P 1236 – Quantification of the toughness influence on crack arrest properties of modern linepipe steels
The resistance to brittle fracture initiation and long-running ductile fracture propagation are the most important safety requirements for gas transporting pipelines. To meet these requirements, pipeline steels with excellent toughness properties have been developed.
However, due to the brittle fracture phenomena occurring in the Charpy impact tests and the Battelle drop weight tear tests during the experimental toughness quantification the conventional design procedures for a comprehensive use of these excellent properties have to be reviewed. The phenomena include so called separations and inverse fracture behaviour, both of which have been assigned to the brittle failure mechanism. In addition, the ductile crack arrest needs to be ensured by a sufficient material resistance to fracture propagation, which is defined by the Charpy upper-shelf toughness. Studies in the past showed non-conservative design results for modern pipeline steels, which are addressed with appropriate safety factors. For the design, this results in a potential that still has to be exploited.
Therefore, damage mechanics models were developed, modified and parameterized in this project based on an extensive laboratory test program, that captured the ductile damage and fracture behavior on the upper shelf as well as the brittle fracture in the lower shelf. The ductile and brittle fracture models were verified by the simulations of the Charpy impact tests as well as the drop weight tear tests and the applied to predict the fracture behavior of pipe sections. The developed methodology enabled the simulation of the separations and the inverse fracture in the Battelle drop-weight tear tests for the first time.
In the pipes, the fracture speed was found increasing with the formation of separations. At the same time, less critical stress conditions regarding the formation of brittle fracture were obvious ahead of the crack tip. Based on the analysis of local stress states, the inverse fracture was attributed to the loading scenario of the laboratory specimen.
Additionally, a coupled FSI pipe model was developed to predict the fluid structure interaction effects during the ductile fracture propagation. The model considers the inelastic deformation and ductile fracture in the pipe material, the decompression behavior of one- and two-phase gases and the soil backfill in onshore application. The numerical FSI model was applied to predict the three-dimensional pressure field in the area of the crack tip and the flaps, which strongly differ for different types of mixtures. The FSI model was verified by simulations of full-scale burst tests.
The project results achieved support the secure handling of modern line pipe materials and expand their range of applications. The correlations between the material behavior measured from laboratory experiments and on component level, which are used in the conventional design standards, can be evaluated or disproved based on the findings of the numerical studies in this project. Furthermore, the damage mechanics and multi physical models represent innovative concepts and tools for the design of various pressure-vessels applications.
Prof. Dr.-Ing. S. Münstermann, David Lenz, M.Sc., Prof. Dr.-Ing. A. Nonn, V.Keim, M.Sc.