Analysis of melt flow in thick-walled heated element butt welded thermoplastic joints by FE simulation.

点击翻译按钮获取中文摘要

Due to the further development of pipe winding technology and extrusion technology in pipeline construction, pipe dimensions of more than 3,000 mm diameter and wall thicknesses of more than 100 mm can be produced. The joining of semi-finished products of these wall thicknesses has led to a need to re-evaluate current strength theories. As investigations and cases of damage have shown, the effective relationships of thin-walled components cannot be transferred to thick-walled components by means of the law of similarity. The significantly longer flow channel of thick-walled components leads to higher rheological flow velocities and melt contact times. This, in conjunction with the long shear of the melt as a result of the joining process, possibly leads to the formation of a high state of order and thus to anisotropic material behavior in the edge regions. It is therefore conceivable that, in addition to the criterion of the minimum flow rate of the melts, there must also be a criterion of the maximum flow rate in conjunction with a maximum time-dependent shear or melt movement. The technical/scientific reason for the research application is the increased occurrence of weld fractures on weld seams in components with large wall thicknesses, in some cases with short-term loading in installation situations. The scientific necessity is based on the fact that the structurally relevant shear and strain flow velocities in the joining of large wall thicknesses encompass much larger scale ranges than have been investigated in scientific work to date. In conjunction with the more inhomogeneous cooling conditions, this leads to weld morphologies and residual stress states that were not previously understood. Preliminary investigations show that the melt in thick-walled components is in the plastic state for longer than previously assumed. Currently, this is not taken into account in German DVS 2207-1, English WIS 4-32-08 or American PPI-TR33. In the project, the gaps in the understanding of the process are to be closed by setting up numerical models for the heating behavior and the melt flow in the joining phase, as well as linking the influences to the short- and long-term strengths. This requires the experimental elaboration of the complex dependencies, e.g. on melt layer thickness and flow velocity. The simulation serves to improve the understanding of the process and to minimize the exp. effort. In the cooperation of the two applicants, this research idea has high prospects of success and makes a significant contribution to the in-depth understanding of the solidification and crystallization processes in thick-walled components and extends the understanding of the strength mechanisms. Furthermore, the simulative description of the thermal/rheological processes in the weld seam enables the optimal process design with regard to strength, service life and residual stress-free weld seam formation.