Cohesive zone modelling of hydrogen induced cracking on the interface of clad steel pipes

Abstract

A coupled finite element and cohesive zone modelling approach has been applied to simulate hydrogen induced fracture initiation in a hot rolled bonded clad steel pipe. The results are compared in terms of experimental fracture mechanical testing in air and under in situ electrochemical hydrogen charging. A best fit to the experimental CTOD fracture initiation toughness value in air was achieved for an initial cohesive stiffness k n = 4 • 10 6 MPa/mm and a critical cohesive stress σ c = 1210 MPa. For simulating under hydrogen influence, the hydrogen induced lowering of the cohesive strength was computed both in terms of the lattice concentration and the total hydrogen concentration. Two different formulations for calculating the dislocation trap density was considered. The simulated results revealed that both hydrogen in lattice and hydrogen trapped at dislocations can be responsible for the observed hydrogen induced reduction in CTOD fracture initation toughness. The choice of trap density formulation appeared significant only under the assumption that both lattice and trapped hydrogen infer an influence on the hydrogen induced lowering of the cohesive strength. Further effort is needed to provide a reliable description of the interface hydrogen content and distribution, providing a model able to transfer between different material systems.

A fully coupled implementation of hydrogen embrittlement in FE analysis

“…Since the C L is the dominant contribution, the method adopted to estimate the C T does not affect the results. Same considerations were found by [38], for clad steel pipes in API X60 steel. In this case, the total concentration, C, coincides with C L that shows a peak value of 3.3 ppm, as for the weakly coupled case, but located at 0.219 mm ahead of the crack tip.…”

Investigation of Hydrogen Embrittlement Susceptibility and Fracture Toughness Drop after in situ Hydrogen Cathodic Charging for an X65 Pipeline Steel

“…After applying the in situ hydrogen cathodic charging process at 10 and 20 mA/cm 2 current densities, a severe increase in surface hardness was detected (Figure 5). This effect is attributed according to international literature to the interaction between dislocation networks and hydrogen Cottrell atmospheres, to the creation of high volume fraction of hydrogenated vacancies (V-H clusters), and the expanded development of interstitial solid solution [27][28][29][30][31][32][33][34][35]. In particular, the above phenomenon is attributed to the fact that hydrogenated interstisial vacancies of the crystalline structure impede the movement of dislocations, resulting in their interlocking and immobilization effect.…”

Models of hydrogen influence on the mechanical properties of metals and alloys

“…Сразу необходимо отметить, что все расчеты с помощью HEDEмодели в известной нам литературе выполнялись на модельных примерах -цилиндрических или призматических образцах с надрезом или инициирующей трещиной известных размеров. Как правило, образцы выполнялись из калящихся мартенситных сталей, так как такой моделью определяется хрупкое разрушение, но есть работы, посвященные и обычной нержавеющей стали [50].…”