STUDY ON THE EFFECT OF MATERIAL CONSTRAINT ON THE STRESS-STRAIN BEHAVIOR OF PIPELINE GIRTH WELD

For the study of the pipeline girth weld, the key to ensure the accuracy of the finite element analysis result is to obtain the accurate constitutive relationships. However, due to the complex material composition of the girth weld, it is difficult to obtain the accurate stress-strain curves. At present, the digital image correlation (DIC) testing is an important method to obtain the stress-strain curves of the girth weld. However, when this method is used for testing, there will be the material constraint effect caused by weld mismatch. The material constraint will have an important effect on the accuracy of the test result. Especially for the undermatched weld and the softened heat affected zone, the effect is more significant. This effect will make the measured stress significantly deviate from the true value and ultimately affect the accuracy of the finite element analysis result. However, there is still no sufficient study on this issue. Therefore, it is necessary to further study the effect of the material constraint on the stress-strain behavior of the weld. In this study, the finite element analysis method is used to simulate the data processing process during the DIC test, including strain extraction and stress calculation. The girth weld is simplified to include the weld metal (WM) and the base metal (BM), that is, the properties of the heat affected zone (HAZ) and BM are the same. The effects of the geometric dimension and the mismatch degree of the weld are analyzed. The stress-strain curves inputted into the finite element model are compared with the stress-strain curves calculated based on the test method, and then the effect of the material constraint is determined. The results show that for the undermatched weld, the calculated stress based on the test method is greater than the true stress of the weld, and this difference is related to the weld size and the mismatch degree. When the weld width is large, the calculated stress is close to the true stress. With the decrease of the weld width, the calculated stress deviates from the true stress gradually. With the increase of the weld mismatch degree, the deviation of the calculated stress from the true value gradually increases. For the overmatched weld, the calculated stress is less than the true stress of the weld, but the difference is relatively small. This is due to the fact that the overmatched weld specimen will fracture in BM, and the fracture position is relatively far away from the weld, which makes the effect of the material constraint relatively small. In summary, the material constraint effect makes BM have an important effect on the deformation of the weld and changes the stress state of the weld. The results obtained in this study are important for further analyzing the weld material constraint effect and proposing a correction method for DIC test result.