On the suitability of single-edge notch tension (SENT) testing for assessing hydrogen-assisted cracking susceptibility

TL;DR

The paper evaluates the suitability of single-edge notch tension (SENT) testing for hydrogen-assisted cracking by combining constant-load SENT experiments on C110 steel in two H$_2$S environments with hydrogen permeation measurements and a phase-field deformation-diffusion-fracture model. The model links environmental hydrogen uptake to local crack-tip concentrations and predicts $K_{ ext{th}}$ values that agree well with experiments in aggressive environments, while highlighting larger scatter under milder conditions. It shows that peak crack-tip hydrogen content is reached rapidly (about 10 h) and that corrosion-product layers mainly affect longer-term uptake, suggesting an optimal SENT test duration of less than a day. The study concludes that SENT can be informative for hydrogen embrittlement susceptibility in severe environments but offers limited advantages over higher-triaxiality tests when $K_{ ext{th}}$ approaches $K_{Ic}(C)$, and it proposes a route to augment SENT with virtual testing and simplified protocols for pipeline-relevant conditions.

Abstract

Combined experiments and computational modelling are used to increase understanding of the suitability of the Single-Edge Notch Tension (SENT) test for assessing hydrogen embrittlement susceptibility. The SENT tests were designed to provide the mode I threshold stress intensity factor ($K_{\text{th}}$) for hydrogen-assisted cracking of a C110 steel in two corrosive environments. These were accompanied by hydrogen permeation experiments to relate the environments to the absorbed hydrogen concentrations. A coupled phase-field-based deformation-diffusion-fracture model is then employed to simulate the SENT tests, predicting $K_{\text{th}}$ in good agreement with the experimental results and providing insights into the hydrogen absorption-diffusion-cracking interactions. The suitability of SENT testing and its optimal characteristics (e.g., test duration) are discussed in terms of the various simultaneous active time-dependent phenomena, triaxiality dependencies, and regimes of hydrogen embrittlement susceptibility.

Figures (12)

• Figure 1: SENT specimen configuration: geometry (left), with dimensions in mm, picture of the specimen under constant load using a proof ring (centre), and optical microscope image of fractured SENT specimen indicating the 5 equidistant locations where the pre-crack measurements are taken (right).
• Figure 2: Uniaxial stress-strain relation: experimental data and power law fit considered in the numerical model.
• Figure 3: Sensitivity of the fracture toughness to the hydrogen concentration in low-alloy steels. $J_{Ic}$ (or $G_c$) versus $C$ data taken from Refs. Cancio2010Vera1997. The fitting curve corresponds to Eq. (\ref{['eq:hydrogen-Gc']}) with the parameters $q=0.5$ and $G_c^{\text{min}}=2$ N/mm.
• Figure 4: Time-dependent Dirichlet hydrogen concentration at the exposed boundaries of the sample, for each of the considered H$_2$S solutions. These numerical boundary conditions are based on the permeation experiments discussed in Section \ref{['Sec:permeation']}.
• Figure 5: Geometry and numerical model of the single edge notched tensile (SENT) test. A representative distribution of the hydrogen concentration $C$ and the phase field fracture parameter $\phi$ are also shown. The initial crack, of length $a_0$, is introduced through a combination of a geometrical definition and a Dirichlet $\phi=1$ boundary condition. All geometrical dimensions are in mm.