Assessing Hydrogen-Assisted Cracking Susceptibility Using Single-Edge Notch Tension (SENT) Testing

Core Concepts

Combined experiments and computational modeling are used to evaluate the suitability of the Single-Edge Notch Tension (SENT) test for assessing hydrogen embrittlement susceptibility.

Abstract

The study investigates the suitability of the Single-Edge Notch Tension (SENT) test for assessing hydrogen embrittlement susceptibility of a C110 steel in two corrosive environments. Hydrogen permeation experiments were conducted to relate the environments to the absorbed hydrogen concentrations. A coupled phase-field-based deformation-diffusion-fracture model was then employed to simulate the SENT tests, predicting the mode I threshold stress intensity factor (Kth) in good agreement with the experimental results.
The key findings include:

Permeation experiments enabled quantifying hydrogen absorption as a function of the environment (H2S content) and revealed the role of corrosion products in reducing sub-surface hydrogen concentration.

The numerical model provided reliable predictions across a wide range of applied loads, environments, and geometries.

SENT tests and computational simulations enabled determining a critical stress intensity factor threshold Kth for each environment. However, data scatter was observed for the low H2S content scenario, attributed to the higher sensitivity of fracture toughness to hydrogen content at low concentrations.

For severe environments, hydrogen uptake is such that Kth is close to the toughness saturation value, and triaxiality effects do not play a significant role, negating the advantage of using SENT over other tests with higher crack tip constraint.

The hydrogen peak near the crack tip reaches 90% of its maximum value after 10 hours, shorter than the time for corrosion product formation, suggesting a suitable testing time of less than a day. However, late failures were observed in the low H2S scenario, potentially due to intermittent crack growth and its dependency on diffusion.

Stats

The material employed had a yield strength of 820 MPa and an ultimate tensile strength of 883 MPa.
The hydrogen diffusion coefficient of the material was 1.4 × 10^-4 mm^2/s.

Quotes

"An accurate determination of the mode I threshold stress intensity factor (Kth) is essential for the safe and cost-effective design of metallic structures exposed to environments prone to hydrogen absorption."
"The main challenges are intrinsically related to the mechanism of hydrogen-assisted cracking, as the high sensitivity of the fracture toughness to the hydrogen content makes global quantities such as Kth highly dependent on the local hydrogen concentration around the crack tip."

Key Insights Distilled From

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

by L. C... at arxiv.org 04-30-2024

https://arxiv.org/pdf/2404.18223.pdf

Deeper Inquiries

How can the SENT test protocol be further optimized to provide more reliable and consistent Kth measurements, especially for milder corrosive environments?

The optimization of the SENT test protocol for more reliable and consistent Kth measurements in milder corrosive environments can be achieved through several strategies:

Standardization of Testing Conditions: Ensuring consistent testing conditions, such as temperature, loading rates, and specimen preparation, can help reduce variability in results.

Controlled Environment: Maintaining a controlled environment during testing, including precise control of hydrogen concentration and exposure conditions, can minimize external factors influencing the results.

Improved Sample Preparation: Ensuring uniformity in specimen preparation, including notch geometry and surface finish, can reduce variability in test outcomes.

Extended Testing Duration: While shorter test durations may be sufficient for aggressive environments, longer testing durations may be necessary for milder environments to capture delayed failure events accurately.

Post-Test Analysis: Conducting detailed post-test analysis, including fractographic examination of failed specimens, can provide insights into failure mechanisms and help validate Kth measurements.

Calibration with Additional Testing Methods: Comparing SENT results with other testing methods, such as DCB or CT tests, can help validate Kth measurements and ensure consistency across different test configurations.

Model Validation: Validating experimental results with numerical modeling, ensuring that the model accurately represents the material behavior and failure mechanisms in milder corrosive environments.

What are the potential limitations of the phase field-based modeling approach in capturing the complex interplay between hydrogen diffusion, crack growth, and microstructural heterogeneities?

While phase field-based modeling offers a powerful tool for simulating the interplay between hydrogen diffusion, crack growth, and microstructural heterogeneities, it also has some limitations:

Simplifying Assumptions: The model relies on simplifying assumptions and empirical parameters to represent complex material behavior, which may not capture all the nuances of the actual physical processes.

Computational Intensity: Phase field models can be computationally intensive, requiring significant computational resources and time for simulations, especially for complex geometries and loading conditions.

Material Parameters: The accuracy of the model heavily depends on the material parameters used, and obtaining precise values for these parameters can be challenging, especially for complex alloy systems.

Microstructural Representation: Representing microstructural features accurately in the model, such as inclusions or segregated zones, can be challenging and may require additional calibration and validation.

Validation and Calibration: Ensuring that the model predictions align with experimental observations and real-world behavior requires thorough validation and calibration, which can be time-consuming and resource-intensive.

Assumption of Continuum Behavior: The model assumes continuum behavior, which may not fully capture the discrete nature of crack propagation and interactions with microstructural features.

Could the insights gained from this study be extended to other hydrogen-assisted cracking scenarios, such as those involving different alloy systems or loading conditions?

The insights gained from this study on hydrogen-assisted cracking susceptibility, as assessed through SENT testing and numerical modeling, can be extended to other scenarios involving different alloy systems or loading conditions in the following ways:

General Principles: The fundamental principles of hydrogen embrittlement and its interaction with crack growth and material properties are applicable across different alloy systems, providing a basis for understanding hydrogen-assisted cracking in various materials.

Model Adaptation: While specific material parameters may vary, the modeling approach and methodology can be adapted to different alloy systems by adjusting the material properties and hydrogen degradation laws accordingly.

Testing Optimization: The optimization strategies identified for SENT testing, such as standardization of conditions and post-test analysis, can be applied to different alloy systems to improve the reliability and consistency of Kth measurements.

Limitation Awareness: Understanding the limitations of phase field modeling in capturing complex interactions can guide researchers in adapting the model to different alloy systems and loading conditions effectively.

Cross-Validation: Insights gained from this study can be used as a reference point for cross-validating experimental and numerical results in other hydrogen-assisted cracking scenarios, ensuring the robustness of the findings.

By leveraging the foundational knowledge and methodologies established in this study, researchers can apply similar approaches to investigate hydrogen-assisted cracking in diverse alloy systems and loading conditions, expanding the understanding of this critical failure mechanism.