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New calibration method for high and low triaxiality and validation on SENT specimens of API X70

  • The determination of the exact mechanical properties of material is essential for an optimal and safe design of linepipes. It is especially important for the prevention of over-engineering and the reliable assessment of complex accidental loading, such as extreme bending due to loss of buoyancy of support, or abrupt ground movement. Currently, the focus of research in offshore deepwater installations and linepipes is towards pre-cracked structures with high triaxiality stress states and complex loading histories. At the same time, low triaxiality stress states must be correctly studied in order to represent shear dominated failure in pipes. A comprehensive experimental and numerical program was undertaken to determine the mechanical properties of the traditional API X70 grade of steel. The material was characterized for anisotropic plasticity, fracture initiation and uncracked ductility for various states of stress. The same material was also used for pre-cracked fracture toughness assessment. The experimental program included flat and round specimens. The first type of tests on flat butterfly-shaped, central hole, notched and circular disk specimens; were selected to address the low stress triaxiality range. Tests on round notched bar specimens and SENT fracture mechanics tests extended the characterization and verification process to higher stress triaxiality values. This program covered a wide range of stress conditions and demonstrated their effect on the material resistance to crack extension. Each test conducted was numerically simulated using solid finite element models, matching the exact geometric and loading history features. The numerical simulation provided information on the local stress and strain fields around the location of the potential or existing cracks. Based on the above hybrid experimental/numerical technique tailored for pipe applications, the MMC fracture model was calibrated. The model relates the material ductility not only to stress triaxiality but also to the Lode parameter. The predictive capabilities of the MMC were then evaluated in the case of SENT testing, used extensively in the pipeline industry. It was shown that the present fracture model calibration can describe fracture behavior of SENT experiments.

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Metadaten
Author:Kirki Kofiani, Aida NonnORCiDGND, Tomasz Wierzbicki
DOI:https://doi.org/10.1016/j.ijpvp.2013.07.004
Parent Title (English):International Journal of Pressure Vessels and Piping
Document Type:Article
Language:English
Year of first Publication:2013
Release Date:2021/03/17
Tag:Ductile fracture; Modified Mohr – Coulomb model; Offshore API grade; SENT tests; Triaxiality and Lode angle dependence; X70
Volume:111-112
First Page:187
Last Page:201
Institutes:Fakultät Maschinenbau
Technologie-Campus Neustadt an der Donau
Fakultät Maschinenbau / Computational Mechanics and Materials Lab (CMM)
Begutachtungsstatus:peer-reviewed
Publication:Externe Publikationen
research focus:Produktion und Systeme