Abstract
A possible solution to assess the embrittlement phenomenon associanted with hydrogen transportation using existing piepeline steel grades is to extract small coupons without interrupting supply operations. From these coupons, it is possible to machine sub-size smooth tensile (mST), sub-size notched tensile (mNT), and sub-size compact tension test specimens (mDCT) to characterize ductility and fracture toughness. Due to the small size of these specimens (thickness ranging from 2 to 4 mm), the miniaturized CT specimens do not meet the requirements for validating test results (see: ASTM E1820 recommendations). In the first part of this work, size effects on the mechanical properties of an API 5L X52 vintage steel are investigated under air using NT and CT specimens. The second part presents a new test setup developed to test sub-size specimens under pressurized gaseous hydrogen.
The results show that, compared to the standard tensile specimens, the sub-size tensile specimens exhibit comparable mechanical properties with a slightly higher ultimate tensile strength and ductility. Although the tensile mechanical properties are almost similar, the toughness values obtained on sub-size and standard specimens show a size effect on crack growth resistance properties. The crack growth resistance obtained using sub-size CT specimens is largely underestimated compared to that obtained using standard specimens.
The new gaseous pressurized hydrogen machine is designed to perform low strain rate tests for subsize specimens. The volume of the chamber has been reduced with the development of a new optical extensometer using Edge Tracing technique (ET) outside the chamber to control the machine displacement during mechanical tests. Preliminary tests under air were performed on uncracked and cracked subsize samples. The first results obtained for mST and mNT10 show the same effect of triaxiality on mechanical properties observed for standard specimens. For the crack growth resistance tests, preliminary results show that the displacement measurement with ET technique is stable enough to calculate the crack extension using the unloading compliance method.