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00372 LABORATORY TESTS REPRODUCE TRANSGRANULAR STRESS CORROSION CRACKING OBSERVED IN FIELD

Product Number: 51300-00372-SG
ISBN: 00372 2000 CP
Author: G.Gabetta, S.Di Liberto and A.Bennardo
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Slow Strain Rate (SSR) and Load Control (LC) tests were performed aiming to investigate Transgranular Stress Corrosion Cracking (TGSCC) which can occur on the outside surface of underground pipelines where the protective coating is damaged and where the ground water is in direct contact with the pipe surface. A comparison between the two loading methodologies is proposed with the purpose to identify peculiar conditions, which allow to better reproducing TGSCC as observed in field. Fractographic examinations of C(T) (Compact Tension) notched specimens cut from pipeline steel and tested in an environment simulating field conditions, showed that laboratory tests were able to reproduce the "quasi cleavage" fracture morphology observed in the field. This morphology suggests, as claimed by many authors, that hydrogen concentration at the crack tip is involved in TGSCC. By operating under LC condition, subcritical crack grow generates fracture surfaces with a very short affected zone and a very brittle morphology. In SSR condition, on the contrary, the subcritical crack zones were one order of magnitude larger, but their morphology was less brittle (more dimples), indicating that Load Control methodology (with respect to Slow Strain Rate) offers a better separation between mechanical and environmental components both contributing to crack grow. Integration of the two techniques allows a better understanding of the material behaviour. Keywords: Transgranular Stress Corrosion Cracking (TGSCC), Load Control (LC), Slow Strain Rate (SSR), Quasi Cleavage, Gas Transmission, Environmental Assisted Cracking (EAC).
Slow Strain Rate (SSR) and Load Control (LC) tests were performed aiming to investigate Transgranular Stress Corrosion Cracking (TGSCC) which can occur on the outside surface of underground pipelines where the protective coating is damaged and where the ground water is in direct contact with the pipe surface. A comparison between the two loading methodologies is proposed with the purpose to identify peculiar conditions, which allow to better reproducing TGSCC as observed in field. Fractographic examinations of C(T) (Compact Tension) notched specimens cut from pipeline steel and tested in an environment simulating field conditions, showed that laboratory tests were able to reproduce the "quasi cleavage" fracture morphology observed in the field. This morphology suggests, as claimed by many authors, that hydrogen concentration at the crack tip is involved in TGSCC. By operating under LC condition, subcritical crack grow generates fracture surfaces with a very short affected zone and a very brittle morphology. In SSR condition, on the contrary, the subcritical crack zones were one order of magnitude larger, but their morphology was less brittle (more dimples), indicating that Load Control methodology (with respect to Slow Strain Rate) offers a better separation between mechanical and environmental components both contributing to crack grow. Integration of the two techniques allows a better understanding of the material behaviour. Keywords: Transgranular Stress Corrosion Cracking (TGSCC), Load Control (LC), Slow Strain Rate (SSR), Quasi Cleavage, Gas Transmission, Environmental Assisted Cracking (EAC).
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