Session: MF-11-01 Small-Scale and Miniature Mechanical Testing (Joint with CS-19)

Paper Number: 105789

105789 - Use of Miniature Specimens to Characterize the Ductile Failure of Welds in Pipelines 

Welds in structures such as pipes or pressure vessels must be assessed against catastrophic ductile failure initiated on, e.g.,  fatigue or corrosion cracks. It is, therefore, necessary to obtain fracture data (especially J-R curves) of the different zones of the weld area, such as the weld metal (WM) or the heat-affected zones (HAZ). HAZ can be reproduced using a Gleeble machine, but specimens are small, and it is not fully guaranteed that the synthetic HAZ material is similar to the actual HAZ. The WM can be obtained in large quantities by depositing the WM on plates, but it is also not guaranteed that the synthetic WM is similar to the one obtained while welding structures. In addition, in the case of « vintage » pipes, the weld metal may no longer be available. In this study, it is proposed to extract specimens from the actual welds directly. As in many cases, the weld width is relatively small, the size of the specimens must be reduced compared to usual testing procedures.

A longitudinal weld in a « vintage » X52 line pipe steel is investigated in this work. The pipe thickness is 11.3 mm. The  WM and the HAZ have a width of about 10 and 3 mm, respectively. The following specimens are used : (i) smooth tensile specimens (ST), (ii) notched axisymmetric tensile bars (NT), and Disk Compact Tension (DCT). The specimens are loaded along the pipe's hoop direction. Before machining, slices of the pipe are polished and etched to position the various specimens precisely. In all cases, extensometry is performed using video tracking of predefined points. The DCT specimens have a diameter of 10.8 mm, and a thickness of 4 mm. Side grooves (20% of the thickness) are machined in the specimens after fatigue precracking. The DCT specimens can be located in the WM or the HAZ. In that last case, the crack is located at the center of the HAZ. However, some areas of the specimens lay in the WM and base metal (BM). Tests are conducted using the unloading compliance procedure. ST specimens have a minimum diameter of 1.2 mm and a total length of 11 mm. They can be used in the WM but do not fit in the HAZ (unless the test is performed along the pipe's thickness direction). NT specimens (minimum diameter of 1.4 mm) must then be used to test the HAZ in the hoop direction. In that case, the notched part of the specimen can be entirely located in the HAZ.

Tests on ST and NT specimens are used to obtain the hardening behavior of the various zones. It was first checked on the BM that similar results are obtained using sub-size and standard specimens. For results obtained with NT specimens (i.e., in the HAZ), an inverse numerical procedure must be used based on the finite element simulation of the specimens to obtain the behavior. DCT tests are processed using the ASTM E1820 standard. Due to the small size of the specimens, tests are not considered valid following the standard. Tests can be used, however, to compare the different zones of the weld. It is shown that the HAZ and the BM have similar J-R curves, whereas larger values are obtained for the WM. The various tests will be used in the near future to develop constitutive equations accounting for ductile damage. Such a model can then be used to compute large specimens and obtain valid simulated J-R curves.

Presenting Author: Jacques Besson Mines Paris PSL -- Centre des Materiaux --- CNRS UMR 7633

Presenting Author Biography: Jacques Besson (59) currently holds a permanent position at CNRS (directeur de recherche) working at Centre des Matériaux, a common Mines ParisTech / CNRS (UMR 7633) research center.
He is an expert in the field of damage and fracture of materials. His interest is mainly in modeling and simulations but he has been continuously working in collaboration with researchers carrying out experimental investigations. He is one of the main contributors to the Z-set finite element program which is currently co-developed with ONERA, the French aerospace laboratory. He was a visiting scientist at GKSS (Germany, 1999-2000) and spent one year working in industry at EdF R&D. He was responsible for the EdF/GdF chair on “Durabilité des matériaux et des structures pour l’énergie” (2008-13). From 2012 to 2016 he was head of Centre des Matériaux. He is currently the chair holder of the MESSIAH ANR industry chair (Mini-test specimens for In-Service Monitoring of Structures with Application to Hydrogen Transport).
He has established long-term collaborations with industry (EdF, Framatome, Arcelor Mittal, PSA, Constellium, Dassault, Safran…) and research centers (CEA, ONERA)
Author or co-author of 150 publications in peer-reviewed journals.

Authors:

Jacques Besson Mines Paris PSL -- Centre des Materiaux --- CNRS UMR 7633
Said Belkacemi Mines Paris PSL -- Centre des Materiaux --- CNRS UMR 7633
Yazid Madi Mines Paris PSL -- Centre des Materiaux --- CNRS UMR 7633
Clément Soret GRTGAZ
Magali Poloe GRTGAZ