constraint M decreases from positive to negative with the increase of mismatch ratio as for pipes and the corresponding SENT specimens,and with crack growth,the value of M varies in a relatively similar range,which indicates the effect of the strength mismatch on resistance curves is not quite associated with the amount of crack extension.At last,the ductile tearing resistancefrom the standard SENB specimen is obviously conservative than that from SENT with respect to the pipes for all different mismatched cases considered in this study.As for various grade pipeline steel,the influence of strength mismatch on fracture toughness is of complete consistency,that's to say,the larger mismatch ratio decreases the mismatch constraint M and raises the fracture toughness.And with the increase of steel grade,the influence of the mismatch constraint on fracture toughness increases.Finally,these analyses were extended to assess the effect of strength mismatch on the fracture toughness of 3D SENB and SENT models under large scale yielding.For 3D SENB models,the CTOD-CMOD(Crack Mouth Opening Displacement)curves exhibit obviously 3D effect.The 3D effect of CTOD-CMOD curve is much more pronounced with the increase of mismatch ratio,and the plane near to the symmetric plane has a higher curve.The CTOD-CMOD curve of symmetry plane for3 D SENB model varies according to the strength mismatch ratio.By comparison,the over-mismatched one has a lower CTOD-CMOD curve and a higher crack growth resistance curve.For SENB and SENT,the crack growth resistance curve,obtained by2 D plane strain model and 3D model,is approximate to each other.In other words,2D plane strain model can be used in place of 3D model to evaluate the fracture toughness,which can reduce the difficulty of modeling and improve the efficiency of calculation.